Microstructure of compositionally-graded (Ba_1-xSr_x) TiO₃thin films epitaxially grown on La0.5 Sr0.5 CoO₃-covered (100) LaAlO₃substrates by pulsed laser deposition

Compositionally-graded (Ba1-x Srx) Ti O3 (BST) epitaxial thin films (with x decreasing from 0.25 to 0.0) were deposited by pulsed laser deposition on (100) LaAl O3 (LAO) single-crystal substrates covered with a conductive La0.5 Sr0.5 Co O3 (LSCO) layer as a bottom electrode. X-ray and electron diffraction patterns demonstrate that the entire graded film has a single-crystal cubic structure. The epitaxial relationship between BST, LSCO, and LAO can be described as (100)BST ∥ (100)LSCO ∥ (100)LAO; [001]BST ∥ [001]LSCO ∥ [001]LAO. Cross-sectional transmission electron microscopy (TEM) images reveal that both the BST films and the LSCO bottom electrodes have sharp interfaces and overall uniform thickness across the entire specimen, and that they grow with a columnar structure. Planar TEM images show that the graded films exhibit granular and/or polyhedral morphologies with an average grain size of 50 nm. High-resolution TEM images reveal aligned rectangular-shaped voids in the graded BST film, with length size of 12-17 nm, and width of 5-8 nm along the 〈001〉 direction in the (100) plane.