Composition dependence of electronic defects in CuGaS2

CuGaS₂ films grown by physical vapor deposition were studied by photoluminescence (PL) spectroscopy, using excitation intensity and temperature-dependent analyses. We observed free and bound exciton recombinations, three donor-to-acceptor (DA) transitions, and deep-level transitions. The DA transitions at ~2.41, 2.398, and ~2.29 eV are attributed to a common donor level ~35 meV and two shallow acceptors at ~75 and ~90 meV and a deeper acceptor at 210 meV above the valence band. This electronic structure is similar to those of other chalcopyrite materials. The observed DA transitions are accompanied by several phonon replicas. The Cu-rich and near-stoichiometric CuGaS₂ films are dominated by transitions involving the acceptor at 210 meV. All films show deep-level transitions at ~2.15 and 1.85 eV due to broad deep defect bands. The slightly Cu-deficient films were dominated by intense transitions at ~2.45 eV, which were attributed to excitonic transitions, and a broad defect transition at 2.15 eV.