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JAPAN NANONET BULLETIN
-- 66th Issue -- March 16, 2006
Nanotechnology Researchers Network Center of Japan
Ministry of Education, Culture, Sports, Science and Technology (MEXT)
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IN THIS ISSUE
Nanonet Interview:
"Room-temperature continuous-wave lifetime exceeding 10,000 hours
- A quantum-wire laser realized by a top-down fabrication method -"
Shigehisa ARAI, Professor, Quantum Nanoelectronics Research Center,
Tokyo Institute of Technology
Young Researchers' Introduction:
"Realization of magnetization reversal by carrier-spin-injection
into nano-scale ferromagnetic alloy semiconductors"
Akira OIWA, Lecturer, Department of Applied Physics, The University
of Tokyo and Researcher, Precursory Research for Embryonic Science and
Technology (PRESTO), Japan Science and Technology Agency (JST)
-- 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
Room-temperature continuous-wave lifetime exceeding 10,000 hours
- A quantum-wire laser realized by a top-down fabrication method -
(Issued in Japanese: June 23, 2004)
Shigehisa ARAI, Professor, Quantum Nanoelectronics Research Center,
Tokyo Institute of Technology
In 1978, the possibility reducing transmission loss in optical fibers
at a wavelength of 1.5 to 1.6 microns was only theoretical prediction.
Prof. Arai, who was then a graduate student, developed a laser with a
wavelength of 1.5 microns under the supervision of Professor Emeritus
Yasuharu Suematsu (the former president of Tokyo Institute of
Technology). Around the time, a Japanese company developed low
transmission loss optical fibers with a wavelength of 1.5 microns. The
progress in communication devices led to the enhancement of long
distance optical communication. Since then, Prof. Arai has devoted his
life to the development of optical communication devices.
In 2003, Prof. Arai developed a quantum-wire laser with a wire width
of 23 nm and a period of 80 nm. GaInAsP active layers and InP cladding
layers are stacked alternatively up to five layers thick using organo-
metallic vapor-phase-epitaxy, and the layers are patterned by using
electron beam lithography. Then, vertical gratings are fabricated by
reactive ion etching, and the gratings are filled with InP. This is
how quantum wires are fabricated. This laser with a quantum-wire
structure showed no noticeable performance degradation even after more
than 10,000 hours (22,600 hours as of 27th Oct. 2005) of operation at
room temperature. He says, "I have been challenging myself to see how
narrow I can draw patterns using a top-down method. So far, I have
been able to draw a pattern with a period of 80 nm".
Prof. Arai's 5-layered quantum-wire lasers with a wire width of 43 nm
perform better than commercialized quantum-film lasers. He says
"However, my team's desire is to fabricate quantum-wire lasers with
narrower wires and make use of quantum effects at room temperature.
For wires wider than 30 nm, which cannot be expected to show clear
quantum effects at room temperature, we do not use the word, 'quantum'
effect." Currently, the threshold current density of a quantum-wire
laser with a wire width of 23 nm is 1.6 times higher than that of a
quantum-film laser. This is partly because the threshold gain for
laser oscillation increases due to a decrease in the volume of an
active layer based on narrow wires and the increase of the optical
loss. Prof. Arai thinks that the volume ratio of quantum-wires to
other parts of the laser of 1 to 1 is the best solution to the problem.
He says, "The performance of a quantum-wire laser won't be maximized
unless patterns are drawn very densely. It is important to develop
technology to fabricate fine patterns in desired positions and with
desired densities." In a quantum-wire laser with a wire width of 23 nm,
the wire width fluctuation is currently about 8%. "The width of its
emission spectrum is about the same as that of a normal quantum-film
laser. I would like to make it half the current emission spectrum
width. Thus, I have to obtain wire width fluctuations lower than 5%,"
says Prof. Arai.
However, increasing the precision of patterning and etching is not a
satisfactory solution to all of the problems. Currently, strained
layer epitaxy with a strain of about 1% is used in laser film
fabrication in order to decrease the threshold current. Prof. Arai
says, "It seems that the strain in the first layer is different from
that in the second layer when strained layer epitaxy is used.
Theoretically, it is possible to fabricate a quantum-wire laser with
half of the current emission spectral width when a size fluctuation is
lower than 5%; however, unexpected things such as a broader spectral
width happen with fabricated lasers." He, then, fabricated a quantum-
wire laser with only one active layer to eliminate the influence of
strain on the properties of the laser. It requires strong reflectors
because the optical gain is not high enough in a laser with one active
layer. Then, his team thought of increasing the refractive index
difference between the active layer and the cladding layers to 50% by
using cladding layers made of SiO2 glass or benzocyclobutene, which
has a low refractive index of 1.5. He says, "Placing electrodes on
both sides of the layered film makes the device as thin as 0.1 microns.
We call it a membrane laser." It took three years for his team to make
the laser operating under a room temeperature continuous wave (RT-CW)
condition by photoexcitation. He says, "However, we would like to
fabricate a high performance laser that is operated by current
injection and would like to know how much we can increase its
performance by both increasing optical confinement in an active layer
and reducing the size of an active layer." The optical gain in an
ideal quantum-wire laser is two times larger than that of a quantum-
film and four times larger when the refractive index difference
between the active layer and cladding layers increases from 5% to 50%.
In optical device development, Prof. Arai insists on using a top-down
method to fabricate ultrafine structures. He says, "In 1994, when a
German and Russian team collaborated together to fabricate a quantum-
dot laser using a bottom-up method, those who had crystal growth
equipment rushed into developing devices using the method. However,
even if you were able to fabricate a desired device using a bottom-up
method, you are just one of the followers. If you fabricate a device
using a top-down method, you have your own new field to explore." When
he fabricated the laser with a wavelength of 1.5 microns while he was
in school, he felt that he played a role in the era of rapidly
developing technology because his laser was commercialized within a
short time. Now he advises young researchers as a professor that,
although they may not be able to get good results, they should pursue
their research interests in order to establish their own new fields.
Prof. Arai says, "If you give up, you set your own limit there. You
may not be able to get what you want or may find phenomena that you
cannot understand, but that is only natural because you are doing
something nobody has ever done before."
(Interviewer: Kuniko Ishiguro, Cosmopia Inc.)
For more information,
http://www.nanonet.go.jp/english/mailmag/2006/066a.html
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YOUNG RESEARCHERS' INTRODUCTION
Realization of magnetization reversal by carrier-spin-injection into
nano-scale ferromagnetic alloy semiconductors
(Issued in Japanese: October 27, 2004)
Akira OIWA, Lecturer, Department of Applied Physics, The University
of Tokyo and Researcher, Precursory Research for Embryonic Science
and Technology (PRESTO), Japan Science and Technology Agency (JST)
In spintronics, ferromagnetism is an indispensable property in
designing non-volatile memories, magnetoresistance devices, and so on.
Control of the magnetization orientation in ferromagnetic substances
without using an external magnetic field is crucial for low-power
consumption and high-density integration in future spintronics devices.
To pursue this goal, we are studying optical/electrical spin-injection
magnetization reversal in III-V-based magnetic alloy semiconductors
(III-V-MAS). III-V-based magnetic alloy semiconductors show
ferromagnetic ordering mediated by the exchange interactions between
carriers and magnetic atoms. The exchange interaction offers
opportunities to manipulate the magnetism and the magnetization
reversal by controlling the carrier number and spin.
We are trying to perform spin-injection magnetization reversal by
optical and electrical means. In optical spin-injection experiments,
when the spin-polarized holes are generated in a III-V-based magnetic
alloy semiconductor (Ga, Mn)As thin film with circularly polarized
light, the magnetization is rotated in the direction along the hole
spin polarization (Fig. 1a). From time-resolved magneto-optical effect
measurements using a fs-Ti:sapphire laser, the signal of photoinduced
magnetization rotation rises very rapidly within the pulse width (150
fs) immediately after the excitation (Fig. 1b). This indicates the
possibility of ultrafast photoinduced magnetization rotation. On the
other hand, in electrical control experiments (Fig. 2a), partial
magnetization reversal has been observed when spin-polarized holes are
injected into a free layer in (Ga, Mn)As-based tunnel
magnetoresistance device (Fig. 2b). The observed reversal current
density (middle of 10^5 A/cm^2) is significantly lower than the value
reported in ferromagnetic metals. These results indicate that III-V-
based magnetic alloy semiconductors can be used as spin-injection
magnetization reversal materials.
Our goals have been to establish optical/electrical spin-injection
magnetization reversal and to reduce further the optical intensity and
reversal current density in III-V-based magnetic alloy semiconductors.
Moreover, the control of magnetization reversal dynamics is an
interesting subject.
For more information,
http://www.nanonet.go.jp/english/mailmag/2006/066b.html
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