Photoluminescence and sub band gap absorption of CuGaSe 2 thin films

Abstract

A detailed study on the optical properties of polycrystalline thin film CuGaSe 2 as a function of chemical composition has been carried out using photoluminescence and photothermal deflection spectroscopy. The CuGaSe 2 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 CuGaSe 2 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-dependent 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.