Effects of Fluorine Plasma Treatment on Au-Free Ohmic Contacts to Ultrathin-Barrier AlGaN/GaN Heterostructure

Effects of fluorine plasma etching on Au-free ohmic contacts to SiNx-passivated ultrathin-barrier AlGaN/GaN heterostructure were investigated. It is found that overetching of SiNx passivation layer, which was grown by low-pressure chemical vapor deposition (LPCVD), resulted in strong accumulation of fluorine at the junction between ohmic metal, SiNx passivation, and the (Al)GaN barrier layer. The possible depletion effect of fluorine will cutoff the current conduction path between the ohmic metal and the 2-D electron gas (2DEG) in the access region, leading to degraded contact resistance and 2DEG sheet resistance. The Au-free ohmic contact mechanism can be well fitted by thermionic field emission model, and the extracted Schottky barrier height is increased by ~0.038 eV with F overetching.