Electrical characterization and Poole-Frenkel effect in sol-gel derived ZnO:Al thin films

Electrical properties of ZnO:Al thin films, prepared by sol-gel dip-coating technique were studied in the range of 0.32% to 1.62% Al concentrations in the films. Room temperature electrical conductivity was found in the range of 0.08 to 1.39 S/cm for different aluminium concentrations in the films. I-E characteristics of the films at a constant temperature showed non-linearity, while non-linearity becomes more and more pronounced with increase in temperature and this could be explained by Poole-Frenkel model of thermionic emission. Presence of adsorbed oxygen and excess Al atoms at grain boundaries is assumed to be the cause of this effect. These atoms produce defect levels, which trapped electrons and created a potential barrier across the grain boundaries. In the presence of an external field, the barrier height was attenuated, resulting in the thermionic emission of electrons from the trapped level to the conduction band. The trapped potentials (φt) were calculated for different doping concentrations in the films. The thermoelectric power (TEP) measurement confirmed the n-type nature of the films and the room temperature Seebeck coefficient was found to be −91.65 μV/K.