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

The growth of GaN buffer layers of thickness 10-25 nm directly on 6H-SiC(00 01) substrates was studied using low energy electron microscopy, atomic for ce 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 l ayers were grown on hydrogen-etched SiC substrates at temperatures in the r ange of 600-700 degreesC. The growth proceeds via nucleation of small flat islands at the step edges of the 6H-SiC(0001) substrate surface. The island s increase in size with a lateral-to-vertical growth ratio of similar to 10 and eventually coalesce into a quasicontinuous layer. A highly defective s ubstrate surface was found to be detrimental to the growth of flat buffer l ayers.