Interface formation of Bi on ceramic ZnO: A simple model varistor grain boundary

Abstract

The Schottky barrier formation between Bi and polycrystalline, ceramic ZnO has been studied with photoelectron spectroscopy (PES) under ultrahigh-vacuum conditions. This system is a simple model of a varistor compound. Evaporation of Bi on highly n-doped (Al) sintered ZnO surfaces, fractured in situ and held at room temperature, results in considerable upward band bending. After evaporation of 10 Å of Bi (approximately 3 monolayer coverage), the Bi-induced band bending amounts to 0.43 eV, as evident from the energy shift of the Bi 5d emission in PES. From valence-band and band-gap studies using ultraviolet PES, the states responsible for the observed band bending could be identified: Bi induces states in the ZnO band gap at 0.9 eV above the valence-band maximum. The filling of this high density of band-gap states results in a pinning of the surface Fermi level which makes the band bending proportional to the Bi coverage, with a rapid increase during the formation of the first monolayer and markedly slower thereafter. These results show the importance of Bi in the formation of the varistor Schottky barriers.