Electrical conduction and phase transitions in vacuum-deposited Cu_2-xS films

Abstract

The structure, phase transitions and electrical conductivity of Cu_2-xS films deposited by vacuum evaporation at different substrate temperatures were studied. It was found that the deposition parameters significantly affect the composition and the structure of evaporated Cu_2-xS films. The stoichiometry changes from copper rich to copper deficient as the substrate temperature is increased. This is explained in terms of the mechanism of growth of the Cu_2-xS films. The occurence of phase transitions in these films was studied through measurement of the electrical conductivity as a function of temperature. Films deposited at 300 K exist in the γ phase and undergo a γ →β phase transition at 325K and a β→α a phase transition at 350 K. Films deposited at 400 K exhibit only one phase transition, the β → α a transition at 345K. A very sluggish transition from the orthorhombic to the tetragonal phase of Cu_1.96S was observed at 340 K in films deposited at 475 K. These results were corroborated by electron microscope and diffraction studies on annealed Cu_2-xS films.