Scanning tunneling microscopy of the effect of incident energy upon chemisorption sites for O2/Si(111)-7×7

Scanning tunneling microscopy and molecular beam techniques were used to investigate the chemisorption sites of O2 onto Si(111)-7×7 surfaces as a function of O2 incident energy (Ei). It is shown that there are two different O-containing sites being formed, i.e., bright and dark sites. The distribution of dark sites is not sensitive to Ei. However, the distribution of bright sites is a strong function of O2 incident energy. At Ei=0.02 eV, there is no preference between bright corner-adatom and center-adatom sites (about 1.1:1) while at Ei≳0.06 eV, the bright sites are preferentially located at corner-adatom sites over center-adatom sites by about 2:1. It is proposed that difference in the spatial distribution of adsorbates at different incident energies results from different chemisorption mechanisms: precursor-mediated (physisorption) chemisorption at low incident energy and direct activated chemisorption at high incident energies.