Effect of various pre-treatments on Ti/Al/Ti/Au ohmic contacts for AlGaN/GaN HFET devices

The effect of a number of different treatments on the contact resistance for Ti/Al/Ti/Au (30/80/120/55 nm) contacts to Al0.3Ga0.7N (20 nm)/GaN HFET structures were investigated. The structures were grown by MBE. Neither preannealing of the surface at temperatures up to 1100 \ub0C in N2, nor etching of the surface using chemically assisted ion beam etching (CAIBE) was effective in reducing the contact resistance. Both light CAIBE etching ( 485 nm) to partially remove the AlGaN layer, and deeper etching ( 4850 nm and 100 nm) to remove the AlGaN and contact the GaN channel layer directly, resulted in a substantial increase in the measured contact resistance. The only treatment that was found to decrease the contact resistance was a brief wet etch in K2S2O8/KOH solution under 254 nm UV irradiation. A resist stabilization treatment was developed so that the process was self-aligned. The contact resistance in \u3a9 mm was decreased by a factor of 1.7 for the optimum etch time. Scanning electron microscopy showed that the etching resulted in the formation of discrete holes perforating the AlGaN layer.NRC publication: Ye