High-Mobility Electron Conduction in Oxynitride: Anatase TaON

Abstract

We report on a new route for synthesizing metastable anatase tantalum oxynitride (TaON) in thin film form on lattice-matched (LaAlO3)0.3-(SrAl0.5Ta0.5O3)0.7 (LSAT) single crystals by using nitrogen plasma assisted pulsed laser deposition. Epitaxial stress from the substrate stabilized the anatase structure without the need for doping an impurity, such as Sc or Mg, which is required in conventional bulk synthesis by ammonolysis. X-ray diffraction measurements and cross-sectional transmission electron microscope (TEM) observations demonstrated the growth of phase-pure anatase TaON thin films with the epitaxial relationships (001)TaON ∥ (001)LSAT and [100]TaON ∥ [100]LSAT. A high growth temperature (≥750 °C) and a balanced supply of oxygen and nitrogen are crucial for obtaining high-quality anatase TaON thin films. The films grown at 800 °C exhibited good n-type conduction with a resistivity of ∼1 × 10–2 Ω cm. The source of the carrier electrons was likely anion vacancies. The Hall mobility of anatase TaON (∼17 cm2 V–1 s–1 at 300 K) is comparably high to that of anatase TiO2, which is a well-known oxide semiconductor with the same crystal structure and d0 electronic configuration. The bandgap and refractive index of anatase TaON thin films were 2.37 eV and approximately 3.0, respectively, in the visible region.