Insights into annealing-induced ohmic contact formation at graphene/p-GaN interface with a NiO_x contact layer

The effect of rapid thermal annealing on the graphene/p-GaN interface with a thin NiO_x contact layer in a GaN light-emitting diode (LED) was investigated. Raman and ultraviolet photoemission measurements revealed charge transfer in graphene on a NiO_x contact layer. Rapid thermal annealing at temperatures above 350 °C induced the formation of metallic Ni with a simultaneous increase of Ni³⁺ ions within the NiO_x layer covered by the graphene. As a result, the graphene electrode on a NiO_x/p-GaN surface of a blue GaN LED formed a low-resistance ohmic contact with a specific contact resistance of 5.3 × 10^-4 ohm cm². The ensuing LED chip offered a low forward voltage, comparable to that of the Ni/Au counterpart. This is attributed to a combination of distinct phenomena arising due to a work function modulation in graphene, increase in the carrier concentrations at the near surface region incited during the formation of NiO_x, and thermal reduction induced modifications within the NiO_x contact layer. At elevated annealing temperature, however, oxidation of graphene led to poor current spreading and thus a low optical output, but both these constraints were eliminated by using a few layer graphene.