Study of Ti-Rich and Al-Rich Contact Metallization for AlGaN/GaN HEMT Power Devices

Abstract

Ohmic contacts on AlGaN/GaN HEMT are sensitive to metals and annealing temperature. In this paper, we have studied a Ti/Al metallization on nitrogen-rich substrate (SiNx and AlGaN). Al-rich metallization (meaning Al in excess regarding TiAl3 stoichiometry) is shown to be not suitable for contact metallization because a detrimental porous AlN layer is formed during the 900°C annealing step. In the case of Ti-rich metallization, it is demonstrated that a nitridation appears at the interface between TiAl3 and substrate (in both SiNx and AlGaN substrates). This nitrogen pumping from the substrate appears from 600°C and is amplified at 900°C. In conclusion, it is shown that nitrogen pumping is impacted by Ti/Al ratio and by annealing temperature. Nitrogen pumping is known to be a key parameter for the formation of low resistivity ohmic contact in AlGaN/GaN HEMT. This explains why Ti/Al ratio and annealing temperature must be properly optimized.