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.
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