Sodium induced secondary phase segregations in CuGaSe2 thin films

We report on sodium induced secondary phase segregations in CuGaSe2-thin fi lms, grown by multi-source evaporation on soda-lime glass. The films are ch aracterized by electron microscopy, micro-Raman, microphotoluminescence and secondary ion mass spectroscopy with high spatial resolution. The segregat ions show a crater-like structure of a typical diameter of 3-10 mum 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 con centration is increased by a factor of seven compared to that in the matrix . Micro-Raman spectroscopy within the craters suggests a nonchalcopyrite ph ase with a structure very similar to that of CuGa3Se5. As a possible explan ation we propose the formation of a quaternary Na-Cu-Ga-Se compound which i s based on the defect-chalcopyrite structure but having the Cu-vacancies pa rtially filled by Na atoms. (C) 2001 Elsevier Science B.V. All rights reser ved.