X-RAY REFLECTIVITY STUDY OF THE SI(111)7X7 SURFACE

We have measured the X-ray reflectivity profile of the clean 7 x 7 reconstructed surface of Si(111) from small momentum transfer to beyond the (333) reflection of the bulk. The vertical structure is thereby determined to an accuracy of about 0.03 angstrom. While this result is completely consistent with the dimer-adatom-stacking fault (DAS) model of Takayanagi et al., it disagrees somewhat on the exact positions of the layers determined by LEED, RHEED and X-ray standing waves. The absolute position of he surface layers relative to the distant bulk naturally emerges from the analysis and shows a significant expansion, a result which agrees with a local density approximation theoretical calculation, but differs from the results of these previous experiments.