Surface phase separation of vicinal Si(111)

The structure of a vicinal Si(111) surface, misoriented by 6^∘{} toward the [11{}0] direction, has been measured as a function of temperature. At T{}810^∘{}C the surface steps have short-range order with an average step-step spacing of 28.4±{}1.6 A{}. Upon cooling through the temperature where the 7×{}7 reconstruction appears, the spacing begins to decrease although the step height remains the same. This corresponds to clustering of the steps to form a new face whose misorientation changes continuously and reversibly from 6.3^∘{}±{}0.4^∘{} at high temperature to 17.0^∘{}±{}3.0^∘{} at low temperature. The transition can be described in terms of changes in the equilibrium crystal shape of Si induced by the formation of the 7×{}7 reconstruction.