Temperature dependence of vicinal Si(111) surfaces

Using low-energy electron diffraction, we have examined the temperature dependence of the structure of surfaces of Si which are misoriented from the (111) plane. Our observations are on surfaces misoriented by 4^∘{} to 12^∘{} toward or away from the [1{}10] direction and by 6^∘{} toward the [2{}11]. At high temperatures all these surfaces contain a uniform density of steps which have heights approximately equal to the distance between pairs of neighboring Si(111) planes. At low temperatures, the surfaces break up into two types of regions: one without steps but with a (7×{}7) reconstruction, the other with a high step density. This phase separation is reversible. We interpret our results in terms of the formation of a sharp edge in the equilibrium crystal shape. We support this picture by showing that the orientation of the stepped regions at low temperatures is independent of the net misorientation: all the data fall on the same temperature-orientation phase diagram. The sharp edge appears to vanish at the same temperature as the (7×{}7) to ``(1×{}1)'' transition on the unstepped (111) surface. We suggest a thermodynamic mechanism for this coincidence. On the surfaces misoriented toward or away from the [1{}10] direction, when the temperature is lowered approximately 200 ^∘{}C below the temperature of the (7×{}7) to ``(1×{}1)'' transition, we find evidence for the beginning of a region of coexistence between two types of stepped surfaces; one with an ordered array of kinks, misoriented towards an azimuth slightly away from the [1{}10] direction, and the other with poor step order misoriented towards the [1{}21{}] direction.