Anchoring of nematic liquid crystals on terraced silicon surfaces

Anchoring of nematic liquid crystals on silicon surfaces slightly tilted with respect to the (001), (110), or (111) crystollographic planes is studied. It is found that the silicon crystal and the terraced structure of these surfaces play different roles in the determination of the anchoring directions of the nematic liquid crystals. The first one determines the azimuthal orientation of the anchoring directions that are found to be that of the twofold axes of the silicon crystal parallel to the surface of the terraces. For [001] orientation, this leads to a bistable anchoring; for [111] orientation, the expected tristable anchoring is not observed and only one anchoring direction is induced. The terraced structure determines the tilt of the anchoring directions with respect to the mean surface of the substrate. The experimentally measured tilt is not the one expected if the nematic molecules simply lay parallel to the terraces composing such vicinal surfaces. It is shown that besides this contribution of the terraces, one must also take into account the contribution of the steps separating these terraces.