Surface pyrolysis, silane

The growth rate of silicon crystals by either MBE or CVD is relatively slow, and so there is ample time for adsorbed atoms and molecules to diffuse to energetically more favorable sites. Experimental rates of diffusion of silicon on silicon and the activation barriers, however, are not known well. Experimental estimates of the activation barrier for silicon atoms diffusing on the Si(lll) surface have ranged from 58 kcal/mole when the pyrolysis of silane is used to produce surface silicon atoms , to 4.6 kcal/mole for the direct deposition of silicon atoms under ultrahigh vacuum Furthermore, a comparison of these... 

When a heterogeneous reaction is considered, the partial pressures included in the "Law of Mass Action," Equation (1), are only those for the gaseous reactants. For example, when silicon is deposited on a surface due to silane pyrolysis, we have... 

In silicon based MEMS processing, common CVD films include polysilicon, silicon oxide, and silicon nitride. For polysilicon films (usually the structural layer), an LPCVD pyrolysis method is generally used with silane (SiH4) as the source gas [see Eq. (1)]. To obtain a imiform film across the wafer, the process is carried out at low pressure to ensure that the deposition is surface reaction controlled and not diffusion limited. Typical process temperatures are in the range of 580-650°C, and pressures between 0.1 and 0.4Torr. 

CO2 laser pyrolysis of silane in a gas flow reactor and the extraction of the resulting silicon nanoparticles into a cluster beam apparatus has been shown to offer an excellent means for the production of homogeneous films of size-separated quantum dots. Their photoluminescence varies with the size of the crystalline core. All observations are in perfect agreement with the quantum confinement model, that is, the photoluminescence is the result of the recombination of the electron-hole pair created by the absorption of a UV photon. Other mechanisms involving defects or surface states are not operative in our samples. 

Two different methods, based on flame and laser pretreatments, were investigated. These methods are generally applicable under atmospheric conditions. The first is based on the surface reaction of low-molecular, silane-based precursors which are activated by pyrolysis in a propane gas flame. XPS analysis of the pretreated surface shows a thin silicate layer on the surface which remains active for about 10 days. After a careful parameter optimization, adhesion strength can be greatly improved, especially on steel surfaces. A similar method is commercially available as the Silicoater process. The second method, CLP, is based on a laser pretreatment in combination with a specific primer [4]. With parameters optimized for the specific substrate, adhesion strength can be greatly improved, especially on aluminum surfaces. 

In this paper, we have presented test results of two methods of adhesion improvement on a specific aluminum alloy and a stainless steel. The methods are based on flame pyrolysis of silane precursors and a laser pretreatment in connection with a primer. While the first method seems to be most favorable for steel surfaces, the latter gives good results on both aluminum and steel. Both methods are well suited for application in the railroad industry since they are effective under atmospheric conditions and are therefore promising for integration in an automated production line. 

In laser pyrolysis, a precursor in the gaseous form is mixed with an inert gas and heated with CO2 infrared laser (continuous or pulsed), whose energy is either absorbed by the precursor or by an inert photosensitizer such as SFs. Swihart [84], Ledoux et al. [116,117], and Ehbrecht and Huisken [118] prepared Si nanoparticles by laser pyrolysis of silane. By using a fast-spinning molecular beam chopper, Si nanoparticles in the size range of 2.5-8 nm were deposited on quartz substrates to study quantum confinement effects [116]. Li et al. [119] improved the stability of the Si nanoparticles ( 5 nm) by surface functionalization and obtained persistent bright visible photoluminescence. Hofmeister et al. [120] have studied lattice contraction in nanosized Si particles produced by laser pyrolysis. The method has been used to synthesize metal nanoparticles as well (see Table 2.1). Zhao et al. [121] obtained Co nanoparticles by laser pyrolysis of Co2(CO)s vapor at a relatively low temperature of 44° C. Ethylene was used as a photosensitizer for CO2 laser emission. Nanoparticles... 

Although silicon nanocrystals are now more commonly prepared by a variety of means which are easier to scale up, e.g., pyrolysis of silanes (Xuegeng et al. 2004), thermal treatment of silsesquioxanes (Hessel et al. 2006, 2010), and from reactions of molecular silicon compounds (Wilcoxon et al. 1999 Bley and Kauzlarich 1996), this review will concentrate on routes which proceed via the formation of porous silicon. More general reviews of silicon nanocrystals Irom physics and chemistry perspectives are available (Shirahata 2011 Kang et al. 2011 Heitmann et al. 2005). Derivatization of porous silicon and SiNCs usually relies on the chemistry of the hydrogen-terminated silicon surface, which shares some of the organic reactivity of hydrosilanes (Buriak 2002). Reaction with alcohols results in Si-O-C bonded monolayers (Sweryda-Krawiec et al. 1999), but these are suseeptible to hydrolysis under ambient conditions. Alternately, addition of surface Si-H aeross a C = C double bond produces Si-C bonded monolayers, which are very stable. 

Yang, R. Liu, Y. Wang, K. Yu, J. (2003). Characterization of surface interaction of inorganic fillers with silane coupling agents. Journal of Analytical and Applied Pyrolysis. Vol. 70, No.2, pp.413-425, ISSN 0165-2370. 

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