Kikuchi-band formation in medium-energy electron-diffraction patterns

Abstract

of close-packed ~110! planes. The formation of the Kikuchi bands is shown to be governed by two types of electron scattering in a crystal. The dominant contribution to enhanced electron-scattering intensity within a band comes from the forward-focusing effect as the electrons move along the numerous interatomic directions in the ~110! planes. The other mechanism responsible for the sharp edge regions in the Kikuchi bands involves electron scattering from the nearest planes. It is proposed to use the specific profile of the Kikuchi bands in estimating the shape and size of light-element crystallites forming during initial stages of island-film growth. © 1999 American Institute of Physics.@S1063-7834~99!00903-X# Studies of surfaces and interfaces have been making increasing use of methods of atomic structure analysis based on medium-energy electron diffraction ~hundreds of eV to ;2 keV!. This relates primarily to the diffraction of x-ray generated photo- and Auger electrons, 1‐3 as well as of incoherently scattered primary electrons. 3‐6 The spatial distributions of all these three groups of electrons have a similar structure for close energies. As a rule, it may be considered as a superposition of maxima oriented along the closepacked directions in a crystal. This specific pattern of the diffractograms allows their fairly straightforward interpretation and provides information regarding the object under study in real space. Besides, one usually observes bands of a higher intensity oriented along the projections of the closestpacked atomic planes. For crystals made up of light elements, these bands exhibit a high contrast and have sharp boundaries. The maxima lying along interatomic directions are usually associated with the forward electron focusing effect in crystals. 1‐3 The mechanism responsible for formation of the higher-intensity bands, which in the electron backscattering patterns are traditionally called Kikuchi bands, and in photoand Auger-electron diffraction, Kikuchi-like or Bragg features, is less clear. In the early stages of these studies, when the potential of using these patterns for structural analysis of surfaces was not yet recognized, the Kikuchi bands were considered within the dynamical theory of electron diffraction. 7 The corresponding calculations were performed, as a rule, in the simplest two-wave approximation, which permitted interpretation of the observed Kikuchi-band profile and their width equal to twice the Bragg angle. Later studies, 8‐12 where the emphasis shifted to the electron focusing effect and numerical simulation of the patterns in terms of the single-scattering cluster approximation, put forward various suggestions concerning the formation mechanism of these features. The most widely used viewpoint associates the sharp drops in intensity at the Kikuchiband edges with electron Bragg diffraction from a system of planes, which is taken into account by introducing large clusters. For instance, the cluster used to analyze photoelectron diffraction from CaF2~111! at 1139 eV was 303303 30 Ai n