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

We report on a study of Al and Ti/Al contacts to n-type GaN. Al contacts on n-GaN (7×1017 cm−3) annealed in forming gas at 600 °C reached a minimum contact resistivity of 8×10−6 Ω cm2 and had much better thermal stability than reported by previous researchers. Ti/Al (35/115 nm) contacts on n-GaN (5×1017 cm−3) had resistivities of 7×10−6 and 5×10−6 Ω cm2 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.