Control of crystal orientation of ferroelectric SrBi2Ta2O9 thin films withmulti-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 diffe rent ratios of Bi to Ta, three different structures were observed after hea t treatment at 800 degreesC: gamma -Bi2O3, Bi3TaO7 and Bi1.4Ta2O6.75, gamma -Bi2O3 With a sillenite structure exhibited a plate-like surface morpholog y. Bi3TaO7 with a fluorite structure exhibited a flat surface morphology. B i1.4Ta2O6.75 With a pyrochlore structure exhibited a rough surface morpholo gy. Because a flat surface morphology with a fluorite structure is desirabl e for a seeding layer, the Bi-Ta seeding layers, with a Bi to Ta ratio equa l to 3 to 1, were used as multi-seeding layers. When the seeding layers wer e-sandwiched between SET thin films, multi-seeding layers provided, after h eat treatment at 800 degreesC, a higher degree of crystal orientation in th e 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 SET thin films to obtain improved ferroelectric properties, especially high remanent polar ization values. The SET 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 SET thin film has a good potential for low-voltage operation. The multi-seeding method is thus effective for improving ferroelectric properties of SET thin films far nonvolatile memori es.