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 t hickness of the films was varied from 14 to 500 nm to produce a systematica lly decreasing level of in-plane tensile stresses. The microstructural and crystallographic features of the films were determined via transmission ele ctron microscopy and x-ray diffraction. A theoretical treatment of the in-p lane 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 constan t with the misfit strain is in accordance with a thermodynamic model develo ped. 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 t o study the effect of strain on tunability. (C) 2001 American Institute of Physics.