Hall mobility of electrons in quantized accumulation layers on ZnO surfaces

Hall-effect measurements were performed on the (0001̄) face of ZnO single crystals under extreme accumulation layer conditions with surface electron densities ranging from {~}10¹² up to {~}10¹⁴ cm^-2. The measurements were performed as a function of the surface-electron density and temperatures in the temperature range of 2-300 K. The electron density in the accumulation layer was found to be essentially temperature independent over the whole temperature range. The electron mobility, on the other hand, is temperature dependent for surface-electron densities up to {~}10¹³ cm^-2, decreases with decreasing temperatures and at low temperatures carrier localization occurs. As a function of the surface-electron density (at a fixed temperature) the mobility initially increases, reaches a maximum, and then slowly decreases. The results suggest scattering and/or localization by charged scattering centers which apparently conglomerate into large clusters.