LONG-RANGE COHERENCE AND MACROSCOPIC PHASE-SEPARATION OF STEPS ON VICINAL SI(111)

We present a synchrotron-radiation x-ray-scattering study of a vicinal Si(111) surface misoriented by 3.3-degrees toward the (112BAR) direction. At temperatures above the first-order 7 x 7 to 1 x 1 reconstruction transition temperature the surface contains coherent, thermally rough atomic steps in domains larger than 0.4 x 1-mu-m2. Below this temperature the surface phase separates into macroscopic (111) facets larger than 0.4 x 1-mu-m2, and stepped regions where the step density increases as the temperature is decreased.