Silicon diffusion

In some materials, semiconductors in particular, interstitial atoms play a crucial role in diffusion. Thus, Frank and Turnbull (1956) proposed that copper atoms dissolved in germanium are present both substitutionally (together with vacancies) and interstitially, and that the vacancies and interstitial copper atoms diffuse independently. Such diffusion can be very rapid, and this was exploited in preparing the famous micrograph of Figure 3.14 in the preceding chapter. Similarly, it is now recognised that transition metal atoms dissolved in silicon diffuse by a very fast, predominantly interstitial, mechanism (Weber 1988). 

Silicon diffusion (Ihrigising) is not commonly applied to steels, but is increasingly used to protect the refractory metals - (disilicide coatings). 

An older process to form silicides is siliconizing which is a relatively simple CVD process used to provide oxidation and chemical resistance to refractory metals. The siliconizing reaction uses the substrate itself, (such as Mo or Ti), as the metal source. Silicon diffuses readily in... 

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. 

Aravamudhan, Rahman, and Bhansali. [70] developed a micro direct ethanol fuel cell with silicon diffusion layers. Each silicon substrate had a number of straight micropores or holes that were formed using microelec-tromechanical system (MEMS) fabrication techniques. The pores acted both as microcapillaries/wicking structures and as built-in fuel reservoirs. The capillary action of the microperforations pumps the fuel toward the reaction sites located at the CL. Again, the size and pattern of these perforations could be modified depending on the desired properties or parameters. Lee and Chuang [71] also used a silicon substrate and machined microperforations and microchannels on it in order to use it as the cathode diffusion layer and FF channel plate in a micro-PEMFC. 

Bejina F. and Jaoul O. (1997) Silicon diffusion in silicate minerals. Earth Planet. Sci. Lett. 153, 229-238. 

Silicon diffusion in forsterite aggregates implications for diffusion ac-... 

At pressures below ca. 0.05 atm. it was not possible to photograph the propellant strand through the bell jar window because white fumes evolved during the burning and coated the window (recall the discussion about combustion inefficiency at very low pressure). The problem was alleviated by washing a thin film of oil (silicone diffusion pump fluid) down the inside of the window. 

Without zinc, silicon diffusion to the surface is slow under MCS conditions. Furthermore, when only tin is used as a promoter, no Cu is observed at the surface. Table 3 shows the elemental concentration under various conditions. Under MCS reaction conditions, when zinc was present silicon was not depleted from the subsurface, and when zinc was absent the subsurface was depleted in silicon. Zinc causes the rate of silicon diffusion and copper dispersion to increase. Zinc accumulates at grain boundaries and lowers the free energy of CU3SL Tin and zinc appear to work synergistically but tin does not enhance silicon diffusion on its own. Tin does appear to lower the surface energy of silicon/copper. 

O. Thomas, C.S. Petersson, F.M. d Heurle. The reaction of scandium thin films with silicon diffusion, nucleation, resistivities // Appl.Surf.Sci.- 1991.- V.53.- P.138-146. 

Silicon diffusion pump can coat and damage the filament of a mass spectrometer. 

The stability of the tungsten-silicon interface with respect to silicon diffusion and electrical integrity has been reported to be in the range 450-600°C [Joshi et al.107, Shioya et al.108, Pauleau et al.109, Thomas et al.110]. This is very compatible with Al alloy temperatures (400-450°C) in VLSI processes. 

The activation energy for silicon diffusion during the formation of mullite from fused couples at 1,600 < T < 1,800°C [55] is in the range of 730 diffusion coefficients are much higher than those of silicon at temperatures above the mullite-silica eutectic [56],... 

An EM spectrum attributed to the singly-ionized state of a (Mg,0) complex has been observed in O-containing B-doped silicon diffused with Mg at higher energy than the Mg+ spectrum, and the ionization energy of this centre is 274.90 meV [102]. This seems to show that the (Mg,0) complex is a double donor. 

The observed concentration profiles can be theoretically explained on the basis of the relationships (7-13) and (7-15) [23]. First of all we note that the limiting case of eq. (7-17) applies here. That is, silicon diffuses via vacancies, and carbon dilfuses in the interstices of the fee lattice. The partial differential equations for this case are as follows ... 

One of the very best ways for producing NPN and PNP junction transistors is by diffusing dopants into the silicon. Diffusion of impurity atoms into silicon is a slow process and must be done at high temperatures without melting the silicon. Double diffusions and triple diffusions have been found useful. One double diffusion method might start with n-type silicon into which a p-type impurity is diffused (such as boron). This produces one junction. An n-type impurity (such as phosphorus) can be diffused on top of the p-type diffusion. 

Diffusion coating is also done using a gatseous source of coating metal. For example, silicon diffusion coatings are applied by heating steel in the presence of hydrogen and silicon tetrachloride. 

Silicon diffusion was measured in natural quartz under dry low-pressure (0.1 MPa) conditions using a 20si tracer. The sources of diffusant consisted of 30si-enriched silica powder. Distributions of Si were measured using Rutherford back-scattering... 

Based upon experimentally determined parabolic growlli constants, silicon diffusion in the WSi2 and MoSi2 phases was determined ... 

If equation (6.2) applies, then silicon diffuses via a liquid or gaseous state towards the N2 molecule and steric hindrance from N=N may result in the more strained a-form. Impurities which reduce the likelihood of atomic nitrogen being present—i.e., O2, H2, and H2O, which scavenge N atoms—lead to a preponderance of a-Si3N4 in powder preparations of silicon nitride. 

To make a transformer steel with the proper hysteresis characteristics, it has to be loaded with silicon up to a certain prescribed content. This is accomplished by exposing a steel sheet that is low in silicon content and of 2 mm thickness to an atmosphere of SiQ4 that dissociates to Si(g) and Cl2(g). The silicon gas dissolves in the steel up to 3 wt% at equilibrium. The treatment is to be carried out at 1255 K. Silicon diffusivity in steel at this temperature is 8.2 X 10 m /s. 

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