Films minces à base de Si nanostructuré pour des cellules ...

T d (°C) ν T O3 x = 0/Si
from FTIR
Si excess (at.%)
from FTIR
(unbonded Si)
x = 0/Si
from ellipsometry
Si excess (at.
%)
from
ellipsometry
(agglomerated
Si)
200 1059.6 1.77 4.15 1.92 2.15
300 1061 1.79 3.8 1.92 2.15
400 1064 1.82 3.15 1.91 2.37
500 1067 1.86 2.539 1.76 4.98
Table 3.2: Si excess estimation by FTIR and refractive index (Bruggeman method- analysis
with regard to T d .
(d) Growth mechanism
tel-00916300, version 1 - 10 Dec 2013
It has been reported that r d increases with increasing T d [Gates 89, Kaiser 98]. Our
results indicate a steadily decreasing r d which can be related to a continuous increase
in the etch rates. But, n 1.95eV increases linearly with T d indicating an increase in
the Si content. This necessitates an understanding of the temperature dependent
growth mechanism. The following mechanism is proposed based on the kinetic
theory of gases and temperature dependence of SiH x etch products. A diagrammatic
representation of the proposed explanations are shown in gure 3.3.
According to classical mechanics, the kinetic energy is a function of the particle
mass (m) and velocity (v) expressed as,
E k = (1/2)mv 2 ⋍ (3/2)kT Eqn. (3.9)
where (3/2)kT is the averaged kinetic energy of a classical ideal gas per degree
of freedom related to temperature T, and k is the Boltzmann constant. Considering
our sputter gases, Ar is monoatomic having three degrees of freedom and hydrogen
being a diatomic gas has 6 degrees of freedom.
It can be seen from equation 3.9 that an increase in temperature increases the
average kinetic energy and therefore the velocity of the species in the sputter chamber.
When the substrate is heated, it cannot be denied that a certain amount of
energy is introduced in the plasma leading to the plasma temperature. Thus it can
be said that with increasing T d , E k(avg) and velocity of species in the plasma increases
leading to an enhancement in the kinetics of sputtering process. Moreover,
in a mixture of particles with dierent masses, the heavier atom will have a lower
velocity than the lighter one but have the same average kinetic energy. Hence, at a
given T d hydrogen having the lowest mass acquires the greatest velocity.
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