Experimental and theoretical characterization of Cu adsorption sites on the Si(1 1 1)-7 × 7 surface

Scanning tunneling microscopy (STM) experiments were used to study Cu adsorption on Si(1 1 1)-7 × 7. The experimental results suggest that Cu atoms appear as dark spots while Si adatoms adjacent to them are imaged as gray or bright protrusions in the filled states images. We observed a mutual contrast reversal in the empty states between these bright and gray spots. Based on these experimental findings, we propose that Cu is located below the Si adatom layer. In order to verify this hypothesis, we performed total energy calculations and simulated STM maps by means of Density Functional Calculations. We tested different chemisorption geometries of Cu on the Si surface: on top of a rest atom and a corner adatom, at the so-called T4 and H3 sites as well as at positions situated halfway between the above adsorption positions. The theoretical results lead to the conclusion that Cu is located between the T4 and H3 adsorption sites.