Quantum confined silicon doped

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. 

The quantum confinement model reasonably explains the formation of crystallites of a few nanometers in size. However, it does not provide an explanation of what determines pore diameter. If quantum confinement, which is not related to doping type and concentration, were to occur, it should also occur on all types of silicon substrates. However, quantum size PS is not found in many types of PS, e.g., the PS formed on n-Si in the dark. 

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