Ti/Al/Ni/Cu Ohmic Contact with Low Contact Resistance and Smooth Surface Morphology for AlGaN/GaN HEMT

The feasibility of replacing the Au layer in a conventional ohmic contact of Ti/Al/Ni/Au for AlGaN/GaN structure with Cu has been investigated. While the thicknesses of Ti, Al and Cu layers were fixed throughout the study, the thickness of Ni layer was varied to examine its influence on the Cu-based ohmic contacts. It is showed that the Ni layer thickness played an important role in the Ti/Al/Ni/Cu metallization for achieving low contact resistance and smooth surface morphology. With a 50-Å Ni layer, a low specific contact resistance (r c ) of 1.35x10 -6 ohm-cm 2 has been realized. This result is comparable with the conventional ohmic contact (r c = 1.19x10 -6 ohm-cm 2 ). High electron mobility transistor (HEMT) devices fabricated using both Au- and Cu-based ohmic contact exhibited similar DC characteristics, suggesting that the electrical performance did not degrade with the used of Cu in the ohmic contact. Furthermore, in the absence of Au, the surface roughening caused by Au-Al alloy in the conventional ohmic contact was also prevented. As a result, the root-mean-square roughness of the optimized Cu-based contact was only 7.62 nm as compared to 134 nm for the conventional structure.