Investigation of Ti/Al/Ni/Au ohmic contacts for AlScN/GaN HEMTs

The high bandgap of AlScN makes it difficult to achieve ohmic contacts with a low specific contact resistance (ρc) to AlScN/GaN heterostructures. High ρc increases the on-resistance of the high-electron-mobility transistor fabricated from these heterostructures and reduces the achievable current densities. This work presents an ohmic recess process and Ti/Al/Ni/Au metallizations with a ρc of 8.83×10−5 Ω cm2 after annealing at 900 °C. Interestingly, time-of-flight secondary ion mass spectrometry reveals that out-diffusion of Sc from the barrier to the metal surface occurs at anneal temperatures above 700 °C. While this structural metamorphosis does not show a strong impact on ρc, it leads to an increase in the on-resistance and gate leakage currents, as well as to a decrease in the maximum drain current of HEMTs. At an anneal temperature of 600 °C, no thermal degradation was observed and ρc as low as 13.4×10−5 Ωcm2 are achieved.