We present a theoretical analysis of the atomic and electronic structure of Si-substitutional defects on the a-Sn/Si(1 1 1)-
3 p ) surface. We use a first-principles DFT local-orbital method and analyze Si-defects on a large Sn/Si(1 1 1) surface
unit-cell, corresponding to a defect concentration as low as 3.7%. We also calculate the theoretical STM images, and compare
with the experimental results.We find that a single Si-defect induces an upward displacement of its six nearest Sn-adatoms that
shows up in filled state STM images as a bright ring of adatoms around the defect.We also analyze the atomic structure and STM
images associated with three nearby Si-defects. These results are compared with those found for the Sn/Ge(1 1 1) surface. We
conclude that the atomic distortion induced by Si (Ge) substitutional defects is very local, the differences found for these two
surfaces being basically due to their different ground state geometries.
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