FACETING, ROUGHNESS, AND STEP DISORDERING OF VICINAL SI(111) SURFACES - AN X-RAY-SCATTERING STUDY

The equilibrium and nonequilibrium behavior of the step structure of vicinal Si(111) surfaces has been studied in synchrotron x-ray-scattering experiments. Below the 7 X 7 reconstruction transition temperature of 1100 K, the surface phase separates into macroscopic (111) facets larger than 0.4 X 1 mum2 and stepped regions with increased step density. At temperatures above the transition up to 1275 K where there is significant sublimation, the surface is thermally rough, characterized by a uniform step lattice with an algebraically decaying correlation function. The algebraic decay exponent eta was measured to be 0.18+/-0.13, in agreement with the theory of entropically interacting steps. Around 1275 K under dc Joule heating, the surface undergoes a sharp and reversible morphological phase transition. The steps bunch and disorder while large (111) terraces are generated. The surface roughness at high temperatures and the sharp nonequilibrium transition are explained in terms of kinetic roughening under electromigration.