Size-dependent Electronic and Polarization Properties of Multi-Layer InAs Quantum Dot Molecules - Quantum Dot Molecules - page 166

Tabl e 5. 2 The comparison of the theoretically calculated values of the DOP for various
QD geometry configurations. Column 1: the multi-layer QD system under study. Column
2: the dimensions of the QD layers in the molecules: B is the base diameter and H is the
height of the QD. Column 3-6: the values of the DOP calculated from our model
QD Geometry

Theoretical calculations

(B, H) (nm) DOP [ 100 ] DOP [ 010 ] DOP [ 110 ] DOP [ 110 ]

SQD (20, 3.5) 0.995 0.996 0.996 0.996

(20, 4.0) 0.999 0.999 0.999 0.999

QDM-2 (20, 3.5) 0.922 0.884 0.9046 0.933

(20, 4.0) 0.77 0.77 0.833 0.836

QDM-3 (20, 3.5) 0.594 0.594 0 0.747

−

(20, 4.0) 0.548 0.46 0.244 0.72

QDM-4 (20, 3.5) 0.6 0.6 0.445 0.652

(20, 4.0) 0.38 0.371 − 0.45 0.603

(20, 4.5) 0.45 0.446 − 0.47 0.43

5.5.1 Height of the QD Layers

Since the height of the QD layers (L i ) inside the QDMs is not very clear from
the TEM images (see Fig. 5.1 ainRef.[ 4 ]), so we simulate different height
configurations of the QDMs and provide the values of the DOP in Table 5.2 .This
data serve as a measure of the sensitivity of the DOP with respect to the QDM height
parameter and provide a guide to the experimentalists to explore the design space
of such complex multi-million atom systems. From Table 5.2 , as the QDM height
increases in the SQD QDM-4, the values of the degree of

polarization reduce. The reduction in the value of the DOP is larger for the QDMs
with H

→ QDM-2 → QDM-3 →

5 nm. This is due to the
fact that the larger height of the QD layers in the QDM results in stronger coupling
between the QD layers. This implies a stronger HH-LH intermixing resulting in
larger magnitude of the TM [ 001 ] mode.
The dependence of the DOP on the height ( H ) of the QD layers inside the
molecules is generally an unknown factor. The calculated values of the DOP in
Tab le 5.2 show that the DOP becomes very sensitive to the height of the QDs
inside the molecule as the size of the molecule grows larger. For the systems SQD
and QDM-2, the increase in the height ( H ) from 3.5 to 4.0 nm results in a small
decrease in the values of the DOP. However, for a same change in the value of
H, the DOP 110 significantly decreases from 0 to

= 4 . 0 nm as compared to the QDMs with H = 3 .

45
for the QDM-3 and QDM-4 systems, respectively. This implies that an isotropic
polarization response (DOP

− 0 . 244 and from 0.445 to − 0 .

∼ 0) can either be achieved from the QDM-3 with

H 0nm.

We therefore propose that the polarization response of the QDMs can be tuned by
not only increasing the number of QD layers (a parameter typically tuned in the past
experimental studies [ 8 , 9 , 23 , 24 , 45 ]), but also by controlling the height ( H )ofthe
individual QD layers inside the molecules.

= 3 . 5 nm, or from a molecule with fewer number of QD layers and H = 4 .