A dynamical theory for the contrast of perfect and imperfect crystals in the scanning electron microscope using backscattered electrons

Abstract

A many-beam dynamical theory, which apporoximately takes into account multiple inelastic scvattering, has been formulated and used to calculate the intensity of electrons backscattered from perfect and imperfect crystals. The contrast of channelling pattern lines is discussed, and the occurrence of ‘missing’ lines is explained. the backscattered intensity due to an inclined stacking fault is calculated. The image consists of damped fringes of periodicity ζ ^g ; the nature of the first fringe depends on the sign of g . R and no contrast occurs for integral values of g . R. Similar calculations have been made for dislocation. Reversing the sign of g . b reverses the image contrast for dislocations near the surface, and the usual TEM invisibility criteria hold, namely invisibility for g . b = g . b _∧ u = 0. The experimental conditions necessary for the imaging of defects are discussed. Imaging defects in solid specimens should be possible using cold field emission electron sources.