Behaviour of Ti/Al/Ti/Au contacts to AlGaN/GaN heterostructures at low temperature

Ohmic contacts to wide bandgap nitrides have been realised, but little is known about their behaviour at low temperatures. To address this, an established Ti/Al/Ti/Au contact stack on AlGaN/GaN heterostructures has been characterised from 320 to 80 K. Two structures were investigated, with very similar ambient 2D electron gas transport characteristics despite their difference in AlGaN barrier thickness and composition. This allowed for direct comparison of contact behaviour across different heterostructures. Upon annealing at <800 degrees C for samples with 29 nm AlGaN barriers, contacts which had Ohmic characteristics at room temperature exhibited a gradual onset of Schottky behaviour as the measurement temperature was lowered. When non-Ohmic behaviour was observed, a combination of direct tunnelling, Fowler-Nordheim tunnelling and a thermally assisted Fowler-Nordheim mechanism is suggested to describe the carrier transport. In this case, annealing at 800 degrees C for 30 s proved sufficient to ensure Ohmic behaviour when tested from 320 to 80 K. For a heterostructure with 8 nm AlGaN, the required annealing temperature to maintain consistent Ohmic behaviour across the temperature range was reduced to 750 degrees C. From these observations, the determining factor for Ohmic behaviour is suggested to be the thickness of the AlGaN barrier-either as-grown, or the effective thickness following the formation of TiN protrusions into the AlGaN barrier during annealing. The understanding provided here allows tailoring of either the processing conditions or the heterostructure, and may aid with design of novel devices for low temperature operation.