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

5.4 Discussion on the choice of input parameters
5.4.1 Absorption and Emission cross-sections
tel-00916300, version 1 - 10 Dec 2013
We assume that the largest contribution towards emission in our SRSO/SiO 2 and
SRSO/SRSN MLs is from Si-np. Since no Si-np were observed in SRSN, the SRSO
layer is considered to be primarily responsible for emission. It has been reported
that the absorption cross-section of Si-np usually ranges between 10 −22 - 10 −18 m 2
[Khriachtchev 02, Daldosso 06, Pavesi 00, Huda 09, Garcia 03]. From these literature
values, we may assume our SRSO layers to have σ abs in the above mentioned
range.
The absorption intensity depends on the population of emitters in the ground
state. Therefore, the ground state population N 1 is xed at 10 26 emitters/m 3 relating
to the highest possible density of Si-np attainable in our SRSO layers. We keep the
degree of freedom on emission cross-sections and do not link it with σ abs.max during
these simulations. Various simulations were made and as illustrated in gure 5.14,
it was observed that a higher emission cross-section is needed to obtain gain with
our chosen σ abs values and to witness emission peak in the modeled curves.
5.4.2 Lifetime of Si-np in dierent MLs
The lifetime of band to band recombinations of electron-hole pairs in Si-np is reported
to be 10-100 µs [Wilson 93, Littau 93]. In order to have an estimate of
carrier lifetime in our MLs, PL lifetime measurements 2 were made on SRSO/SiO 2
and SRSO/SRSN MLs after CA and STA treatments respectively since these annealing
treatments resulted in the highest emission intensity as demonstrated in
chapters 3 and 4.
Considering the emission peak around 830 nm (∼ 1.5 eV) in CA SRSO/SiO 2 ,
lifetime measurements were done by using this as the detection wavelength. Decay
time was estimated to be 88 µs by tting the experimentally obtained decay curve
with a single exponential decay law.
It was discussed in chapter 4 that the PL spectra of SRSO/SRSN MLs mostly
possess three peaks. The sample chosen for investigations had three emission peaks
centered around 675 nm (∼ 1.83 eV), 800 nm (∼ 1.55 eV) and 900 nm (∼ 1.37
eV). The decay time detected at each of these wavelengths were estimated to be 51
µs, 56 µs and 51 µs respectively by tting the experimental and theoretical curves.
These measurements give an estimate of the carrier lifetime, that well suits with
2 Lifetime measurements were by Dr. J. Cardin and Prof. Christophe Labbé of our NIMPH
team at CIMAP.
153