Sublimation and phase transitions on Si(111) surface observed by ultrahigh vacuum scanning electron microscopy

We studied the atomic step behavior on Si(lll) during sublimation using ultrahigh vacuum scanning electron microscopy. A (111) plane with step spacings as large as several tens of micrometers could be obtained at the bottom of a crater by heating a vicinal Si(lll) substrate with craters at around 1200 degrees C in an ultrahigh vacuum. The step spacing on the plane was determined by nucleation of macrovacancies at the center of the plane while steps moved in a step flow manner, and was related to the adatom diffusion length. Above 1200 degrees C, we found a transition-like increase in the step spacing. The electric current direction that induced step bunching changed at around the transition temperature. We attributed these phenomena to incomplete surface melting on the Si(lll) surface. We also investigated the influence of heating current on the 7 x 7 phase transition using the wide Si(lll) plane and found that the size of the 7 x 7 phase just below the transition temperature depended on the current direction.