Journal of Crystal Growth 201/202 (1999) 407 } 410 Structural properties of GaN epilayers directly grown on on-axis 6H-SiC(0 0 0 1) by plasma-assisted MBE A. Trampert*, O. Brandt, B. Yang, B. Jenichen, Klaus H. Ploog Paul-Drude-Institut fu ( r Festko ( rperelektronik, Hausvogteiplatz 5}7, D-10117 Berlin, Germany Abstract We report on the structural properties of a series of thin GaN epilayers directly grown on on-axis 6H-SiC(0 0 0 1) by plasma-assisted MBE. X-ray measurements show that the crystalline perfection of the layers steadily improves with "lm thickness. In fact, the density of threading defects detected by transmission electron microscopy is found to decrease drastically with the distance away from the GaN/SiC interface, "nally reaching a value of less than 510 cm at a layer thickness of 0.5 m. The formation mechanisms of the threading dislocations in the GaN "lms are discussed in consideration of the speci"c GaN/SiC interface structure.  1999 Elsevier Science B.V. All rights reserved. PACS: 81.15.Hi; 68.35.Ct; 61.16.Bg; 78.55.Cr Keywords: GaN epilayers; GaN/SiC interface structure; 6H-SiC(0 0 0 1) 1. Introduction The direct growth of GaN epilayers on doped 6H-SiC(0 0 0 1) substrates is attractive since it of- fers the potential to fabricate vertically conductive devices. However, the direct nucleation of GaN on 6H-SiC(0 0 0 1) substrates is impeded by a poor wetting behavior that frequently results in GaN "lms of low crystal quality [1]. The use of an AlN bu!er layer was demonstrated to be e!ective in improving the crystallinity as well as in reducing the defect density in the GaN "lms [1}3], but * Corresponding author. Fax: #49-30-20377-515; e-mail: trampert@pdi-berlin.de.  Present address: Department of Physics, University of Il- linois, Chicago, IL 60607, USA. simultaneously inhibits carrier injection across the AlN/SiC interface due to the insulating nature of AlN and its high band gap. In some of the few reports on the growth of GaN directly on 6H-SiC(0 0 0 1), amorphous interlayers [4] or inclusions of the zinc-blende phase [5] were observed close to the GaN/SiC interface by trans- mission electron microscopy (TEM) despite the fa- vorable lattice mismatch when compared to the case of sapphire. Furthermore, the question arises in general why thin GaN layers directly grown on 6H-SiC(0 0 0 1) exhibit inferior crystal quality (higher defect density) compared to corresponding GaN layers grown on AlN-bu!ered 6H-SiC or even sapphire substrates [6,7]. In the present paper, we report on the structural properties of a series of thin (0.2}0.6 m) GaN epilayers grown directly on on- axis 6H-SiC(0 0 0 1). 0022-0248/99/$ } see front matter  1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 9 8 ) 0 1 3 6 3 - 3