**3.13. High-density plasma deposition of silicon**

Mesoporous silicon structures can be fabricated by deposition of high void density crystalline silicon films using low-temperature high-density plasma. An electron cyclotron resonance (ECR)-PECVD tool with hydrogen diluted silane at about 100°C has been utilized for the preparation of thin films composed of nanoscale silicon columns in a void matrix [53]. Porous layers has been realized in the pressure ranges between 5 and 12 mTorr corresponding to microwave power between 640 and 340 W. Formation of the porous structure stems from low mobility and therefore low diffusion length of the deposition species compared to the average distance between the physisorption sites. The low mobility of deposition species can be attributed to the low substrate temperature as well as the low kinetic energy of the impinging ions

enclosed by the surrounding amorphous silicon matrix. Such structures spoil the available enormous surface area of porous silicon and restrict applications of the material. Indeed, this

**Figure 17.** Schematic illustration of the oblique-angle deposition process and a cross-sectional SEM image of the obtained

Porous Silicon

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

Another method for realization of porous silicon is unidirectional solidification of molten silicon in hydrogen ambient. As schematically illustrated in **Figure 18**, high-purity silicon pieces are placed in an alumina crucible and inductively heated in the hydrogen atmosphere. The setup is then tilted and the molten is poured into a specifically designed mold [58]. The sidewalls of the mold are made of molybdenum and its bottom is made of copper which can be cooled down by circulated water. Due to the significant difference of thermal conductivities and heat capacities of the sidewalls and the bottom of the mold, unidirectional solidification can take place in the vertical direction. Both melting and solidification steps were performed in a constant pressure of hydrogen. It is believed that hydrogen dissolves in the molten and generates pores during solidification. Porosity and average pore size of the ingot change from 10 to 34% and 100 to

fabrication route has mainly been used to prepare antireflection coatings in solar cells.

300 μm, respectively, depending on the hydrogen pressure alteration from 1 to 0.1 MPa.

**Figure 18.** Setup used for preparation of porous silicon by unidirectional solidification of molten silicon [58].

Using unidirectional solidification of molten silicon in hydrogen atmosphere, porous cylindrical silicon ingots, 25 mm in diameter and about 30 mm in height, with elongated spheroidal "lotus-type" pores have been fabricated (**Figure 19**). The average pore size in the obtained ingots is at least two orders of magnitude greater than that of porous silicon structures real-

**3.15. Unidirectional solidification of molten silicon**

porous silicon layer [56, 57].

ized by other fabrication routes.

**Figure 16.** Schematic illustration of the laser ablation setup.

of the ECR plasma (< 50 eV) [53]. Moreover, H radicals of the plasma also have a major role in the formation of the structure. Although the deposition process is performed at low temperatures, ECR plasma has high concentrations of H radicals which promote the crystallinity of the structure. The strained regions between the crystallites cannot withstand the severe H radical etching, leading to the formation of a void volume.

Using high-density plasma deposition, uniform porous layers up to 1 μm can be realized on silicon wafers, glass and plastic substrates as well as metal foils. Increasing the plasma power lowers the void fraction of the layer and decreases the porosity. Similarly, increasing the substrate temperature also decreases the porosity. By controlling substrate temperature and microwave power, porosities between 25 and 90% can be achieved by this bottom-up method. The obtained porous silicon layers show luminescence in the red portion of the spectrum and are sensitive to water vapor.
