Gallium hydride vapor phase epitaxy of GaN nanowires
© Zervos and Othonos; licensee Springer. 2011
Received: 9 December 2010
Accepted: 28 March 2011
Published: 28 March 2011
Straight GaN nanowires (NWs) with diameters of 50 nm, lengths up to 10 μm and a hexagonal wurtzite crystal structure have been grown at 900°C on 0.5 nm Au/Si(001) via the reaction of Ga with NH3 and N2:H2, where the H2 content was varied between 10 and 100%. The growth of high-quality GaN NWs depends critically on the thickness of Au and Ga vapor pressure while no deposition occurs on plain Si(001). Increasing the H2 content leads to an increase in the growth rate, a reduction in the areal density of the GaN NWs and a suppression of the underlying amorphous (α)-like GaN layer which occurs without H2. The increase in growth rate with H2 content is a direct consequence of the reaction of Ga with H2 which leads to the formation of Ga hydride that reacts efficiently with NH3 at the top of the GaN NWs. Moreover, the reduction in the areal density of the GaN NWs and suppression of the α-like GaN layer is attributed to the reaction of H2 with Ga in the immediate vicinity of the Au NPs. Finally, the incorporation of H2 leads to a significant improvement in the near band edge photoluminescence through a suppression of the non-radiative recombination via surface states which become passivated not only via H2, but also via a reduction of O2-related defects.
Group III-nitride (III-N) compound semiconductors such as GaN, InN, and AlN have been investigated intensively over the past decades in view of their successful application as electronic and optoelectronic devices . In particular, III-N semiconductors are attractive since their band-gaps vary between 0.7 eV in InN  and 3.4 eV in GaN  up to 6.2 eV in AlN , allowing the band-gaps of Al x Ga1-x N or In x Ga1-x N to be tailored in between by varying x which is very important for the realization of high-efficiency, full spectrum solar cells. In addition III-N nanowires (NWs) have also been investigated in view of the up surging interest in nanoscale science and technology. More specifically, InN , GaN  NWs and also In x Ga1-x N NWs  have been grown and their transport and optical properties have been investigated. However, the use of III-N NWs for the fabrication of NWSCs has not yet been demonstrated. To date NWSCs have not only been fabricated from a single p-i-n core-shell Si NW , but also using disordered arrays of Si NWs . Evidently the growth of high-quality GaN NWs is crucial for the fabrication of NWSCs based on III-N NWs. So far GaN NWs have not only been grown by a variety of methods including metal organic chemical vapor deposition (MOCVD), molecular beam epitaxy (MBE), but also via the direct nitridation of Ga with NH3 between 900 and 1100°C on a broad variety of substrates, e.g., SiC, Al2O3, and Si using various catalysts such as In, Fe, Ni, Au, and NiO, reviewed elsewhere . The GaN NWs have a hexagonal-wurtzite crystal structure and their diameters vary between 10 and 50 nm. Nevertheless despite this broad variety of investigations there are still many issues pertaining to their growth and properties that need to be clarified and understood to improve crystal quality and to enable the fabrication of nanoscale devices such as NWSCs. Recently, hydride vapor phase epitaxy (HVPE) has been used to grow GaN layers  and also GaN NWs . The use of H2 first of all eliminates O2 and secondly leads to the formation of Ga hydride, which in turn reacts with NH3 giving GaN. This method is cleaner compared to MOCVD or halide-VPE . Previously, we showed that the use of a few % of H2 leads to the growth of straight GaN NWs with lengths of 2-3 μm and diameters of 50 nm [6, 10]. More recently, Lim et al.  investigated the effect of H2 on the initial stages of growth of GaN NWs by varying the ratio of N2:H2 up to 0.6 and found that the density and growth rate of the GaN NWs decreased with increasing % H2. In this article, we have carried out a study into the growth of GaN NWs on Au/Si(001) via the reaction of Ga with NH3 and N2:H2 where the H2 content was varied between 10 and 100%. It has been find that the growth of straight GaN NWs on Au/Si(001) is critically dependent on the thickness of the Au and the Ga vapor pressure while no deposition occurs on plain Si(001). Increasing the H2 content leads to an increase in the growth rate, a reduction in the density of the GaN NWs and a clear suppression of the amorphous (α)-like GaN layer that forms without H2. A growth mechanism is proposed to explain these findings, where the effect of H2 is clarified in detail. Finally, we show that the incorporation of H2 leads to a significant improvement in the near band edge photoluminescence (PL) through a suppression of the non-radiative recombination via surface states and their passivation by H2.
Summary of HVPE growth conditions for GaN NWs carried out on 0.5 nm Au/Si(001) at T = 900°C for 60 min via the reaction of Ga with 20 sccms of NH3 and N2:(10-100%) H2
Results and discussion
The GaN NWs were not as straight as a direct consequence of the excessive Ga which is consistent with the morphology of the GaN NWs obtained under Ga-rich conditions by LPCVD . A high yield, uniform distribution of straight GaN NWs over 1 cm2 under these Ga-rich conditions was obtained by using 40% H2 while we observed a reduction in the areal density of the GaN NWs using 100% H2 and a significant enhancement in the growth rate.
This reduction in the areal density of the GaN NWs is consistent with the findings of Lim et al.  who observed a monotonic drop in the number of GaN NWs with increasing content of H2 which they attributed to the agglomeration of Au NPs. An alternative explanation for the observed reduction maybe the catalytic dissociation of H2 over the Au NPs which gives H that reacts with incoming Ga or Ga spreading out from the Au NPs to be explained in more detail below.
At the same time, the Ga hydride released from the surface or generated upstream will promote one-dimensional growth via its reaction with NH3 at the tops of the GaN NWs as shown schematically in Figure 3c thereby enhancing the growth rate. The latter is essentially governed by the availability of reactive species at the tops of the GaN NWs in accordance with the self-regulated, diameter selective growth mechanism of Kuo et al. . Finally, the reduction in the super saturation of the Au NPs will limit extreme fluctuations of the Ga in the Au NPs resulting in GaN NWs with uniform diameters and smooth surfaces. This in turn implies a reduction of surface states which are passivated by H2 giving stronger band edge PL emission.
Straight GaN NWs with diameters of 50 nm, lengths up to 10 μm, and a hexagonal wurtzite crystal structure have been grown at 900°C on Au/Si(001) via the reaction of Ga with NH3 and N2:H2 where the H2 was varied between 10 and 100%. We find that the growth of high-quality GaN NWs can be achieved with Au having a thickness <1 nm. A growth mechanism was described whereby H2 reacts with Ga giving Ga hydride thereby promoting one-dimensional growth via its reaction with NH3 at the tops of the GaN NWs. Hydrogen may therefore be used not only to control the growth rate and obtain straight GaN NWs, but also to suppress the formation of the underlying α-like GaN under Ga-rich conditions.
hydride vapor phase epitaxy
molecular beam epitaxy
metal organic chemical vapor deposition
scanning electron microscope
This study was supported by the Research Promotion Foundation of Cyprus under the grant no. BE0308/03.
- Nakamura S, Mukai T, Sengh M: Candela-class high brightness InGaN/AlGaN double heterostructure blue light emitting diodes. Appl Phys Lett 1994, 64: 1687. 10.1063/1.111832View Article
- Wu J, Walukiewicz W, Yu KM, Ager JW, Haller EE, Lu H, Schaff WJ, Saito Y, Nanishi Y: Unusual properties of the fundamental bandgap of InN. Appl Phys Lett 2002, 80: 3967. 10.1063/1.1482786View Article
- Levinshtein MichaelE, Rumyantsev SergeyL, (Editor), Shur MichaelS: Properties of Advanced Semiconductor Materials GaN, AlN, InN. Wiley-Interscience; 2001. ISBN-10: 0471358274 ISBN-10: 0471358274
- Li J, KB Nam, Nakarmi ML, Lin JY, Jiang HX, Carrier P, Wei S-H: band structure and fundamental optical transitions in wurtzite AlN. Appl Phys Lett 2003, 83: 5163. 10.1063/1.1633965View Article
- Othonos A, Zervos M, Pervolaraki M: Ultrafast carrier relaxation of InN nanowires grown by reactive vapor transport. Nanoscale Res Lett 2009, 4: 122–129. 10.1007/s11671-008-9211-8View Article
- Tsokkou D, Othonos A, Zervos M: Defect states of CVD grown GaN nanowires: Effects and mechanisms in the relaxation of carriers. J Appl Phys 2009, 106: 054311. 10.1063/1.3212989View Article
- Kuykendall T, Ulrich P, Aloni S, Yang P: Complete compositional tunability of InGaN nanowires grown using a combinatorial approach. Nat Mater 2007, 6: 951. 10.1038/nmat2037View Article
- Tian B, Zheng X, Kempa TJ, Fang Y, Yu N, Yu G, Huang J, Lieber CM: Coaxial silicon nanowires as solar cells and nanoelectronic power sources. Nature 2007, 449: 885. 10.1038/nature06181View Article
- Tsakalakos L, Balch J, Fronheiser J, Korevaar BA, Sulima O, Rand J: Silicon nanowire solar cells. Appl Phys Lett 2007, 91: 233117. 10.1063/1.2821113View Article
- Zervos M, Othonos A: Hydride assisted growth of GaN nanowires grown on AuSi(001) via the direct reaction of Ga with NH 3 and H 2 . J Cryst Growth 2010, 312: 2631. 10.1016/j.jcrysgro.2010.05.040View Article
- Imade M, Yamada N, Kitano Y, Kawamura F, Yoshimura M, Kitaoka Y, Mori Y, Sasaki T: Increase in the growth rate of GaN single crystals grown by gallium hydride vapor phase epitaxy method. Phys Status Solidi 2008, 5: 1719. 10.1002/pssc.200778602View Article
- Hou WC, Hong FCN: Controlled surface diffusion in plasma enhanced chemical vapor deposition of GaN nanowires. Nanotechnology 2009, 20: 055606. 10.1088/0957-4484/20/5/055606View Article
- Seryogin G, Shalish I, Moberlychan W, Narayanamurti V: Catalytic hydride vapor phase epitaxy growth of GaN nanowires. Nanotechnology 2005, 16: 2342. 10.1088/0957-4484/16/10/058View Article
- Lim SK, Crawford S, Gradečak S: Growth mechanism of GaN nanowires: preferred nucleation site and effect of hydrogen. Nanotechnology 2010, 21: 345604. 10.1088/0957-4484/21/34/345604View Article
- Ganley JC, Thomas FS, Seebauer EG, Masel RI: A priori catalytic activity correlations: The difficult case of hydrogen production from ammonia. Catal Lett 2004, 96: 117. 10.1023/B:CATL.0000030108.50691.d4View Article
- Chisholm JA, Bristowe PD: Formation energies of metal impurities in GaN. Comput Mater Sci 2001, 22: 73. 10.1016/S0927-0256(01)00168-9View Article
- Kuo CK, Hsu CW, Wu CT, Lan ZH, Mou CY, Chen CC, Yang YJ, Chen LC, Chen KH: Self-regulating and diameter-selective growth of GaN nanowires. Nanotechnology 2006, 17: S332. 10.1088/0957-4484/17/11/S17View Article
- Bertness KA, Roshko A, Mansfield LM, Harvey TE, Sanford NA: Mechanism for spontaneous growth of GaN nanowires with molecular beam epitaxy. J Cryst Growth 2008, 310: 3154. 10.1016/j.jcrysgro.2008.03.033View Article
- Fujitani T, Nakamura I, Akita T, Okumura M, Haruta M: Hydrogen dissociation by Au clusters. Angew Chem Int Ed 2009, 48: 9515–9518. 10.1002/anie.200905380View Article
- Bus E, Miller JT, van Bokhoven JA: Hydrogen chemisorption on Al 2 O 3 -supported Au catalysts. J Phys Chem B 2005, 109: 14581–14587. 10.1021/jp051660zView Article
- Kawamura F, Imade M, Yoshimura M, Mori Y, Sasaki T: Synthesis of GaN crystal using gallium hydride. Jpn J Appl Phys 2005, 44: 1. 10.1143/JJAP.44.L1View Article
- Yazdanpanah MM, Harfenist SA, Cohn RW: Gallium-driven assembly of gold nanowire networks. Appl Phys Lett 2004, 85: 1592. 10.1063/1.1787938View Article
This article is published under license to BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.