Si - substitutional defects on the a - Sn / Si ( 1 1 1 ) - ( ffiffiffi 3 p ffiffiffi 3 p ) surface

We present a theoretical analysis of the atomic and electronic structure of Si-substitutional defects on the a-Sn/Si(1 1 1)( ffiffiffi 3 p ffiffiffi 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. # 2004 Elsevier B.V. All rights reserved.