Phase change and optical band gap behavior of Ge-Zn-Te chalcogenide films

Glassy alloys with composition of Ge₂₀Zn_xTe_80-x (x=0, 5, 10, 15at%) were prepared by conventional melt-quenching method, which were subsequently used as target for thermal evaporation (TE) technique under vacuum. The Ge-Te-Zn chalcogenide thin films were characterized with XRD and Transmittance spectra. XRD measurements indicate that there is an amorphous-to-crystalline phase transition. The GeTe crystallines were suppressed in the film with x=5 after annealing while the GeTe crystallines become predominant and the formation of Te crystallines were suppressed with Zn content increasing. Optical band gap is estimated using Tauc´s extrapolation and is found to decrease from 0.509 to 0.395eV in as-deposited films with x=0, 5, 10., while the optical band gap increases to 0.555eV in as-deposited films with x=15. The similar behavior is also observed in the annealed films. A comparison of optical band gap of various films illustrates the thermally-induced effects.