**3.21. Annealing of ultrathin films of amorphous silicon**

trated with nitric acid solution leads to galvanic deposition of mesoporous silicon layers on metallic substrates like pure aluminum or aluminum alloy [70]. The reaction pathway of this

**Figure 21.** Schematic illustration of the apparatus used for realization of porous silicon layers by laser-induced decom-

<sup>2</sup><sup>−</sup> <sup>+</sup> 4Al <sup>+</sup> <sup>6</sup> <sup>F</sup><sup>−</sup>→3Si <sup>+</sup> <sup>4</sup> AlF6

The obtained porous layer has an amorphous structure with pore sizes between 3 and 8 nm [70]. In addition to the limited pore size achievable by electrodeposition, this bottom-up synthesis has a slow rate. Indeed, a 12-μm-thick layer is grown in 6 h. Additionally, the layer has

Porous silicon structures can also be formed by compacting and sintering of silicon powder [71]. This fabrication method usually consists of a top-down stage in which silicon powder is formed and a bottom-up pressing process followed by sintering. Silicon powder is usually synthesized by grinding monocrystalline silicon wafers or polycrystalline silicon pieces by a high-energy ball mill. The milling reactor and balls can be made of either hardened steel or ceramic materials. The duration of the milling process might vary from several minutes to a few days depending on the desired particle size, degree of amorphization, agglomeration, and stress of the synthesized powder. Although high-energy ball milling is now extensively used to prepare silicon powder, the material can also be synthesized by precipitation of silicon from silane, especially when high purity powder is required [72]. Silicon powder is then pressed so that a porous green body is formed. The typical compaction pressure varies between 50 and 1000 MPa and uniaxial or isostatic pressing can be applied [73]. As milling and pressing stages generate highly defective silicon matrix, high-temperature treatment is necessary to release the strain and remove the defects. This high-temperature treatment also promotes a transition from amorphous to crystalline structure. The following sintering stage leads to porous matrices with

<sup>3</sup><sup>−</sup> (11)

to silicon and forms a porous film on the substrate surface.

electrodeposition process is as follows:

2−

3Si F6

26 Porosity - Process, Technologies and Applications

in which aluminum reduces Si F6

position of silane [69].

a poor adhesion to the substrate.

**3.20. Mechanical synthesis**

Ultrathin low porosity silicon membranes can be fabricated by rapid thermal annealing of predeposited ultrathin amorphous silicon layers [76]. The fabrication process is schematically shown in **Figure 22**. It starts with thermal oxidation in which 500 nm layers of SiO2 are grown on both sides of the wafer. These oxide layers are not required for porous silicon formation and grown so that the membrane can be realized. After removing the front-side and pattering the back-side oxide layers, a three layer stack of 20 nm SiO<sup>2</sup> /15 nm amorphous silicon/20 nm SiO2 is deposited on the front-side of the wafer. The structure is then placed in a rapid thermal annealing setup and exposed to high temperature (770°C) for 30 s. As a result

**Figure 22.** Fabrication process of porous nanocrystalline silicon membranes for ultrafiltration [76].

strong photoluminescence in porous silicon that the material attracted broad attention. Since then porous silicon has been used for the fabrication of gas sensors, humidity sensors, biosensors, light emitting structures, optical waveguides, distributed Bragg reflectors, Fabry-Pérot resonators, photonic crystals, flat panel displays, optical and acoustic filters, ultrasound generators, and many other devices. Even though optoelectronics has remained the main research area of porous silicon, recently, the material has found application in other areas like medicine, diagnostics, cosmetics, consumer care, and nutrition. In contrast to the conventional chip-based applications, these new areas rely on porous silicon powders and independent structures. In this section, we briefly discuss the application domains of porous silicon and the reader is encouraged to refer to the literature dedicated to porous silicon applications (for example, see [78]).

Porous Silicon

29

http://dx.doi.org/10.5772/intechopen.72910

Since Canham's report on strong photoluminescence in porous silicon at room temperature in 1990 [79], the material has attracted broad attention. Indeed, most of the knowledge we now have about this material is due to the interest arose from this observation. Although bulk silicon is a poor emitter of light due to its indirect bandgap, quantum confinement effect makes radiative transition possible in porous silicon. Accordingly, light emitting devices, luminescing in the infrared, visible, and ultraviolet part of the spectrum have been fabricated. There has also been success in integrating porous silicon LEDs with electronic components offering

hope for the realization of silicon-based monolithic optoelectronic integrated circuits.

fraction gratings are among other optical applications of porous silicon.

antireflection coatings in solar cells due to its tunable refractive index.

Several optical components have also been realized by porous silicon. For instance, optical waveguides have been prepared using alternating low porosity and high porosity porous silicon layers. In such a structure, light would be trapped inside the low porosity layer which has a higher refractive index in comparison to the adjacent low refractive index layers due to total internal reflection. Based upon the thickness and refractive indices of the layers, the waveguide would support up to several number of propagating modes. Although the light is guided inside the low porosity layer, decaying fields existed in the adjacent layers facilitate coupling of light into and out of the waveguide. Photonic crystals, optical resonators, and dif-

Gas sensing, gettering, lithium-ion batteries, and solar cells antireflection coatings are the main application areas of porous silicon in the field of electronics. The large amount of surface area makes porous silicon a promising candidate for realization of gas sensors. Almost a dozen structures have been proposed for porous silicon gas sensors based on alteration of the electrical and optical characteristics of the material in presence of more than 50 chemical species. Porous silicon has also been utilized as an anode in lithium-ion batteries. These anodes not only have more capacity than conventional anodes used in lithium-ion batteries but also are devoid of mechanical instability due to the expansion/contraction problem of bulk silicon with lithiation and delithiation process. Another electronic application area of porous silicon is gettering, i.e., deportation of impurities from active regions of the electronic devices. The great deal of surface area in porous silicon absorbs the impurities and makes the material suitable as a getter medium. Finally, porous silicon is an appropriate candidate to be used as

**4.1. Optic and optoelectronic applications**

**4.2. Electronic applications**

**Figure 23.** Fabrication of porous silicon nanotubes by using ZnO nanowire templates [77].

of high temperature annealing, nanocrystals nucleate leaving voids behind. Then, the voids span the molecularly thin membrane to create pores. Finally, the membrane is fabricated by anisotropic etching of the back-side of the wafer with ethylenediamine pyrocatechol (EDP) and removal of the protecting oxide layers. This method can only be used for the fabrication of ultrathin layers of porous silicon which is applicable for ultrafiltration. The porosity of the layers cannot exceed 7% and the pore sizes ranges only between 5 and 55 nm.
