Alloying can be complex to understand because at least three different elements are involved: Au, As, Ga. Therefore a ternary phase diagram must once again be studied in order to decide temperatures that would allow for eutectic stable liquid or solid alloys (eutectic point is the lowest melting point achieved by a mixture of metals). \cite{Joyce_2011}

Vapor-liquid-solid-hydride vapor phase epitaxy (VLS-HVPE) is a technique that has allowed for the growth of exceptionally long (tens of micrometers) and defect free NW with a mean diameter of 50 nm. \cite{Gil_2014} A gold monoloayer is placed on top of a GaAs substrate. The GaCl precursors allow for exceptionally fast decomposition and hence delivery of Ga for NW growth. In addition, the Cl species suppress radial growth of the NW. The assumption of fast Ga atom delivery to the liquid catalyst has been modeled by Andre and co-workers, resulting in good agreement with experiment.\cite{Andr__2014} Dubrovskii and coworkers have presented a new expression for the Zeldovich nucleation rate in Au-catalyzed VLS III−V NWs, taking into account the nucleation kinetics in a ternary Au−III−V alloy and the self-consistency renormalization of the Zeldovich nucleation rate changes.\cite{Dubrovskii_2015} In their studies it was found that after taking the new expression into account for zinc blend and wurzite structure, the long zinc blend structure of VLS-HVPE can be explained.

Although VLS and MOCVD are techniques that have been commonly used for decades there are still unknowns about the mechanism of growth mentioned earlier. The original VLS mechanism was proposed in 1964.\cite{Wagner_1964} In 2005 a Vapor Solid Solid mechanism was described by Dick and coworkers.\cite{Persson_2004} \cite{Dick_2005} In particular a solid phase diffusion mechanism in addition to the liquid phase mechanism for GaAs NW growth has been proposed. \cite{Persson_2004} An experiment growing two types of GaAs NWs, differing only in method growth termination, was conducted. Method one included switching off the Ga pressure while maintaining As pressure, and method two involved switching off the gas supply of both at the same time. X-ray energy dispersive spectrometry reveals the ratio of Ga and As in the NW body and seed particle, and results conclude that the concentration of Ga for eutectic melt is never reached, suggesting a solid phase mechanism. \cite{Persson_2004} More recently it is believed that Ga species is adsorbed at the NW sidefacets and substrate surfaces and diffuse to the growth region as adatoms, while As contribute to the axial growth primarily as secondary species. \cite{Kogstrup_2011} \cite{Ramdani_2013} In addition to these traditional vertical growth mechanism, a planar mechanism has been discovered and its details suggested.\cite{Zhang_2014} The wetting nature of seed droplet on the substrate and adhesion energy during growth are important factor responsible for the planar type of VLS GaAs NW growth.\cite{Zhang_2014}

Gas Source Molecular Beam Epitaxy (GS-MBE) coupled with Au particles has been successfully used to grow GaAs NWs.\cite{Plante_2006}. Briefly, GS-MBE works as follows. Samples are carefully prepared so that a clean single crystal face is oriented towards the beam source. The beam source is then placed at a sufficient distance to allow uniform deposition onto the surface. The growth rates of the NW depend on the flux of gas used and formation rate of clusters.\cite{9780199544219} The mechanism of NW growth reveals that the role of the Au catalyst is different than that described above for VLS and MOCVD. MBE GaAs NW growth was studied as a function of temperature, As:Ga ratio, and Au seed particle size (20-800nm), revealing two main mechanism with different rates of NW formation.\cite{Plante_2006} For Au droplet catalyst below the diameter size of 105-130nm, the height of the wire is driven by the diffusion of Ga/As gas from the walls up to the tip of the wire. For catalyst with diameters greater than 103nm, bulk diffusion through the metal catalyst is responsible for NW growth.\cite{Plante_2006}

Bulk synthesis of GaAs NWs include an “electrodeposition of gold particles" method. While Au catalyst have been used in evaporated, aerosol and colloid particle forms, these methods make it challenging to grow NW in a pattern, making the techniques less dependable for high reproducibility and consistency. Electrodeposition in conjunction with lithography has succesfully been used to produce nanoscale patterns.\cite{Jafari_Jam_2015} The method begins with nanoimprint lithography to define a pattern on a substrate with resist, resist residues are then removed by ion etching. Then electrodeposition (a process that uses electric current to reduce dissolved gold metal cations so that they form a coherent metal coating on the electrode’s exposed parts) occurs by placing the patterned substrate/resist wafer into a solution of gold. After the gold deposition, resist stripping is performed once more so that only the substrate and gold pattern are left.\cite{Jafari_Jam_2015} Advantages compared to other methods such as thermal evaporation include higher throughput, and reduced gold consumption by a factor of 300.

Another method of bulk synthesis of GaAs NW is the aerotaxy method.\cite{Heurlin_2012} Au agglomerates are formed by an evaporation–condensation process in a high-temperature furnace between 1,750 and 1,850\(^{\circ}\)C. Then the Au nanoparticles are size selected using a differential mobility analyzer and passed through a sintering furnace. The gold particles along with the precursors (GaCH\(_3\) and AsH\(_3\)) are pumped into the same furnace by a carrier gas and NW begin to form. After the allowed growth time has passed the NW are then deposited onto a substrate. The largest advantage of this method is the speed of production. While MOVPE can produce \(2.0*10^{9}\) NWs in an hour’s time on average, the aerotaxy method can produce \(1.02*10^{11}\) NWs per hour. But this difference in production rate grows even more drastic considering the scale up potentials of each method. While MOVPE would require a larger growth reactor, and larger/more substrates, which is difficult and expensive to do. The aerotaxy method is only limited by the number of Au particles produced therefore scale up is as easy as connecting several Au particle producing furnaces in parallel.

Attempts at modulating the morphology of GaAs NWs for bulk synthesis via CVD has seen some improvement. Han and coworkers have discovered that by annealing the Au catalyst film in air before GaAs NW growth instead of in H\(_2\), Ostwald ripening of the catalysts is minimized yielding uniform NW of \(>\)10\(\mu m\) length. \cite{Han_2015}