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Atomic Scale Characterization of Complex Oxide Thin Films

Author : Meng Gu

Publisher :

Page : pages

File Size : 10,73 MB

Release : 2011

Category :

ISBN : 9781267240491

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Book Description

Materials with the ABO3 perovskite structure possess a wide variety of properties including superconductivity, ferroelectric, and magnetic properties. These properties are highly tunable due to the fact that the B site cation can assume multiple valence states and its high structural stability allows for large scale doping and strain. Due to a reduced dimensionality, two dimensional thin films and superlattices grown using techniques such as pulsed laser deposition (PLD) often possess novel properties which differ from the bulk perovskite materials. The origins of these novel properties can be traced to interfacial chemical intermixing, electronic reconstruction, strain as well as defect formation, which cause significant changes in the electronic structures. Therefore, it is crucially important to investigate the atomic and electronic structures of the functional materials in order to understand the correlation between microstructures and physical properties. Chemically-sensitive Z-contrast imaging and bonding-sensitive electron energy loss spectroscopy (EELS) in aberration corrected scanning transmission electron microscopes (STEM) can directly characterize the local structure, strain, composition and bonding on the atomic scale. Determination of the atomic and electronic structures of the interfaces and defects in the thin films can then be correlated with the magnetic and transport properties. Therefore, the understanding of the structure-property relationship for several different systems of perovskite oxide thin films and superlattices were developed on the atomic scale. Multifunctional superlattices composed of ferromagnetic (FM) La(0.7)Sr(0.3)MnO3 (LSMO) and antiferromagnetic (AFM) La(0.7)Sr(0.3)FeO3 (LSFO) have potential applications for next generation data storage and logic devices. Defect formation, driven by strain relaxation in the LSMO/LSFO superlattices can modify not only the structure and surface sharpness, but also the functional properties of the superlattice. Stacking faults were found as one efficient way of strain relaxation while maintaining robust antiferromagnetic properties for a thin [3LSMO][6LSFO] superlattice (repeating motif composed of 3 unit-cell LSMO sublayer and 6 unit-cell LSFO sublayer). On the other hand, for a fully strained [3LSMO][6LSFO], large inter-diffusion across the interface between the LSMO and LSFO layers was detected in EELS line scans, resulting in deteriorated AFM properties. When a [6LSMO][6LSFO] superlattice with one micron thickness, a high density of nanoflowers and cracks/pinholes were observed to result from strain relaxation. The formation of these nanoflowers and cracks/pinholes was suppressed by increasing the growth rate and thereby reducing the growth time and overall thermal treatment of the sample. Strain relaxation was shown to be directly related to the growth conditions and have a large effect on both the structure and functional properties of the superlattices. A series of superlattices composed of non-magnetic La(0.5)Sr(0.5)TiO3 (LSTO) and ferromagnetic LSMO were grown on single crystal oxide substrates with different amounts of misfit strain. No significant electronic structure changes along the interfaces was observed in this series of superlattices as revealed by atomic resolution EELS. In comparison, charge transfer effect was reported for the LSMO/STO superlattices and was shown to cause an ultrathin magnetic dead layer along the interfaces. Thus, compared with the LSMO/STO superlattice, composition tuning of the sublayers was proven to be efficient in controlling the interfacial charge transfer effects in a superlattice. In addition, tetragonal distortion was found to reduce the ferromagnetic ordering, decrease the Tc, increase the resistivity, and even lead to metal-insulator transitions of the superlattices. The strain relaxation defects such as dislocations and low angle grain boundaries serve as important pinning sites for magnetic domains, leading to enhanced coercive field strength. In order to determine the properties of an intermixed interface layer, we have performed a detailed study of the solid solution between LSMO and LSFO, i.e. La(0.7)Sr(0.3)Mn(0.5)Fe(0.5)O3 (LSMFO). A large target-substrate distance during the PLD growth led to cation segregation in the LSMFO film. Cation segregation could cause the formation of diverse local magnetic ordering and B site valence states due to the different local stoichiometry and coordination environment. For the cation segregated LSFMO films, robust ferromagnetic and antiferromagnetic coupling was observed at 150K and room temperature. Decreasing the target-substrate distance resulted to a homogeneous cation distribution in the film, without any ferromagnetic ordering as expected. This result suggests the important role of target-substrate distance and the kinetic energy of the plume species on the crystalline quality and functional properties of perovskite oxide thin films. La(x)Sr(1-x)TiO3 possesses a wide range of functional properties which make it an attractive candidate material for applications such as the conductive buffer for high temperature superconductor growth, transparent conductors, and anodes in solid oxide fuel cells. La(0.5)Sr(0.5)TiO3 thin films were grown using PLD and the resistivity was found to be highly dependent on the O2 background pressure used in the deposition. However, a thin film which was deposited as a single phase film was transformed into a semi-ordered superlattice with TiO2 rich stacking faults and distorted lattices upon exposure to high oxygen pressure (~200torr) during the cooling procedure after deposition. This phase change stabilized Ti4+ ions and dramatically increased the resistivity of the film. In addition, a two dimensional free electron gas could be constructed by confining a few unit cells of La doped STO with STO spacer layers. Our study showed that charge transfer over a distance of ~2 u.c. was present in Sr(0.75)La(0.25)TiO3/STO superlattices. This thickness defined the lower limit for the thickness of the STO spacers in order to confine the charge carriers into two dimensions; secondly, the La dopants were shown to be less localized in thicker superlattice (~100nm) due to interdiffusion upon extended thermal exposure. This information provided important feedback on the fabrication and utilization of this material.In conclusion, several perovskite thin film systems with fascinating properties have been explored in this thesis. Strain states and strain relaxations, defect formation, interfacial atomic mixing, charge transfer, and cation segregation were shown to have profound effect on the functional properties of complex oxide thin film systems. Atomic resolution Z-contrast imaging and EELS provide extremely useful information on the structural and electronic structure variations, which enable us to see the whole picture of growth, structure and properties' interactions.



Metal Oxide-Based Thin Film Structures

Author : Nini Pryds

Publisher : Elsevier

Page : 562 pages

File Size : 12,87 MB

Release : 2017-09-07

Category : Technology & Engineering

ISBN : 0081017529

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Book Description

Metal Oxide-Based Thin Film Structures: Formation, Characterization and Application of Interface-Based Phenomena bridges the gap between thin film deposition and device development by exploring the synthesis, properties and applications of thin film interfaces. Part I deals with theoretical and experimental aspects of epitaxial growth, the structure and morphology of oxide-metal interfaces deposited with different deposition techniques and new developments in growth methods. Part II concerns analysis techniques for the electrical, optical, magnetic and structural properties of thin film interfaces. In Part III, the emphasis is on ionic and electronic transport at the interfaces of Metal-oxide thin films. Part IV discusses methods for tailoring metal oxide thin film interfaces for specific applications, including microelectronics, communication, optical electronics, catalysis, and energy generation and conservation. This book is an essential resource for anyone seeking to further their knowledge of metal oxide thin films and interfaces, including scientists and engineers working on electronic devices and energy systems and those engaged in research into electronic materials. Introduces the theoretical and experimental aspects of epitaxial growth for the benefit of readers new to the field Explores state-of-the-art analysis techniques and their application to interface properties in order to give a fuller understanding of the relationship between macroscopic properties and atomic-scale manipulation Discusses techniques for tailoring thin film interfaces for specific applications, including information, electronics and energy technologies, making this book essential reading for materials scientists and engineers alike



Atomic Resolution Studies of Oxide Superlattices and Ultrathin Films

Author : Amish B. Shah

Publisher :

Page : pages

File Size : 31,80 MB

Release : 2011

Category :

ISBN :

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The ability to grow ultrathin films layer-by-layer with well-defined epitaxial relationships has allowed research groups worldwide to grow a range of artificial films and superlattices, first for semiconductors, and now with oxides. In the oxides thin film research community, there have been concerted efforts recently to develop a number of epitaxial oxide systems grown on single crystal oxide substrates that display a wide variety of novel interfacial functionality, such as enhanced ferromagnetic ordering, increased charge carrier density, increased optical absorption, etc, at interfaces. The magnitude of these novel properties is dependent upon the structure of thin films, especially interface sharpness, intermixing, defects, and strain, layering sequence in the case of superlattices and the density of interfaces relative to the film thicknesses. To understand the relationship between the interfacial thin film oxide atomic structure and its properties, atomic scale characterization is required. Transmission electron microscopy (TEM) offers the ability to study interfaces of films at high resolution. Scanning transmission electron microscopy (STEM) allows for real space imaging of materials with directly interpretable atomic number contrast. Electron energy loss spectroscopy (EELS), together with STEM, can probe the local chemical composition as well as local electronic states of transition metals and oxygen. Both techniques have been significantly improved by aberration correctors, which reduce the probe size to 1 ©5, or less. Aberration correctors have thus made it possible to resolve individual atomic columns, and possibly probe the electronic structure at atomic scales. Separately, using electron probe forming lenses, structural information such as the crystal structure, strain, lattice mismatches, and superlattice ordering can be measured by nanoarea electron diffraction (NED). The combination of STEM, EELS, and NED techniques allows us to gain a fundamental understanding of the properties of oxide superlattices and ultrathin films and their relationship with the corresponding atomic and electronic structure. In this dissertation, I use the aforementioned electron microscopy techniques to investigate several oxide superlattice and ultrathin film systems. The major findings are summarized below. These results were obtained with stringent specimen preparation methods that I developed for high resolution studies, which are described in Chapter 2. The essential materials background and description of electron microscopy techniques are given in Chapter 1 and 2. In a LaMnO3-SrMnO3 superlattice, we demonstrate the interface of LaMnO3-SrMnO3 is sharper than the SrMnO3-LaMnO3 interface. Extra spectral weights in EELS are confined to the sharp interface, whereas at the rougher interface, the extra states are either not present or are not confined to the interface. Both the structural and electronic asymmetries correspond to asymmetric magnetic ordering at low temperature. In a short period LaMnO3-SrTiO3 superlattice for optical applications, we discovered a modified band structure in SrTiO3 ultrathin films relative to thick films and a SrTiO3 substrate, due to charge leakage from LaMnO3 in SrTiO3. This was measured by chemical shifts of the Ti L and O K edges using atomic scale EELS. The interfacial sharpness of LaAlO3 films grown on SrTiO3 was investigated by the STEM/EELS technique together with electron diffraction. This interface, when prepared under specific conditions, is conductive with high carrier mobility. Several suggestions for the conductive interface have been proposed, including a polar catastrophe model, where a large built-in electric field in LaAlO3 films results in electron charge transfer into the SrTiO3 substrate. Other suggested possibilities include oxygen vacancies at the interface and/or oxygen vacancies in the substrate. The abruptness of the interface as well as extent of intermixing has not been thoroughly investigated at high resolution, even though this can strongly influence the electrical transport properties. We found clear evidence for cation intermixing through the LaAlO3-SrTiO3 interface with high spatial resolution EELS and STEM, which contributes to the conduction at the interface. We also found structural defects, such as misfit dislocations, which leads to increased intermixing over coherent interfaces.



Strain-induced Phenomenon in Complex Oxide Thin Films

Author : Ryan Haislmaier

Publisher :

Page : pages

File Size : 31,77 MB

Release : 2016

Category :

ISBN :

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Book Description

Complex oxide materials wield an immense spectrum of functional properties such as ferroelectricity, ferromagnetism, magnetoelectricity, optoelectricity, optomechanical, magnetoresistance, superconductivity, etc. The rich coupling between charge, spin, strain, and orbital degrees of freedom makes this material class extremely desirable and relevant for next generation electronic devices and technologies which are trending towards nanoscale dimensions. Development of complex oxide thin film materials is essential for realizing their integration into nanoscale electronic devices, where theoretically predicted multifunctional capabilities of oxides could add tremendous value. Employing thin film growth strategies such as epitaxial strain and heterostructure interface engineering can greatly enhance and even unlock novel material properties in complex oxides, which will be the main focus of this work. However, physically incorporating oxide materials into devices remains a challenge. While advancements in molecular beam epitaxy (MBE) of thin film oxide materials has led to the ability to grow oxide materials with atomic layer precision, there are still major limitations such as controlling stoichiometric compositions during growth as well as creating abrupt interfaces in multi-component layered oxide structures. The work done in this thesis addresses ways to overcome these limitations in order to harness intrinsic material phenomena.The development of adsorption-controlled stoichiometric growth windows of CaTiO3 and SrTiO3 thin film materials grown by hybrid MBE where Ti is supplied using metal-organic titatnium tetraisopropoxide material is thoroughly outlined. These growth windows enable superior epitaxial strain-induced ferroelectric and dielectric properties to be accessed as demonstrated by chemical, structural, electrical, and optical characterization techniques. For tensile strained CaTiO3 and compressive strained SrTiO3 films, the critical effects of nonstoichiometry on ferroelectric properties are investigated, where enhanced ferroelectric responses are only found for stoichiometric films grown inside of the growth windows, whereas outside of the optimal growth window conditions, ferroelectric properties are greatly deteriorated and eventually disappear for highly nonstoichiometric film compositions. Utilizing these stoichiometric growth windows, high temperature polar phase transitions are discovered for compressively strained CaTiO3 films with transition temperatures in excess of 700 K, rendering this material as a strong candidate for high temperature electronic applications. Beyond the synthesis of single phase materials using hybrid MBE, a methodology is presented for constructing layered (SrTiO3)n/(CaTiO3)n superlattice structures, where precise control over the unit cell layering thickness (n) is demonstrated using in-situ reflection high energy electron diffraction. The effects of interface roughness and layering periodicity (n) on the strain-induced ferroelectric properties for a series of n=1-10 (SrTiO3)n/(CaTiO3)n superlattice films are investigated. It is found that the stabilization of a ferroelectric phase is independent of n, but is however strongly dominated by the degree of interface roughness which is quantified by measuring the highest nth order X-ray diffraction peak splitting of each superlattice film. A counter-intuitive realization is made whereby a critical amount of interface roughness is required in order to enable the formation of the predicted strain-stabilized ferroelectric phase, whereas sharp interfaces actually suppress this ferroelectric phase from manifesting. It is shown how high-quality complex oxide superlattices can be constructed using hybrid MBE technique, allowing the ability to control layered materials at the atomic scale. Furthermore, a detailed growth methodology is provided for constructing a layered n=4 SrO(SrTiO3)n Ruddlesden-Popper (RP) phase by hybrid MBE, where the ability to deposit single monolayers of SrO and TiO2 is utilized to build the RP film structure over a time period of 5 hours. This is the first time that a thin film RP phase has been grown using hybrid MBE, where an a stable control over the fluxes is demonstrated during relatively long time periods of growth, which advantageously facilitates the synthesis of high-quality RP materials with excellent structural and chemical homogeneity.Additionally, this work demonstrates some major advancements in optical second harmonic generation (SHG) characterization techniques of ferroelectric thin film materials. The SHG characterization techniques developed here proved to be the bread-and-butter for most of the work performed in this thesis, providing a powerful tool for identifying the existence of strain-induced ferroelectric phases, including their temperature dependence and polar symmetry. The work presented in this dissertation will hopefully provide a preliminary road map for future hybrid MBE growers, scientists and researchers, to develop and investigate epitaxial strain and heterostructure layering induced phenomena in other complex oxide systems.



Defects at Oxide Surfaces

Author : Jacques Jupille

Publisher : Springer

Page : 472 pages

File Size : 22,25 MB

Release : 2015-02-09

Category : Science

ISBN : 3319143670

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Book Description

This book presents the basics and characterization of defects at oxide surfaces. It provides a state-of-the-art review of the field, containing information to the various types of surface defects, describes analytical methods to study defects, their chemical activity and the catalytic reactivity of oxides. Numerical simulations of defective structures complete the picture developed. Defects on planar surfaces form the focus of much of the book, although the investigation of powder samples also form an important part. The experimental study of planar surfaces opens the possibility of applying the large armoury of techniques that have been developed over the last half-century to study surfaces in ultra-high vacuum. This enables the acquisition of atomic level data under well-controlled conditions, providing a stringent test of theoretical methods. The latter can then be more reliably applied to systems such as nanoparticles for which accurate methods of characterization of structure and electronic properties have yet to be developed. The book gives guidance to tailor oxide surfaces by controlling the nature and concentration of defects. The importance of defects in the physics and chemistry of metal oxide surfaces is presented in this book together with the prominent role of oxides in common life. The book contains contributions from leaders in the field. It serves as a reference for experts and beginners in the field.



Physics of Ferroelectrics

Author : Karin M. Rabe

Publisher : Springer Science & Business Media

Page : 395 pages

File Size : 12,30 MB

Release : 2007-07-20

Category : Technology & Engineering

ISBN : 3540345914

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Book Description

The past two decades have witnessed revolutionary breakthroughs in the understanding of ferroelectric materials, both from the perspective of theory and experiment. This book addresses the paradigmatic shifts in understanding brought about by these breakthroughs, including the consideration of novel fabrication methods and nanoscale applications of these materials, and new theoretical methods such as the effective Hamiltonian approach and density functional theory.



Integration of Functional Oxides with Semiconductors

Author : Alexander A. Demkov

Publisher : Springer Science & Business Media

Page : 284 pages

File Size : 43,56 MB

Release : 2014-02-20

Category : Technology & Engineering

ISBN : 146149320X

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Book Description

This book describes the basic physical principles of the oxide/semiconductor epitaxy and offers a view of the current state of the field. It shows how this technology enables large-scale integration of oxide electronic and photonic devices and describes possible hybrid semiconductor/oxide systems. The book incorporates both theoretical and experimental advances to explore the heteroepitaxy of tuned functional oxides and semiconductors to identify material, device and characterization challenges and to present the incredible potential in the realization of multifunctional devices and monolithic integration of materials and devices. Intended for a multidisciplined audience, Integration of Functional Oxides with Semiconductors describes processing techniques that enable atomic-level control of stoichiometry and structure and reviews characterization techniques for films, interfaces and device performance parameters. Fundamental challenges involved in joining covalent and ionic systems, chemical interactions at interfaces, multi-element materials that are sensitive to atomic-level compositional and structural changes are discussed in the context of the latest literature. Magnetic, ferroelectric and piezoelectric materials and the coupling between them will also be discussed. GaN, SiC, Si, GaAs and Ge semiconductors are covered within the context of optimizing next-generation device performance for monolithic device processing.



Epitaxial Growth of Complex Metal Oxides

Author : Gertjan Koster

Publisher : Woodhead Publishing

Page : 534 pages

File Size : 11,20 MB

Release : 2022-04-22

Category : Science

ISBN : 0081029462

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Book Description

Epitaxial Growth of Complex Metal Oxides, Second Edition reviews techniques and recent developments in the fabrication quality of complex metal oxides, which are facilitating advances in electronic, magnetic and optical applications. Sections review the key techniques involved in the epitaxial growth of complex metal oxides and explore the effects of strain and stoichiometry on crystal structure and related properties in thin film oxides. Finally, the book concludes by discussing selected examples of important applications of complex metal oxide thin films, including optoelectronics, batteries, spintronics and neuromorphic applications. This new edition has been fully updated, with brand new chapters on topics such as atomic layer deposition, interfaces, STEM-EELs, and the epitaxial growth of multiferroics, ferroelectrics and nanocomposites. Examines the techniques used in epitaxial thin film growth for complex oxides, including atomic layer deposition, sputtering techniques, molecular beam epitaxy, and chemical solution deposition techniques Reviews materials design strategies and materials property analysis methods, including the impacts of defects, strain, interfaces and stoichiometry Describes key applications of epitaxially grown metal oxides, including optoelectronics, batteries, spintronics and neuromorphic applications





Scanning Transmission Electron Microscopy

Author : Stephen J. Pennycook

Publisher : Springer Science & Business Media

Page : 764 pages

File Size : 16,79 MB

Release : 2011-03-24

Category : Technology & Engineering

ISBN : 1441972005

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Book Description

Scanning transmission electron microscopy has become a mainstream technique for imaging and analysis at atomic resolution and sensitivity, and the authors of this book are widely credited with bringing the field to its present popularity. Scanning Transmission Electron Microscopy(STEM): Imaging and Analysis will provide a comprehensive explanation of the theory and practice of STEM from introductory to advanced levels, covering the instrument, image formation and scattering theory, and definition and measurement of resolution for both imaging and analysis. The authors will present examples of the use of combined imaging and spectroscopy for solving materials problems in a variety of fields, including condensed matter physics, materials science, catalysis, biology, and nanoscience. Therefore this will be a comprehensive reference for those working in applied fields wishing to use the technique, for graduate students learning microscopy for the first time, and for specialists in other fields of microscopy.