Dependence of dielectric properties on internal stresses in epitaxial barium strontium titanate thin films

A series of heteroepitaxial BaxSr1-xTiO3 thin films with composition x = 0.50 were deposited on (001) MgO substrates by pulsed-laser deposition. The thickness of the films was varied from 14 to 500 nm to produce a systematically decreasing level of in-plane tensile stresses. The microstructural and crystallographic features of the films were determined via transmission electron microscopy and x-ray diffraction. A theoretical treatment of the in-plane misfit strain as a function of film thickness is in agreement with the measured out-of-plane lattice parameters. Electrical measurements indicate a drop in the dielectric constant from 2350 for highly stressed thin films to 1700 for relaxed thicker films. The variation in the dielectric constant with the misfit strain is in accordance with a thermodynamic model developed. The relationship between the dielectric constant and electric field is also described by extending the thermodynamic model and taking the effect of electric field into account. A new definition of tunability is adopted to study the effect of strain on tunability. (C) 2001 American Institute of Physics.