Phase Boundary Enabled High Dielectric Tunability in Ba_{1 - x}Sr_xTiO₃ Thin Films and their Integration on Silicon

Achieving ultra-high dielectric tunability with robust temperature and frequency stability poses a key challenge for next-generation microwave electronics and telecommunications devices. Likewise, the integration of such materials with silicon is critical for scalability, yet it remains a complex task. This work addresses these challenges by engineering high-quality, lead-free Ba_{1- x}Sr_xTiO₃ (BST; x = 0.2-0.8) epitaxial thin films. Through systematic control of composition and epitaxial strain, we have experimentally revealed the coexistence of cubic, tetragonal, rhombohedral, and orthorhombic phases, forming a mixed-phase state analogous to a morphotropic phase boundary (MPB). This phase coexistence results in exceptional dielectric properties, including ultra-high tunability (∼91%) and a high breakdown electric field (∼800 kV/cm) at room temperature (10 kHz). The films exhibit good thermal (from 330 to 473 K) and frequency (10 kHz-1 MHz) stability. The robust dielectric tunability being associated with a diffuse-phase transition at higher strontium concentrations, arising from dipole dispersion, leading to relaxor-like behavior. Theoretical studies using effective-Hamiltonian approaches confirm the emergence of the MPB-like state and its role in enhanced dielectric permittivity and tunability. Finally, integration of these BST thin films onto silicon is demonstrated, highlighting the potential for scalability. These findings bridge the gap between material innovation and industrial implementation.