Thickness-Driven Phase Reorientation and Tunability in Ba0.8Sr0.2TiO3 Thin Films on SrTiO3 and Silicon

The ability to realize stable dielectric tunability in ferroelectric thin films requires control over strain, thickness, and polar domain structure, particularly in materials exhibiting diffuse ferroelectric behavior. Here, we demonstrate that film thickness provides a key control parameter for domain configuration, phase-transition character, and dielectric response in epitaxial Ba0.8Sr0.2TiO3 thin films. Using atomic-scale polarization mapping and dielectric spectroscopy, we show that increasing thickness drives a transition from coherently strained, tetragonal-dominated polarization at 60 nm to partially relaxed films at 150 nm with mixed orthorhombic and rhombohedral in-plane domains, with strain relaxation resolved by reciprocal space mapping between 100 and 150 nm. This domain reconfiguration produces a diffuse ferroelectric phase transition accompanied by a systematic enhancement of dielectric permittivity and large electric-field tunability reaching ∼79–85%, stable from 10 kHz to 1 MHz and up to ∼473 K on SrTiO3. The observed dielectric diffuseness evolves systematically with film thickness, together with corresponding changes in ferroelectric domain configuration, transition temperature, and dielectric tunability. Importantly, the same thickness–domain–tunability correlations persist after silicon integration, yielding ∼70% tunability with comparable frequency and thermal stability up to ∼453 K. These findings establish a direct experimental link between dimensionality, domain evolution, diffuse ferroelectricity, and dielectric functionality in silicon-compatible oxide thin films.