Ferroelectric polarization and bandgap engineering in perovskite oxide via sulfur diffusion

Surface and interface conditions strongly influence the ferroelectric properties of oxide films because such regions could significantly affect the output performance by controlling the internal physics of ferroelectric capacitors. In this work, we recovered the dead layer, probably formed due to oxygen vacancies, at the ferroelectric film surface using sulfur-diffusion-based surface modification. For this purpose, Pb(Zr,Ti)O3 (PZT) thin films were deposited via chemical solution deposition, with subsequent sulfur incorporation for surface chemical modification. Specifically, an as-deposited PZT film on Pt(111)/TiO2/SiO2/Si(100) was spin-coated with a thiourea (CH4N2S) solution, with sulfur atoms added to the oxygen-deficient dead surface layer through thermal diffusion. This sulfur diffusion substantially enhanced the ferroelectric polarization and dielectric characteristics of the films. Such enhancements were associated with an increase in the tetragonality of the PZT crystal structure, as confirmed by X-ray diffraction analysis. Furthermore, X-ray photoelectron spectroscopy depth profiling revealed efficient sulfur diffusion within the dead layer to depths around 150 nm. The chemical incorporation of sulfur atoms at oxygen vacancies was responsible for enhancing ferroelectric and dielectric properties and reducing the band gap. This study provided an approach for surface defect engineering in oxide ferroelectrics, offering new insights into the restoration of dead layers and their role in enhancing ferroelectric-photovoltaic performance.