Silicon divalent dissolution

Fig. 4.3 Reaction scheme proposed for the anodic, divalent dissolution of silicon electrodes in HF.

Divalent dissolution is initiated by a hole from the bulk approaching the silicon-electrolyte interface which allows for nucleophilic attack of the Si atom (step 1 in Fig. 4.3). This is the rate-limiting step of the reaction and thereby the origin of pore formation, as discussed in Chapter 6. The active species in the electrolyte is HF, its dimer (HF)2, or bifluoride (HF2), which dissociates into HF monomers and l ions near the surface [Okl]. The F ions in the solution seem to be inactive in the dissolution kinetics [Se2], Because holes are only available at a certain anodic bias, the Si dissolution rate becomes virtually zero at OCP and the surface remains Si-H covered in this case, which produces a hydrophobic silicon surface. 

For the electrochemical dissolution of Si in electrolytes composed of anhydrous HF and an organic solvent a reaction is proposed that is similar to the divalent dissolution in aqueous HF. However, molecular hydrogen is not observed and four charge carriers are consumed per dissolved silicon atom, as in the tetravalent case [Pr7, Ril]. 

The origin of the electron injection peak at the end of the dissolution of an oxide film is not understood in detail. Silicon interface atoms with three Si-O bonds and a single Si-Si bond are proposed to be responsible for the effect [Mai]. On the other hand, during the dissolution process silicon interface atoms with one Si-O bond and three Si-Si bonds lead to a configuration identical to the one for which electron injection is observed during divalent dissolution (Fig. 4.3, step 2). In any case, the injected charge exceeds by a factor of 3 to 5 the charge expected... 

For homogeneously doped silicon samples free of metals the identification of cathodic and anodic sites is difficult. In the frame of the quantum size formation model for micro PS, as discussed in Section 7.1, it can be speculated that hole injection by an oxidizing species, according to Eq. (2.2), predominantly occurs into the bulk silicon, because a quantum-confined feature shows an increased VB energy. As a result, hole injection is expected to occur predominantly at the bulk-porous interface and into the bulk Si. The divalent dissolution reaction according to Eq. (4.4) then consumes these holes under formation of micro PS. In this model the limited thickness of stain films can be explained by a reduced rate of hole injection caused by a diffusional limitation for the oxidizing species with increasing film thickness. 

FIGURE 5.62. Mechanism of the anodic dissolution of silicon in concentrated hydrofluoric acid solutions (divalent dissolution). After Memming and Schwandt. ... 

The divalent dissolution of Si was then described by a two-dimensional reaction scheme at a kink site on a silicon surface (not shown) assuming the surface to be covered by fluorine instead of hydroxyl groups [8]. It was further postulated that the unstable silicon difluoride changes into a stable tetravalent form by a disproportionation reaction [9] ... 

Jakubowicz J, Jungblut H, Lewerenz HJ (2003) Initial surface topography changes during divalent dissolution of silicon electrodes. Electrochim Acta 49 137-146 Jakubowicz J, Smardz K, Smardz L (2007) Characterization of porous sihcon prepared by powder technology. Physica E 38 139-143... 

As already shown by Uhlir [10] and 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 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 formed by reaction (8.2) is expected to be unstable. Originally the divalent dissolution of Si was then described by a two-dimensional reaction scheme at a kink site on a silicon surface (not shown) assuming the surface to be covered by fluorine instead of... 

This is the regime of anodic current densities below JPS. A hole approaching the interface initiates the divalent electrochemical dissolution of a silicon surface atom at the emitter. The dissolution proceeds under formation of H2 and electron injection, as shown in Fig. 4.3. The formation of PS structures is confined to this region. 

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. 

The anodic behavior of p-type Si electrodes is quite different for lower HF concentrations. The current increases, but not really exponentially, with rising anodic polarization, it passes a maximum and increases again slowly at higher anodic potentials [8] (Fig. 8.6). The current increases with the rotation speed to of the electrode. Since the current does not follow a tu /z dependence (Levich relation [11]) the relationship cannot be determined entirely by diffusion. At electrode potentials below the peak, silicon is dissolved again in the divalent state, as already reported above in the case of high HF concentrations. Here also H2 formation was observed. At electrode potentials beyond the current peak, as shown in Fig. 8.6, the dissolution was found to occur via the tetravalent state of Si and the H2 evolution disappeared at p-type electrodes [8]. These results were confirmed 25 years later [12]. Experiments performed using the thin slice arrangement (see Chapter 4) have shown that the anodic reactions occur only via the valence band at all electrode potentials [8]. 

As with metals, semiconductors are also subject to passivation. Figure 22.9 shows the anodic dissolution and the passivation of n-type and p-type silicon electrodes in sodium hydroxide solution [13]. Silicon dissolves in basic solution in the form of soluble divalent silicon, Si(OH),iq or Si(OH)2jaq, and passivates forming a silicon dioxide film. 

The anodic passivation of semiconductors in aqueous solution occurs in much the same way as that of metals and produces a passive oxide film on the semiconductor electrodes. Figure 22.25 shows the anodic dissolution current and the thickness of the passive film as a function of electrode potential for p-type and n-type silicon electrodes in basic sodium hydroxide solution [32,33], As mentioned earlier, silicon dissolves in the active state as divalent silicon ions and in the passive state a film of quadravalent insoluble silicon dioxide is formed on the silicon electrode. The passive film is in the order of 0.2-1.0 nm thick with an electric field of 106 107 V cm 1 in the film within the potential range where water is stable. 

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