Excitonic luminescence of Cu(In,Ga)Se2

In the present work, we report the results of photoluminescence (PL) studies on Cu(In,Ga)Se-2 thin films with varying Ga contents. The samples are epitaxially grown on a GaAs substrate by means of metal-organic vapour phase epitaxy (MOVPE) at slight Cu excess ([Cu]/[III] approximate to 1,1). The polycrystalline samples are prepared by coevaporation on glass. To determine the nature of the observed emissions, PL experiments are performed with varying excitation intensities and temperatures. For the first time, excitonic luminescence is observed in Cu(In,Ga)Se,. It is seen for [Ga]/([Ga]+[In]) ratios (GGI) <= 0.25 and >= 0.75 that exciton binding energies and band gaps are found to rise with increasing Ga content and to be in good accordance with the hydrogen model. Emissions at lower energies are shown to be due to a donor-acceptor pair (DAP) transition equivalent to the DA1 transition in CuInSe2 and CuGafe(2), as well as their phonon replicas. The defect depth was found to increase with increasing Ga content, as expected from the hydrogen model. The study is supplemented by cathodoluminescence (CL) investigations of epitaxial layers and by PL measurements of polycrystalline thin films. (c) 2004 Published by Elsevier B.V.