Characteristic features in RHEED patterns of disordered surfaces: Theoretical considerations

A638 strength of screened 2D excitons in these structures for the first few exciton levels. The binding energy and the oscillator strength are both found to decrease with increasing values of the screening parameter q~. For arbitrary values of the screening.parameter, the ground state of the exciton remains bound although the binding energy becomes arbitrarily small. However, calculations of the energies of some of the higher lying exciton states show that they become u n b o u n d at finite values of the screening parameter. Also, the oscillator strengths for both the ground and excited exciton states are found to decrease with increasing screening.

Surface Science 224 (1989) 591-612 North-Holland. Amsterdam CHARACTERISTIC

FEATURES

591 IN RHEED

PATTERNS

OF D I S O R D E R E D SURFACES: T H E O R E T I C A L

CONSIDERATIONS

G. MEYER-EHMSEN

Fachbereich Physik, Unwersitiit Osnabriick, Postfach 4469, 4500 OsnabriJck, Fed. Rep. of German.v B. B I D L G E R

and P.K. LARSEN

Philips Research Laboratories, P.O. Box 80000, 5600 JA Eindhoven, The Netherlands Received 5 May 1989; accepted for publication 22 August 1989 The diffraction of fast electrons from crystals in the reflection geometry ( R H E E D ) is in m a n y respects similar to low energy electron diffraction (LEED), because the components of the wave vectors normal to the surface are of the same order of magnitude. Due to the grazing incidence of the electron beam and its higher energy, however, R H E E D is much more sensitive to long range perturbations of the surface in the direction parallel to the beam. The present paper studies scattering from a system consisting of a disordered surface sheet and an underlying bulk substrate with a perfect crystal structure. After some intuitive considerations a scattering matrix formulation is given, which shows that the diffuse scattering distribution from the surface m a y be modified strongly by Bragg diffraction features of the substrate. Eventually this leads to enhancements of the diffuse streaks perpendicular to the surface at crossings with the Kikuchi lines or envelopes of the substrate in agreement with experimental observations. The effect is further demonstrated in model calculations of the diffuse streak intensity.