Abstract
The miniaturization and integration of frequency-agile microwave circuits—tunable filters, resonators, phase shifters and more—with microelectronics offers tantalizing device possibilities, yet requires thin films whose dielectric constant at GHz frequencies can be tuned by applying a quasi-static electric field . Appropriate systems, e.g., BaxSr1–xTiO3, have a paraelectric-to-ferroelectric transition just below ambient temperature, providing high tunability1– . 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—Srn+1TinO3n+1 phases , —where (SrO)2 crystallographic shear , planes provide an alternative to point defect formation for accommodating non-stoichiometry , . Here, we report the experimental realization of a highly tunable ground state arising from the emergence of a local ferroelectric instability in biaxially strained Srn+1TinO3n+1 phases with n ≥ 3 at frequencies up to 120 GHz. In contrast to traditional methods of modifying ferroelectrics—doping or strain in this rather unique system increasing the separation between the (SrO)2 planes 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.