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Optimization of self-catalyzed InAs Nanowires on flexible graphite for photovoltaic infrared photodetectors

View Article: PubMed Central - PubMed

ABSTRACT

The recent discovery of flexible graphene monolayers has triggered extensive research interest for the development of III-V/graphene functional hybrid heterostructures. In order to fully exploit their enormous potential in device applications, it is essential to optimize epitaxial growth for the precise control of nanowire geometry and density. Herein, we present a comprehensive growth study of InAs nanowires on graphitic substrates by molecular beam epitaxy. Vertically well-aligned and thin InAs nanowires with high yield were obtained in a narrow growth temperature window of 420–450 °C within a restricted domain of growth rate and V/III flux ratio. The graphitic substrates enable high nanowire growth rates, which is favourable for cost-effective device fabrication. A relatively low density of defects was observed. We have also demonstrated InAs-NWs/graphite heterojunction devices exhibiting rectifying behaviour. Room temperature photovoltaic response with a cut-off wavelength of 3.4 μm was demonstrated. This elucidates a promising route towards the monolithic integration of InAs nanowires with graphite for flexible and functional hybrid devices.

No MeSH data available.


Aspect ratio (top), length (LNW) and diameter (DNW) (bottom) of InAs nanowires grown on graphitic substrate as a function of varied growth temperature (TG) (a); growth rate (b) and V/III flux ratio (c).
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f2: Aspect ratio (top), length (LNW) and diameter (DNW) (bottom) of InAs nanowires grown on graphitic substrate as a function of varied growth temperature (TG) (a); growth rate (b) and V/III flux ratio (c).

Mentions: The scanning electron microscope (SEM) images in Fig. 1 shows the dependence of InAs NWs density and morphology on growth temperature (TG) in sample series A. The NWs exhibit a homogeneous diameter along their entire length without any measurable tapering. At the lowest temperature (400 °C), a dominant cluster (islands) growth was observed. A slight increase in TG to 420 °C yielded a sparse NW distribution (∼5.28 × 108 cm−2), and a maximum yield of ∼8.09 × 108 cm−2 was obtained for a further increase in temperature to 435 °C (Fig. 1b). Increasing TG into the 450–475 °C range led to a decrease in NW density as shown in Fig. 1(c,d). The observed dependence of NW density on TG can be explained by the kinetic modifications to the distribution of pre-deposited In droplets30. A kinetically inhibited adatom mobility at a low temperature (400 °C) promotes the development of small but highly dense droplets leading to the growth of surface clusters at the expense of NWs. An increase in TG in the range of 420–435 °C results in the formation of slightly large but well seperated droplets which favour the growth of high density of NWs. It is therefore not surprising that the highest yield of vertically-aligned NWs was obtained at 435 °C. However, for higher temperatures (435 °C < TG ≤ 475 °C), the nucleation droplets merge to form clusters and are consumed at the early stages of NW growth. In addition, the desorption rate of the droplets is significantly increased leading to less nulceation of NWs43. Figure 2(a) shows the variation of NWs length (LNW) as a function of TG. Over 70% of measurable NWs in each sample were analyzed for each point. Gaussian approximations were then used for the determination of the error bars of the NWs geometry (LNW and DNW) which is expressed as the deviation from the mean geometry of normally distributed NWs. It can be seen that LNW increases for a TG increase in the range of 400–435 °C while it decreases in the temperature range of 435 °C < TG ≤ 475 °C. Maximum LNW was thus obtained at a TG of 435 °C which is consistent with a previous report44. It is worth noting that the dependence of LNW on TG is accompanied by an inverse dependence of diameter (DNW) on TG, e.g. the DNW decreases first (up to 435 °C) then increases with a further rise in TG. Consequently, long and thin NWs with a high aspect ratio (~83) was obtained at an optimal temperature of 435 °C (length and diameter of 2.58 ± 0.34 μm and ~31.21 ± 6.59 nm respectively). This also corresponds to the temperature which yields a high density of NWs. Notably, despite the high aspect ratio, the NWs remain vertically well-aligned without the presence of randomly oriented NWs. This demonstates the feasibility of fabricating InAs NWs with good geometry which would allow for fundamental studies such as size-dependent quantum confinement effects. The observed phenomenon can be interpreted in terms of the diffusion-limited growth of NWs in MBE. It has been theoretically and experimentally shown that the axial growth of NWs by MBE is strongly dependent on adatom diffusion from the substrate to the droplet4546.


Optimization of self-catalyzed InAs Nanowires on flexible graphite for photovoltaic infrared photodetectors
Aspect ratio (top), length (LNW) and diameter (DNW) (bottom) of InAs nanowires grown on graphitic substrate as a function of varied growth temperature (TG) (a); growth rate (b) and V/III flux ratio (c).
© Copyright Policy - open-access
Related In: Results  -  Collection

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getmorefigures.php?uid=PMC5385536&req=5

f2: Aspect ratio (top), length (LNW) and diameter (DNW) (bottom) of InAs nanowires grown on graphitic substrate as a function of varied growth temperature (TG) (a); growth rate (b) and V/III flux ratio (c).
Mentions: The scanning electron microscope (SEM) images in Fig. 1 shows the dependence of InAs NWs density and morphology on growth temperature (TG) in sample series A. The NWs exhibit a homogeneous diameter along their entire length without any measurable tapering. At the lowest temperature (400 °C), a dominant cluster (islands) growth was observed. A slight increase in TG to 420 °C yielded a sparse NW distribution (∼5.28 × 108 cm−2), and a maximum yield of ∼8.09 × 108 cm−2 was obtained for a further increase in temperature to 435 °C (Fig. 1b). Increasing TG into the 450–475 °C range led to a decrease in NW density as shown in Fig. 1(c,d). The observed dependence of NW density on TG can be explained by the kinetic modifications to the distribution of pre-deposited In droplets30. A kinetically inhibited adatom mobility at a low temperature (400 °C) promotes the development of small but highly dense droplets leading to the growth of surface clusters at the expense of NWs. An increase in TG in the range of 420–435 °C results in the formation of slightly large but well seperated droplets which favour the growth of high density of NWs. It is therefore not surprising that the highest yield of vertically-aligned NWs was obtained at 435 °C. However, for higher temperatures (435 °C < TG ≤ 475 °C), the nucleation droplets merge to form clusters and are consumed at the early stages of NW growth. In addition, the desorption rate of the droplets is significantly increased leading to less nulceation of NWs43. Figure 2(a) shows the variation of NWs length (LNW) as a function of TG. Over 70% of measurable NWs in each sample were analyzed for each point. Gaussian approximations were then used for the determination of the error bars of the NWs geometry (LNW and DNW) which is expressed as the deviation from the mean geometry of normally distributed NWs. It can be seen that LNW increases for a TG increase in the range of 400–435 °C while it decreases in the temperature range of 435 °C < TG ≤ 475 °C. Maximum LNW was thus obtained at a TG of 435 °C which is consistent with a previous report44. It is worth noting that the dependence of LNW on TG is accompanied by an inverse dependence of diameter (DNW) on TG, e.g. the DNW decreases first (up to 435 °C) then increases with a further rise in TG. Consequently, long and thin NWs with a high aspect ratio (~83) was obtained at an optimal temperature of 435 °C (length and diameter of 2.58 ± 0.34 μm and ~31.21 ± 6.59 nm respectively). This also corresponds to the temperature which yields a high density of NWs. Notably, despite the high aspect ratio, the NWs remain vertically well-aligned without the presence of randomly oriented NWs. This demonstates the feasibility of fabricating InAs NWs with good geometry which would allow for fundamental studies such as size-dependent quantum confinement effects. The observed phenomenon can be interpreted in terms of the diffusion-limited growth of NWs in MBE. It has been theoretically and experimentally shown that the axial growth of NWs by MBE is strongly dependent on adatom diffusion from the substrate to the droplet4546.

View Article: PubMed Central - PubMed

ABSTRACT

The recent discovery of flexible graphene monolayers has triggered extensive research interest for the development of III-V/graphene functional hybrid heterostructures. In order to fully exploit their enormous potential in device applications, it is essential to optimize epitaxial growth for the precise control of nanowire geometry and density. Herein, we present a comprehensive growth study of InAs nanowires on graphitic substrates by molecular beam epitaxy. Vertically well-aligned and thin InAs nanowires with high yield were obtained in a narrow growth temperature window of 420&ndash;450&thinsp;&deg;C within a restricted domain of growth rate and V/III flux ratio. The graphitic substrates enable high nanowire growth rates, which is favourable for cost-effective device fabrication. A relatively low density of defects was observed. We have also demonstrated InAs-NWs/graphite heterojunction devices exhibiting rectifying behaviour. Room temperature photovoltaic response with a cut-off wavelength of 3.4&thinsp;&mu;m was demonstrated. This elucidates a promising route towards the monolithic integration of InAs nanowires with graphite for flexible and functional hybrid devices.

No MeSH data available.