Participation of Bands and Rate-Limiting Processes

The conduction of the charge carriers in the space charge layer during anodization of silicon can occur either by electron injection into the conduction band or by hole capturing from the valence band. For j9-type silicon, the anodic reactions in HF 

The eharge transfer meehanism aeross the interfaee barrier layer is different for lowly doped and heavily doped p-type silieon. For lowly doped p-type siheon the proeess is by thermal emission of holes to go over the barrier layer whereas it is by Zener tunneling for heavily doped materials. For n-Si the eonduetion band proeesses depend on doping density and on illumination intensity. For heavily doped n-Si it is by Zener tunneling and the i-V eurve is identical to that forp-Si. For moderately or lowly doped -Si in the dark the reaetion is limited by the minority holes, which are required to initiate the dissolution proeess. Significant dissolution of n-Si can proceed when a large number of holes are generated by illumination. 

TABLE 5.6. Participation of Conduction Band and Valence Band in Carrier Conduction 

Only Che electrochemical part of the reactions is considered. The chemical part, which is responsible for almost 100% of the dissolution reaction at OCP in KOH solutions and about half at OCP V Pp in HF solutions, is not considered. Contribution from minority carriers in the dark is neglected. This table is constructed based on the results presented in Section 3-5 in this chapter and Refs. 8, 22, 34, 73, 74, 108, 207, 269, 695, 700, and 873. 

In KOH solutions the rate-limiting process at OCP is of chemical nature, i.e., only rg is involved. Electrochemical processes, that is, r4 and rs, are increasingly involved 

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