An STM and STS study on Iridium modified Si(111) Surface

The structure of Si(111) $\sqrt 7 \times \sqrt 7 \,R\,19.1^{0}-Ir$ reconstructed surface have been investigated with the help of scanning tunneling microscopy/spectroscopy and low energy electron diffraction. We propose a model based on the experimental data. The model defines a unit cell containing one surface substitutional iridium atom centered under six silicon ad-atoms. Once the sample is annealed at 1200 $^{\circ}$C, a low density lattice gas of these ring clusters forms on top of an impurity stabilized $'1\times 1'$ domains. These ring clusters and $'1\times 1'$ domains co-exist with $7\times 7$ domains of clean Si(111) surface. The local density of states graphs measured on Si(111) $\sqrt 7 \times \sqrt 7 \,R\,19.1^{0}-Ir$ reconstructed surface contains an asymmetric peak at the edge of the valence band suggesting that there is a surface state exhibiting a Rashba type spin-orbit coupling.