Using scanning tunneling microscopy (STM), we studied the dimer structure of the $\mathrm{Si}\mathrm{}\left(001\right)2\times 1$ surface at low temperature $(<\mathrm{}10\mathrm{}\mathrm{}\mathrm{K}).\mathrm{}$ Asymmetric (buckled) dimer structure, locally forming $c(4\mathrm{}\times 2)$ or $p(2\mathrm{}\times 2)$ periodicity, was observed with positive sample bias voltages, while most of the dimers appear symmetric with negative bias voltages. Our observation indicates that actual dimer structure is asymmetric and that the apparent symmetric dimer observation is due to an artifact induced by STM imaging. Since a transition temperature between the buckled- and symmetric-dimer imaging, which is found to be $\sim 40\mathrm{K},$ corresponds to the temperature where the carrier density changes dramatically from intrinsic to saturation range, the apparent symmetric-dimer imaging should be related with the reduced carrier density and ensuing charging effect.

• Received 26 December 2002

DOI:https://doi.org/10.1103/PhysRevB.67.201306