Nickel vacancy acceptor in nickel oxide: Doping beyond thermodynamic equilibrium

Abstract

This work reports on temperature-induced out-diffusion and concentration decay of the prominent intrinsic point defect ${\mathrm{V}}_{\mathrm{Ni}}$ (nickel vacancy) in the wide-gap $p$-type semiconductor nickel oxide (NiO). ${\mathrm{V}}_{\mathrm{Ni}}$ can easily be introduced into NiO thin films by offering high oxygen reactivity during film growth, rendering nonstoichiometric semiconducting material. However, exposure to lower oxygen reactivity after growth, e.g., in a standard atmosphere, usually leads to a gradual decrease of film conductivity, because the vacancy concentration returns to its thermodynamic equilibrium value. In this study we observe this process in situ by performing temperature-dependent measurements of the electrical conductivity on a room temperature-grown NiO film. At a temperature of 420 K under exclusion of oxygen, the doping level decreases by a factor of 8 while the associated room temperature DC conductivity drops by six orders of magnitude. At the same time, out-diffusion of the mobile ${\mathrm{V}}_{\mathrm{Ni}}$ species can be indirectly observed through the occurrence of electrode polarization characteristics.