The optical properties of InAs/GaSb heterostructures under applied magnetic fields are studied in experimental and theoretical detail. The InAs/GaSb system is a type-II "crossed-gap" system, where the valence band edge of GaSb lies higher in energy than the conduction band edge of InAs. This leads to a region of energy above the InAs conduction band where conduction and hole states mix. Thin-layer superlattices remain semiconducting due to confinement effects, but thick-layer superlattices experience charge transfer which leads to intrinsic carrier densities approaching 10¹² cm^-2 per layer. Existing multi-band modeling techniques based on the <strong>k·p</strong> formalism are discussed, and a method of solving superlattice band structure (the "momentum-matrix" technique) is presented. The quantizing effects of the superlattice layers and applied magnetic fields are investigated, and the selection rules for optical transitions are derived. Standard cyclotron resonance (CR) is used to study effective masses in InAs/GaSb structures. The heavy hole mass is found to be strongly orientation-dependent, with a mass in the [111] orientation reduced 25% from the [001] mass. The electron mass is found to be roughly isotropic with respect to growth orientation, but shows variation with the InAs width due to quantum confinement effects. CR of InAs/GaSb heterojunctions display hitherto unexplained oscillations in linewidth, intensity, and effective mass. A model is proposed which explains the oscillations, based on the intrinsic nature of the InAs/GaSb system. CR is performed on an InAs/GaSb heterojunction using a free-electron laser, where due to the high intensities (on the order of MW/cm²) the absorption process saturates. This saturation allows for a determination of non-radiative relaxation lifetimes, and through the energy dependence of these lifetimes the magnetophonon effect is observed, allowing a direct measurement of LO-phonon-assisted energy relaxation rates. Coupling is introduced into the standard CR experiment, either by tilting the sample with respect to the magnetic field, or by applying a metal grating to the surface. These coupled CR experiments have striking qualitative results which allow for determination of subband separation energies and coupling matrix elements. Photoconductivity experiments are performed on thin-layer (semiconducting) superlattices, showing optical response at far-infrared wavelengths (5-20 μm). The results are compared with <strong>k·p</strong> calculations. One sample is processed for vertical transport, in which conduction occurs perpendicular to the superlattice layers. Strong optical response from this sample indicates the viability of InAs/GaSb-based far-infrared detectors. The momentum-matrix technique is used to predict optimum parameters for semiconducting superlattices with band gaps in the far-infrared. Semimetallic structures are studied via a multi-band self-consistent model, with results corroborating with and extending previous work. Intrinsic structures under applied magnetic field are modeled theoretically for the first time.

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Thesis (M.S.)--Oregon State University, 2009. / Printout. Includes bibliographical references (leaves 145-153). Also available on the World Wide Web.

Thesis (M.Mat.S.E.)--University of Delaware, 2007. / Principal faculty advisor: Robert L. Opila, Dept. of Materials Science & Engineering. Includes bibliographical references.

Thesis (Ph. D.)--University of Texas at Austin, 2007. / Vita. Includes bibliographical references.

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The development of state-of-the-art AlGaN/GaN high electron mobility transistors (HEMTs) has shown much promise for advancing future RF and microwave communication systems. These revolutionary devices demonstrate great potential and superior performance and many commercial companies have demonstrated excellent reliability results based on multiple temperature accelerated stress testing. However, a physical understanding of the various reliability limiting mechanisms is lacking and the role and relative contribution of the various intrinsic material factors, such as physical stress and strain has not been clearly explained in the literature. Part of issues that impact device reliability are the mechanical stresses induced in the devices as well as the self-heating that also limit device performance. Thus, quantification of stress and temperature in AlGaN/GaN HEMTs is of great importance. To address some of the needs for metrology to quantify stress in AlGaN/GaN HEMTs, micro-Raman spectroscopy and micro-photoluminescence (micro-PL) were utilized to quantify the residual stress in these devices. Through the use of micro-Raman and micro-PL optical characterization methods, mapping of the vertical and lateral stress distributions in the device channels was performed. Results show that stress can be influenced by the substrate material as well as patterned structures including metal electrodes and passivation layers. Previously developed and reported micro-Raman thermometry methods require an extensive calibration process for each device investigated. To improve the implementation of micro-Raman thermometry, a method was developed which offers both experimental simplicity and high accuracy in temperature results utilizing a universal calibration method that can be applied to a broad range of GaN based devices. This eliminates the need for performing calibration on different devices. By utilizing this technique, it was revealed that under identical power dissipation levels, the bias conditions (combination of Vgs and Vds) alter the heat generation profile across the conductive channel and thus influence the degree of device peak temperature. The role of stress in the degradation of AlGaN/GaN HEMTs was also explored. A combined analysis using micro-Raman spectroscopy, coupled electro-thermo-mechanical simulation, and electrical step stress tests was conducted to investigate the link between performance degradation and the evolution of total stress in devices. It was found that in addition to stresses arising from the inverse piezoelectric effect, the substrate induced residual stress and the operational themo-elastic stress in the AlGaN layer play a major role in determining the onset of mechanically driven device degradation. Overall, these experiments were the first to suggest that a critical level of stress may exist at which point device degradation will start to occur. The optical characterization methods developed in this study show the ability to reveal unprecedented relationships between temperature/stress and device performance/reliability. They can be used as effective tools for facilitating improvement of the reliability of future AlGaN/GaN HEMTs.

Issues of major relevance to the n-type conductivity of Al0.77Ga0.23N associated with Si and O incorporation, their shallow donor or deep donor level behavior, and carrier compensation are elucidated by allying (i) study of Si and O incorporation kinetics at high process temperature and low growth rate, and (ii) electron paramagnetic resonance measurements. The Al0.77Ga0.23N composition correlates to that Al content for which a drastic reduction of the conductivity of AlxGa1−xN is commonly reported. We note the incorporation of carbon, the role of which for the transport properties of AlxGa1−xN has not been widely discussed.

GaN-based light emitting diodes (LEDs) face several challenges if the technology is to make a significant impact on the solid state lighting market. The two most pressing of these challenges are cost and efficiency. The development of alternative substrate technologies shows promise toward addressing both of these challenges, as both GaN-based device technology and the associated metalorganic chemical vapor deposition (MOCVD) technology are already relatively mature. Zinc oxide (ZnO) and silicon (Si) are among the most promising alternative substrates for GaN epitaxy. This work focuses on the development of MOCVD growth processes to yield high quality GaN-based materials and devices on ZnO and Si. ZnO, because of its similar lattice constant and thermal expansion coefficient, is a promising substrate for growth of low defect-density GaN. The major hurdles for GaN growth on ZnO are the instability of ZnO in a hydrogen atmosphere and out-diffusion of zinc and oxygen from the substrate. A process was developed for the MOCVD growth of wurtzite GaN and InxGa1-xN on ZnO, and the structural and optical properties of these films were studied. High zinc and oxygen concentrations remained an issue, however, and the diffusion of zinc and oxygen into the subsequent GaN layer was studied more closely. Silicon is the most promising material for the development of an inexpensive, large-area substrate technology. The challenge in GaN growth on Si is the tensile strain induced by the lattice and thermal mismatch between GaN and Si. A thin atomic layer deposition (ALD)-grown Al2O3 interlayer was employed to relieve strain while also simplifying the growth process. While some strain was still observed, the oxide interlayer leads to an improvement in thin film quality and a reduction in both crack density and screw dislocation density in the GaN films. A comparison of GaN-based LEDs grown on sapphire and Al2O3/Si shows similar performance characteristics for both devices. IQE of the devices on silicon is ~32%, compared to ~37% on sapphire. These results show great promise toward an inexpensive, large-area, silicon-based substrate technology for MOCVD growth of GaN-based optoelectronic devices.