The effect of deep surface acceptor states on the temperature-dependent conductivity of zinc oxide nanoparticles

An experimental and theoretical study was conducted on the temperature dependence of the conductivity of zinc oxide (ZnO) layers in dry air and ethanol. The study found that the conductivity of ZnO depends non-linearly on temperature, and its type varies depending on experimental conditions. Scanning speed directly affects the type of the temperature dependence, with a local minimum in conductivity between 250±5°C at a scanning rate of 0,4°C/min. However, monotonic dependencies are observed at slower scanning rates, such as 30°C/min, and a mechanism is proposed to explain the effect of the scanning velocity on the type of dependency. This mechanism involves the process of the thermal activation of oxygen molecules to form atomic oxygen at adsorption sites. The key idea behind this mechanism is that atomic oxygen forms deeper acceptor levels on zinc oxide surfaces compared to molecular oxygen. At a high scanning rate, the relaxation time for filling the surface with adsorbed oxygen is significantly longer than the experimental time. Therefore, metastable «frozen» states of atomic forms of adsorbed oxygen appear on the surface of zinc oxide layers. As a result, metastable «frozen» states of adsorbed atoms appear on the surface of zinc oxide layers. Based on calculations using this model, we propose a method for reducing the number of nonequilibrium atomic oxygen states by heating samples in a reducing medium.