MOVPE of CuGaSe2 on GaAs in the presence of a CuxSe secondary phase

We have grown epitaxial CuGaSe2 layers by MOVPE on GaAs substrates. CuGaSe2 is the highest band gap member of the chalcopyrite Cu(In,Ga)Se2 family (CIGSe). CIGSe is used as an absorber in polycrystalline thin film solar cells. From growth experiments interrupted at different time steps we find that Ga diffusion from the substrate plays a major role in the early stages of growth. The epi-layers start to grow Ga-rich although the final film is dominated by Cu-excess and shows a homogeneous composition profile. After about 300 nm film thickness Ga diffusion becomes less dominant and the Cu-excess leads to the formation of Cu selenides on the surface. The structure of the films and intentional conversion experiments show that those crystalline Cu selenides are transformed into epitaxial CuGaSe2 during growth.