Photoanodic dissolution, silicon

Fig. 8.16. Reaction scheme for photoanodic dissolution of silicon in low intensity limit illustrating the competition between hole capture steps (rate constants k to k ) and electron injection steps (rate constants k to k,). The nominal valence states of the silicon intermediates are indicated. The final product Si(IV) is the soluble hexafluorosilicate species.

The photoanodic dissolution of n-silicon in acidic fluoride media provides an example of the complexity of multistep photoelectrochemical reactions [33, 34]. The reaction requires the transfer of four electrons, but it is clear that not all of the steps involve photogenerated holes because the photocurrent quantum efficiency is between 2 and 4. The explanation of the high quantum efficiencies is that the initial hole capture step can be followed by a series of steps in which intermediates with low electron affinity inject electrons into the conduction band. These intermediates can be assigned nominal oxidation states as shown in the following scheme. 

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. 

Assuming that the single Si-F ligand at the kink site is sufficiently polar to allow successive reaction of the two Si-Si backbonds with HF, then the overall reaction is consistent with pore formation in n- and p-type silicon where two charges are transferred per silicon atom. For the case of photoanodic dissolution of n-type silicon in NH4F [120-124] the characteristic photomultiplication effect at low light intensities is also consistent with the hole capture/electron injection sequence shown in Fig. 29 a. 

Electron injection from reaction intermediates of the oxidation of n-type semiconductors can be observed as quantum efficiency larger than unity in photocurrent-potential measurements. There are two striking examples in the literature the photoanodic dissolution of n-type silicon in HE solution [91, 92] and of n-type InP in HCl solution [54]. In these cases the quantum efficiency at low light intensity is exceptionally high, four for silicon and two for InP. In the case of Si, this means that only one photon (and thus one hole) is required to dissolve each silicon atom three electrons are injected into the conduction band... 

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