Exploiting dimensionality and defect mitigation to create tunable microwave dielectrics

A new family of tunable microwave dielectrics with unparalleled performance at frequencies up to 125 GHz at room temperature has been created, using dimensionality to add and control a local ferroelectric instability in a system with exceptionally low dielectric loss. Microwave-proof insulators Tunable dielectric materials are valuable components for complex microwave circuitry, yet such materials tend to suffer losses when operated at microwave frequencies owing to intrinsic defects in their structures. Che-Hui Lee and colleagues have selected a family of dielectrics known to exhibit exceptionally low loss, and now show how these materials can be engineered to boost their tunability and attain levels of performance that rival all known 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 x Sr 1− x TiO 3 have a paraelectric–ferroelectric transition just below ambient temperature, providing high tunability 1 , 2 , 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 n +1 Ti n O 3 n +1 phases 4 , 5 —in which (SrO) 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 biaxially strained Sr n +1 Ti n O 3 n +1 phases with n ≥ 3 at frequencies up to 125 GHz. In contrast to traditional methods of modifying ferroelectrics—doping 1 , 2 , 3 , 11 , 12 or strain 13 , 14 , 15 , 16 —in this unique system an increase in the separation between the (SrO) 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 .