Low-resistance Ta/Ti Ohmic contacts for p-type GaN

Although extensive efforts have continued to develop Ohmic contacts for p-type GaN, which have specific contact resistance ${\mathrm{(\rho }}_c)$ lower than that ${\mathrm{(\rho }}_c{\text{∼10}}^{\text{−2}}\mathrm{ \Omega }{\mathrm{cm}}^{\mathrm{2}})$ of conventional Ni/Au contacts, to the best of our knowledge no breakthrough has been reported in open literature. We demonstrated that bilayered Ta/Ti contacts have a ${\rho }_c$ value of around ${\text{3×10}}^{\text{−5}}\mathrm{ \Omega }{\mathrm{cm}}^{\mathrm{2}}$ for p-type GaN with a hole concentration of ${\text{7×10}}^{17} {\mathrm{cm}}^{\mathrm{-3}}.$ This contact has resistance low enough to manufacture blue laser diodes, but deterioration of the ${\rho }_c$ value during room-temperature storage is the key issue.