Enhancement of ferroelectricity in perovskite BaTiO3 epitaxial thin films by sulfurization

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 (similar to 34.8%) and saturated (similar to 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.