Transparent Ohmic contacts of oxidized Ru and Ir on <i>p</i>-type GaN

Low-resistance, high-transparency, and thermally stable Ohmic contacts on p-type GaN were achieved using Ru and Ir. Oxidation annealing under O2 atmosphere led to the reduction of contact resistivity by greater than one order of magnitude, compared to annealing under N2. The bilayer contacts of Ru (50 Å)/Ni (50 Å) and Ir (50 Å)/Ni (50 Å) exhibited a low contact resistivity of ∼4×10−5 Ω cm2 and high light transmittance of ∼85% after annealing at 500 °C for 1 min under O2. The barrier height for hole injection could decrease via contact formation of RuO2 (or IrO2) on p-type GaN. The Au-free contact structure of NiO/RuO2 (IrO2)/GaN led to high light transmittance and good thermal stability.