Low thermal budget Hf/Al/Ta ohmic contacts for InAlN/GaN-on-Si HEMTs with enhanced breakdown voltage

The authors have studied the electrical characteristics of Hf/Al/Ta ohmic contacts on In0.18Al0.82N/GaN heterostructure grown on Si (111) substrate. With annealing at 600 °C in vacuum (which is ∼200 °C lower than that for traditional Ti/Al/Ni/Au contacts), a minimum ohmic contact resistance of ∼0.58 Ω·mm and specific contact resistivity of ∼6.75 × 10−6 Ω·cm2 are obtained. The minimum contact resistance of Hf/Al/Ta contacts is comparable to that of Ti/Al/Ni/Au contacts. Owing to the lower annealing temperature, Hf/Al/Ta contacts exhibit better surface morphology and edge acuity. More importantly, Hf/Al/Ta contacts show a smooth interface with In0.18Al0.82N/GaN, whereas spike structures that penetrate the In0.18Al0.82N layer are observed for Ti/Al/Ni/Au contacts. As a result, the source–carrier-injection induced breakdown mechanism is reduced in the In0.18Al0.82N/GaN-on-Si high electron mobility transistors (HEMTs) with Hf/Al/Ta ohmic contacts, thereby leading to an improved three-terminal off-state breakdo...