| Realization of the anticipated potential of GaN-based technology for electronic and optoelectronic applications has been hampered, due in large part to the inordinately high density of defects incorporated into even device-grade material. The adverse impact of deep levels arising from defects impedes device performance, and a fundamental understanding of deep level defects in GaN is necessary in order to optimize device performance. The focus of this dissertation investigates the physical origin and nature of deep levels in GaN and pseudomorphic AlGaN/GaN with particular emphasis on films that serve as component layers in AlGaN/GaN heterojunction field effect transistor (HFET) devices. To elucidate the origin of observed deep levels, an array of spectroscopy techniques were employed to track deep level incorporation as a function of growth methods, growth conditions, doping levels, and Al mole fraction. Several innovative techniques were devised to address the challenges of quantitative analysis of deep level concentrations in wide bandgap, semi-insulating semiconductors, and discerning the deep level spectrum of a nanoscale, pseudomorphic AlGaN layers grown on much thicker GaN films.; In the study of GaN:C films, which find application as highly resistive buffers in AlGaN/GaN HFET devices, prominent deep level states at Ec - 3.0 eV and Ec - 3.28 eV were identified with carbon impurities, the latter being ascribed to C N. The carbon-related deep level at Ec - 3.0 eV was also associated with a carbon-related yellow luminescence band at 2.2 eV through application of a self-consistent configuration-coordinate model. The site selectivity of carbon incorporation and thus the mechanism for semi-insulating behavior in GaN:C:Si films was observed to depend on substrate temperature of films grown by molecular beam epitaxy.; Deep level incorporation of p-type GaN:Mg was also studied. Deep levels were found to incorporate near both band edges in GaN:Mg, and major deep levels were found at Ev + 3.05, 3.22 and 3.27 eV, and the Ev + 3.27 eV state was attributed to residual CGa. Additional bandgap states were evident at Ec - 2.97 and 3.24 eV, and the latter was attributed to MgGa itself.; Because spectroscopy techniques geared exclusively for devices do not distinguish among intimate component layers of a heterostructure that may consist of different materials and deep level properties, methods were established for discriminating the deep level spectra of thin pseudomorphic AlGaN layers of nanoscale dimensions (∼ 30 nm) grown upon much thicker GaN (∼ 1 mum) films. In this manner, deep levels at Ec - 2.00 eV and Ec - 3.85 eV were associated with the AlGaN layer while other deep levels at Ec - 2.65 eV and Ec - 3.30 eV were attributed to the underlying GaN region. Near-Ev bandgap states in were evident in all studied AlGaN/GaN films, and thus appear prototypic for Al mole fraction 0 < x < 0.21.; Spectroscopy techniques were extended from capacitance-mode measurements to current-mode measurements applicable to HFET devices directly. Preliminary investigation found a near-Ev bandgap state associated with the AlGaN layer. Further, despite the inclusion of a SI GaN:C buffer in the HFET device, no major carbon-related deep level signatures were observed in the I-DLOS spectra. This demonstrates that with a properly tailored growth, a GaN:C layer can be incorporated in an HFET without deleterious effects. |
