Composition and electric field effects on the transport properties of Bi doped chalcogenide glasses thin films

The temperature dependence of the DC electrical conductivity sigma(DC) was measured in the temperature range from 300-500 K. It was found that there are double activation energies, E-sigma for Ge20Se80-xBix (x = 0, 2.5 and 5 at%) films, while there is single activation energy for Ge20Se72.5Bi7.5. when incorporation of Bi = 7.5 at%, the pre-exponential value sigma(0) decreases by about six order of magnitude, the activation energy in the extended states E-sigma decreases from 0.96 to 0.09eV. Also the effect of applied electric field was studied and observed that, activation energy in high temperature region increases with increasing electric field; this behavior can be understood assuming that the contribution to the conductivity activation process is due to conduction in the extended states and also due to hopping in the localized states. With the increasing electric field, as former process, which is having high activation energy, becomes more predominant due to the dumping of the carriers in the extended states, the effective activation energy of the system increases, in spite of the fact that the activation energy of the extended states conduction may remain constant. Finally, the electrical data suggests that the addition of bismuth produces localized states near the conduction band edge so that the electrical transport is due to hopping of electrons after being excited into localized states at the conduction band edge. (c) 2006 Elsevier B.V. All rights reserved.