Sodium induced secondary phase segregations in CuGaSe 2 thin films

Abstract

We report on sodium induced secondary phase segregations in CuGaSe 2-thin films, grown by multi-source evaporation on soda-lime glass. The films are characterized by electron microscopy, micro-Raman, micro-photoluminescence and secondary ion mass spectroscopy with high spatial resolution. The segregations show a crater-like structure of a typical diameter of 3–10 μm with an elevated rim and a reduced thickness of the film in the crater bottom. The Ga/(Ga+Cu) ratio in the craters is approximately 0.75 and the sodium concentration is increased by a factor of seven compared to that in the matrix. Micro-Raman spectroscopy within the craters suggests a non-chalcopyrite phase with a structure very similar to that of CuGa 3Se 5. As a possible explanation we propose the formation of a quaternary Na–Cu–Ga–Se compound which is based on the defect-chalcopyrite structure but having the Cu-vacancies partially filled by Na atoms.