Microstructure and current transport in Ti/Al/Ni/Au ohmic contacts to n-type AlGaN epilayers grown on Si(111)

The microstructure and the current transport in Ti/Al/Ni/Au ohmic contacts on AlGaN were investigated in this paper. Significant structural changes of the metal stack occurred upon annealing and ohmic contacts were obtained after thermal treatments above 700 degrees C, with specific contact resistance values p(c) similar to 10(-5) Omega cm(2), which depend on the thickness of the Ti layer. Although the formation of a TiN interfacial layer is independent of the Ti thickness, different contact structures, i.e. the phases and grain location, were found as a function of the Ti layer, which were crucial for the nanoscale current transport through the contact stack. In particular, conductive atomic force microscopy combined with the resistivity measurement of the main phases formed upon annealing (AINi, AlAu4, Al2Au) indicated that the low resistivity Al2Au phase provides preferential conductive paths for the nanoscopic current flow through the contact.