Thickness, strain, and temperature-dependent properties of barium strontium titanate thin films

Barium strontium titanate thin films prepared by MOCVD with Pt top and bottom electrodes have been investigated as a function of film thickness between 15 and 600 nm. Macroscopic temperature- and field-dependent electrical properties have been measured for the set of BST films with consistent composition and microstructure. All films are predominantly 001 oriented, have a fiber-textured microstructure, exhibit loss tangent values less than 0.01 (above T_max), and have negligible dielectric dispersion between 10³ and 10⁶ Hz. With decreasing film thickness, the permittivity, the transition temperature, and the electrical tunability are reduced. This reduction in properties is accompanied by an increasingly diffuse ferroelectric transition. Using a lift-off process developed at NCSU the BST thin films (with bottom electrodes) have been removed from the substrates. After substrate removal, the films were characterized both electrically, i.e., the temperature dependence of the permittivity, and structurally, i.e., precision lattice constant measurements. These "substrateless" measurements were compared to the as deposited data, and the differences between the two data sets are believed to be related to the effects of residual strain. In general, with substrate removal, the permittivity values and transition temperatures increase. X-ray measurements reveal a release of approximately 0.1% strain in a direction parallel to the measurement axis (normal to the substrate). The electrical data and strain data are considered within the context of measurements and models for the behavior of ferroelectric crystals under the influence of an applied equi-biaxial stress.