Biomedical Engineering Reference
In-Depth Information
For the SQD system containing only a single QD layer, our calculated DOP
values in all the directions are very close to 1.0, exhibiting a very strong polarization
anisotropy (TE mode
TM mode). This is similar to other theoretical studies
basedonk
p method [ 8 , 46 ] where the values of DOP close to 1.0 have been
reported for the single QD layers. However the experimentally measured values for
the single QD layer are relatively low (
·
0.7). Since the values for both DOP [ 110 ]
and DOP
[
are very close so we believe that the reason for this discrepancy
between the theory and experiment is not related to the shape of the QD. This is
because any elongation in the QD shape will result in considerably enhanced in-
plane anisotropy (DOP [ 110 ]
110
]
being different) [ 10 ]. We speculate that
one possible reason for this could be due to In-Ga intermixing and In-segregation
effects as recently suggested [ 7 ]. However, further information on the composition
profiles of the experimental samples [ 9 ] is required to confirm this understanding.
Our calculated value of the DOP [ 110 ] for the QDM-3 is also lower than the
experimental measurement, but as we will show later in Sect. 5.5.1 that the
polarization response of the QDM-3 is highly sensitive to the height of the QD
layers and even a small variation such as 0.5 nm results in a significant increase in
the value of the DOP. Therefore, we believe that this discrepancy may be due to the
assumed height of the QD layers (4.0 nm) or circular-base shape for the QDM-3.
However, our theoretical results exhibit excellent agreement with the experimental
measurements for the QDM-4 and therefore confirm that the isotropic polarization
response can be achieved by engineering the size of the QDMs.
and DOP
[
110
]
5.5
Effect of Geometry Parameters
In the last two sections, we have studied the electronic and polarization properties
of the multi-layer QDMs assuming an ideal dome-shape for the QD layers with
circular-base. However in reality, the actual shape of the SK self-assembled QDs
is far from being perfectly circular or square. Several experimental investigations
have suggested that the actual shape of the QDs significantly deviates from the ideal
circular-base (for dome or lens) or square-base (for pyr am id), and usually tends
to elongate either along the [110] [ 12 - 15 ]oralongthe[110] [ 16 - 19 ] directions.
Therefore it is essential to investigate the impact of the elliptical shapes on the
polarization properties of the QDMs.
Furthermore, we have simulated the QDM geometries with the base diameter and
height of the QD layers directly extracted from the recent experimental TEM images
[ 4 , 9 ], which suggest that the height and the base diameter of the QD layers are
roughly 4.0 nm and 20 nm, respectively. However, as the experimental TEM images
are not very clear, so it is worthwhile to analyze the impact of small variations in
the base diameter and the height of the QD layers on the calculated polarization
properties. Therefore in this section, we provide a quantitative description of the
impact of the variations in the geometry parameters on the calculated values of
the DOP.
 
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