The paper reports on the phase separation of the AlGaN layers in an AlGaN/GaN/AlGaN heterostructure grown on GaN/sapphire by metalorganic chemical vapor deposition (MOCVD). It is found that complex-facetted islands, of about 10 mum in diameter, were found during the growth of a 100 nm thick AlGaN. The density of the islands is about 4 x 10(4) cm(-2). When a GaN layer of thickness 100 nm was grown over the AlGaN layer, the complex-facetted islands were almost covered to give a smooth surface. When the second AlGaN layer was grown on the GaN, those complex-facetted islands reappear. Energy dispersion X-ray spectrometer shows that the AlN molar fraction is less on the islands than in other flat areas, which is further confirmed by micro-photoluminescence. These Ga-rich islands are formed during the growth of AlGaN in contrast to the earlier reports on AlN phase separation. A possible cause of the island formation is the difference of surface migration length. (C) 2004 Elsevier B.V. All rights reserved.
