St. Frohnhoff

Formation techniques for porous silicon superlattices

Abstract Porosity superlattices (SLs) are a new type of Si-based heterostructures which exhibit a periodical variation of the porosity in depth. These structures have been investigated by transmission electron microscopy. Different formation techniques for porous Si SLs will be presented: SLs on p-type doped Si were formed by periodic variation of the formation current density or by using periodically doped Si substrate layers. An influence of the substrate quality on the interface roughness has been found. On n-type Si the illumination intensity has been periodically changed during the etching process which leads to a periodical variation in the macropore radii. An explanation for this dependence is suggested.

An Extended Quantum Model for Porous Silicon Formation

Porous silicon formed by anodization of a p-type silicon substrate is characterized by a distribution of crystallites with diameters smaller than about 100 A. The corresponding size distributions obtained from Raman measurements show distinct peaks which are explained for the first time by the tunneling of holes through crystallite barriers during the formation process of porous silicon. The theoretical description is based on quantum mechanical calculations of the tunneling probability of the holes through small crystallites into the electrolyte. This tunneling probability shows oscillations as a function of crystallite size which are comparable to the structures observed in the size distributions. The calculations presented provide a deeper understanding of these size distributions and of the basic formation mechanism of porous silicon

Characterization of supercritically dried porous silicon

Porous silicon (PS) has been formed by electrochemical etching of p-type silicon. During drying in air the sponge-like structure of the porous layers is exposed to capillary forces which will partially destroy the microstructure of highly PS. One possibility for avoiding the partial structural collapse is by supercritically drying the PS. This technique is already known from the formation of highly porous aerogels and has now been applied to PS. Porosities of 90% can be achieved. These highly porous layers were investigated using photoluminescence, Raman, reflectance and X-ray photoemission spectroscopy. The porosity was determined by gravimetric measurements. The effect of the drying process on the properties of PS was also studied.

Influence of the formation conditions on the microstructure of porous silicon layers studied by spectroscopic ellipsometry

Abstract The influence of various parameters of the electrochemical process on the resulting microstructure of porous silicon layers was studied by spectroscopic ellipsometry. The first parameter, the etching time, is known to determine the layer thickness. In addition, although the current density is kept constant, the microstructure of the layers changes, at least in the top part probed by the light, with increasing etching time. A change in the microstructure was also found upon leaving the samples in the electrolyte for a certain time interval after the end of the electrochemical process. The main effect in both situations seems to be an increase in porosity of the layers. For long etching times or applied illumination with short wavelength light during the etching process the spectra indicate that the diameters of the crystals comprising the silicon skeleton are reduced.

Porous Si: From single porous layers to porosity superlattices

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.

The influence of microelectronic processing steps on the properties of porous Si-layers

In the view of possible applications of porous Si layers in optoelectronic devices it is necessary to study the influence of microelectronic processing steps on the properties of porous Si films. Therefore, changes due to photolithography steps were investigated. The influence of contact formation was studied for evaporated and electroplated Au contacts. In addition the preparation of small area porous layers was investigated where the lateral homogeneity of the porous films was controlled by micro infrared spectroscopy.