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

portant to control the formation of Si QDs in various hosts, and their corresponding
optical and electronic properties. Hence, this section deals with a brief discussion on
the widely used congurations of Si-QDs explored thus far towards PV applications.
1.5.1 Porous Silicon (p-Si)
tel-00916300, version 1 - 10 Dec 2013
Porous Si is the rst investigated form of Si-QDs, ever since the discovery of
size tunable visible luminescence in the material. The surface of p-Si is highly textured
which enhances light trapping and reduces the reection losses. The possibility
of having a wide particle size distribution among the QDs helps in the
tunability of the p-Si bandgap. This can be utilized to absorb a wide range of
solar spectrum. But the high resistivity of the material leads to problematic electric
transport. Besides, the poor thermal and mechanical stability of the material
challenges [Nguchi 93, Ruiz 94] its endurance to high temperature processing steps
usually ranging between T=1000°C to T melt =1400°C, the melting temperature of
Si [Reber 99, Bergmann 99] in solar cell fabrications. The emission properties of
p-Si is not stable during storage in air due to various causes such as desorption of
hydrogen or SiH x complexes [Prokes 92, Weng 93, Zoubir 94], oxidation processes
[Stevens 93, Xiao 93], absorption of water and other substances from the atmosphere
[Cullis 97, Canham 91].
1.5.2 Si Quantum Dots embedded in dielectric hosts
In order to overcome the drawbacks of p-Si while taking advantage of the quantum
connement eects observed in it, alternative means of synthesizing Si QDs
were investigated. Embedding Si QDs in wide bandgap insulators is one of the
most convincing solution to obtain strong quantum connement. Various techniques
for the growth of Si-rich layers have been employed such as ion implantation
[Min 96, Bonafos 04], chemical vapour deposition [Lin 00], pulsed laser deposition
[Chen 04], plasma enhanced chemical vapour deposition [Iacona 00], magnetron
sputtering/co-sputtering [Seifarth 98, Gourbilleau 01], evaporation in ultrahigh
vacuum [Miska 10] etc. Subsequent annealing treatments allow the formation
of nanoclusters or crystals from the Si excess in the Si-rich layers. Since the goal is
to concentrate on an all-Si tandem cell, the forthcoming discussions are restricted to
the Si QDs embedded in its dielectric compounds such as oxide, nitride or carbide,
the oxide dielectric providing the strongest connement of all (Fig. 1.12).
(a) Si QDs in SiO 2 : Silicon oxide with silicon excess are termed as Silicon
Rich Silicon Oxides or Silicon Rich Oxides and are often denoted by any of these :
SRSO/SRO/SiO x where x