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 ${\mathrm{(\rho }}_c\mathrm{).}$ The current–voltage measurement suggests that there is an optimal microstructure for the oxidized Ni/Au contact to obtain a minimum ${\rho }_c$ of ${\text{4×10}}^{\text{−6}}\mathrm{ \Omega }{\mathrm{cm}}^{\mathrm{2}}.$ Such low ohmic contact resistance could be related to the formation of the NiO and the specific microstructure. However, high ${\rho }_c$ of about ${\text{2–5×10}}^{\text{−2}}\mathrm{ \Omega }{\mathrm{cm}}^{\mathrm{2}}$ 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 $\mathrm{p-}\mathrm{GaN}$ 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 $\mathrm{metal}\mathrm{/p-}\mathrm{NiO}\mathrm{/p-}\mathrm{GaN}$ heterojunction is reliable to explain the low ${\rho }_c$ to $\mathrm{p-}\mathrm{GaN}.$