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

etween 2.5-25 µm. The former is used for alignment and the latter interacts with
the sample. In the FTIR set-up, the IR radiation is guided through a Michelson's
interferometer (Fig. 2.4a) with a moving mirror. An interferogram consisting of
light intensity versus optical path is recorded by the detector with each movement
of the mirror. This data from the interferogram is converted into the spatial domain
using fourier transformation (Fig. 2.4b). The contributions from the reference substrate
and other atmospheric components such as CO 2 and water were subtracted
from the spectrum to analyse the structural composition of the deposited material.
tel-00916300, version 1 - 10 Dec 2013
Figure 2.4: (a) Schematic representation of FTIR set-up and (b) Interferogram to Fourier
transformed spectra.
Typical spectra of SiO 2 obtained in two dierent angles of incidence are presented
in gure 2.5 as a representative of the main absorption features analysing
methodology which will be followed for all the other samples in this thesis.
The spectrum recorded in normal incidence enables to visualise the transverse
optical modes as shown in gure 2.5a. The spectra of SiO 2 is characterized by
three major absorption bands TO 2 , TO 3 and TO 4 corresponding to Si-O-Si rocking,
symmetric stretching and asymmetric stretching modes respectively. Each transverse
optical mode (TO) has an associated longitudinal optical mode (LO), and
this is called as the LO-TO splitting. LO modes cannot be seen through FTIR in
normal incidence but can be detected if measured in oblique incidence (Fig. 2.5b)
[Berremann 63]. In this thesis, Brewster incidence of 65° is chosen for viewing the
LO modes. Besides these vibrations, there is a fourth vibrational splitting known as
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