We have investigated the correlation between the formation of single-crystal GaN islands in low-temperature (LT)-GaN buffer layer and the crystalline quality of subsequently grown high-temperature (HT)-GaN layer. X-ray diffraction (XRD) analysis showed that the LT-GaN layer was partially crystallized even without an annealing process. The annealing process transformed the nanometer-sized LT-GaN grains into the larger-sized islands whose size and density could be controlled by changing the growth temperature of LT-GaN layer. Atomic force microscopy (AFM) and XRD analyses indicated that the crystalline quality of HT-GaN layer depended strongly on the size and density of the islands in the LT-GaN layer. The full-width at half-maximum (FWHM) values of XRD rocking curves of GaN(0002) and (1 0 (1) over bar 2) reflections were 198 and 232 arcsec, respectively, when the LT-GaN layer was grown at 475 degrees C. (c) 2006 Elsevier B.V. All rights reserved.