Improvement of Ohmic contacts to n-type GaN using a Ti/Al multi-layered contact scheme

An improvement to the conventional Ti/Al/Ti/Au contact scheme has been developed to achieve better Ohmic contact properties to n-GaN using a Ti/Al multi-layered structure. Transmission electron microscopy demonstrates the formation of an AlGaN barrier layer at the metal-GaN interface of the conventional contact scheme, due to the in-diffusion of the excess Al during thermal annealing. X-ray photoelectron spectroscopy shows that the Al diffusion was significantly reduced via the new contact scheme, since the excess Al was tied up by the additional Ti layer. As a result, an order of magnitude smaller specific contact resistivity was realized. The surface morphology of the contact electrodes was also improved, as verified by scanning electron microscopy and atomic force microscopy. These results indicated that the Ti/Al multi-layered contact scheme is an improvement to the conventional counterpart in achieving lower contact resistance as well as higher reliability and uniformity of GaN-based devices.