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\subsection{Hydride Vapor Phase Epitaxy (HVPE)}  HPVE involves the reaction of gallium metal with HCl gas at 750 C, which forms GaCl. A nitrogen gas flow then transports the GaCl to a chamber wherein it reacts with ammonia gas directly upon a sapphire substrate to form nanowires. This Shin's team used this  process formed to form  straight nanowires that were perpendicular to the substrate, with the diameters ranging from 80 to 120 nm. The size of the nanowires was able to be modified by controlling the temperature. One of the more unique aspects of HVPE synthesis is was  that fact that growth temperature chamber was kept around 480 C, compared to temperature temperatures  ranging from 850 – 1050 C as described earlier. for VLS.  \cite{Kim_2002} More recently, a team in 2012 managed to optimize the process to create various morphologies of GaN nanostructures. This was attained by varying the ration of HCl and ammonia gas, as well as the temperature in the reaction chamber. The group also utilized a silicon substrate, with is arguably more inexpensive than sapphire ones. \cite{Shin_2012} More recent advances in HPVE synthesis has have  been developed by the Avit group, who managed to create a growth method that couples high lengths of nanowires with extremely high growth rates. The nanowires grown have defect-free  crystal structures that are similar to previously grown nanowires, and are defect free as well. nanowires.  Another benefit of their process was the usage of a HPVE furnace at atmospheric pressure, which is high beneficial towards scale up due to reducing would reduce  the costand design  of HVPE furnaces. HPVE furnaces for scale-up.  Therefore, by using HPVE, researchers are were  able to develop GaN nanowires efficiently while avoiding impurities that may be have been  present in VLS and other synthetic methods. \cite{Avit_2014}