NTT Technical Review
August 2013 Vol. 11 No. 8
View from the Top
Srini Koushik, Chief Executive Officer
NTT Innovation Institute, Inc.

Feature Articles: Front-line Research on Graphene
Graphene Research at NTT
Basic Principles of Raman Spectroscopy for Graphene
A Novel and Simple Method ofGrowing Atomically Thin Hexagonal Boron Nitride
Exploring Relativistic Physics andBand Gap Detection in Epitaxial Graphene
Plasmon Transport in Graphene
Surface-enhanced Raman Scatteringof Graphene on SiC
Biosensing on a Graphene Oxide Surface

Regular Articles
Monolithic Integration of Polarization-entangled
Photon Pair Source Using Silicon Photonics Technology

Global Standardization Activities
The ITU Workshop on eHealth andthe Fourth Meeting of ITU-T FG-DR&NRR in Tokyo

Practical Field Information about
Telecommunication Technologies
Failure Cases in Access Facilities (Aerial Metallic
Communication Facilities) and Countermeasures

External Awards
External Awards

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Applying Startup Experience with
Silicon Valley Spirit to Make the NTT
Group Universally Known
Srini Koushik
Chief Executive Officer
NTT Innovation Institute, Inc.
Overview

NTT I3 (NTT Innovation Institute, Inc.) is an innovative enterprise with
global aspirations that seeks to leverage its Silicon Valley location and corestrengths in original cloud and security technologies. We asked newly
appointed Chief Executive Officer (CEO) Srini Koushik to tell us more
about NTT I3’s core strengths, and as someone who has had a long careerin the IT industry, to talk about NTT Group’s current standing in the world
and NTT I3’s outlook for the future.

Off to a good start: NTT I3 aims to become amultifaceted NTT Group hub—Mr. Koushik, what are your feelings now that the
opening reception has been completed?
Well, I feel quite excited that the reception went
very well and that it succeeded in getting others toknow NTT I3 and its people. I was very happy to seethat participants included dignitaries not only from
Silicon Valley companies like Cisco and Salesforce. com
but also from renowned global companies.
I remember one particular compliment from a
Washington, D.C. reporter who said that our ceremony was the best of all receptions held by Japanesecompanies that he had seen.
In the light of all this encouragement, I would liketo merge NTT’s long history and solid traditions with
Silicon Valley’s startup culture to make the NTT
Group name well-known throughout North America.
I plan, in particular, to focus our energies on three
key strategies.
The first is to form close partnerships with leading
companies and universities. Our second strategy is to

invite talented people from NTT Group companies
that have bases in North America to stimulate research
activities. My aim here is to create a microcosm of the
NTT Group and its strengths within NTT I3 and topromulgate an image of NTT I3 as a group hub.
Finally, our third strategy is to pursue appropriate
activities as a global market leader. We will work, inparticular, on promoting research and developing
services and products that can be used by our customers in the United States where NTT I3 is located. Of
course, our objective is not just the United States—
we plan a global rollout that will include emerging
countries.
Let me elaborate on our first strategy of partneringwith major companies and universities.
We have already tied up with Georgia Institute ofTechnology (Georgia Tech) and Stanford University,and we are now looking for other universities to form
connections with.
Forming tie-ups with academic organizations isextremely important. Such partnerships can be asource of new ideas and creativity, but they can also
serve another important purpose: university students
who participate in research activities at NTT I3 may
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see for themselves how attractive research and development (R&D) work at our institute can be. This
would be one way for us to secure top-notch, talented
people for the future.
In addition, we already have good partnerships withprominent corporations like Cisco, Salesforce.com,
and Oracle, and we will continue to build upon thosepartnerships.
In terms of our second strategy, let me say that thesuccess of NTT I3 is dependent on the success of the
NTT Group, so I will promote the participation of
personnel from NTT Group operating companies. In
fact, we have already obtained the participation of
COO (Chief Operating Officer) Eiji Kuwana fromNTT laboratories and other talented individuals, andfrom here on, I would like to bring in more talentfrom such group companies as Dimension Data, NTT
DATA, and NTT Communications (NTT Com).
Finally, with respect to our third strategy, we
already have three projects underway geared to putting NTT on a path to becoming a driver in the global
market. One is research and development in the security field, another is the development of software
defined networking in the cloud computing field, andthe third one is the development of frameworks for
migrating applications to the cloud.
So we’re off to a good start. I think that NTT Group,
the parent company, has done all they need to do tohelp us achieve a smooth launch. Looking forward,
we can see that there is great interest in cloud services in the American market, and we are working to
introduce our services and penetrate this market.
Products like Infrastructure as a Service (IaaS) and
Software as a Service (SaaS) are also used quite
extensively in the United States. So, as I just mentioned, I think that the next step in cloud services is
the migration of business applications to the cloud,which is a good business opportunity for us.
To become a global leader with this vision in mind,
we have to show enterprises that we can provide a
cloud environment that is secure and available 24
hours a day.
NTT is one of three companies that can expand
such business in a global manner. The job of NTT I3
is to provide support from the R&D side so that the
NTT Group can roll out these services in the global
market in a smooth and steady manner.
R&D hub where people have a strong desire towork above and beyond remuneration—Can you give us some more specifics on capabiliVol. 11 No. 8 Aug. 2013

ties, talented staff, and anything else that you arefocusing on to provide this support?
Well, to begin with, we already have a strong foundation of technical engineers and designers, but it’s
important that we continue to welcome highly capable people to NTT I3. Over the next few months, wewill be solidifying our direction and projects evenfurther, and we will need engineers and architectswith a variety of capabilities and individuals withproject-management skills as well.
We need project managers who understand business requirements and who know how to provide our
products to our customers on a regular, stable basis.
Architects, meanwhile, must get the technologies
from the various NTT operating companies to work
together to enable the solutions we want to offer.
To this end, I think having a hub like NTT I3 in
North America and particularly in Silicon Valley isvery beneficial. To develop compelling products andbuild attractive solutions, we need strong technology
partnerships in addition to our joint development
efforts within the NTT Group.
—By the way, how do you plan to apply your past
CEO experience to your present position?
As a result of accumulating more than 28 years ofexperience in the technology industry, including
about 16 years in technology companies and morethan 10 years in the financial services and insurance
industry, I have been able to be involved in the information technology (IT) industry in a variety of ways,
including gaining experience in infrastructure andapplications and as a service provider and user.
I feel that my business experience at technologyView from the Top

companies like IBM and HP, which are nearly as big
as NTT, will prove useful not only in understanding
technology but also in working with a big, complex
company and managing NTT I3. I also believe thatmy experience in the financial services and insurance
industry will help me in understanding what consumers want. It is probably a common characteristic oftechnology companies that they want to sell the technologies that they develop, but it is important thatthey obtain a good understanding of what consumers
want to buy. In this regard, I think that penetrating thebuyer’s mind is one ability that I can bring to NTT I3.
My vision of success at NTT I3 is that the industry
accepts our innovative ideas, and that these ideas add
value and drive growth for NTT’s operating companies in North America. Another goal is to create an
outstanding work environment so that anyone whocomes to work at NTT I3 does so because they truly
enjoy working here above and beyond their paycheck.
I will spare no effort in achieving such a workplace,and one year from now, I believe I will have new
dreams as we move forward.
Transforming one’s weakness into strength
in the spirit of judo:
flexibility over stiffness—Why did you select NTT I3 when there are otherorganizations like it in the IT industry? How do you
think NTT is viewed in the world market?
Well, I wanted to work for a technology-relatedcompany but not for a company that pursues technology for technology’s sake. Of course, NTT I3 is also
involved in technology research, but I believe that its
goal is to change society through technology. This iswhat I find to be important!
Despite NTT’s long history of success, NTT I3 is a
new participant in the R&D field in the United Statesmarket, but given my history of solving IT problems
through innovative products and services, this presents me with a great challenge that I am eager toundertake.
So how is the NTT Group positioned in the world
market right now? I would say that NTT Com is recognized first and foremost as a network carrier, thatNTT DATA, while having gained a certain amount ofrecognition, is still viewed as one of the smaller service providers, and that Dimension Data is a company
that is just now starting to make an impact.
I think that these company attributes can also beviewed as strengths. For example, ordinary companies are sometimes reluctant to partner with hugeproviders because they fear that they will not be able
to receive suitable and effective services tailored totheir needs. So I think that NTT DATA can offer great
flexibility precisely because of its small-scale stature.
This is a great business opportunity for NTT.
By the way, two of my hobbies are skydiving andhang gliding. If there is something that is new andexciting that I might be afraid to do, I make sure to try
it so that I am no longer afraid of it.
I also have a black belt in judo, and I think there issomething in common here between judo and NTT I3.
No matter how small one’s body may be, it’s possible
to transform one’s weak point into a strength to use
against your opponent.
In the cloud market, I think that NTT is in a very
good position. NTT Com is viewed as a company thatprovides one of the most advanced carrier-enabledclouds. The advantage of a carrier-enabled cloud isthat it can truly spread the workload across the globe
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in a much easier way.
Dimension Data, meanwhile, is viewed as a company that can provide powerful functions for private
clouds and that has very strong capabilities in implementing managed services on top of private clouds inthe infrastructure space.
And NTT DATA and NTT Centerstance have very
good capabilities in implementing SaaS and integrating it with legacy applications.
In this way, the NTT Group has laid out the framework to promote business support through a full
lineup of services, from cloud services, managed services, and advisory services to application services
too.
So what we have to do at NTT I3 is to help our customers understand that we can tie all these groupcompany strengths together.
Furthermore, we can provide security in a comprehensive manner for a full range of services fromapplications to infrastructures and clouds. We are oneof only a few companies—maybe two or three in theworld—that can do so.
—How do you plan to work with R&D in Japan?
Needless to say, NTT has top-class R&D even froma global perspective. The R&D team is producing
extraordinary intellectual property, and they havebeen helping us to understand what they have.
Looking to the future, I want to do three things withNTT R&D.
The first is to commercialize NTT R&D results for
the American market and to expand them into a tangible business. The second is to do further research in
specific fields. And the third is to maintain a close
relationship with NTT R&D, to know what NTT
researchers are working on at any given time, and tohelp the NTT operating companies in North America
to understand the content of that R&D work.
It’s OK to fail! Acting like astartup to make NTT a global name
—Mr. Koushik, could you leave us with a message forall your colleagues at NTT I3?
First of all, I would like to tell all my colleagues that
I am very excited to be here and very proud to be apart of NTT I3. I would like them to know that I am

Vol. 11 No. 8 Aug. 2013

here to support them and to be a part of NTT I3 as a
key startup in the United States. This is the stance that
I believe is important in getting the NTT name widely known throughout the United States market and inbeing a part of NTT growth. I also want every one at
NTT I3 to know how great a company NTT is, andhow we can leverage that to “put a dent in the universe,” as Steve Jobs was fond of saying.
You can hear many stories about people launching
new companies and rising to the top, but those people
often fail many times before succeeding. Generating
new ideas and being innovative is the essence ofNTT I3. If only three of our ideas out of ten are successful, there is no need for us to lose sleep over theseven failures. What we need to do is focus our effortson the three successful ideas.
I have attempted a number of startups in the past,and some of them worked and some of them didn’t.
Failure is never easy, but it is also the time when you
learn the most. In other words, it is OK to fail, but
repeating the same mistake is not so good!
There is some similarity between Japanese andIndian culture in that there is a tendency to look upon
failure as a bad thing. In the United States, however,
failure is OK—you learn from failures and you get
better, and that is part of being a startup.
This kind of mind frame is essential at the startup
stage, and I would ask our Japanese researchers to
adopt this attitude as well.

Interviewee profile
Career highlights
Srini Koushik joined IBM in 1994 and became
head of e-business development in 2000. He
joined Nationwide Insurance in 2001 and became
its CTO (chief technology officer) in 2009. In
2011, he joined HP and took up the post of chief
development officer spearheading the development of global applications. He founded Right
Brain Systems, a consulting company, in 2012,serving as president and CEO. He took up his
present position at NTT Innovation Institute, Inc.
in May 2013.

Feature Articles: Front-line Research on Graphene

Graphene Research at NTT
Hiroki Hibino
Abstract

Graphene, a two-dimensional sheet of carbon atoms, was experimentally discovered in 2004. Since
then, graphene research has progressed exponentially from basic science to applications. NTT aims to
make a large impact on the information society through the materials science of graphene and is therefore promoting a wide range of graphene research projects from theory to devices. In this Feature Article,
the current status of graphene research underway at NTT laboratories is presented.

1. Introduction

2. Graphene

relatively new. Researchers from the University ofManchester first discovered graphene in 2004 when
they managed to isolate it from thin graphite flakes
that had been mechanically exfoliated from bulk
graphite. It has been researched intensively eversince, owing to its novelty in science and the expectations of its having a wide range of applications. The
Nobel Prize in Physics was awarded to the discoverers in 2010.
The scientific novelty of graphene is symbolized by
its extraordinary electronic structure. Since the equation that describes the states of conduction electrons
in graphene is the same as the relativistic quantum
mechanical equation (Dirac equation) with massequal to zero, conduction electrons in graphene arecalled massless Dirac particles. These electrons constitute a new class of two-dimensional electron system, which is truly different from the two-dimensional electron gas in semiconductors such as silicon(Si) and gallium arsenide (GaAs). For example, theenergy of electrons is normally proportional to thesquare of the momentum, but the energy of conduction electrons in graphene is proportional to themomentum. This property is the same as that of lightand is compatible with massless particles.

2.1 Basic structure
Graphene is a two-dimensional crystal consistingof carbon atoms arranged in a honeycomb lattice(Fig. 1). A three-dimensional stack of graphenesheets is graphite, which is a familiar material in daily
life. Graphene is also the basic element of fullerenes
and carbon nanotubes, which are well-known carbon
nanostructures. However, its experimental history is

2.2 Properties and synthesis
Graphene has excellent mechanical, electrical, andoptical properties. Its typical properties are summarized in Table 1. Graphene is mechanically verystrong and flexible. The breaking strength of graphene is more than 100 times greater than that ofsteel. The crystal structure does not break even after
being stretched up to 20%. From an electrical aspect,

Historically, the development of new materials has
revolutionized our way of life, and we have learned
that an infinite variety of materials can be formed
from different combinations of atoms and molecules.
Research on materials science increases our knowledge of how to create and develop new materials andtherefore has great potential for improving our lives
and society. NTT laboratories have already createdvarious new materials that have had a significant
impact on the information society and are continuing
to search for materials that will contribute to solvingsocial issues in the future.
These Feature Articles focus on graphene, which iscurrently attracting a great deal of attention both at afundamental level and in technological domains.
NTT laboratories are approaching graphene multilaterally from various aspects: theory, synthesis, physical properties, and devices. In this article, I first
explain why graphene is such a promising material
and then present an overview of graphene research atNTT.



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Electronics
Interconnects

Photonics

Electricalproperties

Opticalproperties

Transparent
electrodes

Sensors
Surface/volume
ratio

Mechanicalproperties
Graphene

Capacitors

Membranes

ThermalImpermeability
properties
Gas barriers
Heatdissipaters

Fig. 1. Structure of graphene and its potential applications.

Table 1. Properties of graphene.
Property

Value

Comparison with other materials

References

Breaking strength

42 Nm−1

Elastic limit~20%

Carrier mobility at room temperature

200,000 cm2V−1s−1More than 100 times higher than Si[4]

Thermal conductivity~5000 Wm−1K−1

More than 10 times higher than Cu[2], [5]Maximum current density>108 Acm−1~100 times larger than Cu[6]

Optical absorption coefficient

2.3%~50 times higher than GaAs[7], [8]the carrier mobility of graphene at room temperature
is more than 100 times higher than that of silicon.
Furthermore, graphene is an ultra-wide-band opticalmaterial that interacts strongly with light of a wide
range of wavelengths. Graphene absorbs 2.3% oflight in the visible to infrared region. This absorption
coefficient is one to three orders of magnitude higher
than those of conventional semiconductor materials.
These properties of graphene suggest that various
applications are possible. For example, several-layerthick graphene films are transparent, electricallyconductive, and flexible. Therefore, flexible transparent electrode applications, including touch screensand solar cells, have been extensively studied. Highcarrier mobility makes graphene promising as a channel material of high-speed electronics. However,since it is difficult to turn off an electrical current ingraphene, progress has mainly been achieved inresearch on applications involving analog high-frequency devices. Graphene is also promising for photonics devices such as modulators and photodetectors. In general, the range of potential applications isvery wide and includes sensors, interconnects, capacVol. 11 No. 8 Aug. 2013

More than 100 times greater than steel[1], [2][3]itors, heat dissipaters, gas barriers, and membranes.
High-quality graphene can be obtained by mechanically exfoliating graphene from graphite. Thismethod is therefore ideal for basic research, but interms of productivity and the small size of graphenesheets that can be obtained, it is not suitable for industrial applications. Therefore, in recent years, techniques for growing graphene on a substrate have been
actively studied. Two growth methods are attracting
intense interest: thermal decomposition of siliconcarbide (SiC), in order to grow graphene on it, andchemical vapor deposition (CVD) to grow grapheneon metal catalyst substrates. The quality of the graphene grown using these methods is improving rapidly. If single-crystal graphene substrates could beobtained routinely, they would dramatically advance
graphene research and increase the number of potential applications.
3. Graphene research at NTT
NTT has a comprehensive research scope ranging
from basic theory to applied research, and we areFeature Articles

Devices
Bio-sensors, photonics,
mechanical resonators
External cooperation
Physical properties
Electronic transport,
Plasmons, SERS
Synthesis
SiC thermal decomposition, CVD,
MBE, safe synthesis
External cooperation
TheoryOptical properties, growthmechanism

CVD: chemical vapor deposition
MBE: molecular beam epitaxy
SERS: surface-enhanced Raman scattering

Fig. 2. Areas of research on graphene at NTT.

studying graphene across this entire spectrum. Further, to accelerate the research in order to elucidate
graphene’s fundamental properties and its synthesis,
we are actively collaborating with research institutes
outside NTT. Typical research topics in four research
stages: theory, synthesis, physical properties, anddevices are shown in Fig. 2.
3.1 Theory
The Feature Article in this issue entitled “Basic
Principles of Raman Spectroscopy for Graphene” [9]covers the theory of the optical properties of graphene. Another important research subject is the theory of graphene growth. On the basis of our understanding of the stable structures of carbon atoms on
SiC determined from first principles calculations*1,
we are trying to clarify the mechanism of graphene
growth by thermal decomposition of SiC.
3.2 Synthesis
NTT possesses a low-energy electron microscope(LEEM)*2 that is suitable for in-situ observation of
the graphene growth process and for microscopic
evaluation of the number of graphene layers. We used
LEEM to investigate the growth process of grapheneon SiC, and from the insights gained, we succeededin growing highly uniform monolayer and bilayer
graphene. These graphene substrates enabled us toevaluate the electronic transport properties at eachthickness, as introduced in the Feature Article entitled“Exploring Relativistic Physics and Band Gap Detection in Epitaxial Graphene” [10].
NTT laboratories are working on growing not only
graphene but also monolayer boron nitride (BN), a
two-dimensional crystal consisting of boron andnitrogen, using CVD. In particular, we are promoting
external cooperation with the aim of establishing atechnique for growing single-crystal graphene andBN by using single-crystal metal thin films on a sapphire substrate. In addition to the results obtained in
CVD and thermal decomposition of SiC, we haveachieved many promising results in graphene and BN
synthesis, as exemplified by a safe synthesis methodthat uses a solid material deposited on a metal substrate as a source, which is presented in the Feature
Article entitled “A Novel and Simple Method ofGrowing Atomically Thin Hexagonal Boron Nitride”[11], and by the direct growth of graphene by molecular beam epitaxy (MBE)*3.

*1	 First-principles calculations: Method to calculate electronic stateson the basis of quantum mechanics using only basic physicalquantities.
*2 Low-energy electron microscope: A type of electron microscopethat uses electron beams with an accelerating voltage as small asa few volts and that is therefore sensitive to differences in thesurface structure of a material.
*3 Molecular beam epitaxy: A method of supplying material onto asubstrate like a beam in a vacuum, resulting in growth of a material following the atomic arrangement of the substrate.

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Feature Articles

3.3 Physical properties
The Feature Articles entitled “Exploring Relativistic Physics and Detection of a Band Gap in Epitaxial
Graphene” [10], “Plasmon Transport in Graphene”[12], and “Surface-enhanced Raman Scattering ofGraphene on SiC” [13] report that NTT laboratories
have obtained many notable results on the optical andelectronic properties of graphene. Further, because
the electronic transport properties of graphene grownon SiC are influenced by the substrate, we are also
attempting to control the interface structure to
improve the transport properties. In addition, we areworking to clarify the fundamental physics of graphene by promoting collaboration with external
research institutions that apply unique characterization methods such as high-sensitivity measurementsof polarization rotation of terahertz waves and magneto-optical absorption measurements under an ultrahigh magnetic field.
3.4 Devices
The Feature Article entitled “Biosensing on a Graphene Oxide Surface” [14] explains how graphene
oxide produced by chemical oxidation and exfoliation of graphite can be used for protein detection.
NTT laboratories are also investigating the vibration
characteristics of mechanical resonators fabricatedfrom graphene grown on SiC by electrochemicallyetching the substrate, and exploring the light transmission characteristics of optical waveguides integrated with graphene.
4. Future prospects
There is no doubt that graphene has enormous
potential as a future material, but we face many challenges in achieving industrial applications. The synthesis method is a prime example. The CVD method
is suitable for large-scale and low-cost synthesis ofgraphene, but electronics/photonics applications
require that graphene on metal substrates be transferred onto other insulating substrates. Damage-freetransfer technology is also a difficult challenge. Further, the ideal graphene does not have a band gap,
which impedes the development of logic devices. In
these Feature Articles, we will describe the electricaldetection of a band gap in bilayer graphene on SiC.
Various attempts to create a band gap in graphene
have been made from materials science and device
science approaches. On the other hand, the discoveryof graphene has aroused increasing interest in twodimensional crystals other than graphene, such as
Vol. 11 No. 8 Aug. 2013

insulating BN and semiconducting molybdenum
disulfide (MoS2). It is expected that hybrid structuresof two-dimensional materials with various properties
will bring new functions. Although research on twodimensional crystals containing graphene is very
competitive, we are confident that NTT laboratories,
with NTT’s comprehensive research organization andresearch network outside NTT, will be at the center ofgraphene research and will attain new levels of understanding that will contribute to our society.
References[1]

C. Lee, X. Wei, J. W. Kysar, and J. Hone, “Measurement of the Elastic
Properties and Intrinsic Strength of Monolayer Graphene,” Science,Vol. 321, No. 5887, pp. 385–388, 2008.
[2] Scientific Background on the Nobel Prize in Physics 2010, Royal
Swedish Academy of Sciences, Oct. 2010.
http://www.nobelprize.org/nobel_prizes/physics/laureates/2010/
advanced-physicsprize2010.pdf[3] K. S. Kim, Y. Zhao, H. Jang, S. Y. Lee, J. M. Kim, K. S. Kim, J.-H.
Ahn, P. Kim, J.-Y. Choi, and B. H. Hong, “Large-scale pattern growthof graphene films for stretchable transparent electrodes,” Nature, Vol.
457, pp. 706–710, 2009.
[4] J.-H. Chen, C. Jang, S. Xiao, M. Ishigami, and M. S. Fuhrer, “Intrinsic
and extrinsic performance limits of graphene devices on SiO2,” Nature
Nanotechnol., Vol. 3, pp. 206–209, 2008.
[5] S. Ghosh, W. Bao, D. L. Nika, S. Subrina, E. P. Pokatilov, C. N. Lau,
and A. A. Balandin, “Dimensional crossover of thermal transport in
few-layer graphene,” Nature Mater., Vol. 9, pp. 555–558, 2010.
[6] R. Murali, Y. Yang, K. Brenner, T. Beck, and J. D. Meindl, “Breakdown current density of graphene nanoribbons,”Appl. Phys. Lett.,Vol.
94, No. 24, pp. 243114, 2009.
[7] R. R. Nair, P. Blake, A. N. Grigorenko, K. S. Novoselov, T. J. Booth,T. Stauber, N. M. R. Peres, and A. K. Geim, “Fine Structure ConstantDefines Visual Transparency of Graphene,” Science, Vol. 320, No.
5881, pp. 1308, 2008.
[8] Q. Bao and K. P. Loh, “Graphene Photonics, Plasmonics, and Broadband Optoelectronic Devices,” ACS Nano, Vol. 6, No. 5, pp. 3677–3694,
2012.
[9] K. Sasaki, “Basic Principles of Raman Spectroscopy for Graphene,”
NTT Technical Review, Vol. 11, No. 8, 2013.
https://www.ntt-review.jp/archive/ntttechnical.php?contents=ntr2013
08fa2.html[10] K. Takase and S. Tanabe, “Exploring Relativistic Physics and Band
Gap Detection in Epitaxial Graphene,” NTT Technical Review, Vol.
11, No. 8, 2013.
https://www.ntt-review.jp/archive/ntttechnical.php?contents=ntr2013
08fa4.html[11] S. Suzuki, “A Novel and Simple Method of Growing Atomically Thin
Hexagonal Boron Nitride,” NTT Technical Review, Vol. 11, No. 8,
2013.
https://www.ntt-review.jp/archive/ntttechnical.php?contents=ntr2013
08fa3.html[12] N. Kumada, “Plasmon Transport in Graphene,” NTT Technical
Review, Vol. 11, No. 8, 2013.
https://www.ntt-review.jp/archive/ntttechnical.php?contents=ntr2013
08fa5.html[13] Y. Sekine, H. Hibino, K.Oguri, T. Akazaki, H. Kageshima, M. Nagase,
K. Sasaki, and H. Yamaguchi, “Surface-enhanced Raman Scatteringof Graphene on SiC,” NTT Technical Review, Vol. 11, No. 8, 2013.
https://www.ntt-review.jp/archive/ntttechnical.php?contents=ntr2013
08fa6.html[14] K. Furukawa and Y. Ueno, “Biosensing on a Graphene Oxide Surface,” NTT Technical Review, Vol. 11, No. 8, 2013.

Feature Articles

https://www.ntt-review.jp/archive/ntttechnical.php?contents=ntr2013
08fa7.html

Hiroki Hibino

Executive Manager of the Materials Science
Laboratory and Group Leader of the Low-dimensional Nanomaterials Research Group at NTT
Basic Research Laboratories.
He received the B.S. and M.S. degrees in physics from the University of Tokyo in 1987 and1989, respectively, and the Ph.D. degree in pureand applied physics from Waseda University,
Tokyo, in 2006. In 1989, he joined NTT Basic
Research Laboratories, where he has been studying surface dynamical processes using microscopic techniques. His research interests include
step structures on vicinal surfaces, surface masstransport, step instability during epitaxial growth,
self-assembled nanostructure formation, andepitaxial graphene growth. He spent one year as
a visiting research professor at Arizona State
University during 2000–2001. He also has experience as a visiting professor at Tokyo Institute ofTechnology (2007–), Kyushu University (2009–2010), and the University of Tokyo (2011–).

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Feature Articles: Front-line Research on Graphene

Basic Principles ofRaman Spectroscopy for Graphene
Ken-ichi Sasaki
Abstract

Graphene is a planar honeycomb network composed of carbon atoms. Even though graphene has avery simple structure, it has received considerable attention from researchers in various fields. This
article explains the basic principles of Raman spectroscopy for use with graphene in order to provide
insight into the unique properties of graphene.

1. Overview of Raman spectroscopy
When laser light is applied to a sample, it is scattered with a finite probability. If we plot the intensityof scattered light as a function of its energy, we mayfind that several peaks (or bands) appear in the result,as shown in Fig. 1. The spectrum of scattered light asa function of the energy shift is known as the Raman
spectrum, which is the main data obtained from
Raman spectroscopy. Normally, the intensity is at a
maximum when the energy of scattered light is equal
to that of incident light. However, this is not the only
peak. There are other peaks for which the energy shift
is nonzero. Since the energy is conserved, the energy
difference corresponds to the energy that is absorbed
into the sample. When the peak originates from theinteraction between light and a lattice vibration(called a phonon), the energy shift corresponds to theenergy of a Raman active phonon.
If we know that the energy of a Raman active phonon in sample A is 500 cm−1 and that in sample B is
530 cm−1, we can identify the sample by using Raman
spectroscopy. This describes the basic use of Raman
spectroscopy as a characterization tool. We know thatfor graphene there are two principal Raman bands:
one peak appears at ~1580 cm−1, which is called the
G band, and the other peak is seen at ~2700 cm−1,
which is called the 2D band (or G’ band). If a sample
does not show a G or 2D band, we do not consider it
to be graphene.
Raman spectroscopy is useful not only for characterizing a sample, but also for obtaining information
about a sample. This is partly because there are several parameters that we can change in Raman spectroscopy measurements. For example, the energy ofincident laser light (EL) can be changed. For graphene,

Raman spectrum
Incident light
EL

E′L

2D

Scattered lightIntensity of
scattered light

G

Graphene0

EL – E ′L

Fig. 1. Schematic illustration of Raman spectroscopy.

NTT Technical Review

Feature Articles(a)Electron

Incident light
K

K′
Scattered light

Hole
(b)

(c)

ky
Forward

K

k

−2k

Θ′
K′
Backward

Real spaceΘKkx
Backward

ForwardFig. 2. (a) Schematic of a Raman process in the Dirac cones of graphene. (b) Top view of (a). (c) The A1g phonon is emitted
from an electron through backward scattering.

the G band energy is known to be insensitive to thechange in EL, whereas the 2D band energy increases
linearly as EL increases. What is the physics behindthe dispersive (nondispersive) behavior of the 2D (G)band? This is the central issue that we would like toexplore in this review article.
2. Phonon wave vector
Phonons in graphene form the energy band structures, and a phonon’s energy is a function of the wave
vector q as ω(q). If we assume that a phonon’s
energy does not depend on the EL value used in theexperiment (which is true for most experimental conditions), the nondispersive behavior of the G bandsuggests that the identical phonon with a fixed q value
is created while EL is changed. In fact, the wave vector of the G phonon is q = 0 (the Γ point), and thenondispersive behavior of the G band is reasonably
understood because ω(0) is independent of EL.
Meanwhile, many studies have revealed that the 2Dband consists of two A1g zone-boundary phonons.
The wave vector of each phonon is near the K point;
namely, it is written as K + q, where q represents asmall derivation from K (the wave vector of the K
point). A reasonable solution to explain the dispersive
behavior of the 2D band is to think that the wave vector q is changed by changing EL.
Let us explain the reason for the EL dependence ofq. A process for the 2D band that is induced in theVol. 11 No. 8 Aug. 2013

electronic band structures called Dirac cones by laser
light is shown in Fig. 2(a). Because the wave vectorof light (typically ~1/600 nm) is much smaller than
the wave vector of electrons and holes, optical transitions are possible only when an electron is transferred
between the valence and conduction bands without achange in its wave vector. Thus, a vertical electronhole pair is created by light, and it is annihilated by
the emission of scattered light. An immediate consequence of such a direct transition in the Dirac cones
is that EL is related to the wave vector of a photoexcited electron (and hole) k as EL = 2 νFk, where νFis the electron’s velocity and k = |k|. Thus, increasing
EL is equivalent to increasing the radius of the circle
defined with respect to the K point shown in
Fig. 2(b).
When a photo-excited electron emits an A1g phonon, the valley changes from K to K’ due to momentum conservation. Let the wave vector of the scatteredelectron be k', which is measured from the K’ point*1.
Similarly, the photo-excited hole changes its valley
from the K point to the K’ point when it emits an A1g
phonon. In general, the hole is not scattered into theposition just below the electron because the electron*1	 Strictly speaking, the radius of the circle near the K’ point, i.e.,|k'|, shown in Fig. 1(b) must be a little smaller than that near the
K point k = |k|, because of energy conservation with respect tothe phonon’s energy. We consider the case in which EL = 2 ν Fk ismuch larger than the phonon energy (~0.15 eV); the difference
between the radii is not essential in this discussion.

Feature Articles

and hole are independently scattered by different A1g
phonons. Only when the wave vectors of the scatteredhole and electron are identical (as shown in Fig. 2(a))
can the hole and electron be annihilated by scattered
light emission, which can be observed as the 2D bandin the Raman spectrum.

µ>0

µ=0

3. Dominance of backward scattering
If there is no preferred direction of k' for a given k,
the q (≡ k' – k) value cannot be specified. This is easy
to recognize by projecting the process shown in
Fig. 2(a) onto the two dimensional k-space shown in
Fig. 2(b), in which the magnitude q = |q| is small forthe forward scattering (q ≈ 0), denoted by the dashedarrow in Fig. 2(b), while it can be large for the backward scattering (q ≈ –2k).
Interestingly, there is a strong probability that theelectron will undergo backward scattering when it
emits an A1g phonon, as shown in Fig. 2(c). Namely,
although the forward scattering may be allowed bymomentum conservation, it never takes place because
the corresponding electron-phonon matrix element issuppressed. In fact, the calculated matrix element
squared is proportional to 1– cos(Θ' – Θ), where Θ(Θ') is the angle between k (k') and the kx-axis. The
matrix element squared vanishes for the forward scattering (Θ' = Θ), while it takes the maximum for the
backward scattering (Θ' = Θ + π). Thus, the phonons
whose wave vector satisfies q = |q| ≈ 2k contributesignificantly to the 2D band. As a result, increasing
the EL is equivalent to increasing linearly the magnitude of the wave vector of the phonon, q(EL) ≈ EL/
νF.*2
4. Phonon dispersion and self-energy
The next problem is the q dependence of ω(q).
Normally, the dispersion relation of phonons is calculated by using a classical model in which carbon
atoms are connected by springs with some force constants. If we assume that the force constants are nonzero only for nearest-neighbor carbon atoms, it canbe shown that the q dependence is negligible: ω(q) =constant, for which we fail to explain the dispersive
behavior of the 2D band. Thus, many studies havebeen done concerning the parameters including theforce constants beyond the nearest-neighbor that can
reproduce the experimental results. However, even if
a classical model gives ω(q) = constant, quantum
mechanics can modify the q dependence of ω(q)through the self-energy.
ω + ∏(0)<ω + ∏(µ)

Fig. 3. T he doping dependence of the presence/absenceof intermediate states that contribute to the selfenergy.

The concept of self-energy is also essential in
explaining the experimental result in which the energy of the G band increases as the Fermi energy μ
increases (by doping). Suppose that a phonon withwave vector q hits an electron with wave vector k
through electron-phonon interaction. The electron istransferred from the valence to the conduction bands,
and the wave vector becomes k + q due to momentum
conservation. Since q = 0 for the G band, the electron
undergoes a vertical transition (Fig. 3). There are different vertical transitions depending on the value of k.
These vertical electron-hole pairs are the intermediate (virtual) states of the phonon, so they return to theoriginal phonon after a while. The important point
here is that the presence/absence of the virtual states
gives an observable consequence. Namely, the presence/absence of virtual states decreases/increases thephonon energy. As can be seen in Fig. 3, when μ is
close to the Dirac point (i.e., μ ≈ 0), all possible virtual states exist. As a result, the phonon energy takes
its minimum value because the correction to theenergy, which is a negative quantity as Π(μ) ∝ –∞∫μ dk/(2 νF), is maximal in this case. By contrast,
when the graphene is doped, some of the virtual states
are forbidden by the exclusion principle, and theenergy of the phonon increases. In fact, it can beshown that the self-energy Π(μ) is proportional to thedensity of states at μ, so Π(μ) increases linearly with|μ|.
*2 The origin of the anisotropy of the electron-phonon matrix element is the bonding or antibonding character (or pseudospin) ingraphene, which is beyond the scope of this article. (See reference [1] for details on this point.)

NTT Technical Review

Feature Articles

K

K′

Hardening
4πµ

e

K+q

Π(q, µ )/Gep

K

h

10

1.0

5
0
Translation

q=0

0.0
q≠0

q

Dispersive behavior

0.5ν Fq(eV)

Hardening~π2 ν Fq

0.5 µ (eV)

1.0 0.0

Fig. 5. S elf-energy as a function of q and μ. The self-energy
is scaled by the electron phonon coupling, Gep.
Shifted Dirac cones

Fig. 4. T he concept of shifted Dirac cones, which enables
us to capture the virtual electron-hole pairs that
contribute to the self-energy of an inter-valley
phonon as direct electron-hole pairs.

The dispersive behavior of the 2D band can beunderstood in terms of self-energy in a similar manner to the doping dependence of the G band. Supposethat the A1g phonon with wave vector K + q hits anelectron near the K point with k, the electron is transferred from the valence to the conduction band, andthe wave vector becomes k + (K + q), due to momentum conservation. To understand the self-energy ofthe phonon, we need to discuss an electron-hole pairthat crosses two Dirac cones. However, the fact thatthe electron is very far from the hole makes it very
difficult to consider the presence or absence of a virtual state. Therefore, let us displace this Dirac conefrom the K’ point toward the K point by - K – q. Thenthe wave vector of the electron becomes k so that theelectron is located just above the position where thehole is created. The vertical electron-hole pairs in
shifted Dirac cones will be the virtual states of the A1g
phonon.
Let us assume that for clarity the Fermi energy isfixed at the Dirac point in Fig. 4. For the A1g phononwith q ≈ 0, we can expect from the discussion of the
G band that the energy will reach its minimum due tothe self-energy. For the A1g phonon with q ≠ 0, someof the low-energy virtual states are excluded, so theenergy increases from q ≈ 0. This is very similar tothe situation in which doping prevents the low-energyVol. 11 No. 8 Aug. 2013

virtual states from contributing to the self-energy of
the G band. The number of excluded virtual states
increases almost linearly with the increasing q. In
fact, a direct calculation shows that the self-energy is
proportional to q, as Gep × π 2 νFq, where Gep is theelectron-phonon coupling.
Since q is proportional to EL, the phonon energy
exhibits a linear dependence on EL as ω (EL) ∝π 2Gep EL, which is consistent with the measured dispersive behavior of the 2D band. The self-energy also
depends on μ. More detailed information on the selfenergy is shown in Fig. 5. Note that when μ ≈ 0, theself-energy follows Gep × π 2 νFq.
5. Summary and discussion
We have explored the origin of the dispersive
behavior for the 2D band, which is the most prominent peak in the Raman spectrum of graphene, by
employing two fundamental concepts. One is theself-energy of phonons. The other is the dominanceof backward scattering for the A1g phonon, which is aproperty of electron-phonon interaction.
Some comments on the latter concept are in order.
Strictly speaking, the electron-phonon matrix element squared being proportional to 1 – cos (Θ' – Θ)does not necessarily mean that only the phonons thatsatisfy q = 2k contribute to the 2D band, because theprobability is nonzero not only for the exact backward scattering (Θ' = Θ + π) but also for the approximate backward scattering (Θ' ≈ Θ + π). If we take the
distribution of the approximate backward scatterings
into account, it can be shown that the average q valueFeature Articles

is a little smaller than 2k. In fact, the average q value
is given by introducing a numerical factor (1.3) as q ≈EL /1.3 νF. This prediction can be verified by examining the μ dependence of the width of the 2D band [2].
A reduction factor seems to be necessary to explain
the recent experimental data [3]. Meanwhile, a calculation of the S-matrix, which takes off-resonanceprocesses into account, for the 2D band suggests thatthe contribution of the phonon with q = 2k is strongly
enhanced [4]. Thus, determining the exact q value isan unresolved problem.
The concept of migration of Dirac cones used inunderstanding the virtual states for the self-energy ofan A1g phonon may be important when graphene issubjected to strain. A uniform strain causes a shift of

the Dirac cones, and the phonon’s q value would bechanged by applying strain to a graphene sample.
References
[1]
[2]

[3]

[4]

K. Sasaki, Y. Tokura, and T. Sogawa, “The Origin of Raman D Band:
Bonding and Antibonding Orbitals in Graphene,” Crystals, Vol. 3,
No. 1, pp. 120–140, 2013.
K. Sasaki, K. Kato, Y. Tokura, S. Suzuki, and T. Sogawa, “Decay andfrequency shift of both intervalley and intravalley phonons in graphene: Dirac-cone migration,” Phys. Rev. B, Vol. 86, No. 20, 201403,
2012.
C.-F. Chen, C.-H. Park, B. W. Boudouris, J. Horng, B. Geng, C. Girit,
A. Zettl, M. F. Crommie, R. A. Segalman, S. G. Louie, and F. Wang,“Controlling inelastic light scattering quantum pathways in graphene,” Nature, Vol. 471, pp. 617–620, 2011.
D. M. Basko, “Theory of resonant multiphonon Raman scattering ingraphene,” Phys. Rev. B, Vol. 78, No. 12, 125418, 2008.

Ken-ichi Sasaki

Research Specialist, Quantum Optical Physics
Research Group, Optical Science Laboratory,
NTT Basic Research Laboratories.
He received the M.S. and Ph.D. degrees in science from Tohoku University, Miyagi, in 1999
and 2003, respectively. He joined NTT Basic
Research Laboratories in 2010 and has been
studying theoretical aspect of graphene physics.
He is a member of the Physical Society of Japanand the American Physical Society.



NTT Technical Review

Feature Articles: Front-line Research on Graphene

A Novel and Simple Method ofGrowing Atomically Thin Hexagonal
Boron Nitride
Satoru Suzuki
Abstract

Hexagonal boron nitride (h-BN), a structural analogue of graphene, is an insulating material with alarge band gap. The combination of graphene and h-BN is expected to lead to a wide variety of applications. Thus, the demand for large-area and high-quality growth of h-BN is increasing. This article
describes a novel and very simple method for growing a large-area and atomically thin h-BN film. In thismethod, an amorphous solid source is converted to crystalline h-BN through diffusion of boron andnitrogen atoms in metal foil followed by precipitation at the surface.

1. Overview of hexagonal boron nitride
Planar materials of atomic layer thicknesses, forexample, graphene, are attracting much attention for
their potential use as building blocks of future nanoelectronics. Hexagonal boron nitride (h-BN) is such alayered material that has a honeycomb atomic network similar to graphene. h-BN is an insulator with alarge band gap of about 6 eV [1], which is in strong
contrast to graphene, which has no band gap. Various
applications can be expected by combining grapheneand h-BN, whose structures are similar, but whose
physical properties are largely different. For example,
the mobility of a graphene device is known to belargely improved when h-BN is used as a substratecompared to a conventional SiO2 substrate [2]–[5].
The reason for this is generally considered to bebecause h-BN has a much smaller quantity of charged
impurities, which scatter carriers in graphene andreduce the mobility. Moreover, the combination ofgraphene and h-BN would overcome the greatest
disadvantage of graphene: the poor switching property (low on/off ratio) in a FET (field-effect transistor), which originates intrinsically in the gaplesselectronic structure. A band gap opening is expected
when graphene is transferred onto h-BN because of
the stacking-induced symmetry breaking [6]–[10].
Furthermore, h-BN can also be applied as a gate insu

lator or a tunnel barrier in electronic devices [11],[12].
The exfoliation method has generally been used toobtain h-BN films [2], [3], [5]. In this method, a single crystal is cleaved by using adhesive tape, and apiece of the crystal is transferred onto a substrate byrubbing the tape on the substrate. (Initially, graphene
was obtained exclusively using this method.) However, the typical size of the h-BN film obtained by
exfoliation is only 10 mm, and the lack of scalability
has prevented more common use of h-BN. Thus, thedemand for large-area high-quality h-BN growth isincreasing. Recently, many attempts have been madeto grow large-area h-BN film by using thermal chemical vapor deposition (CVD) [4], [13]–[19]. However,
the CVD method often uses a toxic and explosive gas
as a feedstock.
We are researching a CVD method with the objective of obtaining high-quality and controlled growthof the number of h-BN layers [19]. We have also beendeveloping a new method of growing h-BN thatwould replace the CVD technique [20]. Here, wereport a novel and very simple method of growing an
atomically thin h-BN film, in which an amorphousmaterial is converted to crystalline h-BN.

NTT Technical Review

Feature Articles

h-BN
a-BN

SiO2

Co or Nih-BN

Si
Sputter deposition(a)Annealing in a vacuum
(b)

(c)

Fig. 1. S chematic of the growth method for h-BN film. (a) Initial substrate. (b) Sequential
sputter deposition of a-BN and Co or Ni films on the substrate. (c) Annealing in
vacuum results in the formation of h-BN films on both topmost and bottom surfacesof the metal film. Here, we focus on the h-BN film formed on the topmost surface.

2. Growth of h-BN using a solid source

Vol. 11 No. 8 Aug. 2013

h-BN

50 µm

Fig. 2. O ptical microscope image of a h-BN film transferred
to a SiO2/Si substrate.

70
60
Ni

10 min30 min50
Reflectance (%)

Our method of growing a h-BN film is schematically shown in Fig. 1. A thermally stable SiO2/Si
wafer is used as a substrate (Fig. 1(a)). An amorphousboron nitride (a-BN) film 30 nm thick is deposited on
the substrate by radio frequency (rf) magnetron sputtering using a sintered BN target. Sequentially, apolycrystalline Ni or Co film 300 nm thick is deposited by rf magnetron sputtering (Fig. 1(b)). The sample was inserted in a quartz tube furnace and heatedat 930˚C for 10 to 30 min in a high vacuum. An
atomically thin h-BN film is formed on the surface ofthe sample by the heating process (Fig. 1(c)). With
this method, we can in principle obtain a h-BN film
as large as the substrate that can be set in the furnaceand sputtering machine. Another major advantage isthat this method is much simpler and safer because no
toxic or explosive gases are required.
The grown h-BN film can be transferred onto any
kind of substrate by etching the metal film in an acid.
An optical microscope image of a h-BN film transferred to a SiO2/Si substrate is shown in Fig. 2. The
h-BN film is scarcely visible because of the hightransparency in the visible light. Similarly, graphene
can also be transferred to a substrate. Therefore, we
can fabricate various graphene/h-BN stacking structures by using the transfer technique.
h-BN has a band gap in the ultraviolet (UV) region
and reflects a part of photons whose energy is larger
than the band gap. UV reflectance spectra obtained
from Ni and Co samples after heating are shown in
Fig. 3. The peaks observed at about 6.3 eV are characteristic in h-BN, and thus, the appearance of thepeaks is indicative of the formation of h-BN (Theother part of each spectrum is mainly due to reflection
by a Ni or Co film underneath the h-BN film.).
A cross-sectional transmission electron microscope

SiO2

40
30
20

Co

10 min30 min

10
0
5.0

5.5

6.0

6.5

Photon energy (eV)Fig. 3. U V reflectance spectra of h-BN films grown on
Ni and Co.

image of a Co sample is shown in Fig. 4. The observed
layered structure is also specific to h-BN. The figure
indicates that the h-BN film consists of about fourFeature Articles

10 nm

Co

Fig. 4. Cross-sectional TEM image of a h-BN film grown on Co.

Ni or Co

Ni or Co

C source

BN source(a) Soluble(b) Insoluble

Fig. 5. Carbon is soluble, but BN is insoluble.

atomic layers. This growth method makes it possible
to obtain such an atomically thin film.
3. Growth mechanism
Incidentally, why is a h-BN film formed in thismethod shown in Fig. 1? The B and N source material, a-BN, is initially embedded in the thick metal
film. To begin with, how can B and N atoms reach themetal surface? It is known that a graphite film isformed on the surface by heating when the a-BN filmin Fig. 1(b) is replaced with an amorphous carbon (aC) film. This phenomenon is explained as follows. A
considerable amount (~1 at.%) of carbon is soluble in
Ni or Co at high temperatures near 1000˚C, as schematically shown in Fig. 5(a). Thus, some of the carbon atoms are dissolved in the metal during the heating in the furnace. The carbon atoms dissolved at theinterface move to the inside of the metal by diffusion.
After a certain period, the dissolved carbon atoms are
uniformly distributed in the metal. Here, heating is
stopped and the temperature decreases. Accordingly,
carbon solubility in the metal also decreases. Then,the excess carbon atoms, which can no longer be dissolved in the metal, precipitate on the surface andform a graphite film. One may expect that this scenario can be applied to the h-BN case. Unfortunately,
however, this scenario cannot explain the h-BN formation. This is because BN or N solubility in Ni or
Co is virtually zero even at high temperature. (Balone is known to be slightly soluble at high temperature.) Again, how can N atoms move in the metal andreach the surface? The detailed mechanism is notclear at present, but some experimental results havebeen obtained that provide some insight into theissue. A scanning electron microscope (SEM) imageof a Ni sample after heating is shown in Fig. 6(a).
Although the entire surface is covered by a h-BN
film, it is thin enough for electrons to penetrate, andthe observed contrast is due to the morphology of theNi film. The figure shows that a polycrystalline Ni
film is formed after heating, in which the typicalgrain size is micrometer scale. B(KLL) and N(KLL)
Auger electron* mapping images showing the amountof B and N at the surface are shown in Figs. 6(b) and*Auger electron: An electron emitted from an excited atom having
a core-hole. It has a kinetic energy specific to the atom. In thisstudy, a K shell electron in a B or N atom is excited by a high energy electron from an electron gun, and one of the L shell electrons is emitted when the atom is relaxed.

NTT Technical Review

Feature Articles

0.5 µm(a) SEM

(b) B

(c) N

Fig. 6. ( a) SEM image of a Ni sample. (b) B (KLL) and (c) N (KLL) Auger
mapping images from the same region as (a).

(c), respectively. The two images are largely correlated due to the formation of h-BN. The positiondependent intensity variation shows a thickness variation of the h-BN film. The thick (white) part often
forms a closed curve along the fringe of a Ni grain.
These results suggest that N atoms reach the surface
by the grain boundary diffusion.
4. Future prospects
Currently, we are trying to make this method ofh-BN growth further simpler. In this study, the a-BN
and metal films were deposited by the sputtering
method. Recently, however, we succeeded in growing
an atomically thin h-BN film using a spin-coatedsolid source and commercially obtained metal foil[23]. Thus, an evaporation machine is no longer necessary for growing h-BN. We are also studying themechanism of h-BN formation. Recent experimentssuggest that the diffusion of N atoms is much slower
than B and that the h-BN formation is restricted by
the provision of N atoms at the surface [23]. We will
also try to fabricate stacking structures of h-BN andgraphene with the aim of achieving high performanceand high functionality graphene devices.
References
[1]
[2]

[3]

[4]

K. Watanabe, T. Taniguchi, and H. Kanda, “Direct-bandgap properties
and evidence for ultraviolet lasing of hexagonal boron nitride single
crystal,” Nature Mater., Vol. 3, No. 6, pp. 404–409, 2004.
C. R. Dean, A. F. Young, I. Meric, C. Lee, L. Wang, S. Sorgenfrei, K.
Watanabe, T. Taniguchi, P. Kim, K. L. Shepard, and J. Hone, “Boronnitride substrates for high-quality graphene electronics,” Nature
Nanotechnol., Vol. 5, No. 10, pp. 722–726, 2010.
W. Gannett, W. Regan, K. Watanabe, T. Taniguchi, M. F. Crommie,and A. Zettl, “Boron nitride substrates for high mobility chemical
vapor deposited graphene,” Appl. Phys. Lett., Vol. 98, No. 24, 242105,
2011.
K. Lee, H. Shin, J. Lee, I. Lee, G. Kim, J. Choi, and S. Kim, “LargeScale Synthesis of High-Quality Hexagonal Boron Nitride Nanosheets
for Large-Area Graphene Electronics,” Nano Lett., Vol. 12, No. 2, pp.
714–718, 2012.

Vol. 11 No. 8 Aug. 2013

[5]

N. Petrone, C. R. Dean, I. Meric, A. M. van der Zande, P. Y. Huang, L.
Wang, D. Muller, K. L. Shepard, and J. Hone, “Chemical Vapor Deposition-Derived Graphene with Electrical Performance of ExfoliatedGraphene,” Nano Lett., Vol. 12, No. 6, pp. 2751–2756, 2012.
[6] G. Giovannetti, P. A. Khomyakov, G. Brocks, P. K. Kelly, and J. vanden Brink, “Substrate-induced band gap in graphene on hexagonalboron nitride: Ab initio density functional calculations,” Phys. Rev. B,
Vol. 76, No. 7, 073103, 2007.
[7] J. Slawinska, I. Zasada, P. Kosinski, and Z. Klusek, “Reversible
modifications of linear dispersion: Graphene between boron nitride
monolayers,” Phys. Rev. B, Vol. 82, No. 8, 085431, 2010.
[8] A. Ramasubramaniam, “Tunable Band Gaps in Bilayer Graphene−BN
Heterostructures,” Nano Lett., Vol. 11, No. 3, pp. 1070–1075, 2011.
[9] N. Kharche, and S. K. Nayak, “Quasiparticle Band Gap Engineeringof Graphene and Graphone on Hexagonal Boron Nitride Substrate,”
Nano Lett., Vol. 11, No. 12, pp. 5274–5278, 2011.
[10] R. Quhe, J. Zheng, G. Luo, Q. Liu, R. Qin, J. Zhou, D. Yu, S. Nagase,
W. Mei, Z. Gao, and J. Lu, “Tunable and sizable band gap of singlelayer graphene sandwiched between hexagonal boron nitride,” NPG
Asia Mater., Vol. 4, e16, 2012.
[11] L. Britnell, R. V. Gorbachev, R. Jalil, B. D. Belle, F. Schedin, A.
Mishchenko, T. Georgiou, M. I. Katsnelson, L. Eaves, S. V. Morozov,
N. M. R. Peres, J. Leist, A. K. Geim, K. S. Novoselov, and L. A.
Ponomarenko, “Field-Effect Tunneling Transistor Based on Vertical
Graphene Heterostructures,” Science, Vol. 335, pp. 947–950, 2012.
[12] G. Lee, Y. Yu, C. Lee, C. Dean, K. L. Shepard, P. Kim, and J. Hone,“Electron tunneling through atomically flat and ultrathin hexagonalboron nitride,” Appl. Phys. Lett., Vol. 99, No. 24, 243114, 2011.
[13] L. Song, L. Ci, H. Lu, P. B. Sorokin, C. Jin, J. Ni, A. G. Kvashinin, D.
G. Kvashnin, J. Lou, B. I. Yakobson, and P. M. Ajayan, “Large Scale
Growth and Characterization of Atomic Hexagonal Boron Nitride
Layers,” Nano Lett., Vol. 10, No. 8, pp. 3209–3215, 2010.
[14] Y. Shi, C. Hamsen, X. Jia, K. Kim, A. Reina, M. Hofmann, A. L. Hsu,
K. Zhang, H. Li, Z. Juang, M. S. Dresselhaus, L. Li, and K. Kong,“Synthesis of Few-Layer Hexagonal Boron Nitride Thin Film by
Chemical Vapor Deposition,” Nano Lett., Vol. 10, No. 10, pp. 4134–
4139, 2010.
[15] P. Sutter, J. Lahiri, P. Albrecht, and E. Sutter, “Chemical Vapor Deposition and Etching of High-Quality Monolayer Hexagonal Boron
Nitride Films,” ACS Nano, Vol. 5, No. 9, pp. 7303–7309, 2011.
[16] K. Kim, A. Hsu, X. Jia, S. Kim, Y. Shi, M. Hofmann, D. Nezich, J. F.
Rodriguez-Nieva, M. Dresselhaus, T. Palacios, and J. Kong, “Synthesis of Monolayer Hexagonal Boron Nitride on Cu Foil Using Chemical Vapor Deposition,” Nano Lett., Vol. 12, No. 1, pp. 161–166,
2012.
[17] A. Ismach, H. Chou, D. A. Ferrer, Y. Wu, S. McDonnell, H. C. Floresca, A. Covacevich, C. Pope, R. Piner, M. J. Kim, R. M. Wallace, L.
Colombo, and R. S. Ruoff, “Toward the Controlled Synthesis of Hexagonal Boron Nitride Films,” ACS Nano, Vol. 6, No. 7, pp. 6378–
6385, 2012.
[18] S. Suzuki and H. Hibino, “Chemical Vapor Deposition of Hexagonal
Boron Nitride,” e-J. Surf. Sci. Nanotechnol., Vol. 10, pp. 133–138,Feature Articles

2012.
[19] C. M. Orofeo, S. Suzuki, H. Kageshima, and H. Hibino, “Growth andlow-energy electron microscopy characterization of monolayer hexagonal boron nitride on epitaxial cobalt,” Nano Res. Vol. 6, No. 5, pp.
335–347, 2013.
[20] S. Suzuki, R. M. Pallares, and H. Hibino, “Growth of atomically thin
hexagonal boron nitride films by diffusion through a metal film andprecipitation,” J. Phys. D, Vol. 45, No. 38, 385304, 2012.
[21] M. Zheng, K. Takei, B. Hsia, H. Fang, X. Zhang, N. Ferralis, H. Ko,
Y. Chueh, Y. Zhang, R. Maboudian, and A. Javey, “Metal-catalyzed

crystallization of amorphous carbon to graphene,” Appl. Phys. Lett.,Vol. 96, No. 6, 063110, 2010.
[22] K. L. Saenger, J. C. Tsang, A. A. Bol, J. O. Chu, A. Grill, and C.
Lavoie, “In situ x-ray diffraction study of graphitic carbon formed
during heating and cooling of amorphous-C/Ni bilayers,” Appl. Phys.
Lett., Vol. 96, No. 15, 153105, 2010.
[23] S. Suzuki, R. Molto Pallares, C. M. Orofeo, and H. Hibino, “Boronnitride growth on metal foil using solid sources,” J. Vac. Sci. Technol.
B, Vol. 31, No. 4, 041804, 2013.

Satoru Suzuki

Senior Research Scientist, NTT Basic ResearchLaboratories.
He received the B.S., M.S., and Ph.D. degrees
from Tohoku University, Miyagi, in 1990, 1992,
and 1999, respectively. He joined NTT in 1992.
Since 1994, he has studied synthesis and physicalproperties of carbon-based nanomaterials. He
has also studied the correlation between the electronic structure and electrochemical properties oftransition-metal-based electrode materials of
rechargeable lithium ion batteries. Currently, heis studying the synthesis and properties of graphene and hexagonal boron nitride. He is amember of the Japan Society of Applied Physics,
the Physical Society of Japan, and the Surface
Science Society of Japan.



NTT Technical Review

Feature Articles: Front-line Research on Graphene

Exploring Relativistic Physics andBand Gap Detection in Epitaxial
Graphene
Keiko Takase and Shinichi Tanabe
Abstract

Graphene is a relatively new type of material whose charge carriers exhibit unique relativistic quantum
behavior. Recent progress in graphene research has revealed that high-quality and large-scale epitaxial
graphene can be obtained through thermal decomposition of SiC. This article describes electrical transport measurements of carrier-density-tunable epitaxial graphene devices that shed new light on graphene. For epitaxial monolayer graphene, we first report on the unusual quantum Hall effect derived
from the relativistic nature of the charge carriers and further elucidate its associated physical features.
For epitaxial bilayer graphene, we present a signature of a band-gap opening that can be induced by thefield effect.

1. Introduction
Graphene is a two-dimensional honeycomb latticeof carbon atoms. It was first mentioned by Dr. P. R.
Wallace in the 1940s as a fascinating but imaginary
concept. He found that charge carriers in graphene
were governed by the relativistic quantum theory
instead of by the usual quantum mechanics or classical mechanics that normally describes the motions ofcarriers in a solid. However, it was presumed thatgraphene did not exist in the real world because
researchers thought that it went against the Mermin–
Wagner theorem stating that a macroscopic crystalline order does not exist in two-dimensional systems
at finite temperatures. This consensus on graphene
was broken in 2004 and 2005 with the report statingthat a monolayer of graphite, i.e., graphene, exfoliated from bulk graphite on SiO2/Si, shows transportfeatures of massless Dirac fermions, which arederived from the relativistic theory. Shortly after
these reports were published, the stability of graphene
was found to be compatible with the Mermin–Wagner theorem due to its corrugation structures, and theremarkable discovery of graphene sparked researchin a variety of fields. This is because graphene is morethan just a new material; it opens up possibilities fortable-top experiments demonstrating relativistic
physics, which have conventionally been investigatedusing large accelerators. This breakthrough was so
remarkable that it resulted in the 2010 Nobel Prize inPhysics being awarded to the researchers only five to
six years after their discovery. However, studies ongraphene still have a long way to go. It is likely thatsuch studies will be extended to even more diverse
fields and will also face various problems. One
important issue that should be addressed is the needto develop a means of growing high-quality andlarge-area graphene that is suitable for industrial
applications.
In this article, we present our recent research on thetransport properties of monolayer and bilayer graphene grown on SiC, which we hereafter refer to as
epitaxial monolayer graphene and epitaxial bilayer
graphene, respectively. We first show that devices
fabricated from epitaxial monolayer graphene have ahigh level of quality that is comparable to those fabricated using the standard exfoliation technique. Thisis demonstrated by the observation of a robust halfinteger quantum Hall effect, which is a hallmark of
the relativistic nature of charge carriers [1]. Then weshow that transport measurements of epitaxial monolayer graphene can also be used as a form of energy
NTT Technical Review

Feature Articles

spectroscopy of graphene [2]. Unlike monolayer
graphene, bilayer graphene is predicted to have aband gap, which has previously been investigated by
optical measurements. We report here that transportmeasurements of epitaxial bilayer graphene candetect the signature of the band gap [3].
2. Experimental methods
We obtained epitaxial monolayer graphene andepitaxial bilayer graphene through thermal decomposition of SiC(0001). By choosing the appropriate
growth conditions, we can preferentially grow eithera monolayer or bilayer of graphene on a wafer scale
in a controlled manner. We fabricated top-gated Hallbar devices using these graphene layers as the channel. Schematics of the top and cross-sectional viewsof the devices used in this study are shown in
Figs. 1(a) and (b). A gate electrode was formed above
the graphene channel, separated by a dielectric layer.
Applying a gate voltage between the graphene channel and the gate electrode allows us to tune the carrier
density over a wide range across the charge neutrality
point. Transport measurements were performed attemperatures ranging between ~1.5 and 300 K. Magnetic fields of up to 10 T were applied perpendicular
to the graphene sheet for Hall measurements (Fig. 1(a)).
These experimental conditions are sufficient to study
the physics discussed in this article.
3. Monolayer graphene:
half-integer quantum Hall effectHere, we describe the results of transport measurements of an epitaxial-monolayer graphene device,obtained at a temperature of 1.5 K and in fields from0 to 10 T. The Hall resistance (red line) and longitudinal resistance (black line) as a function of magnetic
field, obtained with gate voltage Vg fixed at zero areshown in Fig. 2(a). At low fields, the Hall resistanceincreases proportionally with the field, while at highfields a region referred to as a plateau appears, which
is characterized by a constant Hall resistance accompanied by zero longitudinal resistance. This is a phenomenon universally observed in two-dimensional
electron or hole systems induced at high-quality
semiconductor interfaces, and is a consequence of thediscrete energy levels referred to as Landau levels
that form at high fields. If we consider for simplicity
spin-split Landau levels, then charge carriers filled in
one Landau level contribute to Hall conductance by
e2/h. Accordingly, the Hall resistance is quantized toVol. 11 No. 8 Aug. 2013

Magnetic field B

Current I

Vxy Hall resistance Rxy = Vxy /I

Vxx
Longitudinal resistance Rxx = Vxx /I(a) Top view.
Gate electrode
ElectrodeDielectric layer
Graphene

SiC substrate(b) Cross-sectional view.

Fig. 1. Schematic of device structure.

h/ie2 with i being an integer, the phenomenon known
as the integer quantum Hall effect (QHE). For the
QHE in monolayer graphene, the Hall resistance ispredicted to take the values h/{4(N+1/2)e2} (N: integer) at the plateaus. Because graphene has a fourfold
degeneracy due to spin and valley degrees of freedom, the integer in the denominator changes by 4N.
Interestingly, a unique feature arises at the first (N =0) plateau. The resistance becomes h/{4×(0+1/2)e2}=h/(2e2), and thus, a half-integer shift seems
to occur. This shift is a consequence of the relativisticenergy spectrum, because at high fields graphene hasa zero-energy state arising from the coexistence ofelectrons and holes, which are equally responsible forthe fourfold degeneracy.
In Fig. 2(a), plateaus labeled by the resistance values of ±h/(2e2) and h/(6e2) are observed, and at thesame time, longitudinal resistance becomes zero atthe ±h/(2e2) plateaus. We can thus say that the n = 2
quantum Hall effect (n: the filling factor, i.e., thenumber of Landau levels filled with electrons orholes) is well developed. Furthermore, we can also
confirm that graphene has bipolar charged carriers byobserving the dependence of Hall resistance on Vg(Fig. 2(b)), which changes from positive to negative
at Vg = −22 V because of changes in the positive andnegative signs of the carriers. These observations inwhich we verified the bipolar half-integer quantum
Hall effect led to the conclusion that our device fabricated from epitaxial monolayer graphene is as high inquality as those fabricated from graphene exfoliatedFeature Articles

(kΩ)
12

h/(2e 2)842−h/(2e 2)

0
−6

−4

−2

12

5

h/(6e 2)

0V

0 Rxy

8

−5

4

Rxx 6

−10
−15
6 (T)

0
2
4
B
(a) Dependence of longitudinal resistance Rxx and Hall
resistance Rxy on magnetic field B.

10

Vg = −16 V

10

10

(kΩ)
15

(kΩ)
16

(kΩ)
15

10 V
Rxy

0
0
(kΩ) 015

B=3Th/(2e 2)

10
Rxx
5

5
Rxy 0

0

−5
−10
−15
−60

−h/(2e 2)
−40

−20
0
20 (V)Vg(b) Dependence of Hall resistance Rxy on gate voltage Vg.

0

2

4

6
8
10 (T)B(c) Dependence of Hall resistance Rxy andlongitudinal resistance Rxx on magnetic
field B at various gate voltages Vg.

Fig. 2. Half-integer quantum Hall effect in epitaxial monolayer graphene.

from bulk graphite [1]. Moreover, in our devices, thecarrier density can be modulated over a wide range
using the gate electrode. Then the type of carrier canbe varied between electrons and holes across the
Dirac point, where electrons and holes ideally vanish.
Such a device using epitaxial monolayer graphene
has not been demonstrated previously. This gives us adefinite advantage in exploring an area of physics thathas not been revealed for epitaxial monolayer graphene.
4. Monolayer graphene: spectroscopy throughtransport measurements
Next, we show an example to illustrate the advantage of the wide tunability offered by our epitaxialmonolayer graphene devices. The results were measured for another sample and show the field-dependence of Hall and longitudinal resistances obtainedfor different gate voltages; these results are shown in
Fig. 2(c). As the graph clearly shows, the n = 2 QH
regime starts from lower fields for lower carrier densities, and it persists up to the highest field for all thecarrier densities shown. This feature has not been
observed for graphene on SiO2/Si, and is thus a feature specific to epitaxial monolayer graphene,referred to as the anomalously wide n = 2 QH state.
The mechanism for this anomalous behavior has
remained elusive but was recently revealed by ourstudy, as reported below.
A mapping of longitudinal resistance Rxx plotted asa function of gate voltage and magnetic field is shownin Fig. 3(a). The trajectories of Rxx peaks displayed inFig. 2(c) are represented in Fig. 3(a) as parabolic
curves centered at Vg = −37 V. Rxx mapping obtained
for a conventional exfoliated graphene device shows
linear trajectories of the Rxx peaks, as presented inFig. 3(b). A comparison of Figs. 3(a) and (b) demonstrates that the n = 2 QH regime is significantly wider
in epitaxial monolayer graphene than in exfoliatedgraphene. This anomalous wideness is intuitively
attributed firstly to the parabolic trajectories of the
Rxx peaks and secondly to the width of the Rxx peak at
n = 0, which is nearly unchanged with increasing
fields.
To clarify the underlying mechanism of the
NTT Technical Review

Feature Articles

(T)

(T)

10

10

8

8

Rxx (kΩ)ν = −2

6

ν=2

6

50

ν = −2ν=2−6

Rxx (kΩ)
6

6

B

B
4

4

4
25

2
0

2

2
0
−80

−60

−40

−20

0

0
−20

20 (V)

−10

0
Vg

Vg
(a) Dependence of Rxx on gate voltage Vg and magnetic
field B in epitaxial monolayer graphene.
(T)

ν=4
ν=2

B

6

4

ν=8
A1

2

−20

0
20
Vg−VD

40

60 (V)(c) Simulation to reproduce experimental results in (a).

Cox

Cc2

Cc1

6

−40

0

Vg

8
ν = −220 (V)(b) Dependence of Rxx on gate voltage Vg and magnetic
field B in exfoliated graphene.

ν=0

10

0

10

Cγ1

Cq

Cγ2

n1
−en1
γ1

−enG

−en2
εF

−eVg
A2

d1
d2
Interface Graphene Interface
state
state

n2
γ2

Gate

(d) Model used for simulation in (c).

Fig. 3. Experimental data and simulation results on the anomalously wide n = 2 quantum Hall state.

anomalously wide n = 2 QH regime, we first examined the Vg dependence of the carrier density deduced
from Hall resistance at low fields. The Vg dependence
would be linear in ideal two-dimensional systems;
however, it is parabolic in our epitaxial-monolayer
graphene devices. In addition, around the Dirac point,
the change in carrier density induced by a change in
Vg is much smaller than that expected from the device
geometry. This may indicate that some additionalcharge reservoir other than graphene exists in thedevices. Likely candidates are interface states such as
dangling-bond states at the interface with the SiC
substrate or those in the gate insulator; the former isknown to inevitably occur in devices fabricated from
epitaxial monolayer graphene grown under standard
conditions. We then developed a model describing
the interplay between graphene quantum capacitanceand interface-state capacitances (Fig. 3(d)). By
Vol. 11 No. 8 Aug. 2013

applying this model to the Vg dependence of the carrier density deduced at low fields, we can deduce theinterface state densities and reproduce the trajectoriesof the Rxx peaks in the gate-voltage vs. field plane(Fig. 3(a), white dashed lines). Furthermore, when
we take into account the fourfold degeneracy of graphene Landau levels, we can calculate the borderlines
separating integer and noninteger n, which are ingood agreement with our experiment. Therefore, ourstudy demonstrates that the origin of the anomalously
wide n = 2 QH state is the interplay between graphene
quantum capacitance and interface-state capacitances[2]. More importantly, when the interface state densities are much larger than the density of states in graphene, the Fermi energy of graphene becomes proportional to the gate voltage. Consequently, the Rxx
peaks plotted vs. Vg and field closely resemble therelativistic LL (Landau level) diagram plotted vs.
Feature Articles

(kΩ)
300

EnergyEnergy2K

250
200
MomentumGapρ
Momentum

150
100
50
0

300 K
−60

−40

−20

0

20

(V)

Vg
(a) Band structure of monolayer
graphene.

(b) Band structure of bilayer graphene
under electric field perpendicular tothe plane.

(c) Dependence of resistivity ρ on gate voltage
Vg in epitaxial bilayer graphene.

Fig. 4. Band structures of monolayer and bilayer graphene together with transport properties of epitaxial bilayer graphene.

energy and field. This indicates that transportmeasurements serve as a kind of spectroscopy, whichallows us to deduce the energy width of the disorderbroadened LLs, which has been an important issue ingraphene research.
5. Bilayer graphene: electrical detection ofband gap opening
We next present a signature of a band gap opening
in epitaxial bilayer graphene. In order to explain our
experimental results, we briefly describe the band
structures of monolayer graphene and bilayer graphene.
The energy band structure of monolayer graphene
is shown in Fig. 4(a). The conduction and valencebands touch each other at the Dirac point, whichmeans that monolayer graphene does not have a bandgap. This becomes a drawback of monolayer graphene when it is used as a channel in logic devices
because a large on/off ratio cannot be expected.
Although graphene that is patterned into nanoribbons, which are strips of graphene in nanometer-scale
width, is predicted to have a band gap, making suchnanoribbons with uniform widths and controlled edge
structures is still a formidable experimental challenge.
However, opening a band gap was more easily
achieved in bilayer graphene, where a band gap
appears between the valence and conduction bands
when there is a potential difference between the twographene layers, as shown in Fig. 4(b). The potential

difference can be easily induced by applying an electric field perpendicular to the two graphene planes ina gated structure, as shown in Fig. 1. Thus, opening aband gap is more feasible in bilayer graphene than inmonolayer graphene.
Our experimental results on the gate-voltage dependences of resistivity of a top-gated Hall bar device
using epitaxial bilayer graphene are shown in
Fig. 4(c). The data at 2 K indicate that when the gate
voltage is reduced from 0 to −30 V, the resistivity
increases and reaches a maximum at −30 V. The resistivity decreases as the gate voltage is further reduced
from −30 to −70 V. In a Hall effect measurement, thepolarity of the Hall voltage indicates that from 0 to
−30 V the carriers are electrons, and their density
decreases as the gate voltage approaches −30 V. The
polarity of the Hall voltage is reversed when the gate
voltage is reduced from −30 to −70 V, which indicatesthat carriers are holes in this gate voltage range. Also,
the hole density increases as the gate voltage
approaches −70 V. These results indicate that theincrease in resistivity is due to the decrease in carrier
density.
It is also clear from Fig. 4(c) that the value of the
maximum resistivity decreases with increasing temperature. The strong temperature dependence of the
maximum resistivity indicates that epitaxial bilayer
graphene exhibits insulating behavior. This is compatible with the theoretical prediction that a band gap
can be opened by the applied field effect, since theobserved features can be understood by the scenario
where the carriers are thermally excited from the
NTT Technical Review

Feature Articles

Fermi level situated in the gap to the conduction orvalence bands. Therefore, we can conclude that wehave observed a signature of the band gap opening inepitaxial bilayer graphene. Such a band gap can betuned by the field effect and scalability of the graphene growth to an arbitrary size, which makes epitaxial bilayer graphene a very promising material for
use as channels in integrated transistors.

is useful for further study of unique graphene physics.
We found that for epitaxial bilayer graphene, a band
gap could be opened because of the applied field
effect, which will enable future applications as channels of electronic devices.
References[1]

6. Summary and future prospects[2]

We have reported our recent experimental resultson charge transport in monolayer and bilayer graphene grown on SiC. We demonstrated that epitaxial
monolayer graphene shows significant relativistic
charge transport and that our field-effect gated device

Keiko Takase

Researcher, Quantum Solid State Physics
Research Group, Physical Science Laboratory,
NTT Basic Research Laboratories.
She received the B.Sc., M.Sc., and Ph.D.
degrees in physics from the University of Tokyoin 2004, 2006, and 2009, respectively. Since joining NTT Basic Research Laboratories in 2009,
she has been engaged in research on the bilayer
quantum Hall state in GaAs and the quantum
Hall effect in graphene. She is a member of the
Physical Society of Japan and the Surface Science Society of Japan.

Vol. 11 No. 8 Aug. 2013

[3]

S. Tanabe, Y. Sekine, H. Kageshima, M. Nagase, and H. Hibino,“Half-Integer Quantum Hall Effect in Gate-Controlled Epitaxial Graphene Devices,” Appl. Phys. Exp., Vol. 3, No. 7, 075102, 2010.
K. Takase, S. Tanabe, S. Sasaki, H. Hibino, and K. Muraki, “Impactof graphene quantum capacitance on transport spectroscopy,” Phys.
Rev. B, Vol. 86, 165435, 2012.
S. Tanabe, Y. Sekine, H. Kageshima, M. Nagase, and H. Hibino,“Observation of Band Gap in Epitaxial Bilayer Graphene Field EffectTransistors,” Jpn. J. Appl. Phys., Vol. 50, No. 4, 04DN04, 2011.

Shinichi Tanabe

Researcher, Low-Dimensional ResearchGroup, Materials Science Laboratory, NTT Basic
Research Laboratories.
He received the B.E. and M.E. degrees in materials engineering from Osaka University in 2007
and 2009, and the Ph.D. degree in physics fromthe University of Tsukuba in 2013. Since joining
NTT Basic Research Laboratories in 2009, hehas been engaged in research on the transport
properties of SiC graphene. He is a member of
the Japan Society of Applied Physics.

Feature Articles: Front-line Research on Graphene

Plasmon Transport in Graphene
Norio Kumada
Abstract

This article describes time-resolved measurements of plasmon transport in graphene. It is demonstrated that the plasmon velocity can be varied over two orders of magnitude by changing the magnetic
field, carrier density, and the gate screening effect.

1. Introduction
Optical interconnections have the advantages of
low-loss and high-speed data transmission over electrical interconnections, and consequently, opticalfibers have replaced the metal cables formerly used
for short-distance connections between racks andboards in supercomputers as well as for long-distancetransmission over the Internet. Furthermore, many
research efforts have recently been directed at controlling optical signals and replacing some electronic
devices with optical ones, with the aim of further
reducing the power consumption. However, the integration of optical devices into electronic chips has
been hampered by the diffraction limit of light, which
is typically 1000 nm.
One possible way to solve this problem is to confine optical signals to nanoscale regions in the formof plasmons. This field of research is called plasmonics, and it is being actively studied, mainly using
surface plasmons on metal surfaces. However, there
are fundamental limitations to using metals in plas-

Sheet plasmon

monics. Namely, the properties of surface plasmons
are fixed by the material and the geometry. Furthermore, the loss during the propagation of plasmons islarge.
NTT Basic Research Laboratories has been investigating plasmons in graphene, where tunable properties and low-loss transport of plasmons are expected.
There are two kinds of plasmons in graphene (Fig. 1).
One is called a sheet plasmon, which propagates inthe two-dimensional plane. The other is called anedge magnetoplasmon (EMP), which appears only inhigh magnetic fields. EMPs propagate along thesample edge.
2. Methods of plasmon measurement
In this work, we carried out time-resolved measurement of plasmon transport in graphene. The device
structure and the experimental setup are shown in
Fig. 2. The graphene used was obtained by thermal
decomposition of a silicon carbide (SiC) substrate.
The devices are about 1 × 1 mm2 and have four

Edge magnetoplasmon

B

Graphene~10 nmGraphene

Fig. 1. Plasmons in graphene.

NTT Technical Review

Feature Articles

Oscilloscope
Detector
Ohmic
contact
1 mm

Injection gate

1 µm(a) Image of graphene on SiC(b) Diagram of experimental setup.

Fig. 2. Graphene on SiC and the experimental technique.

Crosstalk(106 m/s)
10

Plasmon signal

Current

Velocity

8
6
4
2
0

1

2
3
Time

4

5 (ns)(a) Typical current trace

0

0

2

4
6
8
Magnetic field

10 12 (T)(b) Plasmon velocity as a function ofmagnetic field

Fig. 3. Plasmon velocity in the ungated sample.

Ohmic contacts. The surfaces of the devices werecovered with 100-nm-thick hydrogen silsesquioxane(HSQ) and 40-nm-thick silicon dioxide (SiO2) insulating layers. We used two samples, one with and theother without a large top gate.
For the time-resolved transport measurement, apulsed plasmon is generated in graphene by applying
a voltage step to the injection gate deposited acrossthe sample edge. The plasmons propagate in graphene and are detected as a time-dependent current
through the detector Ohmic contact using a sampling
oscilloscope. The plasmon velocity can be determined by measuring the time of flight of plasmons at
a time resolution of 100 ps. Although all data shown
here were taken at 1.5 K in order to investigate thephysics of plasmon transport in detail, we confirmedthat the plasmon signal does not change much even atVol. 11 No. 8 Aug. 2013

much higher levels, for example, up to several ten K.
3. Effects of magnetic field on
plasmon transport
A typical current trace is shown in Fig. 3(a). The
origin of the time corresponds to the time when plasmons are injected. The sharp peak is due to crosstalk
between injection and detection high-frequency lines.
Since the crosstalk propagates with the speed of light,
it appears with virtually zero time delay. The broader
peak that appears with a time delay of about 1 ns is
the plasmon signal. The time delay of the plasmon
signal from the crosstalk roughly corresponds to thetime of flight of plasmons. The plasmon velocity is
obtained from the time of flight and the path length.
The plasmon velocity in the ungated sample as aFeature Articles(106 m/s)(106 m/s)
12 T

8T
Velocity

0.2

Velocity

0.2

0.1

0
−80

0.1

−40

0

40

0
−80

80(V)

Gate bias

−40

0

40

80(V)

Gate bias

Fig. 4. Plasmon velocity in the gated sample.

function of the magnetic field is shown in Fig. 3(b).
The velocity is about 6000 km/s at 3 T and decreases
to 2000 km/s as the magnetic field increases. Thisbehavior can be well reproduced theoretically for
EMPs (blue trace). For the magnetic field lower than
2 T, the plasmons are too fast and the velocity cannot
be determined accurately. This indicates that the plasmon velocity around zero magnetic field is above themaximum measurable value of 6000 km/s.
4. Effects of carrier density and the gate
screening effect on plasmon transport
The plasmon velocity in the gated sample as a function of the gate bias at magnetic fields of 8 and 12 T
is shown in Fig. 4. The velocity is about 100 km/s,
which is one order of magnitude smaller than that in
the ungated sample. The smaller velocity is due to thegate screening effect. When there is a metal gate close
to the graphene, the electric field in plasmons is partially screened, which reduces the velocity. As thegate bias and thus the electron density decrease, thevelocity decreases to several ten km/s with oscillations. This behavior can also be qualitatively
explained by EMP theory.
In summary, the velocity of plasmons in graphene
can be tuned over a wide range from several ten to
thousand km/s by changing the magnetic field, carrier
density, and the gate screening effect. The tunabilityof the velocity indicates the tunability of the refractive index and the impedance. By controlling theseparameters, it would be possible to develop plasmonic devices with functionalities such as plasmon
switching, guiding, and routing.
This work was a collaboration between NTT Basic

Research Laboratories and Tokyo Institute of Technology.
5. Future work
NTT Basic Research Laboratories has succeeded incontrolling the plasmon velocity in graphene over
two orders of magnitude. We will try to exploit thisresult in order to control the plasmon transport. Furthermore, we will estimate the loss of the plasmon
signal during the propagation, which is expected to bemuch smaller than that in metallic systems.
References[1][2]

S. Tanabe, Y. Sekine, H. Kageshima, M. Nagase, and H. Hibino,“Half-integer Quantum Hall Effect in Gate-controlled Epitaxial Graphene Device,” Applied Physics Express, Vol. 3, p. 075102, 2010.
N. Kumada, S. Tanabe, H. Hibino, H. Kamata, M. Hashisaka, K.
Muraki, and T. Fujisawa, “Plasmon transport in graphene investigatedby time-resolved electrical measurements,” Nature Communications,Vol. 4, p. 1363, 2013.

Norio Kumada

Senior Research Scientist (Distinguishedresearcher), Quantum Solid-State Physics
Research Group, Physical Science Laboratory,
NTT Basic Research Laboratories.
He received the B.S., M.S., and Ph.D. degrees
in physics from Tohoku University, Miyagi, in
1998, 2000, and 2003, respectively. He joined
Basic Research Laboratories in 2003. Since then,he has been engaged in the study of highly correlated electronic states realized in semiconductor heterostructures. He received the Young Scientist Award of the Physical Society of Japan in2008 and the Young Scientists’ Prize from the
Minister of Education, Culture, Sports, Scienceand Technology in 2012. He is a member of the
Physical Society of Japan.

NTT Technical Review

Feature Articles: Front-line Research on Graphene

Surface-enhanced Raman Scatteringof Graphene on SiC
Yoshiaki Sekine, Hiroki Hibino, Katsuya Oguri,
Tatsushi Akazaki, Hiroyuki Kageshima, Masao Nagase,
Ken-ichi Sasaki, and Hiroshi Yamaguchi
Abstract

Raman scattering spectroscopy is a well-known optical tool for identifying the properties of graphenesuch as the number of layers, carrier concentration, and strain. Interaction between incident light and a
metallic nanoparticle such as gold or silver enhances the intensity of light, which can be used in Raman
scattering spectroscopy. This article describes our investigation of graphene properties by using surfaceenhanced Raman scattering.

1. Introduction
Graphene can be grown on a silicon carbide (SiC)
substrate by sublimating silicon (Si) in a high-temperature furnace. The use of high-quality and largearea graphene on SiC has enabled a high-frequency
transistor and a metrological resistance standard
based on the quantum Hall effect*1 to be demonstrated. The interaction between graphene and SiC
modifies the graphene carrier concentration andstrain, which are basic properties for electronic
devices. An electron in strained graphene behaves as
if it is in a magnetic field. Strain, along with carrier
concentration, is therefore a prominent property ofelectronic devices. Raman scattering is a non-destructive method for examining these properties. However,the Raman spectrum of SiC is superimposed on thesalient spectrum of graphene, which makes it difficult
to analyze the graphene’s properties. Here, we explain
how surface-enhanced Raman scattering (SERS)
enhances only the graphene spectrum and thereby
makes it possible to quantitatively analyze the Raman
spectrum of graphene on SiC.
2. Raman scattering and Raman peaksof graphene
In Raman scattering, the energy of light scatteredfrom a substance is slightly different from that of theincident light. A schematic view of Raman scattering
is shown in Fig. 1. The process involves (i) the excitation of electrons from a ground state, (ii) the scattering of the excited electrons with energy relaxation
driven by a lattice vibration, and (iii) the transition of
the scattered electrons to the ground state and theemission of scattered light. The energy difference
between the incident and scattered light is called a
Raman shift, which is expressed in an energy unit ofcm–1. The energy of the lattice vibration in a material
is derived from the Raman shift. Furthermore, electron states can be examined because both an electron
and a lattice vibration are related to Raman scattering. The ratio of Raman scattered light to total scattered light is extremely small, and most of the scattered light is Rayleigh scattered light whose energy is
the same as that of the incident light. Nonetheless,
Raman scattering spectroscopy is a powerful tool for
identifying material properties such as the lattice
vibration and electron state.
Here, we explain the origin of the salient Ramanpeaks of graphene on SiC. G (~1600 cm–1) and D*1	 Quantum Hall effect: In a two-dimensional electron system witha high magnetic field, the Hall resistance exhibits a quantizedvalue, i.e., a specified fraction of an integer of a universal valueof h/e2 = 25.8128056 kΩ. The quantized Hall resistance is applied to a metrological resistance standard.

NTT Technical Review

Feature Articles

Excited state(ii)

Incident lightE

Incident light
Scattered light
(i)
(iii)

Ground stateFig. 1. Depiction of Raman scattering.

Fig. 2. Localized surface plasmon resonance.

(~1350 cm–1) peaks are observed in carbon materials.
The D peak originates from defects and bindings toother materials (including carbon). The D' peak(~1630 cm–1) is also due to defects and bindings andis smaller than the G and D peaks. The 2D peak(~2710 cm–1) with a Raman shift nearly twice that of
the D peak is observed in graphene without defects
and bindings. The 2D intensity decreases as the density of defects and bindings increases.

enhance the light intensity. Nanometer-scale metallic
nanoparticles exhibit LSPR, and the intensity of theenhanced light decreases rapidly as the distance fromthe nanoparticles increases. The light enhancement
only occurs in a region equivalent to the size of themetallic nanoparticle.

3. Enhancement of light intensity due tolocalized surface plasmon resonanceSilver (Ag) nanoparticles were deposited on graphene by Ag evaporation in a high-vacuum chamber*4. The deposited Ag stays in the form of particles
rather than forming a layer because of the inertness ofgraphene. These particles can move and they combine to form aggregate particles about 10 nm in size.
A transmission electron microscopy*5 (TEM) imageof hemispherical Ag nanoparticles on graphene isshown in Fig. 3(a). The magnified image in Fig. 3(b)shows that there is no damage to the graphene even

The interaction between incident light and a metallic nanoparticle such as gold or silver enhances thelight intensity in a small region near the nanoparticle,
which is illustrated in Fig. 2. Incident light induces
polarization in a metallic nanoparticle. The polarization generates an electric field outside the metallic
nanoparticle, which can be derived by solving theelectric-field equation with the boundary condition of
the nanoparticle’s spherical surface. The generated
electric field is equivalent to the electric field induced
by an electric dipole*2 in the center of the nanoparticle. The difference between the air and metal dielectric constants, which depends on the wavelength,
results in a remarkable phenomenon: in a resonancewith an appropriate wavelength, i.e., localized surface plasmon*3 resonance (LSPR), the electric dipole
induces a strong electric field compared with theelectric field of the incident light. The LSPR for goldand silver occurs in the visible light range (380–800nm), and many applications exploiting LSPR havebeen developed because of its ability to greatly
Vol. 11 No. 8 Aug. 2013

4. Sample fabrication and transmission
electron microscopy image*2 Electric dipole: Positive and negative electric charges +Q and –Q
are next to each other; these separated charges are referred to as
an electric dipole.
*3 Plasmon: A state where charged particles such as electrons andions move freely is called a plasma state. Collective oscillation of
charged particles is known as plasma oscillation, and a quantumof plasma oscillation is referred to as a plasmon.
*4 Vacuum evaporation: A method for forming a layer on a solid
surface with an evaporating or sublimating material in a vacuum.
The evaporated or sublimated material attaches to a solid surface(substrate), and the deposited material generally takes the form ofa thin film.
*5 Transmission electron microscopy: A microscopy technique inwhich a thin (less than 1 µm) specimen is irradiated with an electron beam and the contrast of the transmitted electron beam is
examined.

Feature Articles

20 nm
(222)
(a)

(111)

1 nm
(c)

(d)

5 nm
(b)

Fig. 3. (a) TEM image of Ag-deposited graphene. (b) Magnified TEM image of Ag-deposited graphene, (c) Fouriertransform image of yellow square in (b), and (d) enlarged image at interface between graphene and Ag
nanoparticles.

though Ag was directly deposited on it.
We first investigated the structure of crystallized Ag
nanoparticles on graphene. In Fig. 3(c), a Fouriertransform*6 image of the yellow square in Fig. 3(b) isdisplayed with the corresponding reciprocal lattice
vector of a point that represents the distance of a lattice plane. It is clearly seen that the (111) plane of Ag(face-centered cubic) attaches to the graphene, whichis reasonable by considering the lattice constants ofAg and graphene. A magnified image at the interface
between an Ag nanoparticle and graphene is shown in
Fig. 3(d), where yellow, red, and green lines indicate
Ag, graphene, and a buffer layer (a non-conductive
carbon layer between the graphene and SiC, where acarbon atom binds to the SiC). We measured the distance between an Ag nanoparticle and the graphenefrom the TEM image and averaged the distance measured at many points to obtain a distance of 3.14 ±0.13 Å. By considering a theoretical calculation thatthe distance is 3.33 Å for physisorption and less than
2.3 Å for chemisorption [1], we conclude that Ag
nanoparticles attach to graphene without chemicalbonds, i.e., in a physisorbed state, which is consistent
with there being no degradation of the graphene
under the nanoparticles.
5. Surface-enhanced Raman scattering ofgraphene on SiC

makes quantitative analysis difficult. With the Ag
deposition, the spectrum of graphene with Ag
nanoparticles (red) overwhelms that of the SiC substrate (gray). A comparison of the signals of graphenewith Ag nanoparticles (red) and without them (green),shows that the enhanced signal of graphene with Ag
nanoparticles exhibits clear graphene features of D,G, and 2D peaks. By comparing the 2D peak intensity of graphene with and without Ag nanoparticles,
we obtain a nominal enhancement of 37.
The large enhancement paves the way to quantitatively analyzing the D and G peaks of graphene on
SiC as shown in Fig. 4(c). Sub-peaks at 1320 and
1400 cm–1 are observed in addition to the main D
peak at 1360 cm–1. Our SERS method reveals the
sub-peak structure, which cannot be resolved in conventional Raman spectroscopy. The G peak also has a
sub-peak structure at 1580 cm–1 below the mostprominent peak at 1600 cm–1. Furthermore, the D'
peak is observed at 1630 cm–1. The peaks at 1580 and
1600 cm–1 are graphene peaks, not SiC peaks. We
know this because those peaks are observed in micromechanically cleaved graphene*7 on a Si substrate[2]. As shown in the graph, three-curve fitting isapplied to the spectrum around the G peak. The yellow curves represent the summation of the three fitting curves, and the yellowish-brown ones represent
each fitting curve.

Without the Ag deposition, the SiC peaks overwhelm the graphene feature of D and G peaks asshown in Fig. 4(a). Enlarged plots of the D, G, and2D peaks of graphene without Ag nanoparticles areshown in Fig. 4(b). Although there is no SiC peakaround the 2D peak of graphene, the SiC peak issuperimposed on the graphene D and G peaks, which*6 Fourier transform: A mathematical transform to investigate a periodic structure. With the Fourier transform, the distance of a lattice plane can be obtained from a crystalline image.
*7 Micromechanically cleaved graphene: Graphene (monolayer) can
be cleaved from graphite (comprising many layers of graphene)
with adhesive tape. Though micromechanically cleaved graphene
can be obtained using only adhesive tape, it is difficult to obtainlarge-area and homogeneously layered graphene.

NTT Technical Review

Feature Articles

Intensity

Graphene/SiC
500

Intensity

G
D

500
SiC

D
Ag/graphene/SiC
Graphene/SiC
SiC

2D

Graphene/SiC

1280

1360

1440

G

Graphene/SiC
500
SiC

2D
SiC

1560 1600 1640
Raman shift

2800 (cm–1)

2700

(b)

5000

Ag/graphene/SiC
Graphene/SiC
2000
Raman shift

2400

(a)

2800 (cm–1)

1280

G

D

Intensity

1600

5000

D

SiC

5000
D'

1320

1360

1400

1440 1560
Raman shift

1600

1640

(cm–1)(c)Fig. 4. Raman scattering of graphene on SiC: (a) Raman spectra of Ag-deposited graphene, graphene, and SiC, (b)magnified graphs of D, G, and 2D peaks of graphene, and (c) enlarged plots of D and G peaks of Ag-depositedgraphene.

The D and D' peaks, which are caused by disorder,
are observed in graphene with Ag nanoparticles. We
attribute them to defects or the buffer layer that was
originally present in our sample because no damage
is found in the TEM images. Quantitative analysis of
the D, D', and G peaks makes it possible to assess
disorder in graphene. The peak intensity ratio of D to
G and that of D' to G are obtained from the fitting, andthey are an order smaller than those of nanocrystalline graphene [3]. Our SERS technique is a powerful
method for analyzing the D and G peaks of grapheneon SiC and resolving the sub-peak structures, whichcannot be observed in conventional Raman spectroscopy.
6. Doping effect
The Ag deposition has a doping effect on graphene,
which can be estimated by SERS. The carrier densityof graphene is derived from the peak intensity ratio of
2D to G [4]. An electron density of 1.9 × 1013 cm–2 isobtained for graphene with Ag nanoparticles. Using
an electrical measurement, we estimated a typicalelectron density of approximately 1 × 1012 cm–2 forgraphene without Ag nanoparticles. Our results are
consistent with a theoretical calculation that shows
Ag-deposited graphene is electron-doped [1].
Vol. 11 No. 8 Aug. 2013

Care should be taken in estimating the SERS
enhancement because the 2D peak intensity decreases
as the carrier density increases [5]. By considering
the doping effect due to the Ag deposition, the nominal enhancement of 37 is modified to the actual
enhancement of approximately 100. The estimationof the correct enhancement is obtained by quantitatively analyzing the electron density of graphene.
7. Enhancing only the graphene signal
The enhancement of only the graphene signal iswhat makes the quantitative analysis possible. The
graphene spectrum is enhanced, while the SiC spectrum is suppressed. The SiC spectrum with Ag
nanoparticles becomes around half that without Ag
nanoparticles, which is clearly seen in the two peaks
at 1700 cm–1 and the shoulder at 1930 cm–1 as indicated by the blue arrows in Fig. 4(a). The light incident to SiC and the light scattered from it are blocked
by the Ag nanoparticles. Furthermore, the small
region of enhanced light due to LSPR has an impact
on the observed phenomenon, as shown in Fig. 5. The
region of enhanced light is on the order of the size of
the Ag nanoparticles, i.e., 10 nm; therefore, theobserved region is also 10 nm in depth. The spot sizeof the laser in our experiments was 1 µm, and theFeature Articles

Laser

Ag

Ag

Ag

Ag

Ag

Graphene

SiC

Fig. 5. Enhancement of graphene by localized surface plasmon resonance.

observed region without Ag nanoparticles was 1 µm
in depth. Here, we consider the volume of graphene(monolayer) to the observed region. The volume withAg nanoparticles is monolayer in 10 nm, and thatwithout them is monolayer in 1 μm, respectively. The
volume of monolayer graphene to the observed
region with Ag nanoparticles was 100 times larger
than that without them. Consequently, our SERS
method makes it possible to enhance only the graphene signal efficiently.

[1]

8. Further study[2]

In this article, we showed that the sub-peak structure of the D and G peaks of graphene on SiC with Ag
nanoparticles can be revealed using SERS. Thanks tothe efficient enhancement of only the graphene signal, the carrier concentration can be obtained byquantitatively analyzing the graphene peaks. We plan
to use our SERS technique to estimate the strain due
to the interaction between graphene and SiC. The buffer layer between graphene and SiC has an impact on
the strain in graphene, and it is removed by high-tem-perature annealing in a hydrogen atmosphere. We
will also investigate the difference between the strainin graphene with and without the buffer layer. Our
SERS technique is therefore effective for quantitatively obtaining the basic properties of graphene on
SiC.
References

[3]

[4]

[5]

G. Giovannetti, P. A. Khomyakov, G. Brocks, V. M. Karpan, J. van den
Brink, and P. J. Kelly, “Doping Graphene with Metal Contacts,” Phys.
Rev. Lett., Vol. 101, No. 2, 026803, 2008.
J. Lee, K. S. Novoselov, and H. S. Shin, “Interaction between Metal
and Graphene: Dependence on the Layer Number of Graphene,” ACS
Nano, Vol. 5, No. 1, pp. 608–612, 2011.
E. H. Martins Ferreira, M. V. O. Moutinho, F. Stavale, M. M. Lucchese, R. B. Capaz, C. A. Achete, and A. Jorio, “Evolution of theRaman spectra from single-, few-, and many-layer graphene withincreasing disorder,” Phys. Rev. B, Vol. 82, No. 12, 125429, 2010.
A. Das, S. Pisana, B. Chakraborty, S. Piscanec, S. K. Saha, U. V.
Waghmare, K. S. Novoselov, H. R. Krishnamurthy, A. K. Geim, A. C.
Ferrari, and A. K. Sood, “Monitoring dopants by Raman scattering inan electrochemically top-gated graphene transistor,” Nat. Nanotechnol., Vol. 3, No. 4, pp. 210–215, 2008.
C. Casiraghi, “Doping dependence of the Raman peaks intensity ofgraphene close to the Dirac point,” Phys. Rev. B, Vol. 80, No. 23,233407, 2009.

NTT Technical Review

Feature Articles

Yoshiaki Sekine

Hiroyuki Kageshima

Researcher, Low-Dimensional Nanomaterials
Research Group, Materials Science Laboratory,
NTT Basic Research Laboratories.
He received the B.S. degree in engineeringfrom Hokkaido University in 1994 and the M.S.
and Ph.D. degrees in science from the Universityof Tokyo in 1996 and 1999, respectively. He
joined NTT Basic Research Laboratories in 1999
and has been studying low-dimensional semiconductor physics. He is currently engaged in anexperimental study of the optical and electricalproperties of graphene. He is a member of the
Physical Society of Japan (JPS) and the Japan
Society of Applied Physics (JSAP).

Senior Research Scientist, Nano-devices
Research Group, Physical Science Laboratory,
NTT Basic Research Laboratories. Joint Graduate Program Visiting Professor in Department of
Nanosystem Science, Graduate School of Nanobioscience, Yokohama City University. Visiting
Lecturer in Graduate School of Science andTechnology, Keio University.
He received the Ph.D. degree in science fromthe University of Tokyo in 1991. He joined NTT
Basic Research Laboratories in 1991 and has
been studying semiconductor physics theory. He
is a member of JPS and JSAP.

Hiroki Hibino

Masao Nagase

Executive Manager of the Materials Science
Laboratory and Group Leader of the LowDimensional Nanomaterials Research Group at
NTT Basic Research Laboratories.
He received the B.S. and M.S. degrees in physics from the University of Tokyo in 1987 and1989, respectively and the Ph.D. degree in pureand applied physics from Waseda University,
Tokyo, in 2006. In 1989, he joined NTT Basic
Research Laboratories, where he has been studying surface dynamical processes using microscopic techniques. His research interests include
step structures on vicinal surfaces, surface masstransport, step instability during epitaxial growth,
self-assembled nanostructure formation, andepitaxial graphene growth. He spent one year as
a visiting research professor at Arizona State
University during 2000–2001. He also has experience as a visiting professor at Tokyo Institute ofTechnology (2007–), Kyushu University (2009–2010), and the University of Tokyo (2011–).

Professor, Material Science and Devices,
Department of Electrical and Electronic Engineering, Faculty of Engineering, the Universityof Tokushima.
He received the B.E., M.S., and Dr.Eng.
degrees in nanometrology for nanodevices fromWaseda University, Tokyo, in 1982, 1984, and1997, respectively. In 1984, he joined the LSILaboratories of Nippon Telegraph and Telephone
Public Corporation (now NTT), where he worked
on R&D of fabrication processes for submicrometer VLSIs. In 1996, he moved to NTT Basic
Research Laboratories, where he researchedfabrication processes for mesoscopic devices
such as single-electron, NEMS, and graphenedevices in the Nanostructure Technology
Research Group of the Physical Science Laboratory. He moved to the University of Tokushima in
2010. His current research field is nanocarbon
engineering. He is a member of JSAP.

Ken-ichi Sasaki

Katsuya Oguri

Senior Research Scientist, Quantum Optical
Physics Research Group, Optical Science Laboratory, NTT Basic Research Laboratories.
He received the B.S., M.S., and Ph.D. degrees
from the University of Tokyo in 1996, 1998, and2006, respectively. He joined NTT Basic
Research Laboratories in 1998. Since then, he
has been engaged in the study of ultrafast x-ray
spectroscopy with laser-produced-plasma x-rays
and high-order harmonics. His current interest is
ultrafast graphene dynamics investigated withultrafast x-ray spectroscopy techniques. He is amember of JSAP and the Optical Society ofAmerica.

Research Specialist, Quantum Optical Physics
Research Group, Optical Science Laboratory,
NTT Basic Research Laboratories.
He received the M.S. and Ph.D. degrees in science from Tohoku University, Miyagi, in 1999
and 2003, respectively. He joined NTT Basic
Research Laboratories in 2010 and has been
studying theoretical aspect of graphene physics.
He is a member of JPS and the American Physical Society.

Hiroshi Yamaguchi
Tatsushi Akazaki

Professor, Department of Electrical Engineering and Information Science, Kochi National
College of Technology.
He received the B.S. and M.S. degrees in physics from Kyushu University, Fukuoka, in 1984
and 1986, respectively, and the Ph.D. degree inelectrical engineering from Osaka University in
1995. He joined NTT Basic Research Laboratories in 1986, where he engaged in research on thetransport properties of semiconductor-coupled
superconducting devices. In 2013, he moved to
Kochi National College of Technology. He is amember of JPS and JSAP.

Vol. 11 No. 8 Aug. 2013

Senior Distinguished Researcher and Group
Leader of the Hybrid Nanostructure Physics
Research Group, Physical Science Laboratory,
NTT Basic Research Laboratories.
He received the B.E. and M.S. degrees in physics and the Ph.D. degree in engineering from
Osaka University in 1984, 1986, and 1993,
respectively. He joined NTT Basic ResearchLaboratories in 1986. His current interests aremicro/nanomechanical devices using semiconductor heterostructures. He has been a guest
professor at Tohoku University since 2006. He isa fellow of the Institute of Physics and a memberof JPS, JSAP, and IEEE.

Feature Articles: Front-line Research on Graphene

Biosensing on a
Graphene Oxide Surface
Kazuaki Furukawa and Yuko Ueno
Abstract

This article describes our recent research on a selective protein detection system built on a graphene
oxide (GO) surface. Our original idea is to fix pieces of GO on a solid support. This enables direct observation of chemical reactions occurring on the GO surface and on-chip device fabrication. The basic
principle can be applied to other proteins as well as other biological molecules simply by changing themolecules to be modified, which makes GO a versatile platform for integrated biosensing devices.

1. Introduction to graphene oxide
Graphene oxide (GO) has a sheet-like structure
similar to graphene, but its preparation method andelectronic properties are very different from those ofgraphene. Let us take a brief look at the similarities
and differences between GO and graphene. The schematic chemical structure of graphene and GO are
shown in Fig. 1. Graphene consists solely of benzene
rings made of carbon (C) atoms with C=C double
bonds (sp2 bonds). In GO, many of the double bonds
in graphene are scissored and linked to oxygen (O) toform C-O bonds, which yields C-C single bonds (sp3bonds). Mobile π electrons that dominate the conductivity in graphene are lost at the oxidized bonds.
Thus, GO becomes an insulator. Many articles in thisspecial issue describe research concerning the conductive nature of graphene, and this is a big difference between the two materials.
The preparation of GO is totally different from thatof graphene. We can chemically synthesize GO byoxidizing graphite in strong acid conditions [1]. This
yields a large quantity of GO in glassware. In contrast
to graphene, GO is well dispersed in water. This isbecause of the many hydroxyl (-OH) groups existingon the GO surface. The synthesized GO has a sheetlike structure about 1 nm thick and with 1–100 µm
edges, which was determined by atomic force microscopy (AFM) [2], [3]. GO is a very large material
compared with molecule-scale materials. It is surprising that such large pieces of GO can float and are notprecipitated in water. Note that GO does not dissolve
in water: it forms a colloid solution. Thus, GO is precipitated when we centrifuge the solution.
2. Mechanism of biomolecule detection
GO is known to behave as a very efficient fluorescence quenching material [4]. When a fluorescentmolecule such as a dye is located in the vicinity of theGO surface, the fluorescence from the dye is notobserved. As the distance between the dye and theGO surface increases, the quenching efficiency drastically decreases, and the dye fluorescence becomes
observable. This is an important feature of the biosensing architecture presented in this article. We
fabricated the protein detection system on the surfaceof GO that is fixed on a solid support through chemical modification [5]. We first explain the fabrication
method and the detection mechanism.
GO that is fixed on a solid support is readily prepared by spin-coating an aqueous dispersion of GO
on a hydrophilic surface such as a silicon (Si) wafer
with silicon dioxide (SiO2) (SiO2/Si) and quartz. The
density of GO pieces can be controlled by the concentration of the dispersion and the spin-coating conditions. For the experiments described here, we used
relatively large pieces of GO fixed on SiO2/Si at low
density in order to access a single piece of the GO.
The function for biological molecule detection canbe built on the GO surface through chemical modification (Fig. 2). There are many sp2 domains, which
NTT Technical Review

Feature Articles(b) Graphene oxide(a) Graphene

Fig. 1. S chematic chemical structure of graphene and GO.

Thrombin aptamer (TBA):
5’-GGTTGGTGTGGTTGG-3’
Pyrene linker

FAM

ThrombinadditionFAM
sp2 domain

Thrombin

Probe dye
Graphene oxide
FAM: proprietary name of the probe dye

Fig. 2. S chematic illustrations of modified GO before and after thrombin
addition. Note that the GO is fixed on a solid support.

remain as graphene-like structures, in GO. Pyrene, amolecule made of four benzene rings, is known tohave a strong affinity to graphene surfaces. We thus
prepared pyrene with a reactive site (a succinimidegroup) and adsorbed the pyrene molecule to the sp2
domains in GO.
Deoxyribonucleic acid (DNA) was linked to thereactive site in the next step. DNA is a polymer composed of four bases that forms a double helix structure with its complementary DNA. The DNA we usedhere is called an aptamer, which is single-stranded
DNA with a determined base sequence. The aptamer
can bind with a specific protein molecule to form acomplex structure. Biosensors that use aptamers areoften referred to as aptasensors. We used a thrombin
aptamer, which forms a complex with thrombin, aprotein important in blood clotting. Thrombin is produced in our body after an injury. It facilitates clotting
in order to prevent the loss of blood. It is not ordinarily present in the blood; if it were, it might induce aninfarct in a blood vessel. An infarct is an area of tissue
that undergoes necrosis as a result of an obstructionin the blood supply.
The base sequence of the thrombin aptamer isshown in Fig. 2. One of the aptamer termini has anVol. 11 No. 8 Aug. 2013

NH2 group that forms a chemical bond with a pyrene
linker. The other is bonded to a dye used for the detection. The intended base sequences with modified termini can be easily synthesized by modern bioengineering. We can therefore order and purchase thedesigned DNA molecule.
The single-stranded DNA has a great affinity to theGO surface and is adsorbed on the surface in both
atmospheric and solution conditions. Thus, the dyemolecule is located in the vicinity of the GO surface,
and its fluorescence is not observed because of theefficient quenching by GO. Since the biomolecule
usually functions in solution conditions, let us consider the case where we add thrombin into a molecular system in water. As shown in Fig. 2 (right), theaptamer recognizes thrombin and forms a threedimensional complex structure. This forces the dye atthe terminus to be separated from the GO surface.
Then the dye fluorescence that was quenched at theinitial stage is recovered. Although the change in distance is very small, the recovery sensitively responds
to the difference. By observing the recovered fluorescence, we can detect the protein, in this case thrombin.

Feature Articles

3. Direct observations of protein
detection on GO surface
We show the experimental results of thrombin
detection in this section. The fluorescence microscope images of a GO aptasensor before and after
thrombin addition are shown in Fig. 3. The image wasdark before the addition; it fluoresces green after the
addition. This is attributed to the fluorescence fromthe dye at the aptamer terminus. We demonstrate theprotein detection using our GO aptasensor in thismanner. Time lapse observations of the green fluorescence revealed that the fluorescence intensities
become about four times more than the background
fluorescence. As we can imagine from the fluorescence images, the increase is significant enough todetect the specific proteins.
A detection system similar to the one we constructed on GO has been reported by other research groups,
but they demonstrated the detection using an aqueous
dispersion of GO [6]. The means of biosensing using
a GO aptasensor fixed on a solid support is NTT’s
original idea [5]. The fixation enables us to performseveral interesting experiments that have not beenpossible with the dispersion system.
The images shown in Fig. 3 were taken by a confocal microscope that detects fluorescence only at thefocused plain. Therefore, the green fluorescence in
Fig. 3 originated from the complex formation between
thrombin and the thrombin aptamer on the GO surface. However, it is impossible to determine howmuch thrombin adsorbs on the surface, and how
densely, only from the fluorescence intensities. To
estimate them, we observed the GO surface using
AFM, which cannot be applied to GO in aqueous
dispersion. By observing the same piece of GO using
a confocal fluorescence microscope and AFM, we
can make a reliable determination.
The AFM topographies before and after thrombin
detection are shown in Fig. 4. The shape of the piececorresponds to that of the green fluorescence in Fig. 3,
which is good proof that we are observing the same
piece of GO. The average thickness of GO is 1.7 nm
before and 3.6 nm after the thrombin detection. Thisobvious increase in thickness is due to the thrombin
adsorption. The thrombin is about 5 nm in diameterin solution conditions. The increase in the thickness
is smaller than the diameter, which suggests that thethrombin does not fully cover the GO surface. In
addition, the AFM topographies were taken in dry
atmospheric conditions, which may cause transformation and shrinkage of the thrombin. Another(a) Before(b) After

10 µm

Fig. 3. F luorescence images of single piece of GO before
and after thrombin addition.

(a) Before(b) After

10 µm

nm

10

0

Fig. 4. A
 FM topographies of single piece of GO before andafter the addition of thrombin. The piece of GO is
the same as that in Fig. 3.

important feature in Fig. 4 is that no thrombin wasobserved on the hydrophilic SiO2 surface. Althoughproteins like thrombin often adsorb on many kinds ofsurfaces, it is suppressed in our system. We would
therefore not have been able to detect the selectivityof thrombin adsorption without the AFM observations.
4. On-chip device application
The unique feature of our system is that the GO
aptasensor is fixed on a solid support. This enables us
to introduce the microchannel device on the GO aptasensor and to extend the technology to on-chip device
applications. The prototype device, the GO aptasensor equipped with two microchannels, is shown in
Fig. 5 [7].
Practical sensor applications always require highsensitivity. We expect that higher sensitivity isachieved with this device by using substrates covered
densely with GO, rather than the isolated GO as in
NTT Technical Review

Feature Articles

10 mm

sensitivity and for estimating the selectivity of proteins.

Microchannel

5. Future perspectives

Thrombin
Channel 1

withChannel 2

without
500 µm(a) Optical microscope(b) Fluorescence microscope

Fig. 5. Optical images of microchannel device (top),
and GO aptasensor equipped with it (bottom
left). Fluorescence microscope image when the
top and bottom channels are respectively filledwith thrombin solution and water (bottom right).

Fig. 3. We prepared such substrates by spin-coating
fine pieces of GO onto them. The fine GO had edges
less than 1 µm, which was obtained by irradiating theultrasound beam to the aqueous dispersion of theoriginal GO. The appropriate conditions will yield
substrates with more than 70% surface coverage by
GO. The substrate was modified with the aptamer andoverlaid with the dual microchannel on the top.
The fluorescence microscope image of the microchannels filled with thrombin solution (channel 1)
and water (channel 2) is shown in Fig. 5. It is clearthat the whole area is green fluorescent in channel 1.
This originates from the dye tethered on the GO surface fixed on a solid support. By contrast, the green
fluorescence is at a background level in channel 2,
which has no thrombin in the solution.
One of the advantages of the microchannel configurations is that we can examine the characteristics ofthe GO aptasensor easily and accurately. This isbecause we can compare the response of the aptasensor to several different samples at the same time,
while maintaining the other identical experimental
conditions. This is a powerful technique for optimizing the molecular structure in order to achieve higher

Vol. 11 No. 8 Aug. 2013

In this article, we presented our protein detection
system built on a GO surface and its use in detecting
thrombin. We combined materials science research
conducted in NTT Basic Research Laboratories andmicrofluidics technology developed in NTT Microsystem Integration Laboratories to develop an original idea that has many advantages and potential
applications in various areas of research.
In future, the protein detection system can beapplied to a variety of target molecules by choosing
the corresponding aptamers. We have already demonstrated the detection of PSA (prostate specific antigen), a protein known as a cancer marker. The detection sensitivity can be increased by designing themolecular structure for GO surface modification. We
have also demonstrated a significant increase in thethrombin detection sensitivity by adding a DNA
spacer. The accumulation and combination of individual technologies is expected to lead to an integrated on-chip GO aptasensor that enables us tosimultaneously detect a wide variety of biologicalmolecules with a small quantity of samples solution.
References
[1]
[2]
[3]

[4]

[5]
[6]
[7]

W. S. Hummers Jr. and R. E. Offeman, “Preparation of Graphitic
Oxide,” J. Am. Chem. Soc., Vol. 80, No. 6, p. 1339, 1958.
K. Furukawa and H. Hibino, “Self-spreading of Supported Lipid
Bilayer on SiO2 Surface Bearing Graphene Oxide,” Chem. Lett., Vol.
41, No. 10, pp. 1259–1261, 2012.
Y. Ueno, E. Tamechika, K. Furukawa, S. Suzuki, and H. Hibino,“Near-Infrared Photoluminescence Spectral Imaging of ChemicallyOxidized Graphene Flakes,” e-J. Surf. Sci. Nanotech. Vol. 10. pp.
513–517, 2012.
E. Treossi, M. Melucci, A. Liscio, M. Gazzano, P. Samorì, and V.
Palermo, “High-Contrast Visualization of Graphene Oxide on DyeSensitized Glass, Quartz, and Silicon by Fluorescence Quenching,” J.
Am. Chem. Soc., Vol. 131, No. 43, pp. 15576–15577, 2009.
K. Furukawa, Y. Ueno, E. Tamechika, and H. Hibino, “Protein Recognition on Single Graphene Oxide Surface Fixed on Solid Support,” J.
Mater. Chem. B, Vol. 1, No. 26, pp. 1119–1124, 2013.
C.-H. Lu, H.-H. Yang, C.-L. Zhu, X. Chen, and G.-N. Chen, “A Graphene Platform for Sensing Biomolecules,” Angew. Chem. Int. Ed.,
Vol. 48, No. 26, pp. 4785–4787, 2009.
Y. Ueno, K. Furukawa, S. Inoue, K. Hayashi, H. Hibino, and E. Tamechika, “Label-Free Thrombin Detection in a Microchannel Using
Aptamer Modified Graphene Oxide Surface,” Proc. of the µTAS2012,
Okinawa, Japan, Oct. 2012.

Feature Articles

Kazuaki Furukawa

Senior Research Scientist, NTT Basic ResearchLaboratories.
He received the B.S. and M.S. degrees in physics and the Dr.Sci. degree from Waseda University, Tokyo, in 1986, 1988, and 2000, respectively. He joined NTT Basic Research Laboratories in 1988. He has been engaged in research on
the chemical physics of soft materials such asfunctional polymers and lipid bilayers. His current interest is in graphene and graphene oxide,
especially for their chemistry and biologicalapplications. He is a member of the Japan Society of Applied Physics, the Physical Society ofJapan, the Chemical Society of Japan (CSJ), andthe Society of Polymer Science, Japan.

Yuko Ueno

Senior Research Scientist, NTT Microsystem
Integration Laboratories.
She received the B.S., M.S., and Ph.D. degrees
in chemistry from the University of Tokyo in
1995, 1997, and 2002, respectively. She joined
NTT Integrated Information & Energy Systems
Laboratories in 1997. She was a post-doctoral
fellow at the University of California at Berkeley
and at Lawrence Berkeley National Laboratory,
Berkeley, CA, USA, from 2004 to 2005. She
received the Encouragement Award (award for ayoung scientist) from the Japan Society for Analytical Chemistry (JSAC) in 2008. She is a member of JSAC, CSJ, and the Spectroscopical Society of Japan.

NTT Technical Review

Regular Articles

Monolithic Integration of
Polarization-entangled Photon Pair
Source Using Silicon Photonics
Technology
Nobuyuki Matsuda, Hiroshi Fukuda, Tai Tsuchizawa,
William John Munro, Kaoru Shimizu, Koji Yamada,Yasuhiro Tokura, and Hiroki Takesue
Abstract

Quantum entanglement is an essential resource for quantum information processing technologies,
including quantum communication and quantum computation. In this article, we present the world’s
first-ever monolithically integrated polarization-entangled photon pair source on a silicon chip.

1. Introduction
Quantum entanglement, or in other words, nonlocal
quantum correlation between two or more quantummechanical objects, is a quintessential feature ofquantum mechanics. At the same time, quantum
entanglement is a substantial resource for quantum
information processing (QIP) technologies including
quantum cryptography, quantum metrology, andquantum computation [1]–[4].
A number of physical systems are being investigated for their potential application in the development of QIP. Among them, photons are excellent
media to generate, process, and distribute entanglements. This is because quantum states encoded inphysical quantities of light are highly robust, thanks
to an inherent weakness in the interactions between
light and electromagnetic environments. The physicalfreedom of light includes polarizations, optical paths,
frequencies, and time bins. Of these quantum states,
polarization is particularly important, since polarization is a true two-level photonic system, which is easy
to manipulate with conventional bulk optics such aswave plates. Thus, many QIP protocols have been
proposed [3], [5] and experimentally demonstrated
[4], [5] using polarization-encoded quantum states oflight. In such experiments, sources of polarizationentangled photons have been inevitably playing a keyrole [6], [7].
Very recently, integrated optical circuits have been
attracting much attention as the next platform of QIP
using photons [8]. Such an approach promises toscale up the experiments by exploiting the miniaturephysical size and highly stable interferometers ofintegrated waveguides. To achieve such integrated
photonic QIP systems, it is essential to develop integrated subsystems to generate, manipulate, and measure the quantum states on a chip. In particular, anintegrated polarization-entanglement source, which
generates polarization-entangled photons into awaveguide mode on a chip, is necessary in order for
us to utilize the wealth of QIP protocols. However, nosuch demonstration of an integrated polarizationentanglement source has been reported to date; nevertheless, it is necessary for a practical integrated QIP
system. In this article, we present the world’s firstever polarization-entanglement source, which NTT
achieved using silicon photonics technology [9](Fig. 1). We have implemented the source as a simple
and stable silicon-on-insulator photonic circuit so the
NTT Technical Review

Regular Articles

effects. The process occurs under the energy conservation of photons as follows
2νp = νs + νi

Fig. 1. P icture of the chip of monolithic polarizationentangled photon pair sources. By injecting pumppulses from the left hand side, we can obtainpolarization-entangled photon pairs to be collectedby the output optical fiber on the right hand side.

source could successfully generate a high degree ofpolarization entanglement.
2. Building blocks of polarization-entangled
photon pair source
An entangled photon pair source requires twobuilding blocks: a source to generate photon pairs
having a time correlation, and a device to manipulate
the polarization state of photons to ensure that photon
pairs are entangled in the polarization degree of freedom. NTT has already realized both components byusing on-chip silicon photonics technology.
A silicon-wire waveguide is a highly efficient
source of time-correlated photon pairs (Fig. 2(a))[10]–[15]. The silicon-wire waveguide is a singlemode waveguide whose core is made of single-crystalline silicon with a typical cross-sectional size of
400 nm (width) and 200 nm (height). Thanks to thisextremely small core, the optical power density in the
waveguide becomes very high, and as a result, we canobserve a huge nonlinear optical effect. The nonlinear
coefficient of our silicon-wire waveguide is fiveorders of magnitude larger than that of a silica opticalfiber. This coefficient represents the optical-nonlinearity strength of a waveguide.
When we input a telecom-wavelength pump light
whose frequency is νp into the silicon-wire waveguide, we can generate a photon pair with frequenciesof νs and νi through spontaneous four-wave mixing(SFWM), which is one of the third-order nonlinear
Vol. 11 No. 8 Aug. 2013

(1)

This indicates that two pump photons are annihilated,and subsequently a signal-idler photon pair is created.
Since the two photons are created simultaneously, we
can generate time-correlated photon pairs. In addition, the use of the single-crystalline silicon coreenables us to avoid background noise photons causedby spontaneous Raman scattering, which has been aserious problem for photon pair sources based onother types of nonlinear waveguides such as fusedsilica [1] whose core has an amorphous structure. A
single-mode waveguide supports two propagating
modes with orthogonal polarization states of light:
transverse-electric (TE) and transverse-magnetic(TM) fields. In the silicon wire waveguide, TE-polarized photon pairs are efficiently generated by a TEpolarized pump field. This is because the waveguide
has a field confinement of TE-polarized light into thecore that is stronger than that of TM-polarized light[13].
The second building block is a silicon polarization
rotator [16], which is used in polarization-diversity
optical circuits for telecommunication applications[17]. The device has an off-axis double-core structure
consisting of a 200-nm2 silicon wire core and a second 840-nm2 silicon-oxynitride (SiON) core on thesilicon core (Fig. 2(b)). The structure exhibits two
orthogonal eigenmodes, which have different effective refractive indices and eigen-axes tilted at 45° tothe normal with respect to the silicon substrate. The
birefringence in the eigenmodes provides an integrated wave plate, which causes the polarization
plane to rotate by an amount that depends on thelength of the rotator. We show the measured polarization rotation characteristics of a 30-μm-long polarization rotator in Fig. 2(c). A TE-polarized input field
was successfully converted to TM polarization withthe rotator. The polarization rotation angle was estimated to be 86.7 ± 0.1° with a polarization extinction
ratio as high as 30 dB. Thus, we obtained a polarization rotation angle of almost 90° with a high polarization extinction ratio. This high performance of therotator enabled us to generate a high degree of polarization entanglement as we show later.
3. On-chip polarization-entanglement source
We combined the two building blocks fabricated onRegular Articles(a)Silica-based
overcladding

Correlated photon pairs
TE, TE s, i

Si core400 nm wide× 200 nm highFour-wave mixing
νpνs

Pump ν
pphotons

νi

SignalphotonIdler
photon2ν p = ν s + ν i

Silica undercladding(b)

Si substrate

SiON 2nd core,n = 1.6
0.84 × 0.84 µm2(Silica-based
cladding,
n = 1.46)(c)Birefringent axes
along ± 45°
Si core, n = 3.4
0.2 × 0.2 µm20Transmittance (dB)Telecom-band
pump laser(TE polarization)

–10
–20
–30
–40

w/o rotator

w/rotator

86.7°
0

90

180

270

360

Output polarizer angle (deg)Fig. 2. Building blocks of polarization-entanglement source. (a) Silicon-wire waveguide source of correlated photon pairs.
(b) Ultrasmall silicon polarization rotator. Claddings are not shown for clarity. (c) Characterization of polarization
rotation angle at the silicon polarization rotator.

the same silicon photonics platform to implement themonolithic polarization entanglement source on achip [9]. Our polarization-entanglement source(Fig. 3) consists of two silicon-wire waveguides connected by a silicon-wire polarization rotator. Bothsilicon wire waveguides are 1.5 mm long. The siliconwire waveguides and the silicon polarization rotator
are connected by 10-μm-long tapered silicon wires.
The over- and under-cladding of the silicon wire
waveguides and the silicon polarization rotator weremade of silica (not shown).
The operating principle of the device is as follows.
We use a pump beam with + 45° linear polarization,
which is a 1:1 combination of the TE and TM modesin the silicon wire waveguides. In the first siliconwire waveguide, the TE component of the pump creates a photon pair in the 1 TE,s 1 TE,i state, which meansthat there is one TE-polarized photon in the signal
frequency channel and one TE-polarized photon in
the idler frequency channel. In the following, werewrite the state as TE s TE i for simplicity. Then, thesilicon polarization rotator rotates this state to the
TM s TM i state as a result of 90° polarization rotation.
Here, the subscripts denote the frequency modes ofthe signal and idler photons. At the same time, thesilicon polarization rotator rotates the TM componentof the pump field to provide it with TE polarization,and the second silicon wire waveguide creates otherTE s TE i photons. Since we cannot distinguish whether the pair was generated in the first or second silicon
waveguides, we obtain the maximally polarizationentangled state:ψ = ( TE TE + e −iφ TM TM ) / 2 ,  (2)sisiat the output of the polarization-entanglement source.
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Polarization-entangled
photon pairs(

1 TE, TE + e−iφ TM, TM
s, i
s, i
2

)

SiON 2nd core

30 µm

45°-polarizedpump pulseTE
pump
TM, TM s, i
Siliconpolarization
rotator=Si wire waveguides
1.5 mm long

TE, TE s, igeneration

+

TE
pump

TM
pump
TE, TE s, igeneration: 90° polarization
rotationFig. 3. Monolithically integrated polarization-entangled photon pairs. The source, fabricated on a silicon-on-insulator
substrate, consists of a silicon 90° polarization rotator sandwiched by two nonlinear silicon wire waveguides. The
device generates the polarization entanglement as a superposition of the two events shown on the right hand side.

Here, the relative phase difference φ is a fixed value
and depends on the difference in length between thefirst and second silicon waveguides.
Generally, it is difficult to precisely control a birefringence of nonlinear waveguides with an ultrasmallcore. The residual birefringence causes significant
polarization-mode dispersion (PMD), which degrades
the degree of polarization entanglement. To avoid thisdegradation, the conventional polarization-entanglement sources adopt off-chip PMD compensators suchas birefringent crystal or polarization-maintaining
fiber. In those cases, photons cannot be polarizationentangled inside the waveguide chip. In our case,since the polarization-entanglement source isdesigned to be symmetric about the polarization
states with respect to the midpoint of the device, thepolarization-mode dispersion of the pump pulses andphoton pairs are completely cancelled out. Therefore,
our polarization-entanglement source generates apolarization-entangled state inside the waveguidechip. This feature enables us to directly integrate thesource with quantum-state controllers and detectors
to construct a quantum information processor on achip. Moreover, the device can automatically balance
the amplitude of the two terms in Eq. (2) even in thepresence of practical waveguide losses, leading tohighly robust polarization entanglement (see Ref. [9]for details).

Vol. 11 No. 8 Aug. 2013

4. Experiment
We generated and analyzed polarization-entangled
photon pairs using the setup shown in Fig. 4. We
injected optical pump pulses whose polarization was
set at + 45° linear polarization and a wavelength centered at 1551.1 nm into the polarization-entanglement source. The photons generated from the chip
were introduced into the wavelength-division-multiplexing (WDM) filter, which suppressed the residual
pump light and separated the signal and idler photons
into different fiber channels. Each output port had acenter wavelength of 1546.4 nm (signal) and 1556.0
nm (idler) with a channel bandwidth of 0.14 nm.
Then, the photons passed through the polarization
analyzers, each of which consisted of a half and aquarter wave plate, and a polarizer. Finally, the photons were received by InGaAs (indium/gallium/arsenide) single photon detectors. The detection signals
from the two detectors were input into the time interval analyzer (TIA), with which we carry out time-correlation measurement of photon pairs.
We first reconstruct the density matrix of the generated polarization-entangled state to evaluate thedegree of entanglement. The density matrix ρ of thestate in Eq. (2) can be expressed asRegular Articles

Quarter-wave plate

Polarizationentanglement
sourcePumppulses90 ps

Half-wave plate

Polarizers

λ p=1551.1 nm

iTime-interval
analyzer

WDM
filterλ s=1546.4 nm
λ i=1556.0 nm

Polarization
analyzers

Single-photon
detectors

Fig. 4. E xperimental setup.

ρ =Σ ψ ψ
1( TE s TE i + e− iφ TM s TM i)2iφ ( TE s TE i + e TM s TM i),
=

 1

1 0
= 
2 0
 e iφ


0

0

0

0

0

0

0

0

(3)

e −iφ 
0 

0 
1 

when the state is purely in the maximally entangledstate. Here, the summation takes over all the generated photon pairs. In this representation, we used
TE s TE i, TE s TM i, TM s TE i, and TM s TM i as a basis
set. Each diagonal element represents the probability(population) of the system, and the off-diagonal elements represent the quantum coherence. If the generated system has no coherence, the density matrix
becomes1
1 0
ρ =Σ ψ ψ = 
2 0

0


0

0

0

0

0

0

0

0

0

0
.
0

1 

(4)

Therefore, the density matrix that has the off-diagonal elements whose absolute values are as high as
those of the diagonal elements shows the full coherence of the entanglement.
To experimentally obtain the density matrix of the

two-photon polarization state, we first carried out thecorrelation measurement of the photon pairs under 16
different polarization combinations that were selectedbased on the angles of the four wave plates in thepolarization projection units [18]. This is analogous
to the measurement of the polarization state of classical light, in which projection measurements of four
different polarization states are at least required toestimate Stokes vectors. In the present case with thetwo-photon state, we require 42 = 16 projection measurement. Then, using a procedure called quantum
state tomography, we can reconstruct the density
matrix. The reconstructed density matrix is shown in
Fig. 5. For simplicity, we have only displayed theabsolute values of each matrix element. The result
exhibits off-diagonal amplitudes as high as 0.5, which
represent a high coherence of the system. Note thatthe results include any background (noise) counts.
Regarding the degree of entanglement, we evaluate
the fully entangled fraction F (ρ ) = maxΨ ρ Ψ , whereΨ
the maximum is taken over all maximally entangledstates |Ψ >. From the measured ρ values in accordancewith the procedure described by Badziag et al. [19],we obtained an F(ρ) value of (94 ± 2)%. This value ismuch higher than the classical limit of 0.5. Furthermore, the F(ρ) value is much greater than 1 2 ~ 71%,
implying that the generated state has strong nonlocal
correlation. Hence, we successfully generated photon
pairs with a high degree of polarization entanglementfrom the chip.

NTT Technical Review

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Experimental result

HH

HV

VH

VV

Ideal
1.0

1.0

0.5

0.5

0.0
VV
VH
HV
HH

0.0
VV
VH
HV
HH

HH

HV

VH

VV

Fig. 5. Reconstructed density matrix. Here, the absolute values of each element are shown. H and V represent the TE andTM bases, respectively.

5. Conclusion
We have demonstrated the world’s first ever sourceof polarization-entangled photon pairs on a chip. The
device we constructed is capable of generating a highdegree of polarization entanglement thanks to nanofabrication technology of silicon photonic devices. To
achieve a photonic QIP system on a chip, it is necessary to integrate our source with subsequent quantum
processors, which manipulate or project the polarization-encoded quantum state of photons. Such integration would also be possible, since our source isequipped with spot-size converters, which can be a
versatile interface with other types of integrated
waveguide platforms [20]. Our monolithic polarization-entangled photon pair source helps pave the way
to the full-scale implementation of a photonic quantum information system on a chip.
References
[1]

[2]
[3]
[4]

[5]

H. Takesue, “Generation of Polarization-entangled Photon Pairs in
1.5-μm Telecommunication Band,” NTT Technical Review, Vol. 3,
No. 12, 2005.
https://www.ntt-review.jp/archive/ntttechnical.php?contents=ntr2005
12052.pdf
K. Edamatsu, “Entangled Photons: Generation, Observation, andCharacterization,” Japan. J. of Appl. Phys., Vol. 46, pp. 7175–7187,
2007.
M. A. Nielsen and I. L. Chuang, “Quantum Computation and Quantum Information,” Cambridge University Press, Cambridge, 2000.
H. Takesue, “Quantum Communication Using Entangled Photon
Pairs––Toward Quantum Repeaters,” NTT Technical Review, Vol. 9,
No. 9, 2011.
https://www.ntt-review.jp/archive/ntttechnical.php?contents=ntr2011
09fa10.html
P. Kok, W. J. Munro, K. Nemoto, T. C. Ralph, J. P. Dowling, and G. J.

Vol. 11 No. 8 Aug. 2013

Milburn, “Linear Optical Quantum Computing with Photonic Qubits,”
Rev. Mod. Phys., Vol. 79, No. 1, pp. 135–174, 2007.
[6] P. G. Kwiat, K. Mattle, H. Weinfurter, A. Zeilinger, A. V. Sergienko,
and Y. Shih, “New High-Intensity Source of Polarization-Entangled
Photon Pairs,” Phys. Rev. Lett., Vol. 75, No. 24, pp. 4337–4341,
1995.
[7] P. G. Kwiat, E. Waks, A. G. White, I. Appelbaum, and P. H. Eberhard,“Ultrabright Source of Polarization-entangled Photons,” Phys. Rev. A,
Vol. 60, No. 2, pp. R773–R776, 1999.
[8] A. Politi, M. Cryan, J. G. Rarity, S. Yu, and J. L. O’Brien, “Silica-onSilicon Waveguide Quantum Circuits,” Science, Vol. 320, No. 5876,
pp. 646–649, 2008.
[9] N. Matsuda, H. Le Jeannic, H. Fukuda, T. Tsuchizawa, W. J. Munro,
K. Shimizu, K. Yamada, Y. Tokura, and H. Takesue, “A monolithicallyintegrated polarization entangled photon pair source on a siliconchip,” Sci. Rep., Vol. 2, 817, 2012.
[10] H. Takesue, Y. Tokura, H. Fukuda, T. Tsuchizawa, T. Watanabe, K.
Yamada, and S. Itabashi, “Entanglement generation using silicon wire
waveguide,” Appl. Phys. Lett., Vol. 91, No. 20, 201108, 2007.
[11] H. Takesue, H. Fukuda, T. Tsuchizawa, T. Watanabe, K. Yamada, Y.
Tokura, and S. Itabashi, “Generation of polarization entangled photon
pairs using silicon wire waveguide,” Opt. Express, Vol. 16, No. 8, pp.
5721–5727, 2008.
[12] K. Harada, H. Takesue, H. Fukuda, T. Tsuchizawa, T. Watanabe, K.
Yamada, Y. Tokura, and S. Itabashi, “Generation of high-purity
entangled photon pairs using silicon wire waveguide,” Opt. Express,
Vol. 16, No. 25, pp. 20368–20373, 2008.
[13] K. Harada, H. Takesue, H. Fukuda, T. Tsuchizawa, T. Watanabe, K.
Yamada, Y. Tokura, and S. Itabashi, “Frequency and polarization
characteristics of correlated photon-pair generation using a siliconwire waveguide,” Selected Topics in Quantum Electronics, IEEE
Journal of, Vol. 16, No. 1, pp. 325–331, 2010.
[14] K. Harada, H. Takesue, H. Fukuda, T. Tsuchizawa, T. Watanabe, K.
Yamada, Y. Tokura, and S. Itabashi, “Indistinguishable photon pairgeneration using two independent silicon wire waveguides,” New J.
Phys., Vol. 13, 065005, 2011.
[15] H. Takesue, “Entangled photon pair generation using silicon wire
waveguides,” Selected Topics in Quantum Electronics, IEEE Journalof, Vol. 18, No. 6, pp. 1722–1732, 2012.
[16] H. Fukuda, K. Yamada, T. Tsuchizawa, T. Watanabe, H. Shinojima,and S. Itabashi, “Polarization rotator based on silicon wire waveguides,” Opt. Express, Vol. 16, No. 4, pp. 2628–2635, 2008.
[17] H. Fukuda, K. Yamada, T. Tsuchizawa, T. Watanabe, H. Shinojima,and S. Itabashi, “Silicon photonic circuit with polarization diversity,”Regular Articles

Opt. Express, Vol. 16, No. 7, pp. 4872–4880, 2008.
[18] D. F. V. James, P. G. Kwiat, W. J. Munro, and A. G. White, “Measurement of qubits,” Phys. Rev. A, Vol. 64, No. 5, 052312, 2001.
[19] P. Badzia̧g, M. Horodecki, P. Horodecki, and R. Horodecki, “Local
environment can enhance fidelity of quantum teleportation,” Phys.
Rev. A, Vol. 62, No. 1, 012311, 2000.

Nobuyuki Matsuda

Research Scientist, Quantum Optical State
Control Research Group, NTT Basic ResearchLaboratories and NTT Nanophotonics Center.
He received the B.E., M.E., and Ph.D. degrees
in electronic engineering from Tohoku University, Miyagi, in 2005, 2006, and 2009, respectively.
During 2009–2010, he was a visiting researchscholar at the University of Bristol, UK. He
joined NTT Basic Research Laboratories in 2010and has since been engaged in research on integrated quantum photonics and single-photon
nonlinear optics. He is a member of the PhysicalSociety of Japan (JPS) and the Japan Society ofApplied Physics (JSAP).

Hiroshi Fukuda

Senior Research Engineer, Photonic Integrated
Interface Research Group, NTT Microsystem
Integration Laboratories and NTT Nanophotonics Center.
He received the B.E. and M.E. degrees innuclear engineering from Tohoku University,
Miyagi, in 1993 and 1995, respectively. In 1995,
he joined NTT Microsystem Integration Laboratories, where he has been engaged in research on
microphotonics devices. He is a member of
JSAP, the Institute of Electronics, and Information and Communication Engineers (IEICE).

Tai Tsuchizawa

Senior Research Engineer, Photonic Integrated
Interface Research Group, NTT Microsystem
Integration Laboratories and NTT Nanophotonics Center.
He received the B.S. and M.S. degrees in physics from Sophia University, Tokyo, and the Ph.D.
degree from the University of Tokyo in 1984,
1986, and 1990, respectively. Since joining NTT
in 1999, he has been engaged in studies on ECR
plasma technology and its application to an etching process for microfabrication. His current
research interest is fabrication technologies forsilicon based optoelectronics devices. He is amember of JSAP.

[20] T. Tsuchizawa, K. Yamada, T. Watanabe, S. Park, H. Nishi, R. Kou, H.
Shinojima, Y. Ishikawa, K. Wada, and S. Itabashi, “Monolithic Integration of Silicon-, Germanium-, and Silica-Based Optical Devices
for Telecommunications Applications,” IEEE J. Sel. Topics Quantum
Electron., Vol. 17, No. 3, pp. 516–525, 2011.

William John Munro

Research Specialist, Quantum Optical State
Control Research Group, NTT Basic ResearchLaboratories.
He received the BSc. degree in chemistry andphysics, the MSc. degree in physics, and theD.Phil degree in quantum optics from the University of Waikato, New Zealand, in 1989, 1991,
and 1994, respectively. In early 1995, he moved
to the computer industry, where he worked on
various projects. In July 1997 he accepted an
Australian Research Council Fellowship in the
Department of Physics at the University of
Queensland, Australia. During this fellowship heinvestigated multiparticle tests of quantum
mechanics and developed an interest in entanglement, methods to characterize it, and its practicaluse in QIP. In 2000, he became a senior researcher at the Australian Special Centre for Quantum
Information Processing. In November 2000, hejoined HP Labs. as a research scientist, and inearly 2010 joined NTT Basic Research Laboratories as a research specialist.

Kaoru Shimizu

Senior Research Scientist, Supervisor, Quantum Optical State Control Research Group
Leader, NTT Basic Research Laboratories.
He received the B.E., M.E, and Ph.D. degrees
in applied physics from Waseda University,
Tokyo, in 1988, 1990, and 1995, respectively. He
joined NTT Transmission System Laboratories
in 1990 and studied lightwave technologies forsurveillance of optical fiber networks. He moved
to NTT Basic Research Laboratories in 1996 andstudied quantum information processing including quantum communication and quantum cryptography. During 2003–2005, he worked at thehuman resource management section of NTT
Science and Core Technology Laboratories andwas mainly engaged in recruitment. Then hereturned to NTT Basic Research Laboratories
and studied quantum memory devices. He was avisiting professor at Nara Institute of Science andTechnology in 2006 and 2009. He is a member of
JPS.

NTT Technical Review

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Koji Yamada

Senior Research Engineer, Supervisor, Distinguished Technical Member, Photonic Integrated
Interface Research Group, NTT Microsystem
Integration Laboratories and NTT Nanophotonics Center.
He received the B.E., M.E., and Ph.D. degrees
in nuclear engineering from Kyushu University,
Fukuoka, in 1986, 1988, and 2003, respectively.
He joined NTT in 1988. In NTT laboratories, he
was engaged in studies on synchrotron lightsources, and since 2000 he has been engaged instudies on silicon photonics platforms. From
2010, he also joined the Photonics Electronics
Technology Research Association (PETRA) as aleader of photonic wiring and the waveguideresearch team of Photonics-Electronics Convergent System Technology research project(PECST). He is a member of JSAP, IEICE, theAtomic Energy Society of Japan, the Particle
Accelerator Society of Japan, and IEEE.

Yasuhiro Tokura

Hiroki Takesue

Senior Research Scientist, Supervisor, Distinguished Researcher, Quantum Optical State
Control Research Group, NTT Basic ResearchLaboratories.
He received the B.E., M.E., and Ph.D. degrees
in engineering science from Osaka University in
1994, 1996, and 2002, respectively. In 1996, hejoined NTT, where he engaged in research onlightwave frequency synthesis, optical accessnetworks using wavelength division multiplexing, and quantum optics. His current researchinterests include quantum optic experiments
using nonlinear waveguides and quantum communication based on entanglement. He hasreceived several awards, including the ITU-T
Kaleidoscope Conference 2nd Best Paper Awardin 2008 and the Commendation for Science andTechnology by the Minister of Education, Culture, Sports, Science and Technology of Japan(The Young Scientist’s Prize) in 2010. From2004 to 2005, he was a visiting scholar at Stanford University, CA, USA. He is a member of
JSAP and IEEE.

Professor, the University of Tsukuba andResearch Professor, NTT Basic Research Laboratories.
He received the B.E., M.E., and Ph.D. degrees
in arts and science from the University of Tokyoin 1983, 1985, and 1998, respectively. He joined
NTT Basic Research Laboratories in 1985 andstudied semiconductor electronic transport innano-structures. During 1998–1999, he stayed at
Technical University of Delft in the Netherlands
as a guest researcher to study transport in carbon
nanotubes. He was an executive manager of the
Optical Science Laboratory of NTT Basic
Research Laboratories from 2005 to 2012. He
became a Professor of the Graduate School ofPure and Applied Sciences at the University ofTsukuba in 2012. He is currently working on thephysics of quantum transport and quantum information science in solid state systems. He is amember of JPS, JSAP, and the American Physical Society.

Vol. 11 No. 8 Aug. 2013

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Global Standardization Activities

The ITU Workshop on eHealth andthe Fourth Meeting of
ITU-T FG-DR&NRR in Tokyo
Hideo Imanaka, Masahito Kawamori, andNoriyuki Araki
Abstract

The International Telecommunication Union (ITU) workshop on eHealth and the fourth meeting of
the ITU-T (ITU-Telecommunication Standardization Sector) Focus Group on Disaster Relief Systems,
Network Resilience and Recovery (FG-DR&NRR) were held in Shinjuku, Tokyo, February 4–8, 2013.
These ITU events were held together in order to promote mutual understanding between working areasin healthcare and disaster response, and to increase NTT’s presence in both fields. This article describes
the main results of these events.

1. Introduction
The ITU workshop on eHealth was held in Tokyo,
Japan, on Monday and Tuesday, February 4–5, 2013.
This workshop was hosted by the Ministry of Internal
Affairs and Communications (MIC), Japan, and wasattended by 135 people from more than 20 countries.
The workshop included an introduction to the needs
and expectations of developing countries with regard
to eHealth and the advanced technology that may
have to be considered for standardization in the
future.
Following this workshop, the fourth meeting of the
ITU-T Focus Group on Disaster Relief Systems, Network Resilience and Recovery (FG-DR&NRR) was
held at the same venue from Tuesday through Friday,
February 5–8, 2013. This meeting was hosted by the
National Institute of Information and Communications Technology (NICT), and attended by approximately 80 people from 15 countries. In addition to theusual discussions on standardization, this event
included a visit to the disaster-affected area in Tohoku
and the NICT Resilient ICT Research Center.
The schedules of the ITU events are shown in
Fig. 1. Part of the workshop (in the afternoon on February 5) was organized as a session on eHealth in theevent of a disaster to promote mutual understanding
between people working in eHealth and in disaster
response.
2. ITU eHealth workshop
2.1 BackgroundIn developed countries including Japan, the aging
society problem is causing a chronic shortage of doctors. Meanwhile, developing countries also have achronic shortage of doctors but for a different reason,
namely, the limited availability of medical services.
Tele-medicine and eHealth are being studied assolutions to these problems. In ITU-T SG16 (Study
Group 16) and ITU-D (ITU-Telecommunication
Development Sector) SG2, studies are being pursued
with the aim of standardizing eHealth and making it
widely available in developing countries. In November 2012, the ITU and World Health Organization(WHO) launched a partnership called the mHealthinitiative, which aims to use mobile phones to deliver
eHealth services to combat non-infectious illnesses[1]. Since 2012, an ITU-T focus group called FGM2M has been studying the standardization ofeHealth as a machine-to-machine (M2M) application.
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Global Standardization Activities

Schedule

Feb. 4 (Mon)e-Health
workshop

Feb. 5 (Tue)Feb. 6 (Wed)Feb. 7 (Thur)

Feb. 8 (Fri)

Held in Tokyo;
about 130 participants(20 countries)
Held in Tokyo;
about 80 participants(15 countries)

FG-DR&NRR

Technical visit to
Sendai

Participants of FG-DR&NRR attended session on
disaster-related eHealth in the afternoon of Feb. 5.

Fig. 1. Schedules of ITU events.

With the aim of ensuring that eHealth standardization proceeds smoothly in the future, ITU-D and
ITU-T held a joint eHealth workshop [2] to provide aforum for dialogue and to exchange information
between their members. In this way, we aim to clarify
the special requirements of developing countries, andto specify the items for future standardization as partof efforts to implement eHealth using advanced technology.
2.2 Overview of the workshop(1) Opening and keynote speeches
Opening speeches were made by Eiichi Tanaka,
Vice-Minister for Policy Coordination of MIC, Japan(Photo 1), and by Sameer Sharma of the ITU AsiaPacific regional office on behalf of the ITU SecretaryGeneral. These were followed by keynote speeches
from Tetsushi Sakamoto, the State Secretary for MIC,
Japan, on the subject of Japan’s eHealth policies, andKiyoshi Kurokawa of the National Graduate Institutefor Policy Studies, who gave a presentation entitled
Global Agenda in Post Fukushima, in which he raised
issues that should be addressed not just by Japan but
by the whole world in the wake of the Great East
Japan Earthquake. This speech stressed the importance of resilience based on the assumption of diverse
risks. It also pointed out that as the world evolves
from the Web 2.0 era into the Web 3.0 era, mobile
devices such as tablet computers will come to play amore important role. Mark Landry of WHO Western
Pacific regional office in the Philippines gave aspeech on behalf of WHO in which he describedsome examples of eHealth policies across Asia, andthe current status of cooperation between WHO andgovernment in Asian countries.
Vol. 11 No. 8 Aug. 2013

Photo 1. O pening speech made by Eiichi Tanaka, ViceMinister for Policy Coordination of MIC, Japan.

(2) Requirements from developing countries
Under the theme of implementing eHealth in a lowresource setting, representatives from India, Sudan,
Uganda, Algeria, the United Arab Emirates, Bangladesh, Vietnam, and Myanmar gave presentations on
the current situation of eHealth in their respective
countries, the issues that need to be addressed, andthe requirements in each case. The requirements ofdeveloping countries are characterized by delayeddevelopment of infrastructure not only for medicalcare but also for insurance, sanitation, and healthmanagement, and by a shortage of healthcare workers
coupled with a poor educational environment. Insteadof the advanced eHealth systems that are being considered in developed countries, these presentations
introduced solutions such as Web-based sharing andeducation of medical information, using videoGlobal Standardization Activities

conferencing to facilitate collaboration between
medical workers including doctors, and using mobile
phones for medical consulting (mHealth), whereby
eHealth is expected to provide a broad range of benefits.
(3)	Items for standardization from developed countries
Representatives from Japan, South Korea, Singapore, and the United States introduced some advanced
examples of eHealth initiatives and discussed thechallenges of implementing eHealth. NTT DATA
gave a presentation introducing use cases of personal
health record (PHR) management and monitoring as
examples of mHealth services in Japan, and stressed
the importance of security and privacy protection.
The representative from Singapore introduced a
Smart TV (television) health management system
based on ITU standards, and showed that interactive
eHealth using TV sets and remote control devices
may be suitable for an aging society since thesedevices can be easily used by elderly people. The US
representative also introduced the importance of considering eHealth for people with disabilities; NICT
introduced the possibility of a body area network(BAN) that people can wear in order to connect tohealthcare equipment; and Fujitsu introduced theresearch of a heart simulator that aims to improvehealthcare technology. These presentations highlighted the need for standardization of the data structures and protocols required for the transmission of
PHR and other data, of the application interfaces andtransmission methods used between medical/healthcare devices and telecommunication networks, wireless devices, and fixed devices, and of security, whichis essential when exchanging PHR data.
(4) eHealth in the event of disasters
The lessons learned after the Great East Japan
Earthquake with regard to the use of eHealth in disaster situations were also introduced. A presentation
was made by a representative of A&D Co., Ltd. on asystem for monitoring health information such as
blood pressure for use in health management of people affected by disasters. This system was actually
put to use after the Great East Japan Earthquake. Professor Isao Nakajima of Tokai University—who isalso the ITU-D vice rapporteur for eHealth and cochairman of this workshop—described eHealth items
that need to be studied in the event of radioactivity
disasters in relation to the nuclear power plant incident. These presentations demonstrated the usefulness of eHealth in the event of a disaster and made acase for the importance of preserving two-way communications.
(5) Future direction of work with the ITU
One of the authors (Kawamori), who also acted asco-chairman of this workshop, drew up the following
summary of the results of this workshop and thefuture direction of eHealth standardization at the
ITU.
-	To promote the spread of eHealth, it is importantto provide education in order to eliminate misconceptions about the circumstances of developing countries.
-	From the viewpoint of standardization, it is necessary to establish cooperation between ITU andrelated organizations with regard to requirements, terminology definitions, and data sets/
applications.
-	In particular, a terminology database is necessary since the technical terminology relating toeHealth covers many fields including medicine,
healthcare, and ICT.
-	For eHealth related regions, it is necessary to
study how this technology can be best applied toelderly patients, disaster victims, and disabled
people.
-	In the future, information should be supplied to
the ITU website including the content of speeches given at this workshop, and an enlightenment
event should be held in cooperation with theWHO.
2.3 Other related events
In addition to the workshop, there were demonstrations related to mHealth by NTT Secure Platform
Laboratories (Photo 2). A simple health management
system was introduced where healthcare equipment
including blood pressure gauges and SMS (shortmessage service) is used to implement mHealth with
low initial investment. These demonstrations drew agreat deal of interest from many countries, including
African nations.
3. The fourth meeting of ITU-T FG-DR&NRR
3.1 B
 ackground and purpose of the fourth
meeting of FG-DR&NRR
In the wake of the Great East Japan Earthquake andTsunami of March 2011, the ITU-T established afocus group (FG) in June 2012 to clarify the role of
ICT in disaster situations and to investigate the need
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Global Standardization Activities

for international standardization at ITU-T [3]. One of
the authors (Araki) was appointed as the chairman of
this FG. This meeting provided a forum to introduce
Japan’s disaster resistance research to the world. In
addition to the usual FG meeting, the event also
included an introduction to eHealth activities needed
during a disaster, demonstrations of disaster-resistant
ICT, and even a technical visit to the region affectedby the Great East Japan Earthquake.
3.2 Overview of the meeting(1) Speeches at special sessions
The Internet Engineering Task Force (IETF) gave apresentation on the provision of information relating
to technical specifications (RFC: Request for Comments) for emergency situations, and demonstrated
technology for transmitting high-priority traffic without delays, which has been proposed as an effective
way of avoiding congestion in a disaster. A representative of the University of Tokyo’s Earthquake
Research Institute proposed an earthquake/tsunami
monitoring system using submarine optical fiber
cables and reported on the installation and planned
operation of a new system off the Sanriku coast (total
length: 120 km, node interval: 25–40 km). NTT
DOCOMO introduced an early warning systemcalled Area Mail that uses CBS (Cell Broadcast Service) to allow alerts and email warning messages tobe transmitted to all mobile terminals in a specified
region. It was agreed that the FG should hold further
discussions on these technologies, and that the
research items should reflect the requirements documents and other deliverables.
(2)	Introduction to the state of disaster response
research in Japan
In Japan, many technologies for strengthening
disaster resistance have been designated for researchand development (R&D) by the MIC, Japan. After theGreat East Japan Earthquake, organizations including
NTT, Tohoku University, SKY Perfect JSAT, NEC,NTT DOCOMO, and KDDI R&D Laboratories proposed technology for the rapid recovery and restoration of telecommunications, which can provide alifeline during a disaster. An emergency communication system for people with disabilities was proposedby the TTC (Telecommunication Technology Committee). In order to share information about this technology and gather information on similar systems inother countries, it was agreed that a liaison statement
would be sent to ITU-D, ITU-R (ITU Radiocommunication Sector), and ITU-T SG16 and JCA-AHF
Vol. 11 No. 8 Aug. 2013

Photo 2. D emonstrations related to mHealth by NTT
Secure Platform Laboratories.

(Joint Coordination Activity on Accessibility and
Human Factors). NTT proposed a way of configuring
networks by assembling and reconfiguring resource
units, and it was confirmed that ongoing discussions
will continue in order to further clarify the requirements for providing people with the means of communication in disaster situations. It was agreed that
this information would be partly reflected in thedeliverables, including the overview and requirements documents.
(3) Updates of the deliverables
The meeting participants discussed the updated
draft overview document based on the results of thismeeting and the previous meeting and added a list oftechnical specifications of IETF emergency/disastercommunications, requirements for disaster message
board and voice messaging services for mobile devices, and requirements relating to technology for theconstruction and reconfiguration of resource units.
The first draft of the requirements document was also
discussed, and the draft table of contents wasapproved. It was also agreed that requirements would
be added for disaster message board services formobile devices and evacuation guidance systems.
3.3 Technical visit to Sendai
This FG included a technical visit to Sendai, including stopovers at NTT offices that had been affectedby the tsunami, and at the NICT’s Resilient ICT
Research Center. It was felt that a visit to the affectedarea during a period of cold snowy weather would
help to highlight people’s fears of disasters and theneed for countermeasures, and would show theGlobal Standardization Activities

Thailand, May 20–24, 2013. In the future, we aim tocontinue holding meetings in countries that haveexperienced major disasters such as floods, hurricanes/typhoons, earthquakes, or tsunamis. Since thelifetime of this FG has been set to one year, it was
confirmed that the extension of this period would be
discussed at the next meeting.
4. Conclusion

Photo 3. Visiting the affected area (Nobiru area).

Photo 4. V isiting NICT’s Resilient ICT Research Center(at Tohoku Univ.).

importance of working towards a rapid recovery aftera disaster occurs. At the NICT center, the visitors
were introduced to the R&D efforts being made by
the MIC, Japan, and by NICT in order to deal withlarge-scale disasters, and they were given the chance
to see the state of research in areas including communication devices and satellite communicationequipment for ensuring regional communications
after a disaster (Photos 3 and 4).
3.4 Future plans
The fifth meeting of the FG-DR&NRR was held ineHealth is a medical care and health management
solution that has been globally recognized as important by developing and developed countries alike. It is
expected that this field will continue to grow in the
future. For its efficient global development, international standards, with appropriate consideration of
the regional characteristics and environmental conditions of each country, are essential. It is hoped that
this workshop will contribute to the expansion ofdeveloping countries, which is the scope of ITU-D, as
well as the further development of ICT standardization, which is the scope of ITU-T.
Disasters such as earthquakes, hurricanes, floods,
tsunamis, and landslides occur all over the world. The
responses to the unprecedented earthquake and tsunami that Japan experienced will be helpful for all thedisaster-affected countries of the world. The establishment of international standards for dealing withdisasters is essential for the construction of a safe
society, so there are great hopes pinned on this FG.
We are sure that the FG meeting in Japan has raised
awareness of standardization efforts relating to disaster response.
References
[1]
[2]
[3]

ITUWHO m-Health.
http://www.itu.int/net/pressoffice/press_releases/2012/77.aspx#.
URNifh0j6So
ITU workshop Tokyo.
http://www.itu.int/en/ITU-T/Workshops-and-Seminars/eHealth/201302/Pages/default.aspx
ITU-T Focus Group on Disaster Relief system, Network Resiliency
and Recover.
http://www.itu.int/en/ITU-T/focusgroups/drnrr/Pages/default.aspx

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Global Standardization Activities

Hideo Imanaka

Senior Manager, R&D Planning Department,
NTT.
He received the B.E., M.E., and Ph.D. degrees
in electrical engineering from Mie University in
1985, 1987, and 2001, respectively. After joining
NTT Telecommunication Network Laboratories
in 1987, he engaged in research on fiber optic
access network architecture and network operation process reengineering methods. From 1996
to 2003, he worked on enterprise resource planning (ERP) systems integration as a consultant in
the Solutions Business Division of NTT Communications. Since 2004, he has been involved in
NGN standardization work at ITU-T. He was the
Rapporteur of Question 1 of Study Group 13 during 2007–2010. He has also played an active rolein IPTV standardization work at ITU-T. He iscurrently in charge of standardization strategy in
the NTT Group. He received the ITU-AJ Award
from the ITU Association of Japan in 2009. He isa member of the Institute of Electronics, Information and Communication Engineers (IEICE)
and the Society of Instrument and Control Engineers.

Noriyuki Araki

Senior Research Engineer, Access Media Project, NTT Access Network Service Systems Laboratories.
He received the B.E. and M.E. degrees in electrical and electronic engineering from Sophia
University, Tokyo, in 1993 and 1995, respectively. He joined NTT Access Network Service Systems Laboratories in 1995. He then worked on
the R&D of operation and maintenance systems
for optical fiber cable networks. Since 2006, hehas been engaged in standardization work foroutside plants in ITU-T SG 6. He has been the
Rapporteur of Question 17 of ITU-T SG 15 since2008. He has also been contributing to the activities of IEC TC86, Fibre Optics, since 2007. He iscurrently serving as the chairman of ITU-T FGDR&NRR. He is a member of IEICE.

Masahito Kawamori

Senior Research Engineer, NTT Service Evolution Laboratories.
He joined the Information Science Laboratory,
NTT Basic Research Laboratories in 1989,
where he first engaged in R&D of artificial intelligence. He then focused on researching multimedia services in the converged environment ofbroadcasting telecommunication and fixed/
mobile systems. Since 2009, he has been the
Rapporteur of Question 28 (Multimedia Framework for eHealth Applications) in SG 16 of
ITU-T. He has spoken at several conferences on
eHealth, including IEEE-ICC, WSIS, and Telecom World, all in 2011. He participated in thecollaboration between ITU-T, ITU-D, and WHO,and in the Joint ITU-WHO Workshop on
e-Health Standards and Interoperability, Geneva,
2012.

Vol. 11 No. 8 Aug. 2013



Practical Field Information about
Telecommunication Technologies

Failure Cases in Access Facilities(Aerial Metallic Communication
Facilities) and Countermeasures
Abstract

In this article, we introduce failure cases in access facilities (aerial metallic communication facilities)
and countermeasures to them. This is the eighteenth in a bimonthly series on the theme of practical field
information on telecommunication technologies. This month’s contribution is from the Access Engineering Group, Technical Assistance and Support Center, Maintenance and Service Operations Department,
Network Business Headquarters, NTT EAST.

1. Introduction
Metallic communication facilities can be said to be
characterized by the following key features: 1) a huge
number of facilities are currently installed in thefield; 2) many years have passed since their construction; and 3) they are easily affected by weather conditions and the peripheral environment. Even today, in
an era in which optical broadband facilities havebecome the shining star of the information-communications infrastructure, metallic communication facilities continue to play an important role as an information-communications infrastructure connecting customers. They must therefore operate effectively andachieve and maintain a certain level of service quality.
With an eye to constructing and maintaining facilities that are robust against extreme weather conditions and the peripheral environment, we present here
failure cases characteristic of aerial metallic communication facilities that have been brought to the attention of the Technical Assistance and Support Center,
and we explain their causes and countermeasures(Fig. 1).
2. Features of aerial metallic communication
facilities and characteristic failures
Metallic communication facilities extend from theexchange (NTT building) all the way to the custom

Lightning29(9%)
Structures
50
(15%)

IP/optical
119
(35%)

338 inquiries in total (2011)

Legacy 140 (41%)

38 cases related to metallic
communication facilities
IP: Internet protocol

Fig. 1. N umber of inquiries made to the Technical Assistance and Support Center regarding characteristic
failures (FY2011).

er’s terminal via aerial and underground metallic
communication facilities.
The aerial metallic communication facilities targeted here for introducing failure cases are broadly
divided into (1) cable intervals and (2) connection
sections. The features and typical failures of both arepresented in the following (Fig. 2).
NTT Technical Review

Practical Field Information about Telecommunication Technologies(1) Cable interval(2) Connection section(a) Outer sheath cracking caused by dancing(c) Wire corrosion caused by degraded insulation coating(b) Damage caused by trees(d) Poor insulation due to poor terminal processing
Cable interval

Connection section

Underground metallic
communication facilities
Aerial metallic communication facilities

Fig. 2. Examples of failures in aerial metallic communication facilities.

2.1 Cable interval
Since the cable interval occupies most of the aerial
metallic interval in terms of length of cable laid, thecable itself incorporates a certain degree of weather
resistance. However, since it is naturally exposed tothe peripheral environment, failures can occur such as
those caused by strong winds or other types ofinclement weather or by contact with trees in theimmediate area.
At the initial stage of a cable-interval failure, there
is often no effect on the quality of communications.
This, combined with the fact that the location of sucha failure is difficult to isolate, means that damage may
have already spread once degraded communication
service becomes noticeable. Although periodic patrols or surveys to check on facility conditions need
to be performed, they require considerable work—
preventive maintenance in itself is beset with problems.
2.2 Connection section
The connection section is an interval limited to thecable terminal box, but it includes coated wires inwhich the cable’s outer sheath is removed as well asbare conductors (copper wires) in hand-soldered connectors. It is consequently easy for the connection
section to be affected by temperature, humidity, seasalt particles, and other environmental conditions.
Characteristic failures include humidity-related corrosion in wires and faulty insulation caused by deterioration of the insulator.
Similar to the cable interval, a failure in the connection section will often have no effect on communicaVol. 11 No. 8 Aug. 2013

tions quality in its initial stage. However, as this section is also easily affected by weather conditions,deterioration across the entire area in question may
have progressed by the time that facility degradation
becomes noticeable. Furthermore, in contrast to thecable interval, failures may also be caused by maintenance work.
3. Failure cases in the cable intervaland countermeasures
Here, we present examples of failures in aerial
metallic communication facilities reported from various regions around Japan, and the countermeasurestaken.
3.1 Outer sheath cracking (ring-shaped cut)
In regions affected by strong winds, dancing* (Fig. 3)
in a self-supporting (SS) cable, especially a smalldiameter one, may cause the outer sheath to crack(forming a ring-shaped cut) at the position where thecable is separated from its support line.
Outer sheath cracking in a color coded polyethylene (CCP) cable due to a ring-shaped cut is shown in
Photo 1, and the mechanism generating this cracking
is shown in Fig. 4. Along seashores or other places
where there are no structures (intermediate pillars) toshield the cable from wind, the cable’s outer sheathcan be damaged in a circumferential direction at the* dancing: A phenomenon in which an SS cable vibrates intensely fromthe horizontal plane due to continuous natural oscillation generated
by strong wind.

Practical Field Information about Telecommunication Technologies

Flat terrain along a seashore with no structures
to shield the cable from wind

Mountainous area with no structures
to shield the cable from wind

Wind

Wind

Sea

Fig. 3. Types of terrain conducive to dancing phenomenon in cables.

Magnified view of outer-sheathcross section

Photo 1. Outer sheath cracking by ring-shaped cut.

Vibration(1) Dancing occurs in cable.
Support line
(2) Strain occurs where cable and support line
separate due to vibration.

CCP-SS
Separated fromsupport line

Separated fromsupport line(3) Vibration fatigue builds up in aluminium tapeof laminated sheath.
Vibration

Location where outer sheath cracking occurs(Strain occurs due to vibration.)(4) Cracks occur in aluminium tape of laminatedsheath resulting in the cable’s outer sheathbecoming damaged from the inside.

Fig. 4. Generation of outer sheath cracking in CCP-SS cable.



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Install binding so that the gap between the support line and cable
does not fluctuate before and after the binding.
Binding stringLine guard
Support line

CCP–SS

Cable
Bind from the upper line guard(six times vertically and as manytimes as needed horizontally).

(1) Use cable binding string to bind the cable to the end of the upper line guard.
(2) Wrap binding string six times vertically and as many times as needed horizontally so that the gap between the support line
and cable does not fluctuate before and after the binding.

Fig. 5. R ing-shaped cut countermeasure using cable binding string.

position where the cable is separated from the support
line. This damage features circular fracture marksthat are similar in appearance to annual growth ringsin trees.
Countermeasures
Apply cable binding string to existing cableThis countermeasure can be applied to cable routes
on which outer sheath cracking has previously
occurred in CCP-SS cables because of strong winds
or other factors or on routes where such damage islikely to occur. The application of cable binding
string disperses the maximum strain occurring at theposition where the cable separates from the support
line and therefore controls the cable’s degree of freedom at the section where it is suspended at the utility
pole. This produces a strain-easing effect, which iswhy binding using cable binding string is important(Fig. 5).
Use high strength (HS) cable when installing new
cable (or upgrading existing cable)
The use of high-strength CCP-HS-SS cable reinforced with a stainless-steel layer is an effective
countermeasure against outer sheath cracking when
installing new cable or upgrading existing cable whenconditions (A) or (B) below apply (Photo 2).
Conditions for applying HS cable:(A)	The route is one on which cable outer sheathdamage has occurred in the past because ofdancing.
(B)	The route is one that satisfies all three of the following conditions, indicating that outer sheathcracking may occur in the future:
Vol. 11 No. 8 Aug. 2013

Stainless steel layer

Photo 2. Appearance of HS cable(1)	The area along the route has an annual averagewind speed of 2 m/s or greater (according todata from the local meteorological weather
station).
(2)	In that area, an average wind speed of 7 m/s orgreater blows in the same direction for at least
half a day from fall to spring over the longterm.
(3)	Dancing is likely to occur at locations on theroute where wind with the speed described in(2) blows with no shielding structures.
String cable through ring fixtures
In addition to the aforementioned methods, suspending the cable through ring fixtures can suppress
cable dancing and prevent ring-shaped cuts.
3.2 Cable outer sheath damage
A cable can come into contact with a tree branchPractical Field Information about Telecommunication Technologies

Corroded wiring caused bywater penetration at damaged
location
Cable damage caused by a tree

Photo 3. Example of cable damage from a tree.

High

Low

Number of disconnections

Disconnection failures in cables at points
below damaged location
High

Low

5

Damaged location

4
3
2
1
0

6

4

2

0

2

4

6

(m)

Distance

Fig. 6. Disconnection failure in cables on inclined routes.

that rubs up against the cable’s outer sheath in such a
way that even core wires can be negatively affected.
An example of cable damage caused by a tree isshown in Photo 3. Although the search for faulty corewires along the span of a suspended cable can be narrowed down through a standard conduction check,isolating the exact failure location often requires avisual inspection and a great deal of labor. Some phenomena have been reported in which the damage issuch that the problem can only be resolved by replacing the cable and not by simply repairing the damaged location. For example, a cable installed on an
inclined route will naturally have a height differencealong its span, and it has been found that core wires
on the side below the damaged location can become
corroded and generate disconnection failures(Fig. 6).
Countermeasures
Some form of protection may be applied in loca

tions where cables and trees come into contact witheach other, or trees in the immediate area may be cut
down as a preventive measure against outer sheathdamage.
4. Failure cases in the connection section
and countermeasures
4.1 Core wire corrosion (insulation fractures)Core wire insulation (polyethylene) in CCP cables
can deteriorate for various reasons including hightemperatures, humidity, and sea-salt particles that get
inside a terminal box. Furthermore, infiltration ofultraviolet light into a terminal box through gaps
caused by deformation in the terminal-box cover orside panels resulting in poor sealing—which was
rarely seen in older types of terminal boxes—canaccelerate such insulation degradation. Moreover,fractures have become quite noticeable in core wires
in non-black colors that absorb a large portion of
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Degradation degree 1
Degradation degree 1
Degradation degree 3
Degradation degree 4

Degradation degree 5

Photo 4. Classification of core wire degradation.

Photo 5. Humidity controlling materials.

Not wrapped aroundsupport line〈〈Photo taken from the directionof the cable〉〉

Photo 6. C ountermeasure to core-wire insulation faults using humidity controlling materials.

Inserted wires

InsertionsectionPhoto 7. Repair technique using a life-extending terminal box.

ultraviolet light (Photo 4).
Countermeasures
The use of humidity controlling materials and lifeextending terminal boxes can be used as countermeasures to core wire corrosion. Humidity controllingmaterials adjust the peripheral humidity and prevent
condensation on core wires, which makes them effective in suppressing insulation degradation and failures. They are applicable to insulation that has aVol. 11 No. 8 Aug. 2013degree of degradation of 1 or 2. In the event of corewire insulation having a degree of degradation of 3,4, or 5, the existing terminal box can be replaced witha life-extending terminal box and wires inserted
accordingly to ensure long-term reliability. Humidity
controlling materials are shown in Photo 5, theimplementation of a countermeasure to core-wireinsulation faults using humidity controlling materials
is shown in Photo 6, and a repair method using a lifeextending terminal box is shown in Photo 7.
Practical Field Information about Telecommunication Technologies

4.2 F aults in terminal processing section (sleevesection)Faults in core-wire insulation in the terminal sleeve
include:(1)	Hardening faults due to insufficient stirring of
the liquid A/B epoxy resin, which is used toprotect the edges of core wires (naked copperwire) against humidity or other materials thatcan cause corrosion or unwanted contact
between wires(2)	End of core wires coming into contact with thebottom of the sleeve(3)	Water penetration due to direction errors at the
top and bottom of the sleeve

Examples of processing the terminal sleeve areshown in Photo. 8.
Countermeasures
A schematic of terminal sleeve processing is shownin Fig. 7. The following points should be kept in mind
when processing the terminal sleeve:(1)	Stir liquid A/B epoxy resin well so that no unreacted hardening agents remain.
(2)	Let the end of core wires float about 1 cm fromthe bottom of the terminal sleeve.
(3)	Turn the terminal sleeve upside down immediately after processing so that its bottom is oriented upward.
5. Concluding remarks

Correct processing

Incorrect processing→Wires touch the bottom of thesleeve, causing mutual conduction.

Photo 8. Examples of terminal sleeve processing.

This article introduced failure cases in aerial metallic communication facilities and countermeasures tothese failures. These types of facilities are affected byweather conditions and the peripheral environment,
and failures can occur for a variety of reasons. It isconsequently very difficult to prevent such failures
before they occur or to efficiently implement countermeasures to failures that have occurred. The AccessEngineering Group of the Technical Assistance andSupport Center hopes that the information provided
here on failures and their countermeasures is useful ina small way to frontline personnel in facility management and in improving the maintenance of service
quality.

Orient bottom of
sleeve upward.
0.5
Mixture1.5

Terminal processing sleeve

5.0

2.0

No. 2 PVC protection tape(half-pilling, covered wrapping)

2.0

No. 4 sealing tape (wrapped around cable
just under the edge of the terminal sleevewith a diameter equal to or greater than theterminal sleeve)(unit: cm)
PVC: polyvinyl chloride

Fig. 7. Schematic of terminal sleeve processing.

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NTT Technical Review

External Awards
The IFIP/IEEE International Symposium on Integrated Network Management (IM 2013) Best Paper Award
Winners: Noriaki Kamiyama and Ryoichi Kawahara, NTT Network
Technology Laboratories
Date: May 30, 2013
Organization: IEEE Communications Society, IFIP
For “Analysis of Content Charge by ISPs”.
When rich content is delivered, a huge amount of traffic is transmitted over the network. For ISPs, the increased investment costrequired to maintain stable quality is a problem. An effective way toaddress this problem is content charge, in which ISPs charge content
providers for each delivery of content. This work models the business
relationship between ISPs and content providers, and we investigate
the conditions for introducing content charge by ISPs.
Published as: N. Kamiyama and R. Kawahara, “Analysis of Content
Charge by ISPs,” Proc. of the IFIP/IEEE International Symposium on
Integrated Network Management, IM 2013, Ghent, Belgium, May
2013.

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NTT Technical Review