Fyzikální ústav Akademie věd ČR

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. Appropriate systems such as Ba_xSr_1−xTiO₃ have a paraelectric–ferroelectric transition just below ambient temperature, providing high tunability. 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+1Ti_nO_3n+1 phases — in which (SrO)₂ crystallographic shear planes provide an alternative to the formation of point defects 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 Sr_n+1Ti_nO_3n+1 phases with n ≥ 3 at frequencies up to 125 GHz. In contrast to traditional methods of modifying ferroelectrics — doping or strain — 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.

Layered crystallic structure of Sr_n+1Ti_nO_3n+1 with  n = 1–6 (left) and dielectric permitivity vs. temperature dependency in thin layers of Sr_n+1Ti_nO_3n+1 (right). Temperature in permitivity peaks is matching the temperature of transition to ferroelectric state.

¹Department of Materials Science and Engineering, Cornell University, Ithaca, New York 14853, USA
²Department of Materials Science and Engineering, Pennsylvania State University, University Park, Pennsylvania 16802, USA
³National Institute of Standards and Technology, Boulder, Colorado 80305, USA
⁴Department of Physics, University of Maryland, College Park, Maryland 20742, USA
⁵School of Applied and Engineering Physics, Cornell University, Ithaca, New York 14853, USA
⁶Institute of Physics ASCR, Na Slovance 2, 182 21 Prague 8, Czech Republic
⁷Department of Signal Theory and Communications, Universitat Politècnica de Catalunya, 08034 Barcelona, Spain
⁸Kavli Institute at Cornell for Nanoscale Science, Ithaca, New York 14853, USA
⁹Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA
¹⁰Leibniz Institute for Crystal Growth, Max-Born-Strasse 2, D-12489 Berlin, Germany
¹¹Materials Science and Engineering Program, The University of Texas at Austin, Austin, Texas 78712, USA
¹²Department of Physics, Temple University, Philadelphia, Pennsylvania 19122, USA
¹³Department of Materials Science and Engineering, University of Maryland, College Park, Maryland 20742, USA