Changes in structures and electrical conduction mechanisms of chemical vapor deposited Ta2O5 thin films by annealing under O3 atmosphere with ultraviolet light radiation

The electrical conduction mechanisms of approximately 8-nm-thick Ta2O5 films grown by metalorganic chemical vapor deposition were investigated by measuring the current density-voltage characteristics at various temperatures. The Ta2O5 films were grown in two steps with or without intermittent annealing at 450 degreesC under an O-3 atmosphere with ultraviolet light radiation (UV-O-3 treatment). High-resolution transmission electron microscopy of the films after post-deposition annealing at 750 degreesC under an O-2 atmosphere showed that the intermittent UV-O-3 treatment improved the crystallization of the film during post-annealing. Auger electron spectroscopy of the variously treated samples showed that the improvement in crystallization was due to the increase in the oxygen concentration of the Ta2O5 films by the UV-O-3 treatment. The Ta2O5 film without the UV-O-3 treatment mostly exhibited a Poole-Frenkel conduction behavior with the electron trap level of 0.62 eV from the conduction band edge. The whole layer UV-O-3 treated Ta2O5 films also showed a Poole-Frenkel conduction behavior with an almost identical electron trap level and a reduced density. The partially UV-O-3 treated Ta2O5 films exhibited a direct tunneling behavior in a relatively low voltage region by the tunneling through the thin (similar to3.8 nm) UV-O-3, treated surface layer. However, these films showed a Poole-Frenkel conduction behavior in the high-voltage region. In general, the UV-O-3 treatment was an efficient method to reduce the leakage current of the high-dielectric Ta2O5 films.