Dimer-vacancy defects on clean (001) 2 x 1 and Ni-contaminated (001) 2 x n surfaces were investigated by means of scanning tunnelling microscopy. It was found that the clean (001) surface exhibited the 2 x 1 reconstruction, regardless of cooling rates that were greater than 150C/s. In the case of (001) 2 x 1 surfaces with a surface dimer-vacancy density of 1.7%, the most abundant dimer-vacancy defect was a randomly distributed 1-dimer vacancy of Wang-Arias-Joannopoulos type. Appreciable numbers of (1+2)-dimer vacancies and 2-dimer vacancies were observed. Ordered defects on the (001) 2 x n surface were composed mainly of (1+2)-dimer vacancies and 2-dimer vacancies. Real-space scanning tunnelling microscopic images revealed that the dimer which was adjacent to the unrebonded side of 2-dimer vacancies was depressed by more than 0.05nm; thus reflecting highly asymmetrical characteristics. A small amount of Ni contamination of the (001) surface markedly increased the dimer-vacancy density, from below 2% to above 20%.

J.Y.Koo, J.Y.Yi, C.Hwang, D.H.Kim, S.Lee, D.H.Shin: Physical Review B, 1995, 52[24], 17269-74