Nanotechnology-Enabled Sensors

4.3 Formation of Thin Films 145
The following affect the condensation rate:
(a) If the accommodation coefficient, α, is different in various faces, the
result can be an asymmetric growth.
(b) J linearly depends on α at low concentrations of the deposition species
and low J. However, at high value for J the increase in α does not cause
any increase in J anymore as the Eq. (4.1) is no longer valid.
(c) High deposition pressure, P, increases, J, and at the same time may increase
the probability of producing defects. This can be beneficial in the
formation of nanostructures. Generally, we would like to produce gaps
which result in separate nanostructures during the deposition process. High
concentrations of the growth species may also generate secondary nucleation.
The secondary nucleation can act as a seed for the growth of multiple
crystal facets and branching (Fig. 4.6).
Fig. 4.6 The formation of nucleation sites and the subsequent branching of nanorods
from these sites during the thermal evaporation of molybdenum oxide onto
the surface of alumina.
The residence time, τs, and diffusion distance, Ds, for the growth of a
species on a substrate are two important parameters which determine the
formation and the dimensions of nanostructures during the deposition
process. τs is given by the equation:
exp(
Edes / kT )
τ s =
, (4.2)
v