We report on successful fabrication of crack-free thick AlGaN layers by introducing a thin AlN interlayer at the high temperature of 1000-1100degreesC on a high quality GaN epitaxial layer. The Al0.52Ga0.48N films grown with the optimized AIN interlayer exhibited mirror-like surface morphology and were free of cracks up to the thickness of 3 mum. X-ray analyses and cathodoluminescence spectra showed excellent structural and optical quality of the AlGaN materials. Si-doped n-type AlGaN films were also grown on the high-temperature AIN interlayer, and excellent n-type conductivity was achieved up to Al0.38Ga0.62N with the electron concentration of 2.0 x 10(18) cm(-3) and the mobility of 79 cm(2)/V s. Furthermore, from monochromatic cathodoluminescence images, we proposed that initial cracking of the AIN interlayer reduces the residual stress, thus preventing cracking in the AlGaN layer by generating misfit dislocations at AlGaN/AIN interface. (C) 2002 Elsevier Science B.V. All rights reserved.