Thermal diffuse low-energy electron diffraction on the Si(111)2 x 1 structure.

Abstract

The theory of thermal diffuse scattering (TDS) in low-energy electron diffraction is derived and applied to the Si(111)2\ifmmode\times\else\texttimes\fi{}1 structure. Both Pandey's parallel \ensuremath{\pi}-bond chain model and Himpsel's buckled \ensuremath{\pi}-bond chain model are investigated. First, calculation of surface vibrations shows that interaction constants between atoms are (3/4 times those of the bulk. Frequencies of vibrations perpendicular to the surface are lower than those parallel to the surface. A root-mean-square amplitude of the first-layer atoms is 0.1 A\r{}. Next, the TDS pattern is calculated to show the appearance of a new diffraction spot at ((\ensuremath{\surd}6 +1)/2,1) for the parallel \ensuremath{\pi}-bond chain model. If the buckling of the chain becomes finite, this spot moves. It appears at (1/2,1) for the buckled \ensuremath{\pi}-bond chain model. It does not appear in patterns for other models such as the Hanneman, buckling, and \ensuremath{\pi}-bond molecular models. Furthermore, the (0,0) Laue spot is accompanied by a gentle and anisotropic TDS wing. The intensities of TDS spots are of the order of 1% of those of Laue spots.