Photoluminescence and sub band gap absorption of CuGaSe2 thin films

A detailed study on the optical properties of polycrystalline thin film CuGaSe2 as a function of chemical composition has been carried out using photoluminescence and photothermal deflection spectroscopy. The CuGaSe2 thin films were grown by a novel open-tube chemical vapor deposition process. This method is suitable for the preparation of films within a wide range of composition. Luminescence due to shallow defect levels can be directly compared with the photoluminescence spectra of MOCVD-grown epitaxial CuGaSe2 thin films. In addition, we have observed a broad excitonic luminescence peak 31 +/- 2 meV below the band gap correlated with a donor-acceptor pair-like luminescence transition at approximately 1:29+/-0:01 eV. By a detailed analysis of the binding nature of the exciton, we were able to identify an acceptor-like defect band at 243 20 meV, presumably caused by Ga-vacancies in slightly Cu-rich films. The composition dependence of the luminescence intensities is correlated with composition-dependent sub band gap absorption measurements performed by photothermal deflection spectroscopy. These data show shallow defect structure-depcndent Urbach-tail energies (varying from 25 +/- 2 to 35 2 meV) as well as variations of defect absorption far below the band edge. By comparing the photoluminescence data with the absorption data, we could attribute this defect absorption to the same acceptor-like defect band. (C) 2002 Elsevier Science B.V. All rights reserved.