X-ray standing wave analysis of the Sn/Si(111)-√3x√3 surface -: art. no. 235416

The 1/3 monolayer (ML) Sn/Si(111)-(root3xroot3)R30degrees surface structure has been extensively studied using low-energy electron diffraction (LEED) and x-ray standing waves (XSW). The summation of several XSW measured hkl Fourier components provides a three-dimensional, model-independent direct-space image of the Sn atomic distribution. While the image demonstrates that the Sn atoms are located at Si(111) T-4-adsorption sites, it alone can not determine whether or not the Sn atomic distribution is flat or asymmetric. However, conventional XSW analysis can make this distinction, concluding that one-third of the Sn atoms are located 0.26 Angstrom higher than the remaining two-thirds. This one up and two down distribution result is consistent with the vertical displacements predicted by a dynamical fluctuations model of the surface. For a second sample with a slightly different surface preparation a root3 LEED pattern was again observed, but in this case the direct space XSW imaging technique clearly revealed that a significant fraction of the Sn atoms were substituting for Si atoms in the bottom of the Si surface bilayer.