AC PROPERTIES OF ZNO THIN-FILMS PREPARED BY RF MAGNETRON SPUTTERING

As part of a programme on the development of ZnO-based varistors, the electrical properties of ZnO/Bi2O3 junctions are currently being studied. In this work, measurements on the a.c. properties of ZnO sandwich structures with ohmic aluminium electrodes are reported. The conductivity sigma(omega) was found to increase with angular frequency omega according to the relation sigma(omega) = A omega(s) where s is an index which increases with frequency and decreases with temperature. In conjuction with activation energy measurements, these results are explained in terms of conduction via a band of localized states at lower temperatures and by a thermally activated free band process at higher temperatures. Both the capacitance and the loss tangent showed a decrease with increasing frequency and an increase with increasing temperature in accordance with existing theory for the case of a thermally activated process using ohmic electrodes. Following recent work on anodic Bi2O3 films, it is shown that there is a linear relationship between the logarithm of the dielectric relaxation time and the inverse temperature; these results were interpreted in terms of the Eyring expression with typical values of enthalpy Delta H = 110 kJ mol(-1) (activation energy 1.14 eV) and entropy Delta S = 135 J mol(-1) K-1.