Optimization of ohmic contacts on thick and thin AlGaN/GaN HEMTs structures

摘要

In this paper, we address the Ohmic contacts comparison and optimization on both thin (18 nm) and thick (25 nm) AlGaN/GaN HEMTs structures. In the conventional metallization scheme of Ti/Al/Ti/Au, several stacks based on Ni, Cr, and Pt metals replacing the middle Ti were tested and compared. Specific Contact Resistance (p_c) strongly depends on the stack ratio. For a particular, stack ratio of 1:5:2:3 tested on thick AlGaN based HEMTs, Cr stack exhibited the least p_c value of 5 × 10~(-5) Ω-cm~2 while the p_c value doubled for Pt and increased by 4 times for Ni. But the morphology comparison shows that Ni is the best choice. Therefore the Ni-based stack was further optimized for low contact resistance. In the optimization process, pre-metallization surface treatments were altered along with the stack ratios. The stack ratio of 1:5:2:2.5 has resulted in lowest specific contact resistance value of 6 × 10~6 Ω-cm~2. Different Ni-based stacks with ratio variations were then deposited and compared for thick and thin AlGaN/GaN HEMTs structures. The same value of p_c was recorded on both thick and thin structures as long as the Ni proportion in the stack is low. With an increase in the Ni proportion, p_c was found to be increased dramatically for thin AlGaN/GaN HEMTs.