Silicon whisker growth

Except for Si02, all other nanowires are well crystalline, mostly oriented along [111] direction. The VLS crystal growth mechanism was first proposed by Wagner et al. [200] for silicon whisker growth. Essentially, they reduced SiCLj using hydrogen... 

The reaction to form silicon carbide whiskers, as described in a later section, will occur at or near the surface of carbon-containing materials. The dependence of the reaction and its kinetics on the concentration of carbon is critical. Figure 4 depicts the phase relationship of silicon and carbon at very dilute concentrations of carbon, which favor whisker growth [126]. The solubility of the carbon in silicon is greatly affected by the concentration of carbon present. For example, the solubility of carbon at 2000°C is estimated to be about 10 wt %. 

The mechanisms of whisker growth vary depending on the substrate used for the reaction. For example, the Si3N4 whiskers grown on mullite show axial defects, characteristic of crystallization by an axial-screw-dislo-cation mechanism [29], In the presence of a catalyst, whisker growth occurs by means of a VLS mechanism, involving crystallization of the nitride from drops of binary or ternary alloys of silicon with iron and aluminum. The iron and aluminum act as solvents for silicon. 

Figure 9 illustrates the VLS whisker growth sequence. At 1400°C the solid catalyst melts and forms the liquid catalyst ball. Carbon and silicon from the gas phase dissolve into the liquid catalyst, which soon becomes supersaturated and solid SiC precipitates at the interface with the carbon substrate. Continued dissolution of gas species into the liquid catalyst allows the whisker to grow, lifting the catalyst ball from the substrate as additional SiC precipitates. These VLS whiskers are typically larger in diameter (4-6 jim) than those formed by the vapor ohd process (VS-whiskers). They frequently grow to length of tens of millimeters. 

Almost all short fibers, which are derived from the vapor phase, grow by a vapor-liquid-solid (VLS) phase transformation, including single crystal silicon whiskers and carbon nanotubes. Only rarely does the growth of short fibers or whiskers occur by a vapor-solid (VS) phase transformation, and the evidence for this type of phase transformations is often difficult to obtain experimentally. 

Figure 3. Growth of irregular arrays of silicon whiskers. Courtesy of Dr. E. I. Givargizov, Russian Academy of Sciences, Moscow.

Regular growth (Figure 5) of silicon whiskers occurs when gold droplets of equal size are deposited on the growth substrate before the reaction is initiated. This result is obtained when the gold droplets are deposited by lithography or similar means. In this case the catalyst particles have will equal diameters and activities and, as a result, forests of individual... 

If this carbothermal process is brought to only partial completion (Equation 11a and 11b), a homogeneous mixture of silicon carbide whiskers and silicon nitride powder [10] is obtained which can be fired directly to yield whisker reinforced ceramics. Silicon carbide reinforced alumina composites and silicon carbide whisker reinforced zirconia composites [31] are also products of the "chemical mixing process". The whisker growth rate in the zirconia process can be accelerated by adding metal particle catalysts such as cobalt chloride, thus potentially facilitating a VLS phase transformation. 

Finally, it can he added that very thin nickel films are used as seed layers for the growth of fullerenes and of silicon carbide SiC whiskers, and that C/Ni multilayers are used as x-ray mirrors. ... 

The formation of silicon nitride whiskers was observed in several different reactions, including vapor deposition, CVD, and growth from a melt. However, only the following techniques are considered to have commercial significance nitriding of metallic silicon or silicon-silica mixture, carbothermal reduction of silica with simultaneous nitridation, and thermal decomposition of silicon halides. 

Milewski JV, Gac FD, Petrovic JJ, Skaggs SR (1985) Growth of beta-silicon carbide whiskers by the VLS process. J Mater Sci 20 1160-1166... 

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