Defect microstructure of thin wurtzite GaN films grown by MBE

Thin films of wurtzite GaN have been grown by molecular beam epitaxy on 6H SiC (basal plane), Si {111} and sapphire (c-plane) substrates with and without various buffer layers. The defect microstructure of the films and the substrate/buffer/GaN interfacial quality have been characterized by cross-sectional transmission electron microscopy. The morphology was dominated by threading defects that originated at the substrate/buffer and/or buffer/film interfaces. Typical defect densities dropped rapidly with distance from the substrate but remained approx.10 ⁸-10 ⁹/cm ², depending on the particular substrate, for film thicknesses approaching one micron or more. The best quality films were grown at 770°C on sapphire with AlN buffer layers, and had X-ray rocking curve full-width at half-maximum values of approx. 55arc-sec.