Biomedical Engineering Reference
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Fig. 1.3 Cross-section images taken by dark field transmission electron microscopy (TEM) of
vertical QDMs formed when ( a )1.4MLand( b ) 1.2 ML of InAs are deposited on the nanohole
template. QD 1 and QD 2 indicate the lower and upper QDs, respectively. While QD 2 forms above
the wetting layer WL 2 ,QD 1 forms below the wetting layer WL 1 , demonstrating the preferential
nucleation of InAs into a nanohole. Adapted from [ 31 ]
whose formation is strain-driven. This is a clear demonstration that the mechanism
involving the formation of QD 1 is the preferential nucleation of InAs material into
the GaAs nanohole. Regarding the sizes of each QD, when depositing 1.4 ML of
InAs (Fig. 1.3 a) the lower QD 1 shows a bigger size than the upper QD 2 .Onthe
other hand, when depositing 1.2 ML of InAs (Fig. 1.3 b) the buried QD 1 is similar in
size to the top QD 2 . These structural results provide a clear indication that the size
of a QD into a vertical QDM structure can be tuned on demand by this fabrication
procedure. However, it is noticeable that despite the engineered change in size
of QD 1 , the nucleation of InAs on top of these nanostructures yields QD 2 with
similar dimensions in all cases. Importantly, this effect provides an independency
in size between the QDs forming the molecule, which opens the door for a real
controllability of a vertical QDM design.
The QDs size tunability in the QDMs is further studied by measuring the QDMs
photoluminescence (PL) at low temperature ( T
30 K). Figure 1.4 shows the PL
spectra for the three different samples fabricated. The convolution of two main
Gaussian peaks is observed in all three cases. They are associated with two different
QDs families in the molecules as verified by excitation power experiments where
no relative saturation effects, corresponding to a given QD family with ground and
excited states, were found in the PL peaks. By combining these optical emission
results and the TEM images, a direct correspondence of the different emission peaks
to the recombination of carriers can be established in either the lower or upper
QDs (QD 1 and QD 2 ) forming the molecule. Figure 1.5 a shows as filled areas the
three PL emission spectra ascribed to the nanostructure QD 1 for the three different
amounts of InAs material deposited on the nanoholes template. As the amount of
=
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