Field-effect mobility in quantized accumulation layers on ZnO surfaces

Measurements are reported of the field-effect mobility FE in strong accumulation layers on the polar surfaces of ZnO crystals. Results are presented of FE as a function of surface electron density N (in the range 1012-1014 cm-2) 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 as a function of N and temperature. At a fixed temperature both FE and initially rise with increasing N, reach a maximum at N (2-5)×1013 cm-2, and then gradually decrease with a further increase in N. For high N (1013 cm-2) FE and are practically temperature independent; for low N ( 3 × 1012) they decrease strongly with decreasing temperature, indicating carrier localization at low temperatures. The results of the conductivity mobility agree well with those of the Hall mobility H reported earlier, and provide further support for the model proposed there of charged scattering centers consisting of large conglomerates of surface ions.