Study of quasi-amorphous to nanocrystalline phase transition in thermally evaporated CuInS₂ thin films

CuInS₂ thin films with thickness ranging from 196 to 1000 nm were prepared from a source containing CuInS₂ nanocrystals by using thermal evaporation method. Annealed films of CuInS₂ show the quasi-amorphous to crystalline phase transition, probed through x-ray diffraction (XRD), UV-visible spectrometer, and Raman spectroscopy. From XRD, the tetragonal distortion (η) is found to be ≈1, confirming the arrangement of an extended double lattice structure of chalcopyrite phase. The surface morphology of quasi-amorphous film exhibits a very smooth surface, whereas crystalline film shows a very rough surface of CuInS₂ as observed from atomic force microscopy. Crystallite size and rms roughness increase from 23 to 310 nm and from 1.5 to 36.5, respectively, with increasing film thickness as well as with increasing annealing temperature due to the crystallization process. Micro-Raman study evidencing the presence of a strong Raman A₁ mode at 303 cm^-1, due to the symmetric vibration of anion sublattice of CuInS₂ structure. A fundamental band edge is observed in as-deposited quasi-amorphous CuInS₂ films, while bulk energy band absorption and excitonic band transition are observed in crystalline films. A sharp drop in both reflectance and transmittance near the energy band gap region is observed in thick films due to a very strong absorption of crystalline phase of CuInS₂.