Biomedical Engineering Reference
In-Depth Information
the QDs along the
c
-axis [
21
]. In the experimental study in [
22
], the influence of the
distance
D
between the dots in a stack of InGaN QDs was analyzed, showing that
with decreasing
D
the PL blue-shifts and the PL lifetime decreases. This indicates a
decrease in the magnitude of the built-in field with decreasing
D
.
Thus, understanding the mechanisms of inter-dot coupling is important not only
for fundamental properties of coupled nitride-based QDs but also for designing
nitride-based optoelectronic devices. However, knowledge about the electronic
structure in quantum dot molecules (QDMs) based on more conventional III-V
materials, e.g. InAs/GaAs systems, cannot be simply transferred to a nitride-based
system. Amongst other reasons, the electrostatic built-in field in wurtzite nitride-
based heterostructures is very different from its III-V zinc blende counterpart.
Not only is the magnitude of the piezoelectric coefficients in group-III nitrides
up to ten times larger than that in conventional III-V and II-VI compounds [
23
]
but also the built-in potential orientation and symmetry in wurtzite nitride-based
heterostructures is very different to that in (001)-oriented zinc blende III-V
heterostructures. While there is no net potential drop along the growth direction in
conventional (001)-zinc blende InAs/GaAs systems, nitride-based heterostructures
show a very pronounced potential drop across the heterostructure. Therefore, when
stacking wurtzite nitride-based QDs along the polar
c
-direction, a complicated
interplay can be expected between the strain fields, built-in potential effects and
electronic coupling.
In this work we present a detailed analysis of the polarization potential in isolated
and coupled In
x
Ga
1
−
x
N/GaN QDs and study its impact on the electronic structure
and consequently the optical properties. In a first step, before turning to the stacked
systems, we discuss the built-in potential of an isolated dot and analyze the different
contributions, spontaneous and piezoelectric polarization, separately and in detail.
We show how the built-in potential evolves when going from a QW to an isolated
QD of the same height and composition. This analysis reveals that the built-in
potential of an isolated QD with a realistic aspect ratio is already strongly reduced
compared to that in a QW. Following this general introduction to the behavior of the
built-in potential in nitride-based heterostructures, we then focus our attention on
the details of the built-in potential. For example, since positive as well as negative
values for the shear strain-related piezoelectric coefficient
e
15
can be found in the
literature [
24
], the impact of
e
15
on the results is also studied. This analysis reveals
that the sign of
e
15
significantly affects the potential outside an isolated dot, and
consequently this will also affect the built-in potential in a stacked QD system.
After this detailed analysis of the influence of
e
15
on an isolated In
x
Ga
1
−
x
N/GaN
QD, we consider the built-in potential in stacked dots. Here, we focus on the impact
of the barrier thickness on the potential and the electronic structure. We show that
by engineering the distance between two non-identical In
x
Ga
1
−
x
N/GaN QDs, the
built-in potential can be further reduced compared to the potential in an isolated
dot, which is already significantly lower than in a QW of the same height and
composition. Consequently, the use of coupled In
x
Ga
1
−
x
N/GaN QDs should provide
a very beneficial reduction in built-in field for optoelectronic devices operating in
the green and yellow spectral regions.
