Ta/Al/CuW low temperature ohmic contacts for GaN-on-Si HEMT

Abstract

We proposed a low temperature Au-free ohmic contacts of GaN-on-Si HEMT with the Ta/Al/CuW metal stack. The CuW was deposited by using the dual-target magnetron sputter deposition method. The annealing conditions and recess depth of ohmic area were systematically investigated. By utilizing the Ta/Al/CuW structure, an improved contact characteristic (0.49 Ω·mm) is obtained following annealing at 550 °C for 10 min in vacuum, with the recess depth of 30 nm(±2 nm). This performance surpasses that of Ta/Al/W Au-free contacts (1.07 Ω·mm). Furthermore, both the Ta/Al/CuW ohmic contacts (RMS = 6.3 nm) and the Ta/Al/W ohmic contacts (RMS = 6.0 nm) exhibit smooth surface morphology. Compared to Ti contact layer, Ta demonstrates superior performance in low temperature contact and breakdown test. Amorphous Ta layer can effectively suppress Cu diffusion. The GaN-on-Si HEMT was also fabricated based on Ta/Al/CuW Au-free ohmic contacts, exhibiting excellent DC characteristics.