other clusters. Based on these calculations we proposed the following SiNW nucleation scheme [52]: First, a silicon suboxide cluster is deposited on the substrate
and some of its highly reactive silicon atoms are strongly bonded to the substrate
(silicon) atoms, limiting the cluster motion on the substrate. Non-bonded reactive
silicon atoms in the same cluster are now exposed to the vapor with their available
dangling bonds directed outward from the surface. They act as nuclei that absorb
additional reactive silicon oxide clusters and facilitate the formation of SiNWs with
a certain crystalline orientation. The subsequent growth of the silicon domain after
nucleation may be crystallographic dependent. Oxygen atoms in the silicon suboxide clusters might be expelled by the silicon atoms during the growth of SiNWs
and diffuse to the edge forming a chemically inert silicon oxide sheath [51]. In a
certain orientation, e.g. [112], the diffusion might be lower and the high reactive
silicon oxide phase can still be exposed to the outside and facilitate the continuous
growth of the wire in such a direction. The oxygen-rich sheath formed in other
directions may however possess lower reactivity and thus does not favor further
stacking of silicon oxide clusters from the gas-phase, leading to growth suppression in such directions. The reactivity of silicon atoms in oxygen-rich clusters becomes very low at a Si:O ratio of 1:2 [51], while the reactivity of oxygen atoms
changes to a lesser extent. The overall reactivity for Si:O ¼ 1:2 is low. 1D growth in
a specific direction is thus facilitated. In summary, the highly reactive SiO x layer
(x > 1) at the tip of nanowires acts as a collector for the vaporized silicon oxide,
while the outer SiO 2 layer of the SiNWs stops the diameter growth of the nanowires.
10.2.3
Oxide-Assisted Growth Mechanism
The Si nanowire growth is determined by four factors: (1) The high reactivity of the
Si x O ðx > 1Þ layer on nanowire tips. (2) The SiO 2 component in the shell, which is
formed from the decomposition of SiO and retards the lateral growth of nanowires.
(3) defects (e.g. dislocations) in the Si nanowire core. (4) The formation of {111}
surfaces, which have the lowest energy among the Si surfaces, parallel to the axis
of the growth direction. The first two factors were discussed earlier. As far as the
first factor is concerned we would like to add to the previous discussion that the
melting temperature of nanoparticles can be much lower than that of their bulk
materials. For example, the difference between the melting temperatures of 2 nm
Au nanoparticles and Au bulk material is over 400
C [46, 53]. The materials in the
SiNW tips (similar to the case of nanoparticles) may be in or near their molten
states, thus enhancing atomic absorption, diffusion, and deposition.
We suggest that the defects of SiNWs are one of the driving forces for the 1D
growth. The main defects in Si nanowires are stacking faults along the nanowire
growth direction of h112i, which normally contain easy-moving 1:6 [112] and nonmoving 1:3 [111] partial dislocations, and micro-twins. The presence of these defects at the tip areas should result in the fast growth of Si nanowires, since dislocations are known to play an important role in crystal growth. The SiNW growth
10 Oxide-Assisted Growth of Silicon and Related Nanowires
316
and some of its highly reactive silicon atoms are strongly bonded to the substrate
(silicon) atoms, limiting the cluster motion on the substrate. Non-bonded reactive
silicon atoms in the same cluster are now exposed to the vapor with their available
dangling bonds directed outward from the surface. They act as nuclei that absorb
additional reactive silicon oxide clusters and facilitate the formation of SiNWs with
a certain crystalline orientation. The subsequent growth of the silicon domain after
nucleation may be crystallographic dependent. Oxygen atoms in the silicon suboxide clusters might be expelled by the silicon atoms during the growth of SiNWs
and diffuse to the edge forming a chemically inert silicon oxide sheath [51]. In a
certain orientation, e.g. [112], the diffusion might be lower and the high reactive
silicon oxide phase can still be exposed to the outside and facilitate the continuous
growth of the wire in such a direction. The oxygen-rich sheath formed in other
directions may however possess lower reactivity and thus does not favor further
stacking of silicon oxide clusters from the gas-phase, leading to growth suppression in such directions. The reactivity of silicon atoms in oxygen-rich clusters becomes very low at a Si:O ratio of 1:2 [51], while the reactivity of oxygen atoms
changes to a lesser extent. The overall reactivity for Si:O ¼ 1:2 is low. 1D growth in
a specific direction is thus facilitated. In summary, the highly reactive SiO x layer
(x > 1) at the tip of nanowires acts as a collector for the vaporized silicon oxide,
while the outer SiO 2 layer of the SiNWs stops the diameter growth of the nanowires.
10.2.3
Oxide-Assisted Growth Mechanism
The Si nanowire growth is determined by four factors: (1) The high reactivity of the
Si x O ðx > 1Þ layer on nanowire tips. (2) The SiO 2 component in the shell, which is
formed from the decomposition of SiO and retards the lateral growth of nanowires.
(3) defects (e.g. dislocations) in the Si nanowire core. (4) The formation of {111}
surfaces, which have the lowest energy among the Si surfaces, parallel to the axis
of the growth direction. The first two factors were discussed earlier. As far as the
first factor is concerned we would like to add to the previous discussion that the
melting temperature of nanoparticles can be much lower than that of their bulk
materials. For example, the difference between the melting temperatures of 2 nm
Au nanoparticles and Au bulk material is over 400
C [46, 53]. The materials in the
SiNW tips (similar to the case of nanoparticles) may be in or near their molten
states, thus enhancing atomic absorption, diffusion, and deposition.
We suggest that the defects of SiNWs are one of the driving forces for the 1D
growth. The main defects in Si nanowires are stacking faults along the nanowire
growth direction of h112i, which normally contain easy-moving 1:6 [112] and nonmoving 1:3 [111] partial dislocations, and micro-twins. The presence of these defects at the tip areas should result in the fast growth of Si nanowires, since dislocations are known to play an important role in crystal growth. The SiNW growth
10 Oxide-Assisted Growth of Silicon and Related Nanowires
316