Enhanced Electrical Conduction in Anatase TaON via Soft Chemical Lithium Insertion toward Electronics Application

Metal oxynitride semiconductors with the d⁰ or d¹⁰ electron configuration are promising materials for nontoxic pigments and photocatalysts, but their electrical properties have scarcely been studied. Anatase TaON (δ-TaON) is a metastable polymorph of TaON, and its epitaxial thin films show good semiconducting properties such as a wide tunability of electrical conductivity and a rather high electron mobility comparable to that of anatase TiO₂. However, the density of carrier electrons (<i>n</i>_e) provided by anion vacancies is limited to ∼1 × 10²⁰ cm^–3, so establishing a method for carrier doping of anatase TaON remains a critical issue for its use in electronics applications. In this report, we used soft chemical insertion of Li into interstitial sites of anatase TaON epitaxial thin films by using an <i>n</i>-butyllithium solution, and the resulting material showed a higher <i>n</i>_e (3.5 × 10²⁰ cm^–3) than anion-deficient anatase TaON films. Additionally, the Li-inserted anatase TaON showed an enhanced Hall mobility (μ_H) of over 30 cm²V^–1s^–1 and a lower resistivity of 6.7 × 10^–4 Ωcm at room temperature. In contrast, direct vapor phase deposition of Li-doped TaON caused Li substitution for Ta, where a large difference in charges between Li⁺ and Ta⁵⁺ was compensated by an increase in the O/N ratio. These results indicate that soft chemical insertion of Li after growth of the host crystal is an effective method for carrier doping of anatase TaON

Reversible Changes in Resistance of Perovskite Nickelate NdNiO₃ Thin Films Induced by Fluorine Substitution

Perovskite nickel oxides are of fundamental as well as technological interest because they show large resistance modulation associated with phase transition as a function of the temperature and chemical composition. Here, the effects of fluorine doping in perovskite nickelate NdNiO₃ epitaxial thin films are investigated through a low-temperature reaction with polyvinylidene fluoride as the fluorine source. The fluorine content in the fluorinated NdNiO_3–<i>x</i>F_<i>x</i> films is controlled with precision by varying the reaction time. The fully fluorinated film (<i>x</i> ≈ 1) is highly insulating and has a bandgap of 2.1 eV, in contrast to NdNiO₃, which exhibits metallic transport properties. Hard X-ray photoelectron and soft X-ray absorption spectroscopies reveal the suppression of the density of states at the Fermi level as well as the reduction of nickel ions (valence state changes from +3 to +2) after fluorination, suggesting that the strong Coulombic repulsion in the Ni 3d orbitals associated with the fluorine substitution drives the metal-to-insulator transition. In addition, the resistivity of the fluorinated films recovers to the original value for NdNiO₃ after annealing in an oxygen atmosphere. By application of the reversible fluorination process to transition-metal oxides, the search for resistance-switching materials could be accelerated