Biomedical Engineering Reference
In-Depth Information
a
c
150
Total
100
c
QD
77
s
p
50
sQD
s
WL
p
20K
b
d
Total
150
sQD
100
s
77
cQD
p
s
50
p
d
d
20K
1.0
1.1
1.2
1.3
1.4
0
50
100
150
Energy (eV)
Temperature (K)
Fig. 3.5
Temperature-dependent PL spectra of (
a
) 1.8/15/1.2 and (
b
) 1.8/25/1.5 QDMs. Text
labels (s, p, and d) indicate cQD emissions; the
arrow
, sQD emissions. The
dotted lines
are multiple
Gaussian function fits to the 20 K data. Graphs are offset for clarity. Temperature-dependent
integrated intensities of constituent peaks of (
c
) 1.8/15/1.2 QDMs extracted from (
a
), and (
d
)
1.8/25/1.5 QDMs extracted from (
b
). The
dotted lines
are guide to the eye. Adapted from [
27
]
with permission from Elsevier
lateral QDMs are due to electrons only; holes are well confined in the QDs due to
high effective mass [
26
]. The macro-PL setup used in our experiments only allow
the observation of the macroscopic PL intensity transfer.
Two QDM samples are grown to study the coupling effects: One sample contains
a single layer of 1.8/15/1.2 QDMs, the other a single layer of 1.8/25/1.5 QDMs
[
27
]. Temperature-dependent PL spectra of the two samples illustrate the differences
in tunneling strength, and the competition between tunneling and thermionic
emissions. The former maintains the total integrated intensity whereas the latter
quenches it due to carrier losses to non-radiative recombination (NRR) channels.
Both samples are excited at a high excitation power density of 20 W/cm
2
sufficient
for the observation of ES as tunneling into ES is easier than into GS due to the wider
spread of ES wavefunctions.
The PL spectra of the 1.8/15/1.2 and 1.8/25/1.5 QDMs are shown in Fig.
3.5
a,
b, respectively. The broad, high-energy peaks indicated by arrows in both figures
are related to sQDs as explained previously. The narrow, low-energy peaks are
