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

tel-00916300, version 1 - 10 Dec 2013
solar cells by forming gas (FG) annealing enhances the eciency of low cost solar
cells [Sana 94, Sopori 96]. This is due to the elimination of dangling bonds which act
as trapping centres for charge carriers. Here, the hydogen atom plays a vital role in
the activation of the radiative recombination centres. Hence a series of investigations
were carried out by annealing under FG and also seeing the eect of preceding or
succeeding this FG annealing by N 2 annealing.
Since the deposition temperature is high (500°C), we may reasonably suspect
that one of the contributing factors to the enhancement (7.4 times) in intensity of
the ML with 100 patterns as compared to 50 patterns after STA (ref. Fig. 4.35a)
may be a consequence of the longer time spent in the deposition chamber. For instance,
in a 100 patterned ML, the rst 50 patterns are subjected to double the time
inside the deposition chamber than the 50 patterned ML under the plasma which
alternates between (argon + hydrogen) and (argon + nitrogen). It was calculated
that 100(3.5/5) ML spent an excess time of 4.75h under high temperature (500 °C)
than 50(3.5/5) in the deposition chamber. This time spent in the chamber at 500 °C
may be considered as a low temperature annealing of the already deposited layers
while the top layers are being deposited. Though the conditions in the deposition
chamber for the excess time is not exactly the same in annealing chamber, long time
annealing of 4.75h under forming gas ow at 500°C (4.75h-FG) was chosen to see
if there could be any inuence of such long time on emission intensity of 50(3.5/5)
ML. The choice of FG annealing is also because it relates more closely to the deposition
chamber ambience than annealing only under nitrogen ux. In addition,
investigations on the inuence of other shorter time FG annealing on enhancing the
emission were also made.
4.9.1 Forming gas annealing versus annealing time
Figure 4.39a shows the PL spectra of 50(3.5/5) SRSO/SRSN ML obtained after
annealing at 500°C under FG atmosphere with t A between 0.5h-1.5h in steps and a
continuous long time annealing (4.75h-FG).
As reported above for the case of annealing under N 2 ow (ref. Sec. 4.6), we
observe the presence of the three emission peaks in the spectrum after FG annealing
also. We focus our analyses on the most intense peak centered around 1.7 eV. An
increase in annealing time favours emission with the highest intensity recorded after
4.75h-FG. Interestingly, it can be noticed that the STA and 4.75h-FG annealing
treatments show similar intensities. This implies that an interplay between the coupled
processes 'N 2 -short time/high temperature' or 'FG-long time/low temperature'
could result in similar emission intensities. Moreover, the annealing process allows
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