Low-resistance ohmic contacts with high transparency to p-type GaN have been developed by oxidizing Ni/Au thin films. Compared to the metallic Ni/Au contacts, the oxidized Ni/Au contacts exhibited lower specific contact resistance and much improved transparency. The transparency was from 65% to 80% in the wavelength of 450–550 nm. A specific contact resistance below 1.0×10−4 Ω cm2 was obtained by oxidizing Ni(10 nm)/Au(5 nm) on p-type GaN. The mechanism of low-resistance ohmic contact could be related to the formation of NiO.

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 4.0×10−6 Ω cm2 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 p-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 Au/p-NiO/p-GaN structure.

We investigate the photo-oxidation process and the corresponding passivation effects on the optical properties of unintentionally doped n-type gallium nitride (GaN). When illuminated with a 253.7 nm mercury line source, oxidation of GaN is found to take place in aqueous phosphorus acid solutions with pH values ranging from 3 to 4. At room temperature, the photo-oxidation process is found reaction-rate limited and has a peak value of 224 nm/h at pH=3.5. Compared with the as-grown GaN layers, threefold enhancement in the photocurrent and photoluminescence response are observed on the oxidized GaN surfaces. These results are attributed to the surface passivation effects due to the deep ultraviolet-enhanced wet oxidation on GaN.

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