Electron diffraction conditions and surface imaging in reflection electron microscopy

Two resonance conditions responsible for enhancement of the specular reflected beam observed in the RHEED pattern have been characterized as Bragg-channelling reflection and Bragg-Bragg reflection, respectively, in terms of different scattering mechanisms. Under the resonance condition, the tremendous increase of both elastic and inelastic electron scattering results in the intensity enhancement of the specular reflected beam. The total reflectivity does not change considerably with the variation of the diffraction condition. The improvement of the topographical contrast in surface imaging is not simply related to increase in the intensity of the specular reflected beam. The surface image obtained from the Bragg-Bragg reflection condition shows a better image contrast for the surface structure among the varieties of resonance conditions. The appearance of the abnormal double-contour contrast for a single-atom-height step is also closely associated with the Bragg-Bragg reflection condition.