Surface and bulk properties of CuGaSe2 thin films

Using complementary techniques, namely X-ray fluorescence (XRF) and XPS, authors present a comparative study of the bulk and surface composition in device grade CuGaSe2 (CGSe) thin films. The films were deposited in two stages by an open-tube CVD process. The first stage leads to a nearly stoichiometric polycrystalline CGSe film of approximately 1.5 mm thickness. During the second stage the film is annealed in a Ga- and Se-rich atmosphere. While the XRF-data show a nearly stoichiometric integrated film composition, the surface composition, as determined by XPS analysis, is Cu-poor, pointing towards a highly non-stoichiometric surface layer. In addition, sodium was found at the film surfaces. The data are discussed in the framework of an ordered defect compound formation and the formation of a (Cu,Na)-Ga-Se compound at the surface of the CuGaSe2 films. Complementary ultraviolet photoelectron- and inverse photoelectron spectroscopy investigations of the film surface derive a widening of the surface energy band gap up to 2.2 eV in comparison with a bulk energy band gap around 1.65 eV (obtained by optical transmission analysis). The observed data are consistent with model of a two layer film structure containing a defect-rich near-surface region and a defect-poor bulk. 21 refs.