Synthesis of InGaN Nanowires: A review
John O. Curry
Affiliation not available
April 21, 2015
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.
1
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[12]. 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 Inx Ga1−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[1] 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[11].
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
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lattice constant[4], making nanowires more attractive for those seeking to fully
utilize the potential of InGaN.
2
Hydride Vapor Phase Epitaxy
Though not termed nanowire synthesis as such, the work of Kim and coworkers
in synthesizing InGaN nanorods[8] (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[14], 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 N2 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 Inx Ga1−x N due to the increased formation of the low-reactivity InCl, as
opposed to InCl3 . As the gaseous HCl entered the system, the In metal would
react to formed the desired InCl3 – but the H2 produced as a byproduct of the
reaction could, at sufficient partial pressures, further react with InCl3 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)[8]. 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 Inx Ga1−x N with constant concentrations of x = 0.25, illustrating that for the wavelengths available, making
a working LED would not be difficult.
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Chemical Vapor Deposition
Focusing on the emissions spectrum, Lieber et al. looked into the potential
of multishell nanowires[13], which had the potential to give a variety of emissions spectra without figuring out how to tune the Inx Ga1−x N concentration
in a single nanowire. Instead the spectrum would be shifted by the relative
concentration of In in a multishell n-GaN/Inx Ga1−x N/GaN/p-AlGaN/p-GaN
structure. To accomplish this , Lieber and company used metal-organic chemical vapor deposition (MOCVD), laying down layers of nanomaterial to get a
2
Figure 1: The HVPE system utilized by Kim, et al.
remarkable variety of spectra, with In concentrations in Inx Ga1−x N of roughly
1, 10, 20, 25, and 35 percent. However, the nanowires were restricted by their
multishell nature, and were not pure Inx Ga1−x N nanowires per se.
To achieve synthesis of pure InGaN nanowires with a higher concentration of
InN, Cai and fellow researchers made use of chemical vapor deposition (CVD)[2],
which circumvents the obstacles presented by HVPE. Instead of relying on a
halide intermediate to carry the metal to the subtrate, the CVD protocol as
pursued by Cai and company placed both of the metals in a quartz tube with
the silicon substrate, which had been covered with gold to serve as a catalyst.
Under low pressure and high temperature, NH3 and Ar were pumped through
the tube, during which the crystal growth took place.
Interestingly, while Cai’s team reported nanowires with the relative Inx Ga1−x N
concentrations they sought, they had very little control over precisely what the
ratios were. While the number of available stoichiometric ratios had increased,
Cai et al. demonstrated no method similar to that available in HVPE, where
one could simply increase intake of the desired metal precursor up to a certain
point. Thus, while they reported InN fractions in their wires of up to 60 per
cent, they were mixed in with all a manner of other concentrations.
4
Molecular Beam Epitaxy
With precision in Inx Ga1−x N ratio so desired, it is no surprise that molecular
beam epitaxy (MBE) has been investigated as a mechanism of synthesis. Operating in an ultra-high vacuum, MBE involves the reaction of high-temperature
molecules as ’molecular beams’ with a crystalline surface[3]. This means that,
similarly to HVPE, the operator has full control over the relative amounts of the
inputs– but the high vacuum reduces the likelihood of any unhelpful competing
reactions.
Illustrative of the basic procedure is the work of Hu and cowworkers, who created InGaN/GaN nanocolumn crystals with an InN interlayer[7], demonstrating
complete control of all InGaN’s components. Grown on a silicon substrate, Hu
3
Figure 2: The wavelengths of nanowires present in the Cai synthesis (left) vs the
Kim synthesis (right). Of note are the lack of definition in the Cai peaks, as well
as the much larger range of the visible spectrum covered by Cai’s nanowires.
and company used InN as an interlayer due to the small lattice constant difference. Once the base layer of InN had been laid down, it was possible to grow
a remarkable InGaN/GaN heterostructure through periodic deposits of InGaN
and GaN. The resulting SEM images revealed a flowered structure with helpful
internal quantum efficiency properties.
Applying this powerful synthesis technique to nanopillars, Vajpeyi et al.
employed nitrogen radiofrequency-plasma source MBE (RF-MBE) to deposit
Inx Ga1−x N directly onto a silicon surface[15]. Varying the substrate temperature revealed that the indium concentration coefficient x was inversely dependent on temperature– but furthermore, that Inx Ga1−x N nanopillars could
not be deposited at temperature below 575 degrees Celsius. Other nanopillar properties–the density and diameter– likewise bore relationships with substrate temperature, indicting that the control granted by this synthesis was
somewhat constrained. But while the temperature cutoff may have limited the
Inx Ga1−x N nanopillars short of the full spectrum possible, Vajpeyi and associates did achieve both range (from 450 to 625 nm) while maintaining limits
on emission wavelength (see figure 3).
More recently, Bhattacharya et al. attempted a comprehensive evaluation
of MBE-produced InGaN nanowires, using plasma-assisted MBE (PA-MBE) to
vary the relative Inx Ga1−x N composition not through substrate temperature,
but rather through changing the In flux during MBE nanowire production[5].
As a result, Bhattacharya’s group managed to not only create nanowires of
varying In compositions previously not seen in MBE synthesis, but also created
wires in which the Inx Ga1−x N ratio varied over the length of the wire. The photoluminescence spectra illustrated a manner of InGaN synthesis which not only
has high precision for different ratios of Inx Ga1−x N, but also could control (to
a degree) the broadness of emission, allowing more or less focused wavelengths
of light, and showing the true potential of MBE.
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Figure 3: Left, the emissions wavelengths achieved by Vajpeyi et al, with their
room temperature wavelengths inset. Right, the relationship between substrate
temperature and indium concentration.
Figure 4: The photoluminescence spectra of the nanowire produced by Bhattacharya et al. The broad, black peak had varied concentrations of In throughout synthesis, while the colored peaks indicate relative purity. Inset is the
temperature dependence of broad luminescence for the varied InGaN peak.
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Halide Chemical Vapor Deposition
But perhaps the most breathtaking development in Inx Ga1−x N synthesis has
come in the field of halide chemical vapor deposition (HCVD), where Kuykendall et al. used an innovative method of synthesis to successfully capture the
entire compositional range of Inx Ga1−x N[9], from x = 0 to x = 1. The synthesis
design carefully circumvented all the obstacles, such as high temperature, metal
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reactivity, and lack of control, that had lain in the way of previous experiments
through a unique furnace design which incorporated not only separate inputs for
each reagent, but also separate zones of temperature control (figure 5). This allowed independent vapor pressure control for both metal sources, keeping them
in the (III) oxidation state, and when combined with an N2 carrier gas permitted Kuykendall and company to tune the In ratio quite finely as the Inx Ga1−x N
grew on a sapphire substrate.
Despite the expected variance in wire morphology, it did not have any deleterious effect on synthesis of certain spectral configurations. The spectra produced
were of unique color and clarity, and presented a brighter, clearer, and more numerous array than even that of Lieber’s multishell compositions. Access to the
full range of combinations allowed full characterization of structural qualities
such as lattice parameter, relative diameter, and shape, explicitly linking them
to the In fraction.
Following up on Kuykendall’s work, Hahn et al. took the HCVD technique
and adapted it for more practical use[6]. Growing the nanowires on an Al2 O3
substrate in the same furnace as Kuykendall et al used, but modified to produce
homegenous arrays rather than compositional gradients, Hahn’s team saw remarkbaly quick growth of roughly 9-12 minutes. The nanowire arrays produced
offered a chance to not only tune large arrays to hit a specific wavelength of
photoluminescence (which was accomplished, with the three arrays offering a
practical RGB spectrum) but also knowledge of the structural nature of homogenous Inx Ga1−x N. With all else held constant, the crystal was observed to
grow in the ¡002¿ direction.
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Conclusion
In less than a decade, the field of InGaN nanowires went from birth to maturity,
with full characterization, clear industrial applications, and a shift in focus from
figuring out how to make the material, to understanding how to make it cheaply
and efficiently. Nonetheless, it would be folly to assume that we have seen
everything InGaN nanowires offer, as efforts so far have narrowly focused on the
admittedly impressive potential for LED design. Yet while LEDs may be the
near future, other researchers are looking into future applications. LEDs may
not be the whole of solid-state lighting, which is certain to rely upon InGaN[12],
and the material is becoming a subject of interest in the study of photovoltaic
cells as well[10]. With a new use comes a desire for new properties– perhaps,
for external quantum efficiency rather than tunability– and the old methods of
synthesis will need reshaping.
References
[1] Growth Properties, and Device Applications of III-Nitride Nanowire Heterostructures. In Zetian Mi, editor, Advances in III-V Semiconductor
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Figure 5: Optical characterization of the nanowires produced by Kuykendall, et
al. a The color CCD images. b Photoluminescence emission from Inx Ga1−x N,
x = 0 to x = 0.6. d Optical absorption spectra.
Nanowires and Nanodevices, pages 22–42. Bentham Science Publishers
Ltd., sep 2011.
[2] X M Cai, Y H Leung, K Y Cheung, K H Tam, A B Djurišić, M H Xie, H Y
Chen, and S Gwo. Straight and helical InGaN core–shell nanowires with a
high In core content. Nanotechnology, 17(9):2330–2333, apr 2006.
[3] A.Y. Cho and J.R. Arthur. Molecular beam epitaxy. Progress in Solid
State Chemistry, 10:157–191, jan 1975.
[4] Elif Ertekin, P. A. Greaney, D. C. Chrzan, and Timothy D. Sands. Equilibrium limits of coherency in strained nanowire heterostructures. J. Appl.
Phys., 97(11):114325, 2005.
7
[5] Wei Guo, Meng Zhang, Animesh Banerjee, and Pallab Bhattacharya.
Catalyst-Free InGaN/GaN Nanowire Light Emitting Diodes Grown on
(001) Silicon by Molecular Beam Epitaxy. Nano Letters, 10(9):3355–3359,
sep 2010.
[6] Christopher Hahn, Zhaoyu Zhang, Anthony Fu, Cheng Hao Wu, Yun Jeong
Hwang, Daniel J. Gargas, and Peidong Yang. Epitaxial Growth of InGaN
Nanowire Arrays for Light Emitting Diodes. ACS Nano, 5(5):3970–3976,
may 2011.
[7] F. R. Hu, K. Ochi, Y. Zhao, and K. Hane. InGaN/GaN quantum-well
nanocolumn crystals on pillared Si substrate with InN as interlayer. physica
status solidi (c), 4(7):2338–2341, jun 2007.
[8] Hwa-Mok Kim, Yong-Hoon Cho, Hosang Lee, Suk Il Kim, Sung Ryong
Ryu, Deuk Young Kim, Tae Won Kang, and Kwan Soo Chung. HighBrightness Light Emitting Diodes Using Dislocation-Free Indium Gallium
Nitride/Gallium Nitride Multiquantum-Well Nanorod Arrays. Nano Letters, 4(6):1059–1062, jun 2004.
[9] Tevye Kuykendall, Philipp Ulrich, Shaul Aloni, and Peidong Yang. Complete composition tunability of InGaN nanowires using a combinatorial
approach. Nat Mater, 6(12):951–956, oct 2007.
[10] Dirk V. P. McLaughlin and Joshua M. Pearce. Progress in Indium Gallium
Nitride Materials for Solar Photovoltaic Energy Conversion. Metall and
Mat Trans A, 44(4):1947–1954, feb 2013.
[11] K P O'Donnell, J F W Mosselmans, R W Martin, S Pereira, and M E
White. Structural analysis of InGaN epilayers. Journal of Physics: Condensed Matter, 13(32):6977–6991, jul 2001.
[12] J.M. Phillips, M.E. Coltrin, M.H. Crawford, A.J. Fischer, M.R. Krames,
R. Mueller-Mach, G.O. Mueller, Y. Ohno, L.E.S. Rohwer, J.A. Simmons,
and J.Y. Tsao. Research challenges to ultra-efficient inorganic solid-state
lighting. Laser & Photon. Rev., 1(4):307–333, dec 2007.
[13] Fang Qian, Silvija Gradecak, Yat Li, Cheng-Yen Wen, and Charles M.
Lieber. Core/Multishell Nanowire Heterostructures as Multicolor HighEfficiency Light-Emitting Diodes. Nano Letters, 5(11):2287–2291, nov 2005.
[14] Guoquan Suo, Shuai Jiang, Juntao Zhang, Jianye Li, and Meng He. Synthetic Strategies and Applications of GaN Nanowires. Advances in Condensed Matter Physics, 2014:1–11, 2014.
[15] A P Vajpeyi, A O Ajagunna, K Tsagaraki, M Androulidaki, and A Georgakilas. InGaN nanopillars grown on silicon substrate using plasma assisted
molecular beam epitaxy. Nanotechnology, 20(32):325605, jul 2009.
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