Pit arrays forming a network structure were observed by an atomic force microscopy (AFM) on an AlGaN surface of AlGaN/GaN heterostructure on a Si(I 1 1) substrate. In order to clarify the origin of these pit arrays, AlGaN/GaN layers were investigated using a transmission electron microscopy (TEM). As a result, it was confirmed that pit arrays of the surface observed by an AFM represent the surface termination of edge dislocations formed at the small-angle boundaries of slightly misoriented crystal domains. The difference in buffer layer structures formed on Si substrates affected the surface pits arrangement. It was also confirmed that the optimization of a buffer structure on a Si substrate is very effective for the reduction of the pit density. (c) 2006 Elsevier B.V. All rights reserved.