H and Li Related Defects in ZnO and Their Effect on Electrical Properties

Abstract

Li and H are important electrically active impurities in ZnO and this work presents a detailed experimental and computational study of the behavior of H and Li in ZnO and their effect on its defect structure. We employ AC conductivity measurements as a function of temperature and partial pressure of O2, H2O, and D2O, which is combined with first principles density functional theory (DFT) calculations and thermodynamic modeling (TDM) of finite temperature defect structures in undoped and Li doped ZnO. Undoped ZnO is dominated by protons as hydroxide defects (OHO•), oxygen vacancies (vO••), and electrons under a large variety of atmospheric conditions, and we also predict from DFT and TDM the substitutional hydride ion (HO•) to dominate concentration-wise under the most reducing conditions at temperatures above 500 °C. The equilibrium concentrations of defects in ZnO are small, and dopants such as Li strongly affect the electrical properties. Experimentally, Li doped ZnO is found to be n-type under all available atmospheric conditions and temperatures, with an n-type conductivity significantly lower than that of as-grown ZnO. The n-type conductivity also increases with decreasing pO2 and with increasing pH2O. The observed electrical properties of Li doped ZnO are attributed to dominance of the ionic defects LiZn/, OHO•, Lii•, vO••, and the neutral complexes (LiZnOHO)× and (LiZnLii)×. Although Li doping lowers the Fermi level of as-grown ZnO significantly, low formation energy of the ionic donors, and passivation of LiZn/ in the form of (LiZnOHO)× and (LiZnLii)×, prevents realization of significant/stable p-type activity in Li doped ZnO under equilibrium conditions.