Elastic Deformations in Thin Freestanding Ferroelectric Films

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We investigated mechanical stress in thin freestanding BaTiO3 films prepared on bare silicon and on silicon, covered by a 120 nm thick, randomly oriented Al2O3 buffer. Films prepared on bare silicon by RF sputtering are essentially stress-free. However, they disintegrate after substrate removal. In contrast, the films prepared on the Al2O3 buffer have high tensile stress, but retain their structural integrity after separation from the substrate. Substrate removal is accompanied by film corrugation; at the same time, the freestanding films resonate mechanically. This seeming contradiction can be understood on the basis of a recently developed theory of 2D clamping in thin ferroelectric films.