Low ohmic contact resistivity realized by in situ SiNx insertion for high Al-composition-AlGaN/GaN heterostructure

Abstract

A low ohmic contact resistance (Rc) is a challenge for an AlGaN/GaN heterostructure with a high Al-composition barrier, which is highly desired to further push the performance of GaN transistors. In this Letter, an effective ohmic contact technique with an in situ SiNx layer inserted between the conventional Ti/Al/Ni/Au metal stack and the III-nitride was proposed and the inside physical mechanisms were investigated for an Al0.65Ga0.35N/GaN heterostructure. The fabricated ohmic contact has a low Rc of 0.17 Ω·mm and a specific contact resistivity value (ρc) of 8.45 × 10−7 Ω·cm2, which is obviously improved compared with the Rc of 0.32 Ω·mm and ρc of 2.84 × 10−6 Ω·cm2 without in situ SiNx inserted. The strong temperature dependency of ρc with in situ SiNx insertion shows that the dominant current transport mechanism is the thermionic field emission. Transmission electron microscopy and energy dispersive x-ray spectroscopy analyses suggest that the enhanced ohmic contact is due to the possible formation of low-work function silicides and more TiN alloys during annealing.