Correlation between electrical properties and point defects in NiO thin films

The wide band-gap semiconductor NiO has p-type characteristics and is an alternative to p-ZnO, due to conduction mechanisms coming from the Ni vacancies and O interstitials. The correlation between the electrical properties and point defects was studied with NiO thin films grown by sputtering at various temperatures and in pure Ar and O₂ atmospheres. The p-type characteristics of the NiO thin films were double-checked by Hall-effect and Seebeck coefficient measurements. Below 300 C, the electrical resistivity of NiO films grown in an O₂ atmosphere was lower than those grown in an Ar atmosphere due to the suppressed point defects. On the other hand, the electrical resistivity over 300 C became semi-insulating due to the relatively stoichiometric NiO. The optical transmittance of the NiO film deposited in an O₂ atmosphere and 600 C was around 80% in the visible region. Finally, the p-NiO/n-ZnO heterojunction diodes showed a well-rectifying current-voltage curve.