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

4. Investigating the origin of photoluminescence
tel-00916300, version 1 - 10 Dec 2013
The fourth major objective was to understand the origin of emission, with regard
to microstructure and optical/geometrical eects. The experimentally obtained PL
spectra consist of one broad band composed of two peaks in the case of SRSO/SiO 2
MLs and three peaks in the case of SRSO/SRSN MLs (as seen from curve tting
operations on the emission spectra).
Experimental investigations: The peaks (1 and 2) in the PL spectra of
SRSO/SiO 2 ML is suggested to be a contribution of bigger Si-np at the interfaces
and Si-np within the SRSO layer respectively. In the case of SRSO/SRSN ML, the
origin of peak (1) is unknown, peak (2) is attributed to Si-np contribution and peak
(3) to defects from SRSN.
Theoretical investigations: Simulations performed on typical PL spectra of
SRSO/SiO 2 conrm the presence of two types of emitters. Investigations indicate
that the optical and geometrical eects on the emission intensities and their peak
positions are not straight-forward. The results caution us about the huge role played
by the pump prole, optical cavity eect, distribution of excited emitters etc. on
emission and demonstrate that the microstructure of the material (eg. density and
size of Si-np) is not the only necessary parameter in achieving the desired emission.
Future perspectives
This thesis lays the foundation for understanding the material growth, structural
and optical properties especially of the new type of ML: SRSO/SRSN. Therefore,
there are a lot of issues that have to be explored in the future some of which are
described below.
(i) It was demonstrated that SRSN enhances the Si-np density and also the
emission from SRSO after short time annealing. It may be assumed that the excess
Si in SRSN may favour this increase in density. However, the exact mechanism that
favours Si-np formation has to be analyzed.
(ii) This thesis shows that SRSN layers when annealed at high temperatures have
a detrimental eect on its emission and also quenches the PL from SRSO sublayer.
The reason for this detrimental eect has to be analyzed.
(iii) Experimental analysis on the origin of emission peaks from SRSO/SRSN
suggests the peak around 1.9 eV (peak 3) to be a contribution from defects in SRSN
sublayers. The type of defects that might lead to emission from SRSN sublayers
have to be investigated. Besides, the origin of low enegy peak is unattributed. PL
life-time measurements may give a deeper insight on these.
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