Investigation of aluminum and titanium/aluminum contacts to n-type gallium nitride

We report on a study of Al and Ti/Al contacts to n-type GaN. Al contacts on n-GaN (7 × 10¹⁷cm^-3) annealed in forming gas at 600°C reached a minimum contact resistivity of 8 × 10^-6 Ωcm² and had much better thermal stability than reported by previous researchers. Ti/Al (35nm/115nm) contacts on n-GaN (5 × l0¹⁷cm^-3) had resistivities of 7 × l0^-6 Ωcm² and 5 × 10^-6 Ωcm² after annealing in Ar at400°C for 5min and 600°C for 15sec, 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 reduction of the native oxide on GaN by Ti and formation of an Al-Ti intermetallic phase in intimate contact with the GaN. Contacts with different Ti/Al layer thicknesses were investigated and those with 50nm/100nm layers had the same low resistance and better stability than 25nm/125nm contacts.