Silicon doping, pore size

The amorphous silica matrixes are porous network structures that allow other species to penetrate [44]. Thus, the doped dye molecules have the ability to react with targets. However, the reaction kinetics is significantly different than the molecules in a bulk solution. In the synthesis of DDSNs, commonly used silicon alkoxides including TEOS and TMOS have tetrahedron structures, which allow compact polycondensation. As a result, the developed silica nanomatrix can be very dense. The small pore sizes provide limited and narrow pathways for other species to diffuse into the silica matrix. 

Figure 8.28 shows the pore size distributions for the PS formed on highly doped / -Si at different current densities." For the PS formed on heavily doped silicon, the pores at a given HF concentration have a narrower distribution at lower current at a given current density, the distribution is narrower at lower HF concentrations. A bimodal distribution of pore diameter is gena-ally associated with two-layer PS on lowly doped / -Si and iUmninated n-Si. The PS formed on p-Si has also been found to have a bimodal distribution of particles sizes small spherically shaped particles with a diameter of a few nanometers and large cylindrically shaped particles oriented with their axis perpendicular to the surface.The distribution of pore size with multiple peaks may be attributed to the fact that PS may have a surface micropore layer and smaller branched pores. Due to the hierarchical structure of the branched pores, the distribution of pore diameters for highly branched PS is found to be fractal-like. ° ... 

The model of Wehrspohn et al. was soon proven to be invalid as macro PS was also found to occur in electrolytes having much higher resistance than the silicon substrate.Alternatively, Lehmann and Ronnebeck postulated that the formation of macro PS on lowly doped p-Si is due to the dominant effect of thermionic emission which is sensitive to barrier height rather than barrier width. However, this does not explain how the two PS layers with a difference of several orders of magnitude in pore size could be determined both at the same time by the space charge layer nor what governs the dimension of the macropores. 

Figure 1. Plan view scanning electron microscopy image of porous silicon. The dark, circular regions are the air pores and the interconnected brighter region is the silicon matrix. The pore size and porosity can be changed by modifying the formation conditions and silicon doping.

Fig. 2 Comparison of characteristic macropore sizes on p-Si in the current-line-driven regime, when changing current density for a substrate resistivity of 100 Q cm (a) and silicon doping for an applied current density of 10 rtiA/cm (b) (After Chazalviel et al. 2002). Triangles refer to the wall width and diamonds to the pore diameter the closed (open) symbols refer to the data obtained in 35 % (25 %) ethanolic HR The solid lines refer to the theoretical prediction (Chazalviel et al. 2002) for the pore diameter, and the dotted line is two times the space-charge width X...

In this handbook we adopt the lUPAC definition of micropores, namely, that this label only refers to extremely small pores of diameter less than 2 nm. We now know that with anodization (see handbook chapter Porous Silicon Formation by Anodisation ), average pore size is lowered when using (a) lightly doped p-silicon, (b) low current densities, and (c) highly concentrated (>40 wt%) hydrofluoric acid. This last parameter has been particularly important in the literature with regard micropore generation. 

Three comprehensive books on porous silicon have been published, wherein detailed information can be found related to silicon anodization (Canham 1997 Lehman 2002 Sailor 2012a). The topics covered include dissolution chemistries and the dependences of porosity, pore morphology, and pore size distribution on various parameters (e.g., wafer type/doping, electrolyte composition, current density, time) additionally, different types of electrochemical cells are discussed (Lehman 2002 Sailor 2012a), as well as some of the more practical aspects related to anodization (Sailor 2012a e.g., wafer preparation, equipment and instrumentation, health and safety). The reader is referred to these references for essential background reading. 

Fig. 6.10 Pore density versus silicon electrode doping density for PS layers of different size regimes. The broken line shows the pore density of a triangular pore pattern with a pore pitch equal to twice the SCR width for 3 V applied bias. Note that only macropores on n-type substrates may show a pore spac-...

Low-doped p-type silicon the porous layer is composed of an interconnected network of nanometer-size silicon ligaments with porosities on the order of 40 - 60% and pore dimensions less than 100 A [87, 88, 90, 92, 94, 98, 99]. At the present time, the detailed morphology of these porous structures has not been resolved. 

The quantum confinement model reasonably explains the formation of crystallites of a few nanometers in size. However, it does not provide an explanation of what determines pore diameter. If quantum confinement, which is not related to doping type and concentration, were to occur, it should also occur on all types of silicon substrates. However, quantum size PS is not found in many types of PS, e.g., the PS formed on n-Si in the dark. 

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