TRANSPORT-PROPERTIES OF VANADIUM GERMANATE GLASSY SEMICONDUCTORS

Measurements are reported for the dc as well as frequency-dependent (ac) conductivities (real and imaginary parts) for various compositions of the vanadium germanate glassy semiconductors in the temperature range 80 450 K. The experimental results are analyzed with reference to various theoretical models proposed for electrical conduction in amorphous semiconductors. The analysis shows that at high temperatures the temperature dependence of the dc conductivity is consistent with Motts model of phonon-assisted polaronic hopping conduction in the adiabatic approximation, while the variable-range-hopping mechanism dominates at lower temperatures. Schnakenbergs model predicts the temperature dependence of the observed activation energy in the intermediate temperature range. The temperature dependence of the ac conductivity is consistent with the simple quantum-mechanical tunneling model at lower temperatures, although this model cannot predict the observed temperature dependence of the frequency exponent. The overlapping-large-polaron tunneling model can explain the temperature dependence of the frequency exponent at low temperature; however, this model predicts a temperature dependence of the ac conductivity much higher than the observed data show. On the other hand, the correlated-barrier-hopping model is consistent with the temperature dependence of both the ac conductivity and its frequency exponent over the entire temperature range of measurements. © 1990 The American Physical Society.