Epitaxy of SrTiO3 on Silicon: The Knitting Machine Strategy

Saint-Girons, Guillaume et al.SrTiO3 (STO) crystalline layers grown on Si open unique perspectives for the monolithic integration of functional oxides in silicon-based devices, but their fabrication by molecular beam epitaxy (MBE) is challenging due to unwanted interfacial reactions. Here we show that the formation of single-crystal STO layers on Si by MBE at the moderate growth temperature imposed by these interface reactions results from the crystallization of a partially separated amorphous mixture of SrO and TiO2 activated by an excess of Sr. We identify the atomic pathway of this mechanism and show that it leads to an antiphase domain morphology. On the basis of these results, we suggest and test alternative STO growth strategies to avoid antiphase boundary formation and significantly improve the STO structural quality. The understanding provided by these results offers promising prospects to crystallize perovskite oxides on semiconductors at moderate temperature and circumvent the issue of parasitic interface reactions.This work was partly supported by the European projects SITOGA (STREP FP7, grant number 619456) and TIPS (ICT H2020, grant number 107347) as well as by the French ANR programs HIRIS and DIAMWAFEL.Peer reviewe

TEM investigation of growth mechanisms and microstructure of model YBCO coated conductor architectures deposited by metalorganic decomposition

Descripció del recurs: el 21-08-2008Consultable des del TDXTítol obtingut de la portada digitalitzadaThis thesis is divided in six chapters. In the first one, we expose an amenable brief introduction to the YBCO compound, as well as to the CSD method. Then, in the second chapter we describe briefly the experimental techniques used for the physical characterization of the CSD thin films presented in this work. The results, chapters 3, 4 , 5 and 6, are presented in two parts: Part I reports many aspects governing the growth mechanisms of CSD films, in particular the identification of those factors controlling the evolution of microstructures, which remain poorly established in comparison with vacuum deposited films. We give first an account of the evolution from a partially oriented granular microstructure to a dense epitaxial one in CeO2 films deposited from chemical solutions (chapter 3), and second the microstructural evolution of YBCO from trifluoroacetate precursors, which follow a complex compositional trajectory (chapter 4). In part II the microstructure of TFA-YBCO thin films with high critical current densities is analysed. We depict intrinsic structural defects occurring within YBCO films, focusing in extended defects which can lead to strong flux-pinning and high critical current density (chapter 5). The microstructure of TFA-YBCO films with artificial defects has been also studied (chapter 6). The inclusion of BaZrO3 results in strong increase of the critical current density, demonstrating that chemical solution growth is a very flexible methodology to nanostructure YBCO films and coated conductors. Finally, we present the general conclusions of this study

Epitaxial stabilization of ε-Fe2O3 (00l) thin films on SrTiO3 (111)

This article may be downloaded for personal use only. Any other use requires prior permission of the author and the American Institute of Physics.Thin films of the metastable and elusive ε-Fe2O3 have been epitaxially stabilized on SrTiO3 (111) substrates. The ε-Fe2O3 films present a (001) orientation perpendicular to the substrate and three in-plane domains measuring a few nanometers and showing atomically sharp interfaces. We argue that this domain structure, rather than the epitaxial-strain, plays an essential role in stabilizing the ε-Fe2O3 by minimizing the energy of (100) surfaces. The ε-Fe2O3 films show a large in-plane coercivity ∼ 8 kOe which combined with the magnetoelectric character claimed for this oxide may lead to novel applications in spintronics

High pinning performance of YBa2Cu3O7-x films added with Y2O3 nanoparticulate defects

Mele, Paolo et al.© 2015 IOP Publishing Ltd. We report the epitaxial growth and superconducting properties of Y2O3-added YBa2Cu3Ox (YBCO) films grown on SrTiO3-buffered MgO substrates by pulsed-laser deposition using surface-modified YBCO targets. Areas of Y2O3 sectors on the YBCO target were increased to 5.44% and 9.22% of the total YBCO pellet in order to find a correlation between the Y2O3 content, morphology, and the pinning properties of YBCO+Y2O3 mixed films. The maximum global pinning forces, FP, at 77 K were 14.3 GN m-3 and 1.15 GN m-3 for the Y2O3 5.44A% and 9.22A%, respectively. The 5.44A% Y2O3-added sample presents a very high value of pinning force at 77 K, approaching the value obtained in YBCO films with added BaZrO3 nanorods, but with less depression in the superconducting critical temperature, Tc. In accordance with scanning transmission electron microscopy (STEM) observations, both films present nanoparticulate Y2O3 dispersed in a YBCO matrix where Y2Ba4Cu8O16 (Y248) intergrowths were also observed. Consistent with the strong pinning theory, the size and distribution of randomly dispersed Y2O3 particles are optimal for the flux pinning of a 5.44A% Y2O3-YBCO film, while in the case of a 9.22A% film, the YBCO matrix is degraded by jam-packed Y248 intergrowth, which leads to a comparatively poor pinning performance. We further used the single-vortex dynamics model to account for vortex pinning in the samples. The 5.44A% Y2O3-YBCO film result shows good agreement with the model fit up to 4 T of the applied magnetic field.This work was supported by CREST-JST and KAKENHI, Grant-in Aid for Science Research (S), Grant number 2322601401. The microscopy work conducted at the Oak Ridge National Laboratory was partially supported by the Oak Ridge National Laboratory’s Shared Research Equipment (ShaRE) User Facility, which is sponsored by the Office of Basic Energy Sciences, US. Department of Energy.Peer Reviewe

High ferroelectric polarization in c-oriented BaTiO3 epitaxial thin films on SrTiO3/Si(001)

Scigaj, M. et al.The integration of epitaxial BaTiO3 films on silicon, combining c-orientation, surface flatness, and high ferroelectric polarization is of main interest towards its use in memory devices. This combination of properties has been only achieved so far by using yttria-stabilized zirconia buffer layers. Here, the all-perovskite BaTiO3/LaNiO3/SrTiO3 heterostructure is grown monolithically on Si(001). The BaTiO3 films are epitaxial and c-oriented and present low surface roughness and high remnant ferroelectric polarization around 6 μC/cm2. This result paves the way towards the fabrication of lead-free BaTiO3 ferroelectric memories on silicon platforms.ICMAB-CSIC authors acknowledge 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 the MAT2014-56063-C2-1-R project, and from Generalitat de Catalunya (2014 SGR 734). Work at Oak Ridge National Laboratory was supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Materials Sciences and Engineering Division. C. H. Chao acknowledges the NSC-CSIC 2014 Summer Program in Spain for Taiwanese PhD students. I. Fina acknowledges Juan de la Cierva – Incorporación postdoctoral fellowship (IJCI-2014-19102) from the Spanish Ministry of Economy and Competitiveness of Spanish Government. INL gratefully acknowledges the European commission and the national French research agency (ANR) for funding, through the projects SITOGA (FP7-ICT-2013-11-619456), TIPS (H2020‐ICT‐02-2014‐1-644453), ANR HIRIS and ANR DIAMWAFEL. INL also acknowledges P. Regreny, C. Botella and J.-B. Goure for MBE technical assistance.Peer reviewe

Epitaxial YBa2Cu3O7-x nanocomposite thin films from colloidal solutions

A methodology of general validity to prepare epitaxial nanocomposite films based on the use of colloidal solutions containing different crystalline preformed oxide nanoparticles (ex situ nanocomposites) is reported. The trifluoroacetate (TFA) metal-organic chemical solution deposition route is used with alcoholic solvents to grow epitaxial YBaCuO (YBCO) films. For this reason stabilizing oxide nanoparticles in polar solvents is a challenging goal. We have used scalable nanoparticle synthetic methodologies such as thermal and microwave-assisted solvothermal techniques to prepare CeO and ZrO nanoparticles. We show that stable and homogeneous colloidal solutions with these nanoparticles can be reached using benzyl alcohol, triethyleneglycol, nonanoic acid, trifluoroacetic acid or decanoic acid as protecting ligands, thereby allowing subsequent mixing with alcoholic TFA solutions. An elaborate YBCO film growth analysis of these nanocomposites allows the identification of the different relevant growth phenomena, e.g. nanoparticles pushing towards the film surface, nanoparticle reactivity, coarsening and nanoparticle accumulation at the substrate interface. Upon mitigation of these effects, YBCO nanocomposite films with high self-field critical currents (J ∼ 3-4 MA cm at 77 K) were reached, indicating no current limitation effects associated with epitaxy perturbation, while smoothed magnetic field dependences of the critical currents at high magnetic fields and decreased effective anisotropic pinning behavior confirm the effectiveness of the novel developed approach to enhance vortex pinning. In conclusion, a novel low cost solution-derived route to high current nanocomposite superconducting films and coated conductors has been developed with very promising features.All authors acknowledge the EU (EU-FP7 NMP-LA-2012-280432 EUROTAPES project). ICMAB acknowledges MINECO (MAT2014-51778-C2-1-R) and Generalitat de Catalunya (2014SGR 753 and Xarmae). UGhent acknowledges the Special Research Fund (BOF), the Research Foundation Flanders (FWO) and the Institute for the Promotion of Innovation through Science and Technology in Flanders (IWT). TEM microscopy work was conducted in the Catalan Institute of Nanoscience and Nanotechnology (ICN2). The authors acknowledge the ICN2 Electron Microscopy Division for offering access to their instruments and expertise. Part of the STEM microscopy work was conducted in 'Laboratorio de Microscopias Avanzadas' at the Instituto de Nanociencia de Aragon—Universidad de Zaragoza. The authors acknowledge the LMA-INA for offering access to their instruments and expertise. JG and MC also acknowledge the Ramon y Cajal program (RYC-2012-11709 and RYC-2013-12448 respectively).Peer Reviewe

Conducting interfaces between amorphous oxide layers and SrTiO3(110) and SrTiO3(111)

Interfaces between (110) and (111)SrTiO3 (STO) single crystalline substrates and amorphous oxide layers, LaAlO3 (a-LAO), Y:ZrO2 (a-YSZ), and SrTiO3 (a-STO) become conducting above a critical thickness tc. Here we show that tc for a-LAO is not depending on the substrate orientation, i.e. tc (a-LAO/(110)STO) ~ tc(a-LAO/(111)STO) interfaces, whereas it strongly depends on the composition of the amorphous oxide: tc(a-LAO/(110)STO) < tc(a-YSZ/(110)STO) < tc(a-STO/(110)STO). It is concluded that the formation of oxygen vacancies in amorphous-type interfaces is mainly determined by the oxygen affinity of the deposited metal ions, rather than orientational-dependent enthalpy vacancy formation and diffusion. Scanning transmission microscopy characterization of amorphous and crystalline LAO/STO(110) interfaces shows much higher amount of oxygen vacancies in the former, providing experimental evidence of the distinct mechanism of conduction in these interfaces

Magnetic Mesoporous Silica Nanorods Loaded with Ceria and Functionalized with Fluorophores for Multimodal Imaging

Multifunctional magnetic nanocomposites based on mesoporous silica have a wide range of potential applications in catalysis, biomedicine, or sensing. Such particles combine responsiveness to external magnetic fields with other functionalities endowed by the agents loaded inside the pores or conjugated to the particle surface. Different applications might benefit from specific particle morphologies. In the case of biomedical applications, mesoporous silica nanospheres have been extensively studied while nanorods, with a more challenging preparation, have attracted much less attention despite the positive impact on the therapeutic performance shown by seminal studies. Here, we report on a sol-gel synthesis of mesoporous rodlike silica particles of two distinct lengths (1.4 and 0.9 μm) and aspect ratios (4.7 and 2.2) using Pluronic P123 as a structure-directing template and rendering ∼1 g of rods per batch. Iron oxide nanoparticles have been synthesized within the pores yielding maghemite (γ-FeO) nanocrystals of elongated shape (∼7 nm × 5 nm) with a [110] preferential orientation along the rod axis and a superparamagnetic character. The performance of the rods as T-weighted MRI contrast agents has also been confirmed. In a subsequent step, the mesoporous silica rods were loaded with a cerium compound and their surface was functionalized with fluorophores (fluorescamine and Cyanine5) emitting at λ = 525 and 730 nm, respectively, thus highlighting the possibility of multiple imaging modalities. The biocompatibility of the rods was evaluated in vitro in a zebrafish (Danio rerio) liver cell line (ZFL), with results showing that neither long nor short rods with magnetic particles caused cytotoxicity in ZFL cells for concentrations up to 50 μg/ml. We advocate that such nanocomposites can find applications in medical imaging and therapy, where the influence of shape on performance can be also assesse

Nanocrystalline Ferroelectric BiFeO3 Thin Films by Low-Temperature Atomic Layer Deposition

© 2015 American Chemical Society. In this work, ferroelectricity is identified in nanocrystalline BiFeO3 (BFO) thin films prepared by low-temperature atomic layer deposition. A combination of X-ray diffraction, reflection high energy electron diffraction, and scanning transmission electron microscopy analysis indicates that the as-deposited films (250 °C) consist of BFO nanocrystals embedded in an amorphous matrix. Postannealing at 650 °C for 60 min converts the sample to a crystalline film on a SrTiO3 substrate. Piezoelectric force microscopy demonstrates the existence of ferroelectricity in both as-deposited and postannealed films. The ferroelectric behavior in the as-deposited stage is attributed to the presence of nanocrystals. Finally, a band gap of 2.7 eV was measured by spectroscopic ellipsometry. This study opens broad possibilities toward ferroelectric oxides on 3D substrates and also for the development of new ferroelectric perovskites prepared at low temperature.This research was supported by MAT2011-28874-C02-01, MAT2014-511778-C2-1-R, SGR753 and Consolider. M.C. and J.G. acknowledge RyC contracts, 2013-12448 and 2012-11709, respectively. I.F. acknowledges the Beatriu de Pinós postdoctoral scholarship (2011 BP-A 00220) from AGAURGeneralitat de Catalunya. Financial support from the ERC Starting investigator grant STEMOX 239739 and Consolider IMAGINE is acknowledged (M.V.).Peer Reviewe

All chemical YBa2Cu3O7 superconducting multilayers: Critical role of CeO2 cap layer flatness

New advances toward microstructural improvement of epitaxial Ce0 2 films grown by chemical solution deposition and their use as buffer layers for YBa2Cu3O7 (YBCO) films are presented. We demonstrate that the degree of epitaxy and the fraction of (001) atomically flat surface area are controlled by the incorporation of tetravalent (Zr4+) or trivalent (Gd3+) cations into the ceria lattice. The degree of epitaxy has been investigated by means of Rutherford backscattering spectroscopy-channeling and reflection high-energy electron diffraction. In addition, we use a new methodology to quantify the fraction of (001) atomically flat area from atomic force microscopy images. Results are further correlated with the superconducting properties, microstructure, and texture of YBCO films grown by the trifluoroacetate route. A comparison with pulsed laser deposition and YBCO films grown on the same ceria layers is also presented. This growth procedure has allowed us to obtain all chemical multilayer films with controlled microstructure and critical current densities above 4 MA cm-2 at 77 K. © 2009 Materials Research Society.We acknowledge the financial support from MEC (MAT2005-02047, and CONSOLIDER NANOSELECT), Generalitat de Catalunya (Catalan Pla de Recerca SGR-0029 and CeRMAE), EU (HIPERCHEM, NMP4-CT2005-516858). J. Gàzquez, M. Coll, and A. Pomar are grateful to Spanish Ministery of Educacion y Ciencia (MEC) for financial support through Formacion de Personal Investigador (FPI), Formacion de Profesorado Universitario (FPU), and “Ramón y Cajal” programs.Peer Reviewe