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  • The synthesis and application of InGaN nanowires: A review

    Abstract In the past few years, the field of indium gallium nitride (InGaN) nanowires has seen a great deal of interest, as the compositional tunability and conveniently placed band-gap of the material coincide quite well with the special physical qualities possessed by nanowire heterostructures. These nanowires can be grown with a variety of methods and in combination with other materials, producing a wide array of possible applications. Among these, light-emitting diodes remain promising as a light source, with research efforts focused on efficiency, while substantial breakthroughs have been made in the field of photoelectrochemical catalysis, primarily through water splitting. Photovoltaic cells too hold promise, though they may be years away from application. In this review, we seek to inform the reader of key developments in the field of InGaN nanowire synthesis and application.

    Introduction

    Humanity’s demand for energy is on a general upward trend, but traditional sources are becoming less viable as long term solutions, given that the vast majority of power generated today comes from nonrenewable sources. As such, the harvesting and efficient use of energy have become very important topics of study in scientific research, with radiant energy– the energy of the light produced by the sun, and that of the light we use to mimic it– taking a central role in this field. From solar power, which seeks convert the sun’s energy into a useful form, to more efficient sources of the lighting we find invaluable in our daily lives, the study of radiant energy and photonics has reach across fields, with numerous methods and applications. In particular, semiconducting nanowires have shown great potential, with applications in light-emitting diodes (LEDs)(Yan 2009)(Chen 2012), photovoltaic cells(Dasgupta 2013), and photoelectrochemical energy conversion(Liu 2014).

    Among the many semiconducting nanowire materials, indium gallium nitride has been a material of practical interest for quite some time. As early as 1998 there had developed an extensive field of research into InGaN laser diodes(Nakamura 1998), and this research has reached fruition in the blue-violet laser diodes currently utilized in modern compact disc technology(Nakamura 2000). More recently, as InGaN has become better characterized, there has been an upsurge in interest in InGaN nanowire structures. This is due to an increase in studies in the general field of nanowires(Law 2004), which have demonstrated that nanowires possess several capacities which make them uniquely suited as structures for InGaN. A one dimensional structure minimizes the large lattice constant mismatch between InN and GaN, and the large amount of surface area present in nanowires is ideally suited to InGaN’s both traditional and emerging applications for InGaN.

    The synthesis of InGaN nanowires was initially focused on 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. 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(O 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.

    Due to these properties, nanowires have drawn attention for both traditional applications of the material, but also for altogether new purposes. Nanowire reviews show the development of InGaN structures intended for well-trod paths such as LED photoluminescence(Zhuang 2011), but also for emerging technologies such as the photochemical conversion(Osterloh 2013) and storage of solar energy(Yang 2010)(Liu 2014). Indeed, emerging interest in InGaN as a solar energy converter– through both photovolataic cells and photochemical conversion– has been the subject of tremendous research in recent years(McLaughlin 2013), due to a band gap that allows the possibility for conversion efficiencies of greater than 50 percent(Sawaki). The pace of this development has only continued in recent years, and recent work in both the synthesis and application of InGaN nanowires is highlighted in this review.

    Synthesis

    Vapor Phase Epitaxy

    Though not termed nanowire synthesis as such, the work of Kim et al.(Kim 2003) in synthesizing InGaN nanorods (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 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)(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 et 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.