IN SI(111)-ROOT-3X-ROOT-3 INTERFACE - AN UNRELAXED T(4) GEOMETRY/

Back reflection x-ray standing waves and surface extended x-ray absorp tion fine structure have determined the atomic coordinates (i.e., the perpendicular displacement and the near-neighbor bond lengths) at the In/Si(111)-square-root 3 x square-root 3 interface. Although the In ad atoms are found to reside at a single position, 2.10 +/- 0.06 angstrom above the first Si bilayer, dual In-Si near-neighbor distances are fo und: 2.73 +/- 0.02 angstrom to the first- and 2.49 +/- 0.03 angstrom t o the second-layer Si atoms, respectively. Contrary to the accepted mo del, our data suggest that the T4 geometry is not relaxed.