Low-resistance ohmic contacts to p-type GaN achieved by the oxidation of Ni/Au films

A contact has been developed to achieve a low specific contact resistance to p-type GaN. The contact consisted of a bi-layer Ni/Au film deposited on p-type GaN followed by heat treatment in air to transform the metallic Ni into NiO along with an amorphous Ni–Ga–O phase and large Au grains. A specific contact resistance as low as ${\text{4.0×10}}^{\text{−6}}\mathrm{ \Omega }{\mathrm{cm}}^{\mathrm{2}}$ was obtained at 500 °C. This low value was obtained by the optimization of Ni/Au film thickness and heat treatment temperatures. Below about 400 °C, Ni was not completely oxidized. On the other hand, at temperatures higher than about 600 °C, the specific contact resistance increased because the NiO detached from $\mathrm{p-}\mathrm{GaN}$ and the amount of amorphous Ni–Ga–O phase formed was more than that of the sample annealed at 500 °C. The mechanism of obtaining low-resistance ohmic contacts for the oxidized Ni/Au films was explained with a model using energy band diagrams of the $\mathrm{Au}\mathrm{/p-}\mathrm{NiO}\mathrm{/p-}\mathrm{GaN}$ structure.