Influence of GaSe deposition temperature on the structural properties and in-depth compositional features of two-step grown Cu(In,Ga)Se 2 thin films

Abstract

In this study, Cu(In,Ga)Se2 thin films were prepared by a classical two-stage growth process, which involved the selenization of thermally evaporated InSe/Cu/GaSe precursors. During the precursor-formation step the InSe and Cu were always deposited at 200 °C, while the GaSe layers were deposited at temperatures between 200 °C and 400 °C. The respective precursors were simultaneously selenized under identical conditions in elemental Se vapor. In cases where the GaSe layers were deposited at low temperatures around 200 °C, X-ray fluorescence (XRF) analysis revealed a large variation in element concentration with sample depth after selenization. In correspondence, X-ray diffraction (XRD) studies revealed the presence of separate CuInSe2 and CuGaSe2 phases in these specific samples. Optimum structural properties were obtained when the GaSe films were deposited at 300 °C, followed by selenization. In general, these films were uniform and dense and XRD studies revealed single-phase Cu(In,Ga)Se2 material. Even more importantly, XRF analysis revealed a remarkable improvement in in-depth compositional uniformity when the GaSe films were deposited at or above 300 °C. An increase in GaSe deposition temperature to 400 °C, however, resulted in a deterioration in the structural features of the Cu(In,Ga)Se2 thin films. In contradiction with other reports, these results indicated that the in-depth composition uniformity and especially the Ga diffusion profile in two-step grown Cu(In,Ga)Se2 thin films can be controlled. The crucial factor influencing the depth profile of these films is the GaSe deposition temperature during the precursor formation step.