Biomedical Engineering Reference
In-Depth Information
It is worth mentioning here that another term quantum dot stack (QDS) is also
widely used in the literature to refer to the vertical arrays of QDs. However, the
distinction between a QDM and a QDS is that the former specifically refers to the
case where the adjacent QD layers are very closely spaced (typically less than 5 nm
apart) and therefore exhibit hybridized molecular states. A QDS, on the other hand,
is a more general term and the QD layers in a QDS may or may not be strongly
coupled. For example, a recent study on the bilayer QDS shows that at 10 nm inter-
layer spacing, the two QD layers are weakly coupled and therefore the electronic
states remain confined inside the individual QD layers and do not form molecular
states. Another theoretical study [
20
] suggested that the transition from weakly
to strongly coupled QDSs roughly occurs at 6 nm inter-layer spacing. This topic
chapter only focuses on the vertical QDSs with the inter-dot spacing of 4.5 nm (as
will be described later in Sect.
5.2.1
), so we will refer these strongly coupled QDSs
as QDMs in the remainder of this chapter.
A special property of the QDMs is that the electronic states are spread over the
whole molecule and therefore provide larger electron-hole wave function overlaps.
Another unique characteristic of the QDMs formed as vertical arrays of QDs is that
they artificially provide very large ARs. Since the electronic states are hybridized
over the whole QDM, so a QDM can be regarded as a single QD nanostructure in
which very large height to base ratios can be realized by controlling their constituent
number of QD layers. This results in the relaxation of biaxial strain close to the
center of QDMs, therefore increasing HH-LH intermixing in the valence band
states. As a result, the TM mode response increases, thereby reducing the value
of DOP towards the isotropic response. This characteristic of the QDMs makes
them very attractive candidates for the design of optical devices where control of
polarization response is critical such as semiconductor optical amplifiers.
During the last few years, several experimental [
4
,
9
,
21
-
25
] and theoretical [
4
,
8
,
10
] studies have been performed to study polarization-dependent optical spectra
from the InAs QDMs. Kita et al. [
25
] and Saito et al. [
8
] demonstrated that an
isotropic polarization response can be achieved by growing columnar QDs—QDMs
with the inter-layer spacing of 0 nm - consisting of nine QD layers.
Recent experiments by Inoue et al. [
9
,
26
,
27
] and a subsequent theoretical study
by Usman et al. [
4
] showed that similar tuning of the polarization properties is
possible in InAs QDMs where the QD layers are geometrically separated by thin
(4.5 nm) GaAs spacers. Such multi-layer QDMs offer twofold advantage over the
columnar QDs: (i) a moderately thick GaAs spacer between the QD layers allows a
precise control of overall QD shape and size and (ii) a reduced strain accumulation
results in isotropic polarization response with fewer number of QD layers in
the molecule. The experimental results [
9
,
26
,
27
] were provided for the QDMs
containing three, six, and nine QD layers. The results indicated that the DOP
[
110
]
takes up the values of
60 for the samples containing six and nine QD
layers, respectively. A change in the sign for the DOP
[
110
]
+
0
.
46 and
−
0
.
implies that the isotropic
polarization response (DOP
[
110
]
→
0) can be achieved by engineering the number
of QD layers between six and nine. The theoretical calculations [
4
] explained this
isotropic polarization response in terms of enhancement of the TM
[
001
]
mode and
