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

We report on a study of Al and Ti/Al contacts to n-type GaN. Al contacts on n-GaN (7 X 10(17) cm(-3)) annealed in forming gas at 600 degrees C reached a minimum contact resistivity of 8 X 10(-6) Omega cm(2) and had much better thermal stability than reported by previous researchers. Ti/Al (35/115 nm) contacts on n-GaN (5X10(17) cm(-3)) had resistivities of 7X10(-6) and 5 X 10(-6) Omega cm(2) after annealing in Ar at 400 degrees C for 5 min and 600 degrees C for 15 s, respectively. Depth profiles of Ti/Al contacts annealed at 400 degrees 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 degrees C range includes Ti reducing the GaN native oxide and an Al-Ti intermetallic coming into intimate contact with the GaN. (C) 1997 American Institute of Physics.