Material grown at highly Zn-rich conditions in reactive sputtering of ZnO thin films resulted in mixed conduction, indicating that stable p-type ZnO could be produced. In n-type conductivity, neutral flaw scattering transport mechanism via VO0 centres seems to be dominant due to the existence of oxygen vacancies in high concentrations. An exponential decrease in electron mobility was observed upon cooling from room temperature to 210K while the concentration of the inactive VO0 state increases. This was also a cause of p-type conduction in the low temperature range (<170K). Mobility of holes had rather usual behaviour as the longitudinal acoustic phonon scattering takes place at 35 to 170K. Ionized acceptor scattering was shown to be effective for holes below 35K. P-type conduction was not affected by VO0 scattering. Quantitative evaluations of VO centres showed that fractional distribution of VO0, VO+ and VO2+ charge states are, respectively, around 4%, 95% and 1% of the total [VO] under the room temperature conditions. The energy of phonons interacting with the centre was estimated to be 38.5meV which was a local phonon mode relaxation, most probably resulting in negative-U behaviour of VO centres.

Determination of the Transport Mechanisms in Mixed Conduction of Reactively Sputtered ZnO Thin Films. Tüzemen, S., Gür, E., Doğan, S.: Journal of Physics D, 2008, 41[13], 135309