Evidence of asymmetric dimers down to 40 K at the clean Si(100) surface

We have studied by synchrotron-radiation photoelectron spectroscopy the apparently p(2×1) structure recently imaged by scanning-tunneling microscopy at low temperatures in the form of seemingly symmetric dimers. Yet we demonstrate that the surface is semiconducting as in the c(4×2) phase and that most dimers remain asymmetric. The p(2×1) structure seen at low temperature may result either from a ferromagneticlike arrangement of static buckled dimers or, eventually, from an artifact of local tip- surface interactions.