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

such a peak was witnessed in [Quiroga 09]. Therefore it may be supposed in our
case, that while forming agglomerates oxygen may be trapped within the Si core
resulting in such a peak. However, at this stage it is not possible to say certainly
if the appearance of this peak with Si excess is due to an increase in disorder or
formation of oxygen trapped Si agglomerates.
It can be seen from the inset of gure 3.6 that both LO 3 and TO 3 peak positions
shift towards lower wavenumbers with increasing r H except for the highest hydrogen
rate. The shift towards lower wavenumbers with increasing Si excess till r H =26%
may be explained with the force constant model. As the Si content increases, the
bond angle of Si-O-Si decreases from 180° as per force constant model (Eqn. 3.13)
and hence the LO 3 mode shifts towards lower wavenumbers [Pai 86, Lucovsky 87]
ν 2 = (k/m 0 )sin 2 (φ/2) Eqn (3.13)
tel-00916300, version 1 - 10 Dec 2013
where ν is the frequency of vibration, k is the nearest neighbour force constant,
m 0 is the mass of oxygen and φ is the Si-O-Si bond angle. The shift of the peak
positions towards higher wavenumbers (towards SiO 2 ) in the case of r H = 57%
may support the previous argument of oxygen trapped by the Si core while forming
agglomerates.
(c) Si excess estimation
In order to consider a balance between r d , n 1.95eV and material composition, it is
important to have an estimate of the Si excess. The Si excess was estimated from
FTIR analysis (unbonded Si) within an uncertainity of ±0.2%, and ellipsometry
(Bruggemann method-agglomerated Si) analysis within an uncertainity of ±3% as
detailed before. Table 3.4 consolidates the value of Si excess obtained by these
methods.
r H (%) 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)
4.6 1068 1.86 2.33 1.99 0.217
11.7 1064 1.82 3.26 1.87 2.33
26 1065 1.82 3.26 1.64 6.76
57 1069 1.87 2.14 1.61 7.55
Table 3.4:
varying r H .
Si excess estimation by FTIR and refractive index analysis with regard to
69