**3.14. Oblique-angle deposition**

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

**Figure 17.** Schematic illustration of the oblique-angle deposition process and a cross-sectional SEM image of the obtained porous silicon layer [56, 57].

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