Abstract
In this study, the formation mechanism of low-resistance ohmic contact to p-type GaN using an alloy of Ni/Cu was investigated. The surface contamination layer on p-type GaN was partially removed by immersing the sample in the aqua regia solution. The Mg-doped samples were deposited with Ni(20nm)/Cu(20nm) and then annealed in nitrogen ambient at different annealing temperatures ranging from 400~700℃. A good ohmic contact with a specific contact resistance of 1.31*10^-4 ohm-cm^2 was obtained when the sample was annealed at 600℃ for 30s. The decrease of specific contact resistance is attributed to (i) the increased contact area and local electric field and (ii) the extraction of hydrogen from the p-type GaN. It is further suggested that the Ni overlayer removes most of the contamination layer during annealing. In the final contact structure, Cu atoms could penetrate throughout the entire conductivity of the interfacial metallic contact on p-type GaN film improves the conductivity of the contact. Based on principles described above, we try to fabricate a novel process (X-method) to form ohmic contacts to p-type GaN using a single metallic layer. However, the high specific contact resistance can be obtained with “X-method”. A possible explanation is the re-growth of the contamination layer upon coating with photoresist on the p-type GaN surface during photolithography, which reduces the roughness of the p-type GaN surface and leads to the increase of the specific contact resistance.