Fyzikální ústav Akademie věd ČR

Scientists from Institute of Physics ASCR contributed to development of a new approach to preparation of highly tunable microwave dielectrics exploring Sr_n+1Ti_nO_3n+1 with layered perovskite crystal structure. This material has in the form of 50 nm thin films and under mechanical strain excellent dielectric properties, which are promising for applications in microwave electronics, e.g. in cellular phones. The paper, describing unique properties and preparation of biaxially strained thin films of Sr_n+1Ti_nO_3n+1, was published in Nature journal on 16^th October 2013.

Modern microwave filters and resonators used in cellular phones are made from materials, whose permittivity¹ is tuned by electric field. Ferroelectrics ² reach highest tunability near temperature of the phase transition from paraelectric to ferroelectric phase, because the permittivity is the highest at this temperature. Nowadays, mainly Ba_1-xSr_xTiO₃. is commercially used, because it exhibits high tuning of permittivity. Unfortunatelly, this material has undesirable high dielectric losses. Scientists from Department of Dielectrics with American colleagues theoretically predicted, physically prepared and experimentally characterized layered perovskite system Sr_n+1Ti_nO_3n+1 with n=1-6. Although this system is non-ferroelectric in the bulk crystals or ceramics, their thin films become ferroelectric under tensile strain and their critical temperatures (T_c) increase with n. Owing to this their permittivity and tunability increase. The best properties were obtained in Sr₇Ti₆O₁₉, where high tunability and exceptionally low dielectric losses were observed. Sr₇Ti₆O₁₉ has one order of magnitude better microwave properties than widely used Ba_1-xSr_xTiO₃. Dielectric losses are usually caused by structural defects, which are always present in the materials. Authors found out that unique layered nanostructure of Sr_n+1Ti_nO_3n+1 „absorbs“ defects and therefore it exhibits low dielectric losses.

Figure of layered crystal structure of Sr_n+1Ti_nO_3n+1 with n=1-6, ∞ and temperature dependence of dielectric permittivity in thin films of Sr_n+1Ti_nO_3n+1. Temperatures of permittivity maxima correspond to paraelectric-ferroelectric phase transition temperatures.

C.-H. Lee, N.D. Orloff, T. Birol, Y. Zhu, V. Goian, E. Rocas, R. Haislmaier, E. Vlahos, J.A. Mundy, L.F. Kourkoutis, Y. Nie, M.D. Biegalski, J. Zhang, M. Bernhagen, N. A. Benedek, Y. Kim, J.D. Brock, R. Uecker, X.X. Xi, V. Gopalan, D. Nuzhnyy, S. Kamba, D.A. Muller, I. Takeuchi, J.C. Booth, C.J. Fennie & D.G. Schlom, Exploiting dimensionality and defect mitigation to create tunable microwave dielectrics, Nature (2013)

This paper is result of a long-term collaboration between Institute of Physics ASCR and Cornell University in USA. Authors from Institute of Physics ASCR are marked in bold.

More information can be obtained from Dr. Stanislav Kamba, kambafzu [dot] cz, phone: +420 266 052 957

¹ Permittivity is material constant expressing capability of electrical polarizability of the given material.
² Ferroelectrics are spontaneously polarized materials whose polarization is switchable under external electric field.