Biomedical Engineering Reference
In-Depth Information
Another interesting case is when the height of the QD layers is 4.5 nm which
is equal to the wetting-layer to wetting-layer separation. In this case, the adjacent
QD layers will be connected to each other and the QDM becomes a columnar QD.
We investigate such structure for the QDM-4 and find that the values of DOP are
relatively higher for the columnar QD when compared to the QDM-4 with 4.5 nm
thick GaAs spacer layer. This indicates that to achieve isotropic polarization, overall
a fewer number of QD layers are required in a QDM with finite GaAs spacer when
compared to a similar diameter columnar QD.
Finally, based on the comparison of the calculated values of the DOP for various
QD height configurations and the measured values provided in Table
5.1
,we
estimate that the dimensions of the QD layers inside the QDM-4 are approximately
B
=
20 nm and
H
=
4nm.
5.5.2
Nonidentical QD Layers
Another design parameter is the base diameter of the QD layers inside the QDM. As
suggested by the experimental studies [
5
,
29
] that the base diameter of the QD layers
during the SK growth process increases as the size of the QDM/QDS increases.
This is due to the fact that the presence of the strain from the lower QD layers
tend to reduce the In out diffusion and therefore results in relatively larger QDs in
the upper layers. In order to investigate its effect on the polarization properties, we
simulate QDM-2v and QDM-3v, where all the geometry parameters are same as
for the QDM-2 and QDM-3, respectively, except that the base diameter of the QD
layers is increased in steps of 1 nm and the height of the QD layers is increased
in steps of 0.25 nm as the size of the QDM increases in the vertical direction. For
example in the QDM-2v, the lowest QD layer L
1
has the base diameter of 20 nm
and the height of 4 nm. The next QD layer L
2
has the base diameter of 21 nm and
the height of 4.25 nm, and so on. Our calculations indicate that such QDMs with the
nonidentical QD layers will exhibit relatively higher values of the DOP. The values
of the DOP
[
110
]
are calculated to increase from 0.833 and 0.836 (for
the QDM-2) to 0.92 and 0.94 (for the QDM-2v), respectively. Similarly, the values
of the DOP
[
110
]
and DOP
[
110
]
244 and 0.72 (for
the QDM-3) to 0.73 and 0.76 (for the QDM-3v), respectively. Hence we deduce that
the QDMs with nonidentical QD layers are not suitable for achieving the isotropic
polarization response. No such increase in the size of the QD layers was reported
from the TEM images [
9
].
and DOP
are calculated to increase from
−
0
.
[
110
]
5.5.3
Elliptical Shape of the QD Layers
The value of the DOP
[
n
]
strongly depends on the shape of the QD layers inside
the QDM. Despite significant experimental evidence for the elongation of the QD
