Anthropogenic climate change is a massive threat to our planet’s stability and habitability. Carbon dioxide makes up the majority of the greenhouse gas emissions leading to rising global temperatures. In order to reduce the global temperature, it is imperative to reduce dependency on fossil fuels by mass adaptation of renewable energy with net-zero carbon emissions. In this work, we present designs to convert incident solar energy to power through scalable nanophotonic systems.
We first introduce a tandem luminescent solar concentrator (LSC). LSCs are of interest due to their ability to concentrate both direct and diffuse light expanding the regions in which LSCs can be deployed. The tandem LSC uses a novel architecture in which InGaP micro-cells lie co-planar and optically coupled to the waveguide as opposed to the traditional edge-lined LSC. The waveguide consists of highly efficient CdSe/CdS quantum dots with emissions tuned to the band edge of the InGaP cells. This LSC is then coupled to a Si sub-cell allowing the tandem LSC to effectively convert a greater portion of the incident solar spectrum. We fabricate and perform outdoor testing on the first co-planar tandem LSC demonstrating a path to high efficiency LSCs.
We then introduce two methods to more efficiently trap light within the LSC. The first is a high contrast grating spectrally selective reflector. By using a high contrast grating, we can achieve high reflectivity with a single layer of high index materially patterned at a sub-wavelength scale on a low index substrate. While we explore both AlSb and a-SiC:H as grating materials, we pursue a-SiC:H and fabricate such a spectrally selective reflector with over 94% reflectivity at 642 nm. We then move to eliminate the need for spectrally selective filters by using photonic crystal waveguides to trap quantum dot emission within the LSC. We present two designs in which over 90% of emission remains trapped in the photonic crystal waveguide and is therefore able to travel to the photovoltaic material. We demonstrate how such a design can be used for LSCs in terrestrial and space solar power applications.
Lastly, we expand on the photonic crystal waveguide and introduce a thermal concentrator for production of scalable solar fuels. The thermal concentrator absorbs incident sunlight and traps the generated heat within the photonic crystal. This elevates the temperature within the thermal concentrator creating conditions under which catalytic reactions producing solar fuels can occur. We design a thermal concentrator that can heat up to 507.3 Kelvin under 1 sun illumination and 729.4 Kelvin under 3 sun illumination.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Atwater, Harry Albert}, } @phdthesis{10.7907/5r7z-zd88, author = {Needell, David Robert}, title = {High-Efficiency Luminescent Solar Concentrators for Photovoltaic Applications}, school = {California Institute of Technology}, year = {2022}, doi = {10.7907/5r7z-zd88}, url = {https://resolver.caltech.edu/CaltechTHESIS:10072021-154010065}, abstract = {Despite an overwhelming abundance of crude solar energy, current photovoltaic systems worldwide harness less than 1% of this available power. As such, emerging solar generation technology must be developed to further spur global adoption – whereby increased sunlight to power conversion efficiency alongside decreased system costs constitute the primary methods to accomplish this goal. The luminescent solar concentrator (LSC) offers a unique approach to collecting and redirecting large areas of incident light onto small-area solar cells. Relying upon photoluminescent materials (i.e., luminophores) suspended within a dielectric waveguide, the LSC absorbs high energy irradiance and re-emits photons at down-shifted energies into optical waveguide modes.
This thesis presents analytical, computational, and experimental work to illustrate the technical power conversion efficiency limits for LSC-based photovoltaic devices. We begin with a technical description of two LSC numerical models – a stochastic Monte Carlo ray-trace and a deterministic closed-form approach. We apply these models to quantify the effects of system and component parameters on power conversion efficiency for a number of end-use applications. To validate our modeling and unveil current practical material limits, we fabricate CdSe/CdS and CuInS2/ZnS core/shell quantum dot waveguides hosting embedded InGaP and GaAs photovoltaic cells, respectively. From these measurements, we observe close model-to-experiment matching and report a world-record LSC power conversion efficiency reaching approximately 10% under 1-sun illumination at modest incident to outgoing radiance areas.
Herein we consider four distinct applications for the LSC: (i) single junction LSC devices for terrestrial-based energy generation, (ii) building-integrated LSC form factors for on-site electricity, (iii) multijunction LSC modules for utility-scale installations at high power conversion efficiency, and (iv) ultra-light structures for on-board power in aerospace settings. We organize each chapter according to its end-use application.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Atwater, Harry Albert}, } @phdthesis{10.7907/kxdy-h496, author = {Glaudell, Rebecca Denise}, title = {Interface Optimization for Improved Photovoltaic Devices}, school = {California Institute of Technology}, year = {2022}, doi = {10.7907/kxdy-h496}, url = {https://resolver.caltech.edu/CaltechTHESIS:06022022-002941282}, abstract = {The wide band gaps and superior conductivity of ZnSₓSe₁₋ₓ semiconductors to amorphous Si suggest an alternative carrier-selective contact in silicon heterojunction solar cells. Electron-selective ZnSₓSe₁₋ₓ front contacts on p-type c-Si solar cells are explored by simulating in Sentaurus TCAD a large design parameter space informed by experimentally determined optoelectronic properties. Comparable performance to experimental and simulated p-SHJ reference devices is shown, with a champion simulated device efficiency of 20.8%. X-ray photoelectron spectroscopy is used to measure band offsets at interfaces for the aforementioned ZnSₓSe₁₋ₓ-c-Si photovoltaic devices as well as various carrier-selective contacts and passivation layers for GaAs photovoltaic devices.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Atwater, Harry Albert}, } @phdthesis{10.7907/pkxj-9584, author = {Kim, Yonghwi}, title = {Light Modulation with Vanadium Dioxide-Based Optical Devices}, school = {California Institute of Technology}, year = {2022}, doi = {10.7907/pkxj-9584}, url = {https://resolver.caltech.edu/CaltechTHESIS:06232021-050358035}, abstract = {This thesis focuses on active material-based tunable optical devices. In particular, I have been working on tunable optical devices based on vanadium dioxide (VO2), which can produce tunable optical responses, such as amplitude, phase, thermal emission, and quantum emission. The modulations of light are achieved by coupling the phase-transition material with the precisely designed resonant structures or by placing it close to quantum emitters. This thesis presents three research streams, which aim at experimentally demonstrating the dynamically tunable optical responses using VO2. First, we propose and experimentally demonstrate an electrically tunable VO2-based reflectarray metasurface that exhibits largely tunable optical responses in the near-infrared region. We incorporate VO2 directly into the plasmonic resonator, which undergoes a phase transition triggered by Joule heating. The induced plasmonic resonance modulation is accompanied by a large and continuous modulation in optical responses, such as amplitude, resonance wavelength, and phase. Second, we propose and demonstrate an active tuning of thermal emission from VO2-based metasurfaces. We introduce a thin VO2 film as an absorbing layer on top of a metal reflector. This layer is coupled with a dielectric resonator, with a dielectric spacer placed between them. Upon undergoing a phase transition triggered by heating, the induced absorption tuning of the VO2 layer is accompanied by modulation in the absorption spectra of the coupled structure. We experimentally show narrowband absorption spectra, which can be tuned by controlling the VO2 temperature. Finally, we experimentally demonstrate the axial position of quantum emitters in a multilayered hexagonal boron nitride (hBN) flake with nanoscale accuracy, which is enabled through the modification of a photonic density of states by introducing VO2. Furthermore, we observe a sharp distance-dependent photoluminescence response by modulating the optical environment of an emitter placed close to the hBN/VO2 interface.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Atwater, Harry Albert}, } @phdthesis{10.7907/pxk0-3d19, author = {Wong, Joeson}, title = {Optoelectronic Physics and Engineering of Atomically Thin Photovoltaics}, school = {California Institute of Technology}, year = {2022}, doi = {10.7907/pxk0-3d19}, url = {https://resolver.caltech.edu/CaltechTHESIS:12132021-004403821}, abstract = {Materials that are atomically thin behave substantially different than those of their bulk counterparts. However, when most materials become thinner, their surface-to-volume ratio increases and the number of unpassivated dangling bonds at the surface approaches the number of internal crystalline bonds, which prevents examining the intrinsic properties of most ultrathin materials. The recent discovery of layered materials, whose crystal structures have naturally passivated basal planes, has enabled the possibility to examine materials’ thicknesses that approach a single atomic layer.
In this thesis, we examine and explore the consequence of this new regime of thickness for active layers in photovoltaic applications. Specifically, we focus on the three aspects that define photovoltaic operation and explore their differences in these ultrathin materials: optical absorption of photons, subsequent carrier generation and transport, and finally, free energy extraction of collected carriers. We first discuss the implications of band-edge abruptness on the maximum efficiency of a solar cell. Then, we show that optical absorption in these ultrathin materials is dominated by cavity wave optics, and design structures that enable near-unity absorption in both ultrathin (~10 nm) and atomically-thin (~7 Å) active layers. Using these optical design rules, we design heterostructures with record incident photon to electron conversion efficiency (>50%). Next, we examine new methods of creating electrical junctions by using thickness to vary the amount of band bending in a material. We spatiotemporally image these ‘band-bending junctions’ for the first time. Finally, we argue that photoluminescence can be used as a direct readout of the open circuit voltage potential, and motivate examination of monolayer materials which have substantially higher radiative efficiency. We therefore examine the strain tuning of photoluminescence properties of both monolayer TMDC and heterobilayer TMDC systems. This work illustrates that van der Waals materials are an ideal system for examining the novel optoelectronic physics of atomically thin photovoltaics.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Atwater, Harry Albert}, } @phdthesis{10.7907/6jhy-2623, author = {Loke, Samuel Pei Hao}, title = {Photovoltaic Technologies Developed for Space-Based Solar Power}, school = {California Institute of Technology}, year = {2022}, doi = {10.7907/6jhy-2623}, url = {https://resolver.caltech.edu/CaltechTHESIS:05142022-060038327}, abstract = {In this thesis, photovoltaic technologies were developed for space-based solar power. Two methods of realizing SBSP were introduced, namely concentrated photovoltaics (CPV) and radiation hard flat panel photovoltaics. Both techniques are instrumental to realizing SBSP as they are pathways to realizing high specific power and lower launch costs. Technologies developed to support these two forms of SBSP were then reported.
In support of CPV, ultralight broadband mid-infrared coatings were developed for the concentrating mirrors used in our project. This was done to create radiative pathways for heat loss to ensure that the solar cells do not overheat. Using the rigorous coupled wave analysis technique, we optimized a backside single-layer coating using 2nm Cr/ 2μm CP1/ 500nm Ag that had an mIR emissivity of 0.6. Adding a second layer of this coating, we predicted that a 0.5nm Cr/ 1.9µm CP1/ 3nm Cr/ 2µm CP1/ 500nm Ag screen could achieve an emissivity of 0.8. We also optimized a 10nm ITO/ 2 μm CP1/ 500nm Ag frontside emitter which had a visible reflectivity of 0.896 and a mIR emissivity of 0.554. A backside emitter coating that was 0.927 emissive in the mIR with areal density 6.0 gm⁻² was successfully fabricated, as was a frontside mirror emitter coating with visible reflectivity of 0.896 and a mIR emissivity of 0.582 with areal density 4.1 gm⁻².
In support of radiation hard photovoltaics, organo-lead halide perovskites (OHLP) were investigated. Challenges facing their fabrication were explored, with special focus on the electron transport layer PCBM as well as OHLP formulation. It was found that doping PCBM with a surfactant CTAB was beneficial, but did not work with all surfaces. An ITO/NiOx/MAPbI3/CTAB+PCBM/Cu device with in-house champion efficiency of 12.41% was achieved, and an ITO/NiOx/FA0.85Cs0.15PbI3/PCBM/Cu device with in-house champion efficiency of 11.81% was achieved. Time-dependant drift diffusion modelling was employed to account for the S-kink arising from poor PCBM carrier concentration.
Finally, the proton degradation of OHLP devices and constituent transport layers were investigated to shed better light on how OHLP devices degrade under proton irradiation. Films of ITO, PEDOT, NiOx, PCBM, and PTCDi were found to degrade under 30keV and 75keV protons of up to 1.4 x 10¹⁴ p⁺cm⁻² fluence, but their electrical resistivity and optical transmissivity were not found to impact the cell as much as the OHLP absorber layer itself. Observing the light IV and EQE degradation of OHLP cells, it is evident that proton deposition in the OHLP layer itself causes the most damage, especially at 30keV and 75keV protons with fluences from 4.3 x 10¹³ p⁺cm⁻² to 1.7 x 10¹⁴ p⁺cm⁻². By considering the discrepancy in trends between Jsc and EQE, we concluded that the protons much accelerate intensity-based metastable photodegradation. Finally, by observing their anneal recovery, we concluded that it was temperature dependant and that maximum irrecoverable damage occurs at the OHLP/HTL interface.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Atwater, Harry Albert}, } @phdthesis{10.7907/btbp-6h76, author = {Jahelka, Phillip Robert}, title = {Progress in Low-Cost Gallium Arsenide Solar Cells}, school = {California Institute of Technology}, year = {2022}, doi = {10.7907/btbp-6h76}, url = {https://resolver.caltech.edu/CaltechTHESIS:05242022-235428954}, abstract = {In order to prevent disastrous global warming the manufacturing capacity of renewable energy power sources must grow rapidly. Solar photovoltaics will likely be one of humanity’s main sources of energy in the future due to the enormous available resource but increasing the manufacturing capacity of solar panels is hamstrung by both the limited profit margins of the highly-competitive renewable energy market and the enormous capital cost of building the factories that convert sand into semiconductor-grade silicon. Gallium arsenide is a material that can potentially help with the capital bottleneck because it absorbs light much more strongly than silicon and so the capital cost per unit weight of making the semiconductor can be spread over a larger number of devices and therefore effectively reduced. We present a number of results aimed at enabling low capital-cost GaAs solar cell manufacturing. First is a technique for open-tube, vapor phase zinc diffusion in GaAs. This method is dramatically simpler than its historical counterparts. Second, we use this technique to fabricate solar cells with Voc’s greater than 960 mV and uncertified efficiencies over 23%, large improvements over the state of the art. We further demonstrate a base-metal, air-tolerant ohmic contact to n-type GaAs which is an improvement over traditional contacts that require noble metals and inert atmospheres. We also found the existence of melt-grown n-type GaAs with minority carrier diffusion length comparable to vapor grown material which helps with the economic viability of these devices. We also performed a technoeconomic analysis on our proposed devices and find that they satisfy the desired properties of both the capital and electricity being cheaper than silicon solar cells. We also demonstrate the first n-on-p diffused junction GaAs solar cells.
As a parallel path to low capital intensity GaAs solar cells we also investigated non-epitaxial heterojunction devices. In the course of this work we both developed and characterized passivation chemistries for GaAs. Results in include the first use of a carbene and dithiothreitol for GaAs passivation and achieving surface recombination velocities comparable to GaInP passivation. With passivated organic heterojunction solar cells we were able to achieve a Voc of 840 mV which is a record for this class of devices, but its unclear how to improve the result to make them competitive with diffused junctions.
We also explored nanowire solar cells as an alternative strategy to reducing material usage by exploiting their strong light-absorption. We developed a computational model for a non-epitaxial GaAs heterojunction nanowire solar cell and predict an optimized efficiency over 30%. Towards fabrication we used metal-assisted-chemical-etching to make nanowire arrays and found we were able to cleanly cleave the nanowires embedded in a polymer from a 110 oriented wafer.
We also share some preliminary work on using total internal reflection in a solar cell encapsulant to mitigate shading loss due to the contacts on the front of a solar cell. We developed a computational model arguing that these structures could increase energy yield by 8% and demonstrated proof-of-principle experiments.
Finally, we share work on designing solar cells for operation on Venus. We developed models for the optical properties and recombination that correctly model the temperature dependence of a reference solar cell and using that model predict that a GaInP single-junction solar cell is a good solar-cell design for general usage in the atmosphere.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Atwater, Harry Albert}, } @phdthesis{10.7907/xrxk-3q08, author = {Went, Cora Margaret}, title = {Two-Dimensional Transition Metal Dichalcogenides for Ultrathin Solar Cells}, school = {California Institute of Technology}, year = {2022}, doi = {10.7907/xrxk-3q08}, url = {https://resolver.caltech.edu/CaltechTHESIS:04082022-171550192}, abstract = {Ultrathin solar cells, with absorber layers less than one micron thick, have the potential to use orders of magnitude less high-quality semiconducting material than current silicon solar cells. This could be advantageous in applications that require high power output per unit weight, such as vehicle-integrated photovoltaics, or where reducing the capital cost of solar cell manufacturing is important. Transition metal dichalcogenides are a promising candidate for the semiconducting absorber layer of ultrathin solar cells due to their intrinsically passivated surfaces and their high absorption per unit thickness.
This thesis explores two-dimensional transition metal dichalcogenides for ultrathin photovoltaics. We start with the simplest type of solar cell, which collects carriers via a Schottky junction formed by sandwiching the absorber layer between two metal contacts with different work functions. To enable this geometry and avoid Fermi-level pinning, we develop a new process for gently transferring van der Waals metal contacts onto transition metal dichalcogenides. We measure an open-circuit voltage of 250 mV and a power conversion efficiency of 0.5% in Schottky-junction solar cells. To improve upon this efficiency, we next make carrier-selective contact solar cells, which employ wide bandgap semiconductors to selectively collect electrons on one side and holes on the other side of the absorber layer. We measure an open-circuit voltage of 520 mV and a power conversion efficiency greater than 2% in devices based on perovskite solar cell geometries, with PTAA and C60 as selective contact layers. We demonstrate that short carrier lifetimes limit the voltage in these solar cells to 750 mV, well below the detailed balance voltage limit. This motivates a more thorough understanding of the carrier dynamics at play, and we use a new pump-probe optical microscopy technique, stroboSCAT, to spatiotemporally track heat and carrier evolution in transition metal dichalcogenides. When paired with a kinetic model, we show that this technique can be used to measure lifetimes and other important material parameters even in materials with low radiative efficiencies.
We conclude by outlining future research directions towards achieving power conversion efficiencies greater than 10% in transition metal dichalcogenide solar cells.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Atwater, Harry Albert}, } @phdthesis{10.7907/4s78-cq55, author = {Welch, Alexandra Justine}, title = {Understanding and Optimizing the Local Catalyst Environment in CO₂ Reduction Electrodes}, school = {California Institute of Technology}, year = {2022}, doi = {10.7907/4s78-cq55}, url = {https://resolver.caltech.edu/CaltechTHESIS:11062021-151820825}, abstract = {Understanding and managing the local microenvironments in carbon dioxide reduction catalysts is crucial for optimizing device performance. In particular a locally high pH can increase catalyst selectivity and activity, as well as indicate which part of the catalyst is most active. In this thesis we begin by studying how nanoporous catalysts can induce this locally high pH in an aqueous system. We observe an increase in both Faradaic efficiency and partial current density for carbon monoxide in the nanoporous system relative to a planar metal film. We then show that this same nanoporous architecture can be used for improved device performance in a gas diffusion electrode configuration. We also perform copper underpotential deposition and secondary ion mass spectroscopy to show that almost half of the catalyst is not in contact with the electrolyte in this configuration. Then we use confocal fluorescent microscopy to image the local pH in a gas diffusion electrode to determine which parts of the electrode are most active. Through a combination of experiment and simulations we find that the catalyst within thin cracks of the microporous layer is most active for carbon dioxide reduction. While the study of local pH and wetting is the main focus of this thesis, we also explore how light can be used to improve selectivity and activity. In particular we study gold nanoparticles on p-type gallium nitride and copper nanoparticles on p-type nickel oxide. Finally, this thesis also explores how carbon dioxide conversion can actually be deployed. We discuss opportunities for combining carbon dioxide capture and conversion, as well as evaluate different pathways for renewable methane generation.
This thesis gives in depth analysis of electrochemical carbon dioxide reduction catalysts as well as putting this research into the larger context of how such devices can be deployed. We hope that by combining systems level thinking and specific device studies better carbon dioxide conversion systems can be realized.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Atwater, Harry Albert}, } @phdthesis{10.7907/m554-as73, author = {Kafaie Shirmanesh, Ghazaleh}, title = {Electro-Optically Tunable Metasurfaces for a Comprehensive Control of Properties of Light}, school = {California Institute of Technology}, year = {2021}, doi = {10.7907/m554-as73}, url = {https://resolver.caltech.edu/CaltechTHESIS:09172020-190836007}, abstract = {The ability to control electromagnetic wavefront is a central key in optics. Conventional optical components rely on the gradual accumulation of the phase of light as it passes through an optical medium. However, since the accumulated phase is limited by the permittivity of naturally existing materials, such a mechanism often results in bulky devices that are much thicker than the operating wavelength.
During the last several years, metasurfaces (quasi-2D nanophotonic structures) have attracted a great deal of attention owing to their promise to manipulate constitutive properties of electromagnetic waves such as amplitude, phase, and polarization. Metasurfaces are ultrathin arrays of subwavelength resonators, called meta-atoms, where each meta-atom imposes a predefined change on the properties of the scattered light. By precisely designing the optical response of these meta-atoms to an incident wave, metasurfaces can introduce abrupt changes to the properties of the transmitted, reflected, or scattered light, and hence, can flexibly shape the out-going wavefront at a subwavelength scale. This enables metasurfaces to replace conventional bulky optical components such as prisms or lenses by their flat, low-profile analogs. Furthermore, a single metasurface can perform optical functions typically attained by using a combination of multiple bulky optical elements, offering tremendous opportunities for flat optics.
The optical response of a metasurface is typically dictated by the geometrical parameters of the subwavelength scatterers. As a result, most of the reported metasurfaces have been passive, namely have functions that are entirely fixed at the time of fabrication. By making the metasurfaces reconfigurable in their phase, amplitude, and polarization response, one can achieve real-time control of optical functions, and indeed, achieve multi-functional characteristics after fabrication. Dynamical control of the properties of the scattered light is possible by using external stimuli such as electrical biasing, optical pumping, heating, or elastic strain that can give rise to changes in the dielectric function or physical dimensions of the metasurface elements.
In this dissertation, we present the opportunities and challenges towards achieving reconfigurable metasurfaces. We introduce a paradigm of active metasurfaces for real-time control of the wavefront of light at a subwavelength scale by investigating different modulation mechanisms and possible metasurface designs and material platforms that let us effectively employ the desired modulation mechanism. We will present multiple electro-optically tunable metasurface platforms. These electronically-tunable schemes are of great interest owing to their robustness, high energy-efficiency, and reproducibility. We will also show the design and experimental demonstration of active metasurfaces for which the tunable optical response can be tailored in a pixel-by-pixel configuration.
The ability to individually control the optical response of metasurface elements has made active optical metasurfaces to be progressively ubiquitous by enabling a wide range of optical functions such as dynamic holography, light fidelity (Li-Fi), focusing, and beam steering. As a result, reconfigurable metasurfaces can hold an extraordinary promise for optical component miniaturization and on-chip photonic integration. Such compact and high-performance devices with reduced size, weight, and power (SWaP) can be used in future free-space optical communications or light detection and ranging (LiDAR) systems.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Atwater, Harry Albert}, } @phdthesis{10.7907/vh7k-4w84, author = {Lin, Wei-Hsiang}, title = {Synthesis of 2D Quantum Materials for Nanoelectronic and Nanophotonic Applications}, school = {California Institute of Technology}, year = {2021}, doi = {10.7907/vh7k-4w84}, url = {https://resolver.caltech.edu/CaltechTHESIS:04262021-085536699}, abstract = {2D materials have attracted tremendous attention for a variety of properties such as ultra-low body thickness, ultra-high mobility, and tunable bandgap. These unique merits of the 2D materials bring in the significant improvements and new perspectives in the digital CMOS scaling, analog performance, as well as the 3D integration of wafer stacking.
In this thesis, we explore van der Waals materials for future CMOS technologies. Chapter 2 introduces a compatible and a single-step method for synthesizing high-mobility monolayer graphene (MLG) in merely a few minutes by means of plasma-enhanced chemical vapor deposition (PECVD) techniques without the need of active heating. This environment enables graphene growth on different surfaces at relatively low temperatures, which paves ways to a CMOS-compatible approach to graphene synthesis. Chapter 3 describes the development of a synthesis method that controls the growth of large-area h-BN films from monolayer to 30 atomic layers, and summarizes the characterizations of the properties of these h-BN films that demonstrate the high-quality of these materials.
New degrees of freedom possess the immense potential and attract huge attentions as the imminent end of “Moore’s Law”. Compared with the traditional charge degree of freedom, spin and valley are the other two additional internal degree of freedom in solid-state electronics which enable the spintronic and valleytronic devices with high integration density, fast processing speed, low power dissipation, and non-volatility. Monolayer transition-metal dichalcogenides (TMDCs) in the 2H-phase are semiconductors promising for opto-valleytronic and opto-spintronic applications because of their strong spin-valley coupling. In chapter 4, we report detailed studies of opto-valleytronic properties of heterogeneous domains in CVD-grown monolayer WS₂ single crystals. By illuminating WS₂ with off-resonance circularly-polarized light and measuring the resulting spatially resolved circularly-polarized emission (Pcirc), we find large circular polarization increases significantly to nearly 90% at 80 K. In Chapter 5, it is reported that valley polarized PL of monolayer WS₂ can be efficiently tailored at room temperature (RT) through the surface plasmon-exciton interaction with plasmonic Archimedes spiral (PAS) nanostructures. The DVP of WS₂ using 2 turns (2T) and 4 turns (4T) of PAS can reach up to 40% and 50% at RT, respectively. Further enhancement and continuous control of excitonic valley polarization in electrostatically doped monolayer WS₂ are demonstrated. Under the circularly polarized light on WS₂-2TPAS heterostructure, 40% valley polarization of exciton without electrostatic doping is icreased to 70% by modulating the carrier doping via a backgate. This enhancement of valley polarization may be attributed to the screening of momentum-dependent long-range electron-hole exchange interactions. The demonstration of electrical tunability in the valley-polarized emission from WS₂-PAS heterostructures provides new strategies to harness valley excitons for application in ultrathin valleytronic devices.
In contrast to future optical switch applications, in Chpater 6, it is reported that Ternary tellurides based on alloying different 2D transition metal dichalcogenides can result in interesting new 2D materials with tunable optical and electrical properties. Additionally, such alloys can provide opportunities for significantly improving the electrical contact properties at the metal-semiconductor interface. In particular, realization of practical devices based on the 2D materials will require overcoming the typical Fermi-level pinning limitations of the electrical contacts at the metal-semiconductor interface and ultimately approaching the ideal Schottky-Mott limit. In this work, we develop a simple method of stacking 3D/2D electrical metal contacts onto dangling-bond-free 2D semiconductors in order to surmount the typical issue of Fermi-level pinning. Specifically, contacts of Au, graphene/Au, and WTe₂/Au are transferred onto WS1.94Te0.06 alloy-based devices via a new transfer method. The WS1.94Te0.06 field-effect transistors (FETs) with WTe₂/Au contacts reveal a field-effect mobility of 25 cm²V⁻¹s⁻¹, an on/off current ratio of 10⁶, and extremely low contact resistance of 8 kΩ μm. These electrical properties are far more superior to similar devices with either Au or graphene/Au contacts, which may be attributed to the fact that the work function of WTe₂ is close to the band edge of the WS1.94Te0.06 alloy so that the resulting metal-semiconductor interface of the FETs are free from Fermi-level pinning. The Schottky barrier heights of the WS1.94Te0.06-FETs with WTe₂/Au contacts also follow the general trend of the Schottky-Mott limit, implying high-quality electrical contacts. Finally, in Chapter 7, several promising opportunities were proposed for future CMOS integrated circuits based on monolayer semiconductors.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Atwater, Harry Albert}, } @phdthesis{10.7907/kd6a-xt88, author = {Cheng, Wen-Hui (Sophia)}, title = {Towards High Solar to Fuel Efficiency: From Photonic Design, Interface Study, to Device Integration}, school = {California Institute of Technology}, year = {2020}, doi = {10.7907/kd6a-xt88}, url = {https://resolver.caltech.edu/CaltechTHESIS:05282020-183139057}, abstract = {Efficient unassisted solar fuel generation, a pathway to storable renewable energy in the form of chemical bonds, requires optimization of a photoelectrochemical device based on photonic design and interface study. We first focused on enhancing absorption via nanophotonic design of light absorbers. Near-unity, broadband absorption in sparse InP nanowire arrays with multi-radii and tapered nanowire array designs are simulated and experimentally demonstrated. Later, a few strategies are introduced to achieved high solar-to-fuel efficiency.
Optically, photoelectrochemical device would require the catalyst ensembles to be highly transparent. We report a record solar-to-hydrogen efficiency by integrating Rh nanoparticle catalysts onto photocathodes with minimal parasitic absorption and reflection losses in the visible range. The other two light management strategies have been developed and experimentally verified to create highly active and effectively transparent catalyst structures: i) arrays of mesophotonic dielectric cone structures that serve as tapered waveguide light couplers to efficiently guide incident light through apertures in an opaque catalyst into the light absorber, and ii) an effectively transparent catalyst consisting of arrays of micron-scale triangular cross-sectional metal grid fingers, which are capable of redirecting the incoming light to the open areas of the PEC cell without shadow loss.
The electronic properties of the surface films exposed to the electrolyte are also critical. The anatase TiO₂ protection layer on the photocathode creates a favorable internal band alignment for hydrogen evolution, promoting the transport of the excess electrons and inhibiting voltage drops. The interfacial conduction mechanism between the defected TiO₂ and metal catalysts is investigated. A combinatorial approach of electrochemistry, X-ray photoelectron spectroscopy, and resonant X-ray spectroscopy reveals the correlation between the interfacial quasi-metal phase with TiO₂ properties. By careful control of gas diffusion electrode assembling to maintain appropriate wetted catalyst interface, another record solar-to-CO efficiency with extended stability can be realized.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Atwater, Harry Albert}, } @phdthesis{10.7907/tg1b-hn35, author = {Tokpanov, Yury}, title = {Towards Next Generation of Optoelectronics: from Quantum Plasmonics and 2D Materials to Advanced Optimization Techniques of Nanophotonic Devices}, school = {California Institute of Technology}, year = {2020}, doi = {10.7907/tg1b-hn35}, url = {https://resolver.caltech.edu/CaltechThesis:06012020-093627645}, abstract = {In this thesis, we explore different novel concepts and materials for the next-generation of nanophotonic and optoelectronic devices that could be used both in classical and quantum settings.
First, we study quantum coherence properties of surface plasmon polaritons (SPPs) in the regime of extreme dispersion. Most experiments to date, that tested quantum coherence properties of SPPs, used essentially weakly-confined plasmons, which experience limited light-matter hybridization, thus restricting the potential for decoherence. Our setup is based on a hole-array chip supporting SPPs near the surface plasma frequency, where plasmonic dispersion and confinement is much stronger than in previous experiments, making the plasmons much more susceptible for decoherence processes. We generated polarization-entangled pairs of photons and transmitted one of the photons through this plasmonic hole array. Our results show that the quality of photon entanglement after the highly-dispersive plasmonic channel is unperturbed. Our findings provide a lower bound of 100 femtoseconds for the pure dephasing time of dispersive plasmons in our materials, and show that even in a highly dispersive regime, surface plasmons preserve quantum mechanical correlations, making possible harnessing the power of extreme light confinement for integrated quantum photonics.
Second, we systematically study different passivation schemes of sulfur vacancies in 2D molybdenum disulfide using first-principles calculations based on density functional theory. We aim at building a microscopic understanding of passivation mechanisms of treatment with TFSI superacid - a popular approach of to improve optical properties. Since superacids have a strong ability to donate protons, we consider hydrogenation and protonation of sulfur vacancies as a possible passivation scheme. Our calculations show that effects of protonation and hydrogenation on properties of 2D molybdenum disulfide are very similar. Moreover, we find that four hydrogen atoms can fully “heal” sulfur vacancies in this material. Our results are an important step towards controllable defects design in 2D transition metal dichalcogenides.
And third, we study applications of advanced methods of optimization and machine learning to the design of different nanophotonic devices. We explore feasibility of using novel multi-fidelity Gaussian processes optimization technique to optimize plasmonic mirror filters for hyperspectral imaging. We compare our results with other common optimization approaches. Then we apply deep-learning inspired techniques to optimize control voltages of individual pixels of active metasurfaces to achieve dynamic beamsteering. We obtain interesting results that pave the way for future experiments both in nanophotonics and machine learning fields.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Atwater, Harry Albert}, } @phdthesis{10.7907/kqaj-ex65, author = {Mitskovets, Anna}, title = {Using DNA Origami to Create Hybrid Nanophotonic Architectures for Single-Photon Emitters}, school = {California Institute of Technology}, year = {2020}, doi = {10.7907/kqaj-ex65}, url = {https://resolver.caltech.edu/CaltechTHESIS:05312020-234931669}, abstract = {The limitations in physical dimensions of silicon transistors give us a stimulus to explore alternative systems for better computational performance. The most promising system that received a lot of attention in the past few years is a quantum computer. Ideally, a nanophotonic quantum computer would consist of hundreds of single-photon emitters, optical or plasmonic resonators, optical waveguides and interconnects. The main difficulty in large-scale production of such quantum photonic networks is the integration and deterministic coupling of single-photon sources to photonic elements.
In the first part of this thesis, we utilize spontaneous parametric down-conversion to create correlated pairs of indistinguishable photons. These photons are generated by bismuth borate nonlinear crystal and then are coupled to a photonic chip where they interfere at directional couplers to produce a path-entangled state. Our photonic chip consists of waveguides, directional couplers, and a single Mach-Zender interferometer with a thermo-optic phase shifter. When a part of the waveguide connecting directional couplers is replace with a plasmonic waveguide, quantum state of photons is converted to plasmonic state. Here we report a measurement of path entanglement between surface plasmons with 95% contrast, confirming that a path-entangled state can indeed survive without measurable decoherence. Our measurement suggests that elastic scattering mechanisms of the type that might cause pure dephasing in plasmonic systems must be weak enough not to significantly perturb the state of the metal under the experimental conditions we investigated.
The second part of this work is dedicated to the study of a novel DNA origami self-assembly technique for creating hybrid nanophotonic architectures to create single-photon emitters. DNA origami is a modular platform for the combination of molecular and colloidal components to create optical, electronic, and biological devices. We present a DNA origami molecule that can be deterministically positioned on a silicon chip within 3.2° alignment. Orientation is absolute (all degrees of freedom are specified) and arbitrary (every molecule’s orientation is independently specified). The use of orientation to optimize device performance is shown by aligning fluorescent emission dipoles within microfabricated optical cavities. Large-scale integration is demonstrated via an array of 3,456 DNA origami with 12 distinct orientations, which indicates the polarization of the excitation light. Following this experiment, we explore how many molecular emitters can be coupled to this DNA origami shape and discover interesting interactions between ssDNA extensions that can cause origami to fold along its seam. Finally, we examine DNA origami self-assembly methods that can be used to deterministically couple single-photon emitters to resonators in order to decrease pure-dephasing rates and increase indistinguishability of emitted photons.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Atwater, Harry Albert}, } @phdthesis{10.7907/KKB7-KN62, author = {Whitney, William Schuyler}, title = {Electrically-Tunable Light-Matter Interactions in Quantum Materials}, school = {California Institute of Technology}, year = {2019}, doi = {10.7907/KKB7-KN62}, url = {https://resolver.caltech.edu/CaltechTHESIS:05252019-154108242}, abstract = {Dynamic control of the flow of light at the nanoscale is critical for next-generation optoelectronic devices that will enable the technologies of the future. Ultra-thin, layered materials are promising building blocks for this functionality, as they are easily fabricated into atom-scale structures, and their optical properties change dramatically under applied electric fields. Many of these material systems, like topological insulators – a subset of layered materials that host spin-polarized surface states, promise more exotic functionality as well. The emerging field of nanophotonics in quantum materials is a route not only to an improved material platform for optoelectronics, but also to new physics, and the potential new device paradigms that follow.
In this work we describe investigations of electrically-tunable light-matter interactions in two different layered materials: few-layer black phosphorus, and bismuth antimony telluride. In few-layer black phosphorus, we demonstrate several in-plane anisotropic optoelectronic phenomena, including Pauli-blocking of intersubband optical transitions under carrier injection, a quantum-confined Stark effect, and a change of quantum well selection rules under applied electric field. We further describe how these optoelectronic phenomena drive anisotropic birefringence and dichroism in few-layer black phosphorus. Lastly, we present theory describing amplitude, phase and polarization control in a black phosphorus integrated microcavity device, with applications that include metasurface beam-steering and more.
We next present experiments demonstrating field-effect control of optical transitions in bismuth antimony telluride. These measurements evidence the merits of topological insulators as optoelectronic materials, and highlight a pathway towards future exploration of spin-plasmon excitations in bismuth antimony telluride.
Lastly, we present a summary of pending work, including initial results of an ongoing study of plasmon excitations in few-layer black phosphorus, and a perspective on next steps for both these projects and nanophotonics in quantum materials at large.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Atwater, Harry Albert}, } @phdthesis{10.7907/VRFE-ZY57, author = {Brouillet, Jeremy Jean}, title = {Graphene-Mediated Light-Matter Interaction}, school = {California Institute of Technology}, year = {2019}, doi = {10.7907/VRFE-ZY57}, url = {https://resolver.caltech.edu/CaltechTHESIS:06022019-011156429}, abstract = {Advances in 2D materials have opened a wealth of possibilities for the control of emission and propagation of light on length scales much smaller than the wavelength of light. Graphene, with highly-confined electrostatically tunable plasmons, provides a strong platform for explore a number of avenues.
We show that graphene that can increase the luminescence of erbium by 80%, can induce population inversion in a three-level system, speed up the response time by over an order of magnitude, and has modulation depth of up to 14 dB for luminescence.
We experimentally demonstrated a tunable epsilon-near-zero metamaterial with a elliptic-to-hyperbolic transition. The device had been theorized for many years and we provide the first experimental realization.
We explore the properties of an isotropic tunable 2D heterostructure composed of black phosphorus, hexagonal boron nitride, and graphene. These symmetry-breaking materials create an effective permittivity that is biaxially anistropic and tunable. This material supports tunable beam steering based on propagation of energy along the hyperbolic dispersion lines.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Atwater, Harry Albert}, } @phdthesis{10.7907/RRYM-VK03, author = {Yalamanchili, Sisir}, title = {Light Management in Photovoltaics and Photoelectrochemical Cells using Tapered Micro and Nano Structures}, school = {California Institute of Technology}, year = {2019}, doi = {10.7907/RRYM-VK03}, url = {https://resolver.caltech.edu/CaltechTHESIS:12182018-111020369}, abstract = {Enhancing the efficiency and reducing the cost of solar photovoltaic (PV) systems is critical for increasing their penetration into energy generation market. The intermittency of energy generation from such systems due to diurnal, seasonal, and weather-related variation of sunlight limits them to low capacity factors (typically ~ 25%). Therefore, despite the cost of electricity from solar PV systems being cost competitive, further reductions are necessary to incorporate storage and increase the fraction of solar energy in total energy generation. An integrated photoelectrochemical (PEC) system that can generate fuel directly from sunlight could potentially reduce the balance of systems cost that dominates current PV systems, and provide an alternative way for energy storage. PEC systems are currently in research stage.
In this work conical and triangular micro-nano structures are utilized to explore optical solutions for maximizing the light absorption and therefore enhancing the efficiencies of both PV and PEC systems. Silicon (Si) based micro conical arrays demonstrate < 1 % Spectrum-and-Angle-Averaged reflection, and absorption nearing ray optic light trapping limit in a 20 µm effectively thick Si substrates. Si microcone based photocathodes prepared for performing hydrogen evolution reaction (HER) show that thick layers of light blocking Pt and Co-P catalysts can be incorporated with only a 6 % photocurrent loss. The light trapping properties of Si micro-cones are a result of efficient coupling of light to available waveguide modes in a conical geometry. Alternatively, TiO2 based dielectric nano-conical arrays are shown to couple the light to waveguide modes and transmit the light into a planar Si substrate despite covering 54 % of the planar front surface with a light blocking Ni catalyst as an alternative way of light trapping without texturing the light absorber.
Triangular silver (Ag) front contacts in place of conventional flat contacts over PV cells are shown as another alternative for reducing front contact reflection losses and enhancing the efficiency by ~ 1% in Si heterojunction solar cells. These structures are implemented using a polymer stamp prepared from a Si master with triangular groves, and by flowing Ag ink through them. A Si master fabrication method is shown for fabrication of multiple configurations of triangular Ag contacts which can potentially be applied to other PV and PEC systems to enhance their efficiency.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Atwater, Harry Albert and Lewis, Nathan Saul}, } @phdthesis{10.7907/RA1G-GS84, author = {Fleischman, Dagny}, title = {Nanophotonic Structures: Fundamentals and Applications in Narrowband Transmission Color Filtering}, school = {California Institute of Technology}, year = {2019}, doi = {10.7907/RA1G-GS84}, url = {https://resolver.caltech.edu/CaltechTHESIS:12032018-173954965}, abstract = {
The optical properties of materials can be manipulated by structures roughly the size of the wavelength of light of interest. For visible wavelengths, many different types of structures sized on the order of 10s-100s of nanometers have been used to engineer materials to produce a targeted optical response. Multilayer stacks of nanoscale metal and dielectric films are a widely explored geometry that has been used to make composite materials with effective optical properties that vary significantly from their constituent films. In this thesis, carefully designed multilayer stacks were used to induce artificial magnetism in non-magnetic materials, opening new directions for tailoring wave propagation in optical media. By perforating these multilayer structures with an array of sub-wavelength slits, these nanophotonic structures were shown to be able to function as narrowband transmission color filters. Using numerical optimization methods, these narrowband filterswere further refined and simplified to only require a single thin film sandwiched between two mirrors to achieve this high resolution spectral filtering. Novel methods were used to fabricate these ultracompact narrowband transmission color filters, which were shown to possess extremely narrow transmission resonances that can be controllably pushed across the visible and near IR parts of the spectrum. These mirrored color filters have footprints as small as 400 nm, well below the size of state-of-the-art CMOS pixels, inviting the possibility for integrating multi- and hyperspectral imaging capabilities into small portable electronic devices.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Atwater, Harry Albert}, } @phdthesis{10.7907/1CDC-HV37, author = {Kim, Laura}, title = {Novel Light Emitting Mechanisms Originating from Graphene Plasmons Near and Far from Equilibrium}, school = {California Institute of Technology}, year = {2019}, doi = {10.7907/1CDC-HV37}, url = {https://resolver.caltech.edu/CaltechTHESIS:05062019-203520662}, abstract = {Graphene supports surface plasmons bound to an atomically thin layer of carbon, characterized by tunable propagation characteristics and distinctly strong spatial confinement of the electromagnetic energy. Such collective excitations in graphene enable the strong interactions of massless Dirac fermions with light. In this work, I explore fundamental properties and applications of graphene plasmons both near and far from equilibrium. I discuss the ability of graphene plasmons to interact with its local environment in various forms of mid-infrared, optically active excitations, demonstrated by tunable graphene plasmon dispersions and an emergence of a new mode via addition of a monoatomic dielectric layer. Furthermore, the viability of graphene for optics-based applications and large-scale integration is epitomized by the experimental demonstration of perfect tunable absorption in a large-area chemically grown graphene by using a noble-metal-graphene metasurfaces. Using these properties of graphene plasmons, electronically tunable thermal radiation is demonstrated. Finally, I present theoretical predictions and experimental validations of nonequilibrium graphene plasmon excitations via ultrafast optical excitation, originating from a previously unobserved decay channel: hot plasmons generated from optically excited carriers. These studies reveal novel infrared light emitting processes, both spontaneous and stimulated, and provide a platform for achieving ultrafast, ultrabright mid-infrared light sources.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Atwater, Harry Albert}, } @phdthesis{10.7907/0HJF-X691, author = {Mauser, Kelly Ann Weekley}, title = {Resonant Thermoelectric Nanophotonics: Applications in Spectral and Thermal Sensing}, school = {California Institute of Technology}, year = {2019}, doi = {10.7907/0HJF-X691}, url = {https://resolver.caltech.edu/CaltechTHESIS:06062019-191307495}, abstract = {Plasmon excitation enables extreme light confinement at the nanoscale, localizing energy in subwavelength volumes and thus can enable increased absorption in photovoltaic or photoconductive detectors. Nonetheless, plasmon decay also results in energy transfer to the lattice as heat which is detrimental to photovoltaic detector performance. However, heat generation in resonant subwavelength nanostructures also represents an energy source for voltage generation, as we demonstrate in the first part of this thesis via design of resonant thermoelectric (TE) plasmonic absorbers for optical detection. Though TEs have been used to observe resonantly coupled surface plasmon polaritons in noble-metal thin films and microelectrodes, they have not been employed previously as resonant absorbers in functional TE nanophotonic structures.
We demonstrate nanostructures composed of TE thermocouple junctions using established TE materials – chromel/alumel and bismuth telluride/antimony telluride – but patterned so as to support guided mode resonances with sharp absorption profiles, and which thus generate large thermal gradients upon optical excitation and localized heat generation in the TE material. Unlike previous TE absorbers, our structures feature tunable narrowband absorption and measured single junction responsivities 4 times higher than the most similar (albeit broadband) graphene structures, with potential for much higher responsivities in thermopile architectures. For bismuth telluride – antimony telluride single thermocouple structures, we measure a maximum responsivity of 38 V/W, referenced to incident illumination power. We also find that the small heat capacity of optically resonant TE nanowires enables a fast, 3 kHz temporal response, 10-100 times faster than conventional TE detectors. We show that TE nanophotonic structures are tunable from the visible to the MIR, with small structure sizes of 50 microns x 100 micons. Our nanophotonic TE structures are suspended on thin membranes to reduce substrate heat losses and improve thermal isolation between TE structures arranged in arrays suitable for imaging or spectroscopy. Whereas photoconductive and photovoltaic detectors are typically insensitive to sub-bandgap radiation, nanophotonic TEs can be designed to be sensitive to any specific wavelength dictated by nanoscale geometry, without bandgap wavelength cutoff limitations. From the point of view of imaging and spectroscopy, they enable integration of filter and photodetector functions into a single structure. Other thermoelectric nanophotonic motifs are also explored.
Generating localized, high electric field intensity in nanophotonic and plasmonic devices has many applications, from enhancing chemical reaction rates, to thermal radiation steering, to chemical sensing, and to photovoltaics. Along with a strongly localized electric field comes a temperature rise in non-lossless photonic materials, which can affect reaction rate, photovoltaic efficiency, or other properties of the system. Measuring temperature rises in nanophotonic structures is difficult, and methods commonly employed suffer from various limitations, such as low spatial resolution (Fourier transform infrared microscopy), bulky and expensive setups (scanning thermal microscopy), intrusive methods that interfere with nanophotonic structures (Pt resistive thermometry), or the need for specialized materials (temperature dependent photoluminescence).
In the second part of this thesis, we overcome these limitations with the first-ever demonstration of temperature measurements of nanophotonic structures by employing both room temperature noise thermometry and the thermoelectric effect under ambient conditions without external probes by utilizing the properties of the materials that make up the nanophotonic structure itself. We have previously estimated the Δ T in a nanophotonic device using the thermoelectric effect, but could not determine the absolute temperature of the system. In the application we will discuss, the absolute electron temperature of the nanophotonic material itself is measured. Because Johnson-Nyquist noise is material independent and is a fundamental measure of absolute temperature, there is theoretically no need for calibration as in the case of resistive thermometry. To measure the temperature rise of a nanophotonic resonant region remotely, the Seebeck coefficient of the material is first carefully measured using noise thermometry, then the thermoelectric voltage generated in the nanophotonic materials themselves is measured from electrical leads spanning the resonantly excited region. To accomplish this, we have developed a metrology technique capable of simultaneously measuring electrical noise at two locations on the nanophotonic structure as well as the electrical potential between the two points, under chopped laser illumination that heats the structure via nanophotonic absorption, thus providing drift-corrected light on/off temperature information.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Atwater, Harry Albert}, } @phdthesis{10.7907/J5FG-1B48, author = {Bukowsky, Colton Robert}, title = {Scalable Nanophotonic Light Management Design for Solar Cells}, school = {California Institute of Technology}, year = {2019}, doi = {10.7907/J5FG-1B48}, url = {https://resolver.caltech.edu/CaltechTHESIS:06012019-002405475}, abstract = {The current trend in wide adoption of solar energy is encouraging in the context of current projections of increasing energy consumption and the dire need to decrease carbon emissions. The solar industry has expanded due to scientific advances in the power conversion efficiency of solar modules. In order maintain a rapid pace of adoption and further decrease electricity costs, converting each photon becomes increasingly important. This work focuses on nanophotonic approaches to increasing the power conversion efficiency of different solar photovoltaic designs. The projects voluntarily impose certain design constraints in order to be compatible with the large scale manufacturing needed by the solar industry. A focus was given to designs that can leverage the promising technology of nanoimprint lithography. Amorphous silicon tandem cells with embedded nanophotonic patterning attempted to increase absorption while minimizing materials and time costs. Simulated designs of Copper Indium Gallium Diselenide absorbers showed that the management of excited carriers is equally as important as light management in decreasingly thin absorber layers. Near perfect anti-reflection structures were given a detailed physical analysis to better describe the fundamental physics of near zero reflection due to nanocones printed on solar cell encapsulation glass. Experimental results agreed with the theoretical analysis, and showed that these nanostructures further increased absorbed photocurrent by trapping light in the encapsulation glass. Finally, a unique device in the form of a tandem luminescent solar concentrator/silicon solar module was proposed and analyzed as a low cost and adaptable technology for increased solar power conversion efficiency. Key to this design was discovery of new, near-perfect components for light management. Exciting and innovative designs are proposed to control the light-matter interaction within these devices. Study of a photonic luminescent solar concentrator predicted that luminescence can be trapped in photonic crystal slab waveguides with near zero loss. Rigorous experimental efforts to characterize a multitude of near-perfect samples help guide these designs toward their final goals.}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Atwater, Harry Albert}, } @phdthesis{10.7907/B6AS-EA22, author = {Omelchenko, Stefan Thomas}, title = {Towards a Net-zero Carbon Energy System: High Efficiency Photovoltaics and Electrocatalysts}, school = {California Institute of Technology}, year = {2019}, doi = {10.7907/B6AS-EA22}, url = {https://resolver.caltech.edu/CaltechTHESIS:06022019-111830413}, abstract = {Modern society is dependent on energy. Despite increases in energy efficiency, human development and economic goals are expected to increase the global demand for energy by almost 30% in the next 20 years. At the same time, anthropogenic carbon dioxide emissions must approach zero to stabilize global temperatures below the 2°C target set out by international climate agreements. Realizing a net-zero carbon energy system will depend on the development of a highly reliable, sustainable electricity grid to power society and the ability to produce chemicals and fuels in a carbon-free manner. Developing cheap, efficient solar photovoltaics and highly active and selective electrocatalysts is thus pivotal to achieving this goal.
In this work, we address issues limiting photovoltaics and electrocatalysts. Our work on photovoltaics analyzes two effects often neglected in the evaluation of efficiency limits for photovoltaic materials. We show that the shape of the band tail and, in particular, the extent of sub-gap absorption, controls the open-circuit voltage, emission, and ultimately the achievable efficiency of a solar cell. These findings are generalizable to any luminescent material and our analysis suggests that efficiency limits for a material can be determined through simple experimental characterization. In addition, we develop a device physics model which accounts for the presence of excitons, which are the fundamental excitation in a host of emerging photovoltaic materials. A case study in cuprous oxide shows that excitonic effects can play a large role in the device physics of materials with large exciton binding energies and that standard models can drastically underestimate the efficiency limits in these systems. Our work on photovoltaics, culminates in the realization of a novel device architecture for tandem silicon/perovskite solar cells that opens the possibility of achieving efficiencies >30%. Finally, we develop a method to tune the catalytic activity of electrocatalysts for the oxygen-evolution and chlorine-evolution reactions. Our method is based on group electronegativity and is likely generalizable to other reactions and catalysts. The analyses and technologies developed herein are promising steps towards a zero-carbon energy system.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, } @phdthesis{10.7907/Z9NZ85V2, author = {Peng, Siying}, title = {2D and 3D Photonic Crystals: Synthesis, Characterization and Topological Phenomenon}, school = {California Institute of Technology}, year = {2018}, doi = {10.7907/Z9NZ85V2}, url = {https://resolver.caltech.edu/CaltechTHESIS:09112017-095117655}, abstract = {Topological photonics has become an increasingly popular research topic in the field of nanophotonics in recent years. Topological phases of light provide opportunities to manipulate light propagation efficiently at the nanoscale volume. Performance of conventional optical elements are limited by back-reflection and bending losses, which hinder their prospect of large scale integration. Topological protection enables unidirectional excitation of edge states or surface states without leaking into the bulk, as well as suppression of scattering when encountering defects and corners. With such advantages, topological photonic elements may surpass conventional photonic design for future generations of ultra-compact efficient computing, imaging, and sensing applications. Due to limitations of fabrication and characterization techniques, previously experimental efforts on topological photonics have been carried out with 2D micron-scale optical design or at the microwave wavelength.
This thesis contributes to the experimental development of topological photonics in two aspects: first, how to fabricate and characterize 3D photonic crystals and therefore extend topological protection into the 3D (Chapters 2-3); and second, how to realize nanoscale topological protection in the visible frequencies (chapters 4-6). Specifically, Chapter 2 reports fabrication of 3D single gyroid structures composed of a-Si and FTIR characterization of a photonic bandgap at the mid-infrared wavelength. This is the foundation to investigate more complex morphologies to introduce topologically nontrivial photonic states. Chapter 3 describe properties of double gyroid photonic crystals, followed by angle resolved characterization method in the mid-infrared. Double gyroid photonic crystals can be designed to possess quadratic degeneracy points, Weyl points, and line nodes. Since Weyl points have non-zero Chern numbers, surface states are topologically protected in double gyroid photonic crystals with parity breaking symmetry. The angle resolved characterization method could be utilize to resolve both Weyl points and surface states. Chapter 4 depicts design, fabrication, and characterization of Dirac-like surface plasmon dispersions in metallic nano-pillars. Chapter 5 presents experimental investigation of coupled silicon Mie resonators, which is the first step towards topological design based on inter-lattice sites coupling in the next chapter. Chapter 6 details photonic bandstructure from angle-resolved cathodoluminescence measurements. We analyze bandstructures collected from the bulk of trivially and topologically gapped lattices, as well as zigzag and arm-chaired edges of domain boundaries. Chapter 7 outlines a method to optically enhance dissociation of hydrazine molecules using ultraviolet plasmons, and attempts to use this method for low temperature GaN growth.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Atwater, Harry Albert}, } @phdthesis{10.7907/Z9J964M8, author = {Sherrott, Michelle Caroline}, title = {Active Infrared Nanophotonics in van der Waals Materials}, school = {California Institute of Technology}, year = {2018}, doi = {10.7907/Z9J964M8}, url = {https://resolver.caltech.edu/CaltechTHESIS:01262018-171457982}, abstract = {Two-dimensional van der Waals materials have recently been introduced into the field of nanophotonics, creating opportunities to explore novel physics and realize first-of-their kind devices. By reducing the thickness of these materials, novel optical properties emerge due to the introduction of vertical quantum confinement. Unlike most materials, which suffer from a reduction in quality as they are thinned, layered van der Waals materials have naturally passivated surfaces that preserve their performance in monolayer form. Moreover, because the thickness of these materials is below typical charge carrier screening lengths, it is possible to actively control their optical properties with an external gate voltage. By combining these unique properties with the subwavelength control of light-matter interactions provided by nanophotonics, new device architectures can be realized.
In this thesis, we explore van der Waals materials for active infrared nanophotonics, focusing on monolayer graphene and few-layer black phosphorus. Chapter 2 introduces gate-tunable graphene plasmons that interact strongly with their environment and can be combined with an external cavity to reach large absorption strengths in a single atomic layer. Chapter 3 builds on this, using graphene plasmons to control the spectral character and polarization state of thermal radiation. In Chapter 4, we complete the story of actively controlling infrared light using graphene-based structures, introducing graphene into a resonant gold structure to enable active control of phase. By combining these resonant structures together into a multi-pixel array, we realize an actively tunable meta-device for active beam steering in the infrared. In Chapters 5 and 6, we present few layer black phosphorus (BP) as a novel material for active infrared nanophotonics. We study the different electro-optic effects of the material from the visible to mid-infrared. We additionally examine the polarization-dependent response of few-layer BP, observing that we can tune its optical response from being highly anisotropic to nearly isotropic in plane. Finally, Chapter 7 comments on the challenges and opportunities for graphene- and BP-integrated nanophotonic structures and devices.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Atwater, Harry Albert}, } @phdthesis{10.7907/Z97H1GS1, author = {Papadakis, Georgia Theano}, title = {Optical Response in Planar Heterostructures: From Artificial Magnetism to Angstrom-Scale Metamaterials}, school = {California Institute of Technology}, year = {2018}, doi = {10.7907/Z97H1GS1}, url = {https://resolver.caltech.edu/CaltechTHESIS:03082018-161639906}, abstract = {The idea of expanding the range of properties of natural substances with artificial matter was introduced by V. G. Veselago in 1967. Since then, the field of metamaterials has dramatically advanced. Man-made structures can now exhibit a plethora of extraordinary electromagnetic properties, such as negative refraction, optical magnetism, and super-resolution imaging. Typical metamaterial motifs include split ring resonators, dielectric and plasmonic particles, fishnet and wire arrays. The principle of operation of these elements is now well-understood, and they are being exploited for practical applications on a global scale, ranging from telecommunications to sensing and biomedicine, in the radio frequency and terahertz domains. Accessing and controlling optical and near-infrared phenomena requires scaling down the dimensions of meta- materials to the nanometer regime, pushing the limits of state-of-the-art nano- lithography and requiring structurally less complex geometries. Hence, within the last decade, research in metamaterials has revisited a simpler, lithography- free structure, particularly planar arrangements of alternating metal and dielectric layers, termed hyperbolic metamaterials. Such media are readily realizable with well-established thin-film deposition techniques. They support a rich canvas of properties ranging from surface plasmonic propagation to negative refraction, and they can enhance the photoluminescence properties of quantum emitters at any frequency range.
Here, we introduce a computational approach that allows tailoring the dielectric and magnetic effective properties of planar metamaterials. Previously, planar hyperbolic metamaterials have been considered non-magnetic. In contrast, we show theoretically and experimentally that planar arrangements com- posed of non-magnetic constituents can be engineered to exhibit a non-trivial magnetic response. This realization simplifies the structural requirements for tailoring optical magnetism up to very high frequencies. It also provides access to previously unexplored phenomena, for example artificially magnetic plasmons, for which we perform an analysis on the basis of available materials for achieving polarization-insensitive surface wave propagation. By combining the concept of metamaterials’ homogenization with previous transfer matrix approaches, we develop a general computational method for surface waves calculations that is free of previous assumptions, for example infinite or purely periodic media. Furthermore, we theoretically demonstrate that hyperbolic metamaterials can be dynamically tunable via carrier injection through external bias, using transparent conductive oxides and graphene, at visible and infrared frequencies, respectively. Lastly, we demonstrate that planar graphene-based van der Waals heterostructures behave effectively as supermetals, exhibiting reflective properties that surpass the reflectivity of gold and silver that are currently considered the state-of-the-art materials for mirroring applications in space applications. The (meta)materials we introduce exhibit an order-of-magnitude lower mass density, making them suitable candidates for future light-sail technologies intended for space exploration.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Atwater, Harry Albert}, } @phdthesis{10.7907/G1CG-E962, author = {Lloyd, John Vickery}, title = {Optoelectronic Design and Prototyping of Spectrum-Splitting Photovoltaics}, school = {California Institute of Technology}, year = {2018}, doi = {10.7907/G1CG-E962}, url = {https://resolver.caltech.edu/CaltechTHESIS:05212018-185007265}, abstract = {Global energy production is dominated by the combustion of fossil fuels but in order to avoid the projected consequences of anthropogenic climate change it is necessary that humankind reduce the carbon intensity of its energy supply. Fortunately the sun supplies a ubiquitous flow of energy of with excellent thermodynamic quality to earth. Massive investment and manufacturing scale has driven the costs of photovoltaic systems to levels competitive with fossil fuel generation, and yet commercial photovoltaic systems convert power from the sun into electricity with less than 20% efficiency. In this thesis we consider the thermodynamic and practical limits to the power conversion efficiency of photovoltaic systems and seek to design systems that address the greatest sources of loss, namely the lack of sub-bandgap absorption and the thermalization of excited carriers. We present several designs of spectrum-splitting systems that utilize optical structures to allocate incident broadband solar radiation into narrower spectral bands which can be converted by multiple distinct photovoltaic cells at greater efficiency. Furthermore, we report on the design and fabrication of thin film III-V single-junction cells at bandgaps spanning the solar spectrum for incorporation within spectrum-splitting systems. These devices were fabricated by utilizing epitaxial lift-off processes from both GaAs and InP wafers as proof of scalability. We additionally report on the fabrication and characterization of series of a spectrum-splitting prototypes. This design featured seven distinct spectral bands with single-junction photovoltaic cells designed to convert them with highest possible efficiency, and the ultimate prototype exhibited an 84.5% spectrum splitting efficiency and 30.2% power conversion efficiency under a standard AM1.5D solar spectrum. We also report a technical pathway to raise the prototype efficiency to a record breaking 45.2%. Finally, we present an optical design of a spectrum-splitting module that is informed by a technoeconomic analysis which drastically reduces the complexity and cost relative to the fabricated prototype.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Atwater, Harry Albert}, } @phdthesis{10.7907/Z9ZW1HXJ, author = {Kim, Seyoon}, title = {Electronically Tunable Light Modulation with Graphene and Noble Metal Plasmonics}, school = {California Institute of Technology}, year = {2017}, doi = {10.7907/Z9ZW1HXJ}, url = {https://resolver.caltech.edu/CaltechTHESIS:02152017-220505611}, abstract = {Graphene is a monolayer of carbon atoms constructing a two-dimensional honeycomb structure, and it has an excellent carrier mobility and a very high thermal conductivity. Remarkably, it has been experimentally demonstrated that a monolayer graphene exhibits an exotic optical properties. To be specific, the plasmonic dispersion relation of a transverse magnetic graphene plasmon is electronically tunable by adjusting carrier density in graphene with external gate bias, and graphene plasmonic nano cavities have been utilized to modulate mid-infrared light.
In this thesis, we present how to efficiently modulate mid-infrared light by combining graphene plasmonic ribbons with noble metal plasmonic structures.
First, we propose and demonstrate electronically tunable resonant perfect absorption in graphene plasmonic metasurface enhanced by noble metal plasmonic effect, which results in modulating reflecting light. In this device, we improve coupling efficiency of free-space photons into graphene plasmons by reducing wavevector mismatching with a low permittivity substrate. In addition, the graphene plasmonic resonance is significantly enhanced by plasmonic light focusing effect of the coupled subwavelength metallic slit structure, which results in strongly fortifying resonance absorption in the graphene plasmonic metasurface. In the proposed device, theoretical calculation expects that perfect absorption in the graphene plasmonic metasurface is achievable with low graphene carrier mobility. We also present an analytical model based on surface admittance in order to fully understand how this enhancement occurs.
In the second device, we propose and demonstrate a transmission type light modulator by combining graphene plasmonic ribbons with subwavelength metal slit arrays. In this device, extraordinary optical transmission resonance is coupled to graphene plasmonic ribbons to create electrostatic modulation of mid-infrared light. Absorption in graphene plasmonic ribbons situated inside metallic slits can efficiently block the coupling channel for resonant transmission, leading to a suppression of transmission. This phenomenon is also interpreted by anti-crossing between the graphene plasmonic resonance in the ribbons and the noble metal plasmonic resonance in the subwavelength metal slit arrays.
Finally, we devise a platform to demonstrate graphene plasmonic resonance energy transport along graphene plasmonic ribbons. In this device, two metal-insulator-metal waveguides are connected by a subwavelength metal slit, and graphene plasmonic ribbons are located inside this slit. Due to the large impedance mismatch at the junction, light coupling efficiency across the junction is poor. If the graphene plasmonic ribbons are tuned to support strong graphene plasmonic resonances, the light energy can be transferred via graphene plasmons along the ribbons, and it leads to significant improvement in the light coupling efficiency across the junction. In addition to enhanced light coupling efficiency, we also present how to totally suppress the transmission by inducing a Fano resonance between a non-resonant propagation mode across the junction and a resonant graphene plasmonic transport mode, which can be utilized to efficiently modulate light in a noble metal plasmonic waveguide with the graphene plasmon resonance energy transfer.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Atwater, Harry Albert}, } @phdthesis{10.7907/Z9DF6P90, author = {Flowers, Cristofer Addison}, title = {Full Spectrum Ultrahigh Efficiency Photovoltaics: System Design, Integration, and Characterization}, school = {California Institute of Technology}, year = {2017}, doi = {10.7907/Z9DF6P90}, url = {https://resolver.caltech.edu/CaltechTHESIS:06082017-225442178}, abstract = {The sun is an immense source of power, radiating more energy than all known non-renewable reserves onto the Earth every year in the form of sunlight. In spite of this abundant availability, photovoltaic electricity conversion provides less than 1% of the of the global energy consumption. This lack of deployment is largely a consequence of the cost of photovoltaics relative to other technologies, but increased efficiency is a strong driver for cost reduction due to its ability to impact both photovoltaic module and balance of systems costs. In this thesis, we present enabling technologies for achieving increased efficiency and energy yield for photovoltaic conversion of sunlight. First, we develop finite element cell modeling and electrical contact optimization tools. These models are used to deploy unconstrained optimization techniques that expand the design space of solar cell contacts. Additionally, constrained optimization techniques are used to design solar cell electrical contacts for lateral spectrum-splitting photovoltaic submodules. The lateral spectrum-splitting submodule uses a series of filters to divide broadband sunlight into seven wavelength bands, sending each onto a solar cell with bandgap chosen to minimize thermalization and sub-bandgap transmission losses. By employing a wholistic design model covering limiting efficiency, material constraints, optical ray tracing, and electrical modeling, we generate designs capable of ultrahigh (>50%) efficiency. We then design, integrate, and prototype the first photovoltaic converter with seven unique bandgaps. Characterization of this prototype and its constituent components shows an integrated 84.5% optical efficiency and 30.2% submodule efficiency. The exemplary optical performance highlights the promise of the design with further development of the cells. Finally, we develop module circuit and power combination topologies that enable independent electrical connection to two or more subcells in a multijunction photovoltaic converter. This circuit architecture enables independent power production from each device, which reduces the module sensitivity to diurnal and seasonal spectral changes and increases panel annual energy yield. The photovoltaic technologies developed herein often break with convention and demonstrate a feasible pathway to very high (>40%) and ultrahigh (>50%) efficiency modules.}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Atwater, Harry Albert}, } @phdthesis{10.7907/Z9MC8X2P, author = {Verlage, Erik A.}, title = {High-Efficiency Solar Fuel Devices: Protection and Light Management Utilizing TiO2}, school = {California Institute of Technology}, year = {2017}, doi = {10.7907/Z9MC8X2P}, url = {https://resolver.caltech.edu/CaltechTHESIS:06012017-152250262}, abstract = {Global climate change coupled with increasing global energy consumption drives the need for renewable and carbon-neutral alternatives to fossil fuels. Photoelectrochemical devices store solar energy in chemical bonds, and have the potential to provide cost-effective fuel for grid-scale energy storage as well as to serve as a feedstock for the production of carbon-neutral transportation fuels. A widely recognized goal is the demonstration of a monolithically-integrated solar-fuels system that is simultaneously efficient, stable, intrinsically safe, and scalably manufacturable. This thesis presents the development of three separate high-efficiency solar fuel devices protected by thin films of amorphous TiO2, and develops light management strategies to increase the performance of these devices.
First, high-efficiency monolithic cells were designed to perform solar water-splitting and CO2 reduction. These designs are driven by high-quality single-crystalline III-V semiconductors that are unstable when placed in direct contact with aqueous electrolytes but can be protected against corrosion by hole-conducting amorphous films. Experimental fabrication and characterization of this tandem device was realized in the form of a fully-integrated water-splitting prototype with a solar-to-hydrogen efficiency of 10% showing stability for over 80 hours of operation. This was followed by the demonstration of water-splitting and CO2 reduction devices enabled by bipolar membranes, which increased stability and alleviated materials-compatibility constraints by creating a pH difference between the anolyte and catholyte, maintained at steady-state. Finally, universal light management strategies were developed using high-aspect-ratio TiO2 nanocones, resulting in an increase in catalyst loading with ultrahigh broadband transmission.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Atwater, Harry Albert}, } @phdthesis{10.7907/Z96W9833, author = {Darbe, Sunita}, title = {Optics for High-Efficiency Full Spectrum Photovoltaics}, school = {California Institute of Technology}, year = {2017}, doi = {10.7907/Z96W9833}, url = {https://resolver.caltech.edu/CaltechTHESIS:01032017-032640614}, abstract = {While the price of solar energy has dropped dramatically in the last few years, costs must be further reduced to reach wide-scale adoption. One strategy to decrease cost is to increase efficiency. Photovoltaic energy conversion is most efficient for a narrow frequency range. Lack of absorption of low energy photons and thermalization of high-energy photons leads lead to a loss of over 40% of incident solar power on a silicon cell. Current-matching and lattice-matching restrictions limit the efficiency of traditional monolithic multijunction solar cells. In order to avoid these limitations and realize ultrahigh efficiency (close to 50%), this thesis explores use of optical elements to split broadband sunlight into multiple spectral bands that can each be sent to physically separated solar cells tuned to best convert that band.
Design of a holographic diffraction grating based spectrum-splitting system resulted in a simulated module efficiency of 37%, meeting the efficiency of state-of-the-art modules. One of four holographic grating stacks is experimentally characterized. Next, a design incorporating dichroic filters, seven subcells with bandgaps spanning the solar spectrum, and concentrators with efficiency potential exceeding 45% module efficiency is presented. While prototyping this design, we also used on-going cost-modeling to ensure that our design was on-track to be a high-volume technology with low lifetime energy cost.
Finally, high-contrast gratings are used as resonant, dielectric spectrally selective mirrors in a tandem luminescent solar concentrator and as alternatives to Bragg reflectors. Gratings can have omnidirectional, high reflectivity by appropriately offsetting grating resonances in nano-patterned subwavelength thickness high-refractive index material. Subwavelength feature sizes suppress diffraction, and the high-refractive index of the grating layer leads to relatively angle-insensitive reflectance. Gratings can be fabricated by nanoimprint lithography, making them a scalable and economical option for photovoltaic applications. Simulations show hemispherically average reflectivity near 90% possible from a single subwavelength thickness layer. These properties are well suited for a variety of applications including multiple spectrum-splitting device architectures.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Atwater, Harry Albert}, } @phdthesis{10.7907/Z98S4MZ4, author = {Batara, Nicolas Anthony}, title = {Spontaneous Pattern Formation in Photoelectrodeposited Semiconductor Films}, school = {California Institute of Technology}, year = {2017}, doi = {10.7907/Z98S4MZ4}, url = {https://resolver.caltech.edu/CaltechTHESIS:06032017-161239736}, abstract = {The ability to manipulate matter with ever-increasing precision has enabled the fabrication of nanoscale structures with unprecedented utility. Scalable patterning technologies have dramatically transformed diverse application spaces such as computing and photonics, in part due to diminishing cost per unit area. The work in this thesis presents a template-free, bottom-up technique based on photoelectrodeposition which allows the direct fabrication of periodically nanostructured thin films of semiconductor material over large areas.
First, we examine the effects of wavelength, polarization and incidence angle of illumination on the film morphology. We develop an understanding of the pattern formation to be the result of interference of light scattered across the surface of the growing interface. We also examine the morphological effects of more complex illumination conditions. For example, when deposited under two different illumination wavelengths, the period of patterned films self-optimizes to concentrate light absorption to the tips of the nanostructures . Additionally, we find that the relative polarization angles and phases of two illumination sources can be tuned to produce film morphologies ranging from isotropic mesh-type patterns to orthogonally arranged, intersecting lamellar structures with independent periodicities.
We deepen our understanding of these observations by building a probabilistic computational model that correlates the local light absorption with a local growth probability at the interface of the film with few material parameters. We find that this model is able to reproduce experimentally observed morphological features for all illumination conditions investigated in this work. Through Fourier analysis, we find quantitative agreement between the simulated and experimental periods. Separately, we use electrodynamic simulations on idealized lamellar structures to understand the effect of two coincident illumination sources on the spatial absorption profile.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, } @phdthesis{10.7907/Z98C9T7Z, author = {Shaner, Matthew Reed}, title = {An Experimental and Technoeconomic Study of Silicon Microwire Arrays for Fuel Production Using Solar Energy}, school = {California Institute of Technology}, year = {2016}, doi = {10.7907/Z98C9T7Z}, url = {https://resolver.caltech.edu/CaltechTHESIS:05312016-151045544}, abstract = {Direct solar energy conversion is one of few sustainable energy resources able to wholly satisfy global energy demand; however, utility scale adoption and reliance are currently limited by the lack of a cost effective energy storage technology. The production of fuel from sunlight (solar fuels) enables solar energy storage in chemical bonds, a volumetrically and gravimetrically dense form compatible with current infrastructure worldwide. Hydrogen production via water splitting is a first generation solar fuel targeted herein that is currently used for hydrocarbon up-grading and fertilizer production and could further be utilized in combustion cycles and/or fuel cells for electricity and heat production and transportation.
This thesis presents achievements that form the foundation for Si microwire array based solar water splitting devices beginning with a tandem junction device design using Si microwire arrays as the architectural motif and one of many active components. Si microwire arrays have potential advantages over two dimensional planar device architectures such as minimized resistance losses, lower semiconductor material usage, and embedment in a polymeric membrane enabling a flexible device.
Experimental fabrication and characterization of this tandem junction device design was realized in the form of a np+-Si microwire array coated by either tungsten oxide (WO3) or titanium dioxide (TiO2) as the second tandem semiconductor. The Si/TiO2 device demonstrated the highest performance with an expected solar-to-hydrogen efficiency of 0.39%. To achieve these demonstrations new processing methods were needed and developed for formation of the np+-Si microwire array homojunction and formation of a low resistance contact between the p+-Si and second semiconductor using sputtered tin- doped indium oxide (ITO) and spray pyrolyzed fluorine-doped tin oxide (FTO).
Another achievement includes demonstration of the longest known (>2200 hours) photoanode stability for water oxidation using a np+-Si microwire array coated with an in-house developed amorphous TiO2 protection layer and NiCrOx electrocatalyst. Additionally, the Si microwire array architecture was used to enable decoupling of semiconductor light absorption and catalytic activity, two performance metrics that ideally are maximized simultaneously. However, all previous demonstrations have shown anti-correlation between these performance metrics because planar architectures are subject to a trade-off where adding electrocatalyst increases catalytic activity, but decreases semiconductor light absorption and vice versa.
Finally, a techno-economic analysis of solar water splitting production facilities was performed to assess economic competitiveness because this is the ultimate metric by which all energy production technologies are currently evaluated. This analysis suggests that a hydrogen production facility that is cosmetically similar to current solar panel installations with hydrogen collection from distributed tilted panels is unlikely to achieve cost competitiveness with fossil fuel derived hydrogen due to the balance of systems costs alone. A cost of CO2 greater than ~$800 (ton CO2)-1 was estimated to be necessary for the least expensive base-case solar-to-hydrogen system to reach price parity with hydrogen derived from steam reforming of methane priced at $3 (MM BTU)-1 ($1.39 (kg H2)-1). Direct CO2 reduction systems were also explored and resulted in even larger challenges than hydrogen production. Accordingly, major facility wide breakthroughs are required to obtain viable economic costs for solar hydrogen production, but the barriers to achieve cost-competitiveness with existing large-scale thermochemical processes for CO2 reduction are even greater.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, } @phdthesis{10.7907/Z9QV3JHT, author = {Brown, Ana Maii}, title = {Classical and Quantum Effects in Plasmonic Metals}, school = {California Institute of Technology}, year = {2016}, doi = {10.7907/Z9QV3JHT}, url = {https://resolver.caltech.edu/CaltechTHESIS:04242016-093536420}, abstract = {The field of plasmonics exploits the unique optical properties of metallic nanostructures to concentrate and manipulate light at subwavelength length scales. Metallic nanostructures get their unique properties from their ability to support surface plasmons– coherent wave-like oscillations of the free electrons at the interface between a conductive and dielectric medium. Recent advancements in the ability to fabricate metallic nanostructures with subwavelength length scales have created new possibilities in technology and research in a broad range of applications.
In the first part of this thesis, we present two investigations of the relationship between the charge state and optical state of plasmonic metal nanoparticles. Using experimental bias-dependent extinction measurements, we derive a potential- dependent dielectric function for Au nanoparticles that accounts for changes in the physical properties due to an applied bias that contribute to the optical extinction. We also present theory and experiment for the reverse effect– the manipulation of the carrier density of Au nanoparticles via controlled optical excitation. This plasmoelectric effect takes advantage of the strong resonant properties of plasmonic materials and the relationship between charge state and optical properties to eluci- date a new avenue for conversion of optical power to electrical potential.
The second topic of this thesis is the non-radiative decay of plasmons to a hot-carrier distribution, and the distribution’s subsequent relaxation. We present first-principles calculations that capture all of the significant microscopic mechanisms underlying surface plasmon decay and predict the initial excited carrier distributions so generated. We also preform ab initio calculations of the electron-temperature dependent heat capacities and electron-phonon coupling coefficients of plasmonic metals. We extend these first-principle methods to calculate the electron-temperature dependent dielectric response of hot electrons in plasmonic metals, including direct interband and phonon-assisted intraband transitions. Finally, we combine these first-principles calculations of carrier dynamics and optical response to produce a complete theoretical description of ultrafast pump-probe measurements, free of any fitting parameters that are typical in previous analyses.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Atwater, Harry Albert}, } @phdthesis{10.7907/Z92V2D22, author = {Tolstova, Yulia}, title = {Cu₂O Heterojunction Photovoltaics}, school = {California Institute of Technology}, year = {2016}, doi = {10.7907/Z92V2D22}, url = {https://resolver.caltech.edu/CaltechTHESIS:06012016-163813213}, abstract = {Cuprous oxide (Cu2O) is an earth abundant semiconductor that has several promising photovoltaic properties, including high absorption in the visible range, high minority carrier diffusion length, and high majority carrier mobility. Cu2O can be easily synthesized by oxidation of copper foils in air. One important advantage that makes Cu2O highly relevant to today’s solar cell markets dominated by crystalline silicon is its wide bandgap of 1.9 eV at room temperature, which makes it an ideal candidate for a top cell in tandem with a crystalline silicon bottom cell. The detailed balance efficiency of such a device exceeds 44%. In this work we aim to understand and address several issues that have limited Cu2O solar cell efficiency. We address the intrinsic p-type nature and chemical instability of Cu2O by pairing it with an appropriate n-type heterojunction partner Zn(O,S), which allows us to achieve devices with open circuit voltages exceeding 1 V. We identify presence of a current blocking layer and reduce it, which results in more than doubling the short circuit current to exceed 5 mA/cm2. Light beam induced current measurements highlight some of the issues inherent to polycrystalline Cu2O solar cells, including grain dependent collection and current losses due to presence of grain boundaries. In order to address the issues affecting Cu2O made by thermal oxidation we also develop thin film growth of Cu2O by molecular beam epitaxy on several substrates including MgO and heteroepitaxial noble metal templates that act as ohmic back contacts. These studies culminate in achievement of the first Cu2O/Zn(O,S) solar cells incorporating an absorber layer grown by molecular beam epitaxy.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Atwater, Harry Albert}, } @phdthesis{10.7907/Z94Q7RXJ, author = {Shing, Amanda M.}, title = {Development of Zn-IV-Nitride Semiconductor Materials and Devices}, school = {California Institute of Technology}, year = {2016}, doi = {10.7907/Z94Q7RXJ}, url = {https://resolver.caltech.edu/CaltechTHESIS:05272016-161721247}, abstract = {This thesis details explorations of the materials and device fabrication of Zn-IV-Nitride thin-films. Motivation in studying this materials series originates from its analgous properties to the III-Nitride semiconductor materials and its potential applications in photonic devices such as solar cells, light emitting diodes, and optical sensors. Building off of initial fabrication work from Coronel, Lahourcade et al., ZnSnxGe1-xN2 thin-films have shown to be a non-phase-segregating, tunable alloy series and a possible earth-abundant alternative to InxGa1-xN alloys. This thesis discusses further developments in fabrication of ZnSnxGe1-xN2 alloys by three-target co-sputtering and molecular beam epitaxy, and the resulting structural and optoelectronic characterization. Devices from these developed alloys are also highlighted.
Initial fabrication was based on the reactive radio-frequency (RF) sputtering technique and was limited to two-target sources and produced nanocrystalline films. Progression to three-target reactive RF co-sputtering for ZnSnxGe1-xN2 (x < 1) alloys is presented, where three-target co-sputtered alloys follow the structural and optoelectronic trends of the initial alloy series. However, three-target co-sputtering further enabled synthesis of alloys having < 10% atomic composition (x < 0.4) of tin, exhibiting non-degenerate doping. The electronic structure of sputtered thin-film surfaces for the alloy series were also characterized by photoelectron spectroscopy to measure their work functions and relative band alignment for device implementation.
Low electronic mobilities, degenerate carrier concentrations, and limited photoresponse may stem from the defective and nanocrystalline nature of the sputtered films. To improve crystalline quality, films were grown by molecular beam epitaxy (MBE). MBE ZnSnxGe1-xN2 films on sapphire and GaN were epitaxially grown, overall displaying single-crystalline quality films, higher electronic mobilities, and lower carrier concentrations. Througout experimentation, devices from both sputter deposited and MBE ZnSnxGe1-xN2 alloys films were constructed. Attempts at solid-state and electrochemical devices are described. Devices exhibited some photoresponse, providing a positive outlook for employment of ZnSnxGe1-xN2 alloys in solar cells or photon sensors.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, } @phdthesis{10.7907/Z9CF9N28, author = {Coronel, Naomi Cristina}, title = {Earth-Abundant Zinc-IV-Nitride Semiconductors}, school = {California Institute of Technology}, year = {2016}, doi = {10.7907/Z9CF9N28}, url = {https://resolver.caltech.edu/CaltechTHESIS:05252016-080726422}, abstract = {This investigation is motivated by the need for new visible frequency direct bandgap semiconductor materials that are abundant and low-cost to meet the increasing demand for optoelectronic devices in applications such as solid state lighting and solar energy conversion. Proposed here is the utilization of zinc-IV-nitride materials, where group IV elements include silicon, germanium, and tin, as earth-abundant alternatives to the more common III-nitrides in optoelectronic devices. These compound semiconductors were synthesized under optimized conditions using reactive radio frequency magnetron sputter deposition. Single phase ZnSnN2, having limited experimental accounts in literature, is validated by identification of the wurtzite-derived crystalline structure predicted by theory through X-ray and electron diffraction studies. With the addition of germanium, bandgap tunability of ZnSnxGe1-xN2 alloys is demonstrated without observation of phase separation, giving these materials a distinct advantage over InxGa1-xN alloys. The accessible bandgaps range from 1.8 to 3.1 eV, which spans the majority of the visible spectrum. Electron densities, measured using the Hall effect, were found to be as high as 1022 cm−3 and indicate that the compounds are unintentionally degenerately doped. Given these high carrier concentrations, a Burstein-Moss shift is likely affecting the optical bandgap measurements. The discoveries made in this thesis suggest that with some improvements in material quality, zinc-IV-nitrides have the potential to enable cost-effective and scalable optoelectronic devices.}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Atwater, Harry Albert}, } @phdthesis{10.7907/Z9KW5CX0, author = {Chen, Christopher Tien}, title = {Heteroepitaxy of Group IV and Group III-V semiconductor alloys for photovoltaic applications}, school = {California Institute of Technology}, year = {2016}, doi = {10.7907/Z9KW5CX0}, url = {https://resolver.caltech.edu/CaltechTHESIS:11062015-161003458}, abstract = {
Photovoltaic energy conversion represents a economically viable technology for realizing collection of the largest energy resource known to the Earth – the sun. Energy conversion efficiency is the most leveraging factor in the price of energy derived from this process. This thesis focuses on two routes for high efficiency, low cost devices: first, to use Group IV semiconductor alloy wire array bottom cells and epitaxially grown Group III-V compound semiconductor alloy top cells in a tandem configuration, and second, GaP growth on planar Si for heterojunction and tandem cell applications.
Metal catalyzed vapor-liquid-solid grown microwire arrays are an intriguing alternative for wafer-free Si and SiGe materials which can be removed as flexible membranes. Selected area Cu-catalyzed vapor-liquid solid growth of SiGe microwires is achieved using chlorosilane and chlorogermane precursors. The composition can be tuned up to 12% Ge with a simultaneous decrease in the growth rate from 7 to 1 μm/min-1. Significant changes to the morphology were observed, including tapering and faceting on the sidewalls and along the lengths of the wires. Characterization of axial and radial cross sections with transmission electron microscopy revealed no evidence of defects at facet corners and edges, and the tapering is shown to be due to in-situ removal of catalyst material during growth. X-ray diffraction and transmission electron microscopy reveal a Ge-rich crystal at the tip of the wires, strongly suggesting that the Ge incorporation is limited by the crystallization rate.
Tandem Ga1-xInxP/Si microwire array solar cells are a route towards a high efficiency, low cost, flexible, wafer-free solar technology. Realizing tandem Group III-V compound semiconductor/Si wire array devices requires optimization of materials growth and device performance. GaP and Ga1-xInxP layers were grown heteroepitaxially with metalorganic chemical vapor deposition on Si microwire array substrates. The layer morphology and crystalline quality have been studied with scanning electron microscopy and transmission electron microscopy, and they provide a baseline for the growth and characterization of a full device stack. Ultimately, the complexity of the substrates and the prevalence of defects resulted in material without detectable photoluminescence, unsuitable for optoelectronic applications.
Coupled full-field optical and device physics simulations of a Ga0.51In0.49P/Si wire array tandem are used to predict device performance. A 500 nm thick, highly doped “buffer” layer between the bottom cell and tunnel junction is assumed to harbor a high density of lattice mismatch and heteroepitaxial defects. Under simulated AM1.5G illumination, the device structure explored in this work has a simulated efficiency of 23.84% with realistic top cell SRH lifetimes and surface recombination velocities. The relative insensitivity to surface recombination is likely due to optical generation further away from the free surfaces and interfaces of the device structure.
Finally, GaP has been grown free of antiphase domains on Si (112) oriented substrates using metalorganic chemical vapor deposition. Low temperature pulsed nucleation is followed by high temperature continuous growth, yielding smooth, specular thin films. Atomic force microscopy topography mapping showed very smooth surfaces (4-6 Å RMS roughness) with small depressions in the surface. Thin films (~ 50 nm) were pseudomorphic, as confirmed by high resolution x-ray diffraction reciprocal space mapping, and 200 nm thick films showed full relaxation. Transmission electron microscopy showed no evidence of antiphase domain formation, but there is a population of microtwin and stacking fault defects.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Atwater, Harry Albert}, } @phdthesis{10.7907/Z9RV0KN6, author = {Emmer, Hal S.}, title = {Paths Towards High Efficiency Silicon Photovoltaics}, school = {California Institute of Technology}, year = {2016}, doi = {10.7907/Z9RV0KN6}, url = {https://resolver.caltech.edu/CaltechTHESIS:04102016-142401400}, abstract = {While photovoltaics hold much promise as a sustainable electricity source, continued cost reduction is necessary to continue the current growth in deployment. A promising path to continuing to reduce total system cost is by increasing device efficiency. This thesis explores several silicon-based photovoltaic technologies with the potential to reach high power conversion efficiencies. Silicon microwire arrays, formed by joining millions of micron diameter wires together, were developed as a low cost, low efficiency solar technology. The feasibility of transitioning this to a high efficiency technology was explored. In order to achieve high efficiency, high quality silicon material must be used. Lifetimes and diffusion lengths in these wires were measured and the action of various surface passivation treatments studied. While long lifetimes were not achieved, strong inversion at the silicon / hydrofluoric acid interface was measured, which is important for understanding a common measurement used in solar materials characterization.
Cryogenic deep reactive ion etching was then explored as a method for fabricating high quality wires and improved lifetimes were measured. As another way to reach high efficiency, growth of silicon-germanium alloy wires was explored as a substrate for a III-V on Si tandem device. Patterned arrays of wires with up to 12% germanium incorporation were grown. This alloy is more closely lattice matched to GaP than silicon and allows for improvements in III-V integration on silicon.
Heterojunctions of silicon are another promising path towards achieving high efficiency devices. The GaP/Si heterointerface and properties of GaP grown on silicon were studied. Additionally, a substrate removal process was developed which allows the formation of high quality free standing GaP films and has wide applications in the field of optics.
Finally, the effect of defects at the interface of the amorphous silicon heterojuction cell was studied. Excellent voltages, and thus efficiencies, are achievable with this system, but the voltage is very sensitive to growth conditions. We directly measured lateral transport lengths at the heterointerface on the order of tens to hundreds of microns, which allows carriers to travel towards any defects that are present and recombine. This measurement adds to the understanding of these types of high efficiency devices and may aid in future device design.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Atwater, Harry Albert}, } @phdthesis{10.7907/Z9PN93HB, author = {Eisler, Carissa Nicole}, title = {Photonic and Device Design Principles for Ultrahigh-Efficiency (>50%), Spectrum-Splitting Photovoltaics}, school = {California Institute of Technology}, year = {2016}, doi = {10.7907/Z9PN93HB}, url = {https://resolver.caltech.edu/CaltechTHESIS:11242015-234332347}, abstract = {The sun has the potential to power the Earth’s total energy needs, but electricity from solar power still constitutes an extremely small fraction of our power generation because of its high cost relative to traditional energy sources. Therefore, the cost of solar must be reduced to realize a more sustainable future. This can be achieved by significantly increasing the efficiency of modules that convert solar radiation to electricity. In this thesis, we consider several strategies to improve the device and photonic design of solar modules to achieve record, ultrahigh (> 50%) solar module efficiencies. First, we investigate the potential of a new passivation treatment, trioctylphosphine sulfide, to increase the performance of small GaAs solar cells for cheaper and more durable modules. We show that small cells (mm2), which currently have a significant efficiency decrease (~ 5%) compared to larger cells (cm2) because small cells have a higher fraction of recombination-active surface from the sidewalls, can achieve significantly higher efficiencies with effective passivation of the sidewalls. We experimentally validate the passivation qualities of treatment by trioctylphosphine sulfide (TOP:S) through four independent studies and show that this facile treatment can enable efficient small devices. Then, we discuss our efforts toward the design and prototyping of a spectrum-splitting module that employs optical elements to divide the incident spectrum into different color bands, which allows for higher efficiencies than traditional methods. We present a design, the polyhedral specular reflector, that has the potential for > 50% module efficiencies even with realistic losses from combined optics, cell, and electrical models. Prototyping efforts of one of these designs using glass concentrators yields an optical module whose combined spectrum-splitting and concentration should correspond to a record module efficiency of 42%. Finally, we consider how the manipulation of radiatively emitted photons from subcells in multijunction architectures can be used to achieve even higher efficiencies than previously thought, inspiring both optimization of incident and radiatively emitted photons for future high efficiency designs. In this thesis work, we explore novel device and photonic designs that represent a significant departure from current solar cell manufacturing techniques and ultimately show the potential for much higher solar cell efficiencies.}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Atwater, Harry Albert}, } @phdthesis{10.7907/Z9513W4S, author = {Narang, Prineha}, title = {Light-Matter Interactions in Semiconductors and Metals: From Nitride Optoelectronics to Quantum Plasmonics}, school = {California Institute of Technology}, year = {2015}, doi = {10.7907/Z9513W4S}, url = {https://resolver.caltech.edu/CaltechTHESIS:06052015-164458210}, abstract = {
This thesis puts forth a theory-directed approach coupled with spectroscopy aimed at the discovery and understanding of light-matter interactions in semiconductors and metals.
The first part of the thesis presents the discovery and development of Zn-IV nitride materials.The commercial prominence in the optoelectronics industry of tunable semiconductor alloy materials based on nitride semiconductor devices, specifically InGaN, motivates the search for earth-abundant alternatives for use in efficient, high-quality optoelectronic devices. II-IV-N2 compounds, which are closely related to the wurtzite-structured III-N semiconductors, have similar electronic and optical properties to InGaN namely direct band gaps, high quantum efficiencies and large optical absorption coefficients. The choice of different group II and group IV elements provides chemical diversity that can be exploited to tune the structural and electronic properties through the series of alloys. The first theoretical and experimental investigation of the ZnSnxGe1−xN2 series as a replacement for III-nitrides is discussed here.
The second half of the thesis shows ab−initio calculations for surface plasmons and plasmonic hot carrier dynamics. Surface plasmons, electromagnetic modes confined to the surface of a conductor-dielectric interface, have sparked renewed interest because of their quantum nature and their broad range of applications. The decay of surface plasmons is usually a detriment in the field of plasmonics, but the possibility to capture the energy normally lost to heat would open new opportunities in photon sensors, energy conversion devices and switching. A theoretical understanding of plasmon-driven hot carrier generation and relaxation dynamics in the ultrafast regime is presented here. Additionally calculations for plasmon-mediated upconversion as well as an energy-dependent transport model for these non-equilibrium carriers are shown.
Finally, this thesis gives an outlook on the potential of non-equilibrium phenomena in metals and semiconductors for future light-based technologies.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Atwater, Harry Albert}, } @phdthesis{10.7907/Z9P26W1K, author = {Fountaine, Katherine Theresa}, title = {Mesoscale Optoelectronic Design of Wire-Based Photovoltaic and Photoelectrochemical Devices}, school = {California Institute of Technology}, year = {2015}, doi = {10.7907/Z9P26W1K}, url = {https://resolver.caltech.edu/CaltechTHESIS:05292015-151831184}, abstract = {The overarching theme of this thesis is mesoscale optical and optoelectronic design of photovoltaic and photoelectrochemical devices. In a photovoltaic device, light absorption and charge carrier transport are coupled together on the mesoscale, and in a photoelectrochemical device, light absorption, charge carrier transport, catalysis, and solution species transport are all coupled together on the mesoscale. The work discussed herein demonstrates that simulation-based mesoscale optical and optoelectronic modeling can lead to detailed understanding of the operation and performance of these complex mesostructured devices, serve as a powerful tool for device optimization, and efficiently guide device design and experimental fabrication efforts. In-depth studies of two mesoscale wire-based device designs illustrate these principles—(i) an optoelectronic study of a tandem Si|WO3 microwire photoelectrochemical device, and (ii) an optical study of III-V nanowire arrays.
The study of the monolithic, tandem, Si|WO3 microwire photoelectrochemical device begins with development and validation of an optoelectronic model with experiment. This study capitalizes on synergy between experiment and simulation to demonstrate the model’s predictive power for extractable device voltage and light-limited current density. The developed model is then used to understand the limiting factors of the device and optimize its optoelectronic performance. The results of this work reveal that high fidelity modeling can facilitate unequivocal identification of limiting phenomena, such as parasitic absorption via excitation of a surface plasmon-polariton mode, and quick design optimization, achieving over a 300% enhancement in optoelectronic performance over a nominal design for this device architecture, which would be time-consuming and challenging to do via experiment.
The work on III-V nanowire arrays also starts as a collaboration of experiment and simulation aimed at gaining understanding of unprecedented, experimentally observed absorption enhancements in sparse arrays of vertically-oriented GaAs nanowires. To explain this resonant absorption in periodic arrays of high index semiconductor nanowires, a unified framework that combines a leaky waveguide theory perspective and that of photonic crystals supporting Bloch modes is developed in the context of silicon, using both analytic theory and electromagnetic simulations. This detailed theoretical understanding is then applied to a simulation-based optimization of light absorption in sparse arrays of GaAs nanowires. Near-unity absorption in sparse, 5% fill fraction arrays is demonstrated via tapering of nanowires and multiple wire radii in a single array. Finally, experimental efforts are presented towards fabrication of the optimized array geometries. A hybrid self-catalyzed and selective area MOCVD growth method is used to establish morphology control of GaP nanowire arrays. Similarly, morphology and pattern control of nanowires is demonstrated with ICP-RIE of InP. Optical characterization of the InP nanowire arrays gives proof of principle that tapering and multiple wire radii can lead to near-unity absorption in sparse arrays of InP nanowires.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Atwater, Harry Albert}, } @phdthesis{10.7907/Z90C4SSB, author = {Weitekamp, Raymond Andrew}, title = {Multifunctional Materials: Bottom-Up and Top-Down}, school = {California Institute of Technology}, year = {2015}, doi = {10.7907/Z90C4SSB}, url = {https://resolver.caltech.edu/CaltechTHESIS:06042015-140901217}, abstract = {This thesis is thematically separated into two complimentary approaches to advanced materials synthesis: bottom-up and top-down. Part I will discuss the self-assembly of photonic crystals, a unique class of periodic nanostructured materials featuring resonant optical response. Chapter 1 will introduce the concepts of self-assembly, specifically in the context of colloidal crystals and block copolymer nanostructures. Chapter 2 summarizes many years of work towards the goal of utilizing brush block copolymers as paintable photonic crystals. We employed 2D colloidal crystals as resonant light-trapping elements to improve the performance of thin-film solar cells; this work is described in Chapter 3. Part II of the thesis is centered around the concept of functional lithography: the ability to directly pattern materials with tailored physical properties and chemically active interfaces. We will briefly provide a history of photolithography in Chapter 4, and outline some of the limitations of the incumbent lithographic methods. The discovery of latent reactivity in ruthenium vinyl ether complexes, and the subsequent development of PhotoLithographic Olefin Metathesis Polymerization (PLOMP), are discussed in Chapter 5. This discovery has since blossomed into a true platform technology. We will discuss improvements to the functional group tolerance of PLOMP, as well as a few of our efforts to use PLOMP towards specific applications in Chapter 6. In the final Chapter 7 we document our attempts to activate PLOMP resists via multiphoton absorption, towards 3D printing of chemically functional microstructures.}, address = {1200 East California Boulevard, Pasadena, California 91125}, } @phdthesis{10.7907/Z92N506Z, author = {Callahan, Dennis Michael}, title = {Nanophotonic Light Trapping In Thin Solar Cells}, school = {California Institute of Technology}, year = {2015}, doi = {10.7907/Z92N506Z}, url = {https://resolver.caltech.edu/CaltechTHESIS:10082014-105407332}, abstract = {Over the last several decades there have been significant advances in the study and understanding of light behavior in nanoscale geometries. Entire fields such as those based on photonic crystals, plasmonics and metamaterials have been developed, accelerating the growth of knowledge related to nanoscale light manipulation. Coupled with recent interest in cheap, reliable renewable energy, a new field has blossomed, that of nanophotonic solar cells.
In this thesis, we examine important properties of thin-film solar cells from a nanophotonics perspective. We identify key differences between nanophotonic devices and traditional, thick solar cells. We propose a new way of understanding and describing limits to light trapping and show that certain nanophotonic solar cell designs can have light trapping limits above the so called ray-optic or ergodic limit. We propose that a necessary requisite to exceed the traditional light trapping limit is that the active region of the solar cell must possess a local density of optical states (LDOS) higher than that of the corresponding, bulk material. Additionally, we show that in addition to having an increased density of states, the absorber must have an appropriate incoupling mechanism to transfer light from free space into the optical modes of the device. We outline a portfolio of new solar cell designs that have potential to exceed the traditional light trapping limit and numerically validate our predictions for select cases.
We emphasize the importance of thinking about light trapping in terms of maximizing the optical modes of the device and efficiently coupling light into them from free space. To further explore these two concepts, we optimize patterns of superlattices of air holes in thin slabs of Si and show that by adding a roughened incoupling layer the total absorbed current can be increased synergistically. We suggest that the addition of a random scattering surface to a periodic patterning can increase incoupling by lifting the constraint of selective mode occupation associated with periodic systems.
Lastly, through experiment and simulation, we investigate a potential high efficiency solar cell architecture that can be improved with the nanophotonic light trapping concepts described in this thesis. Optically thin GaAs solar cells are prepared by the epitaxial liftoff process by removal from their growth substrate and addition of a metallic back reflector. A process of depositing large area nano patterns on the surface of the cells is developed using nano imprint lithography and implemented on the thin GaAs cells.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Atwater, Harry Albert}, } @phdthesis{10.7907/Z9MG7MD3, author = {Fakonas, James Spencer}, title = {Quantum Interference and Entanglement of Surface Plasmons}, school = {California Institute of Technology}, year = {2015}, doi = {10.7907/Z9MG7MD3}, url = {https://resolver.caltech.edu/CaltechTHESIS:12052014-101005469}, abstract = {Surface plasma waves arise from the collective oscillations of billions of electrons at the surface of a metal in unison. The simplest way to quantize these waves is by direct analogy to electromagnetic fields in free space, with the surface plasmon, the quantum of the surface plasma wave, playing the same role as the photon. It follows that surface plasmons should exhibit all of the same quantum phenomena that photons do, including quantum interference and entanglement.
Unlike photons, however, surface plasmons suffer strong losses that arise from the scattering of free electrons from other electrons, phonons, and surfaces. Under some circumstances, these interactions might also cause “pure dephasing,” which entails a loss of coherence without absorption. Quantum descriptions of plasmons usually do not account for these effects explicitly, and sometimes ignore them altogether. In light of this extra microscopic complexity, it is necessary for experiments to test quantum models of surface plasmons.
In this thesis, I describe two such tests that my collaborators and I performed. The first was a plasmonic version of the Hong-Ou-Mandel experiment, in which we observed two-particle quantum interference between plasmons with a visibility of 93 ± 1%. This measurement confirms that surface plasmons faithfully reproduce this effect with the same visibility and mutual coherence time, to within measurement error, as in the photonic case.
The second experiment demonstrated path entanglement between surface plasmons with a visibility of 95 ± 2%, confirming that a path-entangled state can indeed survive without measurable decoherence. This measurement suggests that elastic scattering mechanisms of the type that might cause pure dephasing must have been weak enough not to significantly perturb the state of the metal under the experimental conditions we investigated.
These two experiments add quantum interference and path entanglement to a growing list of quantum phenomena that surface plasmons appear to exhibit just as clearly as photons, confirming the predictions of the simplest quantum models.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Atwater, Harry Albert}, } @phdthesis{10.7907/Z9N58J9H, author = {Wilson, Samantha Stricklin}, title = {Zn-VI/Cu2O Heterojunctions for Earth-Abundant Photovoltaics}, school = {California Institute of Technology}, year = {2015}, doi = {10.7907/Z9N58J9H}, url = {https://resolver.caltech.edu/CaltechTHESIS:05212015-091546304}, abstract = {The need for sustainable energy production motivates the study of photovoltaic materials, which convert energy from sunlight directly into electricity. This work has focused on the development of Cu2O as an earth-abundant solar absorber due to the abundance of its constituent elements in the earth’s crust, its suitable band gap, and its potential for low cost processing. Crystalline wafers of Cu2O with minority carrier diffusion lengths on the order of microns can be manufactured in a uniquely simple fashion — directly from copper foils by thermal oxidation. Furthermore, Cu2O has an optical band gap of 1.9 eV, which gives it a detailed balance energy conversion efficiency of 24.7% and the possibility for an independently connected Si/Cu2O dual junction with a detailed balance efficiency of 44.3%.
However, the highest energy conversion efficiency achieved in a photovoltaic device with a Cu2O absorber layer is currently only 5.38% despite the favorable optical and electronic properties listed above. There are several challenges to making a Cu2O photovoltaic device, including an inability to dope the material, its relatively low chemical stability compared to other oxides, and a lack of suitable heterojunction partners due to an unusually small electron affinity. We have addressed the low chemical stability, namely the fact that Cu2O is an especially reactive oxide due to its low enthalpy of formation (ΔHf0 = -168.7 kJ/mol), by developing a novel surface preparation technique. We have addressed the lack of suitable heterojunction partners by investigating the heterojunction band alignment of several Zn-VI materials with Cu2O. Finally, We have addressed the typically high series resistance of Cu2O wafers by developing methods to make very thin, bulk Cu2O, including devices on Cu2O wafers as thin as 20 microns. Using these methods we have been able to achieve photovoltages over 1 V, and have demonstrated the potential of a new heterojunction material, Zn(O,S).
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Atwater, Harry Albert}, } @phdthesis{10.7907/0K8F-9871, author = {Warmann, Emily Cathryn}, title = {Design Strategies for Ultra-High Efficiency Photovoltaics}, school = {California Institute of Technology}, year = {2014}, doi = {10.7907/0K8F-9871}, url = {https://resolver.caltech.edu/CaltechTHESIS:06052014-163757869}, abstract = {While concentrator photovoltaic cells have shown significant improvements in efficiency in the past ten years, once these cells are integrated into concentrating optics, connected to a power conditioning system and deployed in the field, the overall module efficiency drops to only 34 to 36%. This efficiency is impressive compared to conventional flat plate modules, but it is far short of the theoretical limits for solar energy conversion. Designing a system capable of achieving ultra high efficiency of 50% or greater cannot be achieved by refinement and iteration of current design approaches.
This thesis takes a systems approach to designing a photovoltaic system capable of 50% efficient performance using conventional diode-based solar cells. The effort began with an exploration of the limiting efficiency of spectrum splitting ensembles with 2 to 20 sub cells in different electrical configurations. Incorporating realistic non-ideal performance with the computationally simple detailed balance approach resulted in practical limits that are useful to identify specific cell performance requirements. This effort quantified the relative benefit of additional cells and concentration for system efficiency, which will help in designing practical optical systems.
Efforts to improve the quality of the solar cells themselves focused on the development of tunable lattice constant epitaxial templates. Initially intended to enable lattice matched multijunction solar cells, these templates would enable increased flexibility in band gap selection for spectrum splitting ensembles and enhanced radiative quality relative to metamorphic growth. The III-V material family is commonly used for multijunction solar cells both for its high radiative quality and for the ease of integrating multiple band gaps into one monolithic growth. The band gap flexibility is limited by the lattice constant of available growth templates. The virtual substrate consists of a thin III-V film with the desired lattice constant. The film is grown strained on an available wafer substrate, but the thickness is below the dislocation nucleation threshold. By removing the film from the growth substrate, allowing the strain to relax elastically, and bonding it to a supportive handle, a template with the desired lattice constant is formed. Experimental efforts towards this structure and initial proof of concept are presented.
Cells with high radiative quality present the opportunity to recover a large amount of their radiative losses if they are incorporated in an ensemble that couples emission from one cell to another. This effect is well known, but has been explored previously in the context of sub cells that independently operate at their maximum power point. This analysis explicitly accounts for the system interaction and identifies ways to enhance overall performance by operating some cells in an ensemble at voltages that reduce the power converted in the individual cell. Series connected multijunctions, which by their nature facilitate strong optical coupling between sub-cells, are reoptimized with substantial performance benefit.
Photovoltaic efficiency is usually measured relative to a standard incident spectrum to allow comparison between systems. Deployed in the field systems may differ in energy production due to sensitivity to changes in the spectrum. The series connection constraint in particular causes system efficiency to decrease as the incident spectrum deviates from the standard spectral composition. This thesis performs a case study comparing performance of systems over a year at a particular location to identify the energy production penalty caused by series connection relative to independent electrical connection.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Atwater, Harry Albert}, } @phdthesis{10.7907/8Y5H-AM79, author = {Kosten, Emily Dell}, title = {Optical Designs for Improved Solar Cell Performance}, school = {California Institute of Technology}, year = {2014}, doi = {10.7907/8Y5H-AM79}, url = {https://resolver.caltech.edu/CaltechTHESIS:05222014-154510725}, abstract = {The solar resource is the most abundant renewable resource on earth, yet it is currently exploited with relatively low efficiencies. To make solar energy more affordable, we can either reduce the cost of the cell or increase the efficiency with a similar cost cell. In this thesis, we consider several different optical approaches to achieve these goals. First, we consider a ray optical model for light trapping in silicon microwires. With this approach, much less material can be used, allowing for a cost savings. We next focus on reducing the escape of radiatively emitted and scattered light from the solar cell. With this angle restriction approach, light can only enter and escape the cell near normal incidence, allowing for thinner cells and higher efficiencies. In Auger-limited GaAs, we find that efficiencies greater than 38% may be achievable, a significant improvement over the current world record. To experimentally validate these results, we use a Bragg stack to restrict the angles of emitted light. Our measurements show an increase in voltage and a decrease in dark current, as less radiatively emitted light escapes. While the results in GaAs are interesting as a proof of concept, GaAs solar cells are not currently made on the production scale for terrestrial photovoltaic applications. We therefore explore the application of angle restriction to silicon solar cells. While our calculations show that Auger-limited cells give efficiency increases of up to 3% absolute, we also find that current amorphous silicion-crystalline silicon heterojunction with intrinsic thin layer (HIT) cells give significant efficiency gains with angle restriction of up to 1% absolute. Thus, angle restriction has the potential for unprecedented one sun efficiencies in GaAs, but also may be applicable to current silicon solar cell technology. Finally, we consider spectrum splitting, where optics direct light in different wavelength bands to solar cells with band gaps tuned to those wavelengths. This approach has the potential for very high efficiencies, and excellent annual power production. Using a light-trapping filtered concentrator approach, we design filter elements and find an optimal design. Thus, this thesis explores silicon microwires, angle restriction, and spectral splitting as different optical approaches for improving the cost and efficiency of solar cells.}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Atwater, Harry Albert}, } @phdthesis{10.7907/09NG-5E90, author = {Bosco, Jeffrey Paul}, title = {Rational Design of Zinc Phosphide Heterojunction Photovoltaics}, school = {California Institute of Technology}, year = {2014}, doi = {10.7907/09NG-5E90}, url = {https://resolver.caltech.edu/CaltechTHESIS:06052014-153503097}, abstract = {
The prospect of terawatt-scale electricity generation using a photovoltaic (PV) device places strict requirements on the active semiconductor optoelectronic properties and elemental abundance. After reviewing the constraints placed on an “earth-abundant” solar absorber, we find zinc phosphide (α-Zn3P2) to be an ideal candidate. In addition to its near-optimal direct band gap of 1.5 eV, high visible-light absorption coefficient (>104 cm-1), and long minority-carrier diffusion length (>5 μm), Zn3P2 is composed of abundant Zn and P elements and has excellent physical properties for scalable thin-film deposition. However, to date, a Zn3P2 device of sufficient efficiency for commercial applications has not been demonstrated. Record efficiencies of 6.0% for multicrystalline and 4.3% for thin-film cells have been reported, respectively. Performance has been limited by the intrinsic p-type conductivity of Zn3P2 which restricts us to Schottky and heterojunction device designs. Due to our poor understanding of Zn3P2 interfaces, an ideal heterojunction partner has not yet been found.
The goal of this thesis is to explore the upper limit of solar conversion efficiency achievable with a Zn3P2 absorber through the design of an optimal heterojunction PV device. To do so, we investigate three key aspects of material growth, interface energetics, and device design. First, the growth of Zn3P2 on GaAs(001) is studied using compound-source molecular-beam epitaxy (MBE). We successfully demonstrate the pseudomorphic growth of Zn3P2 epilayers of controlled orientation and optoelectronic properties. Next, the energy-band alignments of epitaxial Zn3P2 and II-VI and III-V semiconductor interfaces are measured via high-resolution x-ray photoelectron spectroscopy in order to determine the most appropriate heterojunction partner. From this work, we identify ZnSe as a nearly ideal n-type emitter for a Zn3P2 PV device. Finally, various II-VI/Zn3P2 heterojunction solar cells designs are fabricated, including substrate and superstrate architectures, and evaluated based on their solar conversion efficiency.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Atwater, Harry Albert}, } @phdthesis{10.7907/1T2B-J793, author = {Jeon, Seokmin}, title = {Structure, Chemistry, and Energetics of Organic and Inorganic Adsorbates on Ga-rich GaAs and GaP(00l) Surfaces}, school = {California Institute of Technology}, year = {2014}, doi = {10.7907/1T2B-J793}, url = {https://resolver.caltech.edu/CaltechTHESIS:10082013-111509695}, abstract = {The work described in this dissertation includes fundamental investigations into three surface processes, namely inorganic film growth, water-induced oxidation, and organic functionalization/passivation, on the GaP and GaAs(001) surfaces. The techniques used to carry out this work include scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), and density functional theory (DFT) calculations. Atomic structure, electronic structure, reaction mechanisms, and energetics related to these surface processes are discussed at atomic or molecular levels.
First, we investigate epitaxial Zn3P2 films grown on the Ga-rich GaAs(001)(6×6) surface. The film growth mechanism, electronic properties, and atomic structure of the Zn3P2/GaAs(001) system are discussed based on experimental and theoretical observations. We discover that a P-rich amorphous layer covers the crystalline Zn3P2 film during and after growth. We also propose more accurate picture of the GaP interfacial layer between Zn3P2 and GaAs, based on the atomic structure, chemical bonding, band diagram, and P-replacement energetics, than was previously anticipated.
Second, DFT calculations are carried out in order to understand water-induced oxidation mechanisms on the Ga-rich GaP(001)(2×4) surface. Structural and energetic information of every step in the gaseous water-induced GaP oxidation reactions are elucidated at the atomic level in great detail. We explore all reasonable ground states involved in most of the possible adsorption and decomposition pathways. We also investigate structures and energies of the transition states in the first hydrogen dissociation of a water molecule on the (2×4) surface.
Finally, adsorption structures and thermal decomposition reactions of 1-propanethiol on the Ga-rich GaP(001)(2×4) surface are investigated using high resolution STM, XPS, and DFT simulations. We elucidate adsorption locations and their associated atomic structures of a single 1-propanethiol molecule on the (2×4) surface as a function of annealing temperature. DFT calculations are carried out to optimize ground state structures and search transition states. XPS is used to investigate variations of the chemical bonding nature and coverage of the adsorbate species.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Atwater, Harry Albert}, } @phdthesis{10.7907/PV2J-1429, author = {Deceglie, Michael Gardner}, title = {Advanced Silicon Solar Cell Device Physics and Design}, school = {California Institute of Technology}, year = {2013}, doi = {10.7907/PV2J-1429}, url = {https://resolver.caltech.edu/CaltechTHESIS:02152013-094838378}, abstract = {A fundamental challenge in the development and deployment of solar photovoltaic technology is a reduction in cost enabling direct competition with fossil-fuel-based energy sources. A key driver in this cost reduction is optimized device efficiency, because increased energy output leverages all photovoltaic system costs, from raw materials and module manufacturing to installation and maintenance. To continue progress toward higher conversion efficiencies, solar cells are being fabricated with increasingly complex designs, including engineered nanostructures, heterojunctions, and novel contacting and passivation schemes. Such advanced designs require a comprehensive and unified understanding of the optical and electrical device physics at the microscopic scale. This thesis focuses on a microscopic understanding of solar cell optoelectronic performance and its impact on cell optimization. We consider this in three solar cell platforms: thin-film crystalline silicon, amorphous/crystalline silicon heterojunctions, and thin-film cells with nanophotonic light trapping. The work described in this thesis represents a powerful design paradigm, based on a detailed physical understanding of the mechanisms governing solar cell performance. Furthermore, we demonstrate the importance of understanding not just the individual mechanisms, but also their interactions. Such an approach to device optimization is critical for the efficiency and competitiveness of future generations of solar cells.}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Atwater, Harry Albert}, } @phdthesis{10.7907/N2JK-5318, author = {Burgos, Stanley P.}, title = {Coupled Plasmonic Systems and Devices: Applications in Visible Metamaterials, Nanophotonic Circuits, and CMOS Imaging}, school = {California Institute of Technology}, year = {2013}, doi = {10.7907/N2JK-5318}, url = {https://resolver.caltech.edu/CaltechTHESIS:06042013-150337959}, abstract = {With the size of transistors approaching the sub-nanometer scale and Si-based photonics pinned at the micrometer scale due to the diffraction limit of light, we are unable to easily integrate the high transfer speeds of this comparably bulky technology with the increasingly smaller architecture of state-of-the-art processors. However, we find that we can bridge the gap between these two technologies by directly coupling electrons to photons through the use of dispersive metals in optics. Doing so allows us to access the surface electromagnetic wave excitations that arise at a metal/dielectric interface, a feature which both confines and enhances light in subwavelength dimensions - two promising characteristics for the development of integrated chip technology. This platform is known as plasmonics, and it allows us to design a broad range of complex metal/dielectric systems, all having different nanophotonic responses, but all originating from our ability to engineer the system surface plasmon resonances and interactions. In this thesis, we demonstrate how plasmonics can be used to develop coupled metal-dielectric systems to function as tunable plasmonic hole array color filters for CMOS image sensing, visible metamaterials composed of coupled negative-index plasmonic coaxial waveguides, and programmable plasmonic waveguide network systems to serve as color routers and logic devices at telecommunication wavelengths.}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Atwater, Harry Albert}, } @phdthesis{10.7907/0KEM-KG56, author = {Darvish, Davis Solomon}, title = {Cu₂O Substrates and Epitaxial Cu₂O/ZnO Thin Film Heterostructures for Solar Energy Conversion}, school = {California Institute of Technology}, year = {2013}, doi = {10.7907/0KEM-KG56}, url = {https://resolver.caltech.edu/CaltechTHESIS:06042013-144822210}, abstract = {Future fossil fuel scarcity and environmental degradation have demonstrated the need for renewable, low-carbon sources of energy to power an increasingly industrialized world. Solar energy with its infinite supply makes it an extraordinary resource that should not go unused. However with current materials, adoption is limited by cost and so a paradigm shift must occur to get everyone on the same page embracing solar technology. Cuprous Oxide (Cu₂O) is a promising earth abundant material that can be a great alternative to traditional thin-film photovoltaic materials like CIGS, CdTe, etc. We have prepared Cu₂O bulk substrates by the thermal oxidation of copper foils as well Cu₂O thin films deposited via plasma-assisted Molecular Beam Epitaxy. From preliminary Hall measurements it was determined that Cu₂O would need to be doped extrinsically. This was further confirmed by simulations of ZnO/Cu₂O heterojunctions. A cyclic interdependence between, defect concentration, minority carrier lifetime, film thickness, and carrier concentration manifests itself a primary reason for why efficiencies greater than 4% has yet to be realized. Our growth methodology for our thin-film heterostructures allow precise control of the number of defects that incorporate into our film during both equilibrium and nonequilibrium growth. We also report process flow/device design/fabrication techniques in order to create a device. A typical device without any optimizations exhibited open-circuit voltages Voc, values in excess 500mV; nearly 18% greater than previous solid state devices.}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Atwater, Harry Albert}, } @phdthesis{10.7907/A8ZZ-Z189, author = {Leenheer, Andrew Jay}, title = {Light to Electrons to Bonds: Imaging Water Splitting and Collecting Photoexcited Electrons}, school = {California Institute of Technology}, year = {2013}, doi = {10.7907/A8ZZ-Z189}, url = {https://resolver.caltech.edu/CaltechTHESIS:12102012-191527683}, abstract = {
Photoelectrochemical devices can store solar energy as chemical bonds in fuels, but more control over the materials involved is needed for economic feasibility. Both efficient capture of photon energy into electron energy and subsequent electron transfer and bond formation are necessary, and this thesis explores various steps of the process. To look at the electrochemical fuel formation step, the spatially-resolved reaction rate on a water-splitting electrode was imaged during operation at a few-micron scale using optical microscopy. One method involved localized excitation of a semiconductor photoanode and recording the growth rate of bubbles to determine the local reaction rate. A second method imaged the reactant profile with a pH-sensitive fluorophore in the electrolyte to determine the local three-dimensional pH profile at patterned electrocatalysts in a confocal microscope. These methods provide insight on surface features optimal for efficient electron transfer into fuel products.
A second set of studies examined the initial process of photoexcited electron transport and collection. An independent method to measure the minority carrier diffusion length in semiconductor photoelectrodes was developed, in which a wedge geometry is back illuminated with a small scanned spot. The diffusion length can be determined from the exponential decrease of photocurrent with thickness, and the method was demonstrated on solid-state silicon wedge diodes, as well as tungsten oxide thin-film wedge photoanodes. Finally, the possibility of absorbing and collecting sub-bandgap illumination via plasmon-enhanced hot carrier internal photoemission was modeled to predict the energy conversion efficiency. The effect of photon polarization on emission yield was experimentally tested using gold nanoantennas buried in silicon, and the correlation was found to be small.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Atwater, Harry Albert}, } @phdthesis{10.7907/59RB-9653, author = {Jang, Min Seok}, title = {Plasmonics and Electron Optics in Graphene}, school = {California Institute of Technology}, year = {2013}, doi = {10.7907/59RB-9653}, url = {https://resolver.caltech.edu/CaltechTHESIS:11262012-160659812}, abstract = {The field of plasmonics has been attracting wide interest because it has provided routes to guide and localize light at nanoscales by utilizing metals as its major building block. Meanwhile, graphene, a two-dimensional lattice of carbon atoms, has been regarded as an ideal material for electronic applications owing to its remarkably high carrier mobility and superior thermal properties. Both research fields have been growing rapidly, but quite independently. However, a closer look reveals that there are actually numerous similarities between them, and it is possible to extract useful applications from these analogies. Even more interestingly, these research fields are recently overlapping to create a new field of research, namely graphene plasmonics.
In this thesis, we present a few examples of these intertwined topics. First, we investigate “rainbow trapping” structures, broadband plasmonic slow light systems composed of single or double negative materials. We clarify the mode-conversion mechanism and the light-trapping performance by analyzing the dispersion relation. We then show that electrons in graphene exhibit photonlike dynamics including Goos-Hanchen effect and the rainbow trapping effect, but quantitatively differently. To study the dynamics of graphene electrons numerically, we develop a finite-difference time domain simulator. We also present a way to enhance electron backscattering in graphene by engineering the dispersion of electron eigenmodes in a Kronig-Penney potential. Finally, we discuss physics of graphene plasmon cavities. We report the resonant mid-infrared transmission across a plasmonic waveguide gap that is governed by the Fano interference between transmission through plasmon modes in graphene and nonresonant background transmission. An ultracompact graphene plasmon cavity, which resonates at near-infrared telecommunication frequencies, is also proposed.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Atwater, Harry Albert}, } @phdthesis{10.7907/8E75-WH21, author = {Turner-Evans, Daniel B.}, title = {Wire Array Photovoltaics}, school = {California Institute of Technology}, year = {2013}, doi = {10.7907/8E75-WH21}, url = {https://resolver.caltech.edu/CaltechTHESIS:05292013-225612828}, abstract = {Over the past five years, the cost of solar panels has dropped drastically and, in concert, the number of installed modules has risen exponentially. However, solar electricity is still more than twice as expensive as electricity from a natural gas plant. Fortunately, wire array solar cells have emerged as a promising technology for further lowering the cost of solar.
Si wire array solar cells are formed with a unique, low cost growth method and use 100 times less material than conventional Si cells. The wires can be embedded in a transparent, flexible polymer to create a free-standing array that can be rolled up for easy installation in a variety of form factors. Furthermore, by incorporating multijunctions into the wire morphology, higher efficiencies can be achieved while taking advantage of the unique defect relaxation pathways afforded by the 3D wire geometry.
The work in this thesis shepherded Si wires from undoped arrays to flexible, functional large area devices and laid the groundwork for multijunction wire array cells. Fabrication techniques were developed to turn intrinsic Si wires into full p-n junctions and the wires were passivated with a-Si:H and a-SiNx:H. Single wire devices yielded open circuit voltages of 600 mV and efficiencies of 9%. The arrays were then embedded in a polymer and contacted with a transparent, flexible, Ni nanoparticle and Ag nanowire top contact. The contact connected >99% of the wires in parallel and yielded flexible, substrate free solar cells featuring hundreds of thousands of wires.
Building on the success of the Si wire arrays, GaP was epitaxially grown on the material to create heterostructures for photoelectrochemistry. These cells were limited by low absorption in the GaP due to its indirect bandgap, and poor current collection due to a diffusion length of only 80 nm. However, GaAsP on SiGe offers a superior combination of materials, and wire architectures based on these semiconductors were investigated for multijunction arrays. These devices offer potential efficiencies of 34%, as demonstrated through an analytical model and optoelectronic simulations. SiGe and Ge wires were fabricated via chemical-vapor deposition and reactive ion etching. GaAs was then grown on these substrates at the National Renewable Energy Lab and yielded ns lifetime components, as required for achieving high efficiency devices.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Atwater, Harry Albert}, } @phdthesis{10.7907/Q4F1-H538, author = {Kimball, Gregory Michael}, title = {Zn₃P₂ and Cu₂O Substrates for Solar Energy Conversion}, school = {California Institute of Technology}, year = {2012}, doi = {10.7907/Q4F1-H538}, url = {https://resolver.caltech.edu/CaltechTHESIS:10302011-182613259}, abstract = {Zinc phosphide (Zn3P2) and cuprous oxide (Cu2O) are promising and earth-abundant alternatives to traditional thin film photovoltaics materials such as CIGS, CdTe, and a-Si. We have prepared high purity substrates of Zn3P2 from elemental zinc and phosphorus, and Cu2O by the thermal oxidation of copper foils, to investigate their fundamental material properties and potential for solar energy conversion. Photoluminescence-based measurements of Zn3P2 substrates have revealed a fundamental indirect band gap at 1.38 eV and a direct band gap at 1.50 eV, with time-resolved data indicating minority carrier diusion lengths of ≥7 μm. Solar cells based on Mg/Zn3P2 junctions with solar energy conversion efficiency reaching 4.5% were examined by composition profiling to elucidate the passivation reaction between Mg metal and Zn3P2 surfaces. Semiconductor/liquid junctions incorporating Cu2O substrates exhibited open-circuit voltage, Voc, values in excess of 800 mV and internal quantum yields approaching 100% in the 400–500 nm spectral range.}, address = {1200 East California Boulevard, Pasadena, California 91125}, month = {June}, advisor = {Atwater, Harry Albert and Lewis, Nathan Saul}, } @phdthesis{10.7907/FAZ1-XZ98, author = {Miller, Gerald Matthew}, title = {Electron Transport in Silicon Nanocrystal Devices: From Memory Applications to Silicon Photonics}, school = {California Institute of Technology}, year = {2012}, doi = {10.7907/FAZ1-XZ98}, url = {https://resolver.caltech.edu/CaltechTHESIS:11062011-172253692}, abstract = {The push to integrate the realms of microelectronics and photonics on the silicon platform is currently lacking an efficient, electrically pumped silicon light source. One promising material system for photonics on the silicon platform is erbium-doped silicon nanoclusters (Er:Si-nc), which uses silicon nanoclusters to sensitize erbium ions in a SiO2 matrix. This medium can be pumped electrically, and this thesis focuses primarily on the electrical properties of Er:Si-nc films and their possible development as a silicon light source in the erbium emission band around 1.5 micrometers.
Silicon nanocrystals can also be used as the floating gate in a flash memory device, and work is also presented examining charge transport in novel systems for flash memory applications. To explore silicon nanocrystals as a potential replacement for metallic floating gates in flash memory, the charging dynamics in silicon nanocrystal films are first studied using UHV-AFM. This approach uses a non-contact AFM tip to locally charge a layer of nanocrystals. Subsequent imaging allows the injected charge to be observed in real time as it moves through the layer. Simulation of this interaction allows the quantication of the charge in the layer, where we find that each nanocrystal is only singly charged after injection, while holes are retained in the film for hours.
Work towards developing a dielectric stack with a voltage-tunable barrier is presented, with applications for flash memory and hyperspectral imaging. For hyperspectral imaging applications, film stacks containing various dielectrics are studied using I-V, TEM, and internal photoemission, with barrier tunability demonstrated in the Sc2O3/SiO2 system.
To study Er:Si-nc as a potential lasing medium for silicon photonics, a theoretical approach is presented where Er:Si-nc is the gain medium in a silicon slot waveguide. By accounting for the local density of optical states effect on the emitters, and carrier absorption due to electrical pumping, it is shown that a pulsed excitation method is needed to achieve gain in this system. A gain of up to 2 db/cm is predicted for an electrically pumped gain medium 50 nm thick. To test these predictions Er:Si-nc LEDs were fabricated and studied. Reactive oxygen sputtering is found to produce more robust films, and the electrical excitation cross section found is two orders of magnitude larger than the optical cross section. The fabricated devices exhibited low lifetimes and low current densities which prevent observation of gain, and the modeling is used to predict how the films must be improved to achieve gain and lasing in this system.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Atwater, Harry Albert}, } @phdthesis{10.7907/ZS04-AH18, author = {Langeland, Krista S.}, title = {Thin-Film Silicon Photovoltaics: Characterization of Thin-Film Deposition and Analysis of Enhanced Light Trapping from Scattering Nanoparticle Arrays}, school = {California Institute of Technology}, year = {2012}, doi = {10.7907/ZS04-AH18}, url = {https://resolver.caltech.edu/CaltechTHESIS:03072012-191033678}, abstract = {Thin-film solar cells have the potential to significantly decrease the cost of a finished device by cutting materials cost, and the characteristics of carrier transport through a thin film can concurrently increase the device performance over that of a wafer-based cell while tolerating a higher defect density in the absorbing material. However, while silicon is an attractive material for use in solar cells due to its nearly ideal band gap for single-junction cells and its relative abundance, its inefficient absorption of infrared light necessitates the development of light-trapping techniques to avoid losses in current generation. This thesis research has focused on two important goals: the development of a scalable thin-film silicon deposition method that produces high-quality material at minimal cost, and the evaluation of light-trapping mechanisms that will increase photon absorption in these films. Hot-wire chemical vapor deposition is used to fabricate silicon thin films with high crystalline fractions even on inexpensive substrates, and films grown with appropriate growth conditions exhibit initial open-circuit voltages above 450 mV, and while challenges in passivation still exist, this research illustrates the potential of this highly scalable and inexpensive deposition technique. Silver and silicon subwavelength structures were then both fabricated and simulated on ultra-thin silicon films on SiO2 to evaluate their potential for increasing light absorption through plasmonic and physical scattering mechanisms, and spectral response measurements demonstrate over a ten-fold increase in carrier generation with a metal nanoparticle surface array. Periodic dielectric structures exhibit Bloch modes in both measurement and simulation, with an increase in overall quantum efficiency of over 11% from both a flat silicon layer and one that is randomly textured. These results highlight the significant role of scattering particle distribution in determining the light trapping characteristics in these devices. Design guidelines have been explored for exploiting resonant modes in these structures, and this thesis demonstrates the potential for both metal and dielectric surface arrays to dramatically increase light absorption in silicon thin films.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Atwater, Harry Albert}, } @phdthesis{10.7907/G19Z-CY24, author = {Briggs, Ryan Morrow}, title = {Hybrid Silicon Nanophotonic Devices: Enhancing Light Emission, Modulation, and Confinement}, school = {California Institute of Technology}, year = {2011}, doi = {10.7907/G19Z-CY24}, url = {https://resolver.caltech.edu/CaltechTHESIS:05312011-175622776}, abstract = {Silicon has become an increasingly important photonic material for communications, information processing, and sensing applications. Silicon is inexpensive compared to compound semiconductors, and it is well suited for confining and guiding light at standard telecommunication wavelengths due to its large refractive index and minimal intrinsic absorption. Furthermore, silicon-based optical devices can be fabricated alongside microelectronics while taking advantage of advanced silicon processing technologies. In order to realize complete chip-based photonic systems, certain critical components must continue to be developed and refined on the silicon platform, including compact light sources, modulators, routers, and sensing elements. However, bulk silicon is not necessarily an ideal material for many active devices because of its meager light emission characteristics, limited refractive index tunability, and fundamental limitations in confining light beyond the diffraction limit.
In this thesis, we present three examples of hybrid devices that use different materials to bring additional optical functionality to silicon photonics. First, we analyze high-index-contrast silicon slot waveguides and their integration with light-emitting erbium-doped glass materials. Theoretical and experimental results show significant enhancement of spontaneous emission rates in slot structures. We then demonstrate the integration of vanadium dioxide, a thermochromic phase-change material, with silicon waveguides to form micron-scale absorption modulators. It is shown experimentally that a 2-µm long waveguide-integrated device exhibits broadband modulation of more than 6.5 dB at wavelengths near 1550 nm. Finally, we demonstrate polymer-on-gold dielectric-loaded surface-plasmon waveguides and ring resonators coupled to silicon waveguides with 1.0±0.1 dB insertion loss. The plasmonic waveguides are shown to support a single surface mode at telecommunication wavelengths, with strong electromagnetic field confinement at the polymer-gold interface. These three device concepts show that diverse materials can be integrated with silicon waveguides to achieve enhanced light emission, broadband modulation, and strong confinement, all while retaining the advantages of the silicon photonics platform.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Atwater, Harry Albert}, } @phdthesis{10.7907/AMD4-Q845, author = {Ferry, Vivian Eleanor}, title = {Light Trapping in Plasmonic Solar Cells}, school = {California Institute of Technology}, year = {2011}, doi = {10.7907/AMD4-Q845}, url = {https://resolver.caltech.edu/CaltechTHESIS:05202011-180941490}, abstract = {Subwavelength nanostructures enable the manipulation and molding of light in nanoscale dimensions. By controlling and designing the complex dielectric function and nanoscale geometry we can affect the coupling of light into specific active materials and tune macroscale properties such as reflection, transmission, and absorption. Most solar cell systems face a trade-off with decreasing semiconductor thickness: reducing the semiconductor volume increases open circuit voltages, but also decreases the absorp- tion and thus the photocurrent. Light trapping is particularly critical for thin-film amorphous Si (a-Si:H) solar cells, which must be made less than optically thick to enable complete carrier collection. By enhancing absorption in a given semiconductor volume, we can achieve high efficiency devices with less than 100 nm of active region.
In this thesis we explore the use of designed plasmonic nanostructures to couple incident sunlight into localized resonant modes and propagating waveguide modes of an ultrathin semiconductor for enhanced solar-to-electricity conversion. We begin by developing computational tools to analyze incoupling from sunlight to guided modes across the solar spectrum and a range of incident angles. We then show the potential of this method to result in absorption enhancements beyond the limits for thick film solar cells. The second part of this thesis describes the integration of plasmonic nanos- tructures with a-Si:H solar cells, showing that designed nanostructures can lead to enhanced photocurrent over randomly textured light trapping surfaces, and develops a computational model to accurately simulate the absorption in these structures. The final chapter discusses the fabrication of a high-efficiency (9.5%) solar cell with a less than 100 nm absorber layer and broadband, angle isotropic photocurrent enhance- ment. Moreover, we discuss general design rules where light trapping nanopatterns are defined by their spatial coherence spectral density.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Atwater, Harry Albert}, } @phdthesis{10.7907/1RB7-ME80, author = {Pryce, Imogen Mary}, title = {Resonant Metallic Nanostructures for Active Metamaterials and Photovoltaics}, school = {California Institute of Technology}, year = {2011}, doi = {10.7907/1RB7-ME80}, url = {https://resolver.caltech.edu/CaltechTHESIS:05282011-153554049}, abstract = {Electromagnetic metamaterials are composites consisting of sub-wavelength structures designed to exhibit particular responses to an incident electromagnetic wave. In general, the properties of a metamaterial are fixed at the time of fabrication by the dimensions of each unit cell and the materials used. By incorporating dynamic components to the metamaterial system, a new type of tunable design can be accessed.
This thesis describes the design and development of resonant metallic nanostructures for use in active metamaterials. We begin by examining passive systems and introduce concepts that are critical for the design of more complex, tunable structures. We show how a simple metamaterial design, a plasmonic nanoparticle array, can be used to enhance the photocurrent of an ultrathin InGaN quantum well photovoltaic cell. We then explore how more complex resonator shapes can be coupled together in a single unit cell in order to access more complex resonant behavior.
In the second half of this thesis, we use several material systems as the basis for the design of active metamaterials. We demonstrate the first tunable metamaterial at optical frequencies using vanadium dioxide, a phase transition material. We exploit this material’s transition from a semiconducting to a metallic state and show how a novel fabrication scheme can be used to achieve a frequency tunable resonant response. We then abandon traditional hard and brittle substrates and develop a lithographic transfer process for adhering metallic nanostructures to highly compliant polymeric substrates. Mechanical deformation is then used to distort the resonator shapes and achieve resonant tunability of a full linewidth. This system is exploited to demonstrate interesting resonant hybridization phenomena, such as Fano resonance modulation, and sets the stage for the more elusive goal of driving two resonant nanostructures into contact. Finally, we describe the use of compliant tunable metamaterials as both refractive index sensors and surface enhanced infrared absorption (SEIRA) substrates. The results highlight the promise of post-fabrication tunable compliant metamaterial sensors and the potential for integration with spectroscopic devices in remote sensing and microfluidic device applications.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Atwater, Harry Albert}, } @phdthesis{10.7907/7J9Z-N927, author = {Diest, Kenneth Alexander}, title = {Active Metal-Insulator-Metal Plasmonic Devices}, school = {California Institute of Technology}, year = {2010}, doi = {10.7907/7J9Z-N927}, url = {https://resolver.caltech.edu/CaltechETD:etd-09222009-133531}, abstract = {As the field of photonics constantly strives for ever smaller devices, the diffraction limit of light emerges as a fundamental limitation in this pursuit. A growing number of applications for optical “systems on a chip” have inspired new ways of circumventing this issue. One such solution to this problem is active plasmonics. Active plasmonics is an emerging field that enables light compression into nano-structures based on plasmon resonances at a metal-dielectric interface and active modulation of these plasmons with an applied external field. One area of active plasmonics has focused on replacing the dielectric layer in these waveguides with an electro-optic material and designing the resulting structures in such a way that the transmitted light can be modulated. These structures can be utilized to design a wide range of devices including optical logic gates, modulators, and filters.
This thesis focuses on replacing the dielectric layer within a metal-insulator-metal plasmonic waveguide with a range of electrically active materials. By applying an electric field between the metal layers, we take advantage of the electro-optic effect in lithium niobate, and modulating the carrier density distribution across the structure in n-type silicon and indium tin oxide.
The first part of this thesis looks at fabricating metal-insulator-metal waveguides with ion-implantation induced layer transferred lithium niobate. The process is analyzed from a thermodynamic standpoint and the ion-implantation conditions required for layer transfer are determined. The possible failure mechanisms that can occur during this process are analyzed from a thin-film mechanics standpoint, and a metal-bonding method to improve successful layer transfer is proposed and analyzed. Finally, these devices are shown to naturally filter white light into individual colors based on the interference of the different optical modes within the dielectric layer. Full-field electromagnetic simulations show that these devices can preferentially couple to any of the primary colors and can tune the output color of the device with an applied field.
The second part of this thesis looks at fabricating metal-insulator-metal waveguides with n-type silicon and indium tin oxide. With the silicon device, by tuning the thicknesses of the layers used in a metal-oxide semiconductor geometry, the device we call the “plasMOStor” can support plasmonic modes as well as exactly one photonic mode. With an applied field, this photonic mode is pushed into cutoff and modulation depths of 11.2 dB are achieved. With the indium tin oxide device, the doping density within the material is changed and as a result, the plasma frequency is shifted into the near-infrared and visible wavelengths. Using spectroscopic ellipsometry, the structure is characterized with and without an applied electric field, and measurements show that when an accumulation layer is formed within the structure, the index of refraction within that layer is significantly changed and as a result, will change the optical modes supported in such a structure.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Atwater, Harry Albert}, } @phdthesis{10.7907/B774-EW86, author = {Hofmann, Carrie Elizabeth}, title = {Optics at the Nanoscale: Light Emission in Plasmonic Nanocavities}, school = {California Institute of Technology}, year = {2010}, doi = {10.7907/B774-EW86}, url = {https://resolver.caltech.edu/CaltechTHESIS:05282010-101941747}, abstract = {
Nanophotonics has greatly benefited from the unique ability of surface plasmons to confine optical modes to volumes well below the diffraction limit of light. Plasmonics is an emerging area of research that opens the path for controlling light-matter interactions on the subwavelength scale, enabling truly nanophotonic technologies that are unattainable with conventional diffraction-limited optical components. Novel surface plasmon devices exploit electromagnetic waves confined to the interface between a metal and a dielectric and permit the researcher to shrink light to dimensions previously inaccessible with optics. The extremely high and localized fields in plasmonic nanocavities are finding applications in research areas such as single-molecule sensing, nano-lasers, and photothermal tumor ablation, among others.
This thesis explores, both experimentally and theoretically, light emission in a number of plasmonic nanostructures. We present cathodoluminescence imaging spectroscopy as a new method of characterizing surface plasmons on metal films and localized in nanocavity resonators, with experimental observations supported by analytical calculations and electromagnetic simulation. This technique enables extremely localized surface plasmon excitation, a feature we exploit in both planar metal geometries and plasmonic nanocavities. We also study a specific nanocavity geometry, the plasmonic core-shell nanowire resonator, investigating both passive and active semiconductor core materials. This geometry allows precise control of the local density of optical states (LDOS), exhibiting the highest LDOS and smallest mode volumes in structures with dimensions as small as λ/50. Moreover, we discuss the Purcell effect as it applies to plasmonic nanocavities, and calculate enhancements in the radiative decay rate of more than 3000× in the smallest structures. These results demonstrate the promise of plasmonics to enable truly nanophotonic technologies and to manipulate light at the nanoscale.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Atwater, Harry Albert}, } @phdthesis{10.7907/JSVM-J029, author = {Putnam, Morgan Charles}, title = {Si Microwire-Array Solar Cells}, school = {California Institute of Technology}, year = {2010}, doi = {10.7907/JSVM-J029}, url = {https://resolver.caltech.edu/CaltechTHESIS:06072010-170215356}, abstract = {By allowing for the fabrication of flexible crystalline-Si (c-Si) solar cells that employ ~1/100th) the Si of a traditional wafer-based c-Si solar cell, while maintaining high photovoltaic efficiencies, vertically aligned arrays of c-Si microwires provide a novel photovoltaic geometry that has the potential to dramatically reduce the cost of solar electricity. In this thesis we report on 1) the growth of Si microwire arrays, 2) the chemical and electrical characterization of Si microwire arrays, and 3) the fabrication of Si microwire-array solar cells.
Using the vapor-liquid-solid (VLS) growth mechanism in combination with photolithographic patterning, vertically aligned arrays of Si microwires, with nominally identical heights and diameters, were fabricated over areas > 1 cm2. Chemical characterization of the Si wires was then performed using secondary ion mass spectrometry to measure the incorporation of the Au VLS-catalyst into the Si wire. The incorporation of the VLS-catalyst into the Si wires at its thermodynamic equilibrium concentration suggested that the use of Cu as a VLS-catalyst was less likely to limit the photovoltaic performance of Si microwire-array solar cells. Switching to the Cu-catalyzed growth of Si wires, in-situ doping with BCl3 was used to demonstrate control of the electrically active dopant concentration from 8 x 1015 to 4 x 1019 dopants cm-3. Scanning photocurrent measurements were then made to measure the minority-carrier diffusion length. The observation of 10 μm minority-carrier diffusion lengths indicated that solar cells with efficiencies of 17.5% should be possible. With the knowledge that highly efficient solar cells were possible, methods for the fabrication of a p-n junction and a transparent top contact in a solid-state solar cell were developed. This culminated in the demonstration of Si microwire-array solar cells with Air Mass 1.5 Global photovoltaic conversion efficiencies of up to η = 7.9%. Through improved device processing and the use of an amorphous Si passivation layer at the top contact, ~15% efficient solar cells should be possible.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Atwater, Harry Albert}, } @phdthesis{10.7907/99RA-7Z65, author = {Kelzenberg, Michael David}, title = {Silicon Microwire Photovoltaics}, school = {California Institute of Technology}, year = {2010}, doi = {10.7907/99RA-7Z65}, url = {https://resolver.caltech.edu/CaltechTHESIS:06082010-074917811}, abstract = {
The favorable bandgap and natural abundance of Si, combined with the large expertise base for semiconductor wafer processing, have led to the use of wafer-based crystalline Si in the vast majority of photovoltaic cells and modules produced worldwide. However the high cost of purifying, crystallizing, and sawing Si wafers has inhibited these photovoltaic energy sources from approaching cost parity with fossil fuels. Crystalline Si microwires, grown by the catalytic vapor-liquid-solid (VLS) chemical vapor deposition process, have recently emerged as promising candidate materials for thin-film photovoltaics–combining low-cost Si deposition techniques with mechanically flexible, high-performance device geometries.
This thesis presents several achievements that have helped to establish the viability of high-performance Si microwire photovoltaics. We begin by developing a comprehensive numerical model of Si microwire-array solar cells, combining finite-element device physics simulations with time-domain optical methods to predict that these devices can exceed 17% solar energy conversion efficiency. We then turn our attention to the optical properties of Si microwire arrays, concerned that the sparsely packed wires might not absorb enough sunlight. However our experiments reveal that simple light-trapping techniques can dramatically improve their absorption, not only permitting them to effectively absorb sunlight using 1/100th as much Si as a wafer, but also leading to an unexpected and fundamentally advantageous absorption enhancement over classical light trapping in planar materials. Techniques are then presented to characterize the material quality of VLS-grown Si wires. Although the growth of these wires is catalyzed by notoriously undesirable metal impurities for crystalline Si (e.g., Au, Ni, and Cu), we find it is nonetheless possible to synthesize high-quality material with remarkable diffusion lengths. By combining these materials with effective surface-passivation and a novel junction-fabrication technique, we realize single-wire solar cells that achieve open-circuit voltages of ~600 mV and with fill factors exceeding 80%. These observations suggest that Si microwires may offer a promising alternative to wafers for cost-effective crystalline Si photovoltaics.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Atwater, Harry Albert}, } @phdthesis{10.7907/WPBT-C144, author = {Dicken, Matthew James}, title = {Active Oxide Nanophotonics}, school = {California Institute of Technology}, year = {2009}, doi = {10.7907/WPBT-C144}, url = {https://resolver.caltech.edu/CaltechETD:etd-05032009-154839}, abstract = {Materials that can be manipulated electrically or mechanically to induce a change in their intrinsic properties are highly relevant when suitably integrated with current technologies. These “active” materials, such as oxide-based ferroelectrics or materials with easily accessible changes of phase, find extensive use as mechanical resonators, solid-state memories, and optical modulators. Barium titanate, a tetragonal ferroelectric at room temperature, is a prime example of a material both mechanically and optically active. This thesis deals primarily with the deposition of active, oxide-based materials and their integration into device structures where either the mechanical or optical properties are exploited.
The technologically interesting paradigms within which these active oxide materials have been investigated are microelectromechanical systems, plasmonics, and metamaterials. Microelectromechanical systems are devices that have been micromachined and rely on an applied voltage to induce a mechanical response. Mechanically active materials, such as piezoelectrics or ferroelectrics, can increase the response of these devices. Plasmonics deals with electromagnetic waves resonantly coupled into free electron oscillations at a metal-dielectric interface or metal nanoparticle. Coupling to these resonant modes allows surface plasmon polaritons to propagate along the metal with a nonlinear dispersion. Metamaterials are ordered, subwavelength, metal inclusions in a dielectric, which respond collectively to electromagnetic radiation. This response can yield a material permittivity or permeability not found in nature. The optical properties of metamaterials lead to effects such as negative index response and super lensing, and can be used to design optical cloaking structures. Here, devices utilizing these effects are investigated with an eye toward tuning or switching their resonant response using optically active oxide thin films.
This manuscript follows the evolution of active oxide thin films from deposition, through design of plasmonic devices and active metamaterials, finite difference modeling of these structures, and finally experimental validation. First, deposition and material integration techniques for oxide-based thin films will be discussed. The role of molecular beam epitaxy, pulsed laser deposition, and ion beam assisted deposition as material growth techniques are investigated. Development of a multitude of oxide materials using these techniques including barium titanate, strontium ruthenate, vanadium oxide, and magnesium oxide will be covered. The following two sections deal with the mechanical and optical properties of barium titanate thin films as they are studied and utilized to design and fabricate active devices. Films were characterized mechanically, using nanoindentation and piezoresponse force microscopy, and optically with variable angle spectroscopic ellipsometry. The subsequent section deals with the design, fabrication, and experimental validation of an active optical device based on surface plasmon polariton wavevector modulation via electrooptic modulation of a barium titanate thin film. Interferometers based on pairs of parallel slits fabricated in silver films on barium titanate are used to investigate optical modulation due to both domain switching and the electrooptic effect. Finally, active metamaterials are discussed through the investigation of a new material, vanadium oxide, as it is deposited and characterized, and the results used to design and fabricate active, split-ring resonator metamaterial structures.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Atwater, Harry Albert}, } @phdthesis{10.7907/3DCC-CZ57, author = {Dionne, Jennifer Anne}, title = {Flatland Photonics: Circumventing Diffraction with Planar Plasmonic Architectures}, school = {California Institute of Technology}, year = {2009}, doi = {10.7907/3DCC-CZ57}, url = {https://resolver.caltech.edu/CaltechETD:etd-10302008-115303}, abstract = {
On subwavelength scales, photon-matter interactions are limited by diffraction. The diffraction limit restricts the size of optical devices and the resolution of conventional microscopes to wavelength-scale dimensions, severely hampering our ability to control and probe subwavelength-scale optical phenomena. Circumventing diffraction is now a principle focus of integrated nanophotonics. Surface plasmons provide a particularly promising approach to sub-diffraction-limited photonics. Surface plasmons are hybrid electron-photon modes confined to the interface between conductors and transparent materials. Combining the high localization of electronic waves with the propagation properties of optical waves, plasmons can achieve extremely small mode wavelengths and large local electromagnetic field intensities. Through their unique dispersion, surface plasmons provide access to an enormous phase space of refractive indices and propagation constants that can be readily tuned with material or geometry.
In this thesis, we explore both the theory and applications of dispersion in planar plasmonic architectures. Particular attention is given to the modes of metallic core and plasmon slot waveguides, which can span positive, near-zero, and even negative indices. We demonstrate how such basic plasmonic geometries can be used to develop a suite of passive and active plasmonic components, including subwavelength waveguides, color filters, negative index metamaterials, and optical MOS field effect modulators. Positive index modes are probed by near- and far-field techniques, revealing plasmon wavelengths as small as one-tenth of the excitation wavelength. Negative index modes are characterized through direct visualization of negative refraction. By fabricating prisms comprised of gold, silicon nitride, and silver multilayers, we achieve the first experimental demonstration of a negative index material at visible frequencies, with potential applications for sub-diffraction-limited microscopy and electromagnetic cloaking. We exploit this tunability of complex plasmon mode indices to create a compact metal-oxide-Si (MOS) field effect plasmonic modulator (or plasMOStor). By transforming the MOS gate oxide into an optical channel, amplitude modulation depths of 11.2 dB are achieved in device volumes as small as one one-fifth of a cubic wavelength. Our results indicate the accessibility of tunable refractive indices over a wide frequency band, facilitating design of a new materials class with extraordinary optical properties and applications.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Atwater, Harry Albert}, } @phdthesis{10.7907/X9WE-V642, author = {Kayes, Brendan Melville}, title = {Radial pn Junction, Wire Array Solar Cells}, school = {California Institute of Technology}, year = {2009}, doi = {10.7907/X9WE-V642}, url = {https://resolver.caltech.edu/CaltechETD:etd-09222008-173738}, abstract = {
Radial pn junctions are potentially of interest in photovoltaics as a way to decouple light absorption from minority carrier collection. In a traditional planar design these occur in the same dimension, and this sets a lower limit on absorber material quality, as cells must both be thick enough to effectively absorb the solar spectrum while also having minority-carrier diffusion lengths long enough to allow for efficient collection of the photo-generated carriers. Therefore, highly efficient photovoltaic devices currently require highly pure materials and expensive processing techniques, while low cost devices generally operate at relatively low efficiency.
The radial pn junction design sets the direction of light absorption perpendicular to the direction of minority-carrier transport, allowing the cell to be thick enough for effective light absorption, while also providing a short pathway for carrier collection. This is achieved by increasing the junction area, in order to decrease the path length any photogenerated minority carrier must travel, to be less than its minority carrier diffusion length. Realizing this geometry in an array of semiconducting wires, by for example depositing a single-crystalline inorganic semiconducting absorber layer at high deposition rates from the gas phase by the vapor-liquid-solid (VLS) mechanism, allows for a “bottom up” approach to device fabrication, which can in principle dramatically reduce the materials costs associated with a cell.
This thesis explores the potential of this design, first theoretically and computationally, and then by exploring the growth of structures with the proposed morphology via methods with the potential for low cost, and finally by the experimental characterization of cells.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Atwater, Harry Albert and Lewis, Nathan Saul}, } @phdthesis{10.7907/4W2B-RD63, author = {Tanabe, Katsuaki}, title = {Low-Cost High-Efficiency Solar Cells with Wafer Bonding and Plasmonic Technologies}, school = {California Institute of Technology}, year = {2008}, doi = {10.7907/4W2B-RD63}, url = {https://resolver.caltech.edu/CaltechETD:etd-05272008-123439}, abstract = {
III-V compound multijunction solar cells enable ultrahigh efficiency performance in designs where subcells with high material quality and high internal quantum efficiency can be employed. However the optimal multijunction cell bandgap sequence cannot be achieved using lattice-matched compound semiconductor materials. Most current compound semiconductor solar cell design approaches are focused on either lattice-matched designs or metamorphic growth (i.e., growth with dislocations to accommodate subcell lattice mismatch), which inevitably results in less design flexibility or lower material quality than is desirable. An alternative approach is to employ direct bonded interconnects between subcells of a multijunction cell, which enables dislocation-free active regions by confining the defect network needed for lattice mismatch accommodation to tunnel junction interfaces.
We fabricated for the first time a direct-bond interconnected multijunction solar cell, a two-terminal monolithic GaAs/InGaAs dual-junction cell, to demonstrate a proof-of-principle for the viability of direct wafer bonding for solar cell applications. The bonded interface is a metal-free n⁺GaAs/n⁺InP tunnel junction with highly conductive Ohmic contact suitable for solar cell applications overcoming the 4% lattice mismatch. The quantum efficiency spectrum for the bonded cell was quite similar to that for each of unbonded GaAs and InGaAs subcells. The bonded dual-junction cell open-circuit voltage was equal to the sum of the unbonded subcell open-circuit voltages, which indicates that the bonding process does not degrade the cell material quality since any generated crystal defects that act as recombination centers would reduce the open-circuit voltage. Also, the bonded interface has no significant carrier recombination rate to reduce the open circuit voltage.
Such a wafer bonding approach can also be applied to other photovoltaic heterojunctions where lattice mismatch accommodation is also a challenge, such as the InGaP/GaAs/InGaAsP/InGaAs four-junction tandem cell by bonding a GaAs-based lattice-matched InGaP/GaAs subcell to an InP-based lattice-matched InGaAsP/InGaAs subcell. Simple considerations suggest that for such a cell the currently-reported interfacial resistance of 0.12 Ohm-cm² would result in a negligible decrease in overall cell efficiency of ~0.02%, under 1-sun illumination.
Engineered substrates consisting of thin films of InP on Si handle substrates (InP/Si substrates or epitaxial templates) have the potential to significantly reduce the cost and weight of compound semiconductor solar cells relative to those fabricated on bulk InP substrates. InGaAs solar cells on InP have superior performance to Ge cells at photon energies greater than 0.7 eV and the current record efficiency cell for 1 sun illumination was achieved using an InGaP/GaAs/InGaAs triple junction cell design with an InGaAs bottom cell. Thermophotovoltaic (TPV) cells from the InGaAsP-family of III-V materials grown epitaxially on InP substrates would also benefit from such an InP/Si substrate. Additionally, a proposed four-junction solar cell fabricated by joining subcells of InGaAs and InGaAsP grown on InP with subcells of GaAs and AlInGaP grown on GaAs through a wafer-bonded interconnect would enable the independent selection of the subcell band gaps from well developed materials grown on lattice matched substrates. Substitution of InP/Si substrates for bulk InP in the fabrication of such a four-junction solar cell could significantly reduce the substrate cost since the current prices for commercial InP substrates are much higher than those for Si substrates by two orders of magnitude. Direct heteroepitaxial growth of InP thin films on Si substrates has not produced the low dislocation-density high quality layers required for active InGaAs/InP in optoelectronic devices due to the ~8% lattice mismatch between InP and Si.
We successfully fabricated InP/Si substrates by He implantation of InP prior to bonding to a thermally oxidized Si substrate and annealing to exfoliate an InP thin film. The thickness of the exfoliated InP films was only 900 nm, which means hundreds of the InP/Si substrates could be prepared from a single InP wafer in principle. The photovoltaic current-voltage characteristics of the In0.53Ga0.47As cells fabricated on the wafer-bonded InP/Si substrates were comparable to those synthesized on commercially available epi-ready InP substrates, and had a ~20% higher short-circuit current which we attribute to the high reflectivity of the InP/SiO²/Si bonding interface. This work provides an initial demonstration of wafer-bonded InP/Si substrates as an alternative to bulk InP substrates for solar cell applications.
Metallic nanostructures can manipulate light paths by surface plasmons and can dramatically increase the optical path length in thin active photovoltaic layers to enhance photon absorption. This effect has potential for cost and weight reduction with thinned layers and also for efficiency enhancement associated with increased carrier excitation level in the absorber layer.
We have observed photocurrent enhancements up to 260% at 900 nm for a GaAs cell with a dense array of Ag nanoparticles with 150 nm diameter and 20 nm height deposited through porous alumina membranes by thermal evaporation on top of the cell, relative to reference GaAs cells with no metal nanoparticle array. This dramatic photocurrent enhancement is attributed to the effect of metal nanoparticles to scatter the incident light into photovoltaic layers with a wide range of angles to increase the optical path length in the absorber layer.
GaAs solar cells with metallic structures at the bottom of the photovoltaic active layers, not only at the top, using semiconductor-metal direct bonding have been fabricated. These metallic back structures could incouple the incident light into surface plasmon mode propagating at the semiconductor/metal interface to increase the optical path, as well as simply act as back reflector, and we have observed significantly increased short-circuit current relative to reference cells without these metal components.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Atwater, Harry Albert}, } @phdthesis{10.7907/9CFK-G982, author = {Archer, Melissa Jane}, title = {Multijunction Solar Cells on Epitaxial Templates}, school = {California Institute of Technology}, year = {2008}, doi = {10.7907/9CFK-G982}, url = {https://resolver.caltech.edu/CaltechETD:etd-05272008-103359}, abstract = {Future ultrahigh efficiency multijunction solar cells will employ designs that feature three or four or more subcells utilizing lattice-mismatched structures to achieve an optimal band gap sequence for solar energy conversion. While lattice-mismatched multijunction cells have been fabricated recently using metamorphic growth approaches, use of direct wafer bonding techniques to enable lattice mismatch accommodation at the subcell interfaces allows considerably more design freedom and inherently higher-quality, defect-free active regions. This thesis presents new results on wafer bonding and layer transfer for integration of materials with large lattice mismatch, as well as modeling work to better understand the key material parameters in the design of new multijunction solar cells.
GaInP/GaAs dual junction solar cells on Ge/Si templates were fabricated using wafer bonding and ion implantation induced layer transfer techniques. Following layer transfer, the surface of the ~1.4 um thick transferred Ge(100) has an as-transferred RMS roughness of ~20 nm and a near surface layer containing a high density of ion implantation-induced defects. The RMS roughness has been reduced to <1 nm. In addition, the effects of changing the strain state of the template substrate on the performance of the devices has been explored by comparing devices grown on Ge/Si and Ge/sapphire. The CTE mismatch between Si and GaAs/GaInP materials induces a tensile strain, whereas the sapphire substrate induces a compressive strain.
An analytical p-n junction device physics model for GaInP/GaAs/InGaAsP/InGaAs four junction solar cells was developed. Real behavior of solar cells is accounted for by including: free carrier absorption, temperature and doping effects on carrier mobility, as well as two recombination pathways: Shockley-Read-Hall recombination from a single mid gap trap level and surface recombination. Upper bounds set by detailed balance calculations can be approached by letting the parameters approach ideal conditions. Detailed balance calculations always benefit from added subcells, current matching requirements in series connected p-n multijunctions indicate a minimum performance required from added subcells for net contribution to the overall device. This model allows novel solar cell structures to be evaluated by providing realistic predictions of the performance limitations of these multijunction devices.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Atwater, Harry Albert}, } @phdthesis{10.7907/8GPE-SB31, author = {Sweatlock, Luke A.}, title = {Plasmonics: Numerical Methods and Device Applications}, school = {California Institute of Technology}, year = {2008}, doi = {10.7907/8GPE-SB31}, url = {https://resolver.caltech.edu/CaltechETD:etd-06112008-051943}, abstract = {
Plasmonics is a rapidly evolving subfield of nanophotonics that deals with the interaction of light with surface plasmons, which are the collective charge oscillations that occur at the interface between conductive and dielectric materials. Plasmonics meet a demand for optical interconnects which are small enough to coexist with nanoscale electronic circuits. Emerging technologies include very small, low-power active devices such as electrooptic or all-optical modulators. Passive plasmonic devices, or “optical antennas”, are being used to enhance the performance of emitters and detectors, and to harvest sunlight for photovoltaics. This manuscript focuses on the process of developing novel plasmonic devices from concept to prototype, with specific emphasis on synthesizing data from numerical simulation and from empirical characterization into an accurate, predictive understanding of nanoscale optical phenomena.
The first part of the thesis outlines the development of numerical methods. In the case of resonant nanostructures such as small metal particles, the principal technique employed is impulse excitation ringdown spectroscopy. This method allows the critical advantage of generating broadband spectra from a single time-domain simulation. For analysis of plasmonic waveguides, Fourier-space analysis is used to reveal the dispersion properties of supported modes, and to perform filtering in the wavevector domain or “k-space”. The remainder of the thesis deals with the design and characterization of plasmonic devices, with the broad and general goal of creating a significant impact in the fields of optoelectronics and photovoltaics.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Atwater, Harry Albert}, } @phdthesis{10.7907/K3BG-N315, author = {Ruglovsky, Jennifer Lynn}, title = {Correlating Microscopic Ferroelectric Properties and Macroscopic Thin Film Device Performance}, school = {California Institute of Technology}, year = {2007}, doi = {10.7907/K3BG-N315}, url = {https://resolver.caltech.edu/CaltechETD:etd-02252007-153131}, abstract = {The relationship between thin film device performance and crystallographic microstructure is one of fundamental importance in materials science. Ferroelectric materials that show an electromechanical response via domain switching, such as the perovskites BaTiO3 and PbTiO3, are discussed. In this work, we focus on thin film MEMS actuators fabricated from four different ferroelectric thin film microstructures: poorly oriented, fiber textured, biaxially textured, and single crystal. The microscale properties of these thin film materials are characterized and correlated to macroscale mechanical device behavior.
We have modeled each of these four microstructures to determine the effect of grain-scale crystallographic texture on device-scale electromechanical constants. The method enables the effective electromechanical properties to be obtained for a polycrystalline film via a self-consistent approach. Using this model, we show that most electromechanical constants depend primarily on the out-of-plane texture of the ferroelectric thin film.
We have used surface micromachining to create free-standing bridge geometries in ferroelectric thin films of polycrystalline and biaxially textured PbTiO3. The material properties of these thin films are characterized with various techniques to confirm the texture at the grain scale. We have utilized a custom experimental apparatus that can apply a loading force to a single microdevice via magnetostatic interaction while measuring the resulting displacement. The force-displacement curves that we measure provide insight into the initial stress and modulus of our composite beam devices and the role of the underlying crystalline microstructure.
In order to study cantilever actuators, BaTiO3 active layers are grown monolithically on SrRuO3 electrodes and devices are patterned via focused ion beam (FIB) milling exclusively or with a subsequent XeF2 etch. Using this fabrication method, we study cantilevers consisting of fiber, biaxial, and single crystalline microtextures. The cantilevers are actuated by applying a voltage across the active layer and the resulting displacement is measured via inspection with optical microscopy. We are able to relate the macroscopic device performance to the microscopic piezoelectric constants via multimorph calculations.
Our experiments show that ferroelectric thin film device performance may be enhanced by improving the underlying grain scale crystalline microstructure - from fiber to biaxial to single crystal texture.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Atwater, Harry Albert}, } @phdthesis{10.7907/GG0F-1775, author = {Richardson, Christine Esber}, title = {Low-Temperature Hot-Wire Chemical Vapor Deposition of Epitaxial Films for Large-Grained Polycrystalline Photovoltaic Devices}, school = {California Institute of Technology}, year = {2007}, doi = {10.7907/GG0F-1775}, url = {https://resolver.caltech.edu/CaltechETD:etd-07052006-123702}, abstract = {Large-grained polycrystalline silicon thin-films on low-cost substrates are an interesting area of research for photovoltaic devices. Such devices, with grain sizes larger than the thickness of the cell, have the potential to achieve multicrystalline-like efficiencies of 15%, but at a much lower cost by taking advantage of thin-film manufacturing techniques. In this thesis, low-temperature epitaxial growth, by hot-wire (or catalytic) chemical vapor deposition, is investigated for the epitaxial thickening of large-grained polycrystalline silicon templates formed by metal-induced crystallization on low-cost substrates. Low-temperature hot-wire chemical vapor deposition allows for the deposition of epitaxial silicon with polycrystalline breakdown and with open-circuit voltages close to that of monocrystalline silicon. This is possible due to the incorporation of hydrogen into the silicon lattice, at temperatures below 350°C, for internal surface and defect passivation. In addition with hot-wire chemical vapor deposition, the critical epitaxial thickness actually increases, with a decrease in the substrate temperature down to temperatures of 270°C. Epitaxial growth of 5.5 micron thick films at 300°C and twinned epitaxial silicon growth of 6.8 micron thick films at 230°C have been achieved, along with arbitrarily thick crystalline films at low temperatures. Since epitaxial and high-quality crystalline silicon can be deposited at such low deposition temperatures, low-cost substrates, such as ordinary soda lime glass and many polymers are possible. In order to work towards achieving an epitaxially-thickened large-grained polycrystalline device, this work studies the mechanisms that lead to epitaxial growth during hot-wire chemical vapor deposition on silicon (100) substrates under various growth regimes, examines the surface evolution of crystalline thin-films grown via hot-wire chemical vapor deposition and their growth mechanisms (including the unusual rough epitaxial growth and arbitrarily thick crystalline films at low temperatures), and concludes by presenting the optical and electrical characteristics of these films and their resultant devices. This thesis demonstrates that low-temperature epitaxial silicon growth by hot-wire chemical vapor deposition is a promising material for low-cost thin-film silicon photovoltaic devices.}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Atwater, Harry Albert}, } @phdthesis{10.7907/TKXK-8930, author = {Walters, Robert Joseph}, title = {Silicon Nanocrystals for Silicon Photonics}, school = {California Institute of Technology}, year = {2007}, doi = {10.7907/TKXK-8930}, url = {https://resolver.caltech.edu/CaltechETD:etd-06042007-160130}, abstract = {In the absence of suitable methods for integrating traditional semiconductor optoelectronic materials in CMOS microelectronic fabrication processes, nanostructured silicon has been actively explored as an alternative light emitter for silicon photonics. This thesis presents new experimental results in silicon nanocrystal photophysics and optoelectronics, including novel device designs for optical memory elements and light-emitting structures.
As quantum dots, silicon nanocrystals exhibit several interesting properties including size-tunable emission over visible and near-infrared wavelengths and improved oscillator strength for radiation. In contrast to bulk silicon, nanocrystals can emit light with quantum efficiencies approaching 100%. Through time-resolved photoluminescence measurements, we first quantitatively establish that the dense ensembles of nanocrystals that are attractive in device applications retain these advantages. We then describe the fabrication of fully CMOS compatible silicon nanocrystal optoelectronic test structures and show that such devices can function as room temperature optical memory elements.
We further demonstrate that electroluminescence can be achieved in our devices through a previously unreported process we call field effect electroluminescence, in which sequential charge carrier injection is used to create excitons in silicon nanocrystals. This mechanism is a promising approach for overcoming the difficulty inherent in electrically exciting silicon nanocrystals, which are necessarily surrounded by an electrical insulator. Finally, we present electrically excited infrared light sources that combine carrier injection through the field effect electroluminescence mechanism with near field energy transfer from silicon nanocrystals to infrared emitters.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Atwater, Harry Albert}, } @phdthesis{10.7907/42Z7-PY73, author = {Holunga, Dean Marcu}, title = {Aerosol Technologies for Fabrication, Collection, and Deposition of Engineered Nanoparticles}, school = {California Institute of Technology}, year = {2006}, doi = {10.7907/42Z7-PY73}, url = {https://resolver.caltech.edu/CaltechETD:etd-09252008-110728}, abstract = {We demonstrate a turbulent mixing reactor capable of producing highly monodisperse, σG ≈ 1.1, heterogeneous oxide-coated silicon nanoparticles from pyrolytic decomposition of silane. Particle concentrations approach 10⁹ cm⁻³ as measured with a radial differential mobility analyzer and fA resolution electrometer. Turbulent mixing power, induced by locally high-momentum jets that actually remain below turbulent Reynolds numbers, induce mechanical mixing within a pathlength comparable to the diameter of the major flow channel. Timescales for transport are enhanced orders of magnitude above laminar processes, enabling nanoparticle evolutionary processes such as densification and crystallization to complete in the absence of significant agglomeration. Use of multiple jets in series may well enable the homogeneous introduction of additional reagents to facilitate additional heterogenous particle development.
Particles formed in the Inconel reactor were further studied using both transmission electron microscopy and photoluminescence measurements. Spherical particle morphology with faceted and unfaceted crystalline cores were observed, and thermal oxides appeared uniform. Particle purity and a high quality passivation of the particles were demonstrated by photoluminescence, although particles occasionally required additional processing to complete O₂ passivation. Photoluminescence measurements are in good agreement with models of quantum-confined exciton recombination, both in emitted wavelength and photoluminescence decay. Particle contamination studies using Electron Energy Loss Spectroscopy and Energy Dispersive X-Ray Spectroscopy found no evidence of metal contamination within particles studied for both native oxide and thermal oxide-coated particles. A phenomenological comparison of size information from the radial differential mobility analyzer and photoluminescence spectra demonstrated that thermally grown oxide shells and native oxide shell have initially opposite trends in the variation of thickness with particle size, although over time, native oxide shells thicken considerably.
A thermophoretic deposition chamber was designed for uniform deposition on wafers ranging in size from 100 mm–300 mm and over a range of flowrates from 500 sccm to 15000 sccm. A power-law hyperbolic inlet nozzle was shown theoretically to minimize separation. A uniform axial temperature gradient is developed using programmable temperature controlled heaters along with active cooling. Characterization by atomic force microscopy studies on 150 mm wafers demonstrated uniform coverage both radially and in the azimuth, in good agreement with model results. Deposition uniformity is predicted on larger wafers, up to 300 mm.
Pyrolysis reactions in small diameter tubular reactors foul the reactors’ walls continuously, with deposition morphology ranging from thin-films to dendritic, filter-like structures. The particle number concentration decays linearly with time. Hybridization of the turbulent mixing reactor with high energy seed reactors, such as a microplasma discharge, shows promise that may significantly reduce fouling, maintain or increase particle number concentration, maintain or increase particle monodispersity, expand chemistries available, and retain the ability to produce heterogeneous particles.
Laminar flow reactors are well suited to the production of monodisperse, σG ≈ 1.1, aerosols. The rate of pyrolytic decomposition of silane precursor is kept relatively slow during a gentle thermal ramp wherein the low temperature favors vapor deposition growth over additional nucleation. The resulting reduction in silane inhibits further nucleation as the temperature is increased. Slow flowrates, wherein diffusional losses of precursor assists the inhibition of additional nucleation, also contributed to maintaining lower nucleation rates, but are not necessary to achieve monodispersity or higher yield.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Flagan, Richard C. and Atwater, Harry Albert}, } @phdthesis{10.7907/B77G-WS08, author = {Biteen, Julie Suzanne}, title = {Plasmon-Enhanced Silicon Nanocrystal Luminescence for Optoelectronic Applications}, school = {California Institute of Technology}, year = {2006}, doi = {10.7907/B77G-WS08}, url = {https://resolver.caltech.edu/CaltechETD:etd-05312006-163455}, abstract = {
On the path toward the realization of silicon-based optical emitters for integrated microelectronics, this thesis studies the optoelectronic properties of silicon nanocrystals as a function of their surface passivation and their interactions with plasmonic materials. The first part of the thesis utilizes controlled oxidation exposures and photoluminescence spectroscopy to verify previous theoretical and computational predictions of oxygen-related surface states that effectively narrow the energy band gap of small silicon nanocrystals. The focus of the second half of the thesis is on experimental and computational studies of enhanced luminescence from silicon nanocrystals in the near field of noble metal nanostructures.
Surface plasmon enhancement is a technique that has only recently been applied to semiconductor nanocrystal luminescence. This thesis thoroughly investigates the emission of silicon nanocrystals coupled to gold and silver nanostructures to achieve a new level of understanding of the enhancement effect. By pairing silicon nanocrystals to metal nanostructures, up to ten-fold increases in the luminescence intensity are realized, concomitant with enhancements of the radiative decay rate, the absorbance cross section, and the quantum efficiency. Moreover, coupling at the plasmon resonance frequency is used to tune the nanocrystal emission spectrum. A computational exploration of these experimental observations indicates that the enhancement effects can be ascribed to emission in the concentrated local field that results from the excitation of metal particle plasmon modes. Finally, the process of coupling silicon nanocrystal emitters to plasmonic metals is applied to a silicon-nanocrystal light-emitting diode, and enhanced electroluminescence is realized.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Atwater, Harry Albert}, } @phdthesis{10.7907/N8JK-ZQ70, author = {Feng, Tao}, title = {Silicon Nanocrystal Charging Dynamics and Memory Device Applications}, school = {California Institute of Technology}, year = {2006}, doi = {10.7907/N8JK-ZQ70}, url = {https://resolver.caltech.edu/CaltechETD:etd-06052006-141803}, abstract = {The application of Si nanocrystals as floating gate in the metal oxide semiconductor field-effect transistor (MOSFET) based memory, which brings many advantages due to separated charge storage, attracted much attention in recent years. In this work, Si nanocrystal memory with nanocrystals synthesized by ion implantation was characterized to provide a better understanding of the relationship between structure and performance – especially charge retention characteristics.
In the structural characterization it was demonstrated that scanning tunneling microscopy (STM) and non-contact atomic force microscopy (nc-AFM) enable much more accurate measurements of the ensemble size distribution and array density for small Si nanocrystals in SiO₂, estimated to be around 2-3 nm and 4 x 10¹² -3 x 10¹³ cm⁻², respectively. The reflection high energy electron diffraction (RHEED) pattern further verified the existence of nanocrystals in SiO₂. Capacitance-voltage (C-V) measurements demonstrated the memory effects. The comparison between charge density and nanocrystal density suggests single charge storage on individual Si nanocrystals.
The electronic property of tunnel oxide layer is a key factor influencing charge retention, and was characterized by conductive atomic force microscopy (C-AFM). An overall high conductance observed between the nanocrystal floating gate and the substrate is believed to be responsible for the relatively short retention time for electrons. A narrowed denuded zone contaminated with nanocrystals is suggested to be the reason for the high conductance, which is further supported by switching events and fluctuations in local current-voltage (I-V) curves. From the results of C-AFM, a better control of nanocrystal distribution close to the channel is shown to be critical for non-volatile nanocrystal memory made via Si ion implantation.
Nanoscale charge retention characteristics of both electrons and holes were probed directly by ultrahigh vacuum (UHV) nc-AFM, in which a highly doped Si tip was applied to inject charges into the nanocrystal layer and monitor subsequent charge dissipation. The results reveal a much longer hole retention time (e.g., >1 day) than that for electrons (e.g., <1 hour), which is consistent with the charge retention characteristics from electrical characterization of nanocrystal floating gate MOS capacitors as well as time-resolved photoluminescence measurements. The large difference in charge retention times for electrons and holes is attributed to the difference in tunneling barrier heights: 3.1 eV and 4.7 eV for electrons and holes, respectively. Based on the charge injection and retention characteristics obtained from UHV nc-AFM and nanocrystal floating gate MOS devices, we suggest that hole programming in Si nanocrystal memory is an interesting choice in improving data retention or in further device scaling.
UHV nc-AFM guarantees high detection sensitivity and stability in charge imaging experiments due to a lack of air damping, so a three-dimensional (3D) electrostatic model can be developed to provide quantitative information regarding the distribution and evolution of the localized charges. For example, a transition from initial complementary error function distribution to Gaussian distribution was suggested in the simulation. In addition, charge detection sensitivity was found to increase with the scanning height, showing much room for further improvement of the sensitivity in UHV nc-AFM. The limitation of the electrostatic model is also discussed, and some knowledge regarding the charge distribution obtained from theoretical analysis and other experimental methods is suggested to be necessary supplements to the quantitative charge analysis by nc-AFM.
Finally, the approach used in the electrostatic simulation of nc-AFM was applied in 3D simulation of Si nanocrystal memory. The dependence of Coulomb charging energy on dielectric environment is analyzed. From the local variation of channel minority carrier density due to separated charge storage, the threshold number density of charged nanocrytals for 1D approximation to break down is shown to be 10¹² cm⁻² in the sample geometry investigated.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Atwater, Harry Albert}, } @phdthesis{10.7907/05F0-K740, author = {Zahler, James Michael}, title = {Materials Integrations for High-Performance Photovoltaics by Wafer Bonding}, school = {California Institute of Technology}, year = {2005}, doi = {10.7907/05F0-K740}, url = {https://resolver.caltech.edu/CaltechETD:etd-06022005-234526}, abstract = {
The fundamental efficiency limit for state of the art triple-junction photovoltaic devices is being approached. By allowing integration of non-lattice-matched materials in monolithic structures, wafer bonding enables novel photovoltaic devices that have a greater number of subcells to improve the discretization of the solar spectrum, thus extending the efficiency limit of the devices. Additionally, wafer bonding enables the integration of non-lattice-matched materials with foreign substrates to confer desirable properties associated with the handle substrate on the solar cell structure, such as reduced mass, increased thermal conductivity, and improved mechanical toughness. This thesis outlines process development and characterization of wafer bonding integration technologies essential for transferring conventional triple-junction solar cell designs to potentially lower cost Ge/Si epitaxial templates. These epitaxial templates consist of a thin film of single-crystal Ge on a Si handle substrate. Additionally, a novel four-junction solar cell design consisting of non-lattice matched subcells of GaInP, GaAs, InGaAsP, and InGaAs based on InP/Si wafer-bonded epitaxial templates is proposed and InP/Si template fabrication and characterization is pursued.
In this thesis the detailed-balance theory of the thermodynamic limiting performance of solar cell efficiency is applied to several device designs enabled by wafer bonding and layer exfoliation. The application of the detailed-balance theory to the novel four-junction cell described above shows that operating under 100 suns at 300 K a maximum efficiency of 54.9% is achievable with subcell bandgaps of 1.90, 1.42, 1.02, and 0.60 eV, a material combination achievable by integrating two wide-bandgap subcells lattice matched to GaAs and two narrow-bandgap subcells lattice matched to InP.
Wafer bonding and layer transfer processes with sufficient quality to enable subsequent material characterization are demonstrated for both Ge/Si and InP/Si structures. The H-induced exfoliation process in each of these materials is studied using TEM, AFM, and FTIR to elucidate the chemical states of hydrogen leading to exfoliation. Additionally, the electrical properties of wafer-bonded interfaces between bulk-Ge/Si and bulk-InP/Si structures are show Ohmic, low-resistance electrical contact. Further studies of p-p isotype heterojunctions in Ge/Si indicate that significant conduction paths exist through defects at the bonded interface. The first known instance of epitaxy of III-V compound semiconductors on wafer-bonded Ge/Si epitaxial templates is demonstrated. Additionally InGaAs is grown on InP/Si templates that have been improved by removal of damage induced by the ion implantation and exfoliation processes.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Atwater, Harry Albert}, } @phdthesis{10.7907/D90P-6821, author = {Casperson, Julie Diane}, title = {Design and Characterization of Layered Tunnel Barriers for Nonvolatile Memory Applications}, school = {California Institute of Technology}, year = {2004}, doi = {10.7907/D90P-6821}, url = {https://resolver.caltech.edu/CaltechETD:etd-05262004-111123}, abstract = {The main limitations of floating gate memory devices (Flash memory) are the long program (microsecond) and erase times (~ 1 µs) inherent to the charging of floating gates using Fowler-Nordheim tunneling. An alternative to the integration of homogeneous dielectric tunnel barriers present in standard Flash memory is to use layered tunnel barriers made of high-k heterostructures. This allows for an effective lowering in barrier height under applied bias, resulting in shorter write/erase times while maintaining long retention times.
To assess these types of dielectric structures, tunneling probability simulations were performed using an effective mass-model, allowing us to predict current-voltage (I-V) characteristics and optimize the layered tunnel barrier structure. Based on our results, we correlated dielectric constants and band offsets with respect to silicon in order to help identify possible materials from which to construct these layered barriers. This survey allowed for the determination of promising high-k materials heterostructures: Si₃N₄ / Al₂O₃ / Si₃N₄ / Si₃N₄ and HfO₂ / Al₂O₃ / HfO₂.
We performed a series of physical and electrical characterization experiments on single-layer as well as two- and three-layer structures of Si₃N₄, Al₂O₃, and HfO₂. Transmission electron microscopy and I-V measurements were used to correlate the physical effects of high-temperature annealing on the electrical properties of the films, allowing us to determine the ideal processing conditions. Construction of Fowler-Nordheim plots from experimental I-V data gave qualitative evidence of barrier lowering in the multi-layer structures.
We developed a bias-dependent photoemission technique for quantitative determination of the band-offsets between silicon and our dielectric barriers, which is found to be highly dependent on the applied bias. For SiO₂ (and other single-layer materials), image potential barrier lowering simulations predict the barrier profile as a function of voltage, allowing us to report the band-offsets for these materials in a more complete way than was previously possible. Also, by characterizing multi-layer structures of HfO₂ and Al₂O₃, we have been able to quantitatively measure the effective barrier height of these structures over a wide range of biases and prove barrier lowering. Analysis by an electrostatic model allowed us to accurately simulate the barrier lowering results over all voltage ranges.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, } @phdthesis{10.7907/J5PY-RS79, author = {Brewer, Rhett Ty}, title = {Quantitative Biaxial Texture Analysis with Reflection High-Energy Electron Diffraction for Ion Beam-Assisted Deposition of MgO and Heteroepitaxy of Perovskite Ferroelectrics}, school = {California Institute of Technology}, year = {2004}, doi = {10.7907/J5PY-RS79}, url = {https://resolver.caltech.edu/CaltechETD:etd-08182003-150957}, abstract = {To facilitate ferroelectric-based actuator integration with silicon electronics fabrication technology, we have developed a route to produce biaxially textured ferroelectrics on amorphous layers by using biaxially textured MgO templates.
Using a kinematical electron scattering model, we show that the RHEED pattern from a biaxially textured polycrystalline film can be calculated from an analytic solution to the electron scattering probability. We found that diffraction spot shapes are sensitive to out-of-plane orientation distributions and in-plane RHEED rocking curves are sensitive to the in-plane orientation distribution. Using information from the simulation, a RHEED-based experimental technique was developed for in situ measurement of MgO biaxial texture. The accuracy of this technique was confirmed by comparing RHEED measurements of in-plane and out-of-plane orientation distribution with synchrotron x-ray rocking curve measurements.
Biaxially textured MgO was grown on amorphous Si3N4 by ion beam-assisted deposition (IBAD). MgO was e-beam evaporated onto the amorphous substrate with a simultaneous 750-1200 eV Ar⁺ ion bombardment at 45° from normal incidence. We observed a previously unseen, dramatic texture evolution in IBAD MgO using transmission electron microscopy (TEM) and RHEED-based quantitative texture measurements of MgO. The first layers of IBAD MgO are diffraction amorphous until the film is about 3.5 nm thick. During the next 1 nm of additional growth, we observed rapid biaxial texture evolution. RHEED and TEM studies indicate that biaxially textured MgO film results from a solid phase crystallization of biaxially textured MgO crystals in an amorphous matrix.
Biaxially textured perovskite ferroelectrics were grown on biaxially textured MgO templates using sol-gel, metallorganic chemical vapor deposition (MOCVD), and molecular beam epitaxy (MBE). Through RHEED-based biaxial texture analysis we observed that the heteroepitaxial ferroelectric in-plane orientation distribution, deposited using ex situ techniques (not performed in the same high vacuum growth environment where the MgO was deposited), narrowed significantly with respect to the in-plane orientation distribution of its MgO template (from 11° to 6° FWHM). Evidence from cross section TEM and RHEED suggest that atmospheric moisture degrades the crystallinity of highly defective, misaligned MgO grains and that heteroepitaxially grown ferroelectrics preferentially nucleate on well-aligned grains and over grow misaligned regions of MgO.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Atwater, Harry Albert}, } @phdthesis{10.7907/7K9R-VX22, author = {Mason, Maribeth Swiatek}, title = {Synthesis of Large-Grained Polycrystalline Silicon by Hot-Wire Chemical Vapor Deposition for Thin Film Photovoltaic Applications}, school = {California Institute of Technology}, year = {2004}, doi = {10.7907/7K9R-VX22}, url = {https://resolver.caltech.edu/CaltechETD:etd-03182004-221215}, abstract = {In this study, we investigate the fabrication of large-grained polycrystalline silicon by hot-wire chemical vapor deposition (HWCVD) and its suitability for thin-film photovoltaic applications. We have devised two strategies for the fast, low-temperature growth of thin polycrystalline silicon films on glass substrates. The first is the direct growth of polycrystalline silicon on SiO₂ by HWCVD. We use atomic force microscopy (AFM) to characterize fully continuous polycrystalline silicon films grown by HWCVD on SiO₂, as well as the nucleation density of silicon islands formed in the early stages of HWCVD growth, as a function of temperature and hydrogen dilution (H₂:SiH₄). Our observations of the nucleation kinetics of Si on SiO₂ can be explained by a rate-equation pair-binding model, from which we derive an estimate for the prefactor and activation energy for surface diffusion of Si on SiO₂ during HWCVD growth and assess the viability of this method for the rapid growth of large-grained polycrystalline silicon on SiO₂.
The second strategy uses large-grained (~100 µm) polycrystalline silicon layers fabricated by selective nucleation and solid-phase epitaxy (SNSPE) on SiO₂ substrates as templates for epitaxial growth by HWCVD. Using reflection high-energy electron diffraction (RHEED) and transmission electron microscopy (TEM), we have derived a phase diagram for Si on Si(100) consisting of epitaxial, twinned epitaxial, mixed epitaxial/polycrystalline, and polycrystalline phases of growth on Si(100) in the 50 nm-2 µm thickness regime. Evidence is also presented for epitaxial growth on SNSPE templates, which use nickel nanoparticles as nucleation sites for the solid-phase crystallization of phosphorus-doped amorphous silicon on SiO₂. Minority carrier lifetimes for films on Si(100), as measured by resonant-coupled photoconductive decay experiments, range from 5.7 to 14.8 microseconds while those for films on SNSPE templates range from 5.9 to 19.3 microseconds. Residual nickel present in the SNSPE templates does not significantly affect the lifetime of films grown on SNSPE templates, making the growth of epitaxial layers by HWCVD on SNSPE templates a possible strategy for the fabrication of thin-film photovoltaics.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Atwater, Harry Albert}, } @phdthesis{10.7907/TKPX-3H40, author = {Maier, Stefan Alexander}, title = {Guiding of Electromagnetic Energy in Subwavelength Periodic Metal Structures}, school = {California Institute of Technology}, year = {2003}, doi = {10.7907/TKPX-3H40}, url = {https://resolver.caltech.edu/CaltechETD:etd-02062003-155401}, abstract = {The ultimate miniaturization of optical devices requires structures that guide electromagnetic energy with a lateral confinement below the diffraction limit of light. In this thesis, the possibility of employing plasmon-polariton excitations in plasmon waveguides consisting of closely spaced metal nanoclusters for this purpose is examined. The feasibility of energy transport with mode sizes below the diffraction limit of visible light over distances of several hundred nanometers is demonstrated.
As a macroscopic analogue to plasmon waveguides, the transport of electromagnetic energy in the microwave regime along closely spaced centimeter-scale metal rods is examined. Full-field electrodynamic simulations show that information transport occurs at a group velocity of 0.65c for fabricated structures consisting of copper rods excited at 8 GHz. A variety of passive routing structures and an all-optical modulator are demonstrated.
The possibility of guiding electromagnetic energy at visible frequencies with mode sizes below the diffraction limit using plasmon waveguides is analyzed using a point-dipole model and finite-difference time-domain simulations. It is shown that energy transport occurs via near-field coupling between metal nanoparticles, which leads to coherent propagation of energy. For spherical gold particles in air, group velocities up to 0.06c are demonstrated, and a change in particle shape to spheroidal particles shows up to a threefold increase in group velocity. Pulses with transverse polarization are shown to propagate with negative phase velocities antiparallel to the energy flow.
Plasmon waveguides consisting of gold and silver nanoparticles were fabricated using electron beam lithography. The key parameters that govern the energy transport are determined for various interparticle spacings and particle chain lengths using far-field measurements of the collective plasmon modes. Spherical gold nanoparticles with a diameter of 50 nm and an interparticle spacing of 75 nm show an energy attenuation of 6 dB/30 nm. This loss can be reduced by one order of magnitude by a geometry change to spheroidal particles. Using the tip of a near-field optical microscope as a local excitation source and fluorescent nanospheres as detectors, experimental evidence for energy transport over a distance of 0.5 micron is presented for plasmon waveguides consisting of silver rods with a 3:1 aspect ratio.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Atwater, Harry Albert}, } @phdthesis{10.7907/Z9S6-8J66, author = {Holt, Jason Knowles}, title = {Hot-Wire Chemical Vapor Deposition of Silicon and Silicon Nitride for Photovoltaics: Experiments, Simulations, and Applications}, school = {California Institute of Technology}, year = {2003}, doi = {10.7907/Z9S6-8J66}, url = {https://resolver.caltech.edu/CaltechETD:etd-11262002-143938}, abstract = {Hot-wire chemical vapor deposition is a promising technique for deposition of thin amorphous, polycrystalline, and epitaxial silicon films for photovoltaic applications. Fundamental questions remain, however, about the gas-phase and surface-kinetic processes involved. To this end, the nature of the wire decomposition process has been studied in detail by use of mass spectrometry. Atomic silicon was the predominant radical formed for wire temperatures above 1500 K, and catalysis was evident for SiH3 production with the use of a new wire. Aged wires appear to produce radicals by a non-catalyzed route and chemical analysis of these wires reveal large quantities of silicon at the surface, consistent with the presence of a silicide layer. This study is the first of its kind to correlate radical desorption kinetics with filament aging for the hot-wire chemical vapor deposition technique.
Threshold ionization mass spectrometry revealed large quantities of the SiH2 radical, attributed to heterogeneous pyrolysis on the walls of the reactor. At dilute (1%) silane pressures of up to 2 Torr, a negligible amount of ions and silicon agglomerates (Si2, Si2H, Si2H6) were detected. Density functional theory calculations reveal an energetically favorable route for the reaction of Si and SiH4, producing Si2H2 and H2. The trace amounts of Si2H2 observed experimentally, however, may suggest that an intermediate spin state transition involved in this reaction is slow under the hot-wire conditions used. Monte Carlo simulations of the hot-wire reactor suggest SiH3 is the predominant growth species under conditions leading to amorphous and polycrystalline growth. The flux of atomic hydrogen, rather than the identity of the precursor, appears to be the more important factor in governing the amorphous-to-microcrystalline transition that occurs upon hydrogen-dilution. Two-dimensional Monte Carlo simulations were used to model a hot-wire reactor for the first time, showing that filament arrays can be used to improve film growth uniformity. Under conditions where agglomerate formation does not occur, continuum simulations predict a maximum growth rate of 10 nm/s for dilute (1%) silane conditions and a rate of 50 nm/s for pure silane.
Hot-wire chemical vapor deposition was used to deposit silicon nitride films with indices of refraction from 1.8-2.5 and hydrogen content from 9-18 atomic %. By tuning the SiH4/NH3 flow ratio, films in which the hydrogen was predominantly bound to N or Si could be produced, each of which reveal different hydrogen release kinetics. Platinum-diffused silicon samples, capped by a hydrogenated silicon nitride layer revealed, upon annealing at 700oC, platinum-hydrogen complexes with a bulk concentration of 1014 cm-3. This constitutes the first direct evidence for bulk silicon passivation by hydrogen release from a silicon nitride layer and hydrogen complex formation. Photovoltaic cells employing a hot-wire nitride layer were found to have comparable electrical properties to those using plasma nitride layers.
Finally, a method for in situ generation of SiH4 by atomic hydrogen etching was evaluated. Using a cooled crystalline silicon target in an H/H2 ambient produced negligible etching, while a cooled amorphous silicon film target was etched at a rate of up to 14 nm/min. In the latter case, net deposition at 0.6 nm/min onto a heated Ge(100) substrate resulted. A method for more efficient etching of crystalline silicon materials was proposed.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Atwater, Harry Albert and Goodwin, David G.}, } @phdthesis{10.7907/1WKJ-RZ66, author = {Ragan, Regina}, title = {Direct Energy Bandgap Group IV Alloys and Nanostructures}, school = {California Institute of Technology}, year = {2002}, doi = {10.7907/1WKJ-RZ66}, url = {https://resolver.caltech.edu/CaltechETD:etd-02142002-211940}, abstract = {Novel group IV nanostructures were fabricated and the optical properties of such nanostructures were investigated for monolithic integration of optically active materials with silicon. The SnxGe1-x alloy system was studied due to the previous demonstration of an indirect to direct energy bandgap transition for strain-relieved SnxGe1-x films on Si(001). In addition, quantum confined structures of Sn were fabricated and the optical properties were investigated. Due to the small electron effective mass of α-Sn, quantum confinement effects are expected at relatively large radii.
Coherently strained, epitaxial SnxGe1-x films on Ge(001) substrates were synthesized with film thickness exceeding 100 nm for the first time. The demonstration of dislocation-free SnxGe1-x films is a step toward the fabrication of silicon-based integrated infrared optoelectronic devices. The optical properties of coherently strained SnxGe1-x/Ge(001) alloys were investigated both theoretically and experimentally. Deformation potential theory calculations were performed to predict the effect of coherency strain on the extrema points of the conduction band and the valence band. The energy bandgap of SnxGe1-x/Ge(001) alloys was measured via Fourier transform infrared spectroscopy. Coherency strain did not change the SnxGe1-x energy bandgap when the strain axis was along [001] but deformation potential theory predicted the absence of an indirect to direct energy bandgap transition when the strain axis was along [111].
In addition to being the only group IV alloy exhibiting a direct energy bandgap, when grown beyond a critical thickness, SnxGe1-x/Ge(001) exhibits an interesting phenomenon during MBE growth. Sn segregates via surface diffusion to the crest of a surface undulation during growth and forms ordered Sn-enriched SnxGe1-x rods oriented along [001]. The SnxGe1-x alloy system was used as a model system to gain insight to the physical mechanisms governing self-assembly and ordering during molecular beam epitaxy.
Sn nanowires were fabricated in anodic alumina templates with lengths exceeding 1 μm and diameters on the order of 40 nm. Anodic alumina templates can be fabricated non-lithographically with ordered domains of hexagonally packed pores greater than 1 μm and pore densities on the order of 1011 cm-2. The achievement of single crystal Sn nanowires fabricated using pressure injection in porous alumina templates was demonstrated.
The fabrication of α-Sn quantum dots embedded in Ge was achieved by annealing 1 μm thick SnxGe1-x films at 750°C. The measured diameter of the quantum dots was 32 nm and a 10% size variation was observed. Quantum size effects were observed in α-Sn quantum dots. Optical transmittance measurements yield a value of 0.45 eV for the direct energy bandgap as a result of quantum confinement. A high degree of tunability of the bandgap energy with the quantum dot radius is expected for α-Sn. Thus quantum-confined structures of α-Sn are promising for optoelectronic device applications.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Atwater, Harry Albert}, } @phdthesis{10.7907/fgh9-ze28, author = {Chen, Claudine Minnie}, title = {Polycrystalline Silicon Thin Films for Photovoltaics}, school = {California Institute of Technology}, year = {2001}, doi = {10.7907/fgh9-ze28}, url = {https://resolver.caltech.edu/CaltechTHESIS:12162010-081444511}, abstract = {Selective nucleation and solid phase epitaxy offers a low temperature method to fabricate large grain, polycrystalline silicon on foreign substrates. Undoped and highly doped silicon films were nucleated with nickel or indium and annealed at 600°C. Indium nucleated crystallization proceeded by conventional solid phase epitaxy. Undoped silicon had grain sizes of 1-2 µm. With doping, although there was enhancement of the growth rate, the grain size did not increase, since the incubation time correspondingly decreased. The exception was the phosphorus-doped silicon that had a maximum grain size of 10 µm. In nickel-nucleated samples, the amorphous silicon layer fully crystallized before the onset of random nucleation, achieving grain sizes on order of tens of microns. Within each grain, however, were many low angle, sub-grain boundaries that came from the needle-like crystal growth. Epitaxy on these layers resulted in strained columnar crystals with dislocations.
Positron annihilation spectroscopy (PAS) was used to study vacancies in solid phase crystallized silicon in four doping cases: undoped, B-doped, P-doped, and P and B-doped. Oxygen-vacancy complexes were seen in all samples and phosphorus-vacancy complexes in the P- and P and B-doped samples. Progressive etchback of a subset of the samples was achieved, and a defect concentration on order of 10¹⁵ cm⁻³ was estimated for all samples.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Atwater, Harry Albert}, } @phdthesis{10.7907/tjep-v350, author = {Min, Kyu Sung}, title = {Synthesis and properties of light-emitting Si-based nanostructures}, school = {California Institute of Technology}, year = {2000}, doi = {10.7907/tjep-v350}, url = {https://resolver.caltech.edu/CaltechTHESIS:10122010-100418753}, abstract = {The concept of silicon-based optoelectronics has attracted much scientific and technological interests over the past decade. The vision of Si-based optoelectronics is based on integration of Si-based photonic components, in which light can be generated, waveguided, modulated, amplified, and detected, with the advanced Si electronics onto the same Si substrate to make monolithically integrated Si-based optoelectronic circuits. The main driving force for development of Si-based optical components comes from unsurpassed qualities of Si as the substrate material on which the electronic components rest: superior native oxide as well as excellent thermal, mechanical, and economic properties. Despite superior substrate properties, the field still remains a frontier at large. The main technological limitation comes from the lack of materials for efficient Si-based light sources such as Si-based lasers and light-emitting devices. Two novel Si-based nanostructures are studied for potential application as visible and infrared light sources: ion-beam synthesized Ge and Si nanocrystals in SiO_2 and coherently strained quantum well and quantum dots based on the Si-Sn system grown by molecular beam epitaxy. The study of Ge and Si nanocrystals is motivated by the prediction that quantum confinement of carriers leads to efficient luminescence despite the indirect nature of the energy gaps. Ge and Si nanocrystals in thermal SiO_2 films are synthesized via precipitation from a supersaturated solid solution of Ge and Si in SiO_2 made by Ge^+ and Si^+ ion implantation. The precipitation of nanocrystals occurs upon thermal annealing in vacuum. It is demonstrated that the SiO_2films containing Ge nanocrystals only exhibit defect-related luminescence and that the Ge nanocrystals do not exhibit luminescence from quantum-confined excitons due to the poor nanocrystal/SiO_2 interface. The visible luminescence from SiO_2 films containing Si nanocrystals, on the other hand, is unambiguously demonstrated to be originating from quantum-confined excitons in Si nanocrystals, based on systematic photoluminescence and photoluminescence decay rate measurements. In agreement with the predictions of the theory of quantum confinement, the peak energy of visible photoluminescence from Si nanocrystals can be continuously tuned throughout most of the visible spectrum by controlling the size distribution of the nanocrystals. The growth of nanostructures based on the Si-Sn system by molecular beam epitaxy is motivated by the fact that diamond cubic α-Sn is a zero band gap semiconductor and that band structure calculations predict a direct and tunable energy gap for Sn-rich Sn_(x)Si_(1-x) alloy system. However, the large lattice mismatch (19%) and severe segregation of Sn to the surface during growth prevent growth of Sn-rich Sn_(x)Si_(1-x) films by ordinary thermal molecular beam epitaxy. The growth of pseudomorphic Sn/Si and Sn_(x)Si_(1-x)/Si heterostructures is demonstrated via a modified molecular beam epitaxy technique employing temperature and growth rate modulations. The growth of pseudomorphic single quantum well structures as well as superlatttice structures is demonstrated. In addition, a novel route for synthesis of coherent Sn-rich Sn_(x)Si_(1-x) quantum dots in Si matrix is presented. Due to chemical instability of the Si-Sn mixture, Stranski-Krastonow growth of coherently strained Sn-rich Sn_(x)Si_(1-x) quantum dot structures using conventional molecular beam epitaxy techniques is very difficult. The novel technique involves phase separation of Sn-rich Sn_(x)Si_(1-x) quantum dots at elevated temperatures from an epitaxially stabilized homogeneous Sn_(x)Si_(1-x)/Si metastable solid solution grown by low temperature molecular beam epitaxy. The dots have been verified to be completely coherent with the surrounding Si matrix by high-resolution transmission electron microscopy.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Atwater, Harry Albert}, } @phdthesis{10.7907/n9df-zc75, author = {Camata, Renato Penha}, title = {Aerosol synthesis and characterization of silicon nanocrystals}, school = {California Institute of Technology}, year = {1998}, doi = {10.7907/n9df-zc75}, url = {https://resolver.caltech.edu/CaltechETD:etd-01182008-131457}, abstract = {NOTE: Text or symbols not renderable in plain ASCII are indicated by […]. Abstract is included in .pdf document. Synthesis and processing of optically active silicon nanocrystals are explored from an aerosol science perspective. Spark ablation, laser ablation and thermal evaporation in inert atmospheres are employed alternatively as vapor phase sources of nanocrystals. Nanocrystals generated employing these techniques comprise a highly polydisperse and morphologically diverse aerosol. After collection on a solid substrate, samples of these nanocrystals exhibit wide-band visible photoluminescence. A system for size classification of the initial polydisperse nanocrystal aerosol is demonstrated employing differential mobility analysis. Working at low nanocrystal concentrations (around […]) size control within 15% to 20% is achieved in the 2 to 10 nm size regime with a radial differential mobility analyzer at the expense, however, of low throughputs which make optical studies challenging. Seeking higher throughputs, the physics of aerosol size classification by this technique is investigated in detail by self-consistent numerical simulations of the particle transport inside the differential mobility analyzer. Our results lead to the identification of critical design characteristics required to maximize the analyzer performance from the viewpoint of semiconductor nanocrystal synthesis. With the guidance of these theoretical predictions, an optimized differential mobility analyzer design is suggested. This instrument has its parameters chosen to perform high resolution, high throughput size classification of nanocrystals in the 0.5 to 10 nm range. Optical characterization studies on polydisperse and size-classified silicon nanocrystal samples are performed. Results suggest that at least two mechanisms for light emission are at work in aerosol synthesized silicon nanocrystals. X-ray photoelectron measurements on size-classified silicon nanocrystals reveal that an oxide layer with thickness in excess of several nanometers forms on the silicon nanocrystals within a few minutes of air exposure. In order to preserve and control the surface chemistry of the nanocrystals, a system for anaerobic transfer of the size- classified silicon nanocrystals is designed and built. The system couples the nanocrystal synthesis experiment with the ultra high vacuum chamber of a surface analysis system via a load lock high vacuum chamber. Optical characterization capabilities are also installed. Preliminary results on nanocrystal synthesis and characterization using this in situ setup are presented and discussed.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Atwater, Harry Albert and Vahala, Kerry J. and Flagan, Richard C.}, } @phdthesis{10.7907/1vfn-ax53, author = {Taylor, Maggie Elizabeth}, title = {Pulsed laser deposition : energetic growth effects in group iv semiconductor materials}, school = {California Institute of Technology}, year = {1998}, doi = {10.7907/1vfn-ax53}, url = {https://resolver.caltech.edu/CaltechETD:etd-02062008-104635}, abstract = {NOTE: Text or symbols not renderable in plain ASCII are indicated by […]. Abstract is included in .pdf document. Pulsed laser deposition is characterized by a broad deposition energy distribution with a mean energy of a few eV or a few tens of eV. The research that comprises this thesis was motivated by a desire to characterize energetic growth effects in pulsed laser deposition of Group IV semiconductor materials in order to understand and manipulate fundamental growth kinetics. This research specifically focuses on energetic effects in growth morphology, in growth on hydrogen-terminated surfaces, and in alloy growth. In Chapter 1, pulsed laser deposition is introduced. In Chapter 2, simulated growth morphologies for Si growth by molecular beam deposition, sputter deposition, and pulsed laser deposition are compared. Feature atom displacement, an energetic effect, was found to significantly decrease roughness at low substrate temperatures. Pulsed roughening, a temporal effect, was found to slightly increase roughness at high substrate temperatures. In Chapter 3, crystalline Si grown by pulsed laser deposition on dihydride-terminated Si (001) surfaces and by molecular beam deposition and sputter deposition on clean Si (001) surfaces are compared. H transfer and Si subplantation, two energetic effects, were found to enable crystalline growth on dihydride-terminated Si (001) surfaces. In Chapter 4, crystalline […] grown by solid phase epitaxy and pulsed laser deposition are compared. Solid phase epitaxy was found to produce alloys with compositions no larger than approximately 0.05. Pulsed laser deposition was found to produce alloys with compositions as large as approximately 0.38. Composition was found to increase with ablation energy density. In Appendix A, actual and simulated growth morphologies for Si growth by molecular beam deposition and pulsed laser deposition are compared. In Appendix B, the simulation code is listed.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Atwater, Harry Albert}, } @phdthesis{10.7907/3693-9p48, author = {Yang, Chih Meng}, title = {Manipulation of Si and Ge Crystallization}, school = {California Institute of Technology}, year = {1997}, doi = {10.7907/3693-9p48}, url = {https://resolver.caltech.edu/CaltechETD:etd-01162008-112621}, abstract = {This thesis discusses methods for altering the crystallization kinetics of Si and Ge in order to obtain large-grained polycrystalline semiconductor thin films or size-selected semiconductor nanocrystals in silicon dioxide. Reduction of grain boundaries in polycrystalline semiconductor thin films is important for improving the performance of microelectronic devices because grain boundaries act as traps for charge carriers. Control of nanocrystal size and concentration in silicon dioxide is important in controlling the nanocrystal photoluminescence and electroluminescence characteristics. Results of modification of crystal nucleation and growth rate via ion beam irradiation, thermal annealing, metal-induced crystallization, and dopant enhanced solid phase epitaxy are presented.
Ion beam irradiation was used to induce amorphization of 1-50 nm Si crystals in amorphous Si. A size-dependent amorphization rate was calculated from the temporal evolution of the crystal size distribution under ion irradiation. A model for irradiation- induced, size-dependent crystal growth/amorphization is developed and it shows good quantitative agreement with the present experiment as well as other experiments.
Precipitation of 1-10 nm Ge nanocrystals and 1-2 nm Si nanocrystals in silicon dioxide was accomplished by ion implantation followed by thermal annealing. Nucleation of 1-2 nm Ge nanocrystals occurred during implantation, and annealing at temperatures higher than 600°C induced coarsening. Increasing the Ge implantation dose resulted in increased nanocrystal concentration but not size. In contrast, no Si nanocrystals were observed in as-implanted samples or samples annealed at less than 1000°C. Samples annealed at 1000°C for 40 min contained 1-2 nm Si nanocrystals.
Large grained polycrystalline Ge thin films, with controlled grain location and size, were synthesized at low temperatures. Grain sizes of 10-20 µm in 50-nm-thick amorphous Ge were obtained at temperatures less than 475°C, which represents a two-orders-of-magnitude improvement over previous efforts. Selective nucleation was achieved by deposition of an array of 5-micron-diameter metal islands on top of amorphous Ge and annealing at low temperatures. During subsequent anneal at higher temperatures crystals that selectively nucleated underneath the metal islands grew tens of microns before random nucleation impeded their growth. The crystal growth rate was enhanced by doping Ge with B or P, resulting in even larger crystal sizes.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Atwater, Harry Albert}, } @phdthesis{10.7907/tett-t906, author = {He, Gang}, title = {Novel group IV alloy semiconductor materials}, school = {California Institute of Technology}, year = {1997}, doi = {10.7907/tett-t906}, url = {https://resolver.caltech.edu/CaltechETD:etd-07222008-093116}, abstract = {NOTE: Text or symbols not renderable in plain ASCII are indicated by […]. Abstract is included in .pdf document. Novel group IV alloy semiconductor materials were investigated to explore their potential applications in silicon-based optoelectronic devices and high speed electronic devices. One system investigated is the ternary […] alloy. Epitaxial […] alloy films with tin and carbon concentrations of up to y=0.02 (x=0.5) were synthesized successfully on silicon substrates by molecular beam deposition followed by solid phase epitaxy. The effect of strain compensation from tin and carbon greatly reduced the epitaxial strain and produced dislocation-free heteroepitaxial films on silicon substrates which may enable high-speed silicon-based low-strain heterojunction devices. Another system investigated is the binary […] alloy. Tight-binding and pseudopotential calculations predicted that […] has a direct energy band gap that is continuously tunable in the mid and long wavelength infrared region […] for tin concentrations in the range of x=0.2 to x=0.6, which makes it an attractive material system for silicon-based integrated infrared optoelectronic devices. Epitaxial […] films were synthesized successfully on silicon substrates by conventional molecular beam epitaxy with tin concentrations of up to x=0.2, beyond which severe tin surface segregation caused a breakdown of epitaxy. To overcome the problem of surface segregation, ion-assisted molecular beam epitaxy was studied. Low energy, high flux ion irradiation of the sample surface during growth greatly reduced tin surface segregation and achieved tin concentrations up to x=0.34. An analytical model was developed to describe surface segregation during energetic beam epitaxial growth and was applied to ion-assisted molecular beam epitaxy growth of […]. Infrared absorption measurements of the […] samples showed that the decrease of […] energy band gap with increasing tin concentration was much faster than predicted by tight-binding and pseudopotential calculations. The measured absorption onset was as low as 0.25 eV for a tin concentration of x=0.15, and the measured absorption strength was comparable to the typical direct band gap infrared semiconductors such as InAs and InSb. The results of the absorption measurements suggest that full access to the tunable […] energy band gap from mid infrared to far infrared may be obtained with a maximum tin concentration of about x=0.25 instead of x=0.6 as predicted by tight-binding and pseudopotential calculations.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Atwater, Harry Albert}, } @phdthesis{10.7907/59ar-ek67, author = {Shcheglov, Kirill Vadim}, title = {Synthesis, optical and electronic properties of group IV semiconductor nanocrystals}, school = {California Institute of Technology}, year = {1997}, doi = {10.7907/59ar-ek67}, url = {https://resolver.caltech.edu/CaltechETD:etd-01172008-081522}, abstract = {NOTE: Text or symbols not renderable in plain ASCII are indicated by […]. Abstract is included in .pdf document. Every operating control system must deal with constraints. On the one hand, the range and rate of change of the input or manipulated variable is limited by the physical nature of the actuator (saturation limits). On the other hand, process state variables or outputs (pressures, temperatures, voltages) may not be allowed to exceed certain bounds arising from equipment limitation, safety considerations, or environmental regulations. A rich theory exists for designing controllers - both linear ([…],LQG, LTR, pole-placement) and nonlinear (nonlinear […], control, feedback linearization, sliding mode control, gain scheduling). However, none of these popular and fashionable controller design techniques account for the presence of input or output constraints. Although occasionally these constraints may be neglected, in general, they lead to design and operating problems unless they are accounted for properly. In traditional control practice, overrides or mode selection schemes are used to deal with output constraints: they switch between a “bank” of controllers, each of which is designed to achieve a specific objective. In both cases (saturation limit and mode selection), a control input nonlinearity is introduced into the operating system. Despite its significance, the study of the constrained control problem has received far less attention than the traditional unconstrained (linear and nonlinear) control theory. With few exceptions, most of the controller design techniques for constrained systems are by-and-large ad-hoc, with very little guarantees of stability, performance and robustness to plant model uncertainty. The objective of this thesis is to take a broad approach towards the constrained control problem. One part of the thesis is devoted to the development of a systematic and unifying theory for studying the so-called Anti-Windup Bumpless Transfer (AWBT) problem. The other part aims towards the development of a general novel approach for the synthesis of a robust model predictive control (MPC) algorithm. NOTE: Text or symbols not renderable in plain ASCII are indicated by […]. Abstract is included in .pdf document. Group IV semiconductor (Si, Ge and Sn) nanocrystals were synthesized in dielectric matrixes by ion implantation of the respective species into the matrix to form a supersaturated solid solution and subsequent precipitation by thermal annealing. The resulting structure was characterized by Transmission Electron Microscopy and Raman spectroscopy. It was found that nanocrystals of these materials can be effectively synthesized with diameters in the nanometer range. Ge nanocrystals in SiO[…] were extensively characterized, particle size distributions were counted from TEM results and were used to compare experimental photoluminescence spectra with theoretical predictions. Unusual nanostructures were formed in samples co-implanted with Ge and Sn and annealed at 600°C. Raman spectroscopy indicated a possibility of significant alloying of Ge and Sn in these nanostructures. Optical properties of Si nanocrystals in silicon dioxide were investigated by photoluminescence spectroscopy as well. It was found that while Ge nanocrystal system luminescence is mostly due to defects in the matrix produced by ion implantation, Si nanocrystal sample luminescence is due to the Si nanocrystals themselves. The luminescence is above the bulk Si bandgap and supports the quantum confined excitonic luminescence theory. Light emitting devices were fabricated using both systems. Electroluminescence was observed for both Si and Ge, albeit with rather low efficiency, in the 10[…] - 10[…] range. Electroluminescence from Si nanocrystal containing devices was spectrally similar to photoluminescence from that system, with a band about 800 nm, consistent with electronic excitation of radiative transitions in Si nanocrystals. Cubic nonlinearities were measured for both Ge and Si nanocrystals and found to be 10[…] - 10[…] esu range. Finally, an interesting interferometric arrangement which has a potential to be useful for investigating nanoscale structures was theoretically described.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Atwater, Harry Albert}, } @phdthesis{10.7907/nt1r-kp46, author = {Brain, Ruth Amy}, title = {Capillary-Driven Reflow of Thin Cu Films with Submicron, High Aspect Ratio Features}, school = {California Institute of Technology}, year = {1996}, doi = {10.7907/nt1r-kp46}, url = {https://resolver.caltech.edu/CaltechTHESIS:11052019-121746850}, abstract = {Conventional sputtering techniques are no longer sufficient for the fabrication of interconnects as trench widths enter the submicron regime and aspect ratios become greater them 1:1. The goal of this thesis is to investigate Cu as a potential interconnect metal for use in integrated circuit technology. Since sputtering is well established and widely used in the integrated circuit industry, we have used current sputtering technology as our deposition technique of choice. An alternative approach to modify the nonconformal deposition profiles obtained by sputtering is to reflow (planarize by capillary-driven surface diffusion) the metal film during a post-deposition anneal. In particular, reflow is performed for thin Cu films deposited on refractory metal barrier layers (Mo, Ta, and W) at temperatures ≤ 500°C.
With a goal of developing and understanding a post-deposition reflow process for Cu, we have studied the following topics. Chapter 1 introduces relevant current concepts in ultra-large scale integration (ULSI) for interconnect technology to motivate the approaches described in this thesis. Chapter 2 investigates several techniques to improve the initial Cu coverage obtained from a magnetron sputtering source. This was found to be necessary since thin Cu films agglomerate on many underlayers, and this constrains the initial Cu thickness requirements for successful reflow. Chapter 3 describes an investigation of the atomic transport mechanism during reflow of Cu films, to understand the kinetics of reflow and measure the appropriate kinetic constants. Extensive transmission electron microscope (TEM) work was done to examine reflowed profiles and to relate the extent of reflow to the morphology, texture, grain size, and orientation of the Cu films. Hot-stage TEM experiments were performed to observe dynamically the reflow of a very low aspect ratio film. Chapter 4 develops a finite-element model to study surface diffusion mediated reflow in high aspect ratio trenches. We have considered (i) reflow of typical continuum, as-deposited profiles from a magnetron sputtering source, (ii) reflow of continuum profiles including an anisotropic surface energy, and (iii) reflow with the inclusion of grain boundaries. We also discuss some limitations of a post-deposition reflow process, and we make recommendations to facilitate the ability to reflow Cu in high aspect ratio trenches. Chapter 5 examines a non-infrared annealing technique to reflow Cu films. In particular, we have examined the possibility of annealing Cu films in a single-mode microwave cavity, which can be advantageous because it is possible to thermally isolate the substrate. Chapter 6 provides a summary of the work in this thesis and suggests some possibilities for future work.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Atwater, Harry Albert}, } @phdthesis{10.7907/1vkn-xh33, author = {Murty, M. V. Ramana}, title = {Ion-surface interactions and limits to silicon epitaxy at low temperatures}, school = {California Institute of Technology}, year = {1995}, doi = {10.7907/1vkn-xh33}, url = {https://resolver.caltech.edu/CaltechETD:etd-10262007-111208}, abstract = {NOTE: Text or symbols not renderable in plain ASCII are indicated by […]. Abstract is included in .pdf document. Low temperature (T […] 400°C) deposition of Si on Si(001) proceeds epitaxially up to a finite thickness followed by a crystal-state-amorphous-state transition. An atomistic model, the twin-boundary/facet (TBF) mechanism, has been proposed for this transition. The increase in surface roughness during film growth has been directly tied to the breakdown of epitaxy. The mechanism involves the nucleation of a twin boundary on a {111} facet (produced by roughening). When the twinned region meets a different part of the perfect crystal, it inevitably leads to the formation of five- and seven-member rings. These act as nucleation sites for amorphous silicon. Adsorbates such as carbon and oxygen can dramatically increase the surface roughness even at small coverages ([…] 0.01 ML). They thus play an indirect role by accelerating the surface roughening rate. Films with improved crystalline quality were deposited by ion beam-assisted molecular beam epitaxy. Atomic force microscopy revealed that the main effect of low energy […] ion irradiation was surface smoothing. Molecular dynamics simulations suggest that epitaxy on hydrogen-terminated silicon surfaces (at high hydrogen coverage) proceeds by subplantation of the incident Si atom and segregation of […] units. The remarkable success of sputter deposition in growing epitaxial films on a dihydride-terminated Si(001) surface is explained by the very rapid rise in the subplantation probability with the incident Si atom energy. An empirical interatomic potential has been developed to describe Si-H interactions. This can be used, with caution, for classical molecular dynamics investigations of hydrogen-terminated silicon surfaces, chemical vapor deposition of silicon and hydrogenated amorphous silicon. A technique for low temperature Si(001)-2x1 substrate preparation was developed to complement the various low temperature processes that are being developed for device fabrication. This was achieved by low energy noble gas ion ([…] or […]) irradiation of a nominally dihydride-terminated Si(001)-1x1 surface. Reconstructed Si(001)-2x1 surfaces were prepared at temperatures as low as 100°C. Silicon films deposited on such surfaces were epitaxial.}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Atwater, Harry Albert}, } @phdthesis{10.7907/TW9D-5657, author = {Shin, Jung Hoon}, title = {Defects in amorphous silicon : dynamics and role on crystallization}, school = {California Institute of Technology}, year = {1994}, doi = {10.7907/TW9D-5657}, url = {https://resolver.caltech.edu/CaltechETD:etd-12052007-131414}, abstract = {Defects play a crucial role in determining the properties of many materials of scientific and technological interest. With ion irradiation, it is possible to controllably inject defects, and thus carefully study the dynamics of defect creation and annihilation, as well as the effects such defect injection has on materials properties and phase transformations. Amorphous silicon is a model system for the study of amorphous solids characterized as continuous random networks. In hydrogenated form, it is an important material for semiconductor devices such as solar cells and thin film transistors. It is the aim of this thesis to elucidate the dynamics of defects in an amorphous silicon matrix, and the role such defects can play on crystallization of amorphous silicon. In the first chapter, the concept of a continuous random network that characterizes amorphous silicon is presented as an introduction to amorphous silicon. Structural relaxation, or annihilation of non-equilibrium defects in an amorphous matrix, is introduced. Also developed are the concept of the activation energy spectrum theory for structural relaxation of amorphous solids and the density of relaxation states. In the second chapter, the density of relaxation states for the structural relaxation of amorphous silicon is measured by measuring changes in electrical conductivity, using ion irradiation and thermal anneal to create and annihilate defects, respectively. A new quantitative model for defect creation and annihilation, termed the generalized activation energy spectrum theory, is developed in Chapter 3, and is found to be superior to previous models in describing defect dynamics in amorphous silicon. In Chapter 4, the effect of irradiation on the crystallization of amorphous silicon is investigated. It is found that irradiation affects crystallization even when the growth kinetics of crystal grains is unaffected, and that defects injected into amorphous matrix by irradiation probably play a role in affecting the thermodynamic quantities that control nucleation. The role of defect injection in affecting the thermodynamic quantities is investigated in Chapter 5, where we estimate the change in the free energy of amorphous silicon under the irradiation conditions of Chapter 4, using the generalized activation energy theory of Chapter 3. The experimental data and its interpretation are consistent with predictions of generalized activation energy spectrum theory.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Atwater, Harry Albert}, } @phdthesis{10.7907/X1XA-W810, author = {Hashim, Imran}, title = {Microstructural and magnetic properties of polycrystalline and epitaxial permalloy (Ni80Fe20) multilayered thin films}, school = {California Institute of Technology}, year = {1994}, doi = {10.7907/X1XA-W810}, url = {https://resolver.caltech.edu/CaltechETD:etd-11302007-112544}, abstract = {NOTE: Text or symbols not renderable in plain ASCII are indicated by […]. Abstract is included in .pdf document. Permalloy ([…]) thin films are of great scientific and technological interest because of their unique soft magnetic properties, and applications to magnetic recording. Chapter 1 provides an introduction to magnetic and magnetotransport properties of […] thin films, and how the film microstructure affects these properties. Chapter 2 discusses the instrumentation used for thin film fabrication, and for magnetic and structural characterization. Further details of instrumentation are discussed in Appendix A. Typically, the […] films for magnetoresistive applications are capped with a refractory metal thin film such as Ta to prevent its oxidation and corrosion. We investigated the interdiffusion kinetics of polycrystalline Ta/[…] thin films and found that for 400 […] T […] 600’C, there was significant grain-boundary interdiffusion which drastically affected soft magnetic properties of […]. In Chapter 3, we present details of the microstructural evolution of these multilayers and the subsequent effects on their magnetic properties. An alternate method for reducing grain-boundary scattering would be to fabricate grain-boundary free epitaxial […] films. The epitaxy of […] on MgO, NaCl and Cu had been demonstrated by investigators as early as the 60s. However, none of these substrates are available with as good atomic flatness as Si wafers. Following reports of epitaxial growth of Cu on Si[1], we proposed using it as a seed layer for growing […] epitaxially on Si. However, there were conflicting reports of Cu epitaxy on Si, as some investigators claimed that Cu epitaxy on Si in UHV was not possible[2]. We were able to resolve some of these controversies (see Chapter 4 for details) and thus fabricate epitaxial Ni80Fe20 films on Cu/Si. Chapter 5 examines the effect of the lattice mismatch between Cu and […] and the subsequent strain, on the soft magnetic properties of […]. To explain these experimentally observed magnetic properties, a micromagnetic model was developed taking into account domain wall interaction with misfit dislocations and film surface roughness especially during the initial stages of epitaxial growth. Finally, epitaxial growth of […]/Cu on Si suggests the possibility of growing grain-boundary free atomically sharp […]/Cu multilayers which exhibit recently-discovered “giant” magnetoresistance. [1] C.A. Chang, Appl. Phys. Lett. 55, 2754 (1989). [2] B.P. Tonner, J. Zhang, X. Chen, Z-L. Han, G.R. Harp, and D.K. Saldin, J. Vac. Sci. Technol. B10, 2082 (1992).}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Atwater, Harry Albert}, } @phdthesis{10.7907/886t-t311, author = {Tsai, Cho-Jen}, title = {Low energy ion-surface interaction and epitaxial growth in the SiGe system}, school = {California Institute of Technology}, year = {1992}, doi = {10.7907/886t-t311}, url = {https://resolver.caltech.edu/CaltechETD:etd-08152007-092600}, abstract = {The structure of a growing epitaxial film is controlled by the relative rate of different surface processes. Low energy ion beams (50-500 eV) can be used to provide energy to adatoms on the surface and atoms in the near-surface region of a growing film. Thus, a low energy ion beam can be employed as a tool to modify surface kinetics. The study of the effects of the beam-induced defects on the epitaxial growth can also provide valuable insights into intrinsic growth processes by altering the relative rate of various surface kinetics. The work contained in this thesis is focused on the role of defects produced by low energy ion bombardment in modification of epitaxial growth of Si, Ge, and SiGe alloy films. In the first two chapters, theoretical and technical aspects of X-ray rocking curve diffractometry which is one of the principal analytic methods used in this thesis is discussed. The high-resolution X-ray diffractometer built at Caltech and the dynamical theory of X-ray diffraction are briefly discussed. The last four chapters are focused on the manner in which low energy ion beam bombardment affect the structural properties and growth kinetics of epitaxial films. One of the most important factors determining the changes that occur in ion-assisted epitaxy is the ion energy used to stimulate the growth processes, which determines the relative number of surface displacements and bulk displacements. The effect of bulk displacement defects on an epitaxial film structure is discussed in Chapter 3. In Chapter 4, a simple moving boundary diffusion model in conjunction with thermal spike activated kinetics is presented to describe the bulk defect incorporation process. The moving boundary diffusion model has also been used to describe the adatom concentration on the vicinal Si surface and the low energy dopant incorporation processes. Surface displacements produced by low energy ion bombardment has a dramatic effect on the growth mode of epitaxial Ge films on Si(100); this is the main theme of Chapter 5. In the initial stages of Ge growth on Si, a layer-by-layer growth followed by island growth was observed in conventional (thermal) molecular beam epitaxial growth. Island formation is inhibited by low energy ion bombardment during epitaxial growth which can prolong the layer-by-layer growth mode to greater thicknesses than for thermal growth. In Chapter 6, the effect of the low energy ion bombardment on the misfit accommodation of the lattice mismatched system is discussed. The point defects injected by the low energy ion bombardment impede dislocation motion in the growing epitaxial film and cause misfit strain to be accommodated by the threading dislocations which greatly enhances the misorientation between a film and its substrate.}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Atwater, Harry Albert}, } @phdthesis{10.7907/cmqv-p871, author = {Sercel, Peter C.}, title = {Semiconductor structures in the quantum size regime}, school = {California Institute of Technology}, year = {1992}, doi = {10.7907/cmqv-p871}, url = {https://resolver.caltech.edu/CaltechETD:etd-08202007-132916}, abstract = {NOTE: Text or symbols not renderable in plain ASCII are indicated by […]. Abstract is included in .pdf document. The physics of quantum wires and quantum dots is investigated theoretically. We develop an analytical formalism for determining the energy eigenstates and bandstructure of spherical quantum dots and cylindrical quantum wires. The technique is based upon a reformulation of second order […] theory in a basis of eigenstates of total angular momentum. We are led by analysis of quantum wires and dots based upon the InAs-GaSb material system to propose a novel class of self-doping nanostructures for carrier transport experiments and possible future application. The polarization dependence of linear optical absorption and gain spectra in cylindrical quantum wires is calculated. Applicability of the results derived for cylindrical quantum wires to the case of wires with lower symmetry is addressed using symmetry group theory. Fabrication of quantum wires and dots is attempted by several techniques. A method for fabricating nanometer-scale GaAs wire structures from quantum well material by selective impurity induced disordering is demonstrated. The technique produces lateral bandgap modifications on a 100 nm scale, as verified by cathodoluminescence imaging and spectroscopy. We demonstrate vapor phase synthesis of nanometer-scale III-V semiconductor clusters in the 5 to 20 nm diameter regime. Clusters form by homogeneous nucleation from a non-equilibrium vapor created by the explosive vaporization of a bulk semiconductor filament in an inert atmosphere. The clusters produced have zincblende crystal structure and are faceted. The optical absorption spectra of the clusters are suggestive of quantum confinement effects. A second method of cluster formation utilizes homogeneous nucleation from volatile metal-organic and hydride precursors to produce nanometer-scale, zincblende GaAs clusters.}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Vahala, Kerry J.}, }