Electrons reflected from a crystal surface are generated by two scattering mechanisms. Bragg reflection, which is purely an elastic scattering process, is responsible for producing sharp peaks governed by Bragg's law. Inelastic scattering, which is dominated by valence-loss and thermal diffuse scattering (TDS), contributes a Kikuchi pattern background in the electron angular distribution and results in electron energy losses and momentum transfers. The excitation of Bragg reflections critically depends on the diffracting conditions, and the intensity distribution in a RHEED pattern is the result of dynamical scattering of electrons by the crystal surface. It is thus important to understand how electrons are reflected from the surface in order to illustrate the surface sensitivity of REM.

In this chapter, the physics of surface resonance in RHEED are systematically illustrated. Experimental results will be shown to exhibit the nature of surface resonance. It will be demonstrated that the surface sensitivity of RHEED will be dramatically enhanced by resonance. Finally, the effect of inelastic scattering on resonance reflection will be discussed.

The phenomenon

Surface resonance is one of the most important scattering processes in RHEED and REM. The surface resonance effect was first observed by Kikuchi and Nakagawa (1933); the reflected-electron intensity suddenly increases when the diffraction spot crosses an oblique Kikuchi line. This is illustrated by the RHEED pattern shown in Figure 4.1. When the resonance condition is not satisfied, there is no strongly reflected beam (Figure 4.1 (a)). The intensity is increased by almost an order of magnitude if the surface resonance is excited (Figure 4.1(b)).

Image calculations for REM are usually difficult because it is necessary to incorporate surface defects, such as steps and dislocations. This, in principle, can be done with the PeTSM theory; but, in practice, the huge amount of computation required and the sensitivity of REM image contrast to focus, beam convergence and diffracting conditions make the calculations rather involved and difficult compared with experimental observations. Attempts have been made to simulate surface step images (Peng and Cowley, 1986; Ma and Marks, 1990), but the results are still not very satisfactory. For this reason, in this chapter, the contrast mechanisms of REM imaging are described based on simplified models, in order to illustrate the physical concepts.

There are four basic contrast mechanisms in REM. Phase contrast (or Fresnel contrast) is produced by the path-length difference (or phase shift) of electron waves due to the change in surface morphology. This contrast dominates the images of atom-high surface steps. High-resolution information can be obtained from the phase contrast mechanism. Diffraction contrast (or Bragg contrast) is produced by the variation of local Bragg reflection angles due to lattice distortion from dislocations, local strain and crystal boundaries. Compositional contrast is produced by a variation in scattering power of different elements. This type of contrast appears on different surface domains or regions, depending on the local composition, and usually has relatively lower resolution. Finally, geometrical (or morphology) contrast is produced by variation in surface geometry, such as large surface steps and facets.

Techniques for studying surfaces in TEM

There are two basic modes of TEM operation, namely the bright-field mode, wherein the (000) transmitted beam contributes to the image, and the dark-field imaging mode, in which the (000) beam is excluded. The size of objective aperture in bright-field mode directly determines the information to be emphasized in the final image. When the size is chosen so as to exclude the diffracted beams, one has the configuration that is normally used for low-resolution defect studies, the so-called diffraction contrast. In this case, a crystalline specimen is oriented to excite a particular diffracted beam, or systematic row of reflections. This imaging mode is sensitive to the differences in specimen thickness, distortion of crystal lattices due to defects, strain and bends. High-resolution imaging is usually performed in bright-field mode by including a few Bragg-diffracted beams within the objective aperture. The lattice images are the result of interference between Bragg reflected beams, the so-called phase contrast. In this section, we outline a few techniques that have been extensively developed for studying surfaces in high-resolution TEM (HRTEM) (Cowley, 1986; Smith, 1987).

Imaging using surface-layer reflections

The dark-field imaging technique is favorable for imaging surfaces when the surface layers have different periodicity from that of the bulk of the crystal (Figure 5.1 (a)). Then ‘superlattice’ satellite reflections appear in the diffraction pattern and dark-field imaging with these spots can give high-contrast images of surface structure.

There are two basic mechanisms for generating the superlattice reflections. A sharp termination of the crystal bulk at the surface may form a stacking sequence that is not a complete unit cell.

There are two basic requirements for REM imaging of surfaces. The specimen is strongly preferred to be a single-crystalline material so that strong Bragg-reflected beams can be generated. The surface has to be flat enough to permit grazing angle imaging. The foreshortening effect along the beam direction, however, is a major disadvantage of REM, which limits the application of REM for imaging a relatively rough surface. It is thus desirable to enhance the potential of this technique by using it in conjunction with other surface imaging and analytical techniques. We first examine the interaction of an electron beam with the surface.

Various processes can be excited when a fine electron probe interacts with a specimen, as shown in Figure 12.1. The reflected primary electrons from the surface can be used to form the surface image. The analysis of electron energy loss can provide rich chemical information about crystal surfaces. Secondary electrons (SE) are generated wherever the electron interacts with the specimen. The energy of secondary electrons is less than about 50 eV. SE signals have been widely used in scanning electron microscopy (SEM) for obtaining surface topographic information because their effect escape depth is about 1 nm. Nanometer-resolution SE imaging has been performed in a dedicated STEM instrument, which generates an electron probe as small as 0.5 nm. Under UHV conditions, Auger electrons emitted from the surface can be analyzed to provide surface-sensitive chemical information. Scanning Auger microscopy allows the detection of small metallic particles as small as 0.5 nm (Liu et al., 1992a, b).

In a conventional TEM, electrons are emitted from an electron gun and focused by the condenser lens as a beam, which illuminates the specimen. The electron beam interacts with the specimen and is scattered (or diffracted) by the crystal atoms. Thus, the electron wave at the exit face of the specimen contains information about the potential distribution in the specimen. Since electrons are charged particles, their interaction with a solid is rather strong in comparison with either X-rays or neutrons, so that multiple scattering effects are always present in electron diffraction. This means that electron diffraction must be described by dynamical scattering theory, especially when quantitative structural analysis is necessary. However, many characteristics of electron diffraction can be qualitatively treated based on the kinematical scattering theory, which is actually a single-scattering theory. The purpose of this chapter is to outline some basic concepts of kinematical electron diffraction theory and to introduce the imaging theory of TEM, which will be applied in the future chapters for REM imaging. A systematic kinematical treatment of electron diffraction for perfect and imperfect crystals has been given by Cowley (1981). A complete description of dynamical electron diffraction theories has been given by Wang (1995).

Electron wavelength

Although TEM was invented based on the optical behavior of charged particles, the diffraction of electrons is purely a result of the wave property of particles.

This book was written following my review article ‘Electron reflection, diffraction and imaging of bulk crystal surfaces in TEM and STEM’, published by Reports on Progress in Physics [56 (1993) 997]. Thanks are due to Dr Simon Capelin, the Editorial Manager of Cambridge University Press, for inviting me to write this book. The book is intended for surface scientists and microscopists who are interested in surface characterizations using reflected electron diffraction and imaging techniques.

Many of the ideas illustrated in the book were collected from my past working experiences with Professor J. M. Cowley, Dr J. Bentley, Professor R. F. Egerton, Dr Ping Lu and Dr J. Liu, to whom I am very grateful. Thanks also go to Professor J. C. H. Spence for his initial suggestions when this book was proposed. I heartily thank Dr Nea Wheeler for her careful and critical reviewing of the manuscript, which significantly improved its quality.

I am grateful to Drs C. C. Ahn, H. Banzhof, E. Bauer, P. A. Crozier, R. Garcia-Molina, J. M. Gibson, H. Homma, T. Hsu, A. Ichimiya, S. Ino, M. Iwatsuki, A. V. Latyshev, G. Lehmpfuhl, J. Liu, L. V. Litvin, H. Marten, G. Meyer-Ehmsen, H. Nakahara, H. Nakayama, N. Osakabe, J. C. H. Spence, Y. Tanishiro, K. Yagi, Y. Yamamoto and N. Yao, for permission to use their micrographs to illustrate the text. Thanks also go to Drs T. Hsu and L. M. Peng, who kindly provided the original REM bibliography text file.

This bibliography lists all papers published on REM, SREM and REELS in the last two decades. The list was based on the bibliography published by Hsu and Peng (1993) and the Science Citation Index. The list includes articles that explicitly deal with the theories, techniques and applications of REM, SREM and REELS for surface studies. Almost exclusively papers published in refereed journals in English are listed.

1995

Wang, Z. L. and Shapiro, A. J. (1995) Studies of LaAlO3 {100} surfaces using RHEED and REM I: twins, steps and dislocations Surf. Sci.328, 141–58.

Wang, Z. L. and Shapiro, A. J. (1995) Studies of LaAlO3 {100} surfaces using RHEED and REM II 5 × 5 surface reconstruction Surf. Sci.328, 159–69.

Wang, Z. L. and Shapiro, A. J. (1995) Microstructure and reconstruction of LaAlO3 {100} and {110} surfaces Proc. Mater. Res. Soc.355, 175–80.

1994

Baba-Kichi, K. Z. (1994) Shadow imaging of surface topography in a conventional scanning electron microscope by forward scattered electrons Ultramicroscopy54, 1–7.

Horio, Y. and Ichimiya, A. (1994) Origin of phase shift phenomena in RHEED intensity oscillation curves Ultramicroscopy55, 321–8.

Houchmandzadeh, B. and Misbah, C. (1994) Localized morphological mode during sublimation of a vicinal pyramid Phys. Rev. Lett.73, 94–7.

Hsu, T. and Cowley, J. M. (1994) Study of twinning with reflection electron microscopy Ultramicroscopy55, 302–7.

Jung, T. M., Phaneuf, R. J. and Williams, E. D. (1994) Sublimation and phase-transitions on singular and vicinal Si(111) surfaces Surf. Sci.301, 129–35.