Suppressed Dependence of Polarization on Epitaxial Strain in Highly Polar Ferroelectrics

Abstract

A combined experimental and computational investigation of coupling between polarization and epitaxial strain in highly polar ferroelectric PbZr 0.2 Ti 0.8 O 3 (PZT) thin films is reported. A comparison of the properties of relaxed (tetragonality c/a ≈ 1.05) and highly-strained (c/a ≈ 1.09) epitaxial films shows that polarization, while being amongst the highest reported for PZT or PbTiO 3 in either film or bulk forms (P r ≈ 82 μC/cm 2 ), is almost independent of the epitaxial strain. We attribute this behavior to a suppressed sensitivity of the A-site cations to epitaxial strain in these Pb-based perovskites, where the ferroelectric displacements are already large, contrary to the case of less polar perovskites, such as BaTiO 3 . In the latter case, the A-site cation (Ba) and equatorial oxygen displacements can lead to substantial polarization increases.Epitaxial strain, induced in thin films due to lattice mismatch between the material and the substrate, results in enhanced properties and device performance for many materials. Examples include a higher operation speed and lower power consumption in strain-engineered semiconductor-based devices [1] and a large enhancement of ferroelectric and dielectric responses in certain complex oxide perovskites [2][3][4][5][6]. Therefore, epitaxial strain is recognized as a useful tool to influence materials properties and it is tempting to assume that such strong sensitivity to epitaxial strain is common to all perovskite compounds.However, recent computational studies suggest that this is not universally true [7], while an experimental confirmation of such nonuniversality is a formidable task due to limitations explained below. Moreover, the fundamental mechanisms responsible for the coupling between epitaxial strain and ferroelectric polarization in thin films are not yet fully understood, although it is quite clear that they should be manifested most profoundly in coherently grown thin films. In such films misfit dislocation formation is energetically and/or kinetically prohibited and therefore reduction of the polarization by pinning of dipoles by defects is unlikely. This is, however, true only for films with thicknesses below a certain critical value (t c ), above which the strain relaxation via defect formation occurs. This thickness depends on the lattice mismatch between the film and the substrate [8,9]: for example, in BaTiO 3 on SrTiO 3 (in-plane mismatch of 2.4%) a t c ≈ 2-4 nm has been experimentally determined [10,11]. But at such small film thicknesses polarization measurements are hindered by high tunnelling and/or leakage currents. A strong influence of depolarization fields, originating from the incomplete screening of polarization [12], and a reduction of atomic displacements in the vicinity of interfaces [13] may also mask the intrinsic effects. Therefore, a careful characterization of strain effects is most reliably achieved in a system with a fairly small lattice mismatch with the substrate and, consequently, large t c (e.g., several tens of nanometr...

Chemistry, growth kinetics, and epitaxial stabilization of Sn2+ in Sn-doped SrTiO3 using (CH3)6Sn2 tin precursor

“…Nevertheless, they are easy to process with standard DFT techniques and can still provide useful information on the basic geometrical parameters of the substituted system. 26 Figure 1(a) shows 2θ − ω scans of Sn-alloyed SrTiO 3 films on SrTiO 3 (001) substrates grown at a fixed Ti BEP (1.27 × 10 6 Torr) and at different Sn BEPs (1.63 × 10 7 -3.5 × 10 6 Torr). No secondary phases were observed.…”

Applying uniform reversible strain to epitaxial oxide films

“…1,2,13,14 However, if additional parameters ͑composition, microstructure, thickness͒ vary among these films, it is impossible to measure the true effect of strain on properties that are highly sensitive to these parameters, such as coercive fields. Thermal expansion differences between substrate and film can yield well-defined but fixed strain states, 8 whereas bending techniques 15,16 provide reversible strain for films on large substrates ͑wafers͒.…”

“…Thus, strain effects can now be explored quantitatively in fundamentally and technologically important oxides, [2][3][4][5][6][7][8][9][10] i.e., ferroelectrics, magnetic and optically active oxides, multiferroics, and superconductors. Ab initio theories are increasingly successful in predicting the large strain effect on ferroelectric polarization 2,8,11 and straininduced ferroelectric phases, 1,12 and domain configurations can be controlled by epitaxial strain. 8,10 In two-phase multiferroics, the magnetization of a magnetic component responds to the strain induced by a piezoelectric component, producing a robust magnetoelectric effect.…”

Misfit strain–film thickness phase diagrams and related electromechanical properties of epitaxial ultra-thin lead zirconate titanate films

“…Published reports show that compressive strains (in the plane of the film-substrate interface) may enhance the tetragonal out-of-plane polarization [6,7], while tensile strains induce in-plane rotational phases of the polarization [3]. It was also shown that the impact of in-plane compressive stresses is significantly more pronounced when the variations in the Curie transition temperature are compared to the polarization [8].…”