Electronic conduction in Bi-modified amorphous thin films of Ge20Te80-xBix exhibiting an absence of a p-n transition

Ac conductivity and dielectric relaxation studies of well-characterized Bi-modified amorphous thin films of Ge20Te80-xBix (x = 0, 0.19, 2.93 and 7.35) prepared by hash evaporation, exhibiting an absence of p-n transitions in the frequency range 500 Hz - 10 kHz and the temperature span 180 K - 450 K, are reported. The ac conductivity, sigma(ac)(omega), is found to be proportional to omega(s). The temperature dependences of sigma(ac)(omega) and the exponents are interpreted using the correlated-barrier hopping model. It is revealed that electronic conduction takes place via bipolaron/single polaron hopping processes at low/high temperatures in all the compositions. At a lower Pi concentration (x = 0.19), Bi-induced defects that take part in single polaron hopping are produced. Further increases in the Pi concentration (x = 2.93 and 7.35) cause little change in the density of such defects. The density of defect states taking part in bipolaron conduction is not significantly affected by addition of Bi atoms to the ahoy. This feature of the Bi impurity is similar to the effect of the addition of a Bi impurity on the thermoelectric power, de conductivity and optical energy gap.