Tuning the polar states of ferroelectric films via surface charges and flexoelectricity

Abstract

Using the self-consistent Landau-Ginzburg-Devonshire approach we simulate and analyze the spontaneous formation of the domain structure in thin ferroelectric films covered with the surface screening charge of the specific nature (Bardeen-type surface states). Hence we consider the competition between the screening and the domain formation as alternative ways to reduce the electrostatic energy and reveal unusual peculiarities of distributions of polarization, electric and elastic fields conditioned by the surface screening length and the flexocoupling strength. We have established that the critical thickness of the film and its transition temperature to a paraelectric phase strongly depend on the Bardeen screening length, while the flexocoupling affects the polarization rotation and closure domain structure and induces ribbon-like nano-scale domains in the film depth far from the top open surface. Hence the joint action of the surface screening (originating from e.g. the adsorption of ambient ions or surface states) and flexocoupling may remarkably modify polar and electromechanical properties of thin ferroelectric films. (A.N.M.) 2 I. INTRODUCTIONFerroelectric materials remain the object of endless fascination for applied and fundamental science alike. The fundamental aspect of these materials is the presence of surface and interface bound charges due to the discontinuity of the spontaneous polarization. Since the early days of ferroelectricity, these charges were recognized as the key aspect of the physics of ferroelectric surfaces and interfaces.Indeed, if the polarization charge were uncompensated, it would provide bulk-like contributions to the free energy of materials, i.e. the corresponding energy would diverge with the system size. These considerations stimulated the search for mechanisms for lowering of this depolarization energy. One such mechanism is formation of ferroelectric domains, extensively analyzed in classical textbooks [1,2 ]. The second is charge screening, either internal "bulk" screening by free charges inside a ferroelecric [3,4] or external "surface" screening by the free charges in the case of the open or electroded ferroelectric film surface. Surprisingly, until now the domain formation and surface screening mechanisms were considered separately, and the competition between these effects almost escaped the attention of scientific community, despite the fact that these processes are intertwined in thin films.Surface screening of the bound charges is typically provided by the mobile charges adsorbed from the ambience in the case of high or normal humidity [5,6,7,8,9] or by internal mobile charges of defect nature [10,11]. In a specific case of the very weak screening, or its artificial absence due to the experimental treatment (cleaned surface in dry atmosphere, ultra-high vacuum or thick dielectric layer at the surface) the screening charges can be localized at surface states caused by the strong bandbending by depolarization field [12,13,14,15,16,17]. For both aforementioned cases the s...

Revealing the role of local stress on the depolarization of BNT-BT-based relaxors

“…However, the electric boundary conditions during heating differed, as the electrodes were kept on for the dielectric and the neutron-diffraction measurements, whereas the electrode on the probed surface had to be removed for the PFM experiment. This could potentially lead to an increased screening length in the PFM measurements compared to the other experiments and a decrease of the transition temperatures, as indicated by calculations on ferroelectric PbTiO 3 thin films [54]. These calculations show, however, that the shift in the characteristic temperatures becomes less pronounced with increasing film thickness and should be negligible for bulk samples.…”

Laminated Modulation of Tricritical Ferroelectrics Exhibiting Highly Enhanced Dielectric Permittivity and Temperature Stability

“…The terms f elas , f elec , and f grad in Equation reflect the long‐range elastic and electrostatic interaction energies and the short‐range exchange interaction energy of gradient term, respectively. The term f elec = f dipole + f depola + f appl , where f dipole is the dipole–dipole interaction caused by polarization, f depola is the depolarization energy density and f appl is the energy density caused by applied electric field. The temporal evolution of the spontaneous polarization field can be obtained by solving the time‐dependent Ginzburg–Landau (TDGL) function as Equation :…”

Defect-driven flexochemical coupling in thin ferroelectric films

“…To illustrate the above mentioned issues, Fig. 2 shows the spatial distributions of the in-plane and out-of-plane polarization components, P x and P z , respectively, corresponding elastic strains u xx and u zz , and von Mises stress Note that the value of the screening length λ strongly affects the polar properties of the film, determines its critical thickness at fixed temperature and the existence as well as the type of the domain structure [30]. In addition, a pronounced minimum at a certain width, which depends on W, temperature and film thickness, appears on the dependence of the system specific energy E on the domain lateral size d when λ increases [see Table II. B.…”