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

Glassy alloys with composition of Ge
₂₀
Zn
_x
Te
_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.