Initial stages of GaN buffer layer growth on (0001) sapphire by metalorganic chemical vapour deposition

Abstract

The initial stages of low temperature GaN buffer layers grown on (0001) sapphire by metalorganic chemical vapour deposition are investigated using conventional and high resolution electron microscopy (HREM). Different deposition times are used in order to observe the evolution of the buffer layer microstructure. Three-dimensional nuclei with the (0001)(GaN)//(0001)(sapphire) and [0110](GaN)//[1120](sapphire) alignment are formed, increasing in height and width with the growth time. Consequently, coalescence is taking place, and the structure of islands is modified. Hexagonal stacking is developing through these islands, but the cubic structure is predominant. A high density of stacking faults and microtwins formed. It is found that GaN islands contain misfit dislocations at the interface with the sapphire substrate. Calculations of residual strain show a transition from a compressive to a tensile strain when the coalescence of islands is operating. (C) 2002 Elsevier Science B.V. All rights reserved.