Influence of annealing temperature on the phase composition and optical properties of CuInS2 films

We studied the growth of CuInS2 thin films by single-source evaporation of CuInS2 powder in a high-vacuum system with a base pressure of 10(-3) Pa. After evaporation, the films were annealed in a sulfur atmosphere at temperatures from 200 to 500 degrees C for 1 h. XRD curves and Raman spectra of the films demonstrated that chalcopyrite CuInS2 was the major crystalline phase. The morphology of CuxS exhibited a star-like structure, which we report for the first time. The phase composition and optical properties of our polycrystalline thin films were effectively modified by annealing in S. For films annealed at 200 and 350 degrees C, a secondary CuIn11S17 phase appeared, which may be related to solid-state reaction in the S atmosphere. This secondary CuIn11S17 phase has not been widely reported in previous studies. After annealing at 500 degrees C, only a chalcopyrite phase was detected, with bandgap energy of 1.46 eV, which is nearly identical to the optimal bandgap energy (1.5 eV) of single-crystal CuInS2. This indicates that the composition of the CuInS2 film annealed at 500 degrees C was nearly stoichiometric. The bandgap of the samples first increased and then decreased with increasing annealing temperature, which may be attributed to an increase in grain size, the secondary CuIn11S17 phase, and deviation from stoichiometry. (C) 2013 Elsevier Ltd. All rights reserved.