Soft-mode hardening in SrTiO3 thin films

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 electrode^2,3. 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.