Reflection Electron Microscopy and Spectroscopy

Reflection Electron Microscopy and Spectroscopy for Surface analysis

by Zhong Lin Wang

(Cambridge University Press, May, 1996)
ISBN: 0-521-48266-6456 (456 pages)
40 West 20th Street, New York, NY 10011-4211
Table of Contents | Introduction | Reviews | Order

Introduction (pdf file)

    In 1986, E. Ruska was awarded the Nobel Physics Prize for his pioneering work of building the world's first transmission electron microscope (TEM) in the late 1920's. The mechanism of TEM was originally based on the physical principle that a charged particle could be focused by magnetic lenses, so that a "magnifier" similar to an optic microscope could be built. The discovery of wave properties of electrons really revolutionized people's understanding about the potential applications of an TEM. In the last 60 years TEM has experienced a revolutionary development both in theory and electron optics, and has become one of the key research tools for materials characterization (Hirsch et al., 1956; Buseck et al., 1989). The point-to-point image resolution currently available in TEM is better than 0.2 nm, comparable to the interatomic distances in solids.

    High resolution TEM is one of the key techniques for real-space imaging of defect structures in crystalline materials. Quantitative structure determination is becoming feasible, particularly with the following technical advances. The installation of an energy-filtering system on an TEM has made it possible to form images and diffraction patterns using electrons with different energy-losses. Accurate structure analysis is possible using the purely elastically scattered electrons, the scattering of which can be exactly simulated using the available theories. The traditional method of recording images on film is being replaced by digital imaging with the use of a charge coupled devise (CCD) camera, which has a large dynamical range with single electron detection sensitivity. Thus, electron diffraction patterns and images can be recorded linearly in intensity, and a quantitative fitting is feasible between an experimentally observed image and a theoretically simulated image. This is the future direction of electron microscopy, which allows quantitative structure determination with an accuracy to be comparable to x-ray diffraction. A modern TEM is a versatile machine which not only can explore the crystal structure using imaging and diffraction techniques but also can perform high-spatial resolution microanalysis using energy dispersive x-ray spectroscopy (EDS) and electron energy-loss spectroscopy (EELS). Thus the chemical composition in a region smaller than a few nanometers can be determined. Therefore, TEM is usually known as high-resolution analytical electron microscopy, which is becoming an indispensable technique for materials research.

    A wide variety of diffraction, spectroscopy, and microscopy techniques are now available for the characterization of thin films and surfaces; but only the microscope methods, primarily those using electrons, are able to provide direct real-space information about local inhomogeneities. Accompanying the extended applications in materials science and thin crystal characterizations, TEM has been employed to image the surface structure. There are several techniques, such as weak-beam dark-field and surface profile imaging techniques (Cowley, 1986; Smith, 1987), that have been developed for studying surface structures in TEM. This book is about the reflection high-energy electron diffraction (RHEED), reflection electron microscopy (REM), scanning REM (SREM) and the associated analytical techniques for studying bulk crystal surfaces and surfaces deposited with thin films. Emphasis is made on real space imaging of surface structures at high-resolution. These techniques can be applied to perform in-situ studies of surfaces prepared in the molecular beam epitaxy (MBE) chamber.

    Surface is a special state of condensed matter, and it is the boundary of materials with vacuum. In the semiconductor device industry, for example, techniques are needed to control surface structures in order to control some specific transport properties. Epitaxial growth of thin films is becoming an indispensable technique for synthesizing new materials, such as superconductor thin films, semiconductor superlattices, metallic superlattices (or multilayers) and diamond films, which have important applications in advanced technologies. Therefore, surface characterization is an essential branch of materials science.

    Techniques which have been applied to investigate surface structures are classified into the following categories: surface crystallography, diffraction and imaging, electron spectroscopy, incident ion techniques, desorption spectroscopy, tunneling microscopy, work function techniques, atomic and molecular beam scattering, and vibration spectroscopy. An introduction to these techniques has been given by Woodruff and Delchar (1994). Table I compares various imaging and diffraction techniques which have been developed for surface studies. Each of these techniques has its unique advantages, and most of the techniques use an electron beam as the probe. As limited by the physical mechanisms and the equipment designs, however, most of these techniques may not be adequate to be applied for imaging in-situ surface phenomena. In this book, we introduce the reflection high-energy electron diffraction (RHEED) and reflection electron microscopy and spectrometry techniques, which can be applied to in-situ observations of thin film nucleation and growth.

Reviews (pdf file)

Professor John F. Watts University of Surrey

    The book describes the analytical techniques based on electron diffraction, reflection and imaging in the TEM (and STEM) for the analysis of materials surfaces. As with most texts dealing with analytical techniques these days it is awash with acronyms and the ones representing the candidate methodologies are frequently encountered. These are RHEED (reflection high energy electron diffraction), REM (reflection electron microscopy), SREM (scanning REM) and REELS (reflection electron energy loss spectroscopy). Many others are included, but the author is considerate enough to define them all in the early pages of his book. The dust-cover notes state that this is 'an entirely self-contained study' in which the 'theories, techniques and applications of REM, RHEED and REELS are comprehensively reviewed'. Inspection of the text reveals that this is, indeed, the case, with three parts (logically Part A, Part B and Part C) of approximately equal length covering the three areas of reflection electron studies. This is preceded by a comprehensive review of the kinematical theory of electron diffraction. This chapter is a very helpful (and necessary) prelude to the rest of the book. It deals with kinematical scattering in the usual numerical manner but the associated text makes this chapter extremely readable. The main body of the text is written in an equally attractive style with the theory of the various topics being introduced alongside the experimental procedures involved. There are many applications of reflection electron microscopy and spectroscopy scattered throughout the book; although inmost cases they serve as illustrations of particular facets of the experimental procedures rather than points which emphasize the relative strengths of the candidate techniques. This is, however, a minor criticism, and the inclusion of examples of real micrographs, diffraction patterns and spectra throughout the text may provide some readers with much needed relief from the undoubted rigour of the theoretical treatments provided.

For those with a TEM background it represents, perhaps, the definitive text for reflection methods.

There are two particularly attractive aspects of this book to be found in the closing pages. The first is an extensive set (ten) of appendices which contain much useful data along with five FORTRAM programs for interpreting spectra and modeling electron beam/specimen interaction. These have presumably been widely tested in the author's laboratory and their inclusion here is to be welcomed. The other feature, warmly welcomed by this reviewer, is the inclusion of a separate index of the materials used to illustrate the various facets of the reflection techniques. Also included as an Appendix is a chronological bibliography of REM, SREM and REELS covering the years 1975-1995. RHEED is presumably excluded as it is the most senior, and widely used, of the methods considered. This book is not one for those with a peripheral interest in RHEED, REM, SREM and REELS. Referring once again to the cover notes it is offered as an 'ideal guide for scientists and graduate students working on quantitative surface structure characterization using reflection electron techniques' and there is no doubt that this target audience will appreciate the publication of such a concise, authoritative and well written text in their chosen area of endeavor. For those with a TEM background it represents, perhaps, the definitive text for reflection methods and provides all the theoretical information necessary for a thorough appreciation of these techniques. At such a reasonable price for a very specialist text one would hope that it will soon find a place on the bookshelf of every electron microscopy unit with a practical need (or even aspirations) to carry out surface structure determination in the TEM or STEM. For those with a need for such a text this book fulfills all the claims made on its behalf. Dr. Wang is to be congratulated on writing a very accessible text. The book is thoroughly recommended.