Polaronic Hopping Conduction in Vanadium Phosphate Glasses

Abstract

Conductivity and dielectric properties of semiconducting glass of composition 80% V2O5:20% P2O5 have been measured as a function of frequency from dc to 3.6 GHz over a temperature range from 77°K to 420°K. A fit of the dc results to a model of polaronic hopping conduction leads to an activation energy due to disorder ΔW=0.09 eV at 77°K and an optical phonon energy of 0.053 eV compared to a calculated Miller-Abrahams energy of approximately 0.12 eV. The real and imaginary parts of the ac conductivity are shown to increase with frequency according to power law σ=Aωs, where s=0.85 to 0.95, and it is found that the dielectric behavior can be described by a Debye-type relaxation behavior only if a broad distribution of relaxation times exists. The results are shown to be consistent with a model of ac conduction normally applied to impurity-doped broad-band semiconductors, and differences between the temperature dependence of the dc and ac conductivity are attributed to a distribution of site energies in the glass. The estimated static dielectric constant is found to change from about 30 at room temperature to about 14 at 77°K and it is suggested that this temperature dependence affects calculations of both the disorder energy and the polaron binding energy. The coupling constant for the small polaron in the glass is about 14, indicating that it is a valid assumption to apply polaron theory to the material.