Epitaxial growth and dielectric properties of functionally graded (Ba1−xSrx)TiO3 thin films with stoichimetric variation

Abstract

Functionally graded (Ba1−xSrx)TiO3 (BST) thin films with stoichimetric variation (x:0.0–0.25) were layer-by-layer grown epitaxially on MgO (100) single-crystal substrates with 100-nm-thick conductive La0.5Sr0.5CoO3 (LSCO) as the bottom electrode by pulsed-laser deposition. X-ray diffraction, rocking curve, and φ scans showed that the graded films are epitaxial grown with an orientation of [001](100)BST//[001](100)MgO. The surface roughness of the up-graded film was larger than that of the down-graded films, and the full width at half maximum of the BST (200) rocking curve of up-graded films was wider than that of the down-graded films. The compositional gradations along the depth in the films were confirmed by Rutherfold backscattering spectroscopy. Dielectric properties measured by vertical structures using LSCO as the bottom electrodes showed that the dielectric constant and dielectric loss at 10 kHz were 532 and 0.010 for the up-graded films, and 715 and 0.0103 for the down-graded films, respectively. An enhanced dielectric behavior was observed in the down-graded films. The graded BST films also exhibit a broad and flat profile of the dielectric constant versus the temperature. Such behavior of the dielectric response is attributed to the presence of the compositional and/or residual strain gradients in the epitaxial graded films. The possible origins of the enhanced dielectric behavior observed in the down-graded films are also briefly discussed.