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

tel-00916300, version 1 - 10 Dec 2013
A detailed analysis by a fellow researcher in our team, Dr. O. Debieu revealed
that emission is obtained only from samples that possess refractive indices between
2.0-2.13 when annealed at temperatures lower than CA. Figure 4.8 shows consolidated
results of his optical investigations. These results also conrmed the absence
of PL for n 1.95eV >2.4 whatever the annealing treatment.
The maximum PL was obtained after annealing at 900°C, and an increase in absorption
coecient with refractive indices is noticed. For the Si 3 N 4 sample (n 1.95ev =
2.01), a drop of α in the absorption spectra is noticed between 2.5-3.5 eV at the PL
excitation energy which may explain the low emission intensity of this sample. For
the other cases, the emission intensity increases with refractive indices till 2.12 and
then begins to fall. This decrease is attributed to the increase in the non-radiative
recombination rates with increasing disorder in the matrix brought by incorporating
higher Si excess [Debieu 12].
4.3 Cosputtering of Si 3 N 4 and Si cathodes
The SiN x layers were grown at 3 mTorr and T d =500°C by using Si 3 N 4 and Si cathodes
in pure Ar plasma. One sample of N-rich silicon nitride (NRSN) was also grown by
sputtering the stochiometric target in N 2 +Ar plasma to compare with Si 3 N 4 and
SRSN samples. The RF power density applied on Si 3 N 4 cathode was maintained at
7.4 W/cm 2 while that on Si cathode was varied between 1.77 and 2.96 W/cm 2 .
4.3.1 Refractive index (n 1.95eV ) and Deposition rates (r d )
Figure 4.9 shows the variation of refractive index (left axis) and deposition rate
(right axis) with respect to P Si . The refractive index increases from 2.3 to 2.82 with
increasing P Si . This is a signature of increasing the Si incorporation in the matrix
with P Si .
A direct sputtering from Si 3 N 4 cathode in pure Ar plasma results in n 1.95eV ∼ 2.30.
This indicates that at our chosen conditions, the sputtering does not yield a stochiometric
material. Hence a couple of samples were grown by sputtering Si 3 N 4 cathode
in N 2 -rich plasma to obtain refractive indices relating to Si 3 N 4 and N-rich SiN x (pink
and brown circles in the gure). Their values of n 1.95eV are also indicated in the
gure at P Si =0W/cm 2 , signifying only the Si 3 N 4 cathode is sputtered for the growth
of these layers. The deposition rate increases with P Si which can be explained by
the increase in the number of reacting species.
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