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

tel-00916300, version 1 - 10 Dec 2013
Initially the laser power
density was kept minimum
(0.14 MW/cm 2 ) to avoid any
possible inuence of laser heating
on the microstructure of our
material. The two broad peaks
around 150 and 480 cm −1 are
attributed the presence of a-
Si and their intensities suggest
a signicant density of amorphous
particles in the material.
It can be seen that even with a
low power density, the width of
the peak between 450-550 cm −1
in the CA lm reduces as compared
to its STA counterpart
and a sharp peak around 510
Figure 4.6: Raman spectra of STA and CA SRSN
sample (n 1.95ev =2.44) grown on fused silica substrate.
(Inset) PL spectra obtained in Raman setup
with λ exc =2.33eV (532 nm) and power density 1.4
MW/cm 2 .
cm −1 begins to appear. This latter peak attests the presence of nanocrystals in
the material and its low intensity indicates a lower density of nanocrystals. There
is no signicant change in this peak on increasing the laser power densities implying
the absence of laser annealing eect. This may be attributed to our sample
whose refractive index 2.44, corresponds to the lower limit required for nanocrystal
formation and to the lower diusion coecient of Si in SRSN as compared to SiO 2 .
Considering the possible inuence of the substrate on this crystallization, the
presence of nanocrystals revealed by XRD from the same material grown on the
Si substrate helps us conrm this sharp peak observed through Raman is from our
SRSN layer. It is also interesting to note from the inset of gure 4.6 that the
PL is quenched in CA sample despite the formation of nanocrystals, suggesting
nanocrystals have a detrimental eect on emission.
4.2.3 Optical properties
The microstructural investigations reveal the presence of Si-np, and the Raman studies
suggest a decrease in emission intensity associated with nanocrystal formation.
In order to understand if such a suggestion is valid, investigations on the photoluminescence
and absorption coecients were made with regard to refractive indices
and annealing.
The absorption coecient studies were initially made on the thinner SiN x as-
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