Enhanced tunability of sandwich-like structural barium strontium titanate thin films on stainless steel substrates

Abstract

Barium strontium titanate (BST) thin films deposited on stainless steel (SS) substrates can be expected for their potential applications in MEMS-structure tunable microwave devices. In this paper, two sandwich-like structures with varied average Ba/Sr ratio of BST thin films, Ba0.6Sr0.4TiO3 layer in the middle or at ends, on LaNiO3 buffered SS substrates were fabricated by sol–gel technique, and meanwhile were theoretically carried out via a modified phenomenological model. Results show that dielectric constant and tunability of such BST films can be tailored by average Ba/Sr ratio and structures. When the average Ba/Sr ratio is unchanged, the structure with smaller lattice parameter of middle layer than that of top/bottom layer can achieve larger tunability resulting from its smaller compressive misfit strain, which has been qualitatively characterized by X-ray diffraction and Raman spectra. Especially, the largest tunability reaches 30 % under the electric field of 300 kV cm−1 by locating Ba0.4Sr0.6TiO3 layer in the middle and Ba0.6Sr0.4TiO3 layers at ends. Furthermore, the experimental dielectric constants and tunabilities are in good agreement with the calculated data, instructing that the dielectric properties of BST thin films with unchanged Ba/Sr ratio can be tailored by changing the structure to adjust the lattice misfit strain of each uniform layer. This implies that multilayer BST thin films also can be designed by phenomenological theory.