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 4.0x10(-6) Omega cm(2) was obtained at 500 degrees C. This low value was obtained by the optimization of Ni/Au film thickness and heat treatment temperatures. Below about 400 degrees C, Ni was not completely oxidized. On the other hand, at temperatures higher than about 600 degrees C, the specific contact resistance increased because the NiO detached from p-GaN and the amount of amorphous Ni-Ga-O phase formed was more than that of the sample annealed at 500 degrees 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 Au/p-NiO/p-GaN structure. (C) 1999 American Institute of Physics. [S0021-8979(99)00820-8].