Silicon vapor-liquid-solid process

The Vapor-Liquid-Solid Process In the VLS process silicon and carbon-rich vapors (usually CH4, SiO, or SiC ) react at 1400°C to from SiC on a liquid... 

Vapor-Liquid-Solid Process In the VLS process, silicon- and carbon-rich vapors (usually CH4, SiO, or SiCU) react at 1400 °C to SiC on a liquid alloy [Eq. (9)]. Microscopic particles of an alloy are first distributed on a substrate (graphite) and then exposed to silicon- and carbon-rich vapors. The presence of a liquid catalyst, such as a transition metal or usually an iron alloy, distinguishes this method from all other whisker preparation methods. 

A vapor-liquid-solid (VLS) technique (Petrovic et al, 1985 Milewski et al, 1985) can be used to grow exceptionally strong and stiff silicon carbide whiskers. The name VLS comes from the fact that the process uses vapor feed gases, a liquid catalyst, and solid crystalline whiskers are the end product. Figure... 

The formation of silicon carbide whiskers can occur through one of three general methods vapor-solid (VS), chemical vapor deposition (CVD), vapor-liquid-solid (VLS). The processes vary widely in the raw... 

Zhang JG et al (2010) Vapor-induced solid-liquid-solid process for silicon-based nanowire growth. J Power Sources 195 1691-1697... 

Silicon carbide whiskers, usually produced from silica and carbon, are widely used for ceramic matrix composites [38]. Tbe vapor-liquid-solid (VLS) process is accepted as the mechanism for SiC whisker production (Fig. 15.1.4). This process requires formation of a liquid phase of a transition metal at the reaction sites. Elemental sources of Si and C are usually supplied in vapor phase form as SiO and CH4, respectively [39]. The vapors are deposited on the surface of the liquid metal and dissolved into the liquid metal droplets. Since the whiskers are precipitated from the dissolved components, metal droplets are often observed at the tip of the whiskers [39]. 

The process of nitride formation during combustion of ferrosilicon in nitrogen can generally be described by four mechanisms solid-gas, liquid-gas, vapor-gas, and vapor-liquid-crystal (VLC). Experimentally measured combustion temperatures (1900 - 2100°C) indicate that silicon will be either in the liquid state as an iron-silicon melt or in the gaseous state during SHS. Under the... 

Figure 4.7 illustrates the VLS whisker growth sequence. At 1400 °C, the soUd catalyst melts and forms the liquid catalyst ball. Carbon and silicon from the gas phase are dissolved in the liquid alloy, which soon becomes supersaturated such that solid SiC precipitates at the interface with the carbon substrate. Continued dissolution of the 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 pm) than those formed by the vapor solid process (VS-whiskers) in fact, they frequently grow to a length of tens of millimeters. 

Epitaxial Layers. Epitaxial deposition produces a single crystal layer on a substrate for device fabrication or a layer for multilevel conductive interconnects which may be of much higher quality than the substrate. The epitaxial layer may have a different dopant concentration as a result of introducing the dopant during the epitaxial growth process or may have a different composition than the substrate as in silicon on sapphire. Methods used for epitaxial growth include chemical vapor deposition (CVD), vapor phase epitaxy (VPE), liquid phase epitaxy (LPE), molecular beam epitaxy (MBE) and solid phase epitaxy (SPE). 

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