Silicide Nanowires

One-dimensional building blocks, such as nanowires and nanotubes, are especially attractive candidates to develop a bottom-up architecture. Furthermore, as nanowires and nanotubes are also promising materials for interconnects, they can act both as intercoimects and as active device elements, for example, as sensors. 

Gallego, J.M. (1990) An approach to the epitaxial growth and characterization on metallic and semiconducting silicides. PD thesis, Universidad Autonoma de Madrid, Spain. 

Hansen, M. and Anderko, K. (1958) Constitution of Binary Alloys, McGraw-Hill. 

ASM International (1992) ASM Handbook Volurrte 3 Alloy Phase Diagrams, ASM International. 

Hultgren, R., Desai, P.D., Hawkins, D.T., Gleiser, M., and Kelley, K.K. (1973) Selected Values of the Thermodynamic Properties of Binary Alloys, ASM International. 

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It should be noted that anisotropic growth resulting from anisotropic lattice mismatch has been invoked to account for the spontaneous growth of silicide nanowires on Si [28] and of Ge islands on high-index Si substrates [29]. It is then expected that growth proceeds preferentially along the direction of lesser lattice mismatch, as observed for AlN growth on (11-20) SiC. 

In many cases, metal silicides may very well be the catalysts. For example, FeSi2 is being considered to be the catalyst in Fe-assisted nanowire synthesis. This is similar to the silicon mono-oxide case, although it is much easier to understand the mechanisms in the FeSi2 case. It is also possible that during the catalytic processes that silicon diffuses relatively freely through the metal catalyst and consequently, the observed silicides at the end of reaction may be different from those during the catalytic reaction. No direct evidence is available to show whether metal or metal silicide nanoparticles are the tme catalyst. 

This leaves to only one possibility, namely that Si gets first into the gas phase to enable the catalytic growth of nanowires via Co or Co silicide nanoparticles. For silicon... 

A schematic of the proposed growth model is shown in Fig. 10.23. In this model, Co nanoparticles play a dual catalytic role. On the one hand, they catalyze silane formation by reacting first with silicon to form Co silicides, and then react with hydrogen to form silane while being reduced to Co metal. The second role of Co nanoparticles is their classic catalytic ability of making nanowires by first dissolving the silane and precipitating out Si nanowires. 

C. Li, N. Wang, S. Wong, C. Lee, and S. Lee, Metal silicide/silicon nanowires from metal vapor vacuum arc implantation, Adv. Mater. 14, 218-221 (2002). 

Si nanowires were first produced using the classical metal catalyst VLS approach [21, 22, 46]. Laser ablation of a metal-containing Si target produces metal/metal silicide nanoparticles that act as the critical catalyst needed for the nucleation of SiNWs. The wires grow further by dissolution of silicon in the metallic nano-cap and concurrent Si segregation from the cap. In a typical experiment, an excimer laser is used to ablate the target placed in an evacuated quartz tube filled with an inert gas, e.g. argon [22]. 

Region I is characterized by a metal catalyst VLS growth, as indicated by the metal caps on top of the nanostructures. The diameter of the nanowires in this growth process is determined by the diameter of the liquid alloy droplet at their tips. Metal silicide clusters of different sizes are present in the flowing gas above... 

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