Gallium arsenide and other III-V materials are well known for their excellent optical and electronic properties and have led to the development of high-performance optoelectronics. Several combinations of III-V semiconductors are now being considered as potentially attractive alternatives to silicon for these applications. However, further development requires fundamental understanding of processes that govern light-matter interactions. Specifically, surface strain and ultrafast dynamics are of great interest to the optoelectronic industry. The research of this dissertation represents an initial exploration of the factors influencing nonlinear optical responses on semiconductor surfaces. The results of this research have the potential to inform the field of nonlinear optics about which lattice behaviors are most likely to contribute to static and transient second harmonic generation (SHG). This information allows for future work to focus on the connection between SHG, dipole contributions, and interatomic potentials in semiconductors under different conditions. This research also provides information about whether strain, resonances, and subpicosecond lattice behaviors can be fit with a simple analytical solution. The results of this research reveal that an analytical fit of polarization-resolved SHG is sensitive to interatomic potential and dipole variations in all three dimensions simultaneously.

The interaction of electrons and phonons with the properties of semiconducting crystals continues to be a fascinating and highly fruitful field of study. This dissertation addresses two research problems under the general heading of electron and phonon effects on the optical properties of indirect gap semiconductors. The first problem concerns nonlinear (multi-photon) absorption in germanium crystals, a topic of interest for the telecommunications industry as well as to the basic scientist. Using a combination of infrared transmittance experiments and numerical analysis, the two- and three-photon absorption coefficients β and γ for germanium have been evaluated over the range of wavelengths from 2.8 µm to 5.2 μm. The ratios of the coefficients across the direct/indirect gap transitions and between the two-and three-photon cases, which are less susceptible to experimental uncertainties than the absolute coefficients, have also been determined. Comparison with theoretical studies shows excellent agreement. The second problem addresses the optical characteristics of ion-bombarded diamond crystals, which is a swiftly developing field due to diamonds current status as the material of choice for hosting photonic and quantum information devices. The ultrafast optical technique known as coherent acoustic phonon interferometry has been applied to He ion irradiated diamond crystals for the purpose of determining the optical modification induced by the implantation damage. The experimental results provide information about the variation at in the complex refractive indices of the implanted specimens as well as the variation in the photoelastic tensor. A simple phenomenological model quantitatively describing the damage-induced optical modification has been developed which accurately predicts the experimental observations, and may prove to be a useful tool for quantum device design.

INTERDISCIPLINARY MATERIALS SCIENCE PHOTOSYSTEM I FROM HIGHER PLANTS ENHANCE ELECTRODE PERFORMANCE DARLENE GUNTHER Thesis under the direction of Professor G. Kane Jennings Photosystem I (PSI) is a supramolecular protein complex found in the thylakoid membranes of higher plants, algae, and cyanobacteria. Recently, researchers from across the world have been interested in extracting PSI from its source and integrating it with electrodes to investigate biologically inspired solar energy conversion. There are two main investigations involving PSI in this thesis. First, adsorbing PSI monolayers onto atomically thin graphene creates a transparent, photoactive electrode that is less than 10 nm thick. Experiments utilizing PSI extracted from spinach and deposited onto graphene results in an enhanced photocurrent density over bare graphene electrodes. Furthermore, choice of opaque mediator with higher concentrations combined with highly transparent graphene produced larger photocurrents than the use of transparent mediator counterparts. Second, PSI is extracted from Pueraria lobata (kudzu) and deposited onto silicon electrodes for the first time. This study investigates the potential for transitioning from traditional food sources, such as spinach, to non-traditional food sources, such as kudzu, as a renewable resource for PSI. The kudzu-PSI-modified-silicon electrodes double the electrical output over bare silicon electrodes. Approved: G. Kane Jennings

Integrated silicon (Si) photonics represents one of the key technologies for developing compact high speed optical systems for computing and telecommunications. In such systems, electric buses are replaced with integrated Si waveguides which transport light across the chip. In order to implement high density networks, it is inevitable that waveguides will need to be crossed to transport information across orthogonal directions. However, when two or more waveguides cross, light is scattered due to the abrupt change in the modal index resulting in losses of up to 40 percent. This loss occurs to both the environment as well as the overlapped waveguide, causing cross-talk into the other channel resulting in false signals. Current Si based waveguide crosses require either a large footprint or are limited in the number of waveguides that can be crossed simultaneously. In this work, we develop integrated gradient index elements based on the Maxwell Fisheye (MFE) to provide low-loss and massively parallel optical waveguide crossings. To realize a crossing, waveguides which are modal index matched to the MFE are coupled across the lens wherein the output of one waveguide is imaged to the input of its partner on the opposite side. Based on this methodology, we present full-wave modeling of the device demonstrating a 0.1 dB loss (97.7% transmission) per crossing for an overall waveguide cross footprint of 28.26 square microns, among the most efficient designs to date. We also propose how this device can be realized using smoothly tapered Si waveguides to provide the required 2D gradient refractive index profile.

As the limits of silicon based transistors are approached, carbon nano-electronics represents a promising alternative to traditional semiconducting transistors. Silicon-dioxide hinders the electron transport through carbon transistors as well as obscuring information about their transport. Diamond is a promising new dielectric for use with carbon transistors which may remedy these problems. This project is concerned with the development of diamond thin films for use as dielectric substrates in FETs. Diamond films were grown, trying to minimize thickness and conductivity. Diamond was chosen as a material due to its wide band gap, as well as its radiation hardness. This would allow inspection of the effects of radiation on the transistor elements in our FETs. These films were characterized using a number of techniques to assess their viability as dielectric substrates. Once the viability was established, the surface was modified to create an optimal interface for the transistors that would be transferred to the surface. Finally, transfer of carbon based transistors was attempted.

White light-emitting diodes (LEDs) are the lighting of the future due to their potential energy savings and the proven success with monochromatic LEDs. However, white LEDs require an expensive fabrication process involving the incorporation of many different monochromatic semiconductors into a single LED; this is often referred to as color mixing. In 2005, a new class of semiconductors was discovered which is called ultrasmall CdSe quantum dots. This new class of material emits perfect, white light, so the integration of ultrasmall CdSe quantum dots into LEDs would result in the eradication of all the costs associated with color mixing. Since its discovery, the brightness of ultrasmall CdSe nanocrystals has increased over time from a ~2% quantum yield to ~31 with a brightening method which has been termed the formic acid treatment. This thesis pertains to the improvement and LED amalgamation of these brighter ultrasmall CdSe quantum dots. In particular, many experiments were done with the goal of improving the formic acid treatment, and in the process, much was discovered about the mechanics of the brightening method. The last chapter of the thesis concludes about the results and gives possible future directions including characterization methods and another possible brightening method.

Single-event burnout in power diodes is studied using coupled electro-thermal simulations. A two-dimensional rectangular diode structure is designed and steady-state electrical characteristics are simulated. Single-event effects are simulated using an ion-strike modeled after data reported in the literature and transient characteristics are simulated following the strike. Ion-strike simulations are performed under isothermal conditions to study temperature-dependent device properties. Coupled electro-thermal simulations are performed to study a thermal feedback loop as a possible failure mechanism. Single-event burnout is not observed for the power diode and a thermal feedback loop is not identified. Highly localized temperature rise near pn junction is observed along the strike direction. Impact-ionization induced charge is responsible for local power generation that leads to this temperature rise. Further, two-dimensional axi-symmetric or 3D simulations could provide more incite into single-event burnout of power diodes.

FePt alloys with the CuAuI L10-ordered structure are important magnetic materials since their large uniaxial magnetocrystalline anisotropy can overcome the thermal stability problem of high-density magnetic recording media. In this study, transmission electron microscopy (TEM) and energy-filtered TEM (EFTEM) are employed to characterize the microstructure of a series of FePt/MgO multilayered thin films with different FePt layer thicknesses. The as-deposited FePt layers transform into the L10-ordered phase after annealing at 700oC for 30 minutes. The laminated FePt/MgO structure effectively preserves a strong L10 [001] texture normal to the thin film plane during annealing. The annealed films exhibit large coercivities from 6.5 kOe to 11.5 kOe due to the formation of the L10-ordered phase. A reduction in coercivity with increasing FePt layer thickness is attributed to the differences in the microstructure after annealing. For the films with thinner FePt layers, annealing produces discontinuous FePt layers that reduce exchange coupling and increase coercivity. As the FePt layer thickness increases, the magnetic layers become more continuous after annealing and thereby the exchange coupled layers have lower coercivity.

Semi-classical transport models based on Boltzmann and Fermi-Dirac statistics have been very effective in identifying the pertinent physical parameters responsible for thermoelectric performance in bulk materials. Reliance on Boltzmann-based models has produced a culture of smaller is better research, where the reduction in size is expected to produce limitless increase in performance. Experimental observations especially in the case of thermoelectric performance of nanoscale devices have not exhibited this behavior. The semi-classical Boltzmann models are based on the relaxation-time approximation and cannot model strong non-equilibrium transport. In addition, wave effects in these models are included through correction terms that cannot suitably capture their influence on transport. <p> A coupled quantum-scattering model to study thermoelectric performance of nanoscale structures is proposed through the nonequilibrium Greens function method. The model includes all the pertinent physics of the wave nature of electrons while coupling electron-phonon scattering effects. The NEGF method is used to study the performance of silicon nano-films and nanowires as well as strained quantum well Si/Ge/Si superlattices as a function of doping, effective mass and in the case of superlattices, substrate strain and superlattice geometry. Results suggest that the power factor of nanostructured materials is dominated by the electrical conductivity which in turn is strongly influenced by quantum confinement effects and electron-phonon scattering effects. No significant improvement in the Seebeck coefficient is observed due to the decrease in dimensionality of the structure. <p> The NEGF method can be used as a tool to design structures with optimized values of doping, effective mass, substrate strain and superlattice geometry taking into consideration the effects of electron confinement and scattering. The model developed in this research can be used as a framework to guide further studies on performance of highly scaled thermoelectric devices in order to obtain optimal value of ZT. This effort represents the first reported use of the nonequilibrium Greens function method to predict thermoelectric performance.

This dissertation presents the radiation-induced charge trapping studies of upcoming material systems of Si and SiC for future low power and high power technologies. HfO2/Si with metal gates has already been announced as the material system that will power the future technology scaling for low power devices. SiO2/SiC based devices are possible candidates for the upcoming high power device technologies. This dissertation provides significant insights into the charge trapping characteristics in these devices exposed to high-energy ionizing radiation. Charge trapping is studied as a function of dose, processing and gate oxide fields with extensive materials characterization performed before irradiations. The results provide additional information for establishing reliable design rules for future MOS devices intended for both, high and low operating voltage when exposed to a radiation environment.