Low resistance ohmic contact on p-type GaN grown by plasma-assisted molecular beam epitaxy

Ohmic contacts on p-type GaN have been investigated. High quality GaN epilayers on c-plane sapphire were prepared using plasma-assisted molecular beam epitaxy that utilized an inductively coupled rf nitrogen plasma source and solid source beams. The resulting film thickness and the doping concentration of the grown samples were in the range of 0.7-1.35 μm and 10¹⁸-10²⁰/cm³ respectively. The metallization consisted of high work function metal bi-layers which included a combinations of 25 nm-thick Ni, Ti, Pt and/or Cr and 200 nm-thick Au on the highly p-doped GaN in a transmission line model pattern. Ohmic contacts were formed by alloying the bi-layers using rapid thermal annealing (RTA) at temperatures in the range of 300-700 °C for 1 min under nitrogen ambient. Current-voltage measurements showed that the specific contact resistance was as low as 1.2×10^-4 Ω-cm² for the sample having 1.4×10²⁰/cm³ p-type doping concentration with a Cr/Au contact annealed at 500 °C for 1 min by RTA. Judging from the scanning Auger microscopy results and the glancing angle x-ray diffraction analysis, this resistance is attributed to Cr diffusion into the GaN layer.