The growth of thick, high-quality GaN layers on GaAs (1 1 I)A substrates was investigated using a two-step growth process by metalorganic hydrogen chloride vapor phase epitaxy (MOHVPE). The thickness and temperature ramping rate of the low-temperature (LT)-grown GaN buffer layer as well as the growth temperature of the subsequent thick GaN layer at high temperatures were found to be the key factors controlling the crystalline quality of the GaN layer on GaAs (I I I)A substrate. Growth of a 50-nm-thick LT-GaN buffer layer at 550degreesC and subsequent heating to 1000degreesC at a temperature ramping rate of 16.1degreesC/min afforded a 100-mum-thick high-quality GaN layer with a specular surface. The results indicate that a thick GaN layer can be successfully grown on a GaAs (I I I)A substrate at high temperatures and represent a promising technique for the preparation of freestanding GaN substrates. (C) 2002 Elsevier Science B.V. All rights reserved.