Annealing and thickness effect on the optical absorption of Ge

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