Variable low-temperature scanning tunneling microscopy study of Si(001): Nature of the 2x1->c(2x4) phase transition

We have studied the Si(001) surface from 120 K to room temperature using a variable low-temperature scanning tunneling microscope, Complementary investigations were carried out on two distinctly different types of surfaces: first, the normal 2 x 1 surface and second, the 2 x n (4<n<12) surface. For the 2 x 1 surface, the defects are scattered randomly, By plotting out the fraction of buckled dimers as a function of temperature, we find a slow transition from predominantly c(2 x 4) at low temperature to mostly 2 x 1 at room temperature for a defect concentration of about 8.5%. For the 2 x n surface, the much larger number of surface vacancies form long-range ordered chains, dividing the surface into many short dimer segments. These dimer segments predominantly appear to be unbuckled at room temperature. Upon cooling to 190 K, we observe very little change in the amount of buckling. The implications of this result are discussed. (C) 1996 American Vacuum Society.