Oxidized Ni/Pt and Ni/Au ohmic contacts to p -type GaN

The Ni/Pt and Ni/Au contacts to p-GaN heat treated in air are investigated to study the effect of the metallic phase of high work function on specific contact resistance (ρc). The current–voltage measurement suggests that there is an optimal microstructure for the oxidized Ni/Au contact to obtain a minimum ρc of 4×10−6 Ω cm2. Such low ohmic contact resistance could be related to the formation of the NiO and the specific microstructure. However, high ρc of about 2–5×10−2 Ω cm2 are obtained for the oxidized Ni/Pt contacts, which is attributed to the different microstructure from that of the oxidized Ni/Pt contact. The NiO phase of the oxidized Ni/Pt contact is separated from p-GaN by a continuous face-centered-cubic Pt–Ni–Ga film. Therefore, the oxidized Ni/Pt contact behaves just like a common metal contact, but not a metal/NiO composite contact. Comparison of the two oxidized contacts implies that the previously proposed model based on metal/p-NiO/p-GaN heterojunction is reliable to explain the low ρc to p-GaN.