STM study of acetylene reaction with Si(111):: observation of a carbon-induced Si(111)√3 x √/3R30° reconstruction

The first stages of acetylene reaction with the Si(l 11)7 x 7 reconstructed surface kept at 600 degreesC are studied by recording scanning tunneling microscopy (STM) images during substrate exposure at a C2H2 pressure of 2 x 10(-4) Pa (2 x 10(-2) mbar). We observed the progressive substitution of the 7 x 7 reconstruction with a carbon induced Si(1 1 1)root3 x root3R30degrees reconstruction characterized by an atomic distance of 0.75 +/- 0.02 nm, very close to that of the silicon 7 x 7 adatoms. This means that a carbon enrichment of the silicon outermost layers occurs giving rise to the formation of a Si-C phase different from the root3 x root3R30degrees reconstruction typical of Si terminated hexagonal SiC(0 0 0 1) surface with an atomic distance of 0.53 nm. To explain STM images, we propose a reconstruction model which involves carbon atoms in T-4 and/or S-5 sites, as occurring for B doped Si(1 1 1) surface. Step edges and areas around the silicon surface defects are the first regions involved in the reaction process, which spreads from the upper part of the step edges throughout the terraces. Step edges therefore, progressively flakes and this mechanism leads, for the highest exposures, to the formation of large inlets which makes completely irregular the straight edge typical of the Si(1 1 1)7 x 7 terraces. These observations indicate that there occurs an atomic diffusion like that driving the meandering effect. Finally, the formation of a few crystallites is shown also at the lowest acetylene exposures. This is the first STM experiment showing the possibility to have carbon incorporation in a Si(1 1 1) matrix for higher amounts than expected, at least up to 1/6 of silicon atomic layer. (C) 2003 Elsevier Science B.V. All rights reserved.