Thallium overlayers on Si(111): Structures of a "new" group III element

The structure and growth of Tl overlayers on Si(111)7X7 surfaces have been investigated using ultraviolet photoelectron spectroscopy, low-energy elect ron diffraction (LEED), Auger electron spectroscopy (AES), and scanning tun neling microscopy (STM). At low coverages Tl adsorption occurs almost exclu sively on the faulted half of the (7X7) unit cell and forms small, two-dime nsional clusters. With increasing Tl coverage the whole surface becomes cov ered while the (7X7) surface periodicity is maintained as evidenced by LEED and STM. After, deposition of the first 2 monolayers (ML) the high mobilit y of Tl adatoms leads to the condensation of few, but large islands. At the surface of the second monolayer of Tl an interesting network of bright con trast lines is observed, which appears to be induced by the (7X7) substrate periodicity lying two layers below and which may be caused by the surface strain mediated by the (7X7) unit cell boundaries. The surface phase diagra m of Tl-Si(111) at elevated temperatures contains(a (1X1) structure at 1 ML coverage and an approximate (6X6) phase. The latter is due to an incommens urate structure of the second Tl layer on top of the (1X1) ML. (C) 1999 Ame rican Vacuum Society. [S0734-2101(99)14804-8].