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 t o 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 in to NiO along with an amorphous Ni-Ga-O phase and large Au grains. A specifi c contact resistance as low as 4.0x10(-6) Omega cm(2) was obtained at 500 d egrees C. This low value was obtained by the optimization of Ni/Au film thi ckness and heat treatment temperatures. Below about 400 degrees C, Ni was n ot completely oxidized. On the other hand, at temperatures higher than abou t 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 mo re than that of the sample annealed at 500 degrees C. The mechanism of obta ining low-resistance ohmic contacts for the oxidized Ni/Au films was explai ned 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].