We have investigated surface morphologies and cluster formation in Mn-incorporated (Galn)As layers grown by metal-organic vapor phase epitaxy (MOVPE) on InP (100) substrates. Whisker growth occurs on the layer surfaces under low V/III ratios and low growth temperature conditions. For temperatures above 500degreesC, MnAs-based cluster structures are formed near the (Galn)As layer surfaces. The MnAs-based clusters show an in-plane anisotropy in the magnetic characteristics. During the overgrowth of the MnAs-based clusters by undoped InP, a change in cluster composition to MnP presumably occurs as indicated by the change in the observed Curie temperature. (C) 2003 Elsevier B.V. All rights reserved.