Temperature dependence of vicinal Si(111) surfaces

Abstract

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\ifmmode^\circ\else\textdegree\fi{} to 12\ifmmode^\circ\else\textdegree\fi{} toward or away from the [1\ifmmode\bar\else\textasciimacron\fi{}10] direction and by 6\ifmmode^\circ\else\textdegree\fi{} toward the [2\ifmmode\bar\else\textasciimacron\fi{}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\ifmmode\times\else\texttimes\fi{}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\ifmmode\times\else\texttimes\fi{}7) to ``(1\ifmmode\times\else\texttimes\fi{}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\ifmmode\bar\else\textasciimacron\fi{}10] direction, when the temperature is lowered approximately 200 \ifmmode^\circ\else\textdegree\fi{}C below the temperature of the (7\ifmmode\times\else\texttimes\fi{}7) to ``(1\ifmmode\times\else\texttimes\fi{}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\ifmmode\bar\else\textasciimacron\fi{}10] direction, and the other with poor step order misoriented towards the [1\ifmmode\bar\else\textasciimacron\fi{}21\ifmmode\bar\else\textasciimacron\fi{}] direction.