**3.12. Laser ablation**

sions between 5 and 10 nm. Although the electromagnetic coupling between localized surface plasmon of silver nanoparticles and porous silicon structures promotes the optical behavior of the material, the presence of metallic nanoparticle is undesirable in other application areas.

**Figure 14.** Synthesis and SEM micrographs of porous Si/C composite synthesized by acid washing of Rochow reaction

This method has been introduced in 2005 for the fabrication of luminescent microporous and mesoporous silicon [47, 48]. In contrast to the fabrication routes already discussed in this chapter, plasma hydrogenation is a bottom-up approach. It starts with the deposition of a thin amorphous silicon (a-Si) layer with a thickness of about 200 nm. The amorphous layer is deposited by physical vapor deposition techniques like evaporation [47, 48] or sputtering [49, 50] instead of chemical vapor deposition to increase the number of voids. Later, the specimens are placed in a DC plasma-enhanced chemical vapor deposition (DC-PECVD) setup to be exposed to DC hydrogen plasma as illustrated in **Figure 15**. After the hydrogenation, a thermal annealing step is performed. It is believed that hydrogen radicals of the plasma replace the dangling bonds of the amorphous silicon layer during the hydrogenation step; then, in the annealing

freed from breaking Si─H bonds promotes the rearrangement of silicon atoms of the specimen and a porous crystalline structure is formed [51]. Although porous silicon can be realized by performing one hydrogenation followed by one annealing step, breaking the process duration into three consecutive repetition of hydrogenation and annealing steps provide more controllability over the properties of the synthesized porous layer. This fabrication process is

is exhausted from the specimen. The energy

**3.11. Plasma hydrogenation**

20 Porosity - Process, Technologies and Applications

byproduct [44].

step, the silicon surface is depassivated and H2

Another bottom-up approach to realize porous silicon is collecting laser-ablated silicon clusters [52]. In this technique, a silicon target is irradiated with a pulsed laser beam in a vacuum chamber as illustrated in **Figure 16**. The laser-ablated silicon clusters are collected by placing the substrate in the vicinity of the target where the ablation plume could reach. The substrate is usually heated to increase adhesion of the porous layer. It is also rotated to increase the uniformity of the deposited film. The porosity and thickness of the porous silicon layer can be controlled by the power of the incident laser, the distance between the substrate and the target, and duration of the ablation. This technique has not attracted much attention for chipbased applications due to its incompatibility with standard technology.
