Morphological control of GaN buffer layers grown by molecular beam epitaxy on 6H-SiC(0001)

The growth of GaN buffer layers of thickness 10-25 nm directly on 6H-SiC(0001) substrates was studied using low energy electron microscopy, atomic force microscopy and cross-sectional transmission electron microscopy. The Ga flux was supplied by an evaporative source, while the NH₃ flux came from a seeded beam supersonic jet source. By monitoring the growth in situ and by suitably adjusting the Ga/NH₃ flux ratio, smooth basal-plane-oriented GaN layers were grown on hydrogen-etched SiC substrates at temperatures in the range of 600-700°C. The growth proceeds via nucleation of small flat islands at the step edges of the 6H-SiC(0001) substrate surface. The islands increase in size with a lateral-to-vertical growth ratio of ∼ 10 and eventually coalesce into a quasicontinuous layer. A highly defective substrate surface was found to be detrimental to the growth of flat buffer layers.