A new facet formation mode in Si lateral solid phase epitaxy (L-SPE) of amorphous Si on SiO//2 is presented. This mode occurs in the case of L-SPE using a two-dimensional seed area. Depending on whether the seed shape is convex or concave, a slowly growing left brace 111 right brace facet or a fast growing left brace 110 right brace facet dominates in the case of left brace 100 right brace Si substrate. This phenomenon can be explained using a microscopic model of crystallization. Similar results are obtained for seeds surrounded by an SiO//2 layer and seeds surrounding an SiO//2 island, which have to be taken into consideration in practical device design.