Thermal effect on the structure and optical properties of Ge-Te-In thin films

Amorphous thin films with composition of Ge61.44Te31.39In7.17 were prepared by magnetron co-sputtering method on chemically cleaned glass substrates. The effect of thermal treatment on structure and optical properties had been investigated. As-deposited and annealed thin films were investigated by X-ray diffraction (XRD), Scanning electron microscope (SEM), Visible/near-infrared transmission spectroscopy and Raman spectra. XRD and SEM results show that an amorphous-to-crystalline phase transition occurs after annealing and the crystalline degree increases with increasing annealing temperature, which in turn, lead to the change in the optical constant. According to the classic Tauc equation, it can be found that the optical band gap of the thin films rises with increasing annealing temperature up to 150 °C and then drastically decreases with temperature beyond 200 °C. This behavior is interpreted on the basis of the change in the structure of the thin films, from amorphous to crystalline state, with increasing the annealing temperature. Raman spectra studies reveal that the formation of c-GeTe is mainly responsible for the macroscopic phase transition during the crystallization process.