Heteroepitaxial growth of GaN on 6H-SiC(0001) by plasma-assisted molecular beam epitaxy

We report on the correlation between growth conditions and defect densities, as well as the defect generation mechanism, for GaN directly deposited on H-etched SiC(0001) by plasma-assisted molecular-beam epitaxy 10 nm thin GaN nucleation layers were deposited at different conditions (600 to 800 degreesC, Ga-lean to Ga-rich). The island size increased from about 50 nm (600 degreesC, Ga-lean) to about 250 nm (800 degreesC, Ga-rich). Additionally, we examine epilayers nucleated under these conditions but overgrown with 300 nm GaN (720 degreesC, Ga-stable). These samples clearly show atomic steps, however, they exhibit rather different defect densities for different nucleation conditions (10(11) cm(-2) for 600 degreesC and Ga-lean to 10(10) cm(-2) for 800 degreesC and Ga-rich) as determined by plan-view transmission electron microscopy. Furthermore, we performed growth-stop experiments using GaN-thicknesses ranging 2 to 2000 nm for studying the defect generation. It appears that island coalescence is the main source of threading dislocations.