Substrate influence on Cu(In,Ga)Se 2 film texture

Abstract

This study reports on the evolution of Cu(In,Ga)Se 2 film surface orientation during growth and on the substrate influence on the resulting film texture. A thermal evaporation process deposits 0.5–1 μm thick Cu-poor Cu(In,Ga)Se 2 layers with constant deposition rates of all elements during growth on Mo single crystals, polycrystalline Mo, and highly oriented pyrolytic graphite. X-ray diffraction measurements allow the determination of the film texture. On highly oriented pyrolytic graphite, on (100)-oriented Mo, and on (111)-oriented Mo surfaces Cu(In,Ga)Se 2 grows with a pure (112) surface orientation, whereas on (110)-oriented Mo single crystals the (220/204) Cu(In,Ga)Se 2 surface orientation dominates. On polycrystalline Mo substrates, no significant preferred film orientation for layers up to 1 μm thickness exists for as-grown films. Annealing of Cu(In,Ga)Se 2 films on polycrystalline Mo substrates leads to an increase of the mean grain size and to an evolution of a (112) preferred film orientation. The pure (112) texture of the Cu(In,Ga)Se 2 film grown on highly oriented pyrolytic graphite which hardly interacts with the adsorbed material proves that the (112) surface orientation is energetically preferred. Among the investigated single crystalline Mo substrates, only (110)-oriented Mo has an impact on the resulting Cu(In,Ga)Se 2 film texture. This substrate influence and the fact that (112)-oriented grains grow at the expense of other orientations reveal why Cu(In,Ga)Se 2 films grow on polycrystalline Mo with no preferred orientation during early stages of growth but result in a (112) texture for later growth stages.