# Synthesis of InGaN Nanowires: A review

Abstract

Nanowires constructed of indium gallium nitride (InGaN) are of great interest due to the wide range of stoichiometries made possible by the flexible composition of the alloy, allowing a range of observable properties in one material. This review article seeks to summarize key developments in the field of indium gallium nitride synthesis, observing breakthroughs in the fields of vapor beam epitaxy, molecular beam epitaxy, halide chemical vapor deposition, metal organic chemical vapor deposition, and vapor-liquid synthesis.

# Introduction

The synthesis of nanowires containing nitrides of both indium and gallium has drawn interest in recent years for applications in light-emitting diodes, due to high levels of quantum efficiency, near the levels necessary for ultra-efficient solid state lighting(Phillips 2007). Equally important, if not moreso, is the wide range of bandgaps available with InGaN, made possible by varying the stoichiometric ratio within the alloy. Any InGaN nanowire is not an equal mixture of the three constituent elements, but rather InN and GaN alloyed, giving possible formulas of the format In$$_{x}$$Ga$$_{1-x}$$N, each having a slightly different bandgap, and thus bridging a very wide emissions spectrum. However, the relative paucity of studies concerning InGaN nanowires until recently(Growth Properties, an...) left researchers in the dark as to the complete properties possessed, or the best methods for efficient and precise synthesis.

Developments in the synthesis of these nanowires have naturally focused on these band gaps, as creating conditions that allow for the full spectrum of InGaN stoichiometries is critical to any major utilization of the material’s bandgap properties. This presented a major challenge to researchers of InGaN, as there is a significant difference in the lattice constants of InN and GaN(ODonnell 2001). However, this may have in fact spurred the development and investigation of InGaN nanowires, due to the unique advantage the nanowire structure has over other arrangements and growths of InGaN. In general, nanowires have shown to be remarkably free of the strain that accompanies alloys with large differences in lattice constant(Ertekin 2005), making nanowires more attractive for those seeking to fully utilize the potential of InGaN.

# Hydride Vapor Phase Epitaxy

Though not termed nanowire synthesis as such, the work of Kim and coworkers in synthesizing InGaN nanorods(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(Suo 2014), the Hydride Vapor Phase Epitaxy (HVPE) as practiced by Kim etc. 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)(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.