Silicon dissolution mechanism

H. GERISCHER (Max Flank Institute) Concerning your mechanism for the dissolution of silicon, can you be sure that the product you analyze by x-ray diffraction is the same as that present during electrolysis If it is a film of silicon it must be insulated from the electrode or else it would undergo dissolution again. 

FIGURE 5.61. Mechanism of the anodic dissolution of silicon in low HF concentrations at higher anodic potentials (tetravalent dissolution). After Memming and Schwandt. " ... 

In this chapter, the conditions for the formation of PS, the relation between the formation conditions and PS morphology, and the mechanisms for the formation of PS and morphology are discussed. The various aspects of surface condition, nature of reactions, and reaction kinetics that are fundamentally involved in the anodic dissolution of silicon are discussed in Chapters 2-5. 

Y. Kang and J. Jorae, Dissolution mechanism for p-Si during porous silicon formation, J. Electrochem. Soc. 144(9), 3104, 1997. 

As already shown by Uhlir [10] and by Turner [9], the dissolution mechanism for Si in concentrated HF is quite different from that of Ge in aqueous solutions. The fact that Si was dissolved in the divalent state is a little surprising because, in general, the stability of a divalent state of an element decreases in the IVth group in the direction Pb-Sn-Ge-Si-C. Therefore, a divalent silicon ion or silicon compound is expected to be unstable. Taking into account this instability of divalent Si, a dissolution mechanism of Si in concentrated HF solutions has been originally proposed as ... 

The growth of this structure at the interface between porous silicon film and silicon substrate has been tentatively explained in [17]. In the silicon dissolution mechanism, it is known that positive carriers (holes) are involved. It is assumed that initially the pore walls are depleted of holes. 

The exact dissolution chemistry of silicon is still a matter of debate however, ignoring the intermediate steps involved in the dissolution mechanism, the overall dissolution reaction for porous Si formation is as follows ... 

The dissolution mechanism has been discussed in detail by Strelko (211), who considers analogous mechanisms for dissolution of silica not only by catechols but also HF, H3PO4, and possibly in acidic acetyl acetone, all of which are known to form silicon compounds in which silicon is coordinated by six Surrounding fluorine or oxygen atoms. 

The tetravalent state is the stable fomi of a silicon when oxidized. The mechanism of oxidation and dissolution of silicon and geimanium was first studied by Turner et al. in an interest to understand the etching and cleaning of silicon and germanium surfaces [2], The mechanism of the oxidation and dissolution of semiconductor is like that of a metal except that (1) two types of charge carriers can be involved, valence band holes and conduction band electrons, and (2) the density of charge carriers at the solid-liquid interface is much smaller for the semiconductor than for a metal. The electrochemical reaction of silicon in an aqueous solution is given by Eq. 1 ... 

Silicones exhibit an apparently low solubility in different oils. In fact, there is actually a slow rate of dissolution that depends on the viscosity of the oil and the concentration of the dispersed drops. The mechanisms of the critical bubble size and the reason a significantly faster coalescence occurs at a lower concentration of silicone can be explained in terms of the higher interfacial mobility, as can be measured by the bubble rise velocities. 

Many theories on the formation mechanisms of PS emerged since then. Beale et al.12 proposed that the material in the PS is depleted of carriers and the presence of a depletion layer is responsible for current localization at pore tips where the field is intensified. Smith et al.13-15 described the morphology of PS based on the hypothesis that the rate of pore growth is limited by diffusion of holes to the growing pore tip. Unagami16 postulated that the formation of PS is promoted by the deposition of a passive silicic acid on the pore walls resulting in the preferential dissolution at the pore tips. Alternatively, Parkhutik et al.17 suggested that a passive film composed of silicon fluoride and silicon oxide is between PS and silicon substrate and that the formation of PS is similar to that of porous alumina. 

A basic requirement for electrochemical pore formation is passivation of the pore walls and passivity breakdown at the pore tips. Any model of the pore formation process in silicon electrodes has to explain this difference between pore tip and pore wall conditions. Three different mechanisms have been proposed to explain the remarkable stability of the silicon pore walls against dissolution in HF, as shown in Fig. 6.1. 

Anodic oxide formation suggests itself as a passivating mechanism in aqueous electrolytes, as shown in Fig. 6.1a. However, pore formation in silicon electrodes is only observed in electrolytes that contain HF, which is known to readily dissolve Si02. For current densities in excess of JPS a thin anodic oxide layer covers the Si electrode in aqueous HF, however this oxide is not passivating, but an intermediate of the rapid dissolution reaction that leads to electropolishing, as described in Section 5.6. In addition, pore formation is only observed for current densities below JPS. Anodic oxides can therefore be excluded as a possible cause of pore wall passivation in PS layers. Early models of pore formation proposed a... 

As exemplified by the silicate profile, all biolimiting elements do not behave identically. In the case of dissolved silicon and TDIC, their concentration maxima lie below the nitrate and phosphate maxima. This reflects the different mechanisms by which the elements are resolubilized. Nitrate and phosphate are regenerated from soft parts. This process seems to occur more readily than the dissolution of hard parts, which releases silicate causing the nitrate and phosphate concentration maxima to lie at shallower depths. Since TDIC is released in nearly equal amounts from soft parts as CO2 and the dissolution of calcareous hard parts as CO3, the resulting concentration maximmn lies below that of nitrate and phosphate. 

The soluble divalent SiF2 compound is in turn transformed by disproportionation into SiF6 and elemental amorphous silicon. This mechanism is responsible for the effective dissolution valence of Si, which was found to be equal to 2 in the range of potential between 0 and +0.4V/SCE. 

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. 

The mechanism of silicon etching in alkaline solutions is a process of material dissolution with a simultaneous hydrogen evolution. The main soluble product is a silicic anion Si02(0H)2 that can further be condensed to form polysilicic anions. In fact, due to the acido-basic ionization of OH radicals in a highly alkaline solution, Eq. (19) should be modified as follows ... 

D. R. TURNER The difference between the two materials lies in the formation of a thick anode film on silicon. Once this film is removed and electropolishing occurs, the mechanism of dissolution is, I believe, quite similar. The amount of material etched electrolytically per coulomb of electricity is related to electrochemical equivalent/density. The electrochemical equivalent for Ge is about twice that of Si, but the density of Ge is about 2 times that of Si so etch rates are about the same. 

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