90 Handbook of Self Assembled Semiconductor Nanostructures for Novel Devices in Photonics and Electronics
Figure 3.6 shows low temperature PL spectra obtained from samples containing two QD lay-
ers separated by GaAs spacer layers of different thickness [44] . The PL excitation intensity is low
(0.1 Wcm
2
) so emission from excited states is negligible. Both QD layers were grown under the
same conditions (by deposition of 2.3 ML InAs at a substrate temperature of 495°C) and are
separated by 40 nm (samples A and B), 20 nm (sample C) or 10 nm GaAs (sample D). For sam-
ples A and D, the GaAs spacer layer was grown at 495°C, the same temperature used for QD
growth, whereas the growth temperature for samples B and C was increased to 580°C after the
fi rst 10 nm GaAs. After the required GaAs thickness had been deposited the growth surface was
annealed under an As fl ux for 10 minutes. This annealing step desorbs segregated In from the
surface [45] and reduces surface undulations inferred from in situ refl ection high-energy electron
diffraction (RHEED) during growth. This was confi rmed by subsequent AFM studies of annealed
and unannealed GaAs surfaces [44] . The importance of the annealing step is illustrated in the
comparison of PL from samples A and B. For QDs grown on the unannealed surface, emission
is blueshifted, despite the large separation between layers. The higher energy peak at 1.20 eV
can be unambiguously assigned to emission from the second QD layer by excitation with either
an Ar
laser or an HeNe laser. The latter source penetrates deeper into the sample and gener-
ates a greater proportion of emission from the lower QD layer. In contrast, only a single peak is
observed from sample B, where the annealing step has successfully recovered a fl at GaAs surface.
As the separation between the QD layers is reduced to 20 nm (sample C), strain from the fi rst QD
layer affects growth of the second layer and emission is blueshifted, despite desorption of surface
In and smoothing of the GaAs surface by annealing. By reducing the separation between lay-
ers to 10 nm (sample D), the two QD layers become electronically coupled and strong emission
is detected only from the QDs in the fi rst layer, due to carrier redistribution to the lowest energy
QD states [38, 39] . For multiple QD-layer laser structures it is crucial to have the gain peak of all
layers coincident. A conclusion gained from the annealing study is that a spacer separation of
at least 40 nm is required for coincident emission. In order to determine whether the integrated
emission scales with the number of QD layers, two samples were compared; one consisting of a
single QD layer, the other containing three QD layers separated by 40 nm (annealed) GaAs spac-
ers. In order to avoid problems associated with carrier diffusion, a metal mask with apertures
60–210 μ m in diameter was deposited on the surface of each sample. A PL study then showed
unequivocally that the PL intensity obtained from the multiple QD layer sample was indeed three
times that observed from the single layer [44] . Thus, by following this recipe, coincident gain
from all layers is assured.
Electronically coupled QDs have attracted attention for structures designed to manipulate
either carrier charge or spin as qubits for quantum information applications and this is covered
more fully elsewhere in this book. For these applications it is desirable for the coupled QDs to be
identical; this is a considerable challenge for the reasons described above. In order to improve the
uniformity of QDs in a multiple layer structure, the so-called “ indium fl ush ” growth technique
can be used [45, 46] . QDs in each layer are partially capped by a thin layer of a few nm GaAs
before growth is interrupted and the substrate temperature is raised to desorb surface indium
before growth of the remaining GaAs spacer layer. This has the effect of truncating the QDs in
each layer so that they have the same height (although there may still be compositional varia-
tion between the QDs). In conclusion, self-assembled QDs require considerable growth expertise
to extract desirable optical properties for use in devices.
3.4 Energy states in QDs
The optical properties of a solid are dictated by the electronic structure; specifi cally the density of
states (DOS). As the dimensionality is reduced, the resulting modifi cation to the density of states
(DOS) is responsible for many of the improvements in the optical properties of QDs, including
higher material and differential gain, lower threshold current densities ( J
th
) in laser structures
[30] and even temperature insensitive operation (corresponding to a characteristic temperature
T
0
) [47] .
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