Semester

Spring

Date of Graduation

2008

Document Type

Dissertation

Degree Type

PhD

College

Eberly College of Arts and Sciences

Department

Physics and Astronomy

Committee Chair

Larry E. Halliburton.

Abstract

Electron paramagnetic resonance (EPR) and electron-nuclear double resonance (ENDOR) studies have been performed on single crystals of aluminum nitride (AlN) and zinc oxide (ZnO), two wide-band-gap semiconductors having the wurtzite crystal structure. These studies were used to characterize point defects in each material. In the first study in AlN, new EPR and ENDOR spectra were acquired from a deep donor. Although observed in as-grown crystals, exposure to x rays significantly increased the concentration of this center. ENDOR identified a strong hyperfine interaction with one aluminum neighbor along the c axis and weaker equivalent hyperfine interactions with three additional aluminum neighbors in the basal plane. These aluminum interactions indicate that the responsible center was located at a nitrogen site. The observed paramagnetic defect is either an oxygen substituting for nitrogen or a nitrogen vacancy. An analysis of the hyperfine data suggests that substitutional oxygen is the most likely candidate.;The second point defect studied in AlN was silicon substituting for aluminum. Silicon is a shallow donor in AlN, and its neutral charge state is paramagnetic. Two samples containing silicon were studied. Only one of the samples was intentionally doped with silicon. The silicon-related EPR signals from these two samples had different behaviors. The signal from the doped sample had behavior similar to that described in previous studies where the silicon was explained as a DX center. The undoped sample had behavior that was inconsistent with a DX center.;In ZnO, EPR was used to monitor oxygen vacancies and zinc vacancies in a ZnO crystal irradiated near room temperature with 1.5 MeV electrons. Out-of-phase detection at 30 K greatly enhanced the EPR signals from these vacancies. Following the electron irradiation, but before illumination, Fe3+ ions and nonaxial singly ionized zinc vacancies were observed. Illumination with 325 nm laser light at low temperature eliminated the Fe3+ signal while producing spectra from singly ionized oxygen vacancies, neutral zinc vacancies, and axial singly ionized zinc vacancies. This light also produced EPR spectra from zinc vacancies having an OH- ion at an adjacent oxygen site. The low temperature response of the irradiated crystal to illumination wavelengths between 350 and 750 nm is described. Wavelengths shorter than 600 nm converted Fe3+ ions to Fe2+ ions and converted neutral oxygen vacancies to singly ionized oxygen vacancies. Neutral zinc vacancies were formed by wavelengths shorter than 500 nm as electrons were removed from isolated singly ionized zinc vacancies. Warming above 120 K in the dark reversed the effect of the illuminations. These wavelength-dependence results suggest that the ground state of the neutral oxygen vacancy is deep, approximately 1.3 eV above the valence band, and that the ground state of the singly ionized zinc vacancy is also deep, about 0.9 eV above the valence band.;The hyperfine associated with the isolated nitrogen acceptor in ZnO was studied with EPR. The sample used in this study was grown by the seeded chemical vapor transport method, with N2 added to the gas stream to serve as the doping source. This study further characterized the hyperfine interactions with the nitrogen nucleus (I = 1) and the nearest-neighbor zinc nuclei (I = 5/2). Angular dependence data were obtained from EPR and were analyzed by complete diagonalizations of the full spin Hamiltonian. Nuclear electric quadrupole effects were included in the nitrogen hyperfine analysis, thus yielding a value for the nuclear quadrupole and more accurate g values and nitrogen hyperfine parameters.

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