The role of strain in Si(111) 7x7 and related reconstructed surfaces.

X-ray diffraction measurements of the sputter-annealed Si(111) 7x7 surface have been made at the National Synchrotron Light Source, Brookhaven. Technical advances have improved the reliability of such data significantly. The 120 structure factor intensities agree with the widely accepted structural model of Takayanagi at the 4 sigma level, but can be made to show complete agreement if the atomic coordinates of this model are allowed to relax. In this way, 15 of the 17 possible 2D positional coordinates in three layers have been determined to an accuracy of about 0.02angstroms. The displacement pattern can be simple interpreted as a superposition of strain fields around the adaptoms, in the dimers, and within the triangular island regions.