Photoemission studies on Cu(In, Ga)Se2 thin films and related binary selenides

The aim of this work is to provide a data basis for both X-ray and UV photoelectron spectroscopy on chalcopyrite thin films. A model for the segregation behaviour at the surfaces of polycrystalline thin films of I–III–VI2-chalcopyrites (I = Cu; III = In, Ga; VI = Se) based on surface analysis data is presented. In situ photoemission measurements on a variety of Cu(In, Ga)Se2 samples, as well as on Cu2−xSe (x ≈ 0.15), In2Se3 and Ga2Se3 thin film samples, clearly prove the existence of Cu2−xSe on the surfaces of all Cu-rich thin films (i.e. Cu(In + Ga)). The surface composition of Cu-poor thin films generally deviates strongly from the bulk stoichiometry. It has been found that the as grown surfaces of Cu-poor thin films are in most cases covered by InGa-rich defect compounds. The surface of bulk Cu-poor CuInSe2 has been identified as CuIn3Se5. For bulk Cu-poor CuGaSe2, the surface composition is determined by the conditions of film growth; it ranges between stoichiometric CuGaSe2 and CuGa5Se8. This study is based on photoelectron distribution curves (EDCs) which have been measured for photon energies of hv = 21.2, 40,8 and 1253.6 eV. The photoionization cross-sections of the atomic levels comprising the valence bands are strongly dependent on hv. This dependence is exploited to correctly interpret the observed valence band features of the different surface species investigated. The energy positions of the valence band maxima, which are of great practical importance considering heterojunction devices have been determined. For Cu-rich thin films these values could be extracted from the measurements by numerical subtraction of the EDC of the overlaying Cu2−xSe from the EDCs of Cu-rich surfaces. The binding energies of the core levels and the kinetic energies of the main Auger structures are given for all the materials examined.