Influence of donor density in ZnO on ZnO–PrCoO_x thin-film junctions: Role of O₂ and Al dopant in varistors

ZnO/PrCoO_x and ZnO/PrCoO_x/ZnO junctions have been fabricated by sputtering as a model of a single grain boundary in a ceramic ZnO varistor. The relations between barrier parameters and varistor characteristics were investigated using voltage-current (V-I) capacitance-voltage (C-V), and deep-level transient spectroscopy (DLTS) measurements. The varistor voltage of the junctions increases as the donor density (N_D) of the ZnO film decreases. The interface states vary according to the method of ZnO sputtering. A clear correlation has been established between the α value and the interface states. The highest α value is obtained when N_D is ≃ 10¹⁸ cm⁻³, the interface level is 0.70 eV, and the breakdown voltage is 3–4 V. Oxygen is effective on control of N_D in ZnO and the interface states. Al added as a dopant is also effective in terms of its ability to increase N_D in ZnO. However, Al doping was found to degrade the interface states and increase the leakage current.