Biomedical Engineering Reference
In-Depth Information
The good qualitative agreement of our theoretical results with the experimental
PL measurements in Table
5.1
even for the assumed ideal circular-base for the
QD layers in the QDM-4 leads to a fundamental question that how much i
s
the
contribution from a realistic shape such as elongated along the [110] or [110]
directions. In order to investigate this effect, we systematically elongate the QD
layers inside the QDM-4 according to the schematic diagram shown in Fig.
5.8
aand
analyze its impact on the values of the DOP in Fig.
5.8
c. In contrast to the SQD,
where the QD base elongations result in a significant tuning of the DOP
,the
[
n
]
magnitude of the DOP
for the QDM-4 is relatively insensitive to the value of
[
n
]
η
is a strong function of the orientation of the
elongation, and even a very small elongation (0.5-1.0 nm) is sufficient to flip the
sign of the DOP
. However, the sign of the DOP
[
n
]
. This is highlighted by using an oval in Fig.
5.8
,whereeven
for a 1 nm [110]-elongation, the DOP
[
110
]
[
n
]
drastically changes its sign from
−
0
.
45
to
6. This implies that the elliptical shape of the QDM-4 cannot be exploited to
simultaneously engineer both DOP
[
110
]
and DOP
+
0
.
below zero.
By comparing Fig.
5.8
b and c, we conclude that although a tuning of the DOP
[
110
]
[
n
]
over a wide range of values is possible by elongation of the SQD, the values of
DOP
for the
QDM-4. Therefore, we expect that the elongation of the QDM-4 would not offer
much improvement in its polarization response. This also explains the reason for
the good agreement between the theory and experiment observed in Sect.
5.4.5
even
for the ideal circular-base of QD layers in the QDM-4.
remain relatively insensitive with respect to the magnitude of
η
[
n
]
5.5.4
Prediction of Geometry Parameters for the QDM-4
from DOP
The above discussion related to the strong dependence of the polarization properties
on the [110]-elongations for the QDM-4 allows us to theoretically probe the
geometrical shape of the QDM-4 as grown by Inoue et al. [
9
]. They reported
that the QDM-4 do not have isotropic shape and the T
EM
images suggested very
little anisotropy in the lateral extent [
28
], possibly a [110]-elongation [
24
]. Our
multi-million-atom calculations show that the polarization response is very sensitive
to the elongation factor (
) and even a 0.5-1.0 nm [110]-elongation increases
DOP
[
110
]
above zero. Therefore according to our model results, the experimentally
me
asured DOP
[
110
]
=
−
η
6 implies that the shape of the QDM-4 can only have
[110] elongated base which also agrees with the findings from the TEM images.
It should also be noted that as the QDM-4 studied here has pure InAs QD layers,
so our finding does not contradict with the conclusions of Mlinar et al. [
49
], where
they report that for the alloyed InGaAs QDs, the alloy random configurations may
significantly impact the polarization properties and make the correlation between
the measured polarization response and the QD geometry unreliable.
0
.
