The Cu-rich CuInSe 2 (CIS) epitaxial films with yΒΌ [Cu]/[In] ΒΌ1.65 are grown on GaAs(0 0 1) substrates using the MBE technique at substrate temperatures ranging from 400 to 600 1C. The unusual background and broad peaks in the XRD spectra are found at 2y between 671 and 691, which are not accounted for any crystal planes of the CIS. They also cannot be removed using the KCN solution and are not observed in the stoichiometric (yΒΌ 1) CIS epitaxial films, suggesting that there exists some other relatively thin epitaxial structures at the interface of the CIS and the GaAs(0 0 1). The broad peaks are found to be in the vicinity of the (0 0 8) crystal plane of the CuGaSe 2 (CGS) even if the Ga flux is not allowed during the growth of CIS. The interface layer is believed to be CGS formed by the diffusion of the excess Cu 2 Γ x Se and Ga atoms at the surface of the GaAs substrate. The formation of the CGS interface layer is directly verified by STEM images and the composition of Cu:Ga:Se obtained from the EDS measurement is approximately 1.1:1.0:2.0. The thickness of the CGS interface layer is dependent on the substrate temperature and found to occur when the substrate temperature is approximately higher than 430 1C.