Role of graphene interlayers in mitigating degradation of Ni/Au ohmic contact morphology on p-type GaN

•We measured the effect of using graphene interlayers in Ni/Au contacts to GaN.•The insertion of graphene interlayers prevented the nickel and gold interchange up to 600 °C and therefore increases the thermal stability.•The results may be beneficial to optoelectronic devices in which reflectivity is critical.

We report an investigation of the effect of graphene interlayers in maintaining good surface morphology in low resistance Ni/Au Ohmic contacts to p-GaN (∼1018 cm−3) annealed at 600 °C. Two different thin-film contact metallizations were compared, namely 20 nm Ni/200 nm Au with and without a graphene layer diffusion barrier placed between the metals. Raman spectroscopy measured at several spots indicated single layer graphene, but subsequent transmission electron microscopy indicated that more generally the graphene was several nm thick. The unannealed contacts showed specific contact resistances of 2.1 × 10−5 Ω-cm2 and 3.1 × 10−4 Ω-cm2 for Ni/Au and Ni/Graphene/Au, respectively. After rapid thermal annealing at 600 °C for 60 s in a flowing N2 ambient, X-Ray Photoelectron Spectroscopy and cross-sectional Transmission Electron Microscopy showed the usual Ni/Au interchange occurred in samples without graphene. By sharp contrast, insertion of graphene interlayers prevented the nickel and gold interchange up to 600 °C and dramatically improved the morphological stability of the metallization stack. The results may be beneficial to optoelectronic devices in which reflectivity is critical.