Split-off dimer defects on theSi(001)2×1surface

Abstract

Dimer vacancy (DV) defect complexes in the $\mathrm{Si}\mathrm{}\left(001\right)2\times 1$ surface are investigated using high-resolution scanning-tunneling microscopy and first-principles calculations. We find that under low-bias filled-state tunneling conditions, isolated “split-off” dimers in these defect complexes are imaged as pairs of protrusions, while the surrounding Si surface dimers appear as the usual “bean-shaped” protrusions. We attribute this to the formation of $\pi$-bonds between the two atoms of the split-off dimer and second-layer atoms, and present charge density plots to support this assignment. We observe a local brightness enhancement due to strain for different DV complexes and provide the first experimental confirmation of an earlier prediction that the 1+2-DV induces less surface strain than other DV complexes. Finally, we present a previously unreported triangular shaped split-off dimer defect complex that exists at $S_{\mathrm{B}}$-type step edges, and propose a structure for this defect involving a bound Si monomer.