ZnO quantum pyramids grown on c-plane sapphire by plasma-assisted molecular beam epitaxy

ZnO quantum disks are found to form on the top surface of a thick ZnO epilayer as a result of a morphology transition occurring during the growth of ZnO single-crystal thin films on c-plane sapphire using plasma-assisted molecular beam epitaxy. These disks manifest themselves as triangular pyramids and organize themselves to conform to the six-fold symmetry of the ZnO epilayer underneath with the base edges along [1 (1) over bar 0 0] and side facets of {1 (2) over bar 1 6}. Atomic-force microscopy measurements show a size distribution with base width centered at 140 and 24 nm height, the height/width ratio remaining constant due to the formation of special facets. Cathodoluminescence studies reveal a strong near-bandgap emission from the regions with a high density of pyramids. (C) 1998 Elsevier Science B.V. All rights reserved.