The correlation between crystallization of low-temperature (LT) GaN buffer layer and crystalline quality of subsequently grown thick GaN layer at high-temperature on GaAs (1 1 1)A substrate has been investigated. During heating of LT-GaN buffer layer to 1000degreesC, the degree of crystallization highly depended on the thickness and the temperature ramping rate of GaN buffer layers. When the LT-GaN buffer layer was less than 40-nm-thick or temperature ramping rate was low, excess crystallization and/or agglomeration of the buffer layer occurred, which resulted in a poor crystalline quality of the GaN epitaxial layer subsequently grown on it. On the other hand, when the thickness of LT-GaN buffer layer exceeded 100 nm, there was relatively little agglomeration of buffer layer, which also resulted in poor crystalline quality of the subsequently grown GaN. To grow high-quality GaN layer at high-temperature, optimum crystallization condition for GaN buffer layer just prior to high-temperature growth was found to be as follows: single crystallites region in the GaN buffer layer occupied a volume 40% as large as the randomly aligned matrix. The trade-off between thickness and temperature ramping rate of GaN buffer layer clearly indicates that the degree of crystallization of the GaN buffer layer is an important parameter for achieving high-quality GaN growth at high-temperature. (C) 2004 Elsevier B.V. All rights reserved.