Session S37: Effect of Strain on Ferroelectrics

First-principles study of novel routes to ferroelectricity using strain

We use first-principles density functional theory (DFT) to explore the use of strain to induce ferroelectricity in otherwise non-polar materials. First, we investigate the polarization induced in LaAlO$_3$ by changing in-plane bondlengths while constraining the unit cell to tetragonal symmetry. We then relax the symmetry constraint to examine the effect of oxygen octahedra rotations on the polarization. Next we explore whether epitaxial strain in the pseudocubic [100] direction can lift the inversion center in otherwise centrosymmetric BiMnO$_3$ and induce the small polarization reported in thin films. By studying these systems with DFT calculations, we have the ability to finely vary the system constraints, predicting new functional materials and providing insight into the underlying physics.   

Single-domain, lattice-tunable, rare-earth scandate templates for strain engineering oxide films

Epitaxial functional oxides have generated excitement due to the improvement in properties over their amorphous and polydomain counterparts. Generally, high quality epitaxy becomes undesirable with large strain. Widely available substrates with site-terminated and atomically flat surfaces unfortunately have large mismatches ($>$1.0{\%}) with oxides like BiFeO$_{3}$ a$_{p}$ = 3.96{\AA}, BaTiO$_{3}$ a$_{p}$ = 4.00{\AA}, and PZT(MPB) a$_{p}$ = 4.07{\AA}. For this reason, orthorhombic Rare-Earth Scandates (REScO$_{3})$ have been developed in bulk and film where tuning the lattice constant is executed by changing the RE ion. Lattice parameters decrease with increasing Z in the range Z = 51, LaScO$_{3}$ a$_{c}$ = 4.05{\AA} to Z=71, LuScO$_{3}$ a$_{c}$ = 3.89{\AA}. In this work, we demonstrate that single domain REScO$_{3}$ template films of LaScO$_{3}$, PrScO$_{3}$, NdScO$_{3}$, SmScO$_{3}$, GdScO$_{3}$, and DyScO$_{3}$ can be deposited with pulsed laser deposition on highly miscut (001) SrTiO$_{3}$ or orthorhombic (110) NdGaO$_{3}$ substrates with crystalline quality approaching that of the bulk crystal. The rocking curve full width at half maximums were typically below 0.1\r{ } and transmission electron microscopy cross sections exhibit large areas of defect free regions near the surface. The result is a lattice tunable template for growth of strain engineered oxide films.   

Effects of Substrate Polarity, Strain, and Chemical Boundary Conditions on Ferroelectricity in PbTiO$_{3}$ on DyScO$_{3}$

Novel substrate materials such as DyScO$_{3}$ have recently been used to control the epitaxial strain imposed on ferroelectric films such as PbTiO$_{3}$, since epitaxial strain effects are predicted to strongly modify the phase transition temperature and domain structure. In addition, the polar nature of these substrates can be expected to impose a different electrical boundary condition than would arise at a traditional non-polar substrate surface such as SrTiO$_{3}$ (001). Chemical conditions at the free surface can also affect polarization in ultrathin films. Here we report synchrotron x-ray investigations of the domain structure and polarization as a function of temperature and oxygen partial pressure for ultrathin films of PbTiO$_{3}$ on DyScO$_{3}$. We observe that the substrate surface charge imposes a strong bias on the net polarization of the film, which can be overcome at low temperatures by the chemical effect of the environment. Work supported by the U. S. Department of Energy under Contract No. DE-AC02-06CH11357.   

In situ LEED-IV characterization of polar distorted ultra-thin BaTiO3 films

Ferroelectric phase stability in nanoscale ferroelectrics is governed by the interplay of electrostatic depolarization energy, domain formation, adsorption, and surface band bending. Predictions for the minimum critical film thickness for ferroelectricity in BaTiO3 have continuously decreased with more complex models to a current value of 6 layers. The thinnest experimental value is 12 layers. Using in situ low energy electron diffraction (LEED) I-V, we have characterized the structure of 4 and 10 ML BaTiO3 films, grown using laser molecular beam epitaxy with fully compressive strain on a SrRuO3/SrTiO3 substrate. Analysis of the LEED-IV reveals a a surface dead layer with a single-domain upward (out of surface) polarized state below. Intrinsic asymmetry and the stability to compensation of depolarizing charges by dipoles induced by surface stress can explain the single domain scenario. Research was sponsored by the Division of Materials Sciences and Engineering and the Center for Nanophase Materials Sciences, Office of Basic Energy Sciences, U.S. Department of Energy with Oak Ridge National Laboratory, managed and operated by UT-Battelle, LLC.   

Phase diagram of Ba$_{0.5}$Sr$_{0.5}$TiO$_3$ thin films

Ba$_{0.5}$Sr$_{0.5}$TiO$_3$ (BST) thin films are a promising material for tunable microwave applications, which require large, tunable dielectric constants and low loss. By controlling the (i) substrate and (ii) oxygen partial pressure, it is possible to tune the tetragonal strain of these films. For example, in-plane strains between -0.48\% (highly compressive) and +0.30\% (highly tensile) have been reported for BST thin films grown on MgO(001) using rf magnetron sputtering. The ability to control the strain makes it possible to optimize the dielectric properties of the film for microwave (and other) applications; however, the strain-temperature phase diagram has not been systematically explored to-date. In this talk, the phase diagram of BST is studied using first-principles calculations, focusing initially on the $T$ = 0 phases. Results for displasive as well as coupled ferroelectric/antiferroelectric phases (recently discussed by Zhang, Cagin, and Goddard in connection to BaTiO$_3$) will be presented.   

Influence of strain relaxation and atomic interface configuration on the dielectric response of BST thin film capacitors

The collapse of the dielectric response which is commonly observed in Ba$_{x}$Sr$_{1-x}$TiO$_{3}$ (BST) thin films is a topic of general physical interest as well as a key issue in terms of a possible application of this material in future DRAM storage capacitors. We addressed the influence of substrates-imposed strain and electrode interface configuration on the dielectric collapse in epitaxial SrRuO$_{3}$/BST/SrRuO$_{3}$ thin film capacitors. The growth mode of BST thin films was analyzed by RHEED and HRTEM from the thickness range of a few unit cells up to hundreds of nanometers where plastic strain relaxation occurs. The crystalline quality of our ultrathin samples enabled us to resolve the atomic arrangement and to identify the terminating layers at the SrRuO$_{3}$-BST interface by STEM. We obtained bulk-like permittivities in the order of 5000 and its thickness dependence can be well described by an extended Ginzburg-Landau-Devonshire model by taking into account plastic strain relaxation in BST thin films and finite screening of depolarizing fields by the SRO electrodes. We will furthermore present relaxor-type behavior of the BST thin films that becomes visible only in samples with sufficient interface quality and hints on nanoscale structural inhomogeneities.   

Strain Effects in Barium Strontium Titanate Films for Tunable Microwave Applications

Recently, it has been demonstrated that control of lattice structure can improve dielectric tuning in epitaxial ferroelectric (FE) films. Understanding the coupling of strain and lattice structure to the dielectric properties is important for FE-based tunable microwave applications. Ba$_{x}$Sr$_{1-x}$TiO$_{3}$ films on (001) MgO substrates were grown by sputter deposition with c$<$a and c$>$a tetragonal distortions, where a and c are the in-plane and out-of-plane lattice parameters, respectively. The dielectric properties were significantly affected by the type of lattice distortion and by the direction of strain-induced permanent polarization if present. Ti K-edge x-ray absorption fine-structure (XAFS) measurements were taken in several orientations. The anisotropy in the spectra with orientation was used to determine the FE phases of the films. The dependence of the in-plane dielectric constant on strain will be discussed in terms of a theoretical model of the phenomenological thermodynamics of the film strain effect.   

An ab initio study of strained PZT films

The potential for creating ferroelectric-based devices such as ferroelectric random-access memories (FeRAMs) has led to an intense interest in ferroelectric perovskite thin films, alloys, and superlattices. Engineering of FeRAMs requires the control of both the spontaneous polarization and the barrier for polarization reversal. The two parameters that are simplest to modify are the composition, which is controlled during the film deposition, and the epitaxial strain, which is controlled by substrate selection. Here, using \emph{ab initio} methods, we investigate how the ferroelectric properties of PbZr$_{(x)}$Ti$_{(1-x)}$O$_{3}$ (PZT) depend upon epitaxial strains ranging from $ -0.02 \leq \epsilon \leq +0.02 $. Compositions ranging from $ 0.0 \leq x \leq 0.5 $ are examined by the creation of special quasirandom structures. In agreement with the literature,\footnote{ H.N. Lee, S.M. Nakhmanson, M.F. Chisholm, H.M. Christen, K.M. Rabe, and D. Vanderbilt, Phys. Rev. Lett. 98, 217602 (2007).} the effect of strain on the polarization is found to be small. We also compute the strain dependence of the barrier for polarization reversal, and discuss the results in the context of experimental measurements of coercive fields and polarization retention.$^1$   

High pressure x-ray diffraction study of single crystal Pb(Sc$_{0.5}$ Nb$_{0.5})$O$_{0.3}$

We employed high pressure single crystal x-ray diffraction to investigate the pressure-induced phase transition in Pb(Sc0.5Nb0.5)O3 (PSN). At 2 GPa and 300 K, PSN undergoes a phase transition as deduced from earlier dielectric measurements. The pressure dependence of diffuse scattering observed around the (110) Bragg peak indicates that pressure suppresses the local distortion that is coupled to the polar nanoregions. We have monitored the pressure dependence of diffuse scattering at temperatures between 300 - 4 K and used this to understand the P-T phase diagram of PSN. In addition, we also investigated the equation of state at various temperatures. Observed changes in the isothermal compressibility at low temperatures indicate that the first order phase transition changes character at 200 K and 4 GPa. The results can be understood in terms of a pressure-induced decrease in the correlation length among polar nanoregions, which is a unique property of relaxor ferroelectrics.   

Strain induced relaxor behavior in PbSc$_{0.50}$Nb$_{0.25}$Ta$_{0.25}$O$_{3}$ thin films: A comparison with the nanoceramics

A comparative studies of the microstructure, micro Raman spectroscopy and dielectric properties of PbSc$_{0.50}$Nb$_{0.25}$Ta$_{0.25}$O$_{3}$ (PSNT) thin films and ceramics were carried out over a wide range of temperature 100-520 K and frequency 100Hz to 1MHz. Microstructure of PSNT films revealed an in plane compressive strain whereas PSNT ceramics showed an average 10-15 nm size nanoordered regions. We observed a shift of 65 K in dielectric maxima temperature towards the lower temperature and frequency dispersion of the dielectric constant in PSNT films compare to bulk that does not exhibit relaxor behavior. We addressed this different dielectric response due to the in plane compressive strain in the films. Temperature dependent micro Raman spectroscopy revealed that the ferroelectric state in PSNT ceramics were accompanied by pronounced changes in both the lowest frequency F$_{2g}$ and highest frequency A$_{1g}$ modes. Micro Raman spectra of the thin films compared to nanoceramics showed shifting of the Raman modes to lower frequencies that confirms the strain state of the films. The in plane compressive strain, dipole arrangement and the size of nanoordered regions change the dielectric response of the PSNT films compare to nanoceramics.   

First Principles Theory of Sub-Monolayer Strontium on Silicon (001)

Conventional attempts to continue transistor scaling consistent with Moore's law will soon result in unacceptable quantum mechanical leakage currents across the dielectric oxide layer. One promising solution to this problem is to replace the current silicon dioxide layer with a thicker crystalline oxide with a higher dielectric constant, grown epitaxially on silicon. Although there has been progress in growing high quality epitaxial interfaces for some materials, the initial stages of growth, including the deposition of the initial metal layer, are not well understood. Using \textit{ab initio} density functional theory, we study the initial stages of the deposition of strontium titanate on silicon (001), a good model system due its successful epitaxial growth. We present the binding energies of several new low energy structures with sub-monolayer converages of strontium which differ significantly from the conventional view of this surface. Additionally, to include finite temperature effects, we calculate vibrational free energies. We compare our results to experimental samples grown by molecular beam expitaxy.   

Sub-monolayer Strontium Phase Diagram on Silicon (100)

Crystalline oxides manifest a number of important phenomena, including magnetism, ferroelectricity, superconductivity, and colossal magnetoresistance. Recently, it has become possible to integrate these materials onto a silicon platform in a fully epitaxial structure. These crystalline oxide-silicon heterostructures bring the promise of integrating the rich functionality present in crystalline oxides with modern silicon device technology. The most successful fully epitaxial oxide-silicon (100) heterostructures have been achieved through a deposition recipe that involves manipulating substrate temperature and oxygen pressure on a layer by layer basis during the deposition of an alkaline earth metal. Motivated by a desire to develop a fundamental understanding of this important transition layer between silicon and oxide, we have mapped out the phase diagram of strontium on silicon as a function of temperature and coverage. In particular, recent work on sub-monolayer strontium deposition on the silicon surface suggests the conventional picture of this structure, upon which the entire crystalline oxides on silicon framework is built, is only a low-temperature phase which plays no role in enabling epitaxial oxide growth. Instead, there is strong evidence that a different high temperature phase is the crucial template for epitaxial oxide growth on silicon.   

Interfacial effects on ferroelectricity in SrTiO$_3$ thin films supported on Si(001)

Experimental and theoretical studies have suggested that the application of a compressive in-plane strain can induce a polar ground state in thin SrTiO$_3$ films. One way to impose such a strain is to grow the film epitaxially on a lattice mis-matched substrate. However, it is unclear whether such films are ferroelectric -- i.e., switchable -- in addition to being polar, and furthermore, what role the substrate plays in determining the switchability. Using density functional theory, we explore the possibility of a ferroelectric ground state in SrTiO$_3$ thin films grown on a Si(001) substrate. In particular, we examine the effects of several proposed interface structures on the interfacial charge compensation and polarization direction, and we discuss the implications of these effects for potential applications in integrated electronics devices.   

Polarization of strained SrTiO$_{3}$ films grown on Si (001)

Perovskite oxides grown on silicon provide powerful new functionalities for device components built upon the ubiquitous silicon platform. A rich set of applications results from the combination of the perovskite's diverse electrical and physical structures and the semiconducting properties of silicon. Understanding how to develop new functionalities, however, requires detailed knowledge of the real space positions of atoms with sub-angstrom resolution. In this work, we have carried out synchrotron x-ray diffraction studies of crystal truncation rods on 4-5 unit-cell-thick SrTiO$_{3}$ films grown epitaxially on silicon, which have been terminated with different gate metals. We determine the precise atomic structure of these materials, in particular the displacements of the TiO$_{2}$ planes relative to the SrO planes that make up the perovskite structure. We show that the polarization and structure of commensurately strained films depend critically upon the electrical and chemical properties of the terminating metal.   

Writing and Reading of Ultrathin Ferroelectric Domains on Commensurate SrTiO$_{3}$ on Silicon

Ferroelectricity in ultrathin epitaxial SrTiO$_{3}$ grown commensurately by oxide-molecular beam epitaxy (MBE) on silicon substrates was investigated using piezoforce microscopy (PFM). A series of samples containing $n$ molecular layers (ML) of SrTiO$_{3}$ ($n$ = 3, 4, 5, 6, 8, 10, 20) was grown on silicon substrates. Room-temperature ferroelectricity was observed for samples containing $n$ = 5, 6, 8, 10 ML. Temperature-dependent measurements indicate that the sample with $n$ = 5 exhibits a ferroelectric phase transition at T$_{c}\sim $317 K. Sample with $n$ = 6 remains ferroelectric up to at least 393K. Polar domains created on the $n$ = 6 was found to be stable at room temperature for more than 72 hours. The implications of these results for fundamental and device-related applications will be discussed briefly.