Biomedical Engineering Reference
In-Depth Information
The second factor is composition profile of the quantum dots that impacts their
polarization properties. During the self-assembly growth process, In-Ga intermixing
and In-segregation effects typically result in a nonuniform composition profile:
large In fraction at the center of the QDs surrounded by low In composition close
to the QD edges [ 11 ]. This nonuniform composition profile results in a decrease
in the value of the DOP as recently demonstrated by our atomistic theoretical
study [ 7 ], where a nonuniform double-layer In composition profile increased the
TM [ 001 ] /TE [ 100 ] ratio from 0.097 (for the pure InAs composition profile) to 0.27,
in accordance with the experimental measurement that indicated TM [ 001 ] /TE [ 100 ] =
0.26. This corresponds to a reduction in the value of the DOP [ 100 ]
from 0.82 to 0.6
(still considerably higher than the desired value of 0).
The third factor that strongly impacts the polarization response of QDs is their
shape. Theoretical studies typically assume perfect circular-base (for dome-shaped
QDs) or square-base (for pyramidal-shaped QDs) shapes, whereas the quantum dots
grown by the SK self-assembly process have generally e llipsoidal shape, elongated
either along the [110] [ 12 - 15 ] direction or along the [110] direction [ 16 - 19 ]. The
elliptical shape of the QDs decreases the value of the DOP as recently predicted by
our atomistic theoretical calculations [ 10 ].
The fourth factor that changes the polarization response of QDs is their aspect
ratio. We have recently shown [ 10 ] that increasing the QD AR from 0.225 to
0.8 reduces the DOP [ 110 ] from 0.97 to 0.83. This decrease is attributed to an
enhanced HH-LH intermixing that increases the TM mode component and a
stronger confinement of the hole wave functions at the QD interface for the tall
QDs that reduces the TE [ 110 ] mode. It should also be noted that the decrease in the
DOP with AR is accompanied by a significant decrease in the transition strength.
By simultaneous increase in th e QD AR from 0.225 to 0.8 and changing the QD
diameters along the [110] and [110] directions from 20 nm and 20 nm to 14 nm and
26 nm, respectively, a net decrease in the value of the DOP [ 110 ] from 0.97 to 0.32 was
calculated [ 10 ]. However, this theoretical prediction has not been experimentally
verified yet.
To conclude this discussion related to the polarization response of the single
QD layers, despite various factors significantly impact their DOP and lead to an
improvement of the polarization response by reducing the value of the DOP towards
zero, still isotropic polarization remains unavailable from these nanostructures.
5.1.1
Multi-Layer Quantum Dot Molecules
The AR of the QD nanostructures can be drastically increased by growing multiple
layers of the QDs in the form of vertical stacks, where the adjacent QD layers
are closely spaced. In these nanostructures, the long-range strain fields couple the
quantum dot layers and therefore result in the formation of molecular electronic
states. Due to this characteristic, these multi-layer QD nanostructures are also
commonly known as “quantum dot molecules” or “artificial molecules.”
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