macropore-formation on n-type-silicon
   
The space-charge-model
The space-charge -model from V. Lehmann and H- Föll (1990) is the fundamental model for the macropores in n-type-silicon.
 
 
  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.

The space-charge-region-model
                                    
Macropores up to 500 Micrometer without pore-fall-out can be optained with setting the etch-parameter according to the space-charge-region-model.
     
       
The limits of the space-charge-model on 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.
       
       
The effective macropore-wall-passivation
  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.
       
       
The anisotropic macropore-growth
  We found that the <100>- and <113>-directions are growth directions of the macropores in n-type-silicon (presented in the paper form S. Rönnebeck et al.). The growth-directions correspond with the kinetics of the hygrogen-passivation. The (100) has got the slowst kinetic - it took a long time to passivate. The TEM-investigations shows (111)-facetts at the pore-walls - macropore-growth is an anisotropic process.