Fabrication and formation mechanisms of Ohmic conducts with low annealing temperature for GaN/AlN superlattice barrier HEMTs

Abstract This work investigates the Ohmic contact mechanism and low resistance Ohmic contact preparation for GaN-based high electron mobility transistors (HEMTs) with GaN/AlN superlattice barriers. The electrical and microstructural characterization of Ti/Al/Ni/Au contacts shows that the formation of Ohmic contacts primarily depends on the TiN islands formed by Ti diffusion in the barrier layer, which is the main reason for the high contact resistance of GaN/AlN SLs HEMTs under conventional high-temperature annealing. A grid deep-recess technique is proposed to establish direct contact between TiN interfacial layer, the barrier, and the two-dimensional electron gas (2DEG) through the sidewalls. This novel technique achieves an Ohmic contact with a low contact resistance of 0.31 Ω·mm at low temperatures (700 ℃). Furthermore, increased grid deep-recess density effectively reduces contact resistance due to the additional contribution from the conduction width. It is also noteworthy that after complete removal of the barrier layer, the impact of recess depth on contact characteristics is minimal, greatly reducing the process complexity of grid deep-recess technique. Consequently, GaN/AlN SLs HEMTs fabricated using the grid deep-recess technique exhibit significant improvements in on-resistance, transconductance and saturation current. These results are expected to expand the potential application of GaN/AlN superlattice barrier layers in RF and power devices.