Comparison of two different Ti/Al/Ti/Au ohmic metallization schemes for AlGaN/GaN Available to Purchase

Two different ohmic metallization schemes for AlGaN/GaN heterostructure field-effect transistor structures are investigated. Each consists of Ti/Al/Ti/Au layers, with the Ti contacting the AlGaN surface, and they differ only in the relative thicknesses of Ti and Al. The “Cornell” metallization consists of 20/100/45/55 nm thicknesses, respectively, while the “NRC” metallization consists of 30/80/120/55 nm thicknesses for the layers. Thus, the Cornell scheme is much richer in Al. The two schemes were compared on the same AlGaN/GaN growth wafer which, because of nonuniformity, provides a variety of contact resistances in the final annealed devices. The Cornell scheme formed good quality ohmic contacts at short anneal times, but clearly contained excess, unalloyed Al, as it showed visible, gas-evolving corrosion in a cleaning solution consisting of 1:1 $HCl:H2O.$ The surface morphology was rough and the edge acuity was relatively poor. Auger depth profiling showed a single, almost uniformly mixed layer. The NRC scheme required a longer anneal time, and the contact resistance remained high for larger grained AlGaN/GaN material. It was stable in the $HCl:H2O$ cleaning solution, and exhibited good edge acuity and a smooth surface morphology. Auger electron depth profiling of the NRC scheme showed two layers present in the annealed metal. The Auger results are consistent with transmission electron microscope results presented elsewhere. The results suggest that the Cornell scheme would be the choice for materials that are more difficult to contact.