The GalnN/GaN layers grown on the sapphire substrates treated with and without nitridation process have been Studied by X-ray CTR scattering measurements. Crystalline quality of the samples with different thicknesses of low-temperature (LT) deposited AlN buffer layers has been investigated. For the samples without the nitridation process, when the buffer layer was 70 nm thick, many defects were generated in the buffer layer and the crystalline quality of the layers on the buffer layer was degraded. On the other hand, for the samples with the nitridation process, the crystalline quality of the GaInN layer was the best when the LT-AlN buffer layer was 70 nm thick. (C) 2002 Elsevier Science B.V. All rights reserved.