Bixbyite-Ta2N3 thin films: Characterization and electrical properties

The structure, thermal stability, composition, morphology, and electrical properties of bixbyite-Ta2N3 films are investigated by combining experimental characterization and ab initio calculations. Ta2N3 films were grown by magnetron sputtering in a pure N2 atmosphere and subsequently annealed. The as grown film consists of poorly ordered bixbyite-Ta2N3 with stretched unit cell due to the excess of N atoms (N/Ta atomic ratio 2.05) which is mostly within the Ta2N3 nanocrystals. Upon annealing at 450 °C, the excess N atoms migrate to the grain boundaries, the cell parameter reduces and the crystallinity improves. At higher annealing temperatures, the N atoms additionally out diffuse from the films, which results in a decrease of the N/Ta ratio to 1.38 at 950 °C. The Ta2N3 phase is stable up to 850 °C where it transforms into fcc δ-Ta0.75N. The resistivity of the as grown film is high (34 Ω cm) and then drastically reduces after annealing at 450 °C (91 mΩ cm). At higher annealing temperatures, the resistivity further decreases (5 mΩ cm at 750 °C), indicating that Ta2N3 is a conductor. We used the percolation theory to quantitatively describe the resistivity variation of the annealed films. Our explanation of the resistivity behavior is supported by ab initio calculations of the electronic structure of Ta2N3, where we found that the Fermi level is within the conduction band. We also found a band gap below the Fermi level and estimated the optical absorption onset energy to be about 2 eV.