Characterization of Pd/Ni/Au ohmic contacts on p-GaN

A low-resistance ohmic contact to Mg-doped p-type GaN grown by metal-organic chemical vapor deposition (MOCVD) with a carrier concentration of 2 × 1017 cm−3 using Pd/Ni/Au metallization was formed. An anneal at 500 °C for 1 min in a flowing N2 ambient produced an excellent ohmic contact with a specific contact resistivity as low as 2.4 × 10−5 Ω cm2. X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES) depth profiles of Pd/Ni/Au contacts annealed at 500 °C demonstrated a strong correlation between Ni and Pd interdiffusion toward the GaN surface and a reduction in specific contact resistance. The low contact resistance is attributed to the reduction of the native oxide by Ni diffusion along with the formation of Pd and Ni related-gallide phases at the p-GaN surface region.