Twin-boundary engineering and thickness effect on strontium-barium niobate thin films on MgO substrate

Abstract

The Sr0.5Ba0.5Nb2O6 (SBN50) films with different thicknesses synthesized by a high-current discharge rf-sputtering in an oxygen atmosphere. All synthesized films have the structure of tetragonal tungsten bronze. The dependence of the unit cell parameters of the film material demonstrates a size effect, the magnitude of which is comparable to the distortions resulting from the ferroelectric relaxor phase transformation of SBN crystals of the same composition. The twinning of the same type is observed in all films when the twinned film components are rotated relative to each other by 36.8 degrees around the fourth-order rotational axis. The orientation of the films relative to the substrate can be written as [001]SBN∥[001]MgO; [100]SBN+∥[310]MgO and [100]SBN−∥[31¯0]MgO, where the sign "+" and "–" marks the twin components. It is noteworthy that such an orientation of the film relative to the substrate corresponds to the maximum possible tensile stress of heteroepitaxial films acting from the MgO single crystal substrate. This type of twinning is not typical for SBN50 films on MgO grown by other synthesis methods. The appearance of this unusual type of twinning is associated with the peculiarities of synthesis. At the initial moment of sputtering, the substrate is additionally heated by the sputtered substance’s plasma. The substrate temperature can also be used in other synthesis methods to control twinning and strain in barium strontium niobate thin films.