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Gallium hydride vapor phase epitaxy of GaN nanowires.

Zervos M, Othonos A - Nanoscale Res Lett (2011)

Bottom Line: 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).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.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.

View Article: PubMed Central - HTML - PubMed

Affiliation: Nanostructured Materials and Devices Laboratory, Department of Mechanical Engineering, Materials Science Group, School of Engineering, University of Cyprus, P,O, Box 20537, Nicosia 1678, Cyprus. zervos@ucy.ac.cy.

ABSTRACT
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.

No MeSH data available.


Related in: MedlinePlus

Growth mechanisms of GaN NWs by VLS (a), self-regulated, diameter selective mechanism [17](b), particle mediated, hydride-assisted growth via the catalytic dissociation of H2 at Au NPs (c).
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Figure 3: Growth mechanisms of GaN NWs by VLS (a), self-regulated, diameter selective mechanism [17](b), particle mediated, hydride-assisted growth via the catalytic dissociation of H2 at Au NPs (c).

Mentions: 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 1b and Table 1. 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 3a. 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 3b. 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 [19,20]. 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.


Gallium hydride vapor phase epitaxy of GaN nanowires.

Zervos M, Othonos A - Nanoscale Res Lett (2011)

Growth mechanisms of GaN NWs by VLS (a), self-regulated, diameter selective mechanism [17](b), particle mediated, hydride-assisted growth via the catalytic dissociation of H2 at Au NPs (c).
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC3211325&req=5

Figure 3: Growth mechanisms of GaN NWs by VLS (a), self-regulated, diameter selective mechanism [17](b), particle mediated, hydride-assisted growth via the catalytic dissociation of H2 at Au NPs (c).
Mentions: 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 1b and Table 1. 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 3a. 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 3b. 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 [19,20]. 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.

Bottom Line: 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).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.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.

View Article: PubMed Central - HTML - PubMed

Affiliation: Nanostructured Materials and Devices Laboratory, Department of Mechanical Engineering, Materials Science Group, School of Engineering, University of Cyprus, P,O, Box 20537, Nicosia 1678, Cyprus. zervos@ucy.ac.cy.

ABSTRACT
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.

No MeSH data available.


Related in: MedlinePlus