Biomedical Engineering Reference
In-Depth Information
Fig. 1.11
Micro-PL contour plot map vs. lateral bias for a QDM. Adapted from [
17
]. Copyright
2011 American Physical Society
The contour map in Fig.
1.11
shows the evolution of the micro-PL spectrum as
a function of the lateral bias
V
applied to QDM. The number of spectral lines is
twice the expected for a single QD [
6
,
16
,
54
]. The spectrum can be understood by
the tentative assignment proposed in the figure, which is roughly that of two single
QDs, A and B, with emission from the negative quarton X
2
−
to the negative trion
X
−
, to the neutral exciton X
0
, and finally to the positive trion X
+
. The two QDs are
asymmetric in size, as expected (see Fig.
1.8
), being A slightly bigger. Starting from
the X
2
−
transitions (singlet and triplet), with increasing external field, the number
of additional electrons is tuned from two to zero, and each QD in the molecule
follows its own Coulomb staircase. This effect shows that net confined charge can be
controlled in a QDM by applying a lateral electric field. The presence of negatively
charged exciton complexes in the absence of an electric field is typical for QDs
grown into nanoholes formed by the droplet “nanodrilling” technique [
30
]. This
is explained by an incomplete crystallization of the Ga droplet, which induces the
presence of arsenic vacancies in the surroundings of the nanohole. These localized
states are close in energy to the electron-confined levels and are occupied by one or
more electrons depending on their state of valence [
55
].
The energy levels ordering in the spectral assignment of Fig.
1.11
is the usual
for isolated QDs. However, there is a remarkable anomaly. While the emission
energy of X
0
(A) is lower than that of X
0
(B), indicating that QD A is bigger than
QD B, the emission energy of X
−
(A) is higher than that of X
−
(B). Importantly, this
result, difficult to explain in terms of isolated QDs, demonstrates electron tunneling
coupling between the QDs in the QDM. A theoretical model explaining this
anomaly trion emission in lateral QDMs has been recently published [
49
]. Overall,
Δ
