**3. Vapor-liquid-solid growth mechanism**

NWs are grown from the vapor-liquid-solid (VLS) method during which an NW is grown from the vapor phase employing a metal seed particle, like Au as shown in **Figure 3a**, that is usually liquid at the expansion temperature (after alloying with the substrate and/or growth species) [56, 70, 71]. The VLS growth technique as its name suggests is the growth method from the combination of the three phases (vapor phase, liquid phase, and solid phase). The vapor phase is the gas phase precursor, the liquid phase is the catalyst and the solid phase is the final grown nanowire. A foreign metal can be used to promote NW growth by forming a liquid eutectic with the desired NW material through this mechanism (**Figure 3a** and **b**). In such NW synthesis, the chemical precursor's vapors are transported into the hot zone by an inert carrier gas and react on a substrate with metal catalyst nanoparticles [72]. With the proper choice of substrates, catalysts, precursors, and growth conditions, various types of vertical NWs, as well as planar NWs can be achieved [73]. While VLS synthesis is the most common with the seed particle in a liquid state, a mechanism is also possible if the catalysts remain solid and do not form a eutectic with the NW

**Figure 3.**

*Schematics for three typical nanowire growth mechanisms: (a) foreign metal–catalyzed growth, (b) self-catalyzed growth, and (c) selective area epitaxy [31].*

material [74]. Such solid-phase diffusion mechanism happens below the catalyst's eutectic point with the metal seeds remaining as solid [75]. Ingvar Aberg et al. [76] have grown GaAs NW array solar cells by the VLS growth Au-assisted method, which demonstrated a 1-sun independently verified solar energy conversion efficiency of 15.3%.

As explained in the previous section the VLS method can also be use self-assisted. The self-assisted method uses a component of the NW itself as shown in **Figure 3b**, which avoids the utilization of foreign metal elements [41]. A foreign particle which found on the top of NWs may cause harmful effects like contamination of the NWs, increased contact resistance, or reflection of sunshine from a photovoltaic device [77]. These foreign metallic particles can be etched after device processing, but etching might cause problem and will have a negative effect on the performance of device. **Figure 3c** shows the oxide-supported growth mechanisms.

Mandl et al. [63] have grown InAs NW by a VLS mechanism employing a liquid In droplet and they identified that the presence of the oxide layer is vital to immobilize In

**Figure 4.**

*The growth mechanism illustration of nanowires: (a) layer before growth, (b) layer during heating and creation of hole, (c) formation of In droplets, (d) growth of the NWs under the In droplets, and (e) droplet solidification during cooling [63].*

### *Solar Energy Conversion Efficiency, Growth Mechanism and Design of III–V Nanowire-Based… DOI: http://dx.doi.org/10.5772/intechopen.105985*

droplets on the surface, restricting the particle size and NW nucleation formed. They have concluded that NWs can be grown in the sequence, as illustrated in **Figure 4**: (a) in the deposited SiOx openings form during heating the substrate; (b) when the trimethylindium (TMI) precursor is activated, the In atoms adsorbed on the surface diffuse into immobile In droplets formed in the openings of the SiOx layer; (c) at the interface between indium particles and the substrate underneath, NW growth is began by enhancing the rate of growth in one direction; and (d) after deactivation of the TMI supply, the droplet forms InAs NW.
