Thermally stable, oxidation resistant capping technology for Ti/Al ohmic contacts to n-GaN

Abstract

The intermetallic TiAl3 has been used as a thermally stable cap for Ti/Al ohmic contacts to n-GaN. The electrical performance of the TiAl3-capped contact is nearly the same as that of a standard Ti/Al/Ni/Au contact processed on the same substrate, but the Ti/Al/TiAl3 contact’s performance is optimized at a much lower temperature. The Ti/Al/TiAl3 contact achieved a lowest specific contact resistance (ρc) of 2.1×10−5 Ω cm2 following 1 min at 700 °C in flowing, oxygen-gettered ultrahigh purity (UHP) Ar. The Ti/Al/Ni/Au contact standard achieved a ρc of 1.8×10−5 Ω cm2 following a 15 s anneal at 900 °C in flowing, oxygen-gettered UHP Ar. The TiAl3-capped contact structure shows little sensitivity to the amount of oxygen in the annealing ambient for optimization, and we found that it could achieve a ρc of 1.1×10−5 Ω cm2 following 5 min at 600 °C in air. This performance is almost identical to that attained when the contact was annealed in oxygen-gettered UHP Ar and ordinary Ar. Anneals were extended to a total time of 20 min in the three ambient atmospheres, and the Ti/Al/TiAl3 contact showed no significant difference in its performance. The fact that this contact structure can withstand optimization anneals in air suggests that it could be annealed alongside a Ni/Au contact to p-GaN in air and still achieve a low contact resistance. The performance of the TiAl3-capped bilayer was found to be stable following thermal aging for more than 100 h at 350 °C in air, which was also comparable to an optimally annealed Ti/Al/Ni/Au contact aged at the same time. The TiAl3 material should be an oxidation cap solution for many other Ti/Al contact structures, almost regardless of the Ti:Al layer thickness ratio, since the TiAl3 will be stable on the upper Al layer. Use of this cap eliminates the need to alter a previously optimized bilayer, thus it is a means of enhancing any existing Ti/Al bilayer contact’s performance without necessitating the reoptimization of the layers to accommodate the cap.