Giant flexoelectric effect in ferroelectric epitaxial thin films

We report on nanoscale strain gradients in ferroelectric HoMnO(3) epitaxial thin films, resulting in a giant flexoelectric effect. Using grazing-incidence in-plane x-ray diffraction, we measured strain gradients in the films, which were 6 or 7 orders of magnitude larger than typical values reported for bulk oxides. The combination of transmission electron microscopy, electrical measurements, and electrostatic calculations showed that flexoelectricity provides a means of tuning the physical properties of ferroelectric epitaxial thin films, such as domain configurations and hysteresis curves.     

Phase transition properties of ferroelectric thin films with two surface layers

The phase diagrams of ferroelectric thin films with two surface layers described by the transverse Ising model have been studied under the mean-field approximation. We discuss the effects of the exchange interaction and transverse field parameters on the phase diagrams. The results indicate that the phase transition properties of the phase diagrams can be greatly modified by changing the transverse Ising model parameters. In addition, the crossover features of the parameters from the ferroelectric dominant phase diagram to the paraelectric dominant phase diagram are determined for ferroelectric thin films with two surface layers.     

Anisotropic metal-insulator transition in epitaxial thin films

By comparing the properties of In and Pb quantum wells in a scanning tunneling microscopy subsurface imaging experiment, we found the existence of lateral bound states, a 2D Mott-Hubbard correlation gap, induced by transverse confinement. Its formation is attributed to spin or charge overscreening of quasi-2D excitations. The signature of the 2D confinement-deconfinement transition is also experimentally observed, with the correlation gap being pinned in the middle of the conduction band. A self-organized 2D Anderson lattice is suggested as a new ground state.     

Stability diagram for elastic domains in epitaxial ferroelectric thin films

The statics of isolated elastic domains (twins) in epitaxial thin tetragonal films grown on a cubic substrate is investigated theoretically. Different possible variants of the geometric shape of a domain are studied: plate, trapezoidal, and triangular. The nonuniform internal stresses, which also exist in polydomain epitaxial systems, are calculated by the effective-dislocation method. Hence the elastic energies stored in heterostructures with different domains are determined. The equilibrium width of a domain is calculated by minimizing the total internal energy of the heterostructure. Next, the stability diagram for isolated domains in epitaxial films is constructed from energy considerations. It is shown that in a large part of this diagram trapezoidal domains are energetically more advantageous than plate-shaped domains. The effect of an external electric field on the stability of 90° domains in epitaxial ferroelectric films is investigated. Fiz. Tverd. Tela (St. Petersburg) 39, 127–134 (January 1997)     

Flexoelectricity induced increase of critical thickness in epitaxial ferroelectric thin films

Flexoelectricity describes the coupling between polarization and strain/stress gradients in insulating crystals. In this paper, using the Landau–Ginsburg–Devonshire phenomenological approach, we found that flexoelectricity could increase the theoretical critical thickness in epitaxial BaTiO₃ thin films, below which the switchable spontaneous polarization vanishes. This increase is remarkable in tensile films while trivial in compressive films due to the electrostriction caused decrease of potential barrier, which can be easily destroyed by the flexoelectricity, between the ferroelectric state and the paraelectric state in tensile films. In addition, the films are still in a uni-polar state even below the critical thickness due to the flexoelectric effect.     

Effect of electromechanical boundary conditions on the properties of epitaxial ferroelectric thin films

The effects of internal stresses and depolarization fields on the properties of epitaxial ferroelectric perovskite thin films are discussed by employing the dynamic Ginzburg-Landau equation (DGLE). The numerical solution for BaTiO₃ film shows that internal stress and the depolarization field have the most effects on ferroelectric properties such as polarization, Curie temperature and susceptibility. With the increase of the thickness of the film, the polarization of epitaxial ferroelectric thin film is enhanced rapidly under high internal compressively stress. With the thickness exceeding the critical thickness for dislocation formation, the polarization increases slowly and even weakens due to relaxed internal stresses and a weak electrical boundary condition. This indicates that the effects of mechanical and electrical boundary conditions both diminish for ferroelectric thick films. Consequently, our thermodynamic method is a full scale model that can predict the properties of ferroelectric perovskite films in a wide range of film thickness.     

Domain wall roughness in epitaxial ferroelectric PbZr0.2Ti0.8O3 thin films

The static configuration of ferroelectric domain walls was investigated using atomic force microscopy on epitaxial PbZr(0.2)Ti(0.8)O(3) thin films. Measurements of domain wall roughness reveal a power-law growth of the correlation function of relative displacements B(L) alpha L(2zeta) with zeta approximately 0.26 at short length scales L, followed by an apparent saturation at large L. In the same films, the dynamic exponent mu was found to be approximately 0.6 from independent measurements of domain wall creep. These results give an effective domain wall dimensionality of d = 2.5, in good agreement with theoretical calculations for a two-dimensional elastic interface in the presence of random-bond disorder and long-range dipolar interactions.     

Polarization instability in polydomain ferroelectric epitaxial thin films and the formation of heterophase structures

A thermodynamic theory is developed for dense laminar domain structures in epitaxial ferroelectric films. It is found that, at some critical misfit strain between the film and substrate, the 90 degrees c/a/c/a domain structure becomes unstable with respect to the appearance of the polarization component parallel to domain walls, which results in the formation of a heterophase structure. For PbTiO3 and BaTiO3 films, the stability ranges of polydomain and heterophase states are determined. Dielectric anomalies accompanying misfit-strain-driven structural transformations are described.     

Leakage currents in ferroelectric thin films

The main mechanisms of leakage currents in thin lead zirconate titanate (PZT) ferroelectric films prepared by the sol–gel method are discussed. Four specific regions are determined in I–V dependencies. At very weak fields (10–20 kV/cm), the current falls with the voltage increase as a result of depolarization. In the low fields region (about 70–100 kV/cm), the leakage current decreases with the decrease of voltage ramp speed and its components are the ohmic and displacement currents. In the high fields region (≥130 kV/cm), the leakage current increases with the decrease of step voltage ramp in contrast to the previous case. Possible conductivity mechanisms are the Poole– Frenkel emission and hopping conduction. In the transition region between above-mentioned ones (from 80–90 to ～130 kV/cm), an abrupt unstable increase of current is observed caused by breakdown of reverse bias Schottky barrier. Depolarization currents are studied for sol–gel PZT films prepared at different preparation conditions.     

Leakage currents in thin ferroelectric films

The basic mechanisms of leakage current components of thin lead zirconate titanate (PZT) ferroelectric films grown by the sol-gel method have been studied. Characteristic regions of current-voltage characteristics with different charge transport mechanisms have been determined. It has been shown that there is an intermediate region which separates such regions. In one of them, the leakage current depends on properties of the contact of electrodes with PZT film at low voltages; in the other, the leakage current is controlled by intrinsic properties of the PZT film bulk, and the basic mechanism of charge transport is Poole-Frenkel emission. In the intermediate region, a stepwise change in the current has been observed, which is caused by relaxing breakdown of the Schottky barrier. Time dependences of the leakage currents have been determined. It has been shown that the leakage current decreases with increasing delay time before the Schottky barrier breakdown, and the dependence becomes opposite in character after the breakdown.     

Phase transitions in ferroelectric polymer films

Phase transitions in ferroelectric films of vinylidene fluoride-trifluoroethylene (70/30) copolymer are studied by differential scanning calorimetry (DSC). Films 40 nm thick are prepared from solution by centrifugation. DSC thermograms reveal peaks typical of the first-order phase transition that are associated with heat absorption and heat release, and the enthalpies of the transitions are determined. Upon cooling, a kink in the slope of the DSC curve that determines the temperature of the glass transition is observed at T = ?30°C.     

Magnetoconductivity of thin epitaxial silver films

The magnetoconductance (MC) of thin epitaxial Ag films on Si(111) surfaces is studied as a function of film thickness (1–125 monolayers (ML)) at 20 K under ultra high vacuum (UHV) conditions. Three different regimes of magnetoconductance are observed depending on the degree of disorder in the films which is controlled by film thickness and annealing procedures. Thick films (d>3 ML) with diffuse electron transport show in the case of large elastic scattering times ₀ a classical, negative MC B ² and in the case of small ₀ a positive MC due to weak localization effects. The MC of thin films (d<2 ML) which have a conductance smaller than e ²/h, i.e. localized electron states, is negative again.     

Formation of the ferroelectric domains in epitaxial PbTiO3 thin films grown by metalorganic chemical vapor deposition

3 thin films have been prepared by metalorganic chemical vapor deposition under reduced pressure. The formation of ferroelectric domains in films grown on SrTiO₃ and LaAlO₃ substrates was investigated by synchrotron radiation and Rutherford backscattering spectroscopy. Single-domain (3000-Å thick) and multi-domain (4500-Å thick) PbTiO₃ films were produced on SrTiO₃. For multi-domain PbTiO₃ film, the c-domain presented epitaxial structure with its c-axis perpendicular to the substrate surface, while a-domains aligned four-fold symmetrically with c-domains by 2.79^° off the c-axis of c-domains. In the film, the measured lattice constants (a, b and c) of the a- and c-domains were different from each other, indicating that the films suffered a modulated strain during domain formation. In contrast, both the a and c domains of films on LaAlO₃ were alternatively aligned on substrate with the a-axis of the a-domain and the c-axis of c-domains perpendicular to the substrate surface. Two-dimensional distribution of these domains is proposed and the formation of these kinds of domains is discussed. The surface morphology and phase transition process of single and multi domain PbTiO₃ film on SrTiO₃ were studied by atomic force microscope (AFM) and high temperature X-ray diffraction, respectively. Received: 15 August 1996/Accepted: 21 January 1997     

Growth and thermal stability of epitaxial BiFeO3 thin films

We have investigated the oxygen pressure and the temperature dependence on BiFeO₃ thin films deposited on SrTiO₃ substrates by pulsed laser deposition. Reflection high energy electron diffraction (RHEED), atomic force microscopy (AFM) and X-ray diffraction measurements indicate that high-quality epitaxial thin films are obtained for and T=650 °C. Outside of this pressure-temperature window, parasitic peaks attributed to β-Bi₂O₃ appear. We find an increase of the out-of-plane lattice parameter with oxygen pressure that we ascribe to Bi-deficiency due to its high volatility at low pressure. Ex-situ anneals have been performed and results show that as-grown single-phase BiFeO₃ thin films degrade after annealing, whereas as-grown BiFeO₃ containing impurity phases evolve toward a single-phase structure. These experiments demonstrate that parasitic phases can stabilize compounds which are usually unstable in air at elevated temperatures.