A comparative microstructural study of compositionally up- and down-graded (Ba,Sr)TiO3 thin films epitaxially grown on (La,Sr)CoO3-covered MgO(100) substrates by pulsed laser deposition

Abstract

We report a comparative study of the microstructure of compositionally graded (Ba1-xSrx)TiO3 (BST) films with two compositionally graded directions, up and down, with respect to the substrate, which were deposited epitaxially on (La,Sr)CoO3 (LSCO)-covered MgO(100) single-crystal substrates by pulsed laser deposition. Cross-sectional transmission electron microscopy (TEM) images and electron diffraction show that the graded films grow epitaxially with their (100) plane parallel to the (100) surface of the MgO single-crystal substrate, and with an in-plane orientation relationship of 〈001〉BST//〈001〉LSCO//〈001〉MgO. The crystalline quality and surface morphology of the graded films are closely related to the direction of the compositional gradient built into the graded films. Down-graded films (starting with a BaTiO3 layer at the film/substrate interface) have a much better crystalline quality and a smoother surface than the up-graded films (starting with a (Ba0.75Sr0.25)TiO3 layer at the film/substrate interface). Obviously, the BaTiO3 bottom layer in the down-graded film acts not only as a part of the graded film but also as an excellent seeding layer to enhance the crystallization of the subsequent film layers, resulting in a high crystalline quality of the down-graded film and an enhanced dielectric behavior. Planar (high-resolution) TEM images also demonstrate that down-graded films have a larger, and more uniform, grain size than up-graded films, and that the latter contain voids.