X-ray diffraction evidence of adatoms in the Si(111)7×7 reconstructed surface

Synchrotron radiation has been employed to measure 73 fractional-order surface Bragg reflections from Si(111)7×7. A novel data-filtering technique is introduced to address the question of which sites in the diamond lattice are filled and which are vacant at the surface. The analysis is greatly simplified because atomic displacements are systematically nulled. Our observations agree with the principle of a stacking fault under half the unit cell. Difference analysis then requires 12 adatoms and one vacancy to complete the structure.