Characterization of homoepitaxially-grown aluminum gallium nitride/gallium nitride heterostructures.

The work in this dissertation is aimed at determining whether homoepitaxial growth of AlGaN/GaN heterostructures will significantly improve the 2DEG transport properties. Van der Pauw Hall measurements at RT and 77K were used to evaluate the 2DEG transport properties of AlGaN/GaN heterostructures grown on on-axis and vicinal surfaces of (0001) semi-insulating bulk GaN and vicinal surfaces of (0001) and (11¯00) n-type bulk GaN. Analytical techniques such as SEM, CV, AFM, HRXRD, PL, CL, Raman spectroscopy, and TEM were used to determine structural, optical, and electronic properties of these heterostructures and to explain the Hall mobility and sheet carrier density. AlGaN/GaN heterostructures with RT 2DEG sheet densities and mobilities modulated by the presence of a large silicon areal density (1.6 x 10 14 cm-2) at the bulk GaN/epitaxial interface were grown. The transport properties of the 2DEG were improved by eliminating Si from the AlGaN barrier and an unintentional 1000 ppm of Al from the n.i.d. GaN buffer. Hall sheet densities of 1013 cm-2 and mobilities of 2065 cm2V-1sec-1 were comparable to the best reported and were only 30% lower than the value predicted for AlGaN/GaN heterostructures grown with atomically smooth interfaces and dislocation densities < 2 x 107 cm-2. Magnesium (Mg) doping was incorporated into the GaN layer near the bulk/epitaxial interface in an attempt to compensate free electrons from the interfacial charge layer After the 800°C ex-situ anneal, sheet carrier densities on the order of 1015 cm-2 and RT and 77K mobilities of 567 cm2V-1sec-1 and 2540 cm 2V-1sec-1, respectively, were determined by Hall measurements. Raman spectroscopy provided evidence for increased Mg donors and nitrogen vacancy formation in these heterostructures and was consistent with expected lower formation energy in the presence of Mg.