Structural and electronic properties of thallium overlayers on the Si(111)-7×7surface

Abstract

We have investigated atomic arrangements and their electronic properties of the well-ordered thallium overlayer structures formed on the Si(111)-$7\ifmmode\times\else\texttimes\fi{}7$ surface. As for other trivalent atoms, Tl is found to form a well-defined $\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3}$ surface, indicating the absence of a so-called ``inert pair effect'' considered only for Tl. Another well ordered $1\ifmmode\times\else\texttimes\fi{}1$ surface at 1.0 monolayer appears to be semiconducting in our angle-resolved photoemission spectra dominated by a unique dispersive surface band near the Fermi level. Our theoretical calculations using density-functional theory show that Tl adatoms occupy the ${T}_{4}$ sites and saturate all the dangling bonds of surface Si atoms to make the surface semiconducting with a band gap of 0.34 eV. The filled surface band observed has been well reproduced in our band calculations.