Improvement of Ti/Al/Ti Ohmic contacts on AlGaN/GaN heterostructures by insertion of a thin carbon interfacial layer

This Letter reports on the improvement of the morphological and electrical behavior in Ti/Al/Ti Ohmic contacts on AlGaN/GaN heterostructures by the insertion of a thin carbon interfacial layer. In particular, the presence of a carbon layer between the Ti/Al/Ti metal stack and the AlGaN surface leads to the lowering of the annealing temperature (down to 450 °C) required to obtain linear I-V curves and to the improvement of the contacts surface morphology. The temperature dependence of the specific contact resistance was explained by the thermionic field emission, with a reduction in the barrier height φB down to 0.62 eV in the annealed contacts with the interfacial carbon layer. The experimental evidence has been justified with the formation of a thin low work function TiC layer, which enhances the current conduction through the metal/AlGaN interface.