Biomedical Engineering Reference
In-Depth Information
peaks, similar to the 2/6/1.4 QDMs spectrum in Fig.
3.4
b. The Gaussian fits (dashed
lines in the lower spectrum) reveal the constituent cQDs and sQDs peaks. The
middle spectrum shows two well-separated cQD and sQD peaks. The four peaks
from the lower and middle spectra are staggered in accordance with the design in
Fig.
3.9
b. The dip in the middle of QDM
1
's spectrum is made up by the rapid rise at
the same energetic position of QDM
2
's spectrum, resulting in a smoothened overall
spectrum in the QDMs bi-layer. The bi-layer spectrum demonstrates that with proper
design a broad Gaussian spectrum can be obtained.
The broken-gap scheme or Type-III chirp depicted in Fig.
3.9
c makes use of
two QDM ensembles with similarly narrow energetic separations where the highest
peak energy of one QDM ensemble is lower than the lowest peak energy of the
other. This can be achieved by designing the bi-layers to have different capping
thickness, to ensure separated cQD peak energies as in Type II, and similarly thick
regrowth, to ensure narrow cQD-sQD separation. Figure
3.9
f shows the PL spectra
of the 2/15/1.7 QDM
1
and 2/6/1.4 QDM
2
bi-layer (upper spectrum) with respect
to the controlled, single layer of 2/6/1.4 QDM
2
(lower). The existent of multiple
peaks in the upper spectrum of the bi-layers begs the question as to whether all
these four peaks are GS. To answer this we reduce the excitation power density
by two orders of magnitude, observe the linear decrease of the four peaks down to
almost the noise floor as shown in the middle spectrum, and thus confirm that all
the peaks in the upper spectrum are indeed GS peaks. Ignoring the small dip in the
middle, the spectrum has a broad FWHM of 170 meV. This non-optimized value
by chirping
two
layers of lateral QDMs compares favorably with 125 meV obtained
from chirping
four
layers of QDs [
47
], or 200 meV from
sixty
stacks of strain-
compensated structure [
50
]. Lateral QDM bi-layers thus provide the best active layer
in terms of cost-performance: a broader FWHM can be achieved for the same stack
number, or the smallest stack number is required for the same FWHM.
3.5.2
Temperature Dependencies
The optical properties of a lateral QDM
single
layer has been shown to follow
the bimodal optical characteristics explained in Sect.
3.4.3
, it is thus expected
that the
bi
-layer should follow the same temperature dependencies since both
layers are separated by a thick 100-nm GaAs spacer layer and hence should be
optically uncoupled. Though reabsorptions (of QDM
1
emissions by QDM
2
,and
vice versa) are a concern, low-temperature PL spectra in the three chirping schemes
above indicate that they do not qualitatively affect the wavelength superposition.
Variable temperature PL spectra in this section additionally indicate that they do
not qualitatively affect the underlying carrier escape and redistribution from and
between cQDs and sQDs either.
The temperature-dependent PL spectra of the Types I-III chirped samples above
are shown in Fig.
3.10
a-c, respectively. The overall spectra are similarly quenched
as the temperature increases, and beyond 250 K no luminescence can be measured.
