Evidence of the Zn Vacancy Acting as the Dominant Acceptor in $n$-Type ZnO

We have used positron annihilation spectroscopy to determine the nature and the concentrations of the open volume defects in as-grown and electron irradiated ($E_{\mathrm{e}\mathrm{l}}=2\mathrm{M}\mathrm{e}\mathrm{V}$, fluence $6\times {10}^{17}{\mathrm{c}\mathrm{m}}^{-2}$) ZnO samples. The Zn vacancies are identified at concentrations of $[V_{\mathrm{Z}\mathrm{n}}]\simeq 2\times {10}^{15}{\mathrm{c}\mathrm{m}}^{-3}$ in the as-grown material and $[V_{\mathrm{Z}\mathrm{n}}]\simeq 2\times {10}^{16}{\mathrm{c}\mathrm{m}}^{-3}$ in the irradiated ZnO. These concentrations are in very good agreement with the total acceptor density determined by temperature dependent Hall experiments. Thus, the Zn vacancies are dominant acceptors in both as-grown and irradiated ZnO.

Figure 1

The average positron lifetime of the as-grown ZnO sample as a function of the measurement temperature. The dashed line shows the temperature dependence of the positron lifetime in the ZnO lattice.

Figure 2

The average positron lifetime of the as-grown and electron irradiated ZnO samples as a function of the measurement temperature. The higher lifetime component corresponding to positrons trapped at vacancies is shown in the top panel.

Figure 3

Lower part: the measured core electron momentum distributions at the defect and in the perfect lattice. Upper part: the calculated core electron distributions at the oxygen vacancy, zinc vacancy, and in the perfect lattice.