Biomedical Engineering Reference
In-Depth Information
will be formed on the previously denuded regions between QDMs. The cQDs and
sQDs exhibit fundamentally different optical behaviors and can interact due to their
proximity as described next.
3.4
Optical Properties of QDMs
The macro-PL results described in this section are obtained at 20 K under a low
excitation power density of 0.45 W/cm
2
, unless otherwise stated. The QDMs are
of high crystalline quality with clear, separate emission peaks from the cQDs and
sQDs as described in Sect.
3.4.1
. Transfers of luminescent intensity between cQDs
and sQDs as temperature varies are reported in Sect.
3.4.2
and explained in terms of
phonon-assisted carrier transfer between the constituent QDs. The QDM ensemble
exhibits a unique, bimodal
optical
characteristics explained in Sect.
3.4.3
which
differ from bimodal
size
distributions generally reported for intermediate sized QDs.
3.4.1
cQDs and sQDs Emissions
This sub-section describes the PL of six QDM samples. Each sample is grown using
a different QDM recipe, i.e. different
x
,
y
, and/or
z
, with identical procedures for the
rest of the structure as described in Sect.
3.3
. The first series of three samples is
grown with the same nanohole template obtained from 2-ML InAs seed QDs and
25-ML GaAs partial-cap layer. The nanoholes are filled with either 1, 2, or 2.5-
ML InAs, resulting in 2/25/1, 2/25/2 or 2/25/2.5 QDMs, respectively. The second
series of three samples is grown with the same 2-ML InAs seed QDs and 1.4-ML
InAs regrown QDMs, but with different GaAs capping layer thickness of 6, 10, or
25 ML, resulting in 2/6/1.4, 2/10/1.4, or 2/25/1.4 QDMs, respectively. Systematic
variation of parameters in the two series enables unambiguous assignment of PL
peaks to the constituent cQDs and sQDs in the QDMs [
23
].
Figure
3.4
a shows, from bottom to top, the PL spectra from the first sample series
which contain 2/25/
z
QDMs where
z
1, 2, and 2.5 ML, respectively. The 2/25/1
QDMs emission is dominated by a single peak at 1.056 eV with a narrow full-
width at half-maximum (FWHM) of 24 meV. The 2/25/2 QDMs emission shows
a low-energy peak also at 1.068 eV with FWHM of 40 meV, and a high-energy
peak at 1.168 eV with FWHM of 68 meV. The 2/25/2.5 QDMs emission show
the same low-energy peak at 1.068 eV but with a slightly narrower FWHM of
25 meV, and a high-energy peak at 1.150 eV with FWHM of 73 meV. The double
peak feature of the latter two samples go up and down together under excitation-
dependent experiments, indicating that the high-energy peak is not an excited state
(ES) arising from the saturation of the low-energy, ground-state (GS) peak. In
addition to confirming GS emission, it is necessary to identify the sources and
predict the characteristics of the two peaks if the QDMs are to be optimized and
employed in devices.
=
