Investigation of the mechanism for Ohmic contact formation in Al and Ti/Al contacts to n -type GaN

Abstract

We report on a study of Al and Ti/Al contacts to n-type GaN. Al contacts on n-GaN ${\text{(7×10}}^{17}\hspace{0.5em}{\mathrm{cm}}^{\text{−3}})$ annealed in forming gas at 600 °C reached a minimum contact resistivity of ${\text{8×10}}^{\text{−6}}\mathrm{\hspace{0.5em}\Omega \hspace{0.17em}}{\mathrm{cm}}^2$ and had much better thermal stability than reported by previous researchers. Ti/Al (35/115 nm) contacts on n-GaN ${\text{(5×10}}^{17}\hspace{0.5em}{\mathrm{cm}}^{\text{−3}})$ had resistivities of ${\text{7×10}}^{\text{−6}}$ and ${\text{5×10}}^{\text{−6}}\mathrm{\hspace{0.5em}\Omega \hspace{0.17em}}{\mathrm{cm}}^2$ after annealing in Ar at 400 °C for 5 min and 600 °C for 15 s, respectively. Depth profiles of Ti/Al contacts annealed at 400 °C showed that low contact resistance was only achieved after Al diffused to the GaN interface. We propose that the mechanism for Ohmic contact formation in Ti/Al contacts annealed in the 400–600 °C range includes Ti reducing the GaN native oxide and an Al–Ti intermetallic coming into intimate contact with the GaN.