Microporous Silicon

Illumination of a microporous silicon layer during anodization changes the PL spectrum significantly, as discussed in Section 7.4, and may also be applied for structuring of microporous layers [As2, Dol]. 

The smallest pores that can be formed electrochemically in silicon have radii of < 1 nm and are therefore truly microporous. However, confinement effects proposed to be responsible for micropore formation extend well into the lower mesoporous regime and in addition are largely determined by skeleton size, not by pore size. Therefore the IUPAC convention of pore size will not be applied strictly and all PS properties that are dominated by quantum size effects, for example the optical properties, will be discussed in Chapter 7, independently of actual pore size. Furthermore, it is useful in some cases to compare the properties of different pore size regimes. Meso PS, for example, has roughly the same internal surface area as micro PS but shows only negligible confinement effects. It is therefore perfectly standard to decide whether observations at micro PS samples are surface-related or QC-related. As a result, a few properties of microporous silicon will be discussed in the section about mesoporous materials, and vice versa. Properties of PS common to all size regimes, e.g. growth rate, porosity or dissolution valence, will be discussed in this chapter. 

The hardness, defined as the resistance to plastic deformation, of microporous silicon decreases with porosity p from the bulk silicon value of about 11.5 GaP to values around 4 GaP for porosities in the order of 75% following a (1-p)2/3 dependence. For porosities above 75% a further decrease in hardness is observed. The hardness of PS formed on highly doped p-type substrates is found to be somewhat less than that observed for low doped substrates, which may be caused by the more columnar structure of meso PS [Du5]. 

The absorption of water in the microporous silicon network was evaluated by exposing the porous samples to HzO and DzO vapor at low pressure (10 s Torr). FTIR spectral analyses revealed that water dissociates at the PS internal surface to SiH (SiD) and SiOH (SiOD). Upon annealing to 350°C, Si-O-Si is formed from the SiOH (SiOD) groups while H2 (D2) is desorbed [Gu3]. 

An extension of this QC model, including tunneling probabilities between the confined crystallites and the bulk, has been developed [Fr6]. The QC model for microporous silicon formation, however, is still qualitative in character, and a quantitative correlation between anodization parameters and the morphology and properties of the porous structure is at yet beyond the capability of the model. 

The broadening of the characteristic peaks of the silicon XRD signal provides information about stress and size of the crystallites. Figure 7.4 shows the diffraction pattern of microporous silicon powders scraped from p-type Si electrodes and of a bulk silicon powder sample. The peak broadening increases with increasing formation current density. For low formation current densities a superposition of... 

Po2]. For microporous silicon a more complex time behavior extending well into the ms range is observed, as shown in Fig. 7.11b. The experimentally observed dependence of intensity P on time t has been fitted to an exponential [Ho3, Fi4, MalO] or a stretched-exponential function according to Eq. (7.4) [Ka6, Pal2, Ool]. 

The minute network structure of microporous silicon is between the two extremes of a single atom and a large crystal. A crystallite of a few hundred silicon atoms is large enough to have a rich electronic band structure but is still small enough to show an increase in the energy of an electron-hole pair (exciton) due to... 

It is possible to draw some conclusions about the physical properties of microporous silicon from the results of the theoretical studies ... 

The cross-section of a macropore may have all shapes between a circle and a four-pointed star, as shown in Fig. 9.12a-e. In addition the pore walls are covered with a microporous silicon layer, as shown in Fig. 9.12h, which makes the determination of AP difficult. In most cases, however, the approximation of the pore cross-section by a square of size d is found to be sufficient. Under this assumption and for a square pattern of pitch i, as shown in Fig. 9.15 a, d becomes simply ... 

Microporous silicon is suitable for sacrificial layer applications because of its high etch rate ratio to bulk silicon, because it can be formed selectively, and because of the low temperatures required for oxidation. PS can be formed selectively if the substrate shows differently doped areas, as discussed in Section 4.5, or if a masking layer is used. Noble metal films can be used for masking as well as Si02, Si3N4 and SiC. Oxidation conditions are given in Section 7.6, while the etch rates of an etchant selective to PS are given in Fig. 2.5 b. 

So far, researchers interested in this topic have had to choose either monographs that deal with the electrochemistry of semiconductors in general or recent editions that deal with special topics such as, for example, the luminescent properties of microporous silicon. The lack of a book that specializes on silicon but which gives the whole spectrum of its electrochemical aspects was my motivation to write the Electrochemistry of Silicon. 

Bao, Z. H., Weatherspoon, M. R., Shian, S., etal, Chemical reduction of three-dimensional silica micro-assemblies into microporous silicon replicas. Nature 2007, 446, 172-175. 

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