P-type macropores in Si

P-type macropores are stright, deep and smooth pores
growing along definite crystallographic directions.

1.2.2. P-type macropores in Si

Macropores in p-type Si were found by Propst and Kohl four years later after the macropores were discovered in n-type Si by Lehmann and Föll. They used the so called organic electrolytes, i.e., HF dissolved in acetonitril, dimethylformamid (DMF) or other organic solvents. The aim of using organic electrolytes is to reduce the amount of water from the electrolytes, i.e. the electrolytes have to be less oxidizing due to the fact that a strong oxidizing electrolyte combined with sufficient holes supplied by the bulk leads to electropolishing and not to pore formation. An example of randomly nucleated pores on p-type Si is presented in Figure 1.7.

Examples of macropores in p-type (100) Si; a) Top view. The pores are randomly nucleated; b) Cross section view;

Figure 1.7: Examples of macropores in p-type (100) Si; a) Top view. The pores are randomly nucleated; b) Cross section view;

As was mentioned above, the SCR model cannot explain the pore formation in p-type substrates. Therefore, new models were developed which can explain the anisotropic dissolution during macropore formation, i.e. explain the difference in dissolution rates at pore tips and pore walls in spite of the fact that at the tips as well as on the pore walls an equal amount of holes is available. A way for explaining the macropore formation in p-type Si could serve the difference in surface passivation against dissolution at the pore walls and tips by the species in the electrolyte. This would mean that the passivation at pore tips is less effective than at pore walls, thus allowing macropore growth. The difference in passivation effectiveness at pore tips and pore walls can be explained by assuming that pore walls and pore tips expose different crystallographic facets, thus the passivation should be also different due to the different number of atoms per unit area on different facets.

One of the models which is based on the passivation behaviour of different facets is Current Burst Model (CBM) [9, 10, 11]. CBM was developed as a general model for Si and other semiconductors like III-Vs. CBM assumes that the passivation of pore walls is accomplished by hydrogen species from the electrolyte. Therefore, this passivation mechanism is also called H-passivation. H-passivation is not a measure of standard chemistry but has nonetheless a well defined meaning within the CBM: it denotes the degree and speed with which a given electrolyte can remove interface states in the band gap of Si by covering a freshly etched surface with hydrogen.

Unfortunately, in spite of the existence of several models explaining the pore formation in p-type semiconductors, all of them are qualitative and there are no clear and simple rules for pore growth in p-type Si like Lehmann's formula for n-type. Nevertheless, there are some well accepted rules:

  • For example, the doping of the substrate is a very important parameter. It was observed that increasing the doping level of the substrate acts to some extent like increasing the potential, i.e. reduces the probability for obtaining macropores;
  • Also, for the formation of macropores in p-type substrates the dielectric constant of the electrolyte must be considered. The dielectric constant determines to which degree HF will be dissociated. Electrolytes with a low dielectric constant tend to have a reduced HF "activity" which slows down the dissolution process.
  • The growth direction of "p"-macropores is similar to that of "n"-macropores, i.e. the pores mainly grow along (100) directions.

It is important to note that macropores formed in both n- and p-type silicon substrates can find a variety of applications in modern microelectronics.

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