Low-resistance ohmic contacts to p-ZnMgO grown by pulsed-laser deposition

Abstract

Electron-beam deposited Ni∕Au is found to produce ohmic contacts to p-type ZnMgO (p∼1016cm−3) after annealing in the range 300–450°C. A minimum specific contact resistance of ∼10−2Ωcm2 at room temperature and of 4×10−5Ωcm2 at 473K from circular transmission line measurements was obtained after annealing at 350°C for 1min in air. Higher anneal temperatures produced higher contact resistances and reaction of the contact metallurgy with diffusion of Ni to the surface of the Au layer where it became oxidized. The effective barrier height obtained from the measurement temperature dependence of the specific contact resistance, assuming thermionic emission, showed a value of 0.39±0.01eV, much lower than the theoretical value of ∼2.4eV. The difference in theoretical and experimental values is a clear indication of the strong role played by surface states, as reported earlier for Au Schottky contacts on ZnO.