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The discharge of negatively charged ZnO powder samples has been measured. The shape of the recorded discharge curves predicts a charge decay which is governed by two different mechanisms. Experiments with a point counter indicate that there is emission of negative charge during the first minutes of the discharge in the dark, as well as under illumination. When, simultaneously with the latter experiments, the discharge curve of the sample is recorded, it can be seen that emission of negative charge cannot be responsible for the complete discharge of the sample. Evidence is presented that there is a second mechanism which corresponds to the final part of the recorded decay curve. A study of the light decay curves of ZnO layers indicates that this second discharge mechanism is very sensitive to light in the fundamental absorption edge (380 nm) Even so yellow ZnO seems to be very sensitive to illumination in the b band (400 nm) We observed the spectral sensitivity of the slope of the final part of the decay curves. As a result of these experiments we concluded that in the second mechanism the negative surface charge is neutralized by means of positive charges present in the surface zone of ZnO Finally the complete discharge in the dark as well as under illumination can be described as follows: after the height of the potential barrier has been decreased considerably as the result of discharge by emission a second mechanism becomes important In this mechanism electrons from the negative surface charge can easily pass the surface barrier and recombine with positive charges in the surface zone of ZnO.
© 1969 Optical Society of America
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