Biomedical Engineering Reference
In-Depth Information
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-
±