Electrical and optical characteristics of Au/Ni ohmic contacts to p-GaN

Optical transmittance, sheet resistance and specific contact resistance of Au/Ni ohmic contacts to p-GaN have been investigated. Au/Ni films of four different thickness combinations with 20 nm total thickness were deposited by e-beam evaporation. Au/Ni/p-GaN were annealed in flowing N₂ ambient for 10 min at temperature range of 300∼700°C. Before annealing Ni film absorbs more visible light than Au film. Ni oxidizes into transparent NiO after annealing and the light transmittance of the whole metal contact increases, however, the transmittance of oxidized Au/Ni contact is limited by Au film thickness. Thicker Au film can retard the oxidation rate of inner Ni film during annealing, enhance the activation efficiency of Mg dopant in p-GaN, and maintain good thermal stability of Au/Ni sheet resistance. Sheet resistance of p-GaN decreases drastically after annealing Au/Ni/p-GaN at temperatures above 500°C, this implies that p-GaN is effectively activated above 500°C. Considering to obtain good combination of optical transmittance, low sheet resistance and low contact resistance, Au(10 nm)/Ni(5 nm)/p-GaN ohmic contacts have better electrical and optical characteristics, and thermal stability. After annealing Au(10 nm)/Ni(5 nm)/p-GaN at 500°C, transmittance is more than 70% in the blue light regime, metal film sheet resistance is about 14 Ω/sq., and specific contact resistance is about 10^-3 Ω-cm².