Structural properties of GaN epilayers directly grown on on-axis 6H-SiC(0001) by plasma-assisted MBE

We report on the structural properties of a series of thin GaN epilayers di rectly grown on on-axis 6H-SiC(0 0 0 1) by plasma-assisted MBE. X-ray measu rements show that the crystalline perfection of the layers steadily improve s with film thickness. In fact, the density of threading defects detected b y transmission electron microscopy is found to decrease drastically with th e distance away from the GaN/SiC interface, finally reaching a value of les s than 5 x 10(9) cm(-2) at a layer thickness of 0.5 mu m. The formation mec hanisms of the threading dislocations in the GaN films are discussed in con sideration of the specific GaN/SiC interface structure. (C) 1999 Elsevier S cience B.V. All rights reserved.