Atomic force microscope (AFM) image of ultra-high surface density quantum dots formed by reducing the amount of gallium irradiation to 3 monolayer at a growth temperature of 30°C. An ultra-high surface density of 7.3 x 1011/cm2 was achieved. |
The
NIMS Photonic Materials Unit is developing an advanced self-assembly
technique for semiconductor quantum dots called droplet epitaxy, which
is an original NIMS technology, and recently succeeded in the
development of a new self-assembly technique for quantum dots with the
world’s highest surface density, greatly exceeding the previously
reported value.
Dr.
Takaaki Mano, a Senior Researcher, Dr. Masafumi Jo, a Post Doctoral
Fellow, and Dr. Yoshiki Sakuma, Group Leader of the Quantum
Nanostructures Group, Photonic Materials Unit (Unit Director: Kazuaki
Sakoda), National Institute for Materials Science (President: Sukekatsu
Ushioda) are engaged in developing an advanced self-assembling
technology for semiconductor quantum dots called droplet epitaxy, which
is an original NIMS technology, and recently succeeded in the
development of a new self-assembling technology for quantum dots with
the world’s highest surface density, greatly exceeding the previously
reported value. In addition, the NIMS researchers observed strong
photoluminescence (PL) emission from the assembled quantum dots groups,
suggesting that the developed technology is also effective for realizing
excellent crystal quality.
Quantum
dots have attracted heightened attention in recent years as a
technology for achieving substantial improvement in the properties of
semiconductor lasers and development of ultra-high efficiency
photovoltaic cells based on a new operating principle. In the
newly-developed technology, (1) use of a substrate with a high index
surface, (2) formation and crystallization of gallium droplets at
near-room temperature, and (3) suppression of the droplet coalescence by
optimization of the amount of supplied gallium were introduced in the
gallium arsenide (GaAs) quantum dot formation by droplet epitaxy. As a
result, the NIMS team succeeded in self-assembly of GaAs quantum dots
with an extremely high surface density of 7.3 x 1011/cm2 in a
lattice-matched system. The team also discovered that defects
originating in crystallization at near-room temperature can be restored
by applying ingenuity to the heat treatment process for the crystallized
quantum dots, and strong PL emission can be observed from the quantum
dots.
Droplet
epitaxy has attracted attention as the only method which enables
self-assembly of quantum dots in lattice-matched systems, and in
principle has the advantage that a large number of high quality quantum
dot layers can be stacked in close proximity with maintaining high
crystallinity. Therefore, if the ultra-high density in-plane quantum
dots developed in this research are stacked in close proximity, it will
be possible to produce quantum dot materials with extremely high
volumetric density, which cannot be realized with the conventional
technology. Thus, it is expected to be possible to achieve higher
performance in optical and electronic devices which use quantum dots as a
result of this research achievement.
Details
of this research were published in the online edition of Applied
Physics Letters, which is an American scientific journal in the field of
applied physics, and are scheduled for publication in Vol. 100, No. 21
of the print edition.
