Interpretation of surface phenomena on ZnO by the compensation model

Field effect mobility and surface photoconductivity on ZnO crystals increase for several orders of magnitude with dark surface conductivity (electron accumulation). An explanation was given previously: Only a small increasing fraction of the injected electrons appears in the conduction band whereas the main part is trapped by surface states. In the new compensation model the trapping action of the surface states is replaced by that of bulk defects changing their condition of charge within an accumulation layer during variations of band bending: From the temperature dependence of con ductivity and Hall effect a set of three bulk defect levels is evaluated. The same set is used for a newcalculation of the accumulation layer and for a quantitative interpretation of photoconductivity and field effect measurements. Also surface photovoltage and photolysis data are discussed. All these experimental results are obtained on single crystals bearing an accumulation layer. Properties of thin polycrystalline ZnO films are also treated. The concept of a drift mobility is of central importance for the compensation model. As a result of compensation space charge layers are much thinner than previously assumed.