As described in detail elsewhere the direct reaction of Ga with NH3
using Ar as a carrier gas at 900°C leads to the growth of a few bent GaN NWs on top of an α-like GaN layer [10
]. Such an α-like GaN layer, shown in the inset of Figure , was obtained on 0.7 nm Au/Si(001) via the reaction of Ga and NH3
using Ar, under Ga-rich conditions at 10-1
mBar. The α-like GaN layer is irregular and consists of connected crystallites that have sizes of ≈ 500 nm. It is important to point out that a low yield, non-uniform distribution of bent GaN NWs was obtained on top of this α-like GaN layer which was readily and clearly observed by SEM. On the contrary, no deposition took place on plain Si(001) in accordance with the findings of Hou and Hong [12
] who found GaN NWs on patterned Au but not on plain Si in between the Au.
Figure 1 SEM image of GaN NWs obtained using 10% H2 (a) and 100% H2 (b) The inset in (a) shows the α-like GaN layer obtained with no H2, while the inset in (b) shows Au NPs obtained by heating 10 nm Au/Si(001) at 900°C using 100% H2. The Au NPs (more ...)
GaN NWs were successfully grown on 0.7 nm Au/Si(001) via the direct reaction of Ga with NH3
at 900°C under a flow of 20 sccm NH3
and 90 sccm N2
:10 sccm H2
. The GaN NWs shown in Figure had diameters of 50 nm and lengths up to 2 μm, confirming that Au does not inhibit their growth. More importantly, the GaN NWs are straight in agreement with the findings of Hou and Hong [12
] who obtained long and bent GaN NWs using N and Ar and straight GaN NWs by adding only a few % H2
. The GaN NWs grown using 10% H2
exhibited clear peaks in the XRD as shown in Figure corresponding to GaN with a hexagonal wurtzite structure and lattice constants of a
= 0.318 nm and c
= 0.518 nm [10
]. Excitation of the GaN NWs shown in Figure using λ = 290 nm resulted in strong RT PL shown in the inset of Figure , where the prominent peak corresponds to band edge emission of GaN at 3.42 eV. Note that there was very little PL around 540 nm commonly referred to as the "yellow luminescence" band of GaN. Despite the improvement in the shape of the GaN NWs obtained with 10% H2
we found that the uniformity was poor over the Au/Si(001) surface due to the high boiling point of Ga, i.e., 1983°C and the resultant low vapor pressure at 900°C. The uniformity was improved significantly by fragmenting the Ga thereby increasing the vapor pressure, but this inadvertently led to the formation of connected crystallites or an α-like GaN layer.
Figure 2 XRD of the GaN NWs grown using 10% H2 with peaks corresponding to the (100), (002), (101) crystallographic planes of the hexagonal wurtzite structure of GaN. The inset shows RT PL with a peak at 3.42 eV (362 nm).
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 [10
]. 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. [14
] 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.
In addition, we find that the growth rate becomes larger for 100% H2
. The lengths of the GaN NWs grown under 100% H2
reached lengths >10 μm as shown in Figure and Table . The growth rate is enhanced significantly because of a higher partial pressure of Ga hydride. Before we describe the growth mechanism which explains the reduction in the areal density of the GaN NWs, suppression of the α-like GaN layer, and higher growth rate, it is instructive to consider other growth mechanisms in more detail. The most commonly invoked mechanism on the growth of GaN NWs is the vapor-liquid-solid (VLS) mechanism whereby the Ga and N are suggested to enter the catalyst NP leading to the formation of GaN NWs as shown in Figure . The poor yield of GaN NWs obtained with Au is usually attributed to the poor solubility of N in Au. Therefore, while Au is an efficient catalyst for the growth of other III-V NWs it has been suggested to be inactive in the case of GaN and Ni is commonly used as an alternative. Here, it should be pointed out that only a small fraction, i.e., ≈5% of NH3
molecules become thermally dissociated at 900°C; so, the availability of reactive N species is limited to begin with but the decomposition of NH3
over different metals is most effective in the following order: Ru > Ni > Rh > Co > Ir > Fe >> Pt > Cr > Pd > Cu >> Te [15
]. Therefore, NH3
dissociates effectively over Ni but not Au, which makes Ni effective in the growth of GaN NWs. However, the formation energies of substitutional metal impurities, i.e., M = Au, Ni, on gallium sites (MGa
) and nitrogen sites (MN
) have been calculated using ab initio
pseudopotential electronic structure calculations and it has been found that Ni has a lower defect formation energy of 1.2 eV in GaN compared to 4 eV of Au [16
]. In addition, Ni may oxidize in contrast to Au. Despite these limitations GaN NWs have been obtained using small Au NPs and a more careful analysis of the relation between the radii of the Au NP and GaN NW, carried out by Kuo et al. [17
], led them to propose an alternative mechanism whereby the Ga enters the Au NP which sits on top of the GaN NW and forms a Au-Ga alloy but Ga also reacts with N at the top of the GaN NW outside and away from the Au NP as shown in Figure . To be specific their GaN NWs had diameters, at least twice as large as the Au NPs and a self-regulated diameter selective growth model was put forward accounting for the stable growth of GaN NWs, where it was argued that the radius of the Au NP must be smaller than the radius of the GaN NW. This is in a way similar to the steady-state growth mechanism of GaN NWs by MBE whereby Ga atoms that impinge on the nanowire tip or within a surface diffusion length of the tip will incorporate. Adatoms arriving farther down the sides are likely to desorb rather than incorporate. Concerning GaN NWs, there is a general agreement concerning their steady-state growth regime but the nucleation process and the subsequent transient regime are, to some extent, a matter of controversy [18
]. Interestingly, the distribution of GaN NWs we obtained with 100% H2
is very similar to that of Kuo et al. [17
]. Now as seen above increasing the H2
content leads to a reduction in the areal density of the GaN NWs and the suppression of the α-like GaN layer. It is well known that noble metal NPs such as Au NPs are efficient in the catalytic dissociation of H2
and the formation of H which will react with incoming Ga around the Au NPs, leading to the formation of Ga hydride which is a gas [19
]. It has also been shown that Ga species prefer to form Ga hydride in the temperature range 800-1000°C [21
], so it is very likely that reactive Ga hydride will form at 900°C over the source of Ga but also in the vicinity of the Au NPs. One ought to recall that no GaN NWs grow on plain Si consistent with Hou and Hong [12
], so Ga must enter the Au NPs and should spread out via alloying during the initial stages of growth [22
]. The dissociation of H2
into H at the Au NP surface and the reaction of H2
, H with incoming Ga or Ga spreading out from the Au NP will suppress the formation of the α-like GaN layer and the areal density of the GaN NWs.
Figure 3 Growth mechanisms of GaN NWs by VLS (a), self-regulated, diameter selective mechanism (b), particle mediated, hydride-assisted growth via the catalytic dissociation of H2 at Au NPs (c).
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 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. [17
]. 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.