Rapid thermal annealing induced changes on the contact of Ni/Au to N-doped ZnO

N-doped p-type ZnO (p ∼ 10¹⁸cm^-3) was grown on sapphire(0 0 0 1) substrate by metal-organic chemical vapor deposition method. Ni/Au metal was evaporated on the ZnO film to form contacts. As-deposited contacts were rectifying while ohmic behavior was achieved after thermally annealing the contacts in nitrogen environment. Specific contact resistance was determined by circular transmission line method and a minimum specific contact resistance of 8 × 10⁻⁴ Ω cm² was obtained for the sample annealed at 650 °C for 30 s. However, Hall effect measurements indicate that, as the rapid thermal annealing temperature increased up to 550 °C or higher the samples’ conductive type have changed from p-type to n-type, which may be due to the instability nature of the present-day p-type N-doped ZnO or the dissociation of ZnO caused by annealing process in N₂ ambient. Evolution of the sample's electric characteristics and the increment of metal/semiconductor interface states induced by rapid thermal annealing process are supposed to be responsible for the improvement of electrical properties of Au/Ni/ZnO.