Structural and electronic properties of thallium overlayers on the Si(111)-7x7 surface

We have investigated atomic arrangements and their electronic properties of the well-ordered thallium overlayer structures formed on the Si(111)-7x7 surface. As for other trivalent atoms, Tl is found to form a well-defined root3xroot3 surface, indicating the absence of a so-called "inert pair effect" considered only for Tl. Another well ordered 1x1 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.