Electrical conductivity and thermoelectric power of a-Ge20 Se80 − xBix thin films

The dc electrical conductivity and thermoelectric power of a-Ge20Se80 − xBix (x = 0, 4, 6, 8, 10, 12) thin films are reported in the present work. The thin films were deposited by flash evaporation at 10− 5 Torr pressure and were well-characterized taking XRD, XRF, DSC and EPMA measurements of the system. The dc conductivity was measured over a temperature range 77 to 476 K. Conduction type and activation energies of electrical conductivity have been determined. The electrical transport takes place via two modes extended state conduction at higher temperatures and variable range hopping at lower temperatures. The conductivity was found to change by few orders of magnitude with Bi doping and the electrical activation energies (Δ Eσ) were found to decrease with increasing Bi content. The density of localized states and pre-exponential factor were determined. The thermopower measurements carried out using differential dc method in the temperature range 4.2 to 300 K and the activation energy (Δ Es) for TEP determined. The change in band gap with increasing Bi content is due to increased band tailing and increase of Bi–Se bonds and decrease of Se–Se bonds thus leading to the modification of the network structure of Ge20Se80 system.