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 Ti/Al/Ni/Au ohmic contact for AlGaN/GaN structure with Cu has been investigated. In this study, the thicknesses of Ti, Al and Cu layers were fixed at 200 Å, 1200 Å and 1000 Å, respectively. On the other hand, Ni layer with different thicknesses were used to examine its influence on the Cu-based ohmic contacts. The results showed that the Ni layer played an important role in the Ti/Al/Ni/Cu metallization for achieving low contact resistance, smooth morphology and excellent edge acuity. With a 50-Å Ni layer, a low specific contact resistance of 1.35x10^-6 ohm-cm² has been realized. This result is comparable with the conventional ohmic contact of Ti/Al/Ni/Au (1.19 x 10^-6 ohm-cm²). 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. 

     The X-ray diffraction (XRD) and transmission electron microscopy (TEM) results suggest that the formation of TiN at the Ti/semiconductor interface has played an important role in achieving low contact resistance similar to the case for the Ti/Al/Ni/Au structure. In addition, the formation of more stable Ti-Al-Cu based alloys in the Cu-based ohmic contact had successfully confined the Cu within the ohmic contact. Both TEM and Auger electron spectroscopy (AES) results confirmed that no Cu was detected in the semiconductor layers. Finally, high electron mobility transistors (HEMTs) were fabricated using the Cu- and Au-based ohmic contacts. Both devices showed the similar DC characteristics suggesting that the Ti/Al/Ni/Au metallization has a great potential to be used as the ohmic contact for AlGaN/GaN devices.