3 - Instrumentation

Summary

Introduction

In RHEED, an electron beam, at an energy usually between 8 and 20 keV for epitaxial growth systems, is incident on a crystal surface at a grazing angle of a few degrees. At the surface there is a scattering process in which there can be energy loss. Diffracted beams leave the crystal, also near grazing incidence, and strike a detector. It is a very open geometry, with the incident beam and detector as much as 20 cm from the sample. It is exceedingly surface sensitive. As a result, RHEED is an ideal measurement to combine with atom deposition, Auger electron spectroscopy, scanning tunneling microscopy, scanning electron microscopy and other surface probes. The appropriate experimental methods depend on the measurements desired and on the sample. In this chapter we describe several designs.

The optimal energy for electron diffraction depends somewhat on the purpose of the measurement. Electron optics become easier as the energy is increased but there does not seem to be any overriding issue. For dynamical analysis, as will be seen later, a planewave expansion is performed since at high energies the scattering is mainly in the forward direction and this is efficient. A spherical-wave expansion could also be used, but this is inefficient since at high energies many diffracted beams will be needed. So for dynamical analysis, energies greater than about 10 keV are essential. However, at lower energies it is possible to go to higher incident glancing angles and still maintain surface sensitivity.