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

With increasing T d , there is a progressive decrease in the LO 3 peak intensity.
As seen from the insets, with increasing T d , the Si-H peak decreases in intensity
and the LO 3 peak position (ν LO3 ) shifts towards higher wavenumbers approaching
the ν LO3 of SiO 2 . There is a similar shift in the TO 3 peak position (ν T O3 ) towards
higher wavenumbers as can be seen from the normal incidence spectra. The inset in
gure 3.2b clearly shows this variation in TO 3 peak positions. These results indicate
that the sample becomes more ordered and the SiO x matrix gradually shifts towards
SiO 2 with increasing T d .
(c) Si excess estimation
c1) From FTIR analysis
tel-00916300, version 1 - 10 Dec 2013
The phase separation of SiO x into SiO 2 and Si particles can be initiated
during the deposition process, or post-annealing treatment. This
is represented by the following equation:
SiO x =
( x
) (
SiO 2 + 1 − x )
Si Eqn (3.2)
2 2
The value x [= O/Si] is calculated using υ T O3 obtained in the FTIR
spectra, from which the atomic percentage of Si excess is estimated using
the formulae specied in section 2.2.1 of chapter 2.
c2) From Refractive index and Bruggeman model
The Si excess and the value of x can also be estimated from the
values of refractive index obtained from ellipsometry using Bruggeman
model (eective medium approximation-[Bruggeman 35]). In our case of
SiO x materials, the eective dielectric function (ε e ) is a combination of
the dielectric functions of Si and SiO 2 . The Bruggeman model gives a
relationship of the eective medium as a volumic fraction of two dierent
materials as follows:
where,
f 1 ( ɛ Si − ɛ e
ɛ Si + 2ɛ e
) + f 2 ( ɛ SiO 2
− ɛ e
ɛ SiO2 + 2ɛ e
)= 0 Eqn (3.3)
ɛ e = n 2 e is the relative permittivity of the eective medium
ɛ Si and ɛ SiO2 are the relative permittivity of Si and SiO 2 respectively,
and f Si and f SiO2 their volumic fractions.
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