Biomedical Engineering Reference
In-Depth Information
to be localized at the top of the lower QD while the hole ground state wave function
is expected to be localized at the bottom of the upper QD, with the built-in potential
change across each dot being very close to that for an isolated QD.
Three key results emerge from this analysis. Firstly, even though we have
assumed a QDM made up of two identical dots, the built-in potential breaks the
symmetry between the two dots. Therefore, one could expect that the electron and
hole ground state wave functions are localized on different QDs in a QDM made up
of two identical dots. Secondly, there can be a reduced potential drop across each
dot for the case of closely stacked dots. Finally, the built-in potential profile outside
an isolated InGaN/GaN QD can lead to electron and hole ground state switching in
the here studied QDMs. Having established these general features, we turn now to
look in more detail at the electronic structure of QDMs.
6.6
Electronic Structure of InGaN/GaN QDMs
Equipped with an understanding of the electrostatic built-in potential in isolated and
stacked InGaN/GaN QDs, we focus now on the electronic structure of InGaN/GaN
QDMs. To build on the results of the previous section, and to keep the analysis
simple and transparent, we first consider molecules made up of two vertically
stacked QDs of identical shape, size and composition with varying inter-dot
distance
D
. We assume here values of
D
≈
1
,
2
,
4
.
1
,
6
.
2, and 8
.
3 nm, following the
experimental findings on stacked InGaN QDs in [
22
].
In a second step, we extend this analysis in Sect.
6.6.2
to investigate InGaN
QDMs made up of two non-identical dots. All other parameters, such as the distance
between the dots, are kept as in the case of the two identical dots. We find that
the results for the non-identical dots are broadly similar to those for identical dots,
except that the electron and hole ground state levels can be found in the same dot
for the case of non-identical dots.
6.6.1
Identical QDs: Influence of Strain and Built-In
Potentials
We consider here the electronic properties of In
0
.
25
Ga
0
.
75
N/GaN QDMs. We assume
for the QD geometry a lens-shaped structure, as suggested by the experimental data
in [
70
]. The molecules considered here are made up of two vertically stacked QDs
of identical shape, size, and composition, but with varying inter-dot distance
D
.
To analyze the electronic structure of In
0
.
25
Ga
0
.
75
N/GaN QDMs in detail we
proceed in the following way. To study the impact of strain and built-in fields sep-
arately, we switch these contributions off in a first step, Sect.
6.6.1.1
. Subsequently,
in Sect.
6.6.1.2
, we investigate the influence of the strain field on the electronic
