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John O. Curry added section_Vapor_Phase_Epitaxy_Though__.tex
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\section{Vapor Phase Epitaxy}
Though not termed nanowire synthesis as such, the work of Kim and coworkers in synthesizing InGaN nanorods\cite{Kim_2004} (differing from nanowires only in degree) cannot be overlooked, as it was a pioneering movement in the field. A common technique in GaN nanowire synthesis\cite{Suo_2014}, the Hydride Vapor Phase Epitaxy (HVPE) as practiced by Kim et al. centered around a furnace containing gallium metal and a sapphire wafer which served as the substrate for the nanorods to grow upon. Into this furnace were pumped the ammonia, nitrogen gas, and metal chloride precursors of InGaN-- synthesized by reacting high-temperature In and Ga with gaseous HCl in a N$_{2}$ carrier gas.
The InGaN nanorods produced as a result of this synthesis illustrates the great potential of the material, but also clearly illuminated the challenges of such a synthesis. Scan electron microscopy (SEM), energy-dispersive x-ray spectroscopy (EDS) and transmission electron microscopy clearly revealed the presence of well-defined nanorods of InGaN. However, when Kim and company investigated the composition of the nanowires, they were revealed to be somewhat limited in their composition, spanning only the range of ${x={0.04-0.2}}$ for In$_{x}$Ga$_{1-x}$N due to the increased formation of the low-reactivity InCl, as opposed to InCl$_{3}$. As the gaseous HCl entered the system, the In metal would react to formed the desired InCl$_{3}$-- but the H$_{2}$ produced as a byproduct of the reaction could, at sufficient partial pressures, further react with InCl$_{3}$ to reduce the indium to an inactive state.
Pushing the synthesis into the practical stage, Kim published further work illustrating a second manner of synthesis: metal organic-halide vapor pressure epitaxy (MOVPE)\cite{Kim_2004}. This manner of synthesis used a new indium source, trimethylindium, which conveniently contained indium in the crucial (III) oxidation state needed for InN formation. Targeting an emission of ~470 nm, Kim el al achieved remarkable specificity, producing In$_{x}$Ga$_{1-x}$N with constant concentrations of ${x=0.25}$, illustrating that for the wavelengths available, making a working LED would not be difficult.
