COBALT SILICIDE NANOSTRUCTURES ON SI(001) SURFACE

Abstract

The structure, growth behaviour and shape evolution of CoSi₂ islands on Si(001) are investigated as a function of Co coverages (0.1 to 0.5ML) and temperatures (500°C to 800°C) using STM and TEM. Two types of 3D CoSi₂ islands are formed: The first type of CoSi₂ island is a flat-type island with a CoSi₂(001)-c(2×2) top-facet morphology. It grows epitaxially into Si(001) such that CoSi₂(001)//Si(001) and CoSi₂[110]//Si[110]. The rectangular islands formed are preferentially elongated along the two orthogonal <110> directions but they become less anisotropic and more square-like as growth temperature increases. The second type of CoSi₂ island is ridge-type that also elongates preferentially along the <110> directions. It grows into the Si(001) substrate such that CoSi₂(221)//Si(001) and CoSi₂[1<span style="text-decoration: overline">1</span>0]//Si[1<span style="text-decoration: overline">1</span>0]. The structure of ridge islands is highly anisotropic and becomes more wire-like with increasing temperature. The growth and shape evolution of the islands are found to be thermally activated and the activation energies of the growth of ridge islands¿ length, width and height are found to be 1.6±0.1eV, 0.8±0.1eV and 0.9±0.1eV respectively, while that of the flat islands are 0.4±0.1eV, 0.9±0.1eV and 1.3±0.1eV respectively. The growth and shape evolution of both types of islands are discussed in terms of crystallographic planes binding the islands and also the influenced of the energy barriers limiting the incorporation of adatoms into the length and width of the islands as well as transport of adatoms around the corners of the islands. The ridge islands are kinetically constrained at low growth temperatures. The islands therefore remain short at low growth temperatures but become more wire-like as temperature increases. In contrast, for flat islands are not kinetically constrained at low growth temperatures, the islands are elongated at lower temperatures due to the net effect of ¿one-way¿ migration of adatoms around the corners of the island. As temperature increases, the migration becomes two-way and this allows the island to achieve a more equilibrium rectangular shape at high growth temperatures. Further analysis of the islands length and width as a function of their area reveal that both types of islands do not follow the Tersoff-Tromp shape transition model, suggesting that the growth behaviours of these islands are not influenced by strain.