Session H5: Focus Session: Interfaces in Complex Oxides - Polar Interfaces and Ferroelectrics

First-principles study of intermixing and polarization at the DyScO$_3$ /SrTiO$_3$ interface

Exploring oxide interfaces is an attractive challenge, due to the emerging novel behaviors which don't exist in the corresponding parent bulk compounds. E. g. joining two simple band insulators LaAlO$_3$ and SrTiO$_3$ with different polarity can induce conductivity at the interface. We carried out density functional theory (DFT) calculations based on the full potential linearized augmented planewave (FLAPW) method as implemented in the {\tt FLEUR} code (www.flapw.de) for studying the polar to non-polar interface of DyScO$_3$ and SrTiO$_3$. Due to the polar discontinuity, arising from nominally charged DyO or ScO$_2$ layers, sharp interfaces induce a strong ferroelectric-like polarization in the SrTiO$_3$ , while in off-stoichiometric interfaces this discontinuity is avoided and no such polarization can be found. In both scenarios the interface remains insulating with only a small reduction of the bandgap. Our calculations show that chemically mixed interfaces are energetically more favorable than sharp ones. Our DFT calculations explore also different configurations of the Dy and Sr atoms within the mixed interface plane. The calculated ground state configuration is confirmed by experimental observations.   

Growth and transport properties of fractionally \textit{$\delta $}-doped oxide superlattices

LaTiO$_{3}$/SrTiO$_{3 }$(LTO/STO) heterostructures are interesting as they show an intriguing 2D conduction, and their bulk counterpart, La$_{x}$Sr$_{1-x}$TiO$_{3}$ (LSTO), exhibits a filling-controlled insulator-metal transition (IMT). In this study, we investigated the filling controlled IMT in 2D geometry by fabricating monolayer-thick fractionally \textit{$\delta $}--doped LSTO/STO superlattices (SLs), in order to find ways to enhancing the carrier mobility of two dimensional electron gas (2DEG). Fractional layers of LSTO have been grown in between STO using advanced PLD. It is found that the SLs' transport properties are governed by a multichannel conduction with at least two distinctly different carriers: (1) High-density-low-mobility carriers presenting at the LSTO interface layer and (2) low-density-high-mobility carriers residing in the STO layers away from the \textit{$\delta $}--doped layer. By optimizing $x$, we could tune the effective mass and carrier density to enhance the carrier mobility by about an order of magnitude, selectively for the high-density-low-mobility carriers. This proves that the fractional \textit{$\delta $}--doping is an effective way to controlling the filling controlled IMT, resulting in highly improved transport properties   

Characterization of extreme-concentration 2DEGs at the SrTiO$_{3}$/GdTiO$_{3 }$interface

Heterostructures of Mott and band insulators exhibit unique interface properties, including two-dimensional electron gases (2DEGs) with extremely high sheet carrier densities due to the polarization discontinuity at the interface. Of paramount importance for the properties is the location, spatial extent and confinement of the 2DEG. Here, we study the 2DEG with carrier densities of 3x10$^{14}$ cm$^{-2}$ formed at GdTiO$_{3}$/SrTiO$_{3}$ interfaces grown by MBE. Using a self-consistent Schr\"{o}dinger-Poisson solver, we estimate the majority of the carriers are confined in a narrow region ($<$ 3 nm) at the SrTiO$_{3}$-side of the interface. Given the large and rapid spatial variation in charge density, experiments are needed to verify the assumptions underlying such models. We measure the admittance as a function of frequency at different fixed DC bias. To extract the carrier distribution in the depletion approximation, a distributed model is used to account for loss and series resistance effects. The resulting CV profile corresponds to a p-type layer. This is explained with a highly conductive space charge layer, resulting in significant depletion only in the p-type GdTiO$_{3}$. We report on the carrier distribution near the interface.   

Dynamical conductivity of the extreme concentration 2DEGs in GdTiO$_3$/SrTiO$_3$ heterostructures

Metallic conductivity with extremely high 2D carrier concentration is observed at the interface between the Mott insulator GdTiO$_3$ and the band insulator SrTiO$_3$. Irrespective of layer thickness or repeats, MBE-grown GdTiO$_3$/SrTiO$_3$ heterostructures have carrier density of approximately 1/2 electron per interface unit cell, or $3 \times 10^{14}$ cm$^{-2}$ per interface, in excellent agreement with the polar discontinuity model. To probe the orbital character, confinement, and correlations of carriers in this system we have measured the static (dc) and dynamical conductivity of a variety of heterostructures, using a combination of THz time-domain and FTIR spectroscopies. Samples with SrTiO$_3$ layers exceeding $\sim$10 nm thickness exhibit a Drude conductivity that may arise from $d_{xz}$ and $d_{yz}$ electric subbands at the interface. A discrepancy between the measured dynamical and dc conductivity measurements indicates the presence of a few additional carriers with very low scattering rate. By contrast, the Drude response of samples with thinner SrTiO3 layers shows an increased scattering rate with excellent agreement between the dc and THz frequency dynamical conductivity.   

Electrostatic carrier doping of GdTiO$_{3}$/SrTiO$_{3}$ heterostructures

Two-dimensional electron gases (2DEGs) at interfaces between Mott insulators and band insulators have attracted significant attention because they can exhibit unique properties, such as strong electron correlations, superconductivity and magnetism. At interfaces between SrTiO$_{3}$ and the rare earth titanates (Mott insulators) an interfacial fixed polar charge arises due to a polarization discontinuity, which can be compensated by a 2DEG, residing in the bands of the Mott/band insulator. In this presentation, we report on intrinsic electronic reconstructions at a Mott/band insulator interface between stoichiometric GdTiO$_{3}$ and SrTiO$_{3}$ that were grown using molecular beam epitaxy. The sheet carrier densities of all GdTiO$_{3}$/SrTiO$_{3}$ heterostructures containing more than one unit cell of SrTiO$_{3}$ are approximately 1/2 electron per unit cell, independent of layer thickness and growth sequence. These carrier densities closely meet the electrostatic requirements for compensating the fixed charge at these polar interfaces. Based on the experimental measurements, insights into the location and confinement of the charge and the influence of different electrostatic boundary conditions are obtained.   

High mobility electron gas in the surface vicinity of the cubic-perovskite KTaO$_3$ via Ar$^+$-irradiation

KTaO$_{3}$ (KTO), like SrTiO$_{3}$ (STO), can be doped to create a high mobility perovskite oxide semiconductor. However, while the low dimensional electronic states of STO have attracted intense interest via spectroscopic [1,2] and transport [3] studies, analogous investigations in KTO have been limited. Of particular interest is the fact that the cubic crystal symmetry in KTO is preserved at low temperatures, in contrast to STO. Here we electron dope KTO via oxygen-vacancy formation by Ar$^+$-irradiation [4]. Below $T$ = 10 K, the Hall mobility ($>$ 10$^4$ cm$^2$/Vs) of the electrons is significantly higher than in previous studies of STO [4]. The angular dependence of the Shubnikov-de Haas oscillations indicates a Fermi surface without cubic symmetry, in contrast to that expected for bulk KTO. We discuss the possible origins of these data, including the formation of quantum well structures, the coexistence of surface and bulk electrons, and the suppression of cyclotron motion by finite size effects.\\[4pt] [1] A. F. Santander-Syro \textit{et al}., Nature \textbf{469}, 189 (2011).\\[0pt] [2] W. Meevasana \textit{et al}., Nat. Mater. \textbf{10}, 114 (2011).\\[0pt] [3] Y. Kozuka \textit{et al}., Nature \textbf{462}, 487 (2009).\\[0pt] [4] J. H. Ngai \textit{et al.}, Phys. Rev. B \textbf{81}, 241307 (2010).   

Memristive behavior of ferroelectric tunnel junctions

Employment of polarization reversal in ultrathin ferroelectric layers opens new possibilities for development of electronic devices with novel functional properties not available in conventional systems. A particularly promising aspect is realization of resistive switching in the ferroelectric tunnel junctions (FTJ), which can be used as non-charge based logical switches in nonvolatile memory devices. Functionality of FTJs is intrinsically linked to a relationship between polarization orientation and tunneling resistance, which brings about a problem of ferroelectric switching and polarization retention in ultrathin ferroelectric barriers. Here, we demonstrate a giant resistive switching effect of more than 105\% at room temperature in the FTJ device structure composed of an epitaxial BaTiO$_3$ ferroelectric barrier sandwiched between bottom and top electrodes. We provide experimental evidence of the memristive behavior of these FTJs where both the low- and high-resistance states can be tuned by the external voltage by several orders of magnitude. The mechanism of the memristive behavior in our junctions is discussed in terms of the modifications of the tunneling potential profile driven by the charge accumulation in the BaTiO$_3$ layer.   

Exploring and alleviating detrimental interface dipole effects in ultra-thin all-oxide metal-ferroelectric-metal heterostructures

Ultrathin-film metal-ferroelectric-metal heterostructures present an exciting prospect for switchable nanoelectronic memories and devices such as ferroelectric tunnel junctions. The main challenge is to realize ferroelectricity in ultrathin-films where detrimental interface effects become increasingly more pronounced as ferroelectric film thicknesses approach the nanoscale. We studied the ferroelectric polarization of BaTiO$_{3}$ in epitaxial SrRuO$_{3}$/BaTiO$_{3}$/SrRuO$_{3}$ junctions by first-principles density functional theory and phenomenological modeling. The calculations show that the presence of a RuO$_{2}$/BaO termination sequence at the SrRuO$_{3}$/BaTiO$_{3}$ interface leads to a pinned interface dipole and is therefore detrimental to the stability of ferroelectricity, leading to the disappearance of switchable polarization under a certain thickness. Here, we propose to alleviate this behavior by depositing a thin layer of SrTiO$_{3}$ at this interface to suppress the RuO$_{2}$/BaO interface termination sequence, thereby eliminating the associated unfavorable pinned interface dipole. By doing this we find, and experiments confirm, that a switchable ferroelectric state can be stabilized in much thinner heterostructures.   

Domains and Electrostatic Coupling in Ferroelectric Superlattices

Superlattices composed of ferroelectric and paraelectric oxides have been the subject of numerous studies, delving into fundamental questions about ferroelectric size effects, revealing novel interfacial phenomena, and opening new possibilities for tailoring the functional properties of these artificially layered ferroelectrics. Here we examine the role of periodic 180 degree ferroelectric nanodomains on the structural and electrical properties of PbTiO$_3$/SrTiO$_3$ superlattices. Using a combination of X-ray diffraction and electrical measurements, nanoscale motion of domain walls under applied field has been detected and linked to the large enhancement of the dielectric response. Electrostatic interactions between the ferroelectric layers have been studied in detail, revealing an unexpected decoupling of the ferroelectric layers once the paraelectric layer thickness exceeds just a few perovskite unit cells. Recent advances in transmission electron microscopy allowed us to map out the local structural distortions across the superlattice using electron energy loss spectroscopy with unit-cell resolution, revealing highly inhomogeneous polarization profiles near the interfaces and giving new microscopic insight into the behavior of these fascinating materials.   

In situ x-ray study of oxide superlattice growth in reactive molecular-beam epitaxy

Improper ferroelectrity found in ultrashort period PbTiO3/SrTiO3 superlattices has attracted interests as one of new `interfacially engineered' materials.[1] At the interface of PbTiO3/SrTiO3 superlattices, the coupling between the ferroelectric mode and antiferrodistortive rotations of oxygen octahedra creates improper ferroelectricity with a large dielectric constant. PbTiO3/SrTiO3 superlattices were grown using reactive molecular-beam epitaxy in a chamber with in situ x-ray diffraction capability at the Advanced Photon Source. The use of in situ surface x-ray diffraction allows one to study the evolution of oxide heterostructures. Here we present initial studies of PbTiO3 and SrTiO3 single layers as well as superlattices. [1] E. Bousquet, M. Dawber, N. Stucki, C. Lichtensteiger, P. Hermet, S. Gariglio, J.-M. Triscone, and P. Ghosez, Nature 452, 732 (2008).   

The memristive magnetic tunnel junction as a nanoscopic synapse-neuron system

Memristors cover a gap in the capabilities of basic electronic components by remembering the history of the applied electric potentials, and are considered to bring neuromorphic computers closer by imitating the performance of synapses. We used memristive magnetic tunnel junctions based on MgO to demonstrate that the synaptic functionality is complemented by neuron-like behavior in these nanoscopic devices. The synaptic functionality originates in a resistance change caused by a voltage-driven oxygen vacancy motion within the MgO layer. The additional functionality provided by magnetic electrodes enabled a current-driven resistance modulation due to spin-transfer torque. We report on memristive magnetic tunnel junctions characterized by the simultaneous occurrence of resistive switching and tunnel magnetoresistance. Since resistivity provides a natural measure of the synaptic strength, and because of the bipolar nature of the resistance change, long term potentiation and long term depression were emulated. Furthermore, we show that the flux is a good variable for describing voltage-induced resistance variation, which provides the scope for the emulation of spike timing dependend plasticity as well.