Tailoring a two-dimensional electron gas at the LaAlO3/SrTiO3 (001) interface by epitaxial strain

Recently a metallic state was discovered at the interface between insulating oxides, most notably LaAlO3 and SrTiO3. Properties of this two-dimensional electron gas (2DEG) have attracted significant interest due to its potential applications in nanoelectronics. Control over this carrier density and mobility of the 2DEG is essential for applications of these novel systems, and may be achieved by epitaxial strain. However, despite the rich nature of strain effects on oxide materials properties, such as ferroelectricity, magnetism, and superconductivity, the relationship between the strain and electrical properties of the 2DEG at the LaAlO3/SrTiO3 heterointerface remains largely unexplored. Here, we use different lattice constant single crystal substrates to produce LaAlO3/SrTiO3 interfaces with controlled levels of biaxial epitaxial strain. We have found that tensile strained SrTiO3 destroys the conducting 2DEG, while compressively strained SrTiO3 retains the 2DEG, but with a carrier concentration reduced in comparison to the unstrained LaAlO3/SrTiO3 interface. We have also found that the critical LaAlO3 overlayer thickness for 2DEG formation increases with SrTiO3 compressive strain. Our first-principles calculations suggest that a strain-induced electric polarization in the SrTiO3 layer is responsible for this behavior. It is directed away from the interface and hence creates a negative polarization charge opposing that of the polar LaAlO3 layer. This both increases the critical thickness of the LaAlO3 layer, and reduces carrier concentration above the critical thickness, in agreement with our experimental results. Our findings suggest that epitaxial strain can be used to tailor 2DEGs properties of the LaAlO3/SrTiO3 heterointerface

A review of molecular beam epitaxy of ferroelectric BaTiO3 films on Si, Ge and GaAs substrates and their applications

SrTiO3 epitaxial growth by molecular beam epitaxy (MBE) on silicon has opened up the route to the monolithic integration of various complex oxides on the complementary metal-oxide-semiconductor silicon platform. Among functional oxides, ferroelectric perovskite oxides offer promising perspectives to improve or add functionalities on-chip. We review the growth by MBE of the ferroelectric compound BaTiO3 on silicon (Si), germanium (Ge) and gallium arsenide (GaAs) and we discuss the film properties in terms of crystalline structure, microstructure and ferroelectricity. Finally, we review the last developments in two areas of interest for the applications of BaTiO3 films on silicon, namely integrated photonics, which benefits from the large Pockels effect of BaTiO3, and low power logic devices, which may benefit from the negative capacitance of the ferroelectric. © 2015 National Institute for Materials Science161711sciescopu

Integration of functional complex oxide nanomaterials on silicon

The combination of standard wafer-scale semiconductor processing with the properties of functional oxides opens up to innovative and more efficient devices with high value applications which can be produced at large scale. This review uncovers the main strategies that are successfully used to monolithically integrate functional complex oxide thin films and nanostructures on silicon: the chemical solution deposition approach (CSD) and the advanced physical vapor deposition techniques such as oxide molecular beam epitaxy (MBE). Special emphasis will be placed on complex oxide nanostructures epitaxially grown on silicon using the combination of CSD and MBE. Several examples will be presented, with a particular stress on the control of interfaces and crystallization mechanisms on epitaxial perovskite oxide thin films, nanostructured quartz thin films, and octahedral molecular sieve nanowires. This review enlightens on the potential of complex oxide nanostructures and the combination of both chemical and physical elaboration techniques for novel oxide-based integrated devicesAC acknowledges the financial support from 1D-RENOX project (Cellule Energie INSIS-CNRS). J.M.V.-F. also acknowledges MINECO for support with a Ph.D. grant of the FPI program. We thank David Montero and L. Picas for technical support. We also thank P. Regreny, C. Botella, J.B. Goure for technical assistance on the Nanolyon technological platform. We acknowledge MICINN (MAT2008-01022 MAT2011-28874-c02-01 and MAT2012-35324), Consolider NANOSELECT (CSD2007-00041), Generalitat de Catalunya (2009 SGR 770 and Xarmae), and EU (HIPERCHEM, NMP4-CT2005-516858) projects. The HAADF-STEM microscopy work was conducted at the Center for Nanophase Materials Sciences, which is a DOE Office of Science User Facility. This research was supported by the European Research Council (ERC StG-2DTHERMS), Ministerio de Economía y Competitividad of Spain (MAT2013-44673-R) and EU funding Project “TIPS” Thermally Integrated Smart Photonics Systems Ref: 644453 call H2020-ICT-2014-1S

Oxide materials at the two-dimensional limit

Emergent phenomena in transition metal oxide films are receiving considerable attention with the development of techniques for the preparation of well-controlled oxide surfaces. On the macroscopic scale, such display novel physics phenomena including superconductivity, magnetism, ferroelectricity, and more. On the nanometer scale, the properties of epitaxial interfaces are further impacted by strain, band alignment, and crystal imperfections that may affect the long-range as well as the short-range order. Furthermore, symmetry lowering at the interface creates entirely new environments that are not accessible in the bulk environment. Thus, thin-film oxide materials are increasingly important in many applications. My work focuses on epitaxial oxides of the perovskite, spinel, and rocksalt structure and covers two main phenomena: (1) the two-dimensional electron gas at epitaxial oxide interfaces, and (2) thin epitaxial electro-optic oxides. Because polar oxides are of prominent interest as a mechanism for the formation of the two-dimensional electron gas, I start with a study of polar semiconductor Co₃O₄. Ellipsometry reveals a direct band gap of 0.75 eV, and magnetic measurements show the signature of antiferromagnetic ordering at 49 K, higher than the typical bulk value. Next, I look closer at the role of defects by studying the highly conducting layer at the crystalline [gamma]-alumina/SrTiO₃ (STO) interface which is attributed to oxygen vacancies. Annealing in oxygen is found to reduce the carrier density and turn a conductive sample into an insulator. Building upon these results, I show that even at room temperature, out-diffusion of oxygen from SrTiO₃ during epitaxy of highly spin-split semiconductor EuO epitaxy creates a highly conductive layer of oxygen vacancies on the SrTiO₃ side of the interface. The films are ferromagnetic with a Curie temperature of 70 K and display giant magnetoresistance below the transition temperature. Leveraging this approach offers an as-yet unexplored route to seamlessly integrate ferromagnetism and the oxide two-dimensional electron gas for the development of novel nano-oxide spintronic devices. The large effective Pockels coefficient for high-quality epitaxial BaTiO₃ (BTO) films on Si distinguishes BaTiO₃ as a highly promising material for integrated silicon nanophotonics. However, the linear electro-optic effect in BaTiO₃ thin films determined in previous experiments clearly shows deteriorated properties compared to bulk BTO crystals. First, I study BaTiO₃ films of varied thickness in order to quantify the Pockels coefficient with respect to crystalline orientation. As a next step, I report on the strong dependence of the Pockels effect in BaTiO₃ thin films on their microstructure, and provide guidelines on how to engineer thin films with strong electro-optic response. The 25× enhancement of the Pockels coefficient indicates a promising route to increase the performance of nonlinear oxides in the two-dimensional limit for the development of novel hybrid silicon photonic platform.Physic

Robust atmospherically stable hybrid SrVO3/Graphene//SrTiO3 template for fast and facile large-area transfer of complex oxides onto Si

Heterogenous integration of complex epitaxial oxides onto Si and other target substrates is recently gaining traction. One of the popular methods involves growing a water-soluble and highly reactive sacrificial buffer layer, such as Sr3Al2O6 (SAO) at the interface, and a functional oxide on top of this. To improve the versatility of layer transfer techniques, it is desired to utilize stable (less reactive) sacrificial layers, without compromising on the transfer rates. In this study, we utilized a combination of chemical vapor deposited (CVD) graphene as a 2D material at the interface and pulsed laser deposited (PLD) water-soluble SrVO3 (SVO) as a sacrificial buffer layer. We show that the graphene layer enhances the dissolution rate of SVO over ten times without compromising its atmospheric stability. We demonstrate the versatility of our hybrid template by growing ferroelectric BaTiO3 (BTO) via PLD and Pb(Zr, Ti)O3 (PZT) via Chemical Solution Deposition (CSD) technique and transferring them onto the target substrates and establishing their ferroelectric properties. Our hybrid templates allow for the realization of the potential of complex oxides in a plethora of device applications for MEMS, electro-optics, and flexible electronics.Comment: 35 pages, 23 figure

Structural, Electrical and Magnetic Properties of CoFe2O4 and BaTiO3 Layered Nanostructures on Conductive Metal Oxides

Multiferroic materials exhibit simultaneously, magnetic and electric order. In a magnetoelectric composite structure, a coupling is induced via an interfacial elastic interaction between magnetostrictive and piezoelectric materials enabling the control of the magnetisation by applying an electric field and vice versa. However, despite the potential of such coupling, experimental limits of theoretical models were observed. This work sheds some light on these limits by focusing the research on the chemistry of nanocomposite CoFe2O4 and BaTiO3, particularly at the interfaces where the coupling predominates. A comparison of the most common conductive oxides, Nb doped SrTiO3 and SrRuO3, was made for the bottom electrode application. The variation of conductive properties in Nb-SrTiO3 thin films at high temperature has been quantified when artificially strained and 60 nm SrRuO3 film was found to be the best bottom electrode choice for room temperature use. Epitaxial growth of magnetic CoFe2O4 was achieved on various metal oxide substrates despite large lattice mismatches. Crystallographic properties and strain evaluation were investigated and a Stranski-Krastanov growth mechanism, arising from the PLD deposition, was predominant. A notable drop of magnetisation was observed depending on the growth template, particularly on BaTiO3 substrates, the piezoelectric counterpart of the magnetoelectric structures. However, an encouraging magnetoelectric coupling induced by thermal phase transition of BaTiO3 was revealed. For BaTiO3, a control of the growth direction was realised by varying the deposition pressure, and the existence of both 180° and 90° ferroelectric domains was observed for films up to 300 nm in thickness. However, both the ferroelectric and piezoelectric properties were reduced in the thin films due to the clamping effect of the substrate. Finally, highly crystalline multilayers of CoFe2O4 and BaTiO3 were prepared on SrRuO3 buffered SrTiO3 substrates. It was found that the degradation of both magnetic and ferroelectric properties was proportional to the increase in the number of interfaces. A thorough microscopic study revealed interdiffusion and chemical instability occurring between CoFe2O4 and BaTiO3 at the interface. This undesired effect was partially recovered by the insertion of an ultra thin layer of SrTiO3, acting as a barrier layer at every interface. This research shows how interfacial chemistry need to be understood to achieve high magnetoelectric coupling in these types of epitaxial engineered structures

The 2016 oxide electronic materials and oxide interfaces roadmap

Lorenz, M. et al.Oxide electronic materials provide a plethora of possible applications and offer ample opportunity for scientists to probe into some of the exciting and intriguing phenomena exhibited by oxide systems and oxide interfaces. In addition to the already diverse spectrum of properties, the nanoscale form of oxides provides a new dimension of hitherto unknown phenomena due to the increased surface-to-volume ratio. Oxide electronic materials are becoming increasingly important in a wide range of applications including transparent electronics, optoelectronics, magnetoelectronics, photonics, spintronics, thermoelectrics, piezoelectrics, power harvesting, hydrogen storage and environmental waste management. Synthesis and fabrication of these materials, as well as processing into particular device structures to suit a specific application is still a challenge. Further, characterization of these materials to understand the tunability of their properties and the novel properties that evolve due to their nanostructured nature is another facet of the challenge. The research related to the oxide electronic field is at an impressionable stage, and this has motivated us to contribute with a roadmap on ‘oxide electronic materials and oxide interfaces’. This roadmap envisages the potential applications of oxide materials in cutting edge technologies and focuses on the necessary advances required to implement these materials, including both conventional and novel techniques for the synthesis, characterization, processing and fabrication of nanostructured oxides and oxide-based devices. The contents of this roadmap will highlight the functional and correlated properties of oxides in bulk, nano, thin film, multilayer and heterostructure forms, as well as the theoretical considerations behind both present and future applications in many technologically important areas as pointed out by Venkatesan. The contributions in this roadmap span several thematic groups which are represented by the following authors: novel field effect transistors and bipolar devices by Fortunato, Grundmann, Boschker, Rao, and Rogers; energy conversion and saving by Zaban, Weidenkaff, and Murakami; new opportunities of photonics by Fompeyrine, and Zuniga-Perez; multiferroic materials including novel phenomena by Ramesh, Spaldin, Mertig, Lorenz, Srinivasan, and Prellier; and concepts for topological oxide electronics by Kawasaki, Pentcheva, and Gegenwart. Finally, Miletto Granozio presents the European action ‘towards oxide-based electronics’ which develops an oxide electronics roadmap with emphasis on future nonvolatile memories and the required technologies. In summary, we do hope that this oxide roadmap appears as an interesting up-to-date snapshot on one of the most exciting and active areas of solid state physics, materials science, and chemistry, which even after many years of very successful development shows in short intervals novel insights and achievements.This work has been partially supported by the TO-BE COST action MP1308. J F acknowledges financial support from the Spanish Ministry of Economy and Competitiveness, through the ‘Severo Ochoa’ Programme for Centres of Excellence in R&D (SEV-2015-0496) and MAT2014-56063-C2-1R, and from the Catalan Government (2014 SGR 734). F.M.G. acknowledges support from MIUR through the PRIN 2010 Project ‘OXIDE’.Peer reviewe

Redox-controlled epitaxy and magnetism of oxide heterointerfaces: EuO/SrTiO3_3

We demonstrate a novel route to prepare thin films of the ferromagnetic insulator Europium monoxide. Key is a redox-controlled interface reaction between metallic Eu and the substrate SrTiO$_3$ as the supplier of oxygen. The process allows tuning the electronic, magnetic and structural properties of the EuO films. Furthermore, we apply this technique to various oxidic substrates and demonstrate the universality and limits of a redox-controlled EuO film synthesis.Comment: 7pages, 6 figures, and supplemetar