The oxidation of silicon

If the movement of ions on the network predominates, the rate will depend on some power of die oxygen pressure, n, which is related to the formation of oxygen ions on the lattice such as 

Experimental data show that tire value of m decreases from 0.6 at 1200°C and 0.3 at 1500°C, indicating a mixed control, as shown in the SIMS analysis. 

For the dilute reactive constituent R which forms the oxide ROn, let the thickness of the internal oxidation zone be , then for the inward diffusion of oxygen, mole fraction No 

Rapp (1961) has confirmed this equation in a study of the oxidation in air of Ag-In alloys at 550°C. The reaction proceeds with the internal formation of ln203 particles over a range of indium concentrations, but at a critical mole fraction of indium in the alloy, external oxidation occurs with the growth of a layer of ln203 covering the alloy. The transition from internal to external oxidation was found by Rapp to occur at the mole fraction of indium corresponding to 

Maak (1961) has obtained the equation governing the oxidation rate of a metal to form both an external oxide and an internally oxidized dilute solute, as for example in Cu-Be alloys, corresponding to the equation given earlier 

The thickness of the internal zone can be calculated from the equation quoted earlier 

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Figure 1 shows a segment of the FTIR absorbance spectrum of a thin film of the oxide of silicon deposited by chemical vapor deposition techniques. In this film, sil-... 

Scheme 23 Possible mechanism for the oxidation of silicon surfaces by molecular oxygen.

Whatever the initial step of formation of surface silyl radicals, the mechanism for the oxidation of silicon surfaces by O2 is expected to be similar to the proposed Scheme 8.10. This proposal is also in agreement with the various spectroscopic measurements that provided evidence for a peroxyl radical species on the surface of silicon [53] during thermal oxidation (see also references cited in [50]). The reaction being a surface radical chain oxidation, it is obvious that temperature, efficiency of radical initiation, surface precursor and oxygen concentration will play important roles in the acceleration of the surface oxidation and outcome of oxidation. 

Again it seems not necessary to discuss the considerations of the chemical versus electrochemical reaction mechanism. It is clear from the extremely negative standard potential of silicon, from Eqs. (2) and (6), that the Si electrode is in all aqueous solutions a dual redox system, characterized by its OCP, which is the resultant of an anodic Si dissolution current and a simultaneous reduction of oxidizing species in solution. The oxidation of silicon gives four electrons that are consumed in the reduction reaction. Experimental results show clearly that the steady value of the OCP is narrowly dependent on the redox potential of the solution components. In solutions containing only HF, alternatively alkaline species, the oxidizing component is simply the proton H+ or the H2O molecule respectively. 

Small changes in impurity content did not affect this rate but the presence of water vapor and changes in partial pressure of oxygen were critical (61,62). Steam and various impurities and binders also affect the oxidation of silicon carbide (63). Differences have been observed in oxygen adsorption on the different SiC crystal faces (64). 

Point Defect Generation During Phosphorus Diffusion. At Concentrations above the Solid Solubility Limit. The mechanism for the diffusion of phosphorus in silicon is still a subject of interest. Hu et al. (46) reviewed the models of phosphorus diffusion in silicon and proposed a dual va-cancy-interstitialcy mechanism. This mechanism was previously applied by Hu (38) to explain oxidation-enhanced diffusion. Harris and Antoniadis (47) studied silicon self-interstitial supersaturation during phosphorus diffusion and observed an enhanced diffusion of the arsenic buried layer under the phosphorus diffusion layer and a retarded diffusion of the antimony buried layer. From these results they concluded that during the diffusion of predeposited phosphorus, the concentration of silicon self-interstitials was enhanced and the vacancy concentration was reduced. They ruled out the possibility that the increase in the concentration of silicon self-interstitials was due to the oxidation of silicon, which was concurrent with the phosphorus predeposition process. 

More recently, Gould and Irene (96) studied surface-cleaning effects on the oxidation of silicon wafers for oxide thicknesses up to 4300 A. They found... 

Evidence for this reaction has been reviewed recently by Revesz (97). Thus, the oxidation of silicon in water vapor involves the diflusion of OH and... 

Schneider, B., Guette, A., Naslain, R., Cataldi, M., Costecalde, A., 1998), Atheoretical and experimental approach to the active-to-passive transition in the oxidation of silicon carbide , J. Mater. Sci., 33, 535-547. 

The oxidation of silicon to SiO is not dependent on the kind of the oxidizing agent used, and the reaction may be carried out with metal oxides, atmospheric oxygen, etc. The finer the grain of the silicon, the better the reaction. 

As x0 can be taken with the plus or minus sign, the experimental data should produce a straight line in the coordinates either (x + x0) - t/(x - x0) or (x - x0) - / (x + x0). This form of presenting the experimental results was used by N.A. Kolobov and M.M. Samokhvalov,17 who found the values of the reaction-diffusion constants for the Si02 layer in the oxidation of silicon by oxygen, listed in Table 1.2. The temperature dependence of both the chemical constant k0 and the physical (diffusional) constant k is well described by the Arrhenius relation (see equation (1.34)) with the activation energy 155 and 120 kJ mol1, respectively. 

Table 1.2. Reaction-diffusion constants for the Si02 layer in the oxidation of silicon by...

The former proceeds at interface 2, while the latter at interface 1 after the backward diffusion of the released nickel atoms. The number of NiSi molecules entering reaction (5.45) is seen to be equal to that occurring as a result of reaction (5.46). Thus, the disappearance of a NiSi molecule at one of the NiSi interfaces is compensated by its formation at the other. Therefore, the thickness of the NiSi layer remains unchanged, whereas that of the Si02 layer increases with passing time. The net reaction is actually the oxidation of silicon from the substrate to form Si02. The NiSi layer of constant thickness simply moves as a whole deeper into the substrate bulk, without loss of its integrity. 

Other forms of silica, such as pyrogenic silica and mineral opals are normally nonporous. Pyrogenic silica is composed of silica particles with a very narrow particle size distribution. This material is obtained by vaporizing Si02 in an arc or a plasma jet, or by the oxidation of silicon compounds [152], Artificial opals are materials characterized by the presence of Si02 microspheres [16]. 

Occurrence and History.—The name ruthenium is due to Osann,1 who obtained what he believed to be the oxide of a new metal from the Ural platinum ores. This oxide was subsequently investigated by Claus,8 who found that, although it contained a high proportion of the oxides of silicon, zirconium, titanium, and iron, it nevertheless possessed a small quantity of a new oxide for which he retained the name used by Osann. 

As described previously, flame s3mthesis reactions include the oxidation of silicon chloride to produce silica the oxidation of titanium chloride to produce titania and the oxidation of other metal chlorides (see Table 7.2 also). 

Fig. 1 presents the oxidation rate versus time for silicon powders differing in preparation procedure, particle shape, and particle size. The images in Fig. 1 illustrate the morphology of the powder particles. The particle shape may have a significant effect on the oxidation process [4,5,7]. The oxidation of silicon powders to silicon dioxide was found to occur in two steps, with a transition at a certain thickness of the oxidized layer, which depended on the oxidation time, particle shape, and particle size. 

Chemical Data. The oxide concentrations given in Appendix C were obtained by DCP-OES. This method of analysis allows easy determination of the oxides of silicon, aluminum, magnesium, calcium, and phosphorus in addition to the 15 oxides originally analyzed by INAA. In the study presented here, this additional concentration data on major constituents was very useful in conjunction with the study of the mineralogy by microscopic examination and by x-ray diffraction analysis. 

Similar experimental conditions allowed the hydroxylation of 1,2-dihydronaphtha-lene in high yield with an ee > 90% [137]. In the latter case, the addition of ferricyanide ion was used also to electrochemically maintain Os concentration in the anolyte. Such reactions were achieved in two-phase systems. Metal complexes can be used for indirect oxidation such as the oxidation of phosphines by Nb species [138] as well as Ni salen used in the oxidation of silicon compounds [139] (Scheme 20). 

According to Seidel et al. the dissolution at OCP is an electrochemical process with concurrent anodic dissolution of sihcon and reduction of hydrogen ions. The oxidation of silicon gives out electrons which are consumed for the reduction of hydrogen. Both OH" and H2O are the active species in that OH" is involved in silicon dissolution and H2O in hydrogen evolution ... 

The reduction of HNO3, which provides holes for the oxidation of silicon, is considered to be a complex process involving multiple reaction steps. 

Silicon is highly unstable in aqueous electrolytes due to the formation of an insulating oxide film which prevents the use of n-Si as photoanode. On the other hand, the silicon electrode has poor kinetics for hydrogen evolution which is not desirable for its use as a photocathode. Many methods have been explored to stabilize Si electrodes in aqueous solutions for possible applications as photochemical cells. They include coating the surface with noble metals, metal oxides, metal silicides, or organic materials as shown in Table 6.6. Also, some redox species, the reduction of which can favorably compete with the oxidation of silicon, can be used to stabilize silicon anodes... 

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