Porous Si: From single porous layers to porosity superlattices

Abstract

Porous silicon has a manifold microscopic structure. Depending on the doping type and level of the substrate used for the anodization, pores with diameters up to 1 μm or down to a few nanometers can be formed [1]. Within this article the discussion will be limited to porous silicon formed on p-type doped substrates. This material has a spongelike microscopic structure which can be observed in transmission electron microscope (TEM) pictures. The typical dimensions of the pores as well as of the remaining silicon crystallites are in the nanometer range. The size of the so-called nanocrystals is of great importance in the view of the quantum confinement model which can explain the luminescence properties of porous silicon [2]. While TEM pictures are very suitable to get a visual impression of the microstructure they are not a good choice to obtain statistical values about the diameters of the silicon nanocrystals. This can be done by means of inelastic light scattering (Raman spectroscopy). The theoretical background mainly based on [3, 4, 5] will be discussed in the following section. Before actually presenting the results achieved by this technique some comments on the experimental conditions will be given.