AVS 45th International Symposium
    Surface Science Division Tuesday Sessions
       Session SS1-TuM

Paper SS1-TuM11
STM of Tl Overlayers on Si(111): Structures of a "New" Group III Element

Tuesday, November 3, 1998, 11:40 am, Room 308

Session: Semiconductor Surface Structure
Presenter: F.P. Netzer, Karl-Franzens-Universität Graz, Austria
Authors: L. Vitali, Karl-Franzens-Universität Graz, Austria
M.G. Ramsey, Karl-Franzens-Universität Graz, Austria
F.P. Netzer, Karl-Franzens-Universität Graz, Austria
Correspondent: Click to Email
Thallium is an unusual group III element because it can adopt a monovalent state in addition to the more conventional trivalent state of the other elements of this group. In this study we investigate for the first time the growth and the structure of Tl overlayers on Si(111)7x7 surfaces from room temperature (RT) to 500°C, using STM and STS in conjunction with LEED and AES. At low coverages Tl adatoms adsorb preferentially on the faulted half of the unit cell, covering the surface with an array of triangular adsorbate islands maintaining the (7x7) periodicity. The Tl adatoms are highly mobile at RT as recognised in consecutive STM images. On completion of the first monolayer a low corrugation surface develops at RT, but at 350°C a well ordered (1x1) structure is observed in LEED and with atomic resolution in the STM. This layer shows a metallic state with a prominent feature at -0.5 eV in STS. Depending on the substrate temperature remarkable contrast phenomena are seen in the STM for the first and the second monolayer of Tl. From RT to 200°C the Si(7x7) substrate arrangement determines the structure of the overlayer, with a network of contrast lines reflecting, presumably, the (electronic?) influence of the (7x7) substrate mesh. Atomic resolution along the lines displays a different distribution of occupied and unoccupied states. From 300-450°C a (6x6) array of broad second layer maxima (~17Å diameter, corrugation ~1Å) on a (1x1) monolayer signals a structural transformation of the overlayer. Tl overlayers on Si(111) thus display a very different structural behaviour to the other group III elements and possible reasons are discussed in terms of valency, bonding geometry and size effects.