Exploiting dimensionality and defect mitigation to create tunable microwave dielectrics

The miniaturization and integration of frequency-agile microwave circuits--relevant to electronically tunable filters, antennas, resonators and phase shifters--with microelectronics offers tantalizing device possibilities, yet requires thin films whose dielectric constant at gigahertz frequencies can be tuned by applying a quasi-static electric field (1). Appropriate systems such as [Ba.sub.x][Sr.sub.1-x]Ti[O.sub.3] have a paraelectric-ferroelectric transition just below ambient temperature, providing high tunability (1-3). Unfortunately, such films suffer significant losses arising from defects. Recognizing that progress is stymied by dielectric loss, we start with a system with exceptionally low loss--[Sr.sub.n+1][Ti.sub.n][O.sub.3n+1] phases (4,5)--in which [(SrO).sub.2] crystallographic shear (6,7) planes provide an alternative to the formation of point defects for accommodating non-stoichiometry (8,9). Here we report the experimental realization of a highly tunable ground state arising from the emergence of a local ferroelectric instability (10) in bi-axially strained [Sr.sub.n+1][Ti.sub.n][O.sub.3n+1] phases with n [greater than or equal to] 3 at frequencies up to 125 GHz. In contrast to traditional methods of modifying ferroelectrics--doping (1-3,11,12) or strain (13-16)--in this unique system an increase in the separation between the [(SrO).sub.2] planes, which can be achieved by changing n, bolsters the local ferroelectric instability. This new control parameter, n, can be exploited to achieve a figure of merit at room temperature that rivals all known tunable microwave dielectrics (3).
Ferroelectric thin films possessing a nonlinear dielectric response to a quasi-static electric field have been widely pursued for tunable dielectric devices (17-20) that work at gigahertz frequencies. [Ba.sub.x][Sr.sub.1-x]Ti[O.sub.3] is the [...]