Morphology and structure of thin epitaxial Cu(In,Ga)S2 films on Si substrates

Cu(In,Ga)S2 (CIGS) thin films were grown epitaxially on Si substrates of various orientations. The sulphur-termination process, utilized to suppress the unfavourable native surface reconstructions of Si, was examined. Thereby, an anisotropic etching of Si by sulphur was observed at high substrate temperatures. Cu(In,Ga)S2 was found to show a strong tendency for nucleation on tips and at step edges of the substrates. Furthermore, the material was observed to agglomerate to existing grains as well. These mechanisms lead to a three-dimensional growth mode. The resulting roughness showed a strong and monotonic dependence on the Ga content of the samples, whereas the influence of the lattice mismatch on the morphology of the samples was found to be less distinct. The coexistence of the highly ordered chalcopyrite structure with the metastable CuAu structure was found to be another prominent feature in epitaxial Cu(In,Ga)S2 thin films on Si.