Figure 1 - The space-charge-region-model
Figure 1 - The space-charge-region-model
The important fact for the pore-formation is the focused on the pore-tip by the space-charge-region. Each photogenerated hole can dissolute silicon at
the pore-tip - the pore-walls are effectiv passivated against dissolution.Macropores up to 500 Micrometer without pore-fall-out can be optained with setting
the etch-parameter according to the space-charge-region-model in n-type silicon.
There is a limit for the doping-level of the silicon on n-type. As show in different paper stabil macropore-growth is easyler to aproache in low doped silicon-wafers.
The space-charge-region-model defines no limit for the doping-level. Sometimes break-trough-mechanismen were used to explain this lack in the model.
In an work about random-macropores in n-type-silicon we found that the space-charge-region defines the distance of macropores by using low doped silicon.
The models breaks down at higher doped silicon - see figure below.
Figure 2 - Macropores
Since we find a strong reorganization of pores in the nucleation phase especially in the case when macropore growth is not very stable. i.e. at high doping levels, we have to discuss an additional fea-ture: Not every pore which can grow under the experimental conditions will grow in the stable phase. There is a certain possibility, that although sufficient minority carriers are present (otherwise no pore tip would have been formed), some pores will cease to grow and thus allow neighboring pores to increase their diameter. In consequence, not each minority carrier which reaches a pore tip will lead the a chemical reaction at this tip; there must be a possibility for a carrier to leave the pore tip again and to diffuse to another pore. This possibility can be quantified by a statistical chemical transfer rate and/or a time dependent chemical passivation of the pore tip by an oxide layer, which must be dissolved, before new minority carriers can be consumed.We thus are lead to believe in some passivation mechanism operative during pore etching. The passivation is the hydrogen passivation at the pore-walls.
Figure 3 - The nucleation takes place at {111} pyramides