Au-free low-temperature ohmic contacts for AlGaN/AlN/GaN heterostructures

Au-free, Ti/Al/Ta ohmic contact on the AlGaN/AlN/GaN heterostructure using low annealing temperature is studied in this paper. With SiCl(4)plasma treatment at the recess-etched contact region, a low contact resistance of 0.52 omega mm and a low sheet resistance of 373 omega/sq are achieved after annealing at 550 degrees C for 30 s. The low annealing temperature also leads to better surface morphology. Furthermore, AlGaN/AlN/GaN high-electron-mobility transistors (HEMTs) are fabricated with the 550 degrees C, 30 s annealed Ti/Al/Ta ohmic contacts, and a maximum transconductance of 123 mS/mm and a maximum drain current of 510 mA/mm are obtained for a gate length of 4 mu m. Based on Silvaco's Atlas device simulation framework, a scaled-down device with a short gate length of 1 mu m would produce a maximum drain current density of 815 mA/mm. It indicates that the direct current performance of the HEMTs with the ohmic metal proposed in this work is considerably better than that with Au-based ohmic contact.