Biomedical Engineering Reference
In-Depth Information
Fig. 3.4 Twenty-Kelvin PL
spectra of ( a ) 2/25/ z QDMs
with z
a
b
2/25/ z
2/ y /1.4
1, 2 or 2.5 ML, and
( b )2/ y /1.4 QDMs with y
=
=
6,
z
= 2.5 ML
y = 25 ML
10 or 25 ML. Spectra are
offset and rescaled for clarity.
Reproduced from [ 23 ]
InAs regrown
GaAs cap
cQDs'
sQDs'
2.0 ML
10 ML
1.0 ML
6 ML
1.0
1.1
1.2
1.3
1.4
1.5
1.0
1.1
1.2
1.3
1.4
1.5
Energy (eV)
Energy (eV)
By comparing the PL spectra with surface morphologies obtained from AFM
images, the origins of the low- and high-energy PL peaks can be attributed to the
cQDs and sQDs, respectively. A typical 2/25/1 QDM contains just a single dot
nucleated inside the nanohole, or cQD, similar to Fig. 3.3 c while 2/25/2 QDM has in
addition another dot or two outside the nanohole, or sQDs, similar to Fig. 3.3 d, and
2/25/2.5 QDM has at least four sQDs, forming a fully-developed molecule similar
to Fig. 3.3 e. The presence of cQDs and absence of sQDs in 2/25/1 QDM ensemble
is a definitive confirmation that the 1.056-eV peak originates from the cQDs. All
three samples share almost identical cQD GS energy at around 1.06 eV which is not
surprising because they all share the same nanohole template and similar size cQDs.
After the nanoholes are saturated, sQDs start nucleating which coincides with the
appearance of the high-energy PL peak. The 1.168-eV peak for 2/25/2 QDMs and
1.15-eV peak for 2/25/2.5 QDMs must therefore originate from the GS of sQDs as
excitation-dependent experiments already rule them out as an ES.
The energetic position, the intensity, and the FWHM of these high-energy GS
peaks agree with surface morphologies and reaffirm their assignments to sQDs.
Figure 3.3 shows that cQDs must first be saturated before sQDs are nucleated. It
is important to note that the base of cQDs is limited by the nanohole template,
while the nominal height of cQDs is always higher than sQDs even though a greater
amount of deposited materials seems to be taken up by the sQDs. This is simply due
to the greater sQD density: four (or more) sQDs per QDM as opposed to one cQD
per QDM. The maximum regrowth thickness for the three samples is 2.5 ML, with
at least 0.5 ML taken up entirely by cQDs and at most 2 ML by sQDs, the per-dot
material accumulated by an sQD is thus much lower than by a cQD. This translates
to smaller sQD and higher GS energy than cQD. It is informative to compare the
energetic positions and integrated intensities of the sQDs in 2/25/2 and 2/25/2.5
QDMs. The sQD PL in 2/25/2 and 2/25/2.5 QDMs peak at 1.168 and 1.150 eV,
respectively. This is simply due to the latter gaining more material, becoming
larger, and thus emitting at a lower GS energy. As more sQDs are nucleated, the
relative intensity of sQDs with respect to cQDs also increases as obvious from the
progressively greater proportion of the sQD peaks in Fig. 3.4 a.
 
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