Ferroelectricity, High Permittivity, and Tunability in Millimeter-Size Crack-Free Ba_1–xSr_xTiO₃ Flexible Epitaxial Sheets

Flexible oxide sheets exhibiting ferroelectricity and high permittivity are crucial for the advancement of various emerging technologies. However, achieving large-area crack-free flexible oxide sheets remains difficult because oxides easily crack when their thicknesses are significantly reduced. In this study, we focused on Ba_1–xSr_xTiO₃ (BST), which is an important material owing to its high permittivity and electric-field-induced tunability. By employing an amorphous AlO_x protective layer with a thickness greater than 10 nm, we successfully fabricated millimeter-sized crack-free BST epitaxial sheets. In contrast, the sheets fabricated without protective layers exhibited breakage. In addition, we observed that a polycrystalline indium tin oxide layer acted as a suitable bottom electrode. The BST sheet with a composition of x = 0.25 exhibited excellent ferroelectric switching behavior and minimal current leakage, even when used with electrodes with a diameter of 100 μm. Furthermore, the BST sheet with a composition of x = 0.5 simultaneously exhibited high permittivity (ε_r ∼ 3500 at 10 kHz) and tunability (56%), combining the desirable characteristics of both bulk and thin-film materials. These improved dielectric properties are attributed to the absence of substrate-induced strain, which is a characteristic not observed in thin-film materials.