Convergence of the incident beam in reflection Electron Microscopy
Authors: Yao Nan , Cowley J. M. 
Published: June 2020

The RHEED (Reflection High Energy Electron Diffraction) patterns as essential indications of the diffraction conditions in relation to REM (Reflection Electron Microscopy) imaging provide a wealth of information about the surfaces. They contain extensive patterns of Kikuchi lines, bands and envelopes resulting from diffuse inelastic scattering processes. They also contain arrays of diffuse spots forbidden by the boundary conditions for elastic scattering but generated by multiple diffuse scattering processes.

However, a word of caution has to be sounded. Strictly speaking, the normal RHEED pattern does not exactly present the diffraction condition for REM imaging in a commercial transmission electron microscope. Practical and theoretical studies of the electron optics of the illuminating system on a Philip-400T transmission electron microscope, in which the specimen is immersed in the magnetic field of the twin objective lens, indicate that the convergent angle of the incident electron beam can be adjusted precisely, in a range from about 0.1 mrad to 5 mrad, with a selection of the second condenser aperture size, by adjusting the excitation current in the second condenser lens. For the best contrast and illumination, the RHEED pattern is generally obtained by focusing the electron beam on the surface with the maximum convergence angle, and the REM image is obtained with an almost parallel illumination with the minimum convergence angle. A typical example obtained from a fresh cleavage (110) surface of InP single crystal is demonstrated in figure 1, in which (a) and (b) are RHEED patterns with the (10,10,0) specular Bragg-reflection condition fulfilled and correspond to the incident electron beam with 2 mrad and 0.2 mrad convergence angles, respectively; (c) is a REM image obtained under exactly the same operation condition as (b) except for changing from diffraction mode to image mode, which indeed has nothing to do with the illumination condition above the specimen position; and (c) is taken from an area consisting of many steps of atomic height. Comparison of (a) and (b) shows that, for the parallel electron illumination, only those diffraction spots are dominant which represent the possible diffracted directions and mark the intersections of Ewald sphere with reciprocal lattice rods of the crystal surface. The extensive Kikuchi lines, bands and envelopes, and even the parabolas appearing in (a), are scarcely visible in (b). This suggests that the channeling effects characterized as the appearance of surface diffraction parabolas showing in RHEED pattern are mainly caused by the portion of electrons with incident direction slightly deviated from the rows of atoms; that is, the inelastically scattered electrons propagating in the directions of rows of atoms only occur when the initial incident electrons interact with the lattice in a direction slightly different from that of the rows of atoms. Following this argument, we may propose that the contrasts observed in REM image are mostly contributed from the diffraction and phase contrasts.