Thin films of Ge20Te80 and Cu6Ge14Te80 of different thicknesses are deposited on glass substrate by thermal evaporation under vacuum. The effect of
incorporation of copper in Ge20Te80 film is studied by measuring the optical
absorption. The mechanism of optical absorption follows the rule of direct
transition. The films are annealed at different elevated temperatures from 370
to 520 K. The measurements were carried on as-deposited and annealed
specimens. The optical energy gap (Eg) was found to decrease with increasing
the annealing temperatures in the case of Ge20Te80 films. But in the case of
Cu6Ge14Te80 films, Eg first increases with annealing temperature up to 410 K,
then decreases sharply after further increasing the annealing temperature above
the glass transition temperature. The decreases of Eg and the increase of the
width of localized states Ee could be attributed to the amorphous - crystalline
transformation. The values of optical energy gap Eg are also found to increase
with thickness of both two-type films. The effect of films thickness on optical
energy gap (Eg) of the films is interpreted in terms of the density of state model
of Mott and Davis. The refractive index n, extinction coefficient k and dielectric
constant $\varepsilon_{\rm i}$ and $\varepsilon_{\rm r}$ are also calculated for all samples.