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}m². Below this temperature the surface phase separates into macroscopic (111) facets larger than 0.4×{}1 {μ}m², and stepped regions where the step density increases as the temperature is decreased.