  In this manuscript, we show through an experimental-computational proof of
concept the native oxide formation into superconducting TaN films. 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 evidence a
face-centered cubic crystalline structure only and exhibits superconductivity
at 2.99 K. To understand the feasibility of the native oxide in TaN, we study
the effect of incorporating different amounts of O atoms in TaN using ab-initio
calculations. A thermodynamic stability analysis shows that a TaOxN1-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.
All analyzed models exhibit metallic behavior. Charge density difference maps
reveal that N and O atoms have a higher charge density redistribution than Ta
atoms. The electron localization function maps and line profiles indicate that
Ta-O and Ta-N bonds are mainly ionic. As expected, stronger ionic behavior is
observed in the Ta-O bonds due to the electronegativity difference between O
and N atoms. Recent evidence points to superconductivity in bulk TaO,
confirming the asseverations of superconductivity in our samples. The results
discussed here 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.
