Biomedical Engineering Reference
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Fig. 1.6 Topography AFM images obtained after depositing 1.5 ML of InAs at an As 2 pressure
of ( a )3.5
10 6 Torr. A direct correlation between the As 2 pressure and
the formation of either a single QD or a QDM into each of the nanoholes is observed. Reprinted
(adapted) with permission from [ 20 ]. Copyright 2009 American Chemical Society
10 7
×
Torr and ( b )1
×
hand, an InAs nucleation enhancement into the nanoholes is found when the As 2
pressure is increased to 1
10 6 Torr, as shown in Fig. 1.6 b. In this case, QDMs
aligned along the GaAs [1 1 0] crystallographic direction are formed inside the
nanoholes with occupancy of 98%, which clearly indicates a higher preferential
nucleation process into them. Detailed 3D rendered images for the initial nanohole,
a single QD, and a QDM inside the nanohole are shown in Fig. 1.7 , together with
AFM profiles along the [1 1 0] direction (dotted lines).
As already mentioned, the nanoholes obtained by the droplet “nanodrilling”
process show an elongated shape along the [1 1 0] direction and the presence of
surrounding GaAs mounds aligned along the [1
×
1 0] direction (Fig. 1.7 a). In the
case of single QDs (Figs. 1.6 aand 1.7 b), a monomodal distribution of QD sizes
is obtained with a mean diameter of 58
0.8 nm, both
quantities referred to the flat surface plane. On the other hand, the QDMs (Figs. 1.6 b
and 1.7 c) show systematic differences in the QDs sizes, as clearly observed in
the distribution of heights represented in Fig. 1.8 . A fit by two Gaussian peaks
shows the presence of a bimodal distribution with height values of 5.3 nm
±
3 nm and a height of 7.4
±
±
17%
and 6.6 nm
24%. The separation edge to edge between QDs in the molecule
is practically zero within AFM resolution, which yields an identical QDs mean
diameter and center-to-center distance of 37
±
4 nm. This QDs separation is short
enough to potentially observe quantum coupling phenomena [ 18 ].
The PL characterization for single QDs and QDMs is shown in Fig. 1.9 .
Interestingly, although the AFM topography showed the single QDs bigger in size
than the QDs in the QDM, a similar PL peak emission energy is found for both
nanostructures. This effect is ascribed to a different In(Ga)As composition of the
single QDs and QDMs, which can be explained by a different filling ratio of Group-
±
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