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 °C under an O3 atmosphere with ultraviolet light radiation (UV-O3 treatment). High-resolution transmission electron microscopy of the films after post-deposition annealing at 750 °C under an O2 atmosphere showed that the intermittent UV-O3 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-O3 treatment. The Ta2O5 film without the UV-O3 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-O3 treated Ta2O5 films also showed a Poole–Frenkel conduction behavior with an almost identical electron trap level and a reduced density. The partially UV-O3 treated Ta2O5 films exhibited a direct tunneling behavior in a relatively low voltage region by the tunneling through the thin (∼3.8 nm) UV-O3 treated surface layer. However, these films showed a Poole–Frenkel conduction behavior in the high-voltage region. In general, the UV-O3 treatment was an efficient method to reduce the leakage current of the high-dielectric Ta2O5 films.