Dimer-vacancy defects on the Si(001)-2 x 1 and the Ni-contaminated Si(001)-2 x n surfaces

Dimer-vacancy defects on clean Si(001)-2\ifmmode\times\else\texttimes\fi{}1 and Ni-contaminated Si(001)-2\ifmmode\times\else\texttimes\fi{}n surfaces are investigated by scanning tunneling microscopy (STM). The clean Si(001) surface shows the 2\ifmmode\times\else\texttimes\fi{}1 reconstruction irrespective of cooling rates faster than 150 \ifmmode^\circ\else\textdegree\fi{}C/sec. On the Si(001)-2\ifmmode\times\else\texttimes\fi{}1 surface with a surface dimer-vacancy density of 1.7%, the most abundant dimer-vacancy defect is a randomly distributed one dimer vacancy (1-DV) of the Wang-Arias-Joannopoulos model. Appreciable amounts of (1+2)-DV and 2-DV are observed. The ordered defects on the Si(001)-2\ifmmode\times\else\texttimes\fi{}n surface are mainly composed of (1+2)-DV and 2-DV. The real-space STM images reveal that the dimer adjacent to the unrebonded side of 2-DV is depressed by more than 0.5 \AA{}, representing the highly asymmetric characteristics. A small amount of Ni contamination on Si(001) drastically increases the dimer-vacancy density from below 2% to above 20%.