Ti/Ag ohmic contacts to n-type GaN

We report on a study of Ti/Ag ohmic contacts to n-type GaN. Ti/Ag contacts on relatively low-doped (7×10 ¹⁷/cm ³) n-type GaN, annealed in N ₂ gas at 850°C, reached a specific contact resistance of 9.5×10 ^-4 Ωcm ² without special surface treatments. This value is similar to that of Ti/Al ohmic contacts fabricated side-by-side. Unlike Ti/Al contacts, however, Ti/Ag contacts showed smooth surface morphology and excellent long-term stability, as the specific contact resistance increased only by 25 % in three months. I-V and SIMS analysis suggest that, in achieving ohmic behavior in our Ti/Ag contacts, the formation of a low-barrier Ti-Ag intermetallic phase at the GaN surface may be more important than the formation of TiN and/or N vacancies near the interface.