Effect of Epitaxial Strain on the Spontaneous Polarization of Thin Film Ferroelectrics

Abstract

Epitaxial strain can substantially enhance the spontaneous polarizations and Curie temperatures of ferroelectric thin films compared to the corresponding bulk materials. In this Letter we use first principles calculations to calculate the effect of epitaxial strain on the spontaneous polarization of the ferroelectrics BaTiO 3 , PbTiO 3 , and LiNbO 3 , and the multiferroic material BiFeO 3 . We show that the epitaxial strain dependence of the polarization varies considerably for the different systems, and in some cases is, in fact, very small. We discuss possible reasons for this different behavior and show that the effect of epitaxial strain can easily be understood in terms of the piezoelectric and elastic constants of the unstrained materials. Our results provide a computational tool for the quantitative prediction of strain behavior in ferroelectric thin films. DOI: 10.1103/PhysRevLett.95.257601 PACS numbers: 77.84.Dy, 77.22.Ej, 77.55.+f, 77.65.ÿj The possible application of ferroelectric materials in microelectronic devices has led to strong interest in the properties of thin film ferroelectrics [1]. One important question in this context is how epitaxial strain, which is incorporated in the ferroelectric material due to the lattice mismatch with the substrate, affects the ferroelectric characteristics of the thin film. It has been demonstrated that epitaxial strain can have drastic effects, such as inducing ferroelectricity at room temperature in otherwise paraelectric SrTiO 3 [2] or increasing the ferroelectric Curie temperature of BaTiO 3 by nearly 500 C and the remanent polarization by 250% compared with the corresponding bulk values [3]. Based on these observations, it is often assumed that the strong sensitivity to epitaxial strain is a common feature of all ferroelectrics.Indeed, this assumption has been supported by first principles calculations for several simple ABO 3 perovskite ferroelectrics [4 -7]. Strain is introduced in these calculations by fixing the lattice constants corresponding to the lateral directions of the substrate while relaxing all remaining structural parameters, according to the minimum of the total energy under the epitaxial constraint. This makes it possible to isolate the pure strain effect from other effects present in real thin film samples such as structural defects, chemical inhomogeneities, and interface effects. For example, in Refs. [6,7] the change in phase stability due to epitaxial strain was investigated by first principles techniques for several simple perovskite systems including the prototype ferroelectrics BaTiO 3 and PbTiO 3 . It was shown that for these systems a series of consecutive phase transitions occur that effectively rotate the polarization from ''out-of-plane'' for compressive epitaxial strain to ''inplane'' for tensile epitaxial strain. Also, the magnitude of the spontaneous polarization within the different phases was shown to be strongly strain dependent. In contrast, we have recently shown that in BiFeO 3 , a multiferroic system where ferroel...

Multiferroic BaCoF4 in Thin Film Form: Ferroelectricity, Magnetic Ordering, and Strain

“…This behavior is the opposite of what is usually observed in oxide perovskites, where a linear dependence of the polarization as a function of the bi-axial strain has been calculated, 44 but is similar to the second order piezoelectric effect found in semiconductors with the zincblende structure. 45 The polarization developed along the crystallographic c-axis, in agreement with bulk behavior.…”

Depth Profiling Charge Accumulation from a Ferroelectric into a Doped Mott Insulator

“…In the vicinity to the Ca1-xCexMnO3/BiFeO3 interfaces the c/a ratio of the last Ca1-xCexMnO3 unit cells slightly increases and in the BiFeO3 it reaches within 3 unit cells the c/a ratio ≥1.25±0.03 which is indicative of the highly tetragonal BiFeO3 phase 19 . The stabilization of this highly tetragonal BiFeO3 phase (with ferroelectric polarization >100 µC/cm 2 ) is induced by the in-plane compressive strain of the YAlO3 substrates 20 .…”

Nanoscale Structure and Mechanism for Enhanced Electromechanical Response of Highly Strained BiFeO3 Thin Films

“…[10,11] Although the structure of BiFeO 3 had been studied for many years, [12,13,14] in 2005 the structural stability of the parent phase had come into question. [15,16] An exhaustive high-temperature neutron-diffraction study of bulk BiFeO 3 powders examined the complex structure and phases in this material and reported that both the β-and γ-phases are orthorhombic and that there is no evidence for a monoclinic phase in unstrained BiFeO 3 . [17] Other thin film studies, however, have shown that 3 a tetragonally-distorted phase (derived from a structure with P4mm symmetry, a ~ 3.665 Å, and c ~ 4.655 Å) with a large spontaneous polarization may be possible.…”

“…[17] Other thin film studies, however, have shown that 3 a tetragonally-distorted phase (derived from a structure with P4mm symmetry, a ~ 3.665 Å, and c ~ 4.655 Å) with a large spontaneous polarization may be possible. [15,18,19] Furthermore, so-called mixed-phase thin-films possessing tetragonal-and rhombohedral-like phases in complex stripe-like structures that give rise to enhanced electromechanical responses were also reported. [7] Since this report, additional information has come forth about these materials including the fact that the tetragonal-like phase is actually monoclinically distorted (possessing Cc, Cm, Pm, or Pc symmetry).…”