Poole-Frenkel effect in nanocrystalline SnO2: F thin films prepared by a sol-gel dip-coating technique

The electrical transport properties of fluorine-doped tin oxide thin films prepared by a sol-gel dip-coating technique have been studied in detail. X-ray diffraction patterns confirm the crystalline nature of the films having a grain size in the nanometer range. The fluorine concentration in the films was varied from 1.62 to 12% and was measured by energy dispersive X-ray analysis. It was observed that the current-voltage(I-V) characteristics of the films were non-linear in nature, which could be explained by the Poole-Frenkel model of thermionic emission. With an increase in temperature, the non-linearity becomes more and more pronounced. The presence of adsorbed oxygen and fluorine atoms at grain boundaries is assumed to be the cause of this effect. These atoms produce defect levels, which trap electrons and create a potential barrier across the grain boundaries. In the presence of an external field, the barrier height is attenuated, resulting in the thermionic emission of electrons from the trapped level to the conduction band. The trapped potentials (phi(t)) are calculated for different doping concentrations in the films. These values range from 589 to 703 meV. The maximum room temperature conductivity of the films was found to be of the order of 1 Omega(-1) cm(-1). Activation energies (E-a), calculated from Arrhenius plots, range from 85 to 130 meV for different fluorine concentrations in the films. (C) 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.