Thermal stability of TaN Schottky contacts on n-GaN

The thermal stability and electrical characteristics of tantalum-nitrogen alloy Schottky contacts on n-GaN were investigated. Non-stoichiometric δ-phase (40 atomic percent nitrogen) tantalum nitride contacts exhibited good electrical properties up to an annealing temperature of 600°C. However, they degrade rapidly above this temperature due to outward diffusion of Ga and presumably nitrogen into the δ-phase tantalum nitride. It is surmised that excess Ta reacts with N at the GaN surface, freeing Ga which then diffuses into the TaN layer. Stoichiometric TaN Schottky contacts were stable at temperatures as high as 800°C and had far superior electrical performance. This stems from the thermodynamic stability of the stoichiometric TaN/GaN interface. δ-phase TaN had I-V and C-V barrier heights of 0.55 eV and 0.8 eV respectively. On the other hand, TaN had an I-V barrier height near 0.7 eV and a C-V barrier height near 1.2 eV. The ideality factors for both δ-phase TaN and TaN were above 1.8 at all annealing temperatures, suggesting tunneling contributes significantly to current transport