Despite the advancement of Au catalyzed synthesis of GaAs NW, Au free synthesis techniques are being investigated. Studies reveal that Au incorporation leads to scattering centers that disrupt desirable optical properties. \cite{1966} Due to this issue an Au free synthesis is desirable. Current techniques range from “Ga-assisted growth", selective area epitaxy (SAE), solution based growth, and oxide assisted growth.\cite{Chatillon_1995}\cite{Yu_2003}\cite{Shi_2001}
Oxide assisted growth works similarly to VLS but instead of an Au catalyst, an oxide species in conjunction with laser ablation is necessary to catalyze NW growth. GaAs NW growth via this method has been reported, with the oxide species being Ga\(_{2}\)O\(_{3}\) and growth being maintained in the same direction as VLS.\cite{Duan_2000}\cite{Shi_2001} The process begins with a quartz tube containing a Si surface mounted in a high-temperature tube furnace containing 25% gallium oxide and 75% GaAs powder. The tube is evacuated, heated to \(930^{\circ}C\), then a laser ablation (10 Hz, 400 mJ/pulse, \(\lambda\) = 248nm) takes place for 5h. NWs with 10-120nm diameters resulted.
“Gallium assisted growth" is also similar to VLS (Ga playing the role of Au).\cite{Morral_2011} \cite{Iyer_2014} \cite{Matteini_2014} The growth conditions are chosen by again analyzing a phase diagram to create a situation in which Ga liquid is in equilibrium with solid GaAs (this temperature is called the “congruent temperature" and was found to be \(630^{\circ}C\)).\cite{Chatillon_1995} Arsenic adatoms (atoms “on top of" the crystal surface plane) diffuse into the Ga liquid droplet to form the GaAs NW, and these adatoms can be deposited via MBE, or MOVPE.
Selective area epitaxy is yet another method of NW growth. A substrate is covered by a film such that NW reactant will not “stick" to it. Then the desired nanoscale pattern is place via lithography which removes the non-reactive film and exposes the substrate for NW growth. Conditions to ensure that atoms will not stick to the film but will grow on the exposed substrate are chosen.\cite{Morral_2011} Studies have shown that the geometry of the NW growth changes during growth based on location on the substrate relative to other NWs.\cite{Bassett_2015}