Phase transition of the Si(001) surface below 100 K

We investigate, with scanning tunneling microscopy, the Si(001) surface at low temperatures. This surface consists of asymmetric (buckled) dimers at 1 10 K as has been reported previously. At 20 K. however, symmetric dimers do minate the surface regardless of the carrier type or the dopant concentrati on. Asymmetric dimers are observed only near defects at 20 K. Our results i ndicate that the Si(001) surface is nut c(4 x 2)-symmetric, but rather p(2 x 1)symmetric at the zero temperature limit, and suggest a reconsideration of its ground state. it is unusual that the higher symmetric phase is more stable than the less symmetric one. (C) 2000 Elsevier Science B.V. All righ ts reserved.