Electrical conduction effects at low temperatures in undoped ZnO thin films grown by Pulsed Laser Deposition on Si substrates

The electrical properties of undoped ZnO films grown by Pulsed Laser Deposition on Si substrates at growth temperatures between 150 and 250 °C and low O2 partial pressures, were studied by resistivity and Hall coefficient measurements in the temperature range of 80 to 350 K. We report acceptor band and hopping conduction effects in these ZnO films, for the first time. P-type conduction is found to be dominant in these films for temperatures higher than approximately 235 K, while at temperatures lower than 235 K the films exhibit a conversion from p- to n-type conductivity. The electrical conductivity studies revealed a conduction mechanism by hopping in the acceptor band in the temperature range between 80 and 270 K. For temperatures higher than 270 K a thermally activated behavior, between the native acceptor band (Zn vacancies) and the valence band is dominant. The acceptor activation energy values extracted from the lnp versus T− 1 curves were found to be of 0.1 eV. The electrical mobility values fall-off rapidly from 90 to 8 cm2/V s for temperatures below 270 K, providing evidence that a mobility edge exists between transport in the valence band and transport in the acceptor band.