A study on electrical transport vis-a-vis the effect of thermal annealing on the p-type conductivity in arsenic-doped MOCVD grown ZnO in the temperature range 10-300 K

摘要

Arsenic-doped p-type ZnO films were grown by MOCVD on semi-insulating GaAs substrates followed by thermal annealing. In order to study the dependence of charge transport on doping concentration, the carrier concentration in the films were varied by varying the post-deposition annealing temperature. The doped films showed p-type conductivity at room temperature with carrier concentration 4.5 × 10~(18)cm~(-3) (Sample A), 2.8 × 10~(19)cm~(-3) (Sample B) and 5.7 × 10~(19)cm~(-3) (Sample C) annealed, respectively, at 650 °C, 700 °C and 750 °C in oxygen ambient. To investigate the temperature dependent charge transport mechanism in the films, the dc conductivity has been measured over a wide range of temperature from 300 K down to 10 K. The experimental data for the films were found to follow the Mott's variable range hopping (M-VRH) type of conduction in the lower temperature region, whereas, in the higher temperature region the conductivity is governed by the thermally activated (TA) type of band conduction. It is found that for sample A, M-VRH is observed in the temperature 10-52 K whereas this upper-end temperature is reduced when the samples are annealed at higher temperatures. Thus for samples B and C, M-VRH is observed at 10-41 K and 10-35 K, respectively. On the other hand, TA type of conduction is observed in the temperature range 130-300 K, 116-300 K and 110-300 K for samples A, B and C, respectively. The decrease in the onset temperature for TA type of conduction with the increase in carrier concentration is due to high density of acceptor impurity levels close to the valence band, which facilitates the band conduction at a comparatively lower temperature. On the other hand, the average dis tance between the localized states near the Fermi level at low temperatures is comparatively small for high carrier concentration facilitating the M-VRH transport.