Influence of composition on electrical and optical properties of new chalcogenide thin films from Ge-Se-Tl system

Bulk Ge20Se80−xTlx (x ranging from 0 to 15 at%) chalcogenide glasses were prepared by conventional melt quenching technique. Thin films of these compositions were prepared by thermal evaporation, on glass and Si wafer substrates at a base pressure of 10−6 Torr. X-ray diffraction studies were performed to investigate the structure of the thin films. The absence of any sharp peaks in the X-ray diffractogram confirms that the films are amorphous in nature. The optical constants (absorption coefficient, optical band gap, extinction coefficient and refractive index) of Ge20Se80−xTlx thin films are determined by absorption and reflectance measurements in a wavelength range of 400-900 nm. In order to determine the optical gap, the absorption spectra of films with different Tl contents were analyzed. The absorption data revealed the existence of allowed indirect transitions. The optical band gap showed a sharp decrease from 2.06 to 1.79 eV as the Tl content increased from 0% to 15%. It has been found that the values of absorption coefficient and refractive index increase while the extinction coefficient decreases with increase in Tl content in the Ge-Se system. These results are interpreted in terms of the change in concentration of localized states due to the shift in Fermi level. DC electrical conductivity of Ge20Se80−xTlx thin films was carried out in a temperature range 293-393 K. The electrical activation energy of these films was determined by investigating the temperature dependence of dc conductivity. A decrease in the electrical activation energy from 0.91 to 0.55 eV was observed as the Tl content was increased up to 15 at% in Ge20Se80−xTlx system. On the basis of pre-exponential factor, it is suggested that the conduction is due to thermally assisted tunneling of the carriers in the localized states near the band edges.