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Metalorganic chemical vapor deposition (MOCVD) growth of AlN, GaN and AlGaN on sapphire substrates was investigated, with the aim of realizing high-quality heterostructures with atomically smooth interfaces. AlN with surfaces constructed of single-atomic-layer steps was grown by the two-step growth technique, in which the thin first layer was grown at a low temperature (1250° C) and then the second layer was grown at a high temperature (1350° C). GaN with surfaces consisting of simple atomic-layer steps were successively grown on the AlN layer by optimizing the growth conditions. Marked degradation of surface flatness was not observed for Al0.2Ga0.8N grown on GaN.