Silicon porosification

A wide variety of metal salts can induce silicon porosification without the addition of any other oxidizer, as their electrochemical potential is sufficiently high to directly inject holes in the valence band of silicon. Those that have been employed for MACE are listed in the following table alongside the nanostructures they have been shown to produce (Table 2). 

Continuous metal films that extend over several tens of microns tend not to sink into the silicon and are unable to form MACE nanostractures, at time impeding MACE altogether as they prevent the silicon underneath from interacting with the solution. In appropriate conditions, silicon porosification can occur underneath and in the vicinity of extended metal films (Geyer et al. 2012). 

Table 1 Silicon porosification via magnesiothermic reduction for different applications ...

Korotcenkov G, Cho BK (2010b) Silicon porosification state of the art. Crit Rev Solid State 35(3) 153-260 Korotcenkov G, Cho BK (2011) Instability of metal oxide-based conductometric gas sensors and approaches to stability improvement. Sens Actuators B 156 527-538... 

Korotcenkov G, Cho BK (2010b) Silicon porosification state of the art. Crit Rev Solid State Mater Sci 35(3) 153-260... 

The as-prepared PS layers were firstly investigated with these complementary methods in order to achieve a complete image of the large internal area chemical map where about 20 % of the silicon atoms are located, looking forward to accomplish a general model for porosification process and also a mechanism for the most renowned property of this material, photoluminescence (see chapter Photoluminescence of Porous Silicon ). Table 2 provides example of the compositional studies of freshly prepared and passivated/oxidized PS. 

Protons 2 MeV (previous to porosification) Control of refiectance in porous silicon microcavities Mangaiyarkarasi et al. (2007)... 

The porosification of the surface of a silicon wafer is generally referred to as anodization and occurs when the wafer is anodically biased in a fluoride-based electrolyte solution. The most commonly used electrolyte component is hydrofluoric acid (HF), with ammonium fluoride (Kuhl et al. 1998) being less common. The degree of anodization is defined by the layer formation rate and porosity and, together with pore morphology, depends on wafer type and resistivity, the applied current density and time, and the electrolyte composition (HF concentration, with or without additives). Secondary parameters include electrolyte temperature and pH. 

Meltzer S, Mandler D (1995) Study of silicon etching in HBr solutions using a scanning electrochemical microscope. J Chem Soc Faraday Trans 91 1019-1024 Menna P, Difrancia G, Laferrara V (1995) Porous silicon in solar-cells - a review and a description of its application as an AR coating. Sol Energy Mater 37 13-24 Mills D, Kolasinski KW (2004) Laser-etched silicon pillars and their porosification. J Vac Sci Technol A 22 1647-1651... 

To prepare porous silicon microarrays in the Laurell group, 380 pm-thick <100> silicon wafers were utilized. The p-type boron-doped wafer with a resistivity of 10-15 Qcm was mounted in an in-house-made electrochemical etch cell. The porosification was made in an electrolyte solution of HF/DMF 1 10, and the wafer was illuminated by a 100 W halogen lamp at a distance of 10 cm. A current density of 2 mA/cm was applied for 1 h. The macropores in the porous layer produced for protein microarrays typically measured 0.5-1.5 pm with a fine side branching network of nanopores (Ressine et al. 2005). 

Nanoscale porosification has been shown capable of introducing new functionality to silicon examples are its visible luminescence and its medical biodegradability (Canham 1997). There are however other properties where nanostructuring alone has not dramatically changed silicon... 

Fig. 26.5 The influence of HF concentration in electrolytes for porosification on the maximum sensitivity of (1) poly silicon and (2) SiC (d-400 to 500 nm) to humidity the sensitivity was measured for a relative humidity (RH) change from 10 to 90 % RH (Adapted with permission from Connolly et al. 2002, Copyright 2002 Elsevier)...

For example, Sharma et al. (2007b) demonstrated that texturizing a silicon surface before porosification is a simple and effective way to form highly porous, highly luminescent, thick films of PSi with reduced stress, improved stability, and superior mechanical properties. Good results may also be obtained by partial oxidation of porous silicon. This approach was discussed in Chap. 22 (Vol.2). Fiirjes et al. (2003) found that, after partial oxidation, the sensitivity of both the surface... 

The production of pSi microparticles generally starts with the porosification of a silicon wafer in an apposite etch tank (Figure 11.4). The tank is composed of HF-resistant materials such as polytetrafluoroethylene or aluminum oxide. The wafer is placed in the tank with the frontside facing the etch solution and the rearmost side in contact with the anode of a power supply. If no backside illumination of the wafer is required, a metallic layer is deposited over the entire wafer backside to provide a better current uniformity otherwise, an annular contact along the wafer edge is employed. As an alternative, a backside electrolytic solution can be used to obtain a better contact, but at the expense of flexibility [12, 19]. 

---