Stacked Self-Assembled Cubic GaN Quantum Dots
Grown by Molecular Beam Epitaxy
Sarah Blumenthal,* Torsten Rieger, Doris Meertens, Alexander Pawlis, Dirk Reuter,
and Donat J. As
We have investigated the stacking of self-assembled cubic GaN quantum
dots (QDs) grown in Stranski–Krastanov (SK) growth mode. The number of
stacked layers is varied to compare their optical properties. The growth is in
situ controlled by reflection high energy electron diffraction to prove the SK
QD growth. Atomic force and transmission electron microscopy show the
existence of wetting layer and QDs with a diameter of about 10 nm and a
height of about 2 nm. The QDs have a truncated pyramidal form and are
vertically aligned in growth direction. Photoluminescence measurements
show an increase of the intensity with increasing number of stacked QD
layers. Furthermore, a systematic blue-shift of 120 meV is observed with
increasing number of stacked QD layers. This blueshift derives from a
decrease in the QD height, because the QD height has also been the main
confining dimension in our QDs.
Group III-nitrides attracted much attention in the development of
optical and quantum optical devices, operating in the UV spectral
range. Especially, quantum dots (QDs) are used for many
applications like QD-lasers, single photon emitters, and QD-
detectors. Stacking of the QDs is an appropriate way to increase the
number of QDs in the active region. Due to the stacked QDs in three
dimensions, quantum dot lasers are a promising candidate for
In the last years, stacked hexagonal GaN
(h-GaN) QDs have already been realized indicating an increase in
room temperature photoluminescence intensity with increasing
number of stacked QD layers.
However, the hexagonal phase
exhibits an internal ﬁeld causing a reduced
This may be
overcome by using zincblende cubic GaN (c-
GaN), where no polarization ﬁelds in (001)
growth direction exist.
Only few groups are
working with this metastable phase. First
results for QD stacking of c-GaN QDs is
shown by Martinez-Guerrero et al.
already published the growth of a
single layerofc-GaN QDs in SK growth mode.
Single-photon emission from these QDs is
The QDs show radiative
lifetimes about one order of magnitude
shorter compared to hexagonal, polar GaN
QDs, which are emitting at the same energy.
The incorporation of these QDs into photonic
structures, like microdisks and two-dimen-
sional photonic crystal membranes with high
quality factors is already realized.
In the InAs/GaAs and InGaAs/GaAs
system, the realization of stacked QDs embedding in micro-
cavities led to an increase of the optical gain and resulted in
lasing at a considerable lower subband at room temperature.
Vertical stacking of QDs with a thin spacer layer led to a coupling
of the QD layers. This coupling induced vertical alignment of the
due to local strain ﬁelds originated by the subjacent
QD layer and prompted preferential nucleation sites vertically
aligned in the subsequent layer. In addition to the inﬂuence on
the structural properties, electronic properties may change due
to the stacking of quantum dots. The reasons include a change of
and electronic coupling between the quantum dot
In the InAs system these effects resulted in a redshift
in PL emission energy in most stacking experiments.
exception is described by Heidemeyer et al.
They observed a
blueshift in PL emission energy of a twofold stacked QD sample
compared to the single layer and trace it back to a growth-related
phenomenon as strain induced intermixing or indium loss and
in addition to complex strain ﬁelds that exist in the closely
In contrast to InAs/GaAs QDs, we observed a blueshift of
the QD emission with increasing layers of stacks in our cubic
GaN/AlN QD structures.
2. Experimental Section
Our samples are grown by molecular beam epitaxy (MBE). For
gallium and aluminum evaporation, standard effusion cells are
used. The nitrogen is derived from dissociation of N
S. Blumenthal, Dr. D. Reuter, Dr. D. J. As
Department of Physics
University of Paderborn
Warburger Str. 100, 33098 Paderborn, Germany
Dr. T. Rieger, Dr. A. Pawlis
Peter Grünberg Institut
52428 Jülich, Germany
Ernst Ruska Centre for Microscopy and Spectroscopy with Electrons
52428 Jülich, Germany
The ORCID identification number(s) for the author(s) of this article
can be found under https://doi.org/10.1002/pssb.201600729.
GaN Quantum Dots www.pss-b.com
Phys. Status Solidi B 2018, 255, 1600729 © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
1600729 (1 of 6)