Photoemission study of evaporated CuInS2 thin films. I. Surface stoichiometry and phase segregation

Abstract

The surfaces of evaporated CuInS2 films were investigated by photoelectron spectroscopy (XPS) with respect to stoichiometry dependent features. The surface composition of In-rich CuInS2 films drastically exceeds the respective bulk value determined by energy dispersive x-ray fluorescence and displays the formation of an In-rich surface layer with a cation ratio of In/(In+Cu)=0.75(2). Sputter profiles obtained by Auger electron spectroscopy confirm the effect of In enrichment and Cu depletion on the surface. The altered surface region is estimated to extend approximately 0.1μm into the film. Films with a Cu-rich bulk composition also exhibited In-rich surfaces, but in addition the segregation of a CuS phase is concluded from XPS investigations. The interrelation of these observations is discussed using simple model considerations.