High temperature induced failure in Ti/Al/Ni/Au Ohmic contacts on AlGaN/GaN heterostructure

Ti/Al/Ni/Au (200/1200/500/2000 Å) Ohmic contact on AlGaN/GaN was prepared and it was subjected to thermal aging experiments. Thermal processing at 400 and 500 °C did not change the contact resistance significantly, while high temperature storage at 600 °C resulted in a surge in the contact resistance. The Al–Au alloy in the contact metal is believed to re-melt because its lowest melting temperature is 525 °C. The liquid of Al–Au alloy is observed to diffuse to the AlGaN surface and consume some AlGaN layer. In addition, voids are found to be produced during thermal process, which can reduce the effective contact area and thus lead to higher contact resistance. The TEM and EDX results of Ohmic contact’s cross sectional images provide evidence for this proposed mechanism.