Dimer-vacancy defects on the Si(001)-2X1 and the Ni-contaminated Si(001)-2Xn surfaces

Dimer-vacancy defects on clean Si(001)-2 X 1 and Ni-contaminated Si(001)-2 X n surfaces are investigated by scanning tunneling microscopy (STM). The clean Si(001) surface shows the 2 X 1 reconstruction irrespective of cooling rates faster than 150 degrees C/sec. On the Si(001)-2 X 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 X 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 Angstrom, 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%.