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

4.2.2 Structural analysis
(a) Fourier transform infrared spectroscopy
tel-00916300, version 1 - 10 Dec 2013
Figure 4.2 shows typical FTIR spectra of our SRSN samples, obtained from a 490
nm thick sample with n 1.95eV = 2.44. The spectra are recorded in both normal and
oblique (Brewster = 65°) incidences.
The normal incidence spectrum contains
only a single absorption peak
around 840 cm −1 , whereas an additional
peak centered around 1020 cm −1 appears
when measurements are made in
Figure 4.2: Typical FTIR spectra of our
SRSN samples recorded in Brewster and normal
incidence obtained from a SRSN sample
with t=490 nm and n 1.95eV =2.44. The TO
mode of Si-N is normalized to unity in the
spectra.
oblique incidence. In the whole spectral
range between 4000-700 cm −1 , only
these two absorption bands can be observed.
This proves the absence of Si-O 1
or Si-H 2 or N-H bonds 3 that are usually
observed in SiN x materials due to oxygen
or hydrogen contamination during
the deposition process. Thus, the two
peaks around 840 cm −1 and 1020 cm −1
observed in our samples can be unambiguously
attributed to the TO and LO
modes of asymmetric Si-N stretching vibrations
respectively [Lin 92, Dupont 97, Bustarret 98, Batan 08]. The contribution
of Si wafer from the interfacial oxide is noticed in thinner SiN x samples around 1107-
1105 cm −1 , whereas in thick samples this position is overlapped by the LO Si−N mode
as seen in gure 4.2.
The evolution of FTIR spectra in SiN x monolayers as a function of refractive index
(i.e. composition) was investigated on samples with thicknesses ranging between
70-100 nm (Fig. 4.3). In the Brewster incidence spectra, the curves are normalized
to 100 nm thickness, and in the normal incidence spectra the TO Si−N peak intensity
is normalized to unity. The spectra is focussed in the region containing the
Si-N bonds since these are the bonds we detect in our material in contrast to other
deposition approaches [Aydinli 96, Wang 03, Vernhes 06, Scardera 08].
1 Si-O (TO 4 ∼ 1200cm −1 , LO 4 ∼ 1160cm −1 ,TO 3 ∼ 1080cm −1 ,LO 3 ∼ 1250cm −1 ,TO 2 ∼
810cm −1 ,LO 3 ∼ 820cm −1 ) [Kirk 88, Bensch 90]
2 Si-H (stretching∼2100 cm −1 ,wagging∼640 cm −1 ,∼890 cm −1 ) [Shanks 80, Shanks 81, Satoh 85]
3 N-H (stretching∼3320-2500cm −1 , bending∼1140-1200cm −1 ) [Coates 00, Lin 02]
93