Field-effect mobility in quantized accumulation layers on ZnO surfaces

Measurements are reported of the field-effect mobility ${\mu }_{\mathrm{FE}}$ in strong accumulation layers on the polar surfaces of ZnO crystals. Results are presented of ${\mu }_{\mathrm{FE}}$ as a function of surface electron density $\Delta N$ (in the range ${10}^{12}$-${10}^{14}$ ${\mathrm{cm}}^{-2\mathrm{}}$) and of temperature (2-300 K). By a suitable integration procedure it has been possible to derive from the field-effect data the ordinary conductivity mobility $\mu$ as a function of $\Delta N$ and temperature. At a fixed temperature both ${\mu }_{\mathrm{FE}}$ and $\mu$ initially rise with increasing $\Delta N$, reach a maximum at $\Delta N\approx \mathrm{}(2-5)\mathrm{}\times {10}^{13}$ ${\mathrm{cm}}^{-2\mathrm{}}$, and then gradually decrease with a further increase in $\Delta N$. For high $\Delta N$ (≳ ${10}^{13}$ ${\mathrm{cm}}^{-2\mathrm{}}$) ${\mu }_{\mathrm{FE}}$ and $\mu$ are practically temperature independent; for low $\Delta N$ (≲ 3 × ${10}^{12}$) they decrease strongly with decreasing temperature, indicating carrier localization at low temperatures. The results of the conductivity mobility $\mu$ agree well with those of the Hall mobility ${\mu }_H$ reported earlier, and provide further support for the model proposed there of charged scattering centers consisting of large conglomerates of surface ions.