Optical properties of co-evaporated CuInSe2thin films

Thin films of CuInSe 2 within a wide composition range around stoichiometry have been deposited by Cu, In and Se co-evaporation. Their optical properties in the near-infrared have been studied in relation to the composition and the deposition process parameters. The refractive index and the absorption coefficient below the absorption edge, depend on the Cu-In percentages ratio and show a minimum at stoichiometry. Moreover, the refractive index increases with the deposition temperature. Beyond the absorption edge the absorption is due to an allowed direct transition and a forbidden direct transition with gap energies around 1.0 and 1.2 eV, respectively. The values of the optical gaps depend on the Cu-In percentages ratio and show a minimum at stoichiometry. These dependences have been correlated with the variations of the crystalline lattice constant with the Cu-In ratio.