Session Q24: Focus Session: Dielectric, Ferroelectric, and Piezoelectric Oxides -- Strain and Interfaces

A new class of atomic-layer-thick ferroelectric films on silicon

As a result of depolarizing field effects, the behavior of nanoscale ferroelectrics often suffers from the suppression of the ferroelectric phase transition temperature, as well as the polarization magnitude, with respect to that of the bulk ferroelectric. Using density functional theory, we design a novel class of ferroelectric heterostructures that, in contrast, display ferroelectric behavior only in the limit of atomically thick films. These heterostructures are composed of a single layer of epitaxial $MX_2$ on a Si(001) substrate, where $MX_2$ is a transition metal chalcogenide or pnictide with a bulk layered crystal structure ({\em e.g.}, CdI$_2$, pyrite). We discuss the chemical and mechanical interactions that stabilize the two polarization states in these materials, and we demonstrate that continuous switching between these states is possible. Our work may thus lead to the realization of new applications based on nanoscale ferroelectrics. We acknowledge financial support by the NSF under MRSEC DMR 0520495, as well as computational support from the NCSA TeraGrid and Yale HPC.   

A Direct Determination of the Structure of Polar SrTiO3 on Silicon

The epitaxial growth of perovskite oxide structures on silicon substrates has opened the door for the integration of a wide range of novel physical properties unique to complex oxides with established silicon-based technologies. A model system is the polar SrTiO3/Si system. Synchrotron based x-ray diffraction measurements allow a direct determination of the structure of the SrTiO3-Si interface, as well as the atomic displacements in the SrTiO3 film. A combination of direct phasing methods and fitting algorithms is used to convert the diffraction data into sub-angstrom resolution real space structural maps. The results can be used to understand the polarization observed in the SrTiO3 thin films and the measured differences between 2.5 and 5 unit cell SrTiO3 deposited on Si.   

Near-surface properties of SrTiO$_{3}$ (001) from resonant soft x-ray reflectance

Resonant reflectivity $R$(\textit{h$\nu $}, $q)$ has been measured from several SrTiO$_{3}$ (001) single crystals across the Ti $L_{2,3}$ edges: we find that this approach senses the near-surface region differently than other techniques including resonant XAS via total electron yield. Modeling $R$(\textit{h$\nu $}) provides absolute resonant optical properties $n$ = 1 -- \textit{$\delta $} -- \textit{i$\beta $} indicating short skin depths ($<$ 10 nm) when tuned to the $t_{2g}$ and $e_{g}$ peaks. We observe unexpected linear dichroism (LD) at 300 K primarily in the $e_{g}$ states, consistent with tetragonal symmetry and possible weak ferroelectric polarization. Below the bulk antiferrodistortive transition at 105 K both $t_{2g}$ and $e_{g}$ states exhibit LD. Resonant $R(q)$ exhibits clear structure indicating a subsurface layer several nm thick having different properties from the bulk, suggesting that the 300 K LD is associated with this layer. We discuss these new results in the context of established studies of the STO surface and their relevance to dielectric and ferroelectric properties of STO surfaces and thin films.   

Interface transformations of SrTiO$_3$ during growth on Si (001)

The integration of crystalline oxides and semiconductors has been made possible by the development of techniques that allow crystalline SrTiO$_3$ to be grown on the silicon (001) surface. This integration allows the wide range of behavior exhibited by crystalline oxides to be combined with the technological advantages silicon exhibits over single crystal complex oxide substrates. Recent studies have established, for specific growth conditions, the precise interface structure between a binary alkaline-earth oxide, BaO, and the silicon (001) surface. Due to the similarities in the crystal structure between SrTiO$_3$ and BaO, the SrTiO$_3$ interface with silicon (001) has been predicted to exhibit the same interfacial features, such as a 2$\times$1 dimerized silicon surface and an initial alkaline-earth oxide atomic plane. However, we find that even though the MBE growth process of SrTiO$_3$ involves forming the alkaline earth interface structure initially, this interface transforms upon deposition of the TiO$_2$ layers. The resulting interface starts with a TiO$_2$ atomic plane and exhibits 1$\times$1 symmetry without silicon dimers. We discuss the implications of this result for the growth of other complex oxides on silicon and for exploiting oxide functionality on a silicon platform.   

Depth resolved x-ray excited optical luminescence from SrTiO$_{3}$

SrTiO$_{3}$ and related perovskite materials are increasingly being utilized in a wide range of electrical applications. Furthermore, SrTiO$_{3}$ is often used as a substrate for growth of thin film structures. Thus, knowledge of its defect structure and, in particular, their depth dependence is crucial for understanding their impact on conductivity and optical phenomena. In the present work we have utilized the limited penetration depth of x-rays to study the near-surface properties of SrTiO$_{3 }$substrates and epilayers For an energy of 600 eV the penetration depth varies between 5 and 144 nm as the incidence angle changes from 2 to 32 degrees. Thus, by obtaining optical luminescence spectra as a function of incidence angle it is possible to probe the near-surface region with nm-scale resolution. We will present angle dependent optical luminescence data from several SrTiO$_{3}$ samples and discuss the results in terms of previous cathodoluminescence studies [1] and theoretical models. [1] J. Zhang, et al, J. Vac. Sci. Technol. B \textbf{26}, 1456 (2008).   

Homoepitaxial off-stoichiometric SrTiO$_{3}$ films studied by ultraviolet Raman spectroscopy

Homoepitaxial Sr$_{1+x}$TiO$_{3+\delta }$ films with -0.2$<$x$<$0.25 grown by reactive molecular-beam epitaxy on SrTiO$_{3}$ (001) substrates have been studied by ultraviolet Raman spectroscopy. Off-stoichiometry for strontium-deficient compositions leads to the appearance of strong first-order Raman scattering at low temperatures, which decreases with increasing temperature and disappears at about 350 K. This indicates the appearance of spontaneous polarization with $T_{c}$ above room temperature. Strontium-rich samples also show strong first-order Raman signal, but the peaks are significantly broader and exhibit a less pronounced temperature dependence, indicating a stronger contribution of the disorder-activated mechanism in Raman scattering.   

Ferroelectric field effect transistors on silicon

The discovery of ferroelectricity in strained SrTiO$_3$ films grown directly on silicon substrates opens the possibility for a variety of devices that exploit direct field effects in this hybrid system. We report the fabrication and characterization of ferroelectric field effect transistors (FeFET) formed by coherently strained SrTiO$_3$ grown on silicon-on-insulator substrates. We observe persistent channel conductance changes of 85\% at large gate bias voltages. A preference for one polarization state is also observed, consistent with a predicted\footnote{M. P. Warusawithana et al., Science \textbf{324},367 (2009).} permanent interface dipole at the SrTiO$_3$/silicon interface.   

Fine tuning strain through composition: Pb$_x$Sr$_{1-x}$TiO$_3$ on DyScO$_3$

Strain tuning, modifying the functional properties by using epitaxial strain as adjustable parameter, has attracted much attention recently. Ferroelectrics are especially suitable for this, thanks to the strong coupling between the polarization and strain. Phase diagrams have been computed for numerous materials, showing low symmetry phases and phase boundaries where interesting properties are expected. From the experimental side, the limited number of suitable substrate materials hampers the application of strain tuning to its full potential. This issue can be circumvented by adjusting the strain state of a ferroelectric material on a particular substrate using cation substitution. We have applied this to the classic ferroelectric PbTiO$_3$ grown on DyScO$_3$. By substitution of Pb with Sr, the polarization direction can be switched from out-of-plane to in-plane. Grazing incidence diffractions (GID) results show a fully strained in-plane a$_1$/a$_2$ domain configuration, with domain walls along the $<$110$>$ directions for films with a Sr content higher than ~20\%. The Pb rich films show a mainly out-of-plane polarization. In contrast to pure materials, the phase boundary between the in-plane and out-of-plane ferroelectric phases is accessible. This shows the potential of combining strain and composition for engineering of functional properties.   

Competition between structural instabilities in strained ABO3 nanostructures

In spite of their simple structure, the family of ABO3 compounds present a large variety of phase transitions involving polar and non polar distortions as well as magnetic orders. Here we will discuss the microscopic origin of these properties and how they are affected in nanostructures through the concept of structural instabilities. We will from the fact that the ferroelectric (FE) and the antiferrodistortive (AFD) instabilities are in competition at the bulk level and are strongly sensitive to pressure and strain. From these considerations we will describe the possibilities to tune this FE/AFD competition by playing with strain and interface engineering. To that end we will first consider the effect of epitaxial strain on BaTiO3, SrTiO3, PbTiO3 and CaTiO3 thin films. In all of these compounds, the epitaxial strain can strongly modify the phase diagrams giving rise to different pure or mixed FE/AFD ground states. We will also extend the discussion on magnetic perovskites like CaMnO3 and will present the different strategies to induce or tune multiferroic properties. Second we will focus on the interface effects as present in bicolor superlattices. As an example we will examine the case of PbTiO3/SrTiO3 superlattice and will show that it exhibits totally unique properties arising from unexpected FE/AFD couplings at the interface between the layers. We will then investigate to which extent similar types of FE/AFD couplings can be induced in other artificially layered systems. We will consider different bicolor superlattices obtained from the combination of PbTiO3, SrTiO3, CaTiO3 and BaTiO3 and discuss how the intrinsic tendency of these compounds to favor either the FE or the AFD instabilities shifts or even suppresses the FE/AFD coupling.   

Ferroelectric properties of epitaxial PbZr$_{0.3}$Ti$_{0.7}$O$_{3}$ thin films as a function of SrRuO$_{3}$ electrode thickness

For the past few years, the one of the main research themes in ferroelectrics is the size effect that should be definitely required for miniaturization of the unit cell in FRAM. Until now, most related researches on the ferroelectric thin film have been merely focused on reducing the film thickness and their lateral size, not the electrode. Changing the point of view to the size effect of electrode, we tried to investigate the bottom electrode thickness-dependent ferroelectric characteristic especially focusing on the coercive field. Epitaxial SrRuO$_{3}$ (SRO) films are successfully grown on TiO$_{2}$ terminated SrTiO$_{3}$ surface by using pulsed laser deposition method, while the PbZr$_{0.3}$Ti$_{0.7}$O$_{3}$ (PZT) (also epitaxially grown) thickness was fixed. It is found that the coercive field of the PZT thin film increases as the SRO electrode thickness decreases. Based on this result, we believe that the depolarization field must be strongly coupled to the electrode thickness.   

Mixed Ising-Bloch-N\'eel Character to 180 $^{\circ}$ Ferroelectric Domain Walls

Ferroelectric 180-degree domain walls are well-known to be predominantly Ising-like. Using density functional theory, and molecular dynamics simulations, the 180 degree domain walls in prototypical ferroelectrics, lead titanate (PbTiO3) and lithium niobate (LiNbO3), are shown to have mixed character: while predominantly Ising-like, they also manifest some Bloch- and N\'eel-like character. Phase-field calculations show that such mixed wall character can be dramatically enhanced in nanoscale thin film heterostructures such as BaTiO3/SrTiO3, where the internal wall structure can form polarization vortices. Such mixed character walls can be expected to exhibit dynamical wall properties distinct from pure Ising walls.