**3.15. Unidirectional solidification of molten silicon**

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

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

Porous amorphous silicon has been fabricated by oblique-angle deposition, aka glancing-angle deposition, using electron-beam evaporation [54, 55] or magnetron sputtering [56]. Obliqueangle deposition is performed by positioning the substrate at a steep angle with respect to the vapor flux in order to achieve geometric shadowing. As schematically shown in **Figure 17**, random growth fluctuation on the substrate surface produces shadow regions where incident vapor flux cannot reach. As deposition continues, areas of larger height variation preferentially grow. Hence, an array of oriented nanorods is formed leaving pores behind. By controlling the vapor flux incident angle during deposition, the porosity can be tailored [57]. The pores of the material realized by oblique-angle physical vapor evaporation techniques are

etching, leading to the formation of a void volume.

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

22 Porosity - Process, Technologies and Applications

are sensitive to water vapor.

**3.14. Oblique-angle deposition**

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

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 realized by other fabrication routes.

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

**Figure 19.** Cross-sectional SEM micrograph of a typical porous silicon ingot prepared by unidirectional solidification of molten silicon [58].
