incorporation in the nanoclusters formed in an N 2 environment probably leads to
incomplete phase separation and to the formation of the remnant spherical particles. The Ar (5% H 2 ) mixture is suspected to exert an intermediate effect between
N 2 and He, thus giving rise to nanowires of intermediate diameters.
10.3.3
Large-Area Aligned and Long SiNWs via Flow Control [62]
Until now we have discussed the effect of two parameters on the SiNW growth:
temperature and carrier gas. Now we show that the carrier gas flow can be exploited as well. In this particular example we use it to grow millimeter-area arrays
of highly oriented, crystalline silicon nanowires of millimeter length.
The growth in this particular case was performed by thermal evaporation of SiO
powder. A carrier gas of argon mixed with 5% H 2 was used with a flow of 50 sccm
at 400 Torr. The furnace temperature was 1300
C, while the growth temperature
was about 930
C.
Large area (about 2 mm  3 mm) of highly oriented, long (up to 1.5–2 cm)
nanowires were grown on the surface of the silicon substrate under these conditions, as detected by SEM (Figure 10.13). The thickness of the oriented nanowire
product was about 10 mm, as estimated from the cross-sectional image (Figure
10.13(b)) of the sample prepared by focused ion beam cutting. EDX shows that the
nanowires are composed of silicon and oxygen with no metal impurities, suggesting an oxide assisted growth. Figure 10.13(c) shows the typical morphology of
SiNWs. TEM shows that the SiNWs are quite clean, with very few particles attached to their surfaces, and are relatively homogeneous. The SiNW diameters vary
from 18 to 46 nm, and the mean value is about 30 nm.
HRTEM shows the typical SiNW Si core encapsulated by a SiO 2 sheath and the
{111} planes of crystalline silicon. The diameters of the crystalline silicon core
varied from 13 to 30 nm, and the mean value was about 20 nm. The thickness of
the amorphous silicon oxide shell varied from 2 to 10 nm, and the mean value was
about 5 nm.
The growth of the oriented SiNWs may be related to the flow of the carrier gas,
because it was found that the orientation direction of the SiNWs is parallel to the
direction of flow of the carrier gas in the alumina tube. A mechanism for the
growth of the oriented silicon nanowires is illustrated below. First, the nucleation
of silicon nanowires from silicon oxide (SiO x ) vapor started at the proper position
on the substrates. Because there was a temperature gradient along the alumina
tube, and the planes with the same temperature were perpendicular to the axial
direction of the tube, only some particular positions with the appropriate temperature may be suitable for the nucleation of SiNWs [21, 27]. These positions should
be located on a line of equal temperature on the substrate and will also be perpendicular to the flow direction of the carrier gas. Once initial nucleation is established, nanowire growth will tend to continue on the substrate. Secondly, the
strength of the flow of the carrier gas will force the growing nanowires to grow in
10 Oxide-Assisted Growth of Silicon and Related Nanowires
328
incomplete phase separation and to the formation of the remnant spherical particles. The Ar (5% H 2 ) mixture is suspected to exert an intermediate effect between
N 2 and He, thus giving rise to nanowires of intermediate diameters.
10.3.3
Large-Area Aligned and Long SiNWs via Flow Control [62]
Until now we have discussed the effect of two parameters on the SiNW growth:
temperature and carrier gas. Now we show that the carrier gas flow can be exploited as well. In this particular example we use it to grow millimeter-area arrays
of highly oriented, crystalline silicon nanowires of millimeter length.
The growth in this particular case was performed by thermal evaporation of SiO
powder. A carrier gas of argon mixed with 5% H 2 was used with a flow of 50 sccm
at 400 Torr. The furnace temperature was 1300
C, while the growth temperature
was about 930
C.
Large area (about 2 mm  3 mm) of highly oriented, long (up to 1.5–2 cm)
nanowires were grown on the surface of the silicon substrate under these conditions, as detected by SEM (Figure 10.13). The thickness of the oriented nanowire
product was about 10 mm, as estimated from the cross-sectional image (Figure
10.13(b)) of the sample prepared by focused ion beam cutting. EDX shows that the
nanowires are composed of silicon and oxygen with no metal impurities, suggesting an oxide assisted growth. Figure 10.13(c) shows the typical morphology of
SiNWs. TEM shows that the SiNWs are quite clean, with very few particles attached to their surfaces, and are relatively homogeneous. The SiNW diameters vary
from 18 to 46 nm, and the mean value is about 30 nm.
HRTEM shows the typical SiNW Si core encapsulated by a SiO 2 sheath and the
{111} planes of crystalline silicon. The diameters of the crystalline silicon core
varied from 13 to 30 nm, and the mean value was about 20 nm. The thickness of
the amorphous silicon oxide shell varied from 2 to 10 nm, and the mean value was
about 5 nm.
The growth of the oriented SiNWs may be related to the flow of the carrier gas,
because it was found that the orientation direction of the SiNWs is parallel to the
direction of flow of the carrier gas in the alumina tube. A mechanism for the
growth of the oriented silicon nanowires is illustrated below. First, the nucleation
of silicon nanowires from silicon oxide (SiO x ) vapor started at the proper position
on the substrates. Because there was a temperature gradient along the alumina
tube, and the planes with the same temperature were perpendicular to the axial
direction of the tube, only some particular positions with the appropriate temperature may be suitable for the nucleation of SiNWs [21, 27]. These positions should
be located on a line of equal temperature on the substrate and will also be perpendicular to the flow direction of the carrier gas. Once initial nucleation is established, nanowire growth will tend to continue on the substrate. Secondly, the
strength of the flow of the carrier gas will force the growing nanowires to grow in
10 Oxide-Assisted Growth of Silicon and Related Nanowires
328