Determination of the transport mechanisms in mixed conduction of reactively sputtered ZnO thin films

Material grown at highly Zn-rich conditions in reactive sputtering of ZnO thin films resulted in mixed conduction, indicating that stable p-type ZnO can be produced. In n-type conductivity, neutral flaw scattering transport mechanism via V-O(0) centres seems to be dominant due to the existence of oxygen vacancies in high concentrations. An exponential decrease in electron mobility is observed upon cooling from room temperature to 210 K while the concentration of the inactive V-O(0) state increases. This is also a cause of p-type conduction in the low temperature range (< 170 K). Mobility of holes has rather usual behaviour as the longitudinal acoustic phonon scattering takes place at temperatures 35-170 K. Ionized acceptor scattering is shown to be effective for holes below 35K. P-type conduction is not affected by V-O(0) scattering. Quantitative evaluations of V-O centres show that fractional distribution of V-O(0), V-O(+) and V-O(++) charge states are, respectively, around 4%, 95% and 1% of the total [V-O] at the room temperature conditions. The energy of phonons interacting with the centre is estimated to be 38.5 meV which is a local phonon mode relaxation, most probably resulting in negative-U behaviour of V-O centres.