Study of phase separation in Ga25Se75−xTex chalcogenide thin films

The effects of composition and thermal annealing in between glass transition and crystallization temperature on the optical and structural properties of Ga25Se75−xTex were investigated. The glass transition and crystallization temperature of the synthesized samples was measured by non-isothermal DSC measurements. Amorphous thin films of Ga25Se75−xTex glasses were grown onto ultra clean glass/Si wafer (100) substrates using the vacuum evaporation technique. The effect of thermal annealing on the optical gap (Eg) for Ga25Se75−xTex thin films in the temperature range 358–388 K is studied. As-prepared and annealed thin films were characterized by X-ray diffraction, field emission scanning electron microscopy, energy dispersive X-ray spectroscopy and optical absorption. Thermal annealing was found to be accompanied by structural effects, which in turn, lead to changes in the optical constants. The optical absorption coefficient (α) for as-deposited and thermally annealed films was calculated from the absorbance data. From the knowledge of absorption coefficient at different wavelengths, the optical band gap (Eg) was calculated for all compositions of Ga25Se75−xTex thin films before and after thermal annealing. Results indicate that allowed indirect optical transition is predominated in as-deposited and thermally annealed thin films. The influence of Te incorporation and thermal annealing in Ga25Se75−xTex thin films results in a gradual decrease in the indirect optical gap, this behavior can be explained as increased tailing. The decrease in optical band gap and an increase in absorption coefficient and extinction coefficient with thermal annealing can be attributed to transformation from amorphous to crystalline phase.