Ferroelectricity, High Permittivity, and Tunability in Millimeter-Size Crack-Free Ba1–xSrxTiO3 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 Ba1–xSrxTiO3 (BST), which is an important material owing to its high permittivity and electric-field-induced tunability. By employing an amorphous AlOx 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 (εr ∼ 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.