ADSORPTION OF SN ON SI(111)7X7 - RECONSTRUCTIONS IN THE MONOLAYER REGIME

Different monolayer phases of Sn on Si(111)7 x 7 have been studied by means of scanning tunneling microscopy (STM), core-level photoelectron spectroscopy (XPS), and Rutherford backscattering spectrometry (RBS). The STM results show that square-root 3 x square-root 3 reconstructio ns are obtained for room-temperature deposition of 1/3 ML of Sn follow ed by sample annealing in a broad temperature range. A T4 Sn adatom sq uare-root 3 x square-root 3 phase is formed for temperatures between 5 00 and 800-degrees-C, with a concentration of defects that is strongly dependent on the temperature and which is as high as 25% for the lowe st temperatures. Above 825-degrees-C a second square-root 3 x square-r oot 3 adatom reconstruction is formed, a mosaic-like phase with a 1: 1 mixture of Si and Sn atoms in T4 positions. The results from investig ations of the higher coverage 2 square-root 3 x 2 square-root 3 recons truction by XPS and RBS support the theory that this phase is a two-la yer epitaxial Sn structure with all Si(111) dangling bonds saturated. The Sn coverage for this phase was determined to be between 1 and 1.2 ML.