Complete Report - University of New South Wales

These results are discussed further elsewhere [4.5.8] and provide tentative evidence for
quantum confi ned energy levels in Si QDs in nitride at a higher energy than in equivalent sized
QDs in oxide. However these are as yet early data. PL on different sized QDs in nitride has
not yet been obtained. These would show whether the 1.8eV peak increases or not with
decreasing dot size, an increase indicating a true confi ned energy level, no increase indicating
a fi xed defect in the interface or matrix. In addition, the intensity of the nitride peaks is
not high, probably indicating a low radiative transition probability. Nonetheless, the fact that
intensity increases with the number of SiQD layers is encouraging. Further data are being
generated.
Figure 4.5.10: Comparison of the PL spectra of
Si QDs in nitride and oxide (300K). Peaks on
the left-hand side are in the nitride matrixes and
higher peaks on the right-hand side are from the
oxide matrixes.
4.5.3.2.2 PL on Si QDs in SiO 2 - Analysis
PL spectra have been used to identify radiative
recombination energy levels in Si QD in SiO 2
nanostructures. In the model used, radiative
contributions are considered to come from one
of three sources: from quantum confi nement
(Q); from SiQD/SiO 2 interfaces (I) and from
amorphous Si clusters (α). The relative strengths of these assumed processes have been
determined by deconvoluting the spectral peaks. The 3 Gaussian deconvolutions of the PL
spectra for Q, I and α are shown in Figure 4.5.11. Initial comparison with similar measurements
on other samples suggests that the position of the Q peak depends on the size of the QDs
whereas the position of the I peak seems to be independent of QD size. Possible mechanisms
for these transitions are being assessed, including a possible recombination due to an oxygen
defi ciency that is at 6.3eV above the valence band of SiO 2 or at 1.65-1.70eV above the
conduction band of the SiQD in the ground state, see Figure 4.5.12.
Figure 4.5.11: Gaussian deconvolution of
PL spectrum for a SiQD/SiO2 sample:
peaks labelled Q, I and α are attributed to
quantum confi nement, interface states and
a-Si clusters, respectively.
Figure 4.5.12: Schematic model of the Q
and I radiative mechanisms.Also shown are
possible hopping conduction mechanisms.
Additional data on the change of the ratio of Q:I peak intensity with the degree of excess
Si in SRO samples and with anneal time are being assessed and promise more detailed
information on these mechanisms.
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