Control and passivation of V_se defect levels in H_2Se-selenized CuInSe_2 thin films

摘要

In this study, we investigated the dominating intrinsic defect levels in CuInSe_2 by means of temperature- and excitation-dependent steady-state photoluminescence (PL). The polycrystalline thin films were prepared by rapid thermal treatment of metallic alloys in H_2Se-Ar. Attention was mainly focused on the influence of post-growth treatments (in Ar-H_2 and O_2) on the defect structure of these films. The PL spectra of slightly In-rich as-grown films were dominated by a donor-acceptor pair transition at 0.92 eV The defect levels are ascribed to V_cu and V_se and their activation energies have been determined to be 45 and 95 meV respectively. Post-growth treatment in Ar-H_2 resulted in a shift of these peaks to higher energies and a significant increase in the intensity. In contrast, annealing in O_2 resulted in a shift of the peak to lower energies and to a dramatic reduction in intensity, compared with the as-grown samples. These effects were also found to be reversible. The change of the PL spectra on oxygen and hydrogen annealing is explained by the change of the density of Vs. defects. This is explained by a model in which oxygen can occupy a V_se site owing to coordinatively unsaturated In at the grain boundaries. The V_se concentration and therefore the device quality of these CuInSe_2 thin films can thus be controlled either by a post-growth annealing step or during the growth process itself.