**3. Indium as catalyst for silicon nanowires' growth**

In the VLS mode, the catalyst surface's properties play an important role in the SiNWs' growth. In order to adsorb the maximum of gaseous species, the surface should be rough. For example, silane dissociation and adsorption are better on the gold-silicon (Au-Si) system's surface than that of silicon. Indeed, the high adsorption and dissociation efficiency of silane at the Au-Si droplet permits the nanowires' growth with a constant radius. Other parameters influence SiNWs' growth, such as the surface tension of the catalyst droplet and the solid–liquid interface tension. The choice of the catalyst metal is very important, matching some criteria:


Gold is the most commonly used metal despite silicon contamination with defects, introducing deep energy levels in the silicon bandgap. Gold, with a simple diagram phase, permits the solubility and nucleation of silicon easily at a relatively low eutectic temperature (363°C). Moreover, gold does not oxidize, increasing its catalytic activity. Gold-catalyzed SiNWs are well controlled and already integrated into prototypes such as transistors [23], biosensors [24], and photo-anodes [25].

Since 2001, post-transition metals like aluminum, indium, and bismuth have been studied. All these metals introduce shallow defects (dopants for silicon). Aluminum, for example, has an eutectic temperature (577°C) higher than that of gold and is very chemically reactive, especially with oxygen.

**Figure 1.** *Phase diagram of indium-silicon system [26].*

Bismuth (Bi) is a promising catalyst. The energy level of the impurity introduced by bismuth into the silicon gap is close to the conduction band, so bismuth is an N-type dopant for silicon. Bismuth exhibits a low silicon eutectic temperature (271°C), which allows working at low growth temperatures and using flexible substrates. Despite these advantages, it is difficult to use it as SiNWs catalyst because it has a high vapor pressure, so it can evaporate easily during growth. It has a low surface tension (0.37 N/m), while gold has a higher surface tension (1.14 N/m). Unidirectional growth has been shown to be difficult with low surface tension catalysts.

Indium is an interesting metal to be used as catalyst. It presents a simple phase diagram with silicon (**Figure 1**), forming an eutectic system at 157°C, permitting low growth temperatures and flexible substrates' use. Indium has a surface tension (0.55 N/m) higher than that of bismuth but lower than gold. However, in the presence of oxygen, indium could be oxidized, reducing thereby its catalytic behavior.
