Ultra-low resistance ohmic contacts to GaN with high Si doping concentrations grown by molecular beam epitaxy

Ti/Al/Ni/Au ohmic contacts were formed on heavily doped n⁺ metal-polar GaN samples with various Si doping concentrations grown by molecular beam epitaxy. The contact resistivity (R_C) and sheet resistance (R_sh) as a function of corresponding GaN free carrier concentration (n) were measured. Very low R_C values (<0.09 Ω mm) were obtained, with a minimum R_C of 0.035 Ω mm on a sample with a room temperature carrier concentration of ∼5 × 10¹⁹ cm⁻³. Based on the systematic study, the role of R_C and R_sh is discussed in the context of regrown n⁺ GaN ohmic contacts for GaN based high electron mobility transistors.