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

Si-np. This phase separation process into Si agglomerates and SiO 2 is witnessed
from v T O3 and decreased intensity of LO 4 −TO 4 peak in the FTIR spectra. This
also explains the decrease of Si excess estimated by FTIR method in table 3.2.
We may also assume that with increasing T d there is an increase in size of the
Si agglomerates favoured by higher Si content and longer diusion time. This could
explain the progressive decrease in the LO 3 peak intensity which reects the Si-SiO 2
interfaces since with increasing sizes, the number of the agglomerates and therefore
the number of interfaces may decrease.
Since the decrease in LO 3 peak intensity indicates a lower total interface area at
higher T d and consequently a lower number of Si-agglomerates, we may write the
following equation,
tel-00916300, version 1 - 10 Dec 2013
N h S h < N l S l ⇒ N h (4πR h 2 ) < N l (4πR l 2 ) Eqn (3.10)
Figure 3.4: Illustration of SRSO layer at
low and high T d .
where N h and N l represent the number
of agglomerates at the highest and
lowest T d , S h and S l their surface area
and V h and V l their volumes.
Due to increase in sizes as well as
refractive index at high T d , the volumic
fraction increases and can be expressed
by,
N l V l < N h V h ⇒ N l ((4/3)πR l 3 ) < N h ((4/3)πR h 3 ) Eqn (3.11)
Thus, from Eqn. (3.10) & (3.11), we deduce,
Rl
3
Rh
3
< N h
N l
< R2 l
R 2 h
< 1 Eqn (3.12)
Equation 3.12 indicates that the radius of Si-np is lower at low T d . This indicates
that there is a high number of small Si-agglomerates and a few large Si-agglomerates
in samples grown at low and high T d respectively as illustrated in gure 3.4.
From the results obtained above, T d =500°C is chosen for all the forthcoming
investigations due to the high refractive index, Si excess and structural ordering
favoured at this temperature.
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