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

tel-00916300, version 1 - 10 Dec 2013
is interesting to note that doubling the number of patterns from 50 to 100 does not
result in a doubling of PL intensity. Instead, the PL intensity shows a huge enhancement
of about 7.4 times and suggests that besides the pattern number there might
be other factors inuencing the emission. The possible reasons will be systematically
analyzed in the forthcoming sections and in chapter 5.
The 50 patterned ML also has three peaks (a, b & c) which are redshifted from
peaks (1, 2 & 3) of the 100 patterned ML (Fig. 4.35b). The positions of peaks (b &
c) fall within the range of peak (1) & peak (2) as seen in previous section and may
be considered as a contribution of Si-np. The peak (3) disappears in 50 patterned
sample, and a new peak (a) appears at lower energy. It is complicated to reason
out:
- if peak (a) has a new origin and if the absence of peak (3) in 50 patterned
sample might be due to a reduced volume of SRSN in the material. This is because
peak (3) is assumed to be arising from Si:Si 3 N 4 interface and/or defect states and
even in 100 patterned ML (more SRSN sublayers than 50(3.5/5)), this peak intensity
is too low.
- if these three curves in 50 patterned ML are only the redshifted (1, 2 & 3)
peaks observed in the 100 patterned sample.
4.8 Eect of SRSN sublayer thickness on structural
and optical properties
The inuence of SRSN sublayer thicknesses (t SRSN ) on the structural and optical
properties were investigated by growing 50(t SRSO /t SRSN ) MLs, where t SRSO = 3 nm
and t SRSN ranges between 1.5 to 10 nm.
Figure 4.36a shows the as-grown FTIR spectra of these set of samples recorded
in Brewster incidence and normalized to 100 nm thickness.
A pronounced increase in the intensity of (LO and TO) Si−N modes can be seen
when t SRSN varies from 1.5 nm - 8 nm which is attributed to an increase in number of
Si-N bonds with sublayer thickness. Such a trend is also observed in the asymmetric
stretching modes of Si-O but to a lower extent. It can be observed in all these
vibrating modes, that with further increase of t SRSN (=10 nm) the peak intensities
decrease. In the spectra recorded with normal incidence (Fig. 4.36b), the Si-O
bonds are more pronounced for lower t SRSN , due to lower amount of Si-N bonds in
the material.
With increasing t SRSN , the number of Si-N bonds in the material increases
whereas the number of Si-O bonds remain constant. Therefore we notice an increase
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