Macropores on high doped n-type silicon
   
It is possible after the predictions of the current burst model to form macropores on high doped silicon
       
  Mesopores are growing with a lack of an anodic oxide into the substrate. Mesopores are growing on high doped n- and p-type silicon. An example of mesopores on high doped n-type silicon (0.020-0.060 Wcm) - see SEM-micrograph below.  
       
    mesopores  
       
  According to this "current-burst-model", macropores form if the balance between direct dissolution and oxidation in a current burst is within a certain bandwidth, and mesopores, generally observed on highly doped p- and n-Si, are observed if the oxidation component is too small. Accordingly, an enhancement of the oxidizing power of an electrolyte, should have the potential to produce macropores under conditions where otherwise only mesopores occur. Strong oxidizing compontes were added to the electrolyte. The macorpores were etched in the dark n-Si: (0.020-0.060 Wcm).  
       
    macropores on high doped n-Si  
       
  The pore tips shows {111}-planes.  
       
    macropore tip  
       
  The nucleation of the macropores takes place at (111)-pyramids forming in a generally rough surface.  
     
    nucleation  
       
  No backside illumination could be used because the small diffusion length of highly doped Si would prevent the hole diffusion to the front side. The holes needed thus must be generated by electric field effects (avalanche break-through) which is easy in highly doped materials. But avalanche break-through so far has been considered to be prime the effect for mesopore generation and thus should not be seen as a prime reason for macropore formation. According to the current burst model, the prime reason for macropore formation in this (and other) cases is the decreased probability for current bursts on H-passivated surfaces coupled with the smoothing action provided by a minimum of oxidation following direct dissolution, and this accounts at least qualitatively for the experimental observations. Other factors, as, e.g., the shape of the space charge region (SCR), or the supply of holes may influence macropore stability, too, by influencing the nucleation probability of current bursts, but the interplay between H-passivation and oxidation must be seen as the prime parameters.  
       
  For detail informations see:
CHRISTOPHERSEN, M., CARSTENSEN, J., FÖLL, H., Macropore-formation on highly doped n-type silicon, PSST 2000, Madrid , March 2000. (to be published in Phys. Stat. Sol. (a))