Enhanced p-type conductivity and band gap narrowing in heavily Al doped NiO thin filmsdeposited by RF magnetron sputtering

Abstract

Stoichiometric NiO, a Mott–Hubbard insulator at room temperature,shows p-type electrical conduction due to the introduction ofNi2+ vacancies(VNi′′) andself-doping of Ni3+ ions in the presence of excess oxygen. The electrical conductivity of this important materialis low and not sufficient for active device fabrication. Al doped NiO thin films were synthesizedby radio frequency (RF) magnetron sputtering on glass substrates at a substrate temperature of250 °C in anoxygen + argon atmosphere in order to enhance the p-type electrical conductivity. X-ray diffractionstudies confirmed the correct phase formation and also oriented growth of NiO thin films.Al doping was confirmed by x-ray photoelectron spectroscopic studies. The structural,electrical and optical properties of the films were investigated as a function of Al doping(0–4 wt%) in the target. The room temperature electrical conductivity increased from0.01–0.32 S cm −1 for 0–4% Al doping. With increasing Al doping, above the Mott critical carrier density,energy band gap shrinkage was observed. This was explained by the shift of the band edgesdue to the existence of exchange and correlation energies amongst the electron–electron andhole–hole systems and also by the interaction between the impurity quasi-particle system.