The effect of a secondary copper selenide phase that appears during epitaxial growth of CuInSe2 (CIS) films on (100) GaAs substrates was investigated. CIS films with different copper to indium molar ratios were deposited on (100) GaAs substrates at 360 degrees C by migration-enhanced epitaxy. Films grown under In-rich conditions were polycrystalline, whereas films grown with a Cu-rich composition were epitaxial, as evidenced by pole figure analysis, even when the copper composition was only slightly higher than the 1:1 Cu:In molar ratio. The epitaxial film growth followed a Stranski-Krastanov mode and contained both matrix and island regions. Grazing incidence XRD revealed a small amount of CIS with the Cu-Au-like structure near the surface of the island regions. A binary Cu-Se secondary phase aggregated near the surface in the island regions of the film, as determined by electron probe nucroanalysis (EPMA) before and after etching in 10% KCN. It is proposed that a Cu-Se phase present during the growth of Cu-rich CIS films enhances the mobility of adatoms on the substrate surface, allowing the film to grow epitaxially to a critical thickness. After that critical thickness is attained, the Cu-Se phase segregates at dislocation sites to nucleate the island regions. TEM-EDX scans of cross-sections also suggested a small amount of the Cu-Se secondary phase segregates on the film surface. (c) 2005 Elsevier B.V. All rights reserved.