The exact charge density of intrinsic point defects in such oxides, and the Volkenstein phenomenological electron theory of chemisorption, were used to treat acceptor-like and donor-like chemisorption on metal-oxide thin films; for various ratios of film thickness to Debye length. The experimentally observed temperature and O partial pressure dependences of the average electron concentrations of polycrystalline ZnO films were explained in terms of 3 different models: the Schottky defect model, the Volkenstein model for electronic equilibrium, and the comprehensive model for complete equilibrium. It was found that a Schottky defect model that involved single-crystal mass action constants did not yield the experimentally observed high-temperature limit. This limit was obtained by using a higher averaged concentration of O vacancies for polycrystalline films; due to the presence of grain boundaries. A comparison of the electronic and complete equilibria showed that screening of the surface charge by positively charged mobile O vacancies had the effect of reducing the surface potential by about 30% within the temperature range that was considered.

H.Geistlinger: Journal of Applied Physics, 1996, 80[3], 1370-80