Session X32: Focus Session: Dielectric, Ferroelectric, and Piezoelectric Oxides - Elastic and Optical Properties

Flexoelectricity in Nanoscale Ferroelectrics

All ferroelectrics are piezoelectric and thus have an intrinsic coupling between polarization and strain. There exists an additional electromechanical coupling, however, between polarization and strain gradients. Strain gradients are intrinsically vectorial fields and, therefore, they can in principle be used to modify both the orientation and the sign of the polarization, thanks to the coupling known as flexoelectricity. Flexoelectricity is possible even in paraelectric materials, but is generally stronger in ferroelectrics on account of their high permittivity (the flexoelectric coefficient is proportional to the dielectric constant). Moreover, strain gradients can be large at the nanoscale due to the smallness of the relaxation length and, accordingly, strong flexoelectric effects can be expected in nanoscale ferroelectrics. In this talk we will present two recent results that highlight the above features. In the first part, I will show how polarization tilting can be achieved in a nominally tetragonal ferroelectric (PbTiO$_{3})$ thanks to the internal flexoelectric fields generated in nano-twinned epitaxial thin films. Flexoelectricity thus offers a purely physical means of achieving rotated polarizations, which are thought to be useful for enhanced piezoelectricity. In the second part, we will show how the large strain gradients generated by pushing the sharp tip of an atomic force microscope against the surface of a thin ferroelectric film can be used to actively switch its polarity by 180$^{\circ}$. This enables a new concept for ``multiferroic'' memory operation in which the memory bits are written mechanically and read electrically.   

Giant Flexoelectric Effect in Ferroelectric Epitaxial Thin Films

The flexoelectric effect describes an electric field that is generated by a strain gradient. Owing to its universal nature, flexoelectricity has inspired broad scientific interest and has application potential, particularly in flexible systems. In solids, however, there has been little investigation into flexoelectricity, due to its minuscule magnitude by limited elastic deformation. In this presentation, we will develop a general framework for realizing and modulating the giant flexoelectric effect in epitaxial oxide thin films, emphasizing the key role of flexoelectricity in solids.\footnote{D. Lee \textit{et al.}, Phys. Rev. Lett. \textbf{107}, 057602 (2011).} In epitaxial oxide thin films, a lattice mismatch between the film and the substrate can result in strain relaxation within tens of nanometers of the film/substrate interface, inducing a large strain gradient. We observed the nanoscale strain gradients in ferroelectric HoMnO$_{3}$ epitaxial thin films, which were 6 or 7 orders of magnitude larger than typical values reported for bulk oxides. Giant flexoelectric effect by the nanoscale strain gradient provides a means of tuning the physical properties of ferroelectric epitaxial thin films, such as domain configurations and hysteresis curves.\footnote{D. Lee \textit{et al.}, Phys. Rev. B \textbf{81}, 012101 (2010).}   

Thermal Stresses in Ferroelectric Thin Films and Their Role on the Dielectric, Pyroelectric, and Electrocaloric Properties

In-plane strains develop in thin films due to thermal stresses that arise from differences between the thermal expansion coefficients of the film and the substrate upon cooling from a growth temperature ($T_{G})$ to room temperature (RT). In ferroelectric (FE) films, there is a coupling between strain and polarization through electrostriction. Therefore, thermal strains may have profound effects on the dielectric, pyroelectric, and electrocaloric (EC) responses of FE films. We provide here a quantitative thermodynamic model to investigate the role of in-plane thermal strains on these properties. We show that there is a substantial degradation in the dielectric response and tunability of SrTiO$_{3}$ films on IC-friendly substrates such as Si and $c$-sapphire due to tensile thermal in-plane strains. Our analysis on (001)-textured polycrystalline Ba$_{x}$Sr$_{1-x}$O$_{3}$ (BST) films with different compositions indicates that for BST 60/40 and BST 70/30 films, the pyroelectric response does not display a significant dependence on$ T_{G}$ if the FE is in a paraelectric state. Furthermore, we show that for BaTiO$_{3}$ films on Si, the thermal stresses are sufficient to shift the zero-field Curie temperature to RT, resulting in a strong enhancement of the EC properties as compared to the bulk material.   

Polarization in Perovskite Manganites induced by Shear Stress

We found static polarization in perovskite manganite films when they are under shear stress. The phenomenon is omnipresent in films deformed in (at least) monoclinic fashion due to the substrate-imposed strain, whereas it is absent in bulk crystals even though they are distorted in a similar manner in thermal equilibrium. The substrate stress of low symmetry is clearly the driving force for the appearance of the polarization. Optical second-harmonic generation (SHG) confirms the loss of inversion symmetry in strained films and pyroelectricity was detected in insulating films confirming the presence of the static polarization. DFT calculations show that the stable atomic positions in the experimentally observed structure is polar with the shift of the center of gravity of anions relative to that of cations as much as $10^{-2} {\AA}$. The calculated polar structure is consistent with the symmetry obtained from the SHG polarimetry.   

Discovery of strain glass transition in non-metallic ferroelastic

Strain glass, a glassy state of lattice strain, has been identified in alloys with shuffle being the principle order parameter and strain being the secondary order parameter. However, it is well known that many non-metallic ferroelastic systems possess long range order with tilt being the first order parameter. But the existence of the glassy state of such strain caused by tilt remains unclear. In the present study, we report that the strain glass indeed exists in the non-metallic ferroelastic material, a Sr and Nb co-doped LaAlO$_{3}$ system, with randomly frozen tilt strain local order. With increasing defect concentration x in La$_{1-x}$Sr$_{x}$Al$_{0.95}$ Nb$_{0.05}$O$_{3}$, the martensitic transition is gradually suppressed and finally strain glass transition occurs. The glassy transition is characterized by a typical frequency dispersion of modulus, a broken of ergodicity for static strain, as well as the formation of nano-domains with R local structure. Due to the strong local barrier caused by the randomly distributed point defects, the ideal freezing temperature $T_{0}$ of strain glass in this system increases with defect concentration, which can be well understood by a modified Landau free energy landscape.   

Enhanced Piezoelectricity in PbTiO$_3$/BaTiO$_3$ Superlattices

First-principles calculations by Cooper and Rabe predict an enhancement of the piezoelectric coefficient ($d_{33}$) in PbTiO$_3$/BaTiO$_3$ (PTO/BTO) superlattices for intermediate values of PTO concentration. PTO/BTO superlattices have been fabricated using an off-axis RF magnetron sputtering technique, enabling x-ray diffraction, electrical measurements and atomic force microscopy on this system. The experimental results agree with the calculated polarization, tetragonality and enhanced piezoelectricity as a function of PTO concentration. Additional first-principles calculations indicate that the enhancement in $d_{33}$ is more pronounced in shorter-period superlattices. By applying a Landau-Devonshire model to this system, we find that the enhanced piezoelectricity is due to the combination of a bulk effect associated with the presence of finite electric fields in each layer and interfacial effects. Implications of our results for future experiments are discussed.   

Interpolation schemes for high-throughput prediction of new piezoelectric alloys

Systematic discovery of materials with optimized properties based on first principles methodologies requires well-defined descriptors, in addition to the automation infrastructure for calculations and data analysis. We have designed a set of computationally affordable descriptors for enhanced piezoelectric performances and analyzed the chemical space for oxides with the perovskite structure. Our results include phase stability for the most promising compositions and ad hoc interpolation schemes that have been exploited to identify 49 alloys with favorable properties.   

Piezoelectric characteristics of PZT thin films on polymer substrate

The goal of piezoelectric energy harvesting is to improve the power efficiency of devices. One of the approaches for the improvement of power efficiency is to apply the large strain on the piezoelectric materials and then many scientists approached using thin films or nano-structured piezoelectric materials to obtain flexibility. However, the conventional thin film processes available for the fabrication of piezoelectric materials as PbZr$_{0.52}$Ti$_{0.48}$O$_{3}$ (PZT) are not compatible with flexible electronics because they require high processing temperatures ($>$700$^{o}$C) to obtain piezoelectricity. Excimer laser annealing (ELA) is attractive heat process for the low-temperature crystallization, because of its material selectivity and short heating time. In this study, the amorphous PZT thin films were deposited on polymer substrate by rf-sputtering. To crystallize the amorphous films, the ELA was carried out with various conditions as function of the applied laser energy density, the number of pulse, and the repetition rate. To evaluate the piezoelectric characteristics, piezoelectric force microscopy (PFM) and electrometer are used. As a result, we obtained the crystallized PZT thin film on flexible substrate and obtained flexible piezoelectric energy harvester.   

Characterizing the Angular Frequency of Radiative Polaritons using Infrared Spectroscopy

Polaritons are important for understanding the optical properties of oxide films and possibly also for energy conversion. The two known types of polaritons: surface phonon polaritons (SPP) and radiative polaritons (RP), form when infrared (IR) photons enter a crystal lattice material and couple with the phonons present. While SPP's are largely studied for their heat transfer properties, RP's are typically not studied; therefore, much is still not understood about RP's. It is known, however, that RP's have a complex angular frequency, which includes a real part, Re($\omega$), and an imaginary part, Im($\omega$). Investigations done by our Group suggests that Im($\omega$) indicates the frequency of the radiated field. What is unknown is the relationship between Re($\omega$) and Im($\omega$). Therefore, we experimentally compare three different oxides deposited on silicon and aluminum by atomic layer deposition (ALD). This allows us to characterize proportionality between Re($\omega$) and Im($\omega$) with respect to oxide film thickness.   

Dielectric and optical properties of SrTiO$_3$ films deposited from metallo-organic solution

SrTiO$_3$ thin films on Si were grown by metallo-organic solution deposition. Spectroscopic ellipsometry was used to determine the ellipsometric angles $\psi$ and $\Delta$ in the 0.6 to 6.6 eV spectral range at three angles of incidence. From the region below 3.5 eV (where the films are transparent), we are able to determine the refractive index and the film thickness, which ranges from 140 to 340 nm for different films. The refractive index is similar for all of our films, but 25\% lower than that of bulk SrTiO$_3$. By contrast, the low-frequency dielectric constant of similar films grown on metalized Si substrates is about the same as bulk SrTiO$_3$. Using a B-spline parametrization, we are able to determine the dielectric function of our films from the ellipsometric angles. We find an onset of absorption (band gap) of about 3.7 eV and similar interband electronic transitions of our films as for bulk SrTiO$_3$. For our films, the interband peaks are broadened due to poly-crystalline disorder and have a lower amplitude. The reason for the discrepancy between the low-frequency (vibrational) and the high-frequency (electronic) dielectric constant is unclear.   

Nonlinear photonic crystals of BaTiO$_{3}$ and their electro-optic properties

Future optical systems will require electro-optic (EO) modulators with bandwidths of 100 GHz and low drive voltage. To achieve this, non-linear photonic crystals using epitaxial BaTiO$_{3}$ have been proposed. In our work two-dimensional photonic crystal (PhC) structures with a hexagonal array were fabricated from an epitaxial BaTiO$_{3}$ thin film using focused ion beam milling. The PhC waveguides were based on Si$_{3}$N$_{4}$/BTO/MgO multilayer epitaxial thin film structure. Simulation shows that sufficient refractive index contrast is achieved at 1550 nm to form a bandgap in the PhC structure by milling through the Si$_{3}$N$_{4}$ and BTO layers. The measured transmission spectrum of the PhC waveguide exhibited a stop-band centered at 1550 nm with well-defined band edges. Photonic crystal electro-optic modulators with a bandwidth of greater than 50 GHz have now been realized. The question of what dielectric properties limit the ultimate bandwidth of the PhC will be addressed.   

Electro-optic contribution of optically generated small bound polarons in nominally undoped, thermally reduced LiNbO$_3$

Recently we have shown that a spatial modulation of optically generated densities of small bound polarons can be applied for hologram recording in LiNbO$_3$ [1]. This new type of recording mechanism is of particular interest for the field of nonlinear and ultrafast photonics because of small bound polaron generation on the fs-scale. The grating recording via the photochromic response of small bound polarons was successfully applied to explain gratings recorded with a grating vector aligned orthogonal to the polar c-axis. In this contribution we study the relation of optically generated small bound polarons with pronounced index changes, that were found with values up to 10$^{-4}$ and a grating vector parallel to the c-axis. The Pockels effect that must be driven by an internal electric space-charge field is taken into account. In contrast to the classical photorefractive effect, where slow and long-ranging transport mechanisms must be considered, we discuss the build-up of the space-charge field on the sub-ps-time scale in the model of optical absorption of small polarons, i.e., the optically generated polaron hopping to next-neighboring lattice sites.\\[4pt] [1] M. Imlau et al., Optics Express 19, 15322 (2011)   

Dispersive properties of small polaron-based hologram recording in nominally undoped, thermally reduced LiNbO$_3$

We recently discovered a new type of hologram recording in nominally undoped, thermally reduced LiNbO$_3$ by means of a single intense ns-laser pulse ($\lambda = 532\,$nm) yielding short-lived volume phase-gratings with unique features [1]: a diffraction efficiency of more than 20\% in the NIR spectral range ($\lambda = 785\,$nm), a stretched-exponential relaxation behavior of the grating efficiency with a lifetime in the ms-range at room temperature and a pronounced dependence on the orientation of the grating vector with respect to the polar c-axis. This type of hologram recording could be successfully modeled by taking into account an optically generated spatial modulation of small electron bipolarons, small bound and free electron polarons. In this contribution we face the unique dispersive properties of this type of hologram recording and particularly present our results for probing light in the blue-green spectral range ($\lambda = 488\,$ nm). We show that the further contribution of small bound hole polarons must be taken into account. Furthermore, we conclude that a considerable diffraction efficiency at the telecommunication wavelength ($\lambda = 1550\,$ nm) can be expected.\\[4pt] [1] M. Imlau et al., Optics Express 19, 15322 (2011)