New experimental results for electron transport in weak accumulation layers of ZnO crystals

Abstract Previous Hall measurements on (0001) and (0001) faces of ZnO have shown a Hall mobility oscillating as a function of Hall surface electron density in the range between N SH =10 6 and 10 11 cm −2 . Here we report on new results obtained by a field effect arrangement for free surfaces in UHV. With donors from H exposure or by illumination weak accumulation layers ( n sh 11 cm −2 ) are established. The field effect shows oscillations in surface conductivity as a function of gate voltage. Also the combination of a field effect with a Hall effect measurement reveals discrete values of Hall surface electron density n sh . Various pretreatments do not change the periodicity of these oscillations. Necessary preconditions are a temperature below 130 K, a Hall surface electron density below 3 × 10 12 cm −2 and a source-drain field of a few V/cm. A model regarding impurity levels in the space charge layer relates the results of the field effect measurements to the results of the Hall effect measurements.