A model for the appearance of chevrons on RHEED patterns from InAs quantum dots

Reflection high energy electron diffraction patterns from quantum dots formed by depositing InAs on to GaAs(0 0 1) show pairs of streaks known as chevrons. A clear understanding of the mechanism by which chevrons are produced does not seem to exist in the literature, and both pure diffraction mechanisms and effects due to the refraction of electrons have been put forward. Scanning tunnelling microscopy has shown that these particular quantum dots have bounding surfaces which are curved, not planar. In this paper, a model is presented for the explanation of the production of chevrons from such quantum dots in terms of pure refraction effects. A method is given for determining the grazing angles of incidence of the various diffracted beams on the curved bounding surfaces of the quantum dots, and hence the refractive deviations have been calculated as a function of the direction of the incident electron beam. It is shown that the model provides a semi-quantitative explanation of the occurrence of the chevrons.