Chemistry Reference
In-Depth Information
where B is a constant related to the material,
γ
is the surface energy of the solid,
k the Boltzmann constant, T is temperature and
σ
the supersaturation ratio:
p
p
0
σ
=
(5.3)
Where p is the actual partial pressure of the vapour specie and p
0
is the equilibrium
partial pressure at temperature T.
This means that the nucleation probability is related to
γ
, the surface energy of
the solid, which can change for different crystallographic planes. An anisotropic
growth can therefore be caused by a wide disparity between the surface energy of
different crystallographic planes of the nanowire and the consequent surface energy
minimization.
Furthermore, any irregularity such as defects or impurities on the substrate can
act as a nucleation site for the nanowire growth (Yang and Lieber
2011
) and, in
particular, a screw dislocation can propagate and promote an axial growth (Burton
et al.
1951
) resulting in the formation of helical SiC nanowires (Zhang et al.
2002
).
The possibility of growing catalyst free SiC nanowires (Yang et al.
2005
) can
be an advantage because the presence of a metal could be deleterious for device
applications.
5.3
Substrate Preparation
The procedure for the growth of silicon carbide nanowires starts with the chemical
cleaning of the substrate, than the metal catalyst is deposed onto it and finally the
sample is inserted into the reactor chamber where the growth takes place.
In vapour phase epitaxy of semiconductor species the substrate plays a funda-
mental role, and the growth interface has a direct influence on the structure of
the epi-layers. In addition, what produces the uniaxial elongation in a catalytic
growth mechanism is the presence of nucleation points on the surface of the
substrate, so, for nanowires synthesis, the presence and the arrangement of the
catalyst holds sway on the final nanostructure morphology and organization.
For that reason a peculiar attention must be paid to substrate preparation and
catalyst deposition.
In this section, after a brief description of the substrate preparation procedure, differ-
ent catalyst deposition techniques are illustrated and the dewetting process is studied
with selected examples.
