| The ferroelectric solid solution BaxSr 1−xTiO3 (BST) has long been studied for both its fundamental properties and for its potential in a wide array of applications, including tunable microwave and RF devices as well as high-density memories. A factor that has limited the performance of devices based on thin film BST and other materials is the differences in dielectric response between bulk ceramic materials and thin films. These differences are typically ascribed, in part, to the presence of residual strain in the film-substrate heterostructures, but surprisingly little work on characterizing this strain has been reported in the literature. Here we present the correlation of residual strain, as measured by x-ray diffraction, with dielectric response in epitaxial BST (x = 0.4) films grown by pulsed laser deposition. By changing the growth parameters, we obtain films with varying biaxial (2-dimensional) and hydrostatic (3-dimensional) strain. The temperature, electric field, and strain dependence of the films' dielectric constants are then modeled using a phenomenological thermodynamic theory appropriate for film-substrate heterostructures. Both the biaxial and hydrostatic strain components produce linear shifts in the transition temperature and, therefore, the entire characteristic temperature dependence. A second effect of the hydrostatic strain is the suppression of the overall dielectric response due to an electric field that is internally generated by growth-induced defects. By means of these strain effects (and the associated internal field), the dielectric response of the entire series of BST films can be seen as being modified from the bulk response. Our modeling also suggests methods of improving the dielectric properties through refinement of the growth parameters that should result in improved device performance. |
