Prediction of a Giant Dielectric Anomaly in Ultrathin Polydomain Ferroelectric Epitaxial Films

Abstract

The amplitudes of electric-field-induced translational vibrations of 90 ± domain walls formed in a tetragonal ferroelectric thin film grown on a cubic substrate are calculated theoretically. The domain wall contribution to the dielectric response of an epitaxial film is evaluated and shown to be important in common heterostructures. In some special film/substrate systems this contribution must increase rapidly with decreasing film thickness, which may result in a giant dielectric anomaly. PACS numbers: 77.80.Dj, 77.22.Ch, 77.55.+f Considerable current interest in ferroelectric thin films is due to their numerous potential applications in electronic devices, which utilize the unique dielectric, pyroelectric, piezoelectric, and electro-optic properties of ferroelectric materials. For many applications, such as dynamic random access memory with very large scale integration, highfrequency transducers, thermistors, and electroluminescent displays, the fabrication of thin films with large electric permittivities is especially important.Recent experimental studies [1][2][3][4][5] have found that, in tetragonal ferroelectric thin films epitaxially grown on cubic substrate, elastic domains (twins) may form during the preparation of heterostructures. The 90 ± domain structure of an epitaxial film has a strong impact on the hysteretic polarization behavior of a ferroelectric layer [4,6]. It may be expected that the formation of 90 ± domain walls in the epitaxial film can also increase markedly its electric permittivity because of the field-induced translational displacements of the walls. Indeed these ferroelastic domain walls, in contrast to the purely ferroelectric 180 ± walls, have a thickness much larger than the unit cell size (about 100 Å in BaTiO 3 [7]) so that the lattice potential barriers hindering their translational motion must be negligible. This supposition is strongly supported by the dielectric measurements on bulk BaTiO 3 single crystals with a laminar 90 ± domain structure [8], which clearly demonstrate that displacements of the 90 ± wall can be induced even by a weak measuring field E ϳ 1 kV͞m. In addition, it has been shown [9-13] that the vibrational motion of 90 ± walls must contribute considerably to the dielectric response of tetragonal ferroelectric ceramics.The above considerations motivated us to perform the first theoretical analysis of the domain wall vibrations in epitaxial ferroelectric thin films. In this Letter, we show that very large permittivities e ϳ 10 5 may arise from these vibrations in the heterostructures involving ultrathin films and pairs of materials having lattice misfits near some special value.Consider a thin tetragonal ferroelectric film epitaxially grown on a cubic substrate with the (001) plane parallel to the surface. Three variants of elastic domains can ex-ist in such a film: c domains with the tetragonal c axis perpendicular to the film surface and a 1 and a 2 domains in which the c axes are parallel to the [100] and [010] axes of the substrate, respectively. In acco...

Thermodynamic theory of epitaxial ferroelectric thin films with dense domain structures

“…If the measuring field E 0 also induces reversible displacements of domain walls (phase boundaries) from their initial equilibrium positions, an additional extrinsic contribution may appear, being caused by rotations of the polarization vector in the part of the film volume swept by moving walls. 27,28 Fortunately, our thermodynamic theory makes it possible to calculate the total permittivity of a polydomain or heterophase film, which results from both intrinsic and extrinsic contributions. To that end, equilibrium polarizations , P P in the domains (phase layers) of two types and their equilibrium volume fractions should be determined numerically as a function of the external electric field E 0 .…”

“…Evaluating in this approximation the field-induced change of the domain population (24) Equation (24) is similar to an analytic expression, which was derived for ' 33 earlier in the approximation of the linear isotropic theory of elasticity. 27,28 The substitution of Eq. (15) for the spontaneous polarization P s into Eq.…”

“…[2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20] The experimental observations triggered intensive theoretical studies of the statics and dynamics of elastic domains (twins) in epitaxial ferroelectric and ferroelastic thin films. 1,[21][22][23][24][25][26][27][28][29][30][31][32][33][34][35][36][37] In the general case, the problem is very complicated because of substantial inhomogeneity of internal mechanical stresses in polydomain (twinned) films. For a domain configuration with an arbitrary geometry, the solution may be found with the aid of the dislocation-disclination modeling of the stress sources existing in epitaxial layers at the film/substrate interface and the junctions of ferroelastic domain walls.…”

Domain boundary engineering

“…In this sense one would like to optimise the interfacial properties rather than the domain structure itself. Once this becomes possible we enter into an area where 'domain boundary engineering' may provide answers where the more traditional 'domain engineering' fails or gives only insufficient solutions [35][36][37]. The term 'engineering' implies that specific domain boundaries can be made which have the desired properties.…”

Piezoelectric properties of domain engineered barium titanate single crystals with different volume fractions of domain walls

“…10 Experimental observations also conformed that a 180°domain wall has a negligible thickness while a 90°domain wall with noticeable energy has elastic strain and a finite width. [11][12][13][14] In a recent theoretical work, domain walls in the PMN-PT crystal have been treated as thin layer conductors, 15 which means that the contribution from domain walls to the macroscopic piezoelectric properties was ignored. Moreover, the dielectric constants of the domain walls were assumed to be infinity in the conducting domain wall model, which is unphysical for an insulating crystal.…”