The transmission and reflection spectra of the chalcogenide Te67.5 Ga2.5 As30 thin film with thickness of
150, 300 and 450 nm are measured. The formed crystalline phases due to the thermal annealing of the
as-prepared film are identified by X-ray diffraction. The as-prepared films have absorption mechanism
which is an indirect allowed transition with a decrease in the value of the optical energy gap (Eg) as
the thickness increases. The films annealed at temperatures higher than the temperature of the onset
of the crystallization have a direct allowed transition with an increase in Eg with annealing temperatures.
The effect of the film thickness on the refractive index and the high-frequency dielectric constant are
studied.