LEED HOLOGRAPHY APPLIED TO A COMPLEX SUPERSTRUCTURE - A DIRECT VIEW OF THE ADATOM CLUSTER ON SIC(111)-(3X3)

For the example of the SiC(111)-(3 x 3)reconstruction we show that a h olographic interpretation of discrete low energy electron diffraction (LEED) spot intensities arising from ordered, large unit cell superstr uctures can give direct access to the local geometry of a cluster arou nd an elevated adatom, provided there is only one such prominent atom per surface unit cell. By comparing the holographic images obtained fr om experimental and calculated data we illuminate validity, current li mits and possible shortcomings of the method. In particular, we show t hat periodic vacancies such as cornerholes may inhibit the correct det ection of the atomic positions. By contrast, the extra diffraction int ensity due re, slight substrate reconstructions, as for example buckli ng, seems to have negligible influence on the images. Due to the spati al information depth of the method the stacking of the cluster can be imaged down to the fourth layer. Finally, it is demonstrated how this structural knowledge of the adcluster geometry can be used to guide th e dynamical intensity analysis subsequent to the holographic reconstru ction and necessary to retrieve the full unit cell structure. [S0163-1 829(98)10539-8].