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

tel-00916300, version 1 - 10 Dec 2013
Figure 4.3: Evolution of FTIR spectra with refractive index as observed in Brewster and
normal incidences; (Inset) TO Si−N peak positions versus refractive index.
The peak around 1106 cm −1 prominently seen in samples with high refractive
indices in Brewster incidence spectra and in all the samples in normal incidence
spectra can be conrmed as Si substrate contribution for two major reasons:
1. The Brewster incidence spectra clearly shows the overlapping of LO Si−N
mode and the 1106 cm −1 peak at lower refractive indices. With increasing refractive
indices, there is a shift of the LO Si−N mode towards lower wavenumbers, whereas
the peak at 1106 cm −1 remains unchanged and is distinctly seen. This peak position
coincides with that observed in Si wafer.
2. A low intensity peak around 1250 cm −1 is observed in Brewster incidence
spectra, suggesting a minimal contribution of Si-O bonds. Hence this peak can be
attributed to the interstitial oxygen in Si substrate (similar to that witnessed at 1107
cm −1 in SRSO samples) or the thermal oxide on the substrate whose contribution
is witnessed better with decreasing sample thickness.
It can be seen from both Brewster and normal incidence spectra that the LO Si−N
and TO Si−N band redshifts with increasing refractive index. Similar shifts of the
LO and TO bands were observed in hydrogenated SiN x lms and were reasoned as
the hydrogen incorporation [Lin 92, Vernhes 06, Bustarret 98, Lucovsky 83]. Since
our samples are hydrogen free, these shifts can be attributed to modication in the
Si-N bonding conguration. A redshift of the TO mode with increasing refractive
index, as observed in our case (Inset of Fig. 4.3) was reported in [Hasegawa 93] and
attributed to a decrease in the Si-N bond length with changing composition of SiN x .
94