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a b s t r a c tDefects in semiconductors and insulators are characterized by their levels, which are defined as the values of the Fermi level at which the charge state of the defect changes. Kohn-Sham density functional theory calculations for charged defects have been widely and successfully used to predict defect levels. Due to their lower computational cost and demonstrated ability to predict levels spanning the measured band gap, semilocal exchange-correlation functionals are widely used in these calculations. However, there is a potential pitfall in using semilocal functionals: although they often predict accurate energies for adding or removing electrons from states that are localized near the defect, the famous band gap error results in overly small energies for adding or removing electrons from extended band edge states. As a result, electrons (or holes) that should occupy localized states may become partially or fully delocalized. In order to help detect and analyze such cases, we introduce bounds on the defect levels that can be obtained using a given functional, supercell, and Brillouin zone sampling. Since these bounds correspond to the charge transition levels of the corresponding defect-free supercell, comparison with the bounds reveals when a calculated level is behaving in a bulk-like rather than defect-like manner. We find that the bounds depend significantly on supercell size due to band-filling effects that arise from the finite charge density created when one electron is added to or removed from a finite-sized supercell, and this size dependence helps explain the success of defect level calculations using semilocal functionals.

We report on issues associated with the metalorganic chemical vapor deposition growth of ScGaN and YGaN. Based on the proposed bandgaps of ScN (2.15 eV), YN (0.8 eV) and the known bandgap of GaN (3.4 eV), we expected that LEDs could be fabricated from the UV (410 nm) to the IR (1600 nm), resulting in LED over all visible wavelengths for solid state lighting However, due to the low volatility of scandium and yttrium metalorganic precursors, we were only able to incorporate doping level concentrations of these atoms into a GaN film. This report highlights the difficulties encountered during the growth of these alloys and how some of these difficulities were overcome. We also investigated the nitridation of thin scandium metal films deposited on GaN using pulsed laser deposition. GaN was also grown on these thin ScN films to as a means to lower the GaN dislocation density. 4This page has been left intentionally blank 5 Extended AbstractThe most energy efficient solid state white light source will likely be a combination of individually efficient red, green, and blue LED. For any multi-color approach to be successful the efficiency of deep green LEDs must be significantly improved. While traditional approaches to improve InGaN materials have yielded incremental success, we proposed a novel approach using group IIIA and IIIB nitride semiconductors to produce efficient green and high wavelength LEDs.To obtain longer wavelength LEDs in the nitrides, we attempted to combine scandium (Sc) and yttrium (Y) with gallium (Ga) to produce ScGaN and YGaN for the quantum well (QW) active regions. Based on linear extrapolation of the proposed bandgaps of ScN (2.15 eV), YN (0.8 eV) and GaN (3.4 eV), we expected that LEDs could be fabricated from the UV (410 nm) to the IR (1600 nm), and therefore cover all visible wavelengths. The growth of these novel alloys potentially provided several advantages over the more traditional InGaN QW regions including: higher growth temperatures more compatible with GaN growth, closer lattice matching to GaN, and reduced phase separation than is commonly observed in InGaN growth. One drawback to using ScGaN and YGaN films as the active regions in LEDs is that little research has been conducted on their growth, specifically, are there metalorganic precursors that are suitable for growth, are the bandgaps direct or indirect, can the materials be grown directly on GaN with a minimal defect formation, as well as other issues related to growth.The major impediment to the growth of ScGaN and YGaN alloys was the low volatility of metalorganic precursors. Despite this impediment some progress was made in incorporation of Sc and Y into GaN which is detailed in this report. Primarily, we were able to incorporate up to 5x10 18 cm -3 Y atoms into a GaN film, which are far below the alloy concentrations needed to evaluate the YGaN optical properties.After a no-cost extension was granted on this program, an additional more "liquid-like" Sc precursor was evaluated and the nitridation of Sc metals on GaN w...

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A solar cell based on a III-nitride hybrid nanowire-film architecture is demonstrated. It consists of a vertically-aligned array of InGaN/GaN multi-quantum well core-shell nanowires, electrically connected by a coalesced p-type InGaN canopy layer. This hybrid structure allows for standard planar device processing, solving an important challenge with nanowire device integration. It also enables various advantages such as higher indium composition InGaN layers via elastic strain relief, efficient carrier collection through thin layers, and enhanced light trapping. This proof-of-concept nanowire-based device presents a path forward for highefficiency III-nitride solar cells. Fabricated III-nitride nanowire solar cells exhibit a photoresponse out to 2.1eV and short circuit current density of ~1 mA/cm 2 (1 sun AM1.5G). 4ACKNOWLEDGMENTS 5