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Ph.D. Defense Wednesday, November 22, 2006 Bourns Hall A171 11:00am Title: Growth and Characterization of p-type ZnO by Plasma-assisted Molecular Beam Epitaxy Abstract: ZnO, as an emerging wide bandgap semiconductor material, is currently receiving much attention due to its fundamental advantages over its main competititor GaN in the request for blue/ultraviolet emitters and detectors, and high-speed electronic devices. One of the greatest difficulties, however, is reliably fabricating p-type ZnO with wide-range hole concentration, high mobility, and low resisvitity. Although substantial research is currently being carried out world wide towards this goal, the effective p-type dopant and its doping process have not yet been identified, which significantly hinders the development of ZnO-based optoelectronic and electronic devices. In this dissertation, group V elements N, P, Sb, and Bi have been used as primary dopants to make ZnO p type. N was found to have limited solubility in ZnO, giving a low hole concentration of ~1016 cm-3. P doping by the decomposition of GaP element in an effusion cell, however, show an amphoteric behavior: the conductivity is dependent on the growth conditions. Sb doping was proved to be the most reliable method so far. Reliable p-type conduction was obtained for all ZnO samples on Si substrates under suitable growth process. One sample possesses high hole concentration of 1.7×1018 cm-3, high mobility of 20.0 cm2/V s, and low resistivity of 0.2 Ω cm. Combining our experiments with the theoretical calculations, it is found that the p-type conductivity arises from the formation of the defect complex of SbZn–2VZn, which serves as a shallow acceptor with an activation energy of about 140 meV. Finally, the last element Bi in group V was attempted. We proved by both experiments and theory that it is fundamentally difficult to realize p-type ZnO by Bi doping because of the high formation energy of 3.9 eV for BiZn–2VZn. In addition, based on the successful Sb-doped p-type ZnO, a p-type Schottky light-emitting diode was fabricated and characterized. The light-emitting properties at room temperature show a promising future for the next-generation ZnO-based solid-state lighting. |
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