Optimization of Ohmic Contact Fabrication for Al0.3Ga0.7N/AlN/GaN HEMTs on 6H-SiC Using Recess Etching and Surface Plasma Treatment Processes
Ohmic contacts to AlGaN/AlN/GaN heterostructures with low contact resistance and smooth surface morphology play a vital role in the development of high power, high frequency GaN transistors. In the present work, two different Ohmic contact fabrication techniques, recess etching and surface plasma treatment, are optimized in order to obtain good Ohmic contact performance on undoped AlGaN/AlN/ GaN heterostructure on 6H-SiC substrate.
For Ohmic contact fabrication, Ti/Al/Ni/Au metallization scheme is studied under optimized Rapid Thermal Annealing (RTA) temperature and time. Three samples are prepared with different recess based process flow and surface plasma treatment. Standard Transmission Line Model (TLM) is used for computation of contact resistance, sheet resistance and specific contact resistance of Ohmic contacts. For the first time, we explored feasibility of surface plasma treatment process for fabricating Ohmic contacts on AlGaN/ AlN/GaN based heterostructures. We achieved contact resistance of around 0.27 Ω*mm. Further to it we realized improvement in contact resistance on AlGaN/AlN/GaN based heterostructures using recess etching and achieved contact resistance of around 0.25 Ω*mm using Ti/Al/Ni/Au metal stack. Based on the characterization results it is also observed that surface plasma treatment process is a good alternative of comparatively complex recess etching process in fabricating Ohmic contacts for AlGaN/AlN/GaN based High Electron Mobility Transistors (HEMTs).