Shape transition in the epitaxial growth of gold silicide in Au thin films on Si(111).

Growth of epitaxial gold silicide islands has been observed when an Au film deposited on a bromine-passivated vicinal (4\ifmmode^\circ\else\textdegree\fi{} misoriented) Si(111) substrate was annealed around the Au-Si eutectic temperature. The islands grow in the shape of equilateral triangles, reflecting the symmetry of the (111) substrate, up to a critical size beyond which the symmetry of the structure is broken, resulting in a shape transition from triangle to trapezoid. The island edges are aligned along the three equivalent {11\ifmmode\bar\else\textasciimacron\fi{}0} directions of the Si(111) surface. The elongated islands, instead of growing along three equivalent {11\ifmmode\bar\else\textasciimacron\fi{}0} directions, grow only along one preferential direction. This has been attributed to the vicinality of the substrate surface as the steps on this substrate are expected to run along the observed preferential growth direction. We have observed, under various conditions, elongated islands with an aspect ratio as large as 15:1.