MICROSTRUCTURE, VIBRATIONAL AND ELECTRONIC-PROPERTIES OF GAN GROWN BYMOLECULAR-BEAM EPITAXY ON AL2O3(0001) AND 6H-SIC(0001)

GaN layers have been grown by molecular beam epitaxy with a rf plasma source on Al2O3(0001) and 6H-SiC(0001). The conductive n-SiC substrate s were employed for the in situ characterization of the grown GaN laye rs by electron spectroscopies (HREELS and XPS). Transmission electron microscopy (TEM) in the conventional and high-resolution mode provides information regarding the structural properties. Plan-view TEM yields a threading defect density of 7 x 10(9) cm(-2) in GaN/6H-SiC and 2 x 10(10) cm(-2) in GaN/Al2O3. Micro-Raman spectroscopy, by analysis of t he coupled A(1)(LO)-phonon-plasmon mode, provides the free carrier con centrations to be n similar to 1-2 X 10(17) cm(-3) for GaN layers grow n on both substrates. The in situ HREEL spectroscopy gives insight int o the electronic and vibrational properties of the GaN surface. Deep l evels at similar to 900 meV above the valence band maximum induce a su rface absorption structure which is reduced after heating at 600 degre es C and which might be assigned, according to the growth process and to the literature, to the presence of Ga vacancies in the as-grown lay ers, in particular in the region close to the surface. Heating the GaN /6H-SiC heterostructures at 900 degrees C causes complete decompositio n of the GaN layer. (C) 1997 American Vacuum Society.