The transmission electron microscopy and atomic force microscopy investigations of the spontaneously formed AlGaN superlattice are presented. It is found that the growth of superlattice takes place for Al content smaller than 20%. Typical period of the AlGaN superlattice is about 30-40 nm. The main finding of the work is a discovery that superlattice appearance depends on initial growth conditions of the AlGaN layer. Superlattice appears when AlGaN layer growth starts in two-dimensional (2D) mode. This could be forced by the growth on a regular GaN layer or on a specially optimized AlN nucleation layer. On the other hand, if AlGaN growth starts in 3D mode, layers with uniform Al distribution are obtained. Therefore, the mechanism responsible for the spontaneous formation of AlGaN superlattice is directly related to the 2D growth mode of the epitaxial layer. (c) 2006 Elsevier B.V. All rights reserved.