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

The effect of rapid thermal annealing on the graphene/p-GaN interface with a thin NiOx contact layer in a GaN light-emitting diode (LED) was investigated. Raman and ultraviolet photoemission measurements revealed charge transfer in graphene on a NiOx 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 NiOx layer covered by the graphene. As a result, the graphene electrode on a NiOx/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 NiOx, and thermal reduction induced modifications within the NiOx 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.