Session V41: Focus Session: Dielectric, Ferroelectric, and Piezoelectric Oxides V

First principles theory of metal-oxide interfaces: the Schottky-Mott theory revisited

Using calculations from first principles, we discuss the interplay between structure and functionality at metal-insulator interfaces using the paradigmatic example of the junctions between various metals (Ag, Pd,Pt,Ni,Cu,Al) and binary crystalline oxides (BaO,CaO and SrO). Our results demonstrate that it is possible to tune the Schottky barrier height in a very broad range of values just manipulating the metal at the interface and elucidate the role of the relative overlap in the density of states of the different components in determining the band alignment. We will also show how we can apply this concepts to interfaces between metals and ferroelectric oxides where the interface structure and charge transfer affect the spontaneous polarization of the ferroelectric.   

Strains observed in 180\r{ } domain walls in LiNbO$_{3}$ with Laue topography

A comparison of the strains associated with domains in congruent LiNbO$_{3}$,$_{ }$previously observed with Bragg x-ray topography and currently with Laue x-ray topography, reveals some surprising differences. The Laue geometry allows imaging through the entire crystal. Using monochromatic collimated synchrotron radiation and a magnifying x-ray camera, we are able to image domains and the strains created by electric field poling, as well the additional strain created by the application of electric fields in real time. The unusual strain patterns can be taken into account with higher order terms in the ferroelectric Ginzberg-Landau theory. Electric fields below the coercive field applied along the c-axis produce Pendell\"{o}sung stripes due to complex strains that are precursors to 180\r{ } domain wall switching. The strains observed with field off and field on in the Laue topographs differ significantly from the surface strains observed previously with Bragg topography.   

Predicting polarization enhancement in multicomponent ferroelectric superlattices

We use ab initio calculations as an input to develop a one-dimensional chain model of the local polarization in epitaxial short-period CaTiO$_3$/SrTiO$_3$/BaTiO$_3$ superlattices grown on a SrTiO$_3$ substrate. The model is then combined with a genetic algorithm optimization technique to predict the arrangement of individual CaTiO$_3$, SrTiO$_3$ and BaTiO$_3$ layers in a superlattice that would simultaneously have the highest possible polarization and a low in-plane lattice constant mismatch with the substrate. This modelling procedure can be applied to a wide range of layered perovskite-oxide nanostructures providing guidance for experimental development of nanoelectromechanical devices with substantially improved polar properties.   

Self-Poling in Strained Asymmetric Superlattices

We have constructed strained dielectric superlattices consisting of three different perovskite titanate phases - BaTiO$_{3}$, SrTiO$_{3}$ and CaTiO$_{3}$ using ozone assisted atomic layer by layer molecular beam epitaxy, where the stacking architecture of the different phases controls the symmetry of the superlattice. To investigate the effect of structural symmetry (or the lack of symmetry) on the dielectric properties of such superlattices, avoiding effects due to asymmetric electrode interfaces, capacitor devices were constructed with symmetric, lattice-matched, conducting oxide electrodes. I will show that the stacking architecture modifies the dielectric and ferroelectric properties of the constituents leading to new electronic properties uncharacteristic of naturally occurring phases. Superlattices with broken inversion symmetry are self poled and generate a second order susceptibility, $\chi ^{(2)}$, as a result of the built-in asymmetric strain fields. The resulting polarization direction and the sign of $\chi ^{(2)}$ are fixed by the symmetry of the superlattice. Sensitive pyrocurrent measurements indicate an increasing polarization as the temperature is lowered while below a temperature, T$_{x}$, an onset of a hysteretic response is observed. I will discuss an unusual ferro-like phase with two unequal polarization states which emerges below T$_{x}$ where the P-E loops are displaced not only along the E-field-axis by an effective polarizing field, but also along the P-axis by an offset polarization.   

X-ray catastrophe focusing with ferroelectrics

Familiar ripple patterns reflected from water waves and the characteristic cusp of light seen in the bottom of a coffee cup are examples of “catastrophe” optics, where focusing occurs without need for highly engineered optical elements; catastrophe theory provides the mathematical tools needed to describe the stability and topology of the caustics that produce these effects. We have discovered catastrophe focusing in the diffraction of hard x-rays from crystalline wafers of ferroelectric lithium niobate, by diffracting monochromated synchrotron undulator radiation from surface planes while applying a voltage across the thin crystal. The presence of anti-phase ferroelectric domains causes a rippling of the surface that produces focusing at an x-ray CCD camera a half meter away. The catastrophe focusing pattern can be uniquely controlled by the applied voltage. These results suggest possible applications of catastrophe optics to x-ray microfocusing, and to the use of voltage-controlled patterned ferroelectrics as a new concept in x-ray optical devices.   

The Effect of Grain Boundary Charge on Ferroelectric Transitions in Nanograin BaTiO$_{3}$

We have recently reported dense polycrystalline BaTiO$_{3}$ that has sub-100 nm grains. As grain size decreases, the electrical field due to grain boundary charge extends to an increasingly larger fraction of the grain giving rise to a size effect that is fundamentally different from those previously considered in the literature. Using Ginsberg Landau theory we have modeled this effect on ferroelectric transition, polarization/phase distribution and dielectric/piezoelectric responses, and compared it with the experimental results of BaTiO$_{3}$ (XRD, TEM, and $\varepsilon (T))$. Such nanograin ferroelectric ceramics, currently available at sub-10 nm grain size, are of considerable interest for the next-generation microelectronic component applications. A related grain boundary effect on phase transition has also been reported in coarse grain and bicrystal SrTiO$_{3}$.   

Epitaxial Growth of BaTiO$_{3}$/SrTiO$_{3}$ and BaO/SrTiO$_{3}$ Superlattices for Phonon Confinement

We discuss the design and material parameters of BaTiO$_{3}$/SrTiO$_{3}$ and BaO/SrTiO$_{3}$ heterostructures relevant for novel phonon devices, including mirrors, filters, and cavities for coherent phonon generation and control. The advantages of using these ferroelectric superlattices include that they have an enormous stop band compared to the GaAs/AlAs superlattices previously reported for this application and that there can be greatly amplified light-sound interaction in these ferroelectric materials. We have grown BaTiO$_{3}$/SrTiO$_{3}$ and BaO/SrTiO$_{3}$ superlattices on TiO$_{2}$-terminated SrTiO$_{3}$ substrates by reactive MBE. Structural characterization by XRD and TEM revealed that the samples studied are of high quality with nearly atomically abrupt interfaces. We have observed folded acoustic phonons at the expected frequencies using UV Raman spectroscopy.   

Site-Selective Studies of Er-doped SiO$_{2}$/SRO layers

Er-doped SiO$_{2}$ has attracted much attention as a possible pathway to realizing Si-based optoelectronic devices. Of particular interest is how the presence of silicon nanocrystals affects the Er emission in this host material. In samples containing nanocrystals, there are three possible environments for the Er defect to occupy: within, near, and far from the nanocrystal silicon; each environment having the possibility for clustered and nonclustered Er. We present site-selective photoluminescence results obtained from samples with and without nanocrystals to analyze the spectral differences between these two types of samples. We can spectrally identify the different Er environments by directly comparing the results from the two types of samples. We observed Er clustering sites which become dominant as the activation anneal temperature increases. This Er clustering site emission is decreased in the samples containing nanocrystals, indicating that the excess silicon inhibits the formation of clusters. Despite our good site-selectivity, we were unable to find any special spectral signature of nanocrystal-related Er suggesting that their local environment is similar to that of isolated Er ions in SiO$_{2}$.   

First principles theory of the local dielectric permittivity: Application to surfaces and interfaces

A new computationally efficient method has been developed within the framework of density functional theory to aid in the study of the dielectric properties of multi-component systems, with explicit treatment of surface and interface effects. The local polarization and permittivity functions, induced due to a finite external electric field, are introduced to describe variation of the dielectric response over length scales of the order of interatomic distances. Specially, we have determined the position dependent dielectric permittivity profiles for Si-SiO$_2$ and SiO$_2$ -polymer systems. We find that at regions close to surfaces and interfaces, the dielectric permittivity is enhanced compared to the corresponding bulk values, while in interior regions it approaches the corresponding bulk values. The calculated optical and static dielectric constant values of these systems are in excellent agreement with experimental results, and other more involved computational treatments.   

Surface Micromachined Ferroelectric MEMS Devices: Correlating Device Performance with Film Microstructure

The realization of free-standing ferroelectric thin film devices compatible with CMOS processing is a priority in achieving sophisticated MEMs systems. To release an active film from a silicon substrate with a back-etch technique requires the removal of hundreds of microns of the substrate as well as a suitable and robust mask for the device layer. A more flexible and process compatible technique is surface micromachining. We present work utilizing a XeF$_{2}$ etch process for surface micromachining silicon with a resistant patterned oxide layer to create free-standing ferroelectric MEMs structures. With surface patterning, we are able to realize both bridge and cantilever geometries. Lead titanate thin films grown by MOCVD for the active layer will be discussed. The devices have been synthesized in three different thin film microstructures: 1) poorly oriented, 2) fiber textured, and 3) biaxially textured. The realization of such a continuum of microstructures allows the switching mechanism to be better resolved and for the correlation between macroscopic device performance and microscopic properties to be more fully understood.   

Interaction between ferroelectric nanodots: a first-principles-based study

We have performed Monte-Carlo simulations within a first- principles-based effective Hamiltonian approach to investigate the consequences (if any) of the interaction between nanodots made of lead titanium zirconate. We found that two dots under open circuit electrical boundary conditions weakly interact with each other, implying that they both adopt a vortex structure for their dipoles that is very similar to the one occurring in a single isolated dot. The situation is dramatically different if the first dot is polarized (e.g., by being under an external field) while the second dot is still under open circuit condition. In that case, the dots interact stronger, with this interaction causing the development of a relatively small spontaneous polarization in the second dot and, more importantly, being able to influence the direction of the toroid moment in this second dot -- which is of technological importance. We further show how this last interaction, and its consequences, depend on the geometry and shape of the two dots, and reveal that all the striking features can be well understood within a multipole expansion of the field produced by the lattice polarization vortex in the free dot. This work is supported by DOE grant DE- FG02-05ER46188, by ONR grants N00014-01-1-0365, N00014-04-1- 0413 and N00014-01-1-0600, and by NSF grants DMR-0404335 and DMR-9983678.   

\emph{Ab-initio} theory of nanoscale capacitors at finite bias

We present a novel technique for calculating the properties of an electric field applied to a periodic heterostructure with alternating metallic and insulating layers. This scheme allows us to investigate fully from first- principles the microscopic properties of a thin-film capacitor at finite bias potential. We demonstrate how the capacitance and local permittivity profiles can be readily obtained by performing calculations for the Ag(100)/MgO(100) and SrTiO$_3$(100)/SrRuO$_3$(100) systems. Applications range from the emerging field of electronic devices based on ferroelectric materials, to the {\em ab-initio} simulation of electrochemical cells.   

Electrophoresis of Ferroelectric Nanoparticles

We have studied the electrophoresis of ferroelectric nanoparticles(Ba1-xSrxTiO3). We used de-ionized (DI) water as solvent and an optical microscope to observe the motion of suspended ferroelectric nanoparticles driven by AC electric fields. The immediate start and stop of motion were noticed when the driving electric field was turned on and off, which was similar to dielectricphoresis. Higher voltage generated higher speed as expected. In some instances, the dielectric constant $\varepsilon $ of ferroelectric materials can increase greatly, which makes it possible that a low driving voltage (no larger than 10V) could induce a relatively high speed. At room temperature, we studied the frequency dependence of the motion speed. By comparing a serial of captured motion movies, we found that higher speeds were corresponding to lower frequencies of driving AC electric field. Further, we use well defined electrodes made by electron-beam lithography and high-vacuum deposition, which may regulate the electric field distribution. Consequently, we can characterize the electric force applied on those nanoparticles.