Enhancement of ferroelectricity in perovskite BaTiO₃ epitaxial thin films by sulfurization

<p>Sulfur is a promising anion dopant for exploring exotic physical phenomena in complex perovskite oxides. However, sulfurization to the epitaxial single-crystal oxide thin films with high crystallinity is experimentally challenging due to the volatility of sulfur element; thus, sulfurization effects on the associated properties have been scarcely studied. Here, we demonstrate an enhancement of ferroelectric polarization of epitaxial BaTiO₃ thin films by sulfur doping. Initially, the epitaxial BaTiO₃ thin films with high crystallinity were grown by pulsed laser deposition (PLD). Then, sulfurization to epitaxial BaTiO₃ films was performed using a precursor of thiourea (CH₄N₂S) solution via a spin-coating technique. The crystalline structure of sulfurized BaTiO₃ films was identified by X-ray diffraction (XRD) and scanning transmission electron microscopy (STEM). The structural distortion with the elongated out-of-plane lattice constant was observed in the sulfurized BaTiO₃ films. Atomic force microscopy (AFM) analyses also confirmed the surface morphology of films after sulfurization. Interestingly, we found an enhanced ferroelectric polarization in sulfur-doped BaTiO₃ films accompanying the improved tetragonality in the crystal structure after sulfurization. The increments in the remnant (~34.8%) and saturated (~30.6%) polarizations of sulfurized BaTiO₃ films were obtained in comparison with pure BaTiO₃ films. Our work could be a primary study for a thorough understanding of the sulfur doping effect in perovskite oxides, opening up the potential of oxysulfide materials.</p>