Thin films, with various Ga/(Ga+In) ratios, were prepared by selenizing stacked Cu, Ga, and In layers in an H2Se reactor at temperatures ranging from 400 to 500C. The resultant films were characterized by means of scanning electron microscopy, X-ray diffraction, energy dispersive spectroscopy, and Auger electron spectroscopy. The films contained a non-uniform distribution of Ga and In. The surface of the selenized films was In-rich, while the Mo/film interface was Ga-rich. Selenized films, with Ga/(Ga+In) ratios greater than 0.25, contained graded Ga and In compositions, and selenized films with Ga/(Ga+In) ratios of less than 0.6 contained a phase-separated mixture of CuInSe2 and CuGaSe2; with the CuInSe2 near to the surface and the CuGaSe2 near to the Mo/film interface. Single-phase homogeneous CuInGaSe2 films were obtained by annealing the as-selenized films in Ar (500 to 600C, 1h). Interdiffusion of In and Ga between the CuGaSe2 and CuInSe2 phases was found to be responsible for homogenization. This process did not occur in the presence of a Se atmosphere. Diffusion measurements yielded similar interdiffusion coefficients for Ga and In. The annealing temperature and time which were required in order to effect homogenization depended upon the Ga/(Ga+In) ratio of the absorber films. Films with lower Ga/(Ga+In) ratios required an homogenization temperature of 600C or above. Films with higher Ga/(Ga+In) ratios were homogenized at lower temperatures (400 to 500C, 1h).

M.Marudachalam, R.W.Birkmire, H.Hichri, J.M.Schultz, A.Swartzlander, M.M.Al-Jassim: Journal of Applied Physics, 1997, 82[6], 2896-905