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JAPAN NANONET BULLETIN
-- 39th Issue -- March 3, 2005
Nanotechnology Researchers Network Center of Japan
Ministry of Education, Culture, Sports, Science and Technology (MEXT)
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**** UK-JAPAN Nanotechnology Symposium ****
- Physics, IT Devices, and Biology -
In order to further promote UK-Japan research collaboration and
researchers exchange program in nanotechnology, UK-Japan
Nanotechnology Symposium will be held on March 9, 2005 at Toranomon
Pastoral (Toranomon, Tokyo) under the support of Grant-in-Aid for
Scientific Research on Priority Areas "Physics of Quantum
Nanoelectronics and Application to Novel Devices" (supervised by Prof.
N. Miura and Prof. Y. Arakawa) of MEXT and Nanotechnology Researchers
Network Center of Japan, MEXT.
For more information on UK-Japan Nanotechnology Symposium,
http://www.nanonet.go.jp/english/event/event20050309.html
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IN THIS ISSUE
Nanonet Interview:
Kenichi IGA, Executive Director, Japan Society for the Promotion of
Science (JPSJ) and Professor Emeritus, Tokyo Institute of Technology
Young Researchers' Introduction:
Shinji YUASA, Group Leader, NanoElectronics Research institute,
National Institute of Industrial Science and Technology (AIST)
-- NANO CALENDAR --
For information on nanotechnology related symposiums and conferences
held in the world,
http://www.nanonet.go.jp/english/calendar/
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NANONET INTERVIEW
Birth of surface emitting laser
--An initiator of nanotechnology in photonics--
(Issued in Japanese: November 18, 2003)
Kenichi IGA, Executive Director, Japan Society for the Promotion of
Science (JPSJ) and Professor Emeritus, Tokyo Institute of Technology
A surface emitting laser can emit light vertically from the surface of
a semiconductor substrate, and it is characterized by low power
consumption, a long lifetime, and a monolithic process to fabricate.
Since 1999, many institutes have been conducting R&D in order to
commercialize the technology. The idea of the surface emitting laser
was first proposed by Prof. Iga in 1977.
In the mid 70's, when it was shown that transmission loss in optical
fibers could be reduced by the use of near infrared light with a
wavelength of longer than 1.3 mm, research involving the development
of a long-wavelength laser which was suitable for optical transmission
began worldwide. Prof. Iga and Prof. Yasuharu Suematsu, who was a
professor at Tokyo Institute of Technology at the time, developed a
long wavelength semiconductor laser with stripe-geometry that emits
light parallel to the surface of the substrate. Its performance was
one of the best in the world. However, Prof. Iga was not satisfied
with it. "To build a stripe geometry laser, a semiconductor substrate
with multi-layered thin films has to be cleaved 300 micrometers in
width by a blade. A laser cannot be tested until after the stripe-
geometry is formed by cleaving. Since cleaving substrates has to be
done manually, they are unsuitable for mass production. As well, they
cannot be formed with two-dimensional arrayed configuration. I
wondered how a laser could be built without having to cleave the
substrate. I thought about it day and night and finally came up with
the idea of changing the light emitting direction from horizontal to
vertical, " says Prof. Iga. That was the moment when the idea of
"Surface Emitting Laser" was born. Thus, a laser can be built without
the cleaving process by placing reflectors above and below the
semiconductor thin films. This method makes large-scale integration
possible as well. However, insufficient light amplification for laser
oscillation became a problem with surface emitting lasers because the
thin active layer caused extremely short resonator lengths. To solve
the problem, he had to increase the reflectivity of the reflectors to
near about 100%.
Prof. Iga and his group began verifying the concept of the surface
emitting laser. When the structure with an active layer consisting of
GaInAsP was cooled with liquid nitrogen and a pulsed current was
applied, it instantly emitted light. However, the threshold current
required for laser oscillation was about 1A, which is 1000 times
higher than the current presently used in the device. The device had
to be cooled to liquid nitrogen temperature; otherwise, it would break.
"It might be impossible, in principle, to obtain laser oscillation at
a practical level with surface emitting lasers because of its
extremely short resonant length or maybe the theory was correct but
there might be a problem with the technology," thought Prof. Iga.
Although he continued research on developing crystal growth technology
as well as the theoretical studies, his research was not going well
because there was only a liquid phase epitaxial method for the crystal
growth at that time, and it was unsuitable for nanoscale processing.
Later, along with the developments in transistor technology, a metal
organic chemical vapor deposition (MOCVD) was developed. In MOCVD, a
metal organic material is transformed into a gas and the gas flows
over the crystal growth substrate. Nanoscale structure control became
possible with this process. He built his own MOCVD for his research,
and in 1988, he prepared the first room temperature surface emitting
laser with continuous oscillation together with his colleague, Prof.
Fumio Koyama of Tokyo Institute of Technology. The threshold current
was 20 to 30 mA and the laser output was 1 to 2 mW.
Continuous oscillation at room temperature attracted worldwide
attention and research concerning surface emitting lasers accelerated.
In the '90s, they were used in transmitters for high-speed LAN. In the
United States, national projects on massively parallel optical
transmissions and optical interconnections using surface emitting
lasers started, and since about 1999, surface emitting lasers have
been used for high-speed networks. Prof. Iga says, "In 1999, during
which surface emitting lasers were first used in the United States, we
were already on the fourth generation of the laser. I verified its
principles from the late 70's to the early 80's, improved and
developed the device for practical use during the '80s, and turned
various ideas such as wavelength conversion into reality in the '90s.
"Tunable lasers were developed by using the reflectors that are
controlled by MEMS technology. The nanofabrication technology used
for surface emitting lasers is the same as that used for LSI.
Nanotechnology including the application of MEMS is under development,
" says Prof. Iga. During the Clinton administration, this laser
technology, developed at Tokyo Institute of Technology, was mentioned
on the Website of the National Nanotechnology Initiative (NNI). The
nanotechnology that Prof. Iga developed will bring about further
developments in surface emitting lasers.
(Interviewer: Shiro Saito, Cosmopia Inc.)
For more information,
http://www.nanonet.go.jp/english/mailmag/2005/039a.html
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YOUNG RESEARCHERS' INTRODUCTION
Coherent spin-polarized electron tunneling in magnetic tunnel
junctions with a single-crystal electrode
(Issued in Japanese: November 11, 2003)
Shinji YUASA, Group Leader, NanoElectronics Research institute,
National Institute of Industrial Science and Technology (AIST)
A new field of electronics called spintronics, in which both the
electric charge and the spin of the conduction electrons are utilized,
has developed rapidly. A magnetic tunnel junction (MTJ), which
consists of two ferromagnetic metal layers (electrodes) separated by a
thin insulating layer (tunnel barrier) and exhibits the tunnel
magnetoresistance (TMR) effect, is especially important for
magnetoresistive random-access memory (MRAM) devices.
The TMR effect originates from spin-polarization of a density of
states (DOS) at the Fermi level in the electrodes. However, agreement
between the observed TMR effects and theories has not been successful.
Additionally, coherent tunneling effects, such as resonant-tunneling,
have never been observed in spin-polarized systems. To clarify the
physical mechanism of the TMR effect, we developed magnetic tunnel
junctions made of well-defined single-crystal electrodes, and have
clarified the following points.
1. Crystal-orientation-dependence of TMR
We fabricated a MTJ that has a single-crystal Fe electrode in
various crystal orientations and observed that TMR significantly
depends on the crystal orientation of the electrode. This phenomenon
possibly reflects the crystal anisotropy of the spin polarization in
the electrode.
2. A large oscillation of TMR due to spin-polarized resonant tunneling
A non-magnetic Cu(001) ultrathin layer (tCu = 0~3nm) was inserted
between a ferromagnetic Co(001) electrode and an Al-O tunnel barrier.
Spin-polarized quantum well (QW) states were formed in the Cu layer,
and spin-polarized resonant tunneling was expected to occur via the QW
states. We observed a large TMR oscillation as a function of tCu due
to spin-polarized resonant tunneling. This phenomenon is a coherent
tunneling effect, which cannot be described simply in terms of the DOS
at the electrode/barrier interface. This result is the first
observation of coherent tunneling in spin-polarized systems and makes
it possible to develop resonant-tunneling-type spintronic devices.
Now we are engaged in the development of all-single-crystal MTJs using
MgO(001) as a tunnel barrier in order to create an extremely large TMR
effect at room temperature.
For more information,
http://www.nanonet.go.jp/english/mailmag/2005/039b.html
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