Ultra-high vacuum scanning electron microscopy revealed the presence of an ordered domain pattern, with 3-fold symmetry, on (111). During the transition between 7 x 7 and so-called 1 x 1 phases on a (111) surface which was misoriented to <11¯2>, triangular 7 x 7 domains nucleated at the upper edge of the step and grew onto the terrace. When the apex of the domain reached the lower edge, the 7 x 7 and 1 x 1 domains formed a triangular-tiled arrangement. This arrangement was explained in terms of elastic-stress relaxation effects.

Triangular-tiled arrangement of 7 x 7 and '1 x 1' domains on Si(111) H.Hibino, Y.Homma, T.Ogino: Physical Review B, 1998, 58[12], R7500-3