Investigation of oxidation mechanism for ohmic formation in Ni/Au contacts to p-type GaN layers

To investigate the function and mechanisms of oxidation, we present the ohmic performances for Ni/Au contacts to p-type GaN treated with various conditions. When the p-type GaN sample was preoxidized at 750 °C for 30 min in air ambient and then treated with (NH₄)₂S_x solution, we routinely obtained a specific contact resistance of 4.5 × 10^-6 Ω cm² for the Ni/Au contacts to samples alloyed at 500 °C for 10 min in air ambient. The fact that, in this configuration, ohmic performance improved one order of magnitude [compared with (NH₄)₂S_x surface treatment], is attributable to the strengthened formation of GaO_x (aided by the preoxidation process), as well as the fact that more holes were induced on the oxidation-free p-type GaN surface.