======================================================================
JAPAN NANONET BULLETIN
-- 77th Issue -- August 17, 2006
Nanotechnology Researchers Network Center of Japan
Ministry of Education, Culture, Sports, Science and Technology (MEXT)
======================================================================
IN THIS ISSUE
Nanonet Interview:
"Combinatorial superlattice engineering
-- Newly developed p-type ZnO --"
Masashi KAWASAKI, Professor, Institute for Materials Research, Tohoku
University
Introduction of Nanonet Research Report:
"Activities on societal implications of nanotechnology in Europe and
US"
-- NANO CALENDAR --
For information on nanotechnology related symposiums and conferences
held in the world,
http://www.nanonet.go.jp/english/calendar/
----------------------------------------------------------------------
NANONET INTERVIEW
Combinatorial superlattice engineering
-- Newly developed p-type ZnO --
(Issued in Japanese: February 23, 2005)
Masashi KAWASAKI, Professor, Institute for Materials Research, Tohoku
University
In December 2004, news about blue light emission by current injection
from a p-n junction made of ZnO was reported as the development of blue
light-emitting diodes made from a cosmetic raw material. Prof. Kawasaki
says, "At that time, many researchers did not think that p-type
semiconductors could be made from ZnO. I think proving its
materialization in a way that convinced every researcher was a
milestone." Prof. Kawasaki has been trying for the past 10 years to
establish combinatorial superlattice engineering. Combinatorial
synthesis is already used widely in organic chemistry to synthesize
compounds exhaustively as new drug candidates. Its concept, combined
with molecular beam epitaxy and mask patterning, was applied to
superlattice engineering for inorganic materials. As a result, it
became possible to synthesize various types of films whose composition
and structure are changed systematically in very small areas on the
surface of a substrate, and to speedily evaluate the films. Making p-
type ZnO is one of the fruits of such an application.
Prof. Kawasaki says ZnO is inexpensive and is used for a wide range of
applications. Almost no one knew that ZnO emits ultraviolet light very
efficiently until his team confirmed its laser oscillation by light
excitation in 1996. ZnO, which has large exciton binding energy, can
theoretically emit light at a high efficiency at room temperature.
However, making p-type ZnO was very difficult because of many
electrons supplied by impurities and lattice defects in ZnO. At one
time, the co-doping method, in which acceptor and donor are injected
simultaneously, was theoretically predicted to work well. By examining
the possibility of making a p-type semiconductor from ZnO through
combinatorial synthesis, Prof. Kawasaki concluded that p-type ZnO would
not be made using the co-doping method as long as the basic way of
thinking remained unchanged. He says, "I started trying to make p-type
ZnO by doping only acceptor nitrogen. ZnO tends to become an n-type
semiconductor because of a large number of electrons supplied by
impurities and lattice defects in ZnO crystals. The key to making p-type
ZnO was reducing the number of these electrons, and I continued
improving the quality of ZnO crystals for three years."
Prof. Kawasaki succeeded in reducing the residual electron
concentration in the crystals to 10^16 cm^-3, and started research on
making p-type ZnO by doping nitrogen. He says, "When the growth
temperature of ZnO crystals is high, nitrogen does not dissolve into
the crystals. I first lowered the substrate temperature to 400 degree C
and made a 15 nm-thick crystal layer containing nitrogen. Then, the
substrate temperature was immediately raised to 900 degree C, and a 1 nm
-thick ZnO crystal layer was deposited on the crystal layer in order to
make the crystal surface smooth. This process was repeated." The hole
concentration of the nitrogen-doped ZnO reached 10^16 ~ 10^17 cm^-3. He
developed a p-n homo-junction light-emitting diode by combining his new
material with n-type ZnO. He observed light emission with a nearly 400
nm-wavelength from the diode, which corresponds to energy slightly
lower than ZnO's band gap energy. The wavelength of the light can be
reduced to 200 nm if the quality of ZnO crystals is improved and the
band gap is widened by doping magnesium. However, Prof. Kawasaki admits
to the difficulty of reducing the wavelength to such a level, saying
that doping magnesium generates defects and thus increases the number
of residual electrons.
The key to the successful development of p-type ZnO was temperature
modulation epitaxy, which was devised based on temperature gradient
epitaxy developed by Prof. Kawasaki. With the temperature gradient
epitaxy, a gradient of 300 degree C can be realized on a 1 cm^2
substrate by heating the substrate locally with a combinatorial
synthesis apparatus equipped with semiconductor laser equipment. He
also used the temperature gradient epitaxy to control the compositions
of materials in order to verify the usefulness of the co-doping method.
When a ZnO film containing a certain volume of gallium with a
temperature gradient is reacted with nitrogen, a gradient in nitrogen
concentration dependent on the temperature gradient is made. This
method can be used to form somewhere on the substrate a film whose
composition is theoretically ideal.
Now that p-type ZnO has been developed, researchers in the field may
think that Prof.Kawasaki's next goal is to develop a laser from such a
material. However, he is not so interested in developing devices using
p-type ZnO. He says, "The objective of my research on combinatorial
synthesis is to create new materials one after another, which may
surprise other researchers." He is determined to continue searching for
new materials, mainly oxides, saying, "There are many types of oxides
and their crystal structures and properties are varied because there
are oxides of almost all elements. In addition, since they have
relatively similar crystal structures, epitaxial growth is possible for
them. This is the most attractive feature of oxides for me. I believe
that their combinatorial syntheses may create very interesting
materials."
Prof. Kawasaki is now working on synthesizing high-temperature
superconductors. Superlattice engineering of strongly correlated oxides
whose atom layers' sequences play a key role is suitable for
combinatorial synthesis. He says, "I know well from my experience that
I won't get lucky as long as I focus my research only on targeted
materials. So, I will also study thermoelectric and other non-
superconductive materials, while considering developing high-
temperature superconductors." Researchers have been searching for new
materials by using innumerable conditions available in the space
created by two axes - choice and composition of the elements - in
combinatorial synthesis. Prof. Kawasaki says, "We use technology to put
one layer of atoms on another. So, combinatorial synthesis is also
useful to search for materials whose atom layers' sequences are
important. This synthesis method can also be applied to looking for the
best parameters of a process as in the case of developing the p-type
ZnO. Through combinatorial synthesis, we can obtain excellent data in a
small number of experiments, and present convincing data for discussion
about a physical mechanism. I think combinatorial synthesis is moving
toward superlattice engineering peculiar to inorganic materials."
(Interviewer: Kuniko Ishiguro, Cosmopia Inc.)
For more information including figures,
http://www.nanonet.go.jp/english/mailmag/2006/077a.html
----------------------------------------------------------------------
INTRODUCTION OF NANONET RESEARCH REPORT
Activities on societal implications of nanotechnology in Europe
and US
(Issued in Japanese: April 20, 2005)
Innovative technology has a wide range of impacts on society through
industrial activities. In addition to initially expected benefits,
such technology sometimes generates unintended effects on society and
puts people at risk. Although no overt risks regarding nanotechnology
have been pointed out, a number of experts have been warning of such
potential risks.
In the United States, the societal implication of nanotechnology,
ranging from the direct impact of nanomaterials on humans and the
environment to ethical implications, has been treated as one of the
most important nanotechnological challenges that society faces since
the National Nanotechnology Initiative (NNI) began in 2000. The
National Science Foundation (NSF) held a workshop on this issue (Roco &
Bainbridge 2001) in September 2000. Since then, US government
departments and agencies have been promoting study programs on the
impact of nanotechnology on the products and technology for which each
department or agency is responsible. These efforts by individual
government offices have been coordinated by the National Nanotechnology
Coordination Office.
In Europe, the NANOSAFE project and other studies on the safety of
nanomaterials were kicked off several years ago. In "Towards a European
Strategy for Nanotechnology," published in July 2004, Europe stated
more clearly than ever before that nanotechnology development through
harmonization with various social aspects, public health, safety and
environment/consumer protection are very important issues. Another key
research report is "Nanoscience and nanotechnologies: opportunities and
uncertainties," published by the Royal Society & Royal Academy of
Engineering of Britain in July 2004.
Risk assessment and risk management of nanomaterials have been
discussed most frequently as a challenge faced by society. Experts have
not yet pointed out any overt risks of such materials. However, we are
not yet at the stage where risk assessment can be conducted because no
standardized test materials or test methods have been established.
Systematic and strategic research on standardization is necessary.
Although the results of some research indicate the toxicity of
nanomaterials, linking such results directly to risk assessment is
inappropriate. According to the conferences and research introduced in
this report, experts have already reached a basic consensus on how to
handle nanomaterials (based on existing chemicals and food). They have
almost always agreed on the definition and classification of
nanomaterials (the largest dimension of up to 100 nm), key issues with
a high priority from the viewpoint of exposure to the materials ((1)
safety and health of workers, (2) safety and health of consumers, and
(3) preservation of ecosystem and environment), risk assessment/
management and public relations.
(Masahiro Takemura, nanonet)
For more information:
http://www.nanonet.go.jp/english/mailmag/2006/077c.html
--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--
Nanotechnology Researchers Network Center of Japan distributes
this e-mail newsletter, "JAPAN NANONET BULLETIN", every other Thursday
with the aim of promoting information exchange and cooperation among
researchers in nanotechnology and related fields.
The next issue of JAPAN NANONET BULLETIN will be delivered on
August 31, 2006.
JAPAN NANONET BULLETIN contains articles, "Nanonet Interview", in
which we interview a leading researcher about current issues and/or
research strategies for the future and "Young Researchers'
Introduction", in which a young researcher in the nanotechnology field
introduces his/her own recent research.
We appreciate your support very much and promise to continue to gather
and disseminate information for your benefit.
Read details on our privacy policy at:
http://www.nanonet.go.jp/english/mailmag/policy.html
Subscribe at:
http://www.nanonet.go.jp/english/mailmag/index.html
Change or cancel your subscription at:
http://www.nanonet.go.jp/english/mailmag/upd_del.html
Inquiry about the newsletter:
----------------------------------------------------------------------
Nanotechnology Researchers Network Center of Japan
Ministry of Education, Culture, Sports, Science and Technology (MEXT)
Our website: http://www.nanonet.go.jp/english/
Inquiry:
Copyright(c) 2003-2006, Nanotechnology Researchers Network Center of
Japan,
All rights reserved.
