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Self-selective Epitaxial Growth of BST Films: toward Nano-composite Structure for Microwave Tunable Devices

  • Tomoaki Yamada (a1), Vladimir O. Sherman (a2), Andreas Noeth (a3), Paul Muralt (a4), Alexander K. Tagantsev (a5) and Nava Setter (a6)...


Ba0.3Sr0.7TiO3 (BST) thin film composite structures were fabricated by means of a selective epitaxial growth process. The epitaxial growth of BST on SrRuO3 electrode surface was selectively achieved at 450∼485°C using a prepatterned ultra-thin amorphous BST layer that locally prevented crystallization. This self build-up mechanism resulted in a columnar composite structure, where epitaxial and amorphous BST columns are electrically connected in parallel. The epitaxial BST showed a high permittivity and a high tunability, whereas the amorphous BST showed a significantly lower permittivity and a very weak tunable behavior. The effective permittivity of the composite capacitors decreased linearly upon increasing the amorphous BST concentration q, while the tunability stayed fairly unchanged until q reached 70%, which agrees with the theoretical ferroelectric/dielectric parallel composite model. The results also give indirect evidence of a good separation between both BST phases.



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6.Since the XRD peaks of underlying SRO and STO are close to those of BST, the amorphicity of the film was also confirmed by XRD of the identical films grown on sapphire substrate.
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10.β of SrTiO3 ( ≍ 8 ×109 JC -4 m -5) is customarily taken as temperature independent and is much greater than that of BaTiO3 (see References in Ref. 2). Therefore, β of BST with a relatively high Sr concentration can be estimated assuming a proportional variation with composition.
11.For accuracy, taking into account both tunabilities of epitaxial and amorphous BSTs, .



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