Ultrahigh transparency of Ni/Au ohmic contacts to surface-treated p-type GaN

We report on ultrahigh-transparency and low-resistance Ni/Au ohmic contacts to surface-treated p-GaN:Mg (3.6×10¹⁷ cm^-3). It is shown that annealing at 500°C for 1 min in a N₂ ambient improves ohmic contact properties. Specific contact resistance is measured to be 5.0(±1.0) ×10^-3 and 2.5(±1.0)×10^-3 Ω cm² for the as-deposited and annealed samples, respectively. It is also shown that the light transmittance is 90.3(±0.6) and 97.3(±0.8) % (at 470 nm) for the as-deposited and annealed contacts, respectively. Furthermore, the surface of the annealed contact is found to be fairly smooth with a root-mean-square roughness of 0.84 nm. These results are compared with those previously reported for the Ni/Au contacts.