Semester

Spring

Date of Graduation

2001

Document Type

Dissertation

Degree Type

PhD

College

Eberly College of Arts and Sciences

Department

Physics and Astronomy

Committee Chair

Thomas H. Myers.

Abstract

A reduced rate for growth of GaN by plasma-assisted molecular beam epitaxy often limits growth to temperatures less than 750°C, with the reduction significantly larger than thermal decomposition rates. Conditions producing a flux consisting predominantly of either atomic nitrogen or nitrogen metastables have been established using various rf-sources. Atomic nitrogen, possibly coupled with the presence of low energy ions, is associated with the premature decrease in growth rate. An active nitrogen flux consisting primarily of nitrogen metastables produces a temperature dependence more consistent with decomposition rates. Growth with molecular nitrogen metastables results in significantly improved electrical properties.;Magnesium incorporation was studied for both (0001), or Ga-polarity and (0001), or N-polarity, orientations for various growth conditions. A significant dependence on surface polarity of Mg incorporation was observed, with up to a factor of twenty-five times more Mg incorporated on the Ga-polarity. Measurements supported surface accumulation of Mg during growth, with stable accumulations of close to a monolayer of Mg. Mg coverage of a monolayer on the Ga-polarity induced a surface polarity inversion. Atomic hydrogen was found to increase the incorporation of Mg without also incorporating potentially compensating hydrogen.;Beryllium incorporation was also studied for both polarities of GaN. Unlike Mg, surface polarity-related incorporation differences were less pronounced for Be. Measurements also support surface accumulation of Be during growth, with stable accumulations approaching a monolayer for heavier doping levels. Transmission electron microscopy studies indicate the surface layer of Be has a significant effect on structure, with severe degradation occurring when accumulation nears monolayer coverage.;High-quality GaN films were grown to study the dependence of controlled oxygen incorporation on polarity and oxygen partial pressure. Oxygen concentrations up to 2.5 x 1022 cm-3 were obtained. About 10 times more oxygen incorporates on N-polar GaN than on the Ga-polarity in high quality epilayers. Oxygen doping is controllable, reproducible, and uncompensated up to concentrations of at least 1018 cm-3 with higher levels showing significant compensation. Layers with oxygen levels above 1022 cm-3 exhibit severe cracking. Oxygen incorporation has a weak dependence on Ga overpressure during Ga-stable growth but dramatically increases for conditions approaching N-stable growth.

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