Dissolution silicon, fluoride solutions

Silicon buffer solution (10 000/ig Si ml-1). Place 21.4 g of Si02 into a 1-1 plastic jar. Add 300 ml of cold water and then slowly add 250 ml of chilled 29 M HF. Allow the solution to approach room temperature slowly in order to dissipate the heat of reaction safely without the loss of silicon as the fluoride and subsequently stand at room temperature (jar loosely capped) until dissolution is complete. Dilute to 11 in a plastic volumetric flask and store in plasticware. 

The reaction steps given by Eqs. (13)-(15) represent a general framework for silicon dissolution in fluoride electrolytes. The nature of the X and X intermediates and the chemical reactions are dependent on solution chemistry and dopant type and are discussed in more detail in subsequent sections. 

HF or H2O. A wide range of processes, including pore formation in n- and p-type silicon in HF solutions, pore formation in n-type silicon in HF solutions under illumination, and photoanodic dissolution of n-type silicon in NH4F solutions, can be explained by these models. In addition, they are consistent with the models developed for open-circuit etching of silicon in fluoride solutions, discussed in Sec. 2.2.2. 

According to Memming and Schwandt, surface states are always present on silicon electrodes in acidic aqueous solutions. The energy levels depend on whether fluoride ions are present the surface states in acidic fluoride solutions are associated with the dissolution of the silicon. The quantity of the surface states depends on the type of silicon and the illumination intensity. When fluoride ions are present in the solution, the n-Si surface, being oxide free and terminated by hydrogen, exhibits a low density of surface states. ... 

The anodic polarization curves of p-Si or strongly illuminated n-Si in fluoride solutions are typically characterized hy two peak currents, Ji and J2, and two plateau currents, J3 and A as shown in Fig. 5.2. At anodic potentials up to that at J], the electrode behavior is characterized hy an exponential dependence of current on potential and by the uneven dissolution of silicon surface leading to the formation of porous... 

The electroless deposition of Ni in fluoride solutions depends on pH it does not occur at pH 1.2 but does occur on both p-Si and n-Si. ° This is attributed to the less favorable position of the Ni " /Ni couple relative to the band edges at low pH than at high pH. Also, the deposition is much faster ( 10 times) on n-Si than on p-Si because the deposition on n-Si proceeds in the conduction band with the majority carriers whereas it is through hole injection on p-Si The rate of deposition is limited by the dissolution of silicon on n-Si and is limited by Ni reduction on p-Si. Figure 6.11 shows the i-V curve for Ni deposition on n-Si. 

D. J. Blackwood, A. Borazio, R. Greef, L. M. Peter, and J. Stumper, Electrochemical and optical studies of silicon dissolution in ammonium fluoride solutions, Electrochim. Acta 37(5), 889, 1992. 

The deposition of platinum and nickel on silicon from fluoride solutions at the open-circuit potential is studied under potentiostatic control. The results are interpreted in terms of the coupling between the anodic dissolution of silicon in fluoride media and the cathodic reactions, including metal deposition and hydrogen evolution. 

The results presented in the previous sections show that the anodic reactions on a silicon electrode may proceed via different paths depending on the conditions and that those in HF solutions and those in KOH solutions are rather different. They also show that the mechanistic models proposed for the reactions in HF and KOH solutions from the many studies in the literature are largely separated. However, in both HF and KOH solutions, the silicon/electrolyte interface is fundamentally similar differing only in the concentrations of hydroxyl and fluoride ions. Thus, a reaction scheme must be coherent with respect to the experimental observations in both FIF and KOH solutions. For comparison. Table 5.8 summarizes the characteristic features of the reactions occurring on silicon in FIF and KOH, in terms of nature of the reaction, rate, effective dissolution valence, photoeffect, and uniformity of the surface. 

The fluoride species such as HF and F" also react directly with the bare silicon surface, and thus in aqueous HF solution, the dissolution of silicon atoms has two principal competing paths, one via the reaction with HF and the other with H2O. 

H. Gerischer and M. Lubke, Electrolytic growth and dissolution of oxide layers on silicon in aqueous solutions of fluorides, Ber. Bunsenges. Phys. Chem. 92, 576, 1988. 

Clearly some form of sample pretreatment is required for soils and sediments. Total levels may be obtained following sodium carbonate-boric acid fusion and the dissolution in hydrochloric acid employing lanthanum as a buffer and releasing agent. If the determination of silicon is not required, it may be volatilized as silicon tetrafluoride using hydrofluoric acid, although some calcium may also be lost as calcium fluoride. For many samples, however, it may be more appropriate to determine the exchangeable cation content of the sample. Here, the sample may be shaken with an extractant solution, for example, 1 mol 1 ammonium chloride, ammonium acetate, or disodium EDTA, prior to filtration and analysis. Where final solutions contain more than - 0.5% of dissolved material, the standards should also contain the major constituents, even where no chemical interference is expected, in order to match the viscosity and surface tension and avoid matrix effects. 

The anodic dissolution of silicon is more complex because an oxide film is easily formed on the Si surfaces which is not soluble in water. This high stabiUty of the oxide film makes Si particulary interesting for many applications. The oxide formation can be avoided, however, by working in fluoride containing solutions. 

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