Optical Properties of Lateral InGaAs Quantum Dot Molecules Single- and Bi-Layers - Quantum Dot Molecules - page 54

3.2.3 QDMs on Nanohole-and-Mound Templates

Similar to the two approaches above, this third approach by Suraprapapich et
al. employs a unique template shaped like the back of a camel (side-view) or a
two-blade propeller (top-view) and is thus originally referred to as a camel-like
nanostructure and nanopropeller [ 13 ]. Overgrowth of InAs on this template create
a unique lateral QDM geometry as summarized in Fig. 3.1 c. The process starts
from the deposition of 1.8-2 ML of InAs QDs at a slow growth rate of 0.01 ML/s,
followed by a short growth interruption to reduce the substrate temperature from
500 to 470 ◦ C before capping the seed QDs by GaAs. The thin GaAs cap layer
does not completely cover the original QDs. The apex is hence thermodynamically
unstable, resulting in In out-diffusion and a shallow dip, or nanohole, in the middle.
The out-diffusing In atoms then react with the incoming Ga and the existing Ga
atoms on the surface to form InGaAs nanomounds with a pronounced elongation
along the [1

1 0] direction. The nanoholes are the most favorable locations for
QD formation, followed by the edges of the surrounding nanomounds. Subsequent
regrowth of InAs always yields InAs QDs inside the nanoholes before the nucleation
and growth on nanomounds. While the number of QDs inside the nanohole is limited
to one, those along the edges of the nanomounds can be varied from 1 to as many
as 13 simply by changing the deposition amount [ 14 ]. These QDMs are also of
high crystalline quality, exhibiting room-temperature luminescence with noticeable
polarization anisotropy [ 15 ].

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3.2.4 QDMs on Strain-Modulated Templates

This approach is fundamentally different from the above approaches by its use of
strain-modulated templates where strains are global in nature, i.e. present on a wafer
scale, as opposed to local strains effective only around nanoholes or nanomounds.
One implementation of this approach by Lippen et al. and with schematic
cross section in Fig. 3.1 d uses a superlattice (SL) template of InGaAs/GaAs on
(3 1 1)B-GaAs to form a slowly modulated strained network that upon overgrowth
results in a lattice of ordered QDMs [ 16 ]. These QDMs are optically active up to
room temperature and exhibit a complex PL behaviors [ 17 ].
Another implementation by Thet et al. and with schematic cross section in
Fig. 3.1 e uses as template a network of orthogonal, interfacial dislocations which
under optical microscopy appears as a cross-hatch pattern (CHP) to form a slowly
varying surface undulations that also supports the growth of QDM lattice [ 18 ].
Despite the presence of dislocation, QD chains grown on these kinds of templates
have been shown to exhibit high crystalline quality [ 19 , 20 ]. QDMs on these kinds
of templates are expected to have high crystalline quality as well though no PL
results have been reported so far. Recently, Seravelli et al. reported PL results of
QDs grown on thick metamorphic InGaAs/GaAs templates and found QDs to be of
high crystalline quality [ 21 ].