**1. Introduction**

The advancement of nanotechnologies has made possible the development of new applications in all fields. In particular, the nanostructuring of materials has paved the way toward new concepts. Important efforts have been dedicated to metallic nanoparticles, thanks to their interesting properties in comparison to bulk materials. Specially, they are widely studied to be used for many purposes in various applications: passivation of silicon for photovoltaics [1], plasmonics [2, 3], and bionanotechnology [4]. Metallic nanoparticles are also used as catalyst for silicon nanowires' (SiNWs) growth by top-down [5, 6] or bottom-up approaches [7].

Silicon nanowires' properties, especially their diameter, could be tuned by controlling the metallic particles properties. In the literature, many metallic nanoparticles are reported as catalysts, such as Au [8], Ti [9], Al [10], Cu [11], Ga [12], and Pt [13]. During last years, indium is considered a very interesting catalyst because it forms a low temperature eutectic with silicon (157°C) and it induces shallow defects as it acts as a p-type dopant encouraging its use as catalyst. An important issue in SiNWs' synthesis is the indium catalyst elaboration.

Indium nanoparticles could be elaborated by different techniques such as vapor deposition technique [14], electrochemical reduction [15], chemical reduction of salts [16], laser ablation [17], and reduction of indium-tin oxide or indium layers by hydrogen or helium plasma [18–20]. Iacopi et al. have obtained indium particles with diameter range of 40–80 nm by electrodeposition on silicon substrate from an aqueous solution (InCl3, KCl, and HCl) [21]. Kumar et al. have reported on the growth of indium droplets obtained with an average diameter of 90 nm by indium evaporation followed by annealing at 300°C during 5 min [22]. The obtained particles' properties are closely related to experimental parameters such as the precursor's concentration, the plasma flow rate, the exposition duration and the substrate temperature, or the annealing parameters.

In this chapter, we report on an ex situ formation of indium particles to be used as catalyst for SiNWs' growth using two annealing processes: a rapid thermal annealing (RTA) and a conventional process. A comparative study is carried out to investigate the annealing process effect on SiNWs' properties. In particular, the effect of indium oxide is presented.
