Surface phase separation of vicinal Si(111)

The structure of a vicinal Si (111) surface, misoriented by 6^ toward the 110 direction, has been measured as a function of temperature. At T810^ the surface steps have short-range order with an average step-step spacing of 28. 41. 6 A. Upon cooling through the temperature where the 77 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. 3⁰. 4^ at high temperature to 17. 0³. 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 77 reconstruction.