Control of crystal orientation of ferroelectric SrBi₂Ta₂O₉ thin films with multi-seeding layers

A Bi-Ta multi-seeding layer system is proposed to improve the ferroelectric properties of SrBi₂Ta₂O₉ (SBT) thin films prepared by a sol-gel method for applications to nonvolatile memories. For seeding layers composed of different ratios of Bi to Ta, three different structures were observed after heat treatment at 800°C: γ-Bi₂O₃, Bi₃TaO₇ and Bi_1.4Ta₂O_6.75. γ-Bi₂O₃ with a sillenite structure exhibited a plate-like surface morphology. Bi₃TaO₇ with a fluorite structure exhibited a flat surface morphology. Bi_1.4Ta₂O_6.75 with a pyrochlore structure exhibited a rough surface morphology. Because a flat surface morphology with a fluorite structure is desirable for a seeding layer, the Bi-Ta seeding layers, with a Bi to Ta ratio equal to 3 to 1, were used as multi-seeding layers. When the seeding layers were sandwiched between SBT thin films, multi-seeding layers provided, after heat treatment at 800°C, a higher degree of crystal orientation in the a- or b-axis direction than a simple SBT thin film without seeding layers. Orientation in the a- or b-axis direction is desirable for SBT thin films to obtain improved ferroelectric properties, especially high remanent polarization values. The SBT thin film with Bi-Ta multi-seeding layers was found to have higher saturation characteristics than that without seeding layers, which suggests that the former type of SBT thin film has a good potential for low-voltage operation. The multi-seeding method is thus effective for improving ferroelectric properties of SBT thin films for nonvolatile memories.