As well-known

ohmic contacts play a very important role in the fabrication of GaN-based devices. Recently

Au/Ni/p-GaNohmic contacts annealed in air or oxygen ambient have been studied extensively because of their low specific contact resistances and high transparency in the fabrication of GaN-based LED s. The roles of Ni

Au layers and their oxidation on the formation of p-GaN ohmic contact are still under dispute. Few works on the structure evolution in the oxidized Ni/Au contact were reported. Moreover

the difference between O

2

and air for the oxidation ambient was not ascertained yet. In this work

thermal annealing in air and oxygen ambient with different time were performed on Ni/Au contact to p-GaN. The structural and electrical properties of the oxidized Ni/Au/p-GaN samples were investigated by synchrotron radiation X-ray diffraction (XRD). The GaNsamples used in our experiments were GaN-based LED grown by metalorganic chemical vapor deposition (MOCVD). p-type GaNlayer is about 200 nm thick. After Ni/Au(20 nm/40 nm) film was deposited by electron-beam deposition system

the samples annealed in oxygen or air for different time

30

60

180

300 and 600 s respectively. 2θ and ω scans XRD of the samples were measured by the synchrotron radiation source. As a result

obvious NiOpeaks were detected by glancing incidence XRD in the annealing samples of Ni/Au contact to p-GaN. When the annealing time was changed from 0 to 600 s

the crystallinity of Au was improved. Apreferentially orientation of Au(111) paralleled with GaN [0001] was observed by ω scan (the rocking curve). It was believed that the Au went down to the interface and form the preferential orientation paralleling to GaN [0001]

and the NiOwas reversed to the surface. It suggested that the Au

Ni

Oand Ga formed a kind of compound that is important to decrease the Schottky barrier. In the process

the dirty on interface was brought out to the surface

and maybe it could take some effect of lowering the Schottky barrier. It was observed that series resistance was reduced when the annealing time increased. The reduction of Au/Ni-p-GaN-Ni/Au series resistance meant the improvement of contact characteristics

which was related with the formation of the epitaxial Au and NiO. The formation speed of NiO annealed in oxygen ambient was faster than that annealed in air.