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Soft-mode hardening in SrTiO3 thin films

Nature volume 404, pages 373–376 (2000)Cite this article

Abstract

Understanding the behaviour of the dielectric constant in ferroelectric thin films remains a challenging problem. These ferroelectric materials have high static dielectric constants, and so are important for their applications in high-storage-density capacitor structures such as dynamic random access memory (DRAM)1. But the dielectric constant tends to be significantly reduced in thin films, thereby limiting the potential benefit of ferroelectrics for memory devices2. Extensive studies have shown that this phenomenon could be caused by a ‘dead layer’ of very low dielectric constant between the ferroeletric film and the electrode2,3. And, although very few direct measurements are in fact available, it has been recognized that the lattice dynamical properties in the thin films should also play a key role in the reduction2 of the dielectric constant. Here we report far-infrared ellipsometry and low-frequency dielectric measurements in SrTiO3 thin films, which demonstrate that the Lyddane–Sachs–Teller relation between the optical-phonon eigenfrequencies and the dielectric constant is fully maintained, as is the case in the bulk material. This indicates that the dramatic reduction of the dielectric constant is a consequence of a profound change of the lattice dynamical properties, in particular of the reduced softening of its lowest optical-phonon mode. Our results therefore provide a better understanding of the fundamental limitations of the dielectric constant values in ferroelectric thin films.

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Figure 1: The effective dielectric function for a 2-µm-thick STO film at 200 K.
Figure 2: The imaginary part of the effective dielectric function, ε″(ω), for the 2-µm-thick STO film at different temperatures.
Figure 3: Comparison of the soft-mode frequencies and dielectric constants of bulk and thin-film STO.
Figure 4: The agreement between the LST relation and the measured ε(0) in 2-µm-thick STO film.

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Acknowledgements

We thank M. Cardona, H. Vogt, L. E. Cross, D. M. Newns, T. M. Shaw, S. B. Desu, J. Humlíček and J. Petzelt for discussions; G. P. Williams and L. Carr for technical support at the U4IR and U12IR beamlines at NSLS at Brookhaven National Laboratory, a US DOE supported user facility; and I. A. Akimov for the Raman scattering measurements. This work was partially supported by the NSF, DOE and DARPA.

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Authors and Affiliations

  1. Department of Physics, The Pennsylvania State University, University Park, 16802, Pennsylvania, USA

    A. A. Sirenko, Anna M. Clark, Jianhua Hao, Weidong Si & X. X. Xi

  2. Max-Planck-Institut für Festkörperforschung, Heisenbergstraße 1, Stuttgart, D-70569, Germany

    C. Bernhard & A. Golnik

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  1. A. A. Sirenko
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Correspondence to A. A. Sirenko.

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Sirenko, A., Bernhard, C., Golnik, A. et al. Soft-mode hardening in SrTiO3 thin films. Nature 404, 373–376 (2000). https://doi.org/10.1038/35006023

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