Oxidation-enhanced diffusion

For example, during oxidation, enhanced diffusion of phosphorus, boron, and arsenic are observed, as well as retarded diffusion of antimony. However, if direct nitridization of the silicon surface occurs, then the inverse effects are observed, that is, enhanced antimony diffusion and retarded phosphorus diffusion. Also, oxidation-enhanced diffusion is significantly affected by doping. As either p- or n-type doping concentration increases above nh oxidation-enhanced diffusion diminishes. If chlorine is introduced into the oxidizing ambient, oxidation-enhanced diffusion is likewise diminished. 

Diffusion in the Presence of Excess Point Defects. Oxidation-Enhanced Diffusion. Oxidation generally enhances the diffusion of group III and group V elements except for antimony (Figure 13). Oxidation-enhanced diffusion is generally observed by depositing a silicon nitride mask on the silicon surface that will prohibit oxidation in the regions that it covers. 

Growth of oxidation-induced stacking faults proceeds by the absorption of the generated self-interstitials. Oxidation-enhanced diffusion can occur as a result of the presence of the excess interstitials via the Watkins (36) replacement mechanism or by an interstitialcy process. 

Dependence of Oxidation-Enhanced Diffusion on Doping. Taniguchi (44) found that oxidation-enhanced diffusion decreases as the concentration... 

Equation 36 is divided into the contributions to the diffusion of substitutional impurity under nonoxidizing conditions, DSI, and the enhanced contribution due to oxidation, AD0. Figure 16 shows the data of Taniguchi et al. (44) for oxidation-enhanced diffusion of P and B versus the total number of dopant impurities per square centimeter, QT. The calculated values of DSI and AD0 are shown in comparison with the experimental data. Reasonable agreement is obtained. Thus, Taniguchi s model of self-interstitial recombination with vacancies is consistent with the models of high-concentration diffusion of B and P used by Fair in his calculations. 

Effect of Chlorine on Oxidation-Enhanced Diffusion. If chlorine is added to oxygen in the furnace in sufficient concentrations such that stacking-fault retrogrowth occurs (46), then oxidation-enhanced diffusion becomes negligible (47). This result is believed to be due to the generation of vacancies at this Si-Si02 interface when Cl reacts with Si atoms on lattice sites to produce SiCl by the following reaction... 

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. 

As the film dissolves more oxide film is formed, i.e. the metal/oxide interface progresses into the metal, and the overall rate may be low enough to be acceptable for a particular process. In other cases, the corrosion products precipitate on the surface of the oxide and either accelerate the overall rate by enhancing diffusion of ions through the porous outer layers or, when less porous layers are formed, access of hydrogen ions to the inner oxide surface is reduced thus decreasing the rate. 

Figure 13. Experiments that illustrate oxidation-enhanced or oxidation-retarded diffusion of dopants in silicon. The supersaturation of self-interstitials associated with the oxidation process drives both effects. (Reproduced with permission from reference 118. Copyright 1984 Noyes Publications.)...

Figure 21. Example of two MOSFET channel B implants performed through a poly gate I oxide structure and annealed at 600 °C for 30 min and at 700 °C for 30 min. The substantial enhanced diffusion is shown modeled with calculations from the Predict program. Data are from Mele et al. (61). Abbreviation and symbols S/MS, secondary ion mass spectrometry I, measured after implant A, measured after anneals. (Reproduced with permission from reference 59. Copyright 1988 Institute of Electrical and Electronics Engineers,...

Keywords Compact fuel cell Proton conductive oxide Radiation induced conductivity Radiation enhanced diffusion... 

Recently Fu et al. [22] considered the state of reduction of the bulk oxide and its inflnence on the SMSI state. They concluded that, although complicated, the bulk must be reduced (or n-type doped) to enable the encapsulation process to occur and their study proposed electric field enhanced diffusion of Ti interstitials as a driver for the formation of the SMSI state. Furthermore, Pd islands on reduced SrTiOjCOOl) single crystals also show hexagonal structures on the flat facets of the islands when vacuum annealed [23]. Here, the reduced SrTiOj(OOl) surfaces are predominantly TiO terminated, so it appears that reduced Ti oxides are a requirement to induce the SMSI state. 

Thus a connection to regions of higher oxygen partial pressure is obtained and instead of nitrides titanium oxide is formed at the metal/oxide interface. As a consequence local temporary regions of high oxidation rate are formed within the scale which may lead to nodules observed on the surface of the oxide scale. At higher temperatures such cracks arc not found possibly because of an increased plasticity of the aluminium depleted subsurface zone or because of a rapid healing rate due to enhanced diffusion... 

The improved electro-oxidation behavior observed with the C-8 and C-12 acid coated electrodes might be attributed to (i) the higher concentration of sulfonic acid groups present in the electrocatalytic layer of the C-8 acid and C-12 acid coated electrode, (ii) enhanced diffusion of reactants and products, and/or (iii) the enhanced wettability" at the catalytic site of oxidation facilitating hydrocarbon adsorbtion. These results are in accordance with earlier findings where the addition of C-8 acid was observed to significantly enhance the ease of oxidation of various oxygenated molecules. 

Another propylene ammoxidation catalyst that was used commercially was U-Sb-0. This catalyst system was discovered and patented by SOHIO in the mid-1960s (26,27). Optimum yield of acrylonitrile from propylene required sufficient antimony in the formulation in order to ensure the presence of the USbaOio phase rather than the alternative uranium antimonate compound USbOs (28-30). The need for high antimony content was understood to stem from the necessity to isolate the uranium cations on the surface, which were presumed to be the sites for partial oxidation of propylene. Isolation by the relatively inactive antimony cation prevented complete oxidation of propylene to CO2. Later publications and patents showed that the activity of the U-Sb-0 catalyst is increased by more than an order of magnitude by the substitution of a tetravalent cation, tin, titanium, and zirconium (31). Titanium was found to be especially effective. The promoting effect results in the formation of a solid solution by isomorphous substitution of the tetravalent cation for Sb + within the catalytically active USbaOio- phase. This substitution produces o gen vacancies in the lattice and thus increases the facility for diffusion of lattice o gen in the solid structure. As is discussed below, the enhanced diffusion of o gen is directly linked to increased activity of selective (amm)oxidation catalysts based on mixed metal oxides. 

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