**4.1. Optic and optoelectronic applications**

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

layers cannot exceed 7% and the pore sizes ranges only between 5 and 55 nm.

heated up to 450°C at the presence of ammonium chloride NH4

applications due to their solubility in water at room temperature.

**3.22. Using sacrificial template**

28 Porosity - Process, Technologies and Applications

ZnCl2

**4. Applications**

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

As depicted in **Figure 23**, firstly, an array of ZnO nanowires is grown on the substrate. The specimen is then placed in a CVD reactor in which silane is used to form a silicon coating around the ZnO nanowires. The thickness of the over-coating silicon shell, which can be determined by the duration of the silane exposure, is extremely crucial for the formation of porous silicon. Indeed, only silicon sidewall thicknesses of about 12 nm or less finally lead to porous nanotubes. The temperature of the CVD process determines the crystallinity of the silicon over-coating layer. Deposition at 500°C results in amorphous silicon, while formation of crystalline shells requires at least 600°C. In order to remove the ZnO template and form silicon nanotubes, the sample is

and decomposes into ammonia and hydrogen chloride gas. The latter reacts with ZnO creating

Due to its high chemical reactivity and rapid oxidation, porous silicon was being utilized for device isolation. By the end of the 1980s, porous silicon had also been used for other purposes like the realization of SOI substrates and the formation of a buffer layer in epitaxial growth of compound semiconductors on silicon substrates. However, it was only after the discovery of

 liquid which is then converted into zinc amide species in the presence of ammonia. If the sidewall would be less than 12 nm, a further thermal annealing process porosifies the nanotubes. Porosification has been attributed to a strain-influenced mechanism [77]. The pore sizes of the obtained structure ranges from 5 to 10 nm. These porous nanotubes can be used for therapeutic

Cl. Ammonium chloride sublimes

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.

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 diffraction gratings are among other optical applications of porous silicon.
