The Electrochemical Dissolution of Silicon

The concepts and basic approach used in studies of electrical fluctuations in corrosion processes proved to be very successful as well in mechanistic studies of electrode reactions taking place at materials covered by passivating films. A typical example is the electrochemical dissolution of silicon. From an analysis of the noise characteristics of this process, it has been possible to identify many features as well as the conductivity of the nanostructures of porous silicon being formed on the original silicon surface. 

For the case of Si02 etching, HF, (HF)2 and HF2- are assumed to be the active species [Vel, Jul]. If HC1 is added to the solution the concentration of the HF2-ion becomes negligible, which leaves HF and its polymers to be the active species [Ve3]. Because for high current densities the electrochemical dissolution of silicon occurs via a thin anodic oxide layer it can be concluded that, at least for this regime, the same species are active. This is supported by the observation that F- is... 

F. Ozanam and J.-N. Chazalviel, In-situ infrared characterization of the electrochemical dissolution of silicon in a fluoride electrolyte, J. Electron Spectrosc. Relat. Phenom. 64-65, 395, 1993. 

The electrochemical reactions and processes involving the anodic dissolution of silicon in HF solutions have been extensively studied in the past. Table 5 provides a summary for the characteristics of the anodic processes that are relevant to the formation of PS (details are documented in Ref.1.)... 

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 mechanisms of the electrochemical reactions of silicon electrodes in alkaline solutions at OCP have been investigated in many smdies due to their importance in the etching processes in micromachining. An important issue involving the reaction mechanisms has been whether the etching process at OCP is of chemical or electrochemical nature, that is, whether charge transfer processes associated with silicon dissolution and hydrogen evolution involve the carriers in the electrode. 

Except for reaction path (3), which is purely chemical in nature, all the other reaction paths are of electrochemical nature, at least partially. These electrochemical reactions depend on the carrier transfer between the states at the interface and those in the semiconductor and thus their rates increase with increasing potential or illumination. While the reaction paths ( ), (3), and (4) result in the direct dissolution of silicon, the reaction paths (2) and (5) result in the formation of Si—O—Si bonds, the dissolution of which results in an indirect dissolution path. The rate of reaction paths (2) and (5) also increase oxide formation with potential. As the coverage of the surface by Si—O—Si bonds increases with increasing potential, the surface becomes increasingly less active and becomes passivated when these bonds fully cover the surface. Further reaction has to proceed via the breaking of Si—O—Si bonds, which is fast in HF solutions but very slow in KOH solutions. 

Ozanam F., Chazalviel J. N., Radi A. and Etman M. (1992), Resonant and non-resonant behavior of the anodic dissolution of silicon in fluoride media—an impedance study , J. Electrochem. Soc. 139, 2491-2501. 

Searson P, Zhang X (1990) The anodic-dissolution of silicon in HF solutions. J Electrochem Soc 137 2539-2546... 

Kettner C, Reimann P, Hanggi P, Muller F (2000) Drift ratchet. Phys Rev E 61 312-323 Kleimann P, Linnros J, Petersson S (2000) Formation of wide and deep pores in silicon by electrochemical etching. Mater Sci Eng B 69-70 29-33 Kooij S, Vanmaekelbergh D (1997) Catalysis and pore initiation in the anodic dissolution of silicon in HF. J Electrochem Soc 144 1296-1301... 

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 ... 

However, prolonged electrochemical treatment in anhydrous methanol electrolyte may also lead to the electrochemical dissolution of bulk silicon ... 

When the surface is completely covered by an oxide film, dissolution becomes independent of the geometric factors such as surface curvature and orientation, which are responsible for the formation and directional growth of pores. Fundamentally, unlike silicon, which does not have an atomic structure identical in different directions, anodic silicon oxides are amorphous in nature and thus have intrinsically identical structure in all orientations. Also, on the oxide covered surface the rate determining step is no longer electrochemical but the chemical dissolution of the oxide.1... 

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. 

As expected from the anisotropy of chemical etching of Si in alkaline solutions, the electrochemical dissolution reaction shows a strong dependence on crystal orientation. For all crystal orientations except (111) a sweep rate independent anodic steady-state current density is observed for potentials below PP. For (111) silicon electrodes the passivation peak becomes sweep rate dependent and corresponds to a constant charge of 2.4 0.5 mCcm-2 [Sm6]. OCP and PP show a slight shift to more anodic potentials for (111) silicon if compared to (100) substrates, as shown in Fig. 3.4. 

The first models for the electrochemical dissolution process of silicon in HF assumed a fluoride-terminated silicon surface to be present in electrolytes containing HF [Ge6, Du3[. However, by IR spectroscopy it was found that virtually the whole surface is covered by hydride (Si-H) [Ni3[. No evidence of Si-F groups is found in IR spectra independent of HF concentration used [Ch9[. This is surprising insofar as the Si-F (6 eV) bond is much stronger than the Si-H (3.5 eV) bond, and so it cannot be assumed that Si-F is replaced by Si-H during the electrochemical dissolution. This led to the conclusion that if a silicon atom at the surface establishes a bond to a fluorine atom it is immediately removed from the surface. 

The quantum efficiency for solid-state devices, e.g. solar cells, is always below unity. For n-type silicon electrodes anodized in aqueous or non-aqueous HF electrolytes, quantum efficiencies above unity are observed because one or more electrons are injected into the electrode when a photogenerated hole enters the electrolyte. Note that energy conservation is not violated, due to the enthalpy of the electrochemical dissolution reaction of the electrode. 

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