Experimental and theoretical assessment of native oxide in the superconducting TaN

The present work shows the native oxide formation into superconducting TaN thin films through experiments and computational simulations. First, TaN was synthesized at an ultra-high vacuum system by reactive pulsed laser deposition and characterized in situ by X-ray photoelectron spectroscopy. The material was also characterized ex situ by X-ray diffraction, transmission electron microscopy, and the four-point probe method. It was detected that TaN contained considerable oxygen impurities (up to 26 %O) even though it was grown in an ultra-high vacuum chamber. Furthermore, the impurified TaN evidences a face-centered cubic crystalline structure only and exhibits superconductivity at 2.99 K. Subsequently, we study the effect of incorporating different amounts of O atoms in TaN using ab initio calculations. A thermodynamic stability analysis shows that a TaO_xN_1-x model increases its stability as oxygen is added, demonstrating that oxygen may always be present in TaN, even when obtained at ultra-high vacuum conditions. Our results highlight the importance of considering native oxide when reporting superconductivity in TaN films since the TaO regions formed in the compound may be key to understanding the different critical temperatures reported in the literature.