Origin of 3×3 diffraction on the Sn1−xSix/Si(1 1 1)3×3 surface

An evolution of an apparent 3×3 low-energy electron diffraction (LEED) pattern has been observed for the Sn1−xSix/Si(1 1 1)3×3 surface alloy. The origin of this additional diffraction has been investigated in detail by scanning tunneling microscopy (STM). The 3×3 diffraction, which appears after annealing, is associated with the arrangement of the Sn and substitutional Si atoms in the surface layer, forming many local structures such as honeycombs, hexagons, and atomic lines. As revealed by Fourier-transforms of the STM-images, these local structures are the origins of the 3×3 diffraction and a weak 23×3 streaky background superposed on the 3×3 LEED pattern.