Au-free ohmic Ti/Al/TiN contacts to UID n-GaN fabricated by sputter deposition

The fabrication and characterization of an Au-free Ti/Al/TiN ( 20/100/100 nm) contact stack to unintentionally doped n-GaN with TiN serving as the diffusion barrier is presented. Sputter deposition and lift-off in combination with post deposition annealing at 850 degrees C are used for contact formation. After annealing, contact shows ohmic behavior to n-GaN and a specific contact resistivity of 1.60 x 10(-3) Omega cm(2). To understand the contact formation on the microscopic scale, the contact was characterized by current-voltage measurements, linear transmission line method, X-ray diffraction, transmission electron microscopy, and X-ray photoelectron spectroscopy. The results show the formation of Ti-N bonds at the GaN/Ti interface in the as-deposited stack. Annealing leads to diffusion of Ti, Al, Ga, and N, and the remaining metallic Ti is fully consumed by the formation of the intermetallic tetragonal Al3Ti phase. Native oxide from the GaN surface is trapped during annealing and accumulated in the Al interlayer. The TiN capping layer, however, was chemically stable during annealing. It prevented oxidation of the Ti/Al contact bilayer successfully and thus proved to be a well suitable diffusion barrier with ideal compatibility to the Ti/Al contact metallization.