Biomedical Engineering Reference
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The FWHM of the 1.168-eV peak in the case of 2/25/2 QDMs (68 meV) and the
1.150-eV peak in the case of 2/25/2.5 QDMs (73 meV) are higher than those of the
1.068-eV peak (25 meV), indicating that the inhomogeneous broadening of sQDs
is worse than those of cQDs. This is expected, considering the almost simultaneous
nucleation of cQDs in the nanoholes across the wafer, and the sequential, random
nucleation of sQDs only after the nanoholes are saturated with cQDs.
The PL spectra from the second sample series affirm the peak assignments above
and elucidate the role of nanohole depth. Figure 3.4 b shows, from bottom to top, the
PL spectra of 2/ y /1.4 QDMs where y
6, 10, and 25 ML, respectively. The 2/6/1.4
QDMs emit a single PL peak at 1.186 eV, the 2/10/1.4 QDMs a double peak at
1.150 and 1.226 eV, and the 2/25/1.4 QDMs a single peak at 1.068 eV. Since the
thicknesses of the seed QDs x and regrown QDs z are identical, the differences in
the three spectra originate from the GaAs capping thickness y . The GaAs capping
thickness y dictates the degree of In out-diffusion from the seed InAs QDs and thus
the nanohole depth which has been carefully characterized and found to vary almost
linearly from 0.4 to 1 nm as y increases from 6 to 25 ML [ 14 ]. The shallower the
nanohole, the easier for it to be saturated, or the smaller the cQDs.
In the case of 2/6/1.4 QDMs with the most shallow nanoholes, both cQDs
and sQDs co-exist and the average sizes of cQDs and sQDs do not much differ,
resulting in a near-merged spectra. Though the bottom spectrum in Fig. 3.4 bshows
a single peak, closer examination of its semi-logarithmic plot reveals different rise
and fall characteristics, indicating different origins. In the case of 2/25/1.4 QDMs
with deepest nanoholes, on the other hand, only cQDs exist, resulting in a single
Gaussian peak at 1.068 eV shown in the top spectrum of Fig. 3.4 b. This peak is
located at the same energetic position as the 2/25/1 QDMs from the first series but is
narrower (FWHM
=
21 meV), possibly due to the better uniformity achieved when
approaching full saturation condition at a thicker regrowth thickness of 1.4 ML.
Finally, in the case of 2/10/1.4 QDMs with nanohole depth intermediate between the
two extreme cases above, both cQDs and sQDs co-exist and their average sizes are
distinctly difference, resulting in two well-separated PL peaks with the low-energy
portion attributed to cQDs and the high-energy to sQDs.
=
3.4.2
cQD-sQD Coupling
The presence of the WL and the proximity between cQDs and sQDs bring
with them the possibility of tunnel coupling, especially in view of the fact that
electron wavefunctions are not well confined and the existent of extended states
has been confirmed [ 5 , 17 ]. Microscopically, coupling gives rise to bonding and
anti-bonding molecular states, resulting in energetic separation or anti-crossings
when the luminescence is measured under varying electromagnetic fields [ 24 , 25 ].
Macroscopically, coupling gives rise to electron transfer between QDs, resulting in
PL intensity transfer between or among related energy levels. Tunnel coupling in
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