Symposium MD3 : Functional Oxide Heterostructures by Design

Ferroelectric oxide superlattices combining ferroelectric and non-ferroelectric materials exhibit an intriguing induced polarization in the non-ferroelectric phase. When a ferroelectric is combined with a non-polar material like SrTiO₃ (STO), the STO layer may become ferroelectric. However, there has been no demonstration of a superlattice where two non-ferroelectric materials combine to produce bulk polarization. We present work studying STO-LaCrO₃ (LCO) superlattices and show that by controlling interfacial termination between layers we can induce a ferroelectric-type polarization in STO. Density functional theory (DFT) predicts that by alternating terminations between positively charged TiO₂-LaO and negative CrO₂-SrO interfaces a polarization will be induced in both materials. Using molecular beam epitaxy, we have synthesized superlattices with such alternating interfaces. X-ray absorption fine structure analysis of the Ti K edge of the superlattices shows that the Ti-O bond lengths along the growth direction differ by ~0.2 Å. Scanning transmission electron microscopy measurements confirmed these results and were used to estimate the polarization within the STO layers. Both results are in good agreement with DFT predictions. A built-in electric field is also observed using laboratory-source and synchrotron x-ray photoelectron spectroscopy (XPS). Broadening of core-level and valence-band peaks for the various species in the superlattice in lab-source measurements is used to estimate a built-in potential gradient of ~1 V across each layer, in close agreement with DFT predictions. We also present standing-wave core-level photoemission and angle-resolved photoemission spectroscopy (SWARPES) measurements that permit probing the STO and LCO layers of the superlattice separately, which show dramatic differences in the valence band electronic structure. These measurements provide excellent depth sensitivity, allowing for unprecedented resolution of the electronic structure of the buried layers within the superlattice.

Manganite is one of the prototype strong correlated systems exhibiting intimate coupling between charge, spin, orbital, and lattice degrees of freedom, resulting in the delicate energy proximity of different phases. Although the ground state of optimal doped bulk La_2/3Ca_1/3MnO₃ (LCMO) is ferromagnetic-metal phase (FMM), LCMO films grown on NdGaO₃(001) (NGO(001)) suffering anisotropic strain demonstrate a tunable antiferromagnetic insulator phase (AFM) or phase separation (PS) in a wide temperature range after annealed in O₂ atmosphere. PS with the coexistence of AFM and FMM was induced in anisotropically strained LCMO/NGO(001) films due to enhanced orthorhombicity. The reentrance of antiferromagnetic charge-order insulator phase (AFM-COI) from a saturated FMM as functions of magnetic field and temperature was observed. The reentrance is mediated by the cooperative MnO₆ octahedral distortions, which is consistent with the Martensitic-like transformation. The quantity of the reentrance of the AFM-COI from the saturated FMM can be controlled by magnetic field and temperature in different processes, however, the resistivity changes a little in a wide range of temperature. Magnetic force microscopy morphologies exhibit anisotropically patterns correlating closely with its magnetic and electrical properties.

High quality superlattices containing SmFeO₃ (SFO) and CaRuO₃ (CRO) with thicknesses as small as 0.8 nm were fabricated. We studied the enhanced conductivity and metal-insulator transitions (MITs) of ultrathin CRO films in (SFO/CRO) _x superlattices. For x=16, All superlattices with the thickness of CRO (t _CRO) more than 0.8 nm are metallic, whereas a 2.4 nm single film is insulating. Even for x=2, with t _CRO = 1.2 nm the sample is still conductive. Moreover, the SFO space layer was replaced by CaRu_0.8Ti_0.2O, which is insulator, the conductivity was further strengthened. Additional “conducting channels” at the interfaces and the relaxation of oxygen octahedral tilts arised from a combination of epitaxial strain and oxygen octahedral connectivity may be the two possible interpretations.
For x=16, a transition temperature (T^*) dependent MITs was observed with the decreasing t _CRO except for 0.8 nm and the T^* was strongly affected by the thickness of SFO. The low temperature insulating behavior can be ascribed to the three-dimensional (3D) weak localization, related to the disorder. Meanwhile the superlattices with t _CRO = 0.8 nm manifest an insulator-like behavior which can be explained well by the Mott-type variable range hopping conduction mechanism. The physical properties of MITs could be understood within a combined picture of the disorder and the electron correlation effects. In addition, the signs of magnetoresistance (MR) were changed at a critical thickness of t _{CRO =} 1.6 nm and no matter with the thickness of SFO. We can’t account for this exactly at present, but a felicitous reason could be the relaxation of octahedral tilts at the critical thickness. To wit: 1.6nm is the critical modulated length by octahedral tilts. More research is needed to have an insight into the physical properties.

Multiferroic materials attract enormous scientific and technological interests due to their ability to exhibit a magnetoelectric (ME) effect which enables the control of the polarization/ magnetization with an applied magnetic/ electric field, respectively. Recently, compared with the bulk multiferroics, the research interest has focused more and more in thin films area with the development of thin film growth techniques, which enable deposition under epitaxial engineering and non-equilibrium conditions. Among the most widely studied two-phase multiferroic composite thin films, the self-assembled epitaxial BiFeO₃-CoFe₂O₄ (BFO-CFO) nanostructured composite thin films, which contain nanopillars of one phase embedded inside the matrix of the other phase, have been found to be able to present different structures and multiferroic properties by depositing on various oriented SrTiO₃ (STO) substrates by pulsed laser deposition (PLD) method.

Here, we have utilized self-assembled BFO nanopillars in a BFO-CFO two phase layer on STO as a seed layer on which to deposit a secondary top BiFeO₃ layer by PLD. The growth mechanism of this secondary BFO layer has been investigated, and its multiferroic properties studied. It has been found from cross-section images of electron microscopy studies that the top BFO layer preferentially grew from the bottom BFO seeds and its grain size could be controlled by these seeds. The multiferroic properties of this new heterostructures have also been studied.

Moreover, based on that above mentioned heterostructures with controlled growth, we optimized the experimental procedures and designed to grow another BFO layer on the top of that heterostructures. Thus, a novel structure with second phase CFO nanoparticles embedded in a primary BFO matrix phase has been fabricated. Improved multiferroic properties have been confirmed by several kinds of characterization methods for this heterostructures. Furthermore, by focusing on switching characteristics of the piezoresponse, we demonstrated that the newly designed oxide film showed magnetic field dependence of piezoelectricity due to the improved coupling enabled by good connectivity amongst the piezoelectric and magnetostrictive phases. The improved connectivity amongst the constituent phases of the composite heterostructure has been examined by the state-of-the-art electron microscopy. We have also provided statistical study for the characteristics and then absolutely confirmed that the yielding notable ME effects result from a better coupling between the ferromagnetic and ferroelectric phases of this special architecture. Such new epitaxial multiferroic oxide heterostructures may open new avenues to the application of multi-functional applications.

Integration of functional oxide thin films with semiconductors has attracted considerable attention in recent years due to their potential applications in future system-on-a-chip devices. GaAs, for example, have a higher saturated electron velocity and mobility allowing transistors based on GaAs to operate at a much higher frequency with less noise compared to Si. In addition, because of its direct bandgap a number of efficient optical devices are possible and by integrating with other III-V semiconductors the wavelengths can be made tunable through hetero-engineering of the bandgap. In this study, we report on the use of SrTiO₃ (STO) films on GaAs (100) substrates by molecular beam epitaxy (MBE) as an intermediate buffer layer for the hetero-epitaxial growth of ferromagnetic La_0.7Sr_0.3MnO₃ (LSMO) and room temperature multiferroic BiFeO₃ (BFO) thin films using pulsed laser deposition (PLD). The properties of the multilayer thin films in terms of growth modes, lattice spacing/strain, interface structures and texture were characterized by the in-situ reflection high energy electron diffraction (RHEED). The crystalline quality and chemical composition of the BFO/LSMO/STO/GaAs heterostructures were investigated by a combination of x-ray diffraction (XRD) and x-ray photoelectron spectroscopy (XPS). Surface morphology, piezo-response with domain structure, and ferroelectric switching observations were carried out on the thin film samples using piezoresponse force microscopy (PFM) in the contact mode. The out-of-plane (OP-PFM) and in-plane (IP-PFM) amplitude images obtained for the BFO by PFM show ferroelectric switching behavior of domains at the nanoscale level. The optical properties and the physical thickness of the multilayers were investigated using variable angle spectroscopic ellipsometry by determining the ellipsometric parameters Ψ and Δ at room temperature. The electrical properties and ferroelectric P-E hysteresis loop were measured using a semiconductor parameter analyzer and a radiant ferroelectric tester system. Saturated ferroelectric hysteresis loop is obtained with P_r~90.68 µC/cm² and low leakage current for our BFO film. The ferromagnetic behavior of BFO/LSMO heterostructure was confirmed with vibrating sample magnetometer (VSM) studies and found to have a saturation magnetization of M_s~650emu\cm³ with improved magnetic hysteresis squareness originating from the BFO/LSMO interface. We found the nanostructure and the physical-composition results obtained from the multilayers are correlated with their corresponding dielectric, piezoelectric, and ferroelectric properties. These result provide an understanding of the heteroepitaxial growth of ferroelectric-antiferromagnetic BiFeO₃ on ferromagnetic La_0.7Sr_0.3MnO₃, integrated on SrTiO₃ buffered GaAs (100) with full control over the interface structure at the atomic-scale. This work also represents the first step toward the realization of magneto-electronic devices integrated with GaAs.

In complex oxide thin films, thickness, charge, and strain all help to define the properties of the material. For example, epitaxial strain can alter the oxygen octahedral tilt structure, which influences the electrical and magnetic properties, and the screening of polar interfaces can give rise to a variety of emergent interfacial phenomena. The overall thickness of a film determines the combination of surface and bulk phenomena that dominate its properties. We have studied the interaction between these factors using in-situ synchrotron x-ray scattering during the off-axis sputter deposition of LaGaO₃ from separate La₂O₃ and Ga₂O₃ sources. The changes in microstructure and oxygen octahedral rotations that occur in these samples as individual layers of LaO and GaO₂ are added to the surface of the film will be described. We will discuss the electrostrictive effect that these nominally charged layers have on the film and will compare these results with predictions from first-principles calculations examining the structure and stability of LaO- and GaO₂-containing heterostructures with varying termination and thickness.

The age of big-data in materials science has arrived. Big-data provides the crucial link between the functional imaging, which has seen vast improvements over the past decade, and modeling, which has also experienced tremendous gains. Here, I will present specific examples of how functional atomic-scale imaging in real-space can be used, through multivariate statistical analysis, to provide deep insight and reveal underlying physics in complex oxides, and how these can be extended to further the progress spurred by the Materials Genome Initiative. Analysis of local crystallography, in addition to clustering of atomic intensities, allows for identification of ‘dopants’ within atomically-resolved images. The use of a sliding fast Fourier transform routine in conjunction with endmember extraction from the N-FINDR algorithm can be used to automatically determine the types and spatial localization of crystallographic phases present in atomically-resolved data. Quantification of adatoms on meandering oxide surfaces, in conjunction with Monte-Carlo modeling, can be used to infer energies of terminations, as well as heights of diffusion barriers. In parallel, in-situ reciprocal space data can be highly useful in determining growth transitions and quality of the final surface, and extension to include chemical information from parallel information streams, combined with deep learning to unravel connections between deposition control parameters and final functional properties, promise a rich tapestry of knowledge that can be harnessed towards a materials by design approach. These examples highlight the utility of big-data for understanding physics of oxides, and show the route towards a shifting paradigm of materials discovery.

This research was sponsored by the Division of Materials Sciences and Engineering, BES, DOE (RKV, SVK). Research was conducted at the Center for Nanophase Materials Sciences, which also provided support (APB) and which is a DOE Office of Science User Facility.

Engineering tailored materials by exploiting emergent interfacial properties in perovskite heterostructures requires understanding of the complex atomic scale influences driving behavior. Properties in these materials develop via indirect interactions of B-cation electrons through a network of corner-sharing oxygen octahedra which are extremely sensitive to both atomic shifts that cause changes in bond angle or length, and to modification of electronic character. As such, we must be able to observe and identify, accurately and with high spatial resolution, individual structural or electrical changes that occur as a result of cation substitution, epitaxial misfit strain and coherence, or charge transfer. In this work, we employ several complementary, state-of-the-art characterization techniques, including aberration corrected scanning transmission electron microscope (Cs-corrected STEM) imaging, electron energy loss spectroscopy (EELS-SI), STEM energy-dispersive x-ray spectroscopy (STEM-EDS), and position averaged convergent beam electron diffraction (PACBED) to investigate the structural, chemical, and electrical interactions at interfaces in heterostructures of La_0.7Sr_0.3MnO₃ (LSMO) and La_0.7Sr_0.3CoO₃ (LSCO) grown on La_0.30Sr_0.70Al_0.65Ta_0.35O₃ (LSAT) substrates, and examine how these interactions mediate functional properties. Imaging allows us to determine that interfaces are abrupt and films are fully strained, and we observe variations in strain accommodation, and measure changes in octahedral distortions, through film thicknesses and layers. In a complementary way, with spectroscopy we analyze the electrical and chemical profile through the film thicknesses and across interfaces. This presentation will discuss the importance of parameters such as layer depth, number of interfaces, or growth order, and their impact on the critical B-O-B bond. In addition, we acknowledge the challenges of data collection and beam-sample interaction by conventional methods in Cs-corrected STEM, and how they can be addressed using compressive sensing. Low dose imaging with high resolution can be realized through the novel application of compressive sensing, which reconstructs under-sampled data into high-resolution images and spectra, thus radically reducing the dose delivered to the sample and the data collection time.

This continues work that was supported by the Swiss National Science Foundation Grant PBFRP2-134402, the Defense Advanced Research Projects Agency Grant N66001-11-1-4135, and is supported in part by the NSF Grant No. DMR 1411250 through UC Davis, LDRD Program: Chemical Imaging Initiative at PNNL, and EMSL, a national scientific user facility sponsored by the DOE’s Office of Biological and Environmental Research and located at PNNL. PNNL is a multiprogram national laboratory operated by Battelle for the DOE under Contract DE-AC05-76RL01830.

One of the most powerful means to tune the interfacial electronic and magnetic ground states of a multilayer system is by the ferroelectric field effect. By using a ferroelectric with a switchable polarization, it becomes possible to either accumulate or deplete holes at the interface of a neighboring film layer. Provided the large sensitivity of the physical properties of magnetic La_1-xSr_xMnO₃ (LSMO) to fluctuations in carrier density by chemical substitution of La³⁺ with lower valent Sr²⁺, these families of compounds are model systems for studying the role of electrostatic doping at the interface with a highly polar ferroelectric such as PbZr_0.2Ti_0.8O₃ (PZT). Conventional characterization techniques such as SQUID magnetometry and transport measurements enable one to explore changes in a material’s bulk properties, but they are not in themselves direct probes of the interface. Here, we report recent results on the magnetic structure of pulsed laser deposited PZT/La_0.8Sr_0.2MnO₃ multilayers probed using the powerful technique of polarized neutron reflectivity (PNR). Since PNR can provide atomic scale information on the magnetic structure and chemical composition as a function of thickness, we are able to investigate the changes at the interfaces resulting from the ferroelectric polarization. Through carefully designed heterostructures, we have confirmed the nanoscale control of interfacial magnetic ground states by ferroelectrics. A clear contrast in PNR results was observed depending on the orientation of the polarization, which can either suppress or enhance the magnetic moment. Specifically, we found that the suppressed magnetization common to the manganite-air interface can enhanced using ferroelectrics. Thus, in this presentation, we will discuss the important role of ferroelectric polarization in modifying interfacial magnetic properties by comparing results from non-ferroelectric/magnetic heterostructures such as LSMO/LaAlO₃.

Acknowledgements The work was supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, Materials Sciences and Engineering Division. The work at ORNL’s Spallation Neutron Source was sponsored by the Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. Department of Energy.

Tantalum oxide memristors promise long endurance in information storage and are frontrunners for next generation memory technology. Recent efforts to uncover the nanophysics behind resistance switching in tantalum oxide and several related materials suggest local conductive channel formation by oxygen-ion-migration as the mechanism of operation, and is supported by indirect experimental observations and theoretical predictions. A more complete understanding and model is critical in enabling widespread implementation of such technology, and our goal was to directly observe this migration.
Here we employed x-ray absorption spectromicroscopy of the O K-edge and Ta-L edge in a planar geometry on micrometer sized crosspoint tantalum oxide memristors. Using a synchronous in-operando technique developed recently, [1] we were able to isolate very weak signals corresponding to extremely localized elemental, chemical and electronic changes to the material.
We electrically cycled tantalum oxide crosspoint memristors between bistable resistance states using voltage pulses of varying amplitudes. Upon using low voltages (This is a direct in-situ observation of nanoscale composition evolution and atomic motion of oxygen in a functioning memristor that describes possible material behavior during switching, repeated cycling and failure. This also highlights the potential of O K-edge x-ray absorption spectromicroscopy as a sensitive probe to study inhomogeneous localized phenomena in a wide variety of oxides with in-situ and in-operando operations.

Next generation solid-state devices may exploit spin degrees of freedom (±½) resulting in large increases in read/write performance and storage capacity. Materials that exhibit high Curie temperatures (above room temperature) and high degrees of spin polarization are good candidates for use in such devices. The family of double perovskites (DPs) with the general formula A₂BB′O₆, have been shown to exhibit favorable properties. In order to use DPs in spintronic devices, high quality, epitaxial thin films with high ordering parameters must be grown, which has proven difficult. Ordering of the B/B′ atoms is essential to maintain the preferred magnetic and electronic properties because of superexchange couplings across B-O-B′ and B-O-B′-O-B bonds.
In this work, we investigate thin films of Sr₂CrReO₆ (SCRO) grown on (LaAlO₃)_0,3(Sr₂AlTaO₆)_0.7 (LSAT) with a buffer layer of SrCr_0.5Nb_0.5O₃ (SCNO). SCRO has been shown to grow epitaxially on SCNO buffer layers and to maintain exceptionally high ordering parameters for the B/B′ sublattice. Because of the high atomic number (Z) contrast, high angle annular dark field scanning transmission electron microscopy (HAADF-STEM) was employed to image thin film interfaces, as well as defects within the films. Fully ordered regions are expected to exhibit clear modulations in intensity from column to column in this imaging mode. To a first approximation, this allows for identification of regions within the film that appear fully ordered versus those that do not for further analysis.
In combination with the experimental STEM images, DP structures were modeled followed by HAADF-STEM image and diffraction simulations. Through careful analysis of the data, it can be shown that regions with the SCRO films that appear highly disordered due to a lack of clear intensity variation are actually a consequence of antiphase domains, which result in a misregistry of the B/B′ sublattice. This implies that the SCRO films are, in fact, highly ordered with some local regions exhibiting wrong nearest-neighbors on the B/B′ sublattice, providing an explanation the favorable magnetic properties.
Image simulation was also used to understand variations in HAADF-STEM image intensities based on the effect of B/B′ ordering. It can be shown that the atomic configurations along the path of the electron beam can have significant effects on the overall intensities seen in the image, demonstrating that HAADF-STEM image contrast alone is not the most reliable way to quantify ordering. Other techniques, such as diffraction or chemical analysis in the microscope are necessary, with simulations, to fully quantify the data.

One highly promising route for future nanoelectronics considers transistors made of Mott insulators [1]. These Mott transistors take advantage of the electronic correlations, enabling higher ON/OFF ratios and lower power consumption than achievable with conventional MOSFET technology. In Mott-insulating CaMnO₃ (CCMO) the chemical substitution of Ca²⁺ by Ce⁴⁺ leads to electron doping, inducing a transition to metallic behavior at much lower doping levels compared to the widely investigated hole-doped manganites. CCMO thus appears as a promising channel material for Mott transistors. In bulk CCMO, this transition occurs at 5% Ce doping and is accompanied by the development of a weak ferromagnetic state with a magnetic moment of ∼0.4 μB/Mn [2]. The resulting rich phase diagram can be further modulated by epitaxial strain in thin films [3, 4].
In this study, high quality thin films of CCMO with different doping levels (0, 2 and 4% Ce content) have been epitaxially grown on YAlO₃(001) substrates. We will present a detailed analysis of their electronic properties based on transport and soft X-ray ARPES experiments, and of their magnetic response (from SQUID magnetometry, XMCD and anomalous Hall measurements). Finally, we will show results on heterostructures combining CCMO as channel and BiFeO₃ as ferroelectric gate oxide, with the aim of controlling the electronic and magnetic response by ferroelectric field effect [5].

The authors acknowledge financial support from the European Research Council (ERC Advanced Grant FEMMES, No. 267579 and ERC Consolidator Grant MINT, No. 615759) and the German Research Foundation (HO 5346/1-1).

[1] D. M. Newns et al., Appl. Phys. Lett. 73, 780 (1998)
[2] E.N. Caspi et al., PRB 69, 104402 (2004)
[3] Xiang et al., Adv. Mater. 23, 5822 (2011)
[4] Xiang et al., JAP 112, 113703 (2012)
[5] H. Yamada et al., Scientific Reports 3, 2834 (2014)

As novel phenomena at materials interfaces draws increasing attention, it is vital to gain atomic control of material growth to further understand and engineer their properties. Currently, Reflection High-Energy Electron Diffraction (RHEED) is one of the most commonly used in situ diagnostic tools for both molecular beam epitaxy and pulsed laser deposition growth techniques. RHEED is frequently used to observe layer-by-layer growth and used to control the growth at the unit cell level. Sputtering is a common, inexpensive, and industrially viable growth technique for many complex oxide materials but lacks the powerful in situ analysis techniques used by pulsed laser deposition and molecular beam epitaxy. Here we demonstrate the integration of RHEED into the sputtering environment as an in situ tool during the growth of oxide materials. We will present on the challenges associated with performing RHEED analysis during sputter deposition and provide a practical and effective approach for mitigating these challenges. Experimentally we observe strong RHEED specular spot oscillations during SrRuO₃ growth for many tens of unit cells, demonstrating the possibility for precise layer-by-layer control of complex oxides during sputter deposition. We apply our approach to the growth of several novel epitaxial heterostructures to demonstrate precise unit cell and interface control.