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

4.3.3 Optical properties
(a) Photoluminescence
The photoluminescence curves of Si 3 N 4 and SRSN (n 1.95eV
= 2.32) samples with
regard to annealing are as shown in gure 4.11. It can be seen that the Si 3 N 4
sample exhibits PL of very low intensity between 1.8-2.2 eV, which increases with
annealing. This can be attributed to some radiative defects in the material. The
spectrum was recorded with a 500 nm high pass lter and hence the curve is cut
o after 2.2 eV. No emission is observed from SRSN sample whereas in the Raman
set-up emission was observed from the same sample. The dierence in emission
intensities of these samples when measured in the PL and Raman set-up, can be
attributed to the dierent excitation power densities used in the experiments (1.4
MW/cm 2 in Raman set-up versus 10 W/cm 2 in the PL set-up).
tel-00916300, version 1 - 10 Dec 2013
(a) Si 3 N 4
(b) SRSN
Figure 4.14: Eect of annealing on photoluminescence of Si 3 N 4 and SRSN layers.
(λ excitation =488 nm and laser power density= 10 W/cm 2 ).
(b) Absorption coecient, (α)
The absorption coecient curves obtained from Si 3 N 4 and SRSN as-grown samples
are as presented in gure 4.15.
It can be noticed from this gure that the trend of the absorption coecient
curves are similar to those obtained in samples grown by the rst deposition approach.
The Si 3 N 4 sample has a low absorption in comparison to SRSN samples
and all the curves show a steadily increasing α values with energy as well as with refractive
index (increasing Si excess). The trend of α for this latter case is attributed
to the increase of Si-np density with Si incorporation.
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