Long-range coherence and macroscopic phase separation of steps on vicinal Si(111)

We present a synchotron-radiation x-ray-scattering study of a vicinal Si(111) surface misoriented by 3.3° todard the 〈112¯〉 direction. At temperatures above the first-order 7×7 to 1×1 reconstruction transition temperature the surface contains coherent, thermally rough atomic steps in domains larger than 0.4×1 μ${\mathrm{m}}^2$. Below this temperature the surface phase separates into macroscopic (111) facets larger than 0.4×1 μ${\mathrm{m}}^2$, and stepped regions where the step density increases as the temperature is decreased.