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

tel-00916300, version 1 - 10 Dec 2013
in SiC, the lower bandgap (∼2.5 eV ) compared to the other two dielectrics discussed
above, oers several advantages [Cho 08, Song 08b]. The lower barrier of the SiC
increases the tunnelling probability among the QDs and also promotes eective
luminescence. The conned energy levels of Si QDs in SiC is quite similar to that of
Si QDs in oxide matrix [Kurokawa 06]. Eorts relating to developing a multi-band
conguration of Si QDs in SiC [Chang 10] and doping for enhanced conductivity
[Lim 08, Cho 08] have been taken up recently which would help signicantly in the
design of PV devices. Depending on the composition of SiC, the QDs vary in shape,
some joining together to form an extended crystal. It is reported that depending
upon the stochiometry, SiC QDs are formed instead of Si QDs [Cho 07]. It also
has to be mentioned that the segregation and formation of Si QDs in SiC are more
dicult that the other two dielectrics mentioned above. Though investigations on
Si QDs in SiC are still at a preliminary level, the results obtained thus far show the
potential use of this material in solar cells.
1.5.3 Si Quantum Dots in multilayers
The successful incorporation of Si QDs in various dielectric matrices and its
various optoelectronic properties reect the advantages of quantum connement in
Si for PV devices, with increased mechanical strength as compared to the p-Si. But
the size dispersion of the QDs in these materials, complicate the understanding of the
material properties [Credo 99]. Moreover, with increase in time and temperatures of
annealing, the nanocrystals (QDs) agglomerate and form bigger crystals (Fig. 1.14).
As a consequence, the material behaves similar to bulk Si with indirect bandgap
nature, due to the loss of quantum connement eect. Nanocrystals with a narrow
size distribution are required to produce a solar cell with a controlled bandgap
energy. Therefore multilayer approaches were adopted to maintain a uniform size
distribution with a precise control over Si QDs size and distribution as represented
in gure 1.14.
Usually a multilayer is formed by sandwiching Si QDs between stochiometric
insulating layers that serve as barriers. The most investigated conguration is
the SRSO/SiO 2 multilayers (MLs) proposed by F. Gourbilleau and M. Zacharias
[Gourbilleau 01, Zacharias 02], due to the relatively well understood mechanisms of
luminescence and electric transports. Since the tunneling probability depends on
the insulating barrier height, Si 3 N 4 or SiC is expected to provide a better carrier
transport. Substantial research eorts are being taken up, in enhancing the carrier
transports, both in the well understood SRSO/SiO 2 MLs, and in the alternative
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