Electron Holography of Potential Barriers in Zno Varistors

ZnO varistor materials are polycrystalline materials that exhibit strong non-linear current-voltage characteristics. Their non-ohmic behaviour makes them suitable for overvoltage protection devices. It is the interfaces between ZnO grains that are the key to the non-linear behaviour of the varistor materials. Previous work has shown that the local microstructure determines the breakdown voltage of the individual ZnO/ZnO interface [1]. The pre-breakdown conductivity and the non-linearity in the breakdown region, which are both crucial to the performance of the varistor, are also affected by the height and the width of the potential barriers. In the present work, electron holography has been used to investigate the potential barriers in ZnO varistor materials.Electron holograms were recorded at 200kV using a Hitachi HF-2000 field emission gun transmission electron microscope [2]. The electrostatic bi-prism was located near the second image plane. The bi-prism voltage was about 9 V and the fringe spacing in the holograms was about 1 nm.

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Electron holography in materials science

Proceedings, annual meeting, Electron Microscopy Society of America ◽

10.1017/s0424820100087665 ◽

1991 ◽

Vol 49 ◽

pp. 670-671

Author(s):

Hannes Lichte ◽

Edgar Voelkl

Keyword(s):

Electron Microscopy ◽

Transmission Electron Microscopy ◽

Materials Science ◽

Fourier Spectrum ◽

Object Wave ◽

Electron Holography ◽

Reconstruction Procedure ◽

Numerical Reconstruction ◽

Transmission Electron ◽

Unique Interpretation

The object wave o(x,y) = a(x,y)exp(iφ(x,y)) at the exit face of the specimen is described by two real functions, i.e. amplitude a(x,y) and phase φ(x,y). In stead of o(x,y), however, in conventional transmission electron microscopy one records only the real intensity I(x,y) of the image wave b(x,y) loosing the image phase. In addition, referred to the object wave, b(x,y) is heavily distorted by the aberrations of the microscope giving rise to loss of resolution. Dealing with strong objects, a unique interpretation of the micrograph in terms of amplitude and phase of the object is not possible. According to Gabor, holography helps in that it records the image wave completely by both amplitude and phase. Subsequently, by means of a numerical reconstruction procedure, b(x,y) is deconvoluted from aberrations to retrieve o(x,y). Likewise, the Fourier spectrum of the object wave is at hand. Without the restrictions sketched above, the investigation of the object can be performed by different reconstruction procedures on one hologram. The holograms were taken by means of a Philips EM420-FEG with an electron biprism at 100 kV.

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Reconstruction of electron holograms recorded in a non-FEG, non-biprism TEM

Proceedings, annual meeting, Electron Microscopy Society of America ◽

10.1017/s0424820100139469 ◽

1995 ◽

Vol 53 ◽

pp. 618-619

Author(s):

Z.L. Wang

Keyword(s):

Electron Microscope ◽

Single Crystal ◽

Experimental Technique ◽

Transmission Electron Microscope ◽

The Other ◽

Electron Holography ◽

Transmission Electron ◽

Coherent Sources ◽

Imaging Theory ◽

Normal Specimen

An experimental technique for performing electron holography using a non-FEG, non-biprism transmission electron microscope (TEM) has been introduced by Ru et al. A double stacked specimens, one being a single crystal foil and the other the specimen, are loaded in the normal specimen position in TEM. The single crystal, which is placed onto the specimen, is responsible to produce two beams that are equivalent to two virtual coherent sources illuminating the specimen beneath, thus, permitting electron holography of the specimen. In this paper, the imaging theory of this technique is described. Procedures are introduced for digitally reconstructing the holograms.

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Interface structures in polycrystalline ceramic materials

Proceedings, annual meeting, Electron Microscopy Society of America ◽

10.1017/s0424820100143900 ◽

1986 ◽

Vol 44 ◽

pp. 468-471

Author(s):

K. J. Morrissey

Keyword(s):

Electron Microscopy ◽

Analytical Electron Microscopy ◽

Physical And Mechanical Properties ◽

High Resolution Electron Microscopy ◽

Ceramic Materials ◽

Polycrystalline Materials ◽

Transmission Electron ◽

Resolution Electron ◽

Interface Structures

Grain boundaries and interfaces play an important role in determining both physical and mechanical properties of polycrystalline materials. To understand how the structure of interfaces can be controlled to optimize properties, it is necessary to understand and be able to predict their crystal chemistry. Transmission electron microscopy (TEM), analytical electron microscopy (AEM,), and high resolution electron microscopy (HREM) are essential tools for the characterization of the different types of interfaces which exist in ceramic systems. The purpose of this paper is to illustrate some specific areas in which understanding interface structure is important. Interfaces in sintered bodies, materials produced through phase transformation and electronic packaging are discussed.

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Electron holography of p-n junctions

Proceedings, annual meeting, Electron Microscopy Society of America ◽

10.1017/s0424820100167330 ◽

1996 ◽

Vol 54 ◽

pp. 974-975

Author(s):

B.G. Frost ◽

D.C. Joy ◽

L.F. Allard ◽

E. Voelkl

Keyword(s):

Electron Beam ◽

Electron Microscope ◽

Field Emission ◽

Ccd Camera ◽

Image Plane ◽

Reference Wave ◽

Field Of View ◽

Control Mode ◽

Objective Lens ◽

Electron Holography

A wide holographic field of view (up to 15 μm in the Hitachi-HF2000) is achieved in a TEM by switching off the objective lens and imaging the sample by the first intermediate lens. Fig.1 shows the corresponding ray diagram for low magnification image plane off-axis holography. A coherent electron beam modulated by the sample in its amplitude and its phase is superimposed on a plane reference wave by a negatively biased Möllenstedt-type biprism.Our holograms are acquired utilizing a Hitachi HF-2000 field emission electron microscope at 200 kV. Essential for holography are a field emission gun and an electron biprism. At low magnification, the excitation of each lens must be appropriately adjusted by the free lens control mode of the microscope. The holograms are acquired by a 1024 by 1024 slow-scan CCD-camera and processed by the “Holoworks” software. The hologram fringes indicate positively and negatively charged areas in a sample by the direction of the fringe bending (Fig.2).

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Application of electron holography to material science studies on magnetic domain states of small particles

Proceedings, annual meeting, Electron Microscopy Society of America ◽

10.1017/s0424820100150976 ◽

1993 ◽

Vol 51 ◽

pp. 1028-1029

Author(s):

T. Hirayama ◽

Q. Ru ◽

T. Tanji ◽

A. Tonomura

Keyword(s):

Magnetic Field ◽

Material Science ◽

Science Studies ◽

Phase Distribution ◽

Carbon Film ◽

Objective Lens ◽

Electron Holography ◽

Transform Method ◽

Transmission Electron ◽

Thin Carbon

The observation of small magnetic materials is one of the most important applications of electron holography to material science, because interferometry by means of electron holography can directly visualize magnetic flux lines in a very small area. To observe magnetic structures by transmission electron microscopy it is important to control the magnetic field applied to the specimen in order to prevent it from changing its magnetic state. The easiest method is tuming off the objective lens current and focusing with the first intermediate lens. The other method is using a low magnetic-field lens, where the specimen is set above the lens gap.Figure 1 shows an interference micrograph of an isolated particle of barium ferrite on a thin carbon film observed from approximately [111]. A hologram of this particle was recorded by the transmission electron microscope, Hitachi HF-2000, equipped with an electron biprism. The phase distribution of the object electron wave was reconstructed digitally by the Fourier transform method and converted to the interference micrograph Fig 1.

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On the Faceting of Polar Ceramic Surfaces: Microscopy and Holography Studies of MGO(111) Surfaces

Proceedings, annual meeting, Electron Microscopy Society of America ◽

10.1017/s0424820100164295 ◽

1996 ◽

Vol 54 ◽

pp. 366-367

Author(s):

M. Gajdardziska-Josifovska ◽

B. G. Frost ◽

E. Völkl ◽

L. F. Allard

Keyword(s):

Electron Microscopy ◽

High Vacuum ◽

Ultra High Vacuum ◽

Electron Holography ◽

Flat Surfaces ◽

Transmission Electron ◽

Excess Surface ◽

Polar Surfaces ◽

Salt Structure ◽

Crystallographic Planes

Polar surfaces are those crystallographic faces of ionically bonded solids which, when bulk terminated, have excess surface charge and a non-zero dipole moment perpendicular to the surface. In the case of crystals with a rock salt structure, {111} faces are the exemplary polar surfaces. It is commonly believed that such polar surfaces facet into neutral crystallographic planes to minimize their surface energy. This assumption is based on the seminal work of Henrich which has shown faceting of the MgO(111) surface into {100} planes giving rise to three sided pyramids that have been observed by scanning electron microscopy. These surfaces had been prepared by mechanical polishing and phosphoric acid etching, followed by Ar+ sputtering and 1400 K annealing in ultra-high vacuum (UHV). More recent reflection electron microscopy studies of MgO(111) surfaces, annealed in the presence of oxygen at higher temperatures, have revealed relatively flat surfaces stabilized by an oxygen rich reconstruction. In this work we employ a combination of optical microscopy, transmission electron microscopy, and electron holography to further study the issue of surface faceting.

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Electron Holography Improving Transmission Electron Microscopy

Proceedings, annual meeting, Electron Microscopy Society of America ◽

10.1017/s0424820100179798 ◽

1990 ◽

Vol 48 (1) ◽

pp. 208-209

Author(s):

Hannes Lichte

Keyword(s):

Electron Microscopy ◽

Transfer Functions ◽

Imaging System ◽

Spherical Aberration ◽

Image Plane ◽

Chromatic Aberration ◽

Object Wave ◽

Objective Lens ◽

Electron Holography ◽

Wave Aberration

Generally, the electron object wave o(r) is modulated both in amplitude and phase. In the image plane of an ideal imaging system we would expect to find an image wave b(r) that is modulated in exactly the same way, i. e. b(r) =o(r). If, however, there are aberrations, the image wave instead reads as b(r) =o(r) * FT(WTF) i. e. the convolution of the object wave with the Fourier transform of the wave transfer function WTF . Taking into account chromatic aberration, illumination divergence and the wave aberration of the objective lens, one finds WTF(R) = Echrom(R)Ediv(R).exp(iX(R)) . The envelope functions Echrom(R) and Ediv(R) damp the image wave, whereas the effect of the wave aberration X(R) is to disorder amplitude and phase according to real and imaginary part of exp(iX(R)) , as is schematically sketched in fig. 1.Since in ordinary electron microscopy only the amplitude of the image wave can be recorded by the intensity of the image, the wave aberration has to be chosen such that the object component of interest (phase or amplitude) is directed into the image amplitude. Using an aberration free objective lens, for X=0 one sees the object amplitude, for X= π/2 (“Zernike phase contrast”) the object phase. For a real objective lens, however, the wave aberration is given by X(R) = 2π (.25 Csλ3R4 + 0.5ΔzλR2), Cs meaning the coefficient of spherical aberration and Δz defocusing. Consequently, the transfer functions sin X(R) and cos(X(R)) strongly depend on R such that amplitude and phase of the image wave represent only fragments of the object which, fortunately, supplement each other. However, recording only the amplitude gives rise to the fundamental problems, restricting resolution and interpretability of ordinary electron images:

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