**3.18. Laser-induced silane decomposition**

Laser-induced decomposition of silane has been used to prepare high porosity mesoporous structures composed of silicon nanocrystals [68]. Nanocrystals have been synthesized by pulsed CO2 laser-induced decomposition of SiH4 in a complex apparatus illustrated in **Figure 21**. The setup consists of three vacuum chambers: a source chamber containing the flow reactor, a differential chamber encasing the chopper for size selection, and an ultrahigh vacuum (UHV) chamber that contains the time-of-flight mass spectrometer. The "source chamber" is indeed a laser-driven CVD reactor where silicon clusters are produced by pulsed CO2 laser pyrolysis of SiH4 . The reaction products are extracted perpendicularly to both the gas flow and the CO<sup>2</sup> laser beam through a conical nozzle projected into the reaction zone. The extracted nanocrystals are then skimmed into a low-pressure vacuum chamber (differential chamber); they form a "molecular beam" of noninteracting clusters. The beam passes through the slits of a rotating chopper, synchronized with the pulsed pyrolysis laser and can be vertically moved from outside. The chopper is used to preselect a small portion of the initially broad cluster pulse to narrow the cluster size distribution. Immediately behind the chopper, is a sample holder that can be moved into the cluster beam to collect the nanocrystals. Thus, silicon particles with preselected size can be deposited on the substrate. If the sample holder is moved out of the path of the beam, the silicon nanoparticles enter the UHV detector chamber equipped with a time-of-flight mass spectrometer [68, 69]. Pores are automatically formed when nanocrystals are collected by the substrate. This fabrication route has not attracted much attention due to the complexity of the apparatus and poor adhesion of the porous layer.

## **3.19. Electrodeposition**

**Figure 19.** Cross-sectional SEM micrograph of a typical porous silicon ingot prepared by unidirectional solidification of

Reduction of porous silica is another bottom-up route to synthesize porous silicon structures [59, 60]. This preparation method is utilized when inexpensive products are required as lowcost silica feedstocks like sand, living plants, and agricultural wastes can be used. As silica reduction generally occurs by magnesium vapor at moderate temperatures, this fabrication

400− ⎯

However, the reduction can also takes place using other reducing agents, e.g., lithium, sodium, aluminum, and calcium [61–64]. The reaction pathway of Eq. (10) is then followed by acid leaching to remove the metal oxide byproduct and obtain porous silicon. The pore size and morphology of the synthesized porous silicon strongly depends upon morphology and moisture content of the feedstock, thermal budget of the process, and byproduct size distribution. In addition to metallothermic porous silica reduction, porous silicon synthesis has also

Porous silica reduction is a simple route for the realization of porous silicon. All classes of porosity can be achieved by this method. However, it is important to precisely control the process to prevent formation of magnesium silicate instead of silicon. Sintering and reintroduction of oxygen, espe-

Using dealloying of Al-Si eutectic system, thin layers of mesoporous silicon with ultrahigh density of cylindrical pores with an average diameter of 5 to 13 nm have been realized [66]. The Al-Si system is deposited by sputtering of an Al0.56Si0.44 target with Ar pressure of 0.1 Torr at low temperatures (below 100°C). Cylindrical pores are generated as a result of combination of nanoscale phase separation of the Al-Si system during deposition and subsequent removal of Al cylinders

<sup>600</sup>⟶°C Si(*s*) <sup>+</sup> 2MgO(*s*) (10)

process is called magnesiothermic reduction of silica (Eq. (10)).

(*s*) <sup>+</sup> 2Mg(*g*) ⎯

been performed by mechanochemical reduction of SiO powder [60].

cially in highly porous products, are also issues [63, 65].

molten silicon [58].

**3.16. Porous silica reduction**

24 Porosity - Process, Technologies and Applications

SiO2

**3.17. Dealloying**

Porous silicon structures can also be synthesized by galvanic deposition, also known as immersion plating. Addition of sodium hexafluorosilicate Na<sup>2</sup> SiF6 to dilute hydrofluoric acid concen-

**Figure 20.** Fabrication of cylindrical pores by dealloying of Al-Si eutectic system and subsequent Al removal (The porous amorphous silicon structure can be crystallized by performing a thermal annealing process.) [67].

**Figure 21.** Schematic illustration of the apparatus used for realization of porous silicon layers by laser-induced decomposition of silane [69].

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 electrodeposition process is as follows:

$$\mathrm{3SiF\_6^{2-}} + \mathrm{4Al} + \mathrm{6F^-} \to \mathrm{3Si} + \mathrm{4AlF\_6^{3-}} \tag{11}$$

typical porosities up to 30%. The limited porosity is the inevitable consequence of the pressure and high temperature of the sintering stage. Indeed, as sintering enhances the strength and density of the material, porosity significantly deceases. However, it is possible to further increase the porosity of the sinter by performing an extra stage of electrochemical or stain etching [73]. All classes of porosity can be realized by mechanical synthesis. Porosity of the sinter can be tailored by tuning compaction pressure and sintering temperature. The method is capable of forming large porous silicon matrices which is impossible by top-down fabrication routes [74]. However, in addition to the limited porosity, lack of control over the morphology of the pores is the main disadvantage of mechanical synthesis. Moreover, high defect density and a great number of impurities of the mechanically made matrix are also important issues. The high compressive stress during ball-powder-ball collisions in the ball milling stage changes the Si─Si bond length, inflicts amorphization, and introduces a great number of defects [75]. Although the structure recrystallizes and most defects are removed during high-temperature treatment, the final product has still significantly higher density of defects and unwanted

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

a rapid thermal annealing setup and exposed to high temperature (770°C) for 30 s. As a result

is deposited on the front-side of the wafer. The structure is then placed in

/15 nm amorphous

Porous Silicon

27

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

impurities in comparison to other porous silicon fabrication methods.

and pattering the back-side oxide layers, a three layer stack of 20 nm SiO<sup>2</sup>

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

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

silicon/20 nm SiO2

in which aluminum reduces Si F6 2− to silicon and forms a porous film on the substrate surface. 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 a poor adhesion to the substrate.

#### **3.20. Mechanical synthesis**

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 typical porosities up to 30%. The limited porosity is the inevitable consequence of the pressure and high temperature of the sintering stage. Indeed, as sintering enhances the strength and density of the material, porosity significantly deceases. However, it is possible to further increase the porosity of the sinter by performing an extra stage of electrochemical or stain etching [73].

All classes of porosity can be realized by mechanical synthesis. Porosity of the sinter can be tailored by tuning compaction pressure and sintering temperature. The method is capable of forming large porous silicon matrices which is impossible by top-down fabrication routes [74]. However, in addition to the limited porosity, lack of control over the morphology of the pores is the main disadvantage of mechanical synthesis. Moreover, high defect density and a great number of impurities of the mechanically made matrix are also important issues. The high compressive stress during ball-powder-ball collisions in the ball milling stage changes the Si─Si bond length, inflicts amorphization, and introduces a great number of defects [75]. Although the structure recrystallizes and most defects are removed during high-temperature treatment, the final product has still significantly higher density of defects and unwanted impurities in comparison to other porous silicon fabrication methods.
