Jun Ouyang

Size effects in ultrathin epitaxial ferroelectric heterostructures

In this letter we report on the effect of thickness scaling in model PbZr0.2Ti0.8O3(PZT)∕SrRuO3 heterostructures. Although theoretical models for thickness scaling have been widely reported, direct quantitative experimental data for ultrathin perovskite (<10nm) films in the presence of real electrodes have still not been reported. In this letter we show a systematic quantitative experimental study of the thickness dependence of switched polarization in (001) epitaxial PZT films, 4to80nm thick. A preliminary model based on a modified Landau Ginzburg approach suggests that the nature of the electrostatics at the ferroelectric–electrode interface plays a significant role in the scaling of ferroelectric thin films.

Thickness dependence of structural and piezoelectric properties of epitaxial Pb(Zr0.52Ti0.48)O3 films on Si and SrTiO3 substrates

We report the structural and longitudinal piezoelectric responses (d33) of epitaxial Pb(Zr0.52Ti0.48)O3 (PZT) films on (001) SrTiO3 and Si substrates in the thickness range of 40nm–4μm. With increasing film thickness the tetragonality of PZT was reduced. The increase in d33 value with increasing film thicknesses was attributed to the reduction of substrate constraints and softening of PZT due to reduced tetragonality. The d33 values of PZT films on Si substrates (∼330pm∕V) are higher than those on SrTiO3 substrates (∼200pm∕V). The epitaxial PZT films on silicon will lead to the fabrication of high performance piezoelectric microelectromechanical devices.

Self-assembled multiferroic nanostructures in the CoFe2O4-PbTiO3 system

The effect of substrate orientation on the morphologies of epitaxial self-assembled nanostructures was demonstrated using multiferroic 0.67PbTiO3-0.33CoFe2O4 thin films. The two-phase composite films were grown by pulsed laser deposition on single crystal SrTiO3 substrates having (001) and (110) orientations. The nanostructures of both orientations consisted of vertical rod- or platelet-like columns of CoFe2O4 dispersed in a PbTiO3 matrix. For the (001) orientation the platelet habits were parallel to the {110} planes, whereas for the (110) orientation the platelets were parallel to the {111} planes. The differences were explained using a thermodynamic theory of heterophase structures.

Formation of 90° elastic domains during local 180° switching in epitaxial ferroelectric thin films

90° elastic domains were observed after local polings in epitaxial ferroelectric Pb(Zr0.2Ti0.8)O3 thin films via piezoresponse force microscopy. An area of internal stress arises under a conductive atomic force microscope tip due to the opposite signs of the converse piezoelectric effects in the switched domain and the unswitched films surrounding. The formation of 90° domains leads to the relaxation of the internal stress and stabilization of 180° domain after turning off the electric field applied by the tip. The criterion that formulates the necessary condition for realization of the relaxation mechanism is presented as well.

Orientation dependence of the converse piezoelectric constantsfor epitaxial single domain ferroelectric films

The orientation dependence of the converse piezoelectric constants in epitaxial single domain piezoelectric films constrained by substrate is calculated. The calculations are supported by the experimental measuring results of longitudinal piezoelectric constants of ferroelectric tetragonal Pb(Zr0.2Ti0.8)O3 thin films with (001), (011), and (111) orientations.

Effect of 90° domain movement on the piezoelectric response of patterned PbZr0.2Ti0.8O3∕SrTiO3∕Si heterostructures

The converse longitudinal piezoelectric responses of PbZr0.2Ti0.8O3∕SrTiO3∕Si heterostructures have been studied for continuous films and for islands patterned by focused ion beam milling (typical dimension is 1×1×1μm). The intrinsic piezoelectric response of the island with an immobile polydomain structure is modeled using finite element analysis. The difference between the results of experimental measurement and modeling is explained by an extrinsic contribution to the piezoresponse from 90° domain movement. The contribution of 90° domain movement to the total piezoresponse is shown to be greatly enhanced in the patterned islands, and leads to an effective piezoelectric coefficient (∼400pm∕V) which is five times larger than the theoretical bulk single domain value.

Intrinsic effective piezoelectric coefficient e31,f for ferroelectric thin films

As a function of film orientation, the intrinsic effective piezoelectric coefficient e31,f is generally formulated for a substrate-constrained ferroelectric film. Numerical results are obtained for Pb(ZrxTi1−x)O3 (PZT) thin films with tetragonal and rhombohedral compositions. It is illustrated that the optimal orientation for e31,f are close to [001] orientation in both tetragonal and rhombohedral PZT films and the maximum calculated e31,f is about −30C∕m2 on the rhombohedral side of the morphotropic phase boundary.

Orientation dependence of the intrinsic converse longitudinal piezoelectric constant for 0.67Pb(Mg1/3Nb2/3)O3–0.33PbTiO3 ferroelectric films with a rhombohedral structure

The general orientation dependence of the intrinsic converse longitudinal piezoelectric constant dnnf in an epitaxial rhombohedral film is calculated by taking into account the effect of substrate clamping. Theoretical predictions are made for dnnf of epitaxial 0.67Pb(Mg1/3Nb2/3)O3–0.33PbTiO3 rhombohedral films, which are compared with experimental results on pseudo-cubic (001), (110) and (111) orientations.

Polydomain structures in ferroelectric and ferroelastic epitaxial films

A review of theoretical models, phase field modeling and experimental studies of domain structures in epitaxial films is presented. The thermodynamic theory of such domain structures is presented within the macroscopic thermo-mechanical framework. The theory allows for the evaluation of the main parameters of the domain structure using the energy minimization approach applied to the energy of elastic interactions. For homophase (polytwin) films, the thermodynamic theory provides a quantitative tool that can be used to estimate domain fractions in the film and the type of domain structure architecture. For heterophase films, the theory describes (a) the conditions under which two-phase structures can be obtained in epitaxial films, and (b) the phase and domain fractions in these films. The thermodynamic theory can also be used to describe the extrinsic contributions from domain structures to the functional properties of epitaxial ferroelectric films. The review of phase field modeling demonstrates that computational results reproduce the predictions of the thermodynamic theory. It is also shown that the phase field modeling that utilizes the energy minimization procedure for elastic and interfacial energies can be used to predict domain morphology for the films with two-phase structures produced either by phase transformation or through the co-deposition of immiscible phases. The experimental data presented in the review validate predictions of the thermodynamic model and the results of phase field modeling.