NTT Technical Review

October 2019 Vol. 17 No. 10

Feature Articles: Creating Novel Functional Materials


Overview of Novel Materials Creation Research at NTTCreation of Novel Material Sr3OsO6 with the Highest Ferromagnetic Transition Temperature
among Insulators

Magnetically

PurifiedErbium-doped

Oxide

Crystal—Towards

CreatingaQuantum-information-manipulation Platform


MBE Growth and Element-distinctive Atomic-resolution Characterization of High Temperature
Superconductors

High-quality Atomic-layer Materials Fabricated by Chemical Vapor Deposition

Development of Next-generation Wide-bandgap Semiconductors



Crystal Growth of Wurtzite GaP Nanowires for Solar-water-splitting Devices

Regular Articles
High Power and High Efficiency Operation of Semiconductor Optical Amplifier Assisted
Extended Reach Electroabsorption Modulated DFB Laser (AXEL) for Extension ofTransmission Distance

Global Standardization Activities


Surviving in the Digital Transformation Era; Technical Trends and Issues from the Perspectiveof The Telecommunication Technology Committee

Practical Field Information about Telecommunication Technologies
Introduction to Troubleshooting Cases Related to Telephone Systems in Customer Premises

External Awards/Papers Published in Technical Journals and Conference Proceedings

Feature Articles: Creating Novel Functional Materials

Overview of Novel Materials Creation
Research at NTT
Hideki Yamamoto and Hideki Gotoh
Abstract

The activities of NTT Basic Research Laboratories include creating novel materials with fascinating
functions through materials design and arrangement control of atoms, molecules, and crystals in materials. The purpose is to make progress in materials science and eventually contribute to the developmentof information and communication technology. While our research covers a vast range of materials, we
introduce some of the latest research accomplishments in inorganic materials, for example, oxides andnitrides, in the Feature Articles in this issue. In particular, we highlight thin films (less than a micrometer
thick), atomic layer materials (less than a nanometer thick), and nanowires.
Keywords: creation of novel materials, functionalization, device fabrication

1. Significance of research on novel
functional materials
The development of novel functional materials has
been contributing to advances in fundamental science
as well as the development of new and/or high-performance devices; the latter has sometimes brought
about breakthroughs even in the design concept ofdevices and has had a substantial impact on our societies. Examples of such novel materials include
superconducting oxides (Nobel Physics Prize in1987); fullerenes, a football-like structure comprising
60 carbon atoms (Nobel Chemistry Prize in 1996);
graphene, an extremely thin graphite layer of singleatom thickness (Nobel Physics Prize in 2010); andnitride semiconductors, the essential constituents ofblue light-emitting diodes (LEDs) (Nobel Physics
Prize in 2014). In fact, some alternative materials
have been proposed to overcome the limits of miniaturization in silicon (Si)-based integrated circuits(Moore’s law limit) [1]. Another good example is InP(indium phosphide), rather new compared with Si
and gallium arsenide (GaAs), which is essential for
ultrahigh-speed-operable optical devices in opticalcommunications [2].
The development of novel functional materials andfabrication of devices exploiting them are always
NTT Technical Review

Vol. 17 No. 10 Oct. 2019

challenges, and one has to start with basic research.
To achieve designated but yet-unrealized functions,
two approaches are possible. One is to create/synthesize brand-new materials/compounds, and the other is
to improve the quality of some already existing materials and alternately stack them followed by processing in the nanometer range.
Tremendous efforts have been made here at NTT
Basic Research Laboratories using both approaches.
We have been exploiting our unique and state-of-theart apparatuses and technologies for materials creation, which, at least partially, we developed throughour own efforts. In the Feature Articles in this issue,
we describe our latest accomplishments in such materials science research. Because of space limitations,
we regrettably had to omit recent developments inbio-soft materials research previously featured in2016 [3].
2. Materials creation using thin-film
synthesis methods
In this section, we explain what thin-film specimens are and how they are prepared.
2.1 Thin-film specimens
The phrase synthesis of novel materials may bring1

Feature Articles

to mind reactions of reagents in beakers and flasks aswell as calcination and sintering of various ingredientpowders in furnaces. However, our approach involves
synthesis by means of thin-film growth methods.
Generally, thin films are specimens whose thicknesses are in the range between 0.1 nm (atomic
monolayer) and 10 µm; here, 1 nm is 1/1000 µm, and1 µm is 1/1000 mm. Researchers and experts use theterm growth for the preparation of films, as our thinfilm specimens are grown on single-crystalline substrates in an atomic- or molecular-layer-by-layermanner, leading to single-crystalline thin films. Notethat solids in which atoms are periodically and orderly arranged and thus form lattices are called crystals,
and crystals that have only a single atomic arrangement over an entire volume are called single crystals.
2.2 Thin-film growth methods
We have mainly adopted two kinds of thin-film
growth methods: molecular beam epitaxy (MBE) andmetal-organic vapor phase epitaxy (MOVPE); theterm epitaxy comes from the Greek roots epi, meaning above and taxis, meaning an ordered manner. In
MBE, thin films are formed on heated single-crystalline substrates in an ultrahigh vacuum (UHV) chamber (~10 trillion times lower than the atmosphericpressure) by reaction between the constituent elements that are supplied as atomic or molecularbeams. In MOVPE, thin films are also formed onheated single-crystalline substrates but in a reactor
furnace, in which metal-organic substances containing the constituent cations are flowed with a carriergas, for example, H2 (hydrogen gas) or N2 (nitrogengas), along with an anion source gas, for example,
NH3 (ammonia gas). The latter method belongs to abroader category called chemical vapor deposition(CVD), where the metal-organic sources are not necessarily used.
Both methods were originally developed for growing high-quality thin films of known materials andalso for fabricating junctions and superlattices consisting of various known materials. To search forbrand-new materials that are non-existent in nature,
MBE is the more suitable method, as the film growth
proceeds under conditions far from the thermodynamic equilibrium. Actually, as exemplified in the
Feature Articles in this issue, MBE is used to fabricate the brand-new ferromagnetic material Sr3OsO6(Sr: strontium, Os: osmium, O: oxygen) [4], as well
as CaCuO2 (Ca: calcium, Cu: copper, O: oxygen)having the infinite-layer structure (IL-CaCuO2) [5],c-BN (cubic-structured boron nitride) [6], and lightly
NTT Technical Review

Vol. 17 No. 10 Oct. 2019

erbium-doped CeO2 (cerium oxide) thin films [7]; ILCaCuO2 and c-BN are thermodynamically metastable, and high pressure is required to synthesize themin bulk forms.
In contrast, in MOVPE and CVD, thin-film growth
proceeds under conditions close to the thermodynamic equilibrium, and, hence, dislocation density inthe thin films can be reduced. This means that highcrystalline-quality films are rather readily achieved.
Accordingly, MOVPE has been widely exploited in
the fabrication of nitride-based light-emitting devices
and transistors, for example, GaN (gallium nitride)based devices. Two of the Feature Articles in thisissue explain the fabrication of atomic-layer-thickgraphene and h-BN (hexagonal boron nitride) by
CVD [8] and GaP (gallium phosphide) nanowires by
MOVPE [9].
3. MBE as a synthesis method sui generis forbrand-new materials
NTT Basic Research Laboratories is one of themost advanced research organizations in the world, inthat some brand-new materials have been synthesizedusing MBE. MBE is a thin-film growth techniquedeveloped in the late 1960s to crystallize and investigate GaAs epilayers and has been applied to growthin epitaxial structures made of various known materials. Here, we start with an explanation of how theunique characteristics of our MBE apparatuses allow
for the creation of novel materials. A schematic illustration of our MBE set-up sui generis, which is
totally tuned up for the growth of multi-cation oxides(complex oxides) [10], is shown in Fig. 1.
One of the unique features of our MBE system isthat the flux rates of multiple cations supplied at a
certain moment are simultaneously monitored andthe results are fed back in real time to the power supplies of the evaporators, which allows for precise
control and stable supply of the atomic fluxes of eachconstituent cation; the flux rates are monitored byelectron impact emission spectroscopy (EIES). In
EIES, atomic fluxes are impacted with accelerated
electrons, and each flux emits light of characteristic
wavelengths. Although the intensity of the emitted
light is too weak to detect with the human eye, the
EIES equipment can measure the intensity of theemitted light in a wavelength-specific manner andtransform the optical signals into electrical ones. In
this manner, one can obtain information on what andhow many atoms are supplied.
It is worth mentioning that the principle of EIES
2

Feature Articles

EIES
sensorEvaporant
fluxesEIES
HeaterSubstrate

Electrons

EIES
QCM 1

ED
Shutter

RHEED
image

RHE

Emittedlight

FilamentO(atomic oxygen)

O3(ozone)PlasmasourceDetectors(PMTs)QCM 2
Movable10 sources

6 electron
guns
UHV
chamber

Wavelength filters
MovablePC
Signal processor and microcontroller

For real-time feedback control

EIES: electron impact emission spectroscopy
PC: personal computer
PMT: photomultiplier tube

Evaporation source power supply

QCM: quartz crystal microbalance
RHEED: reflection high-energy electron diffraction

Fig. 1. Schematic illustration of our MBE system for growing complex oxide thin films.

takes after that of a flame reaction, which may havebeen learned in one’s high school days. Instead ofbeing impacted with electrons, a flame reaction takes
place when materials are heated in a flame. In doing
so, light with element-specific wavelengths is emitted, for example, greenish yellow light for Ba (barium) and bright red light for Sr (strontium). The flame
reaction is utilized in fireworks, where one can enjoy
multiple combinations of beautiful colors achievedby various combinations of pyrotechnic materials.
Another characteristic feature of our MBE systemis the capability of introducing reactive atomic oxygen (O) or ozone (O3) gas, which enables oxidationof the cation species under UHV; typically, oxygen
exists as O2 (oxygen molecules). We can also grownitride thin films, for example, c-BN, if the oxygengas source is replaced by an atomic nitrogen (N) gas
source [11].
Furthermore, modern MBE systems are equippedwith a reflection high-energy electron diffraction(RHEED) capability, which is also the case with ours
and allows for real-time monitoring of crystal structure and crystallinity of thin films during the growth.
In RHEED, a high-energy electron beam is incident
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Vol. 17 No. 10 Oct. 2019

to the sample surface under grazing incident angle,
and hence, this method is surface sensitive. As theoperation of RHEED requires a high vacuum, RHEED
is a method highly compatible with MBE. Note thatin our labs, we can also monitor thin-film growth inreal time with MOVPE using a method other than
RHEED. These advantages of our MBE system haveenabled us to discover novel superconductors [12, 13]
and a magnetic material [14], while some important
growth parameters such as the flux ratio of cations,substrate materials, substrate temperature, and oxidation strength are systematically varied.
Outstanding developments in measurement techniques and equipment are another important factor
that has enabled materials science research using
thin-film specimens. Nowadays, cutting-edge characterization methods can be applied to thin-film specimens due to their sufficiently high sensitivity. Since
the thicknesses of thin-film specimens are limited to,at most, a few hundred atomic/molecular layers,
detailed investigation of their properties had been
elusive until a couple of decades ago even when one
succeeded in synthesizing new materials in thin-film
forms. In stark contrast, atomic-resolution scanning3

transmission electron microscopes as well as magnetometers with an ultrahigh sensitivity adequate forinvestigating the magnetic properties of 300-nmthick specimens have been developed and exploitedin our daily research, as represented by the accomplishments presented in these Feature Articles. The
use of thin-film specimens in the search for novel
materials has the following advantages as comparedwith bulk specimens: less consumption of reagents(less use of natural resources), higher-throughput
screening, and higher compatibility with device fabrication processes.

Far-UV light

Feature Articles

Metalcontact
Semitransparent contact
Hole

Far-UV lightElectron

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Vol. 17 No. 10 Oct. 2019

p-type AlN
AlN
light-emitting layern-type AlN
Metal contact

4. Functionalization: from material synthesis todevice fabrication
To achieve devices that exhibit designated functions by exploiting various properties inherent to
individual materials, just the existence of certain
materials or the availability of their thin films is notenough. We explain here the functionalization anddevice fabrication processes using an aluminium
nitride (AlN)-based LED as an example of a typical
semiconductor device (Fig. 2). The AlN-based diodes
emit light with the shortest wavelength among any
solid-state semiconductor-based light sources, as has
been described in a previous NTT Technical Review
article [15].
The AlN-based LEDs are prepared by MOVPE(Fig. 3). The existence of the chemical substance,
AlN, had been well-known, and accordingly, AlN
itself is not a new material. However, to make AlN
usable as a functional semiconducting material
requires the accumulation of a vast amount ofresearch, which is comparable to that for the creationof new materials.
First, the crystalline quality of AlN itself needs tobe substantially improved (Phase I). This is because
the emergence of inherent functionality is hinderedwhen (thin-film) crystals include polycrystalline portions and/or a high density of defects. Next, doping
control is required (Phase II). To fabricate LEDs, ormore generally, some semiconductor-based devices,
it is necessary to deliberately introduce heterovalent
impurities into pristine materials (here, AlN) andachieve a situation in which positive and negative carriers (holes and electrons, respectively) exist in different regions. Accordingly, in phase II, research
efforts are focused on how to achieve effective doping
without dramatically deteriorating the crystallinity ofthe thin films. Finally, the design and fabrication of
stacking structures (schematically shown in Fig. 2)

p-type superlatticen-type superlatticeHigh-quality AlN epilayer
Substrate
UV: ultraviolet

Fig. 2. S chematic illustration of the structure of AIN-basedLED.

need to be investigated (Phase III). For example, one
should optimize the thickness of each stacking layer
and the number of repetition periods in the superlattice layers. In addition, the stacking layers are lithographed and etched until device structures exhibiting
certain functions are eventually completed.
5. Towards further creation of novel
functional materials
As described above, materials science researchrequires vast amounts of time and effort as well as
long-term strategies [16]; it follows the saying,“Endurance makes you strong.” This is especiallytrue for research on the creation of hitherto-undiscovered materials but with fascinating properties. Such
research may be likened to Phase Zero, and is sometimes compared to a journey for seeking the Holy
Grail, borrowing the phrase of Prof. Emeritus M. R.
Beasley of Stanford University.
Nevertheless, materials science research is a
dreamful activity that inherently has the potential foryielding something exciting, as far as taking methodologically correct approaches. For example, on theassumption that a useful room-temperature superconductor is discovered, it will lead to disruptive innovation
4

Feature Articles

CH3
CH3 AlCH3

Hydrogen purifier(purity: 99.99999%)

H
H N
H

Al(CH3)3 + NH3

MFC



AI N
AlN + 3CH4
Reactor furnace

H2

AlN thin film

TMA

Substrate

NH3
MFC

HeaterMFC
ValvesExhaustAmmonia[NH3]

Metal-organic
compound[Al(CH3)3]

TMA: trimethylaluminium [Al(CH3)3]
MFC: mass flow controller

Fig. 3. Schematic illustration of our MOVPE system and chemical reaction during the growth of AlN.

[17] that allows for lossless direct current power feeding as well as lossless intra- and inter-chip connections.
Materials science research has relied on the three
conventional paradigms of science: experiment, theory, and computation. Individual materials are first
synthesized and characterized. Their molecular/crystal/electronic structures are subsequently determinedwith the aid of theoretical and computational studies.
Finally, functional structures and/or devices aredesigned and fabricated. Recently, a fourth paradigm
called materials informatics (MI) has emerged, where
the compositions and structures of materials having
desired functions are predicted by utilizing machinelearning techniques along with materials databases
and/or theoretical calculations [18]. While further
progress in MI is desirable to see its bona fide capability and potential, we hope to promote our materials
science research further by combining our advantages of thin-film synthesis technologies already in hand
and the newly developed approach, MI.
References
[1]

[2]

H. Hasegawa, M. Akazawa, A. Domanowska, and B. Adamowicz,“Surface Passivation of III-V Semiconductors for Future CMOS
Devices—Past Research, Present Status and Key Issues for Future,”
Appl. Surf. Sci., Vol. 256, No. 19, pp. 5698–5707, 2010.
M. Nagatani, H. Wakita, Y. Ogiso, H. Yamazaki, M. Ida, and H.

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Vol. 17 No. 10 Oct. 2019

[3]
[4]

[5]

[6]

[7]

[8]

[9]

Nosaka, “Ultrahigh-speed Optical Front-end Device Technology for
Beyond-100-GBaud Optical Transmission Systems,” NTT Technical
Review, Vol. 17. No. 5, pp. 27–33, 2019.
https://www.ntt-review.jp/archive/ntttechnical.php?contents=
ntr201905fa4.html“Feature Articles: Forefront Research on Bio-soft Materials,” NTT
Technical Review, Vol. 14, No. 8, 2016.
https://www.ntt-review.jp/archive/2016/201608.html
Y. K. Wakabayashi, Y. Krockenberger, Y. Taniyasu, and H. Yamamoto,“Creation of Novel Material Sr3OsO6 with the Highest Ferromagnetic
Transition Temperature among Insulators,” NTT Technical Review,Vol. 17, No. 10, pp. 7–11, 2019.
https://www.ntt-review.jp/archive/ntttechnical.php?contents=
ntr201910fa2.html
A. Ikeda, Y. Krockenberger, Y. Taniyasu, and H. Yamamoto, “MBE
Growth and Element-distinctive Atomic-resolution Characterizationof High Temperature Superconductors,” NTT Technical Review, Vol.
17, No. 10, pp. 18–23, 2019.
https://www.ntt-review.jp/archive/ntttechnical.php?contents=
ntr201910fa4.html
K. Hirama, Y. Taniyasu, H. Yamamoto, and K. Kumakura, “Development of Next-generation Wide-bandgap Semiconductors,” NTT
Technical Review, Vol. 17, No. 10, pp. 30–35, 2019.
https://www.ntt-review.jp/archive/ntttechnical.php?contents=
ntr201910fa6.html
T. Tawara and T. Inaba, “Magnetically Purified Erbium-doped Oxide
Crystal—Towards Creating a Quantum-information-manipulation
Platform,” NTT Technical Review, Vol. 17, No. 10, pp. 12–17, 2019.
https://www.ntt-review.jp/archive/ntttechnical.php?contents=
ntr201910fa3.html
S. Wang and Y. Taniyasu, “High-quality Atomic-layer Materials Fabricated by Chemical Vapor Deposition,” NTT Technical Review, Vol.
17, No. 10, pp. 24–29, 2019.
https://www.ntt-review.jp/archive/ntttechnical.php?contents=
ntr201910fa5.html
K. Tateno and K. Kumakura, “Crystal Growth of Wurtzite GaP
Nanowires for Solar-water-splitting Devices,” NTT Technical5Feature Articles

Review, Vol. 17, No. 10, pp. 36–41, 2019.
https://www.ntt-review.jp/archive/ntttechnical.php?contents=
ntr201910fa7.html[10] H. Yamamoto, Y. Krockenberger, and M. Naito, “Augmented Methods
for Growth and Development of Novel Multi-cation Oxides,” Proc. of
SPIE OPTO, Vol.8987, 89870V, San Francisco, CA, USA, Feb. 2014.
[11] K. Hirama, Y. Taniyasu, S. Karimoto, H. Yamamoto, and K. Kumakura, “Heteroepitaxial Growth of Single-domain Cubic Boron Nitride
Films by Ion-beam-assisted MBE,” Appl. Phys. Express, Vol. 10, No.
3, 035501/1-4, 2017.
[12] H. Yamamoto, Y. Krockenberger, and M. Naito, “Epitaxial Growth ofSuperconducting Oxides,” Epitaxial Growth of Complex Metal
Oxides, pp. 95–127, Woodhead Publishing, Cambridge, 2015.
[13] M. Naito, Y. Krockenberger, A. Ikeda, and H. Yamamoto, “Reassessment of the Electronic State, Magnetism, and Superconductivity inhigh-Tc Cuprates with the Nd2CuO4 Structure,” Physica C, Vol. 523,
pp. 28–54, 2016.
[14] Y. K. Wakabayashi, Y. Krockenberger, N. Tsujimoto, T. Boykin, S.

Hideki Yamamoto

Senior Research Scientist, Supervisor, Executive Manager of Materials Science Laboratory,
NTT Basic Research Laboratories.
He received a B.S., M.S., and Ph.D. in chemistry from the University of Tokyo in 1990, 1992,
and 1995. He joined NTT in 1995. His principal
research fields are thin film growth, surface science, and superconductivity. He was a visiting
scholar at the Geballe Laboratory for Advanced
Materials, Stanford University, USA (2004–
2005). He received the 2nd Young Scientist Presentation Award (1997) from the Japan Societyof Applied Physics (JSAP) and the 20th Superconductivity Science and Technology Award(2017) from the Forum of Superconductivity Science and Technology, the Society of Non-traditional Technology. He is a member of JSAP, thePhysical Society of Japan, the Japan Society of
Vacuum and Surface Science, the American
Physical Society, and the Materials ResearchSociety.

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Vol. 17 No. 10 Oct. 2019

Tsuneyuki, Y. Taniyasu, and H. Yamamoto, “Ferromagnetism above
1000 K in a Highly Cation-ordered Double-perovskite Insulator
Sr3OsO6,” Nat. Commun, Vol. 10, 535, 2019.
[15] Y. Taniyasu and M. Kasu, “Improved Emission Efficiency of 210-nm
Deep-ultraviolet Aluminum Nitride Light-emitting Diode,” NTT
Technical Review, Vol. 8, No. 8, 2010.
https://www.ntt-review.jp/archive/ntttechnical.php?contents=
ntr201008sf2.html[16] I. Yokohama, “Materials Research Activities,” NTT Technical
Review, Vol. 8, No. 8, 2010.
https://www.ntt-review.jp/archive/ntttechnical.php?contents=
ntr201008sf1.html[17] C. M. Christensen, “The Innovator’s Dilemma: When New Technologies Cause Great Firms to Fail,” Harvard Business Review Press,
Boston, 1997.
[18] T. Lookman, F. J. Alexander, and K. Rajan, “Information Science forMaterials Discovery and Design,” Springer, Cham, 2016.

Hideki Gotoh

Director, NTT Basic Research Laboratories;
Executive Manager of Optical Science Laboratory, NTT Basic Research Laboratories.
He received a B.S., M.S., and Ph.D. in engineering from Hiroshima University in 1991,
1993, and 2000. Since joining NTT Basic
Research Laboratories in 1993, he has beenworking on optical physics and device applications of semiconductor nanostructures. He was avisiting scientist at the University of Illinois at
Urbana-Champaign in 2006. He is a member of
JSAP and the Optical Society.

6Feature Articles: Creating Novel Functional Materials

Creation of Novel Material Sr3OsO6
with the Highest Ferromagnetic
Transition Temperature among
Insulators
Yuki K. Wakabayashi, Yoshiharu Krockenberger,
Yoshitaka Taniyasu, and Hideki Yamamoto
Abstract

We have synthesized a novel material, Sr3OsO6 (Sr: strontium, Os: osmium, O: oxygen) using a uniqueoxide thin-film growth technique that has been developed over many years at NTT Basic ResearchLaboratories. The Curie temperature (TC) value of this material, estimated from the magnetic measurements, is above 780°C, surpassing the TC record among insulators for the first time in 88 years by morethan 100°C. As Sr3OsO6 has been synthesized in a single-crystalline thin film form, this brand-new material is expected to be readily implemented in high-performance magnetic device applications such as
magnetoresistive random access memories and magnetic sensors that work above room temperature.
Keywords: molecular beam epitaxy, magnetic oxide, spin-orbit coupling

1. Introduction
Ferromagnetism is a magnetic state in a material
that gives it the properties of a magnet. In the ferromagnetic state, the net magnetic moment is large
since the magnetic moments of the constituent atoms
are aligned (Fig. 1). A ferromagnetic insulator is amagnet in which electric current cannot flow due toits high resistivity. Ferromagnetic insulators include
maghemite, the first magnet that humans discovered
and used as a compass. Today, ferromagnetic insulators are widely used as permanent magnets and in the
microwave devices incorporated into, for instance,
smartphones, cars, and computers—and such technology could not have been developed without ferromagnetic insulators. Spintronic devices, in which
both the electrical and magnetic properties of electrons are utilized simultaneously, are now being
extensively investigated to achieve high-speed devices with low power consumption. Ferromagnetic insuNTT Technical Review

Vol. 17 No. 10 Oct. 2019

lators will also serve as essential constituents thatmake such spintronic devices viable.
In conjunction with trends in computerization,
there has been a steadily growing demand for practical devices with higher performance. In terms oftemperature, stable operation even above 200°C is
required. However, the record Curie temperature(TC), which is the crucial factor determining the temperature range in which any ferri/ferromagnetic system remains stable, has stood in insulators ever since
ferrite magnets*1 were first developed over eight
decades ago in the 1930s. Therefore, researchers havesought to develop the next generation of ferromagnetic insulators with high TC values as well as toestablish guiding principles to search for such materials.
*1 Ferrite magnets: Ferrite magnets are ferromagnetic insulators developed in the 1930s in Japan. They have been the most widelyused magnets in the world. The major components are iron oxides, and many ferrite magnets also contain Co (cobalt), Ni(nickel), Mn (manganese), and other elements.

7

Feature Articles

Ferromagnetism

Paramagnetism

Fig. 1. Schematic diagram of ferromagnetism and paramagnetism.

Os
Sr
[001]

: Sr

[010]
[100]

: Os
:O

[110]
(a) Schematic diagram

[001]
1 nm

[110]

(b) Microscopy image

O: oxygen, Os: osmium, Sr: strontium

Fig. 2. Crystal structure of Sr3OsO6 (double perovskite).

2. Preparation of high-quality Sr3OsO6
thin films
Solids in which atoms are periodically and orderly
arranged and thus form lattices are called crystals.
Crystals that have only a single atomic arrangement
over an entire volume are known as single crystals.
Samples whose thicknesses range from one atomic
layer to about several tens of micrometers (1 μm =1 × 10–6 m) are called thin films. Single-crystalline
thin films are synthesized on single-crystalline substrates. For microfabrication of high-performancedevices, it is necessary to prepare samples in theform of single-crystalline thin films with submicro­
meter thicknesses. In this study, single-crystalline
Sr3OsO6 thin films with a thickness of 300 nm (1 nm =1 × 10–9 m) were synthesized on single-crystalline
SrTiO3 (strontium titanate) substrates [1].
A schematic diagram of the double perovskite
structure of Sr3OsO6 is shown in Fig. 2(a). The yellow, red, and blue spheres respectively indicate Sr(strontium), Os (osmium), and O (oxygen) atoms.
Sr3OsO6 is a novel material synthesized in this study
for the first time. In crystals, atoms are regularly
NTT Technical Review

Vol. 17 No. 10 Oct. 2019

ordered and form lattices. There are many kinds ofatomic arrangements (crystal structures), and representative ones have specific names. One sucharrangement is known as double perovskite, in which
the lattice is twice as large as the perovskite structure.
Many complex oxides are known to have theperovskite structure. Iodides and chlorides that havethe perovskite structure have also been extensively
studied recently to develop the next generation ofsolar cells [2].
To grow high-quality Sr3OsO6 thin films, precise
control of the flux rate of each constituent cation (Sr,Os) is mandatory. Generally, controlling the flux of
Os is a challenge because of its high melting point(3033°C). Nevertheless, we have succeeded in precisely controlling both the Sr and Os flux rates. We
accomplished this by monitoring the flux rates withan atomic emission spectrometer and feeding themback to the evaporation source power supplies in realtime, which enabled the synthesis of Sr3OsO6 thinfilms with the Sr and Os atoms arranged in a highlyordered structure. An atomic resolution microscopy(scanning transmission electron microscopy) imageof an Sr3OsO6 film viewed along the [110] direction
8

Feature Articles

0
−73°C
127°C
327°C
727°C

−20

Paramagnetism

Magnetization

Magnetization (emu/cc)

20

Magnetization (emu/cc)

Ferromagnetism

40

40

Curie
temperature30
20

Stable operation range(Wide temperature range)

10

−40
0

0
20,000−20,000
Magnetic field (Oe)Magnetic field(a) Magnetization versus
magnetic field(b) Schematic diagram offerromagnetism

−200

0

200 400 600
Temperature (°C)800(c) Magnetization versus temperature

Fig. 3. Magnetic properties of Sr3OsO6.

is shown in Fig. 2(b). We can clearly see the atomicordering depicted in Fig. 2(a).
3. Ferromagnetism above 780°C in Sr3OsO6
As described above, we have synthesized the novel
material Sr3OsO6 having the highest TC among insulators by using a unique oxide thin-film growth technique that we have developed over many years. First,
we measured the electrical and optical properties of
the Sr3OsO6 thin films. The resistivity at room temperature was 75 Ω·cm, which is about 109 times aslarge as typical metals such as Au (gold) and Cu (copper). Also, resistivity increased exponentially as thetemperature decreased. Furthermore, the optical band
gap of Sr3OsO6 was found to be about 2.65 eV. All
these results indicate that Sr3OsO6 is an insulator.
Next, we examined magnetic properties. The magnetization versus applied magnetic field curves of an
Sr3OsO6 film is shown in Fig. 3(a). It shows ferromagnetic behavior (Fig. 3(b)) with a finite magnetization even at the high temperature of 727°C. The
magnetization versus temperature curve of an
Sr3OsO6 film is shown in Fig. 3(c). The applied magnetic field was 2000 Oe. The gradual change in themagnetization up to about 400°C is suitable for highperformance magnetic devices that can be stably
operated at high temperatures (above room temperature). The TC value, at which ferromagnetism disappears, is above 780°C, surpassing the TC record
among insulators for the first time in 88 years bymore than 100°C.
In addition to the experiments, density functional
theory*2 calculations were carried out by the TsuneyuNTT Technical Review

Vol. 17 No. 10 Oct. 2019

ki Research Group at the University of Tokyo. These
calculations revealed that the ferromagnetic insulating state of Sr3OsO6 originates from the large spinorbit coupling of the 5d element Os. The spin-orbit
coupling consists of interactions between the spin
magnetic moment coming from the axial rotation ofelectrons and the orbital magnetic moment comingfrom the revolution of the charged particles (electrons) around the nucleus (Fig. 4). The elements inthe lower rows of the periodic table are heavier thanthose in the higher rows, and they have a larger spinorbit coupling. Thus, the spin-orbit coupling in Os islarger than in Fe (iron) and Co (cobalt), which areused in typical magnets. This insight into the mechanism of the emergent high-temperature ferromagnetism will open a new avenue for developing functional materials in which elements having large spinorbit coupling play a role.
This novel material Sr3OsO6 has been synthesizedin the form of single-crystalline thin films, whichhave high compatibility with device fabrication processes. This is in marked contrast to typical newoxides often synthesized in a powder or sinteredpolycrystalline form. Thus, Sr3OsO6 is expected to bereadily implemented in high-performance magnetic
device applications such as magnetoresistive random*2 Density functional theory: This theory states that the energies ofelectrons in solids can be determined by using spatially dependent electron density n(r). The word functional, which means thefunction of another function, is used since the energy is calculated as a function of n(r), which is already a function of r. The
electronic states of materials can be calculated and predictedfrom the fundamental equation that electrons follow based on thistheory, without experimental data.

9

Feature Articles

Orbital magnetic moment

Nucleus
Spin magnetic moment
Spin

Revolution
ElectronFig. 4. Spin-orbit coupling.

Magnetichead

Antiparallel

ParallelFerromagnet

Ferromagnet

Insulator

Insulator

Ferromagnet

Ferromagnet

Low resistivity

High resistivity(a) Tunnel magnetoresistance effect(b) Magnetic head of harddisc drives

Fig. 5. Tunnel magnetoresistance and its application.

access memories (MRAMs) and magnetic sensors
that work above room temperature.

high-performance magnetic devices that work at hightemperatures.

4. Future outlook

References

In our quest to better understand the fundamentals
of ferromagnetism, we will further investigate theelectronic structures of Sr3OsO6 using advanced
spectroscopy techniques provided by synchrotron
radiation facilities.*3 As part of efforts to develop
high-performance magnetic devices that can be operated at high temperatures, we are working on fabricating some test devices using Sr3OsO6 to examine
the tunnel magnetoresistance effect. This effectoccurs when the tunnel resistance of an insulator
sandwiched between two ferromagnets changes
depending on the magnetic configuration of the ferromagnets (parallel or antiparallel) (Fig. 5(a)). The
tunnel magnetoresistance effect has been widely utilized in commercial devices such as hard disc drives(Fig. 5(b)), MRAMs, and magnetic sensors. Therefore, demonstration of the tunnel magnetoresistanceeffect in Sr3OsO6 will lead to progress in developing
NTT Technical Review

Vol. 17 No. 10 Oct. 2019

[1]

[2]

Y. K. Wakabayashi, Y. Krockenberger, N. Tsujimoto, T. Boykin, S.
Tsuneyuki, Y. Taniyasu, and H. Yamamoto, “Ferromagnetism above
1000 K in a Highly Cation-ordered Double-perovskite Insulator
Sr3OsO6,” Nat. Commun., Vol. 10, 535, 2019.
M. Liu, M. B. Johnston, and H. J. Snaith, “Efficient Planar Heterojunction Perovskite Solar Cells by Vapour Deposition,” Nature, Vol.
501, pp. 395–398, 2013.

*3 Synchrotron radiation facilities: In synchrotron radiation facilities, we can use light with various wavelengths (synchrotron radiation) such as ultraviolet rays and X-rays. Synchrotron radiation is emitted from accelerated electrons that travel in a hugering in an ultrahigh-vacuum environment. The versatile capabilities of such synchrotron facilities include emissions of intenseand variable-wavelength light, and this enables us to performmany kinds of high-resolution spectroscopy to investigate thephysical properties of specimens in detail. Therefore, synchrotron
radiation facilities are also very useful for materials science research. In Japan, there are several synchrotron facilities in operation, including SPring-8 in Hyogo Prefecture and the Photon
Factory in Ibaraki Prefecture.

10Feature Articles

Yuki K. Wakabayashi

Researcher, NTT Basic Research Laboratories.
He received a B.S. in solid-state physics from
Osaka University in 2012, and an M.S. and Ph.D.
in electrical engineering from the University ofTokyo in 2014 and 2017. Since joining NTT in
2017, he has been conducting research on thinfilm growth of ferromagnetic oxides. He received
the English Presentation Award by the Spintronics Committee of the Japan Society of Applied
Physics (JSAP) at the 62nd Spring Meeting of
JSAP in 2015, the Young Researcher Best Poster
Award at the 9th International Conference on
Physics and Applications of Spin-Related Phenomena in Solids in 2016, and the 45th JSAP
Presentation Award at the 66th Spring Meeting of
JSAP in 2019.

Yoshiharu Krockenberger

Senior Researcher, Low-Dimensional Nanomaterials Research Group, NTT Basic ResearchLaboratories.
He received a diploma in physics from theTechnical University of Munich in 2002 and a
Ph.D. in physics from the Darmstadt Universityof Technology, Germany, in 2006. His principal
research areas are superconductivity, strongly
correlated electron systems, and thin film
growth. He has been working at NTT since 2010.
He also worked as a research scientist at RIKEN,
Advanced Science Institutes, in the cross-correlated materials research group of Prof. Y. Tokura(2006–2010).

NTT Technical Review

Vol. 17 No. 10 Oct. 2019

Yoshitaka Taniyasu

Senior Researcher, Group Leader of LowDimensional Nanomaterials Research Group,
NTT Basic Research Laboratories.
He received a B.E., M.S., and Dr. Eng. in electrical engineering from Chiba University in
1996, 1998, and 2001. He joined NTT Basic
Research Laboratories in 2001. He has been
engaged in wide-bandgap semiconductor
research. He received the Young Scientist Awardat the 2007 Semiconducting and Insulating Materials Conference, the Young Scientist Award of
the International Symposium on Compound
Semiconductors 2011, the Commendation for
Science and Technology by the Minister of Education, Culture, Sports, Science and Technologyof Japan (the Young Scientists’ Prize) in 2011,
and the Japan Society for the Promotion of Science Prize in 2019.

Hideki Yamamoto

Senior Research Scientist, Supervisor, Executive Manager of Materials Science Laboratory,
NTT Basic Research Laboratories.
He received a B.S., M.S., and Ph.D. in chemistry from the University of Tokyo in 1990, 1992,
and 1995. He joined NTT in 1995. His principal
research fields are thin film growth, surface science, and superconductivity. He was a visiting
scholar at the Geballe Laboratory for Advanced
Materials, Stanford University, USA (2004–
2005). He received the 2nd Young Scientist Presentation Award (1997) from JSAP and the 20th
Superconductivity Science and Technology
Award (2017) from the Forum of Superconductivity Science and Technology, the Society of
Non-traditional Technology. He is a member of
JSAP, the Physical Society of Japan, the Japan
Society of Vacuum and Surface Science, theAmerican Physical Society, and the Materials
Research Society.

11Feature Articles: Creating Novel Functional Materials

Magnetically Purified Erbium-doped
Oxide Crystal—Towards Creating a
Quantum-information-manipulation
Platform
Takehiko Tawara and Tomohiro Inaba
Abstract

Erbium (Er) is a rare-earth element that is expected to be a platform for manipulation of quantum
information by utilizing telecommunication-wavelength photons. However, high-quality epitaxial
growth of Er-doped host crystals is difficult, and the memory time of quantum information, which determines the performance of quantum manipulation, is much shorter than theoretically predicted. We investigated a good-compatibility rare-earth oxide as a host crystal for Er. The host oxide crystal, cerium oxide(CeO2), had its nuclear spin removed (i.e., magnetically purified). The nuclear spin is the main factor
that shortens the lifetime of quantum-information retention time. In this article, we describe high-quality epitaxial growth of CeO2 and its optical properties as a thin film grown on a silicon substrate.
Keywords: rare-earth oxide, erbium, molecular beam epitaxy

1. Erbium-doped oxide and its application as aquantum-state-manipulation platform
It has long been known that adding rare-earth atoms
to uniform solid crystals forms a determinative, discrete, and less-fluctuating ideal energy quantum levelthat is robust against changes in the external environment such as differences in host materials and temperature. This is because rare-earth atoms have aunique electron configuration; that is, their 4f electron orbit is electrically shielded from the outside
world. In recent years, researchers have been actively
studying the application of such superior quantum
levels of rare-earth atoms as the platforms of quantum-information-manipulation devices (especiallyphoton memories) used in quantum-information
communication.
Quantum-information manipulation is defined asfollows: photons responsible for the transmission ofquantum information are transferred to electrons in amaterial, some operations are added to the electron
NTT Technical Review

Vol. 17 No. 10 Oct. 2019state, and the photons are re-emitted as photons carrying that information. At that time, the electronic
state (quantum level) of the material to which theinformation is transferred must have a slight fluctuation in energy (so that a long time is taken to lose any
quantum information). A rare-earth atom is outstanding as an element that meets this requirement. Of therare-earth elements, only erbium (Er) can interact
with the optical-telecommunication photons (i.e.,
those having a wavelength of 1.55 µm). Accordingly,
Er-doped crystals are promising as a platform forquantum-information-manipulation devices in quantum optical communication using existing opticalfiber networks [1].
The question that arises is what kind of Er-doped
crystals are specifically required. First, Er needs to be
added in a diluted manner. That is because if the distance between the doped Er atoms is too close, energy(i.e., quantum information) will be exchanged
between the Er atoms, and the information will belost instantaneously [2]. The distance between Er
12Feature Articles

Abundance ratio of
nuclear spin (%)

100

B N F

La

Pr

80

Pm

Eu

Tb

Ho

Tm

Lu

60
Dy
40

Sm Gd
Nd

20
C O Ne
0

5

10 55

Yb
Er

Ce
60
65
Atomic number70

B: boron, C: carbon, Ce: cerium, Dy: dysprosium, Er: erbium, Eu: europium, F: fluorine, Gd: gadolinium,
Ho: holmium, La: lanthanum, Lu: lutetium, N: nitrogen, Nd: neodymium, Ne: neon, O: oxygen,
Pm: promethium, Pr: praseodymium, Sm: samarium, Tb: terbium, Tm: thulium, Yb: Ytterbium

Fig. 1. Natural abundance ratio of isotopes with nuclear spin of different atoms.

atoms must therefore be sufficient so that the atoms
are separated (diluted). Moreover, the host crystal towhich Er is added strongly influences the quantum
level characteristics of Er, particularly the degree offluctuation of quantum levels. For example, wheneach atom that composes the host crystal has nuclear
spin, large magnetic fluctuation occurs, and the quantum level fluctuation becomes large. This fluctuationis also a factor contributing to losing quantum information in a short time.
For more efficient manipulation of quantum information, it is necessary to strongly confine the light in
the host crystal and enhance its interaction with Er.
Technology for fabricating optical circuits (e.g., optical resonators, waveguides, and multiplexing/demultiplexing devices) used in silicon (Si) photonics isuseful to achieve this. It is therefore desirable to form
the host crystal as a thin film on a Si substrate.
Some rare-earth oxides are candidate materials thatcan be grown as thin films on a Si substrate with lessmagnetic fluctuation. The crystal structure of a rareearth oxide is similar to that of Si, and its lattice constant is exactly twice that of Si. This indicates that
epitaxial growth of such an oxide on a Si substrate ispossible. Among the many rare-earth atoms, only
cerium (Ce) has no nuclear spin (Fig. 1). Thus, Ce has
excellent characteristics; namely, it has no magnetic
fluctuation and does not affect the quantum state ofEr.
Incidentally, since the natural abundance ratio of
isotopes with nuclear spin is very small for oxygen,cerium oxide (CeO2) is the most promising candidate
NTT Technical Review

Vol. 17 No. 10 Oct. 2019

for an Er-doped host crystal on a Si substrate. However, although CeO2 has been studied as a polishing
agent, reduction catalyst, and other purposes [3],there are no published research examples of it being
used as an Er-doped host crystal—as the basis of aquantum-information manipulation platform—or as
an epitaxial-crystal thin film on a Si substrate.
2. Crystal growth of Er-doped CeO2 thin films
An Er-doped CeO2 film was grown to a thickness of
30 nm at 640°C on a surface-cleaned Si (111) substrate by using molecular beam epitaxy [4]. The rareearth materials used, high-purity (> 99.99%) Er andCe metals, were oxidized with O * (oxygen-atomradicals) to form a rare-earth oxide film. To obtainhigh-quality crystals of CeO2 as a host matrix, it isnecessary to adjust the supply ratio of Ce and O* sothat the formed thin film has a stoichiometric composition (in this case, a stoichiometry of Ce:O = 1:2).
Crystal quality was therefore investigated by changing the amount of Ce supply under a constant O* supply.
Reflection high-energy electron diffraction(RHEED) images after crystal growth are shown in
Fig. 2. When the amount of Ce supplied is small andthe oxidizing power is excessive (Fig. 2(a), (b)), thehalo-like pattern (showing an amorphous state) is
obtained, and when the amount of Ce supplied islarge and the oxidizing power is insufficient (Fig.
2(d), (e)), ring-like patterns (showing a polycrystalline state) appear.
13

Feature Articles

(a)

(b)

Ce = 0.09 Å/s

Ce = 0.03 Å/s
(d)

(c)

Ce = 0.33 Å/s(e)

Ce = 0.60 Å/s(f)

Ce = 1.00 Å/s
O2 = 0.3 sccm

Ce = 0.82 Å/s

Sccm: standard cubic centimeters per minute

Ce4+(CeO2)

106

Si (111)105

Er: CeO2 (111)Ce 3d

XPS intensity

XRD intensity

Fig. 2. RHEED images taken after crystal growth.

104
103
102

Ce3+
(Ce2O3)

101
26

27

28 29 302 (degrees)

31(a) XRD spectrum

32

920

910 900 890
Binding energy (eV)880(b) XPS spectra of Ce 3d orbital

Fig. 3. Structural characteristics of CeO2.

In contrast, in Fig. 2(c), a streak pattern (indicatinga flat surface and single-crystal growth) is observed,
and it can be seen that a thin film close to stoichiometry is formed by supplying that amount of Ce. It canalso be seen that the streak pattern is maintained evenwhen each supply amount is increased while the supply ratio of Ce to O* is kept constant (Fig. 2(f)).
Therefore, when Er is added as a dopant, it is possible
to change the Er concentration while keeping the
crystal quality constant by changing the crystal
growth rate of the host CeO2 crystal.
In addition to the cubic fluorite structure targeted inthis study, CeO2 has multiple crystal structures suchas cubic bixbyite (Ce2O3) and hexagonal. Since theabove-described RHEED images cannot distinguishthese differences in crystal structure, the crystal
NTT Technical Review

Vol. 17 No. 10 Oct. 2019

structure was examined in more detail by using X-ray
diffraction (XRD) spectroscopy and X-ray photoelectron spectroscopy (XPS).
The crystal lattice spacing estimated from the XRD
spectrum shown in Fig. 3(a) is 3.11 Å, which corresponds to cubic CeO2 and indicates that the hexagonal crystal structure does not exist. Moreover, fromthe spacing of the periodic satellite peaks observed
when the crystal is grown uniformly at the atomic
level, it is clear that the thickness of the grown CeO2
film is 26.4 nm, which agrees fairly well with theassumed grown-film thickness. The charge states of
the Ce atoms, estimated from the XPS spectra of theCe 3d orbital shown in Fig. 3(b), are all Ce4+ in thecase of fluorite CeO2, and Ce3+ does not exist in thecase of bixbyite Ce2O3. The above results demonstrate
14

Feature Articles

TEM
Er: CeO2
20 nm

Si

HAADF-STEM
Er: CeO2
2 nm
HAADF-STEM: high-angle annular dark-field scanning TEM

Fig. 4. Cross-sectional TEM image of grown Er-doped CeO2.

1502
1504
1506

PL intensity (arb. unit)

Excitation wavelength (nm)

1500

1508
1510
1512
1514
1516

1525 1530 1535 1540 1545 1550
PL wavelength (nm)1525 1530 1535 1540 1545 1550
PL wavelength (nm)1525 1530 1535 1540 1545 1550
PL wavelength (nm)(a) Er concentration: 4%(b) Er concentration: 2%(c) Er concentration: 1%

Fig. 5. Color plot of dependence of PL on excitation wavelength.

that the grown CeO2 thin film is the desired cubic
fluorite CeO2.
A cross-sectional TEM (transmission electron
microscopy) image of the grown Er-doped CeO2 isshown in Fig. 4. From this image, it can be seen thatin addition to an extremely flat surface, a very highquality Er-doped CeO2 thin-film crystal without crystal defects or different crystal-structure phases isobtained. It is thus concluded that a high-quality Erdoped CeO2 thin film was epitaxially grown on a Si
substrate for the first time.
3. Optical properties of Er-doped CeO2
thin-film crystal
The optical properties of the high-quality Er-doped
NTT Technical Review

Vol. 17 No. 10 Oct. 2019

CeO2 thin-film crystals were investigated. Color plotsof the dependence of photoluminescence (PL) on
excitation wavelength are shown in Fig. 5. In the PL
measurement, energy is applied to a sample by irradiating the sample with a laser to excite electrons in thesample to a high-energy (excitation) state, and thelight emitted when these excited electrons return to alow-energy state is observed. In this investigation, theEr concentration was changed from 4% (Fig. 5(a)) to
1% (Fig. 5(c)), and the PL measurement was performed at a temperature of 4K. It can be seen that for
all Er-doping concentrations, sharp PL emission
appears at a PL wavelength (horizontal axis) of 1533
nm against an excitation wavelength (vertical axis) of
1512 nm. This result indicates that the dopant Er
atoms replace Ce sites in the parent CeO2 crystal and15

Feature Articles

10

2%

1

1%

PL intensity(arb. unit)

Resonant excitation(exc = 1512.2 nm)

PL lifetime (ms)PL intensity

1%

0 2 4 6 8Time (ms)4%15251530 1535 1540 1545
PL wavelength (nm)1550(a) PL spectrum

0.1
0.001

0.01
0.1
Er concentration: x

1(b) PL lifetime

Fig. 6. Dependence of PL spectrum and PL lifetime on Er-doping concentration.

are optically activated. In other words, in this Erconcentration range, Er atoms do not exist between
crystal lattices, or they form Er clusters. Multiple PLemission peaks appear because the energy level of the
doped Er is split due to the crystal field of the CeO2
host.
This emission spectrum is compared in detail in
Fig. 6(a). For all Er concentrations, the emission peak
appears at the same wavelength; at the same time, theemission intensity increases as Er concentration
decreases. This result can be explained by the factthat as Er concentration decreases, the distance
between the Er atoms increases, so the Er atoms
become isolated in the host crystal. As a result, thenon-emission process involving energy transfer by
Er-Er atomic interaction is suppressed (and emission
efficiency is enhanced).
This suppression of the non-radiative process by
diluting the doped Er also changes the PL-emission
lifetime. The dependence of PL-emission lifetime on
Er concentration is shown in Fig. 6(b). The PL-emission lifetime is measured by the intensity change ofthe emission from the sample appearing after thelaser irradiation in the time domain (inset of Fig.
6(b)). It is clear from the figure that the PL-emission
lifetime is extended as the Er concentration is
reduced. The dotted line in the figure shows a PLemission lifetime of about 11 ms when 0.001% of
extremely dilute Er is added to a YSi2O5 host crystal(which has conventionally been used as a quantum
optics crystal), and that value is considered to be theintrinsic PL-emission lifetime of Er in a solid; that is,
the Er-Er atomic interaction can be ignored.
For the Er-doped CeO2 used in this investigation,

NTT Technical Review

Vol. 17 No. 10 Oct. 2019the minimum Er concentration was 1%. By lowering
the Er concentration by another order of magnitude,
it is expected that (i) the Er-Er interaction can becompletely suppressed, and (ii) the Er atoms can becompletely isolated in the solid. Also, in regard to thePL-emission characteristics, we observed the formation of a definite energy state, enhancement of PLemission efficiency, and prolongation of PL-emission
lifetime. In conclusion, Er-doped CeO2 thin films
with high quality in terms of both crystal structure
and optical characteristics were successfully obtained.
4. Future development
It is expected that with the high-quality Er-doped
CeO2 thin film obtained in this investigation, thememory time of quantum information will be longer
than that possible with a conventionally doped host
crystal. We plan to fabricate an optical waveguidestructure on a crystal surface in order to evaluate thequantum-information memory time and achieve onchip manipulation of the quantum state by using
telecom-band photons.
References
[1]
[2]
[3]
[4]

T. Tawara, H. Omi, T. Hozumi, R. Kaji, S. Adachi, H. Gotoh, and T.
Sogawa, “Population Dynamics in Epitaxial Er2O3 Thin Films Grown
on Si(111),” Appl. Phys. Lett., Vol. 102, No. 24, 241918, 2013.
T. Tawara, Y. Kawakami, H. Omi, R. Kaji, S. Adachi, and H. Gotoh,“Mechanism of Concentration Quenching in Epitaxial (ErxSc1-x)2O3
Thin Layers,” Opt. Mat. Express, Vol. 7, No. 3, pp. 1097–1104, 2017.
E. J. Schelter, “Cerium under the Lens,” Nat. Chem., Vol. 5, No. 4, p.
348, 2013.
T. Inaba, T. Tawara, H. Omi, H. Yamamoto, and H. Gotoh, “Epitaxial
Growth and Optical Properties of Er-doped CeO2 on Si(111),” Opt.
Mat. Express, Vol. 8, No. 9, pp. 2843–2849, 2018.

16

Feature Articles

Takehiko Tawara

Senior Research Scientist, Supervisor, Quantum Optical Physics Research Group, Optical
Science Laboratory, NTT Basic Research Laboratories and NTT Nanophotonics Center.
He received a B.S. in applied physics from
Nihon University, Tokyo, in 1996 and an M.S.
and Ph.D. in engineering from Hokkaido University in 1998 and 2001. He joined NTT Basic
Research Laboratories in 2001. Since then, he
has engaged in the study of semiconductor nanostructure fabrication and optical characterizationof semiconductor quantum systems. He is amember of the Japan Society of Applied Physics(JSAP) and the Optical Society (OSA).

NTT Technical Review

Vol. 17 No. 10 Oct. 2019

Tomohiro Inaba

Research Scientist, Quantum Optical Physics
Research Group, Optical Science Laboratory,
NTT Basic Research Laboratories.
He received a B.S., M.S., and Ph.D. in materialscience and engineering from Osaka University
in 2013, 2015, and 2018. He joined NTT Basic
Research Laboratories in 2018. His current
research interests are thin film growth, fabrication technology, and optical properties. He is amember of JSAP.

17

Feature Articles: Creating Novel Functional Materials

MBE Growth and Element-distinctive
Atomic-resolution Characterizationof High Temperature
Superconductors
Ai Ikeda, Yoshiharu Krockenberger, Yoshitaka Taniyasu,
and Hideki Yamamoto
Abstract

Cuprate superconductors are in the material family that have the highest superconducting transition
temperature (Tc) of ~130 K under ambient pressure. The infinite-layer structure is the essential building
block of high-Tc cuprates with a Tc of over 100 K and is therefore vital to understand the mechanism ofhigh-Tc superconductivity. While the infinite-layer phase is inaccessible using bulk single-crystal synthesis methods, we synthesized single-crystalline thin films of the infinite-layer cuprates using ourunique oxide molecular beam epitaxy setup. To clarify the relationship between their microscopic crystal structures and electronic responses, we performed atomic-resolution electron microscopy measurements of the CuO2 (copper peroxide) planes in the infinite-layer cuprates, the playground of high-Tc
superconductivity.
Keywords: superconductivity, oxide molecular beam epitaxy, scanning transmission electron microscopy

1. Introduction
Superconductivity is a phenomenon in quantum
materials that allows for lossless transportation ofelectrical current, and thus of energy and information.
The quantum phenomenon, superconductivity, hadbeen observed only below –140°C (~130 K) untilvery recent reports on hydrides, that is, sulfur hydride(H3S) and lanthanum hydride (LaH10). These materials show significantly higher superconducting transition temperatures (Tcs): –70°C (~200 K) for H3S [1]and even close to room temperature for LaH10 [2].
Synthesizing these hydrides, however, requires
extremely high pressure, specifically, ~2 million
times higher (~200 GPa) than the atmospheric pressure. To make matters more complicated, their crystal
structures are altered when the pressure is reduced
after synthesis. In other words, the superconducting
NTT Technical Review

Vol. 17 No. 10 Oct. 2019phases can exist only under an extremely high pressure comparable to that at the core of the Earth.
Accordingly, practical applications using these
hydrides remain elusive despite the significance of
their discoveries from an academic point of view.
In contrast, cuprates are superconductors thatexhibit the highest Tc under ambient pressure. Amongthem, YBa2Cu3O7-δ (yttrium barium copper oxide) as
well as Bi2Sr2Ca2Cu3O10+δ (bismuth strontium calcium copper oxide), both of which show higher Tc (~90
K and ~110 K, respectively) than the boiling point of
nitrogen (77 K), have already yielded practical applications as superconducting cables and microwave
filters. However, the mechanism of high-Tc superconductivity in cuprates remains unclarified despite
immense efforts for over three decades, which has
been hampering the strategic search of novel superconducting materials with a higher Tc.
18

Feature Articles

Bi

Sr

Ca

Cu

Ca

O
Two-dimensional
CuO2 planes

Sr(a) Bi2Sr2Ca2Cu3O10+(b) Infinite-layer CaCuO2

Bi: bismuth, Ca: calcium, Cu: copper, O: oxygen, Sr: strontium

Fig. 1. Crystal structures of cuprate superconductors.

Nonetheless, the fundamental structural ingredientof high-Tc cuprates is fortunately well known: copper
peroxide (CuO 2 ) planes, as exemplified by
Bi2Sr2Ca2Cu3O10+δ (Fig. 1(a)). Two-dimensional
CuO2 planes are made up of copper (Cu) and oxygen(O) and are separated by bismuth (Bi), strontium (Sr),calcium (Ca), and O. The formal valences of O and
Cu are 2– and 2+, respectively, so the CuO2 plane is
not charge-neutral and cannot exist independently.
Instead, the minimal structural unit that is compassable is charge-neutral calcium copper oxide (CaCuO2)(Fig. 1(b)), which is commonly included in cuprates
whose Tcs are above 100 K; the crystal structure
shown in Fig. 1(b) is called an infinite-layer structure.
That is why we have been focusing our research
efforts on infinite-layer cuprates. Because the synthesis of bulk specimens of CaCuO2 requires high pressure levels (3–5 GPa), single-crystalline CaCuO2
specimens can be prepared exclusively by using thinfilm growth methods, in our case, molecular beam
epitaxy (MBE). Unlike the above-mentioned
hydrides, the infinite-layer structure is stable at ambient pressure once it is formed.
2. Fabrication of ultrahigh-quality infinite-layer
cuprate superconductors
While MBE has been widely used for the growth ofsemiconductors, we have extended it to the growth of
NTT Technical Review

Vol. 17 No. 10 Oct. 2019

complex transition metal oxides. Our customized
MBE method is well equipped not only to synthesize
novel complex transition metal oxides, for example,
Sr3OsO6, a new ferromagnetic insulator with a Curie
temperature > 1000 K [3], but also to push the limitsof common crystal growth methods [4], and this is
illustrated in Fig. 2. The vacuum recipient is 70 cm indiameter and 150 cm in height. A total of 10 metalsources are mounted at the bottom of the vacuum
recipient where the elements can be evaporated byelectron guns operated at 10 kV. The evaporant fluxof each element is monitored and controlled by electron impact emission spectroscopy (EIES). The
details of EIES are explained in another article in thisissue [5].
Molecular oxygen (O 2) is insufficient for thegrowth of complex transition metal oxides, and therefore, the oxide MBE system is equipped with stronger oxidizing agents. This is distinct from conventional MBE. In this work, radio-frequency radical
oxygen (O) was used for the growth of infinite-layer
cuprates. Oxidation/reduction involves the gain/lossof electrons. Radical oxygen has two unpaired electrons and therefore a strong tendency to deprive theneighboring metal atoms of electrons. The absence ofkinetic barriers in atomic oxygen is what drives thegrowth of complex transition metal oxides in an ultrahigh vacuum (~10–9 atm). In addition, the growthorientation is defined by the substrate; this phenomenon
19

Feature Articles

Vacuum recipient
EIES sensor

Shutter
O
zo
ne

n
ge

y

x
lo
ica e
d
c
Raour
s

no

zz

Sr
La

Cu

ron

t
Elec

s

gun

le

Liquid N2 shroud

RHEED

Substrate heater

EIES: electron impact emission spectroscopy
La: lanthanumN2: nitrogen
RHEED: reflection high-energy electron diffraction

Fig. 2. Oxide MBE setup.

Resistivity  (µ·cm)

100

0

0

400
Temperature (K)Fig. 3. Temperature dependence of resistivity for Sr0.9La0.1CuO2 thin film.

is called epitaxy (in Greek, order on top). We used asingle-crystal substrate with a lattice constant closer
to one for infinite-layer cuprates. This mechanism
enables the formation of single-crystalline infinitelayer cuprates.
We used the oxide MBE system to synthesize
superconducting infinite-layer Sr0.9La0.1CuO2 thinfilms [6–9]. In Sr0.9La0.1CuO2, strontium ions (Sr2+
NTT Technical Review

Vol. 17 No. 10 Oct. 2019ions) are partially substituted with La3+ ions (lanthanum ions) to induce superconductivity. The dependence of resistivity on temperature for the
Sr0.9La0.1CuO2 thin film is plotted in Fig. 3. Resistivity decreases as the temperature decreases andabruptly goes to zero at 41 K. The films presented
here are superior to those in other reports on superconducting infinite-layer thin films [10], as evidenced20

Feature Articles

Electron beam

<100>

Ca0.96Nd0.04CuO2 thin film

Lensesyx

Ca+Cu+Nd+O

Incident
electronbeam

Specimen

Ca/Nd
CuO2

Scatteredelectron
beam

Ca

Cu

Nd

O

ABF
detector
EELS
Substrate(a) STEM apparatus

1 nm(b) Atomic position of Ca0.96Nd0.04CuO2 thin film by STEM(c) Atomic column elemental mapping by EELS

ABF: annular bright field
Nd: neodymium

Fig. 4. Identification of superconducting state of infinite-layer CaCuO2.

by the following characteristics:•	sharp superconducting transition (transition
width ΔTc < 1 K)•	metal-like electronic response between 400 K
and 42 K•	low resistivity value in normal state (corresponds
to the intrinsic value for defect-free CuO2 plane).
It is therefore shown that samples grown with our
method are most suitable to reveal the inherent physical properties leading to understanding the mechanism of high-Tc superconductivity.
3. Infinite-layer CaCuO2—elemental-resolved
and atomic-resolution characterization
As already mentioned, CaCuO2 is an essential
structural part of high-Tc cuprates. In contrast to
strontium copper oxide (SrCuO2), however, little is
known about the physical properties of infinite-layer
CaCuO2 due to the difficulty of synthesis. We synthesized infinite-layer Ca1-xNdxCuO2 thin films using
MBE. In Ca1-xNdxCuO2, calcium ions (Ca2+ ions) arepartially substituted with Nd 3+ ions (neodymiumions) by mimicking Sr0.9La0.1CuO2. For a composition of x = 0.06, we found traces of superconductivity
NTT Technical Review

Vol. 17 No. 10 Oct. 2019around 10 K [11]. It is important to note that a truly
superconducting state has not been established.
Therefore, it is of significant importance to determine
the differences between superconducting Sr1-xLaxCuO2
and non-superconducting Ca1-xNdxCuO2 by using
scanning transmission electron microscopy (STEM)
combined with electron energy loss spectroscopy(EELS).
STEM is used to visualize atomic positions in matter. The configuration of our STEM apparatus isshown in Fig. 4(a) [12]. Accelerated electrons at 200
kV are focused on a sample. The scattered electron
beam contains information on the atomic position.
This is shown in Fig. 4(b) for Ca0.96Nd0.04CuO2.
Because the contrast of the original bright-field
image is inverted, the atomic positions appear brightin Fig. 4(b). The observed atomic column arrangements correspond to the infinite-layer structure with
two-dimensional CuO 2 planes. It is evident that
Ca0.96Nd0.04CuO2 thin films are single crystalline. We
used EELS to identify the constituent atomic species.
This is shown in Fig. 4(c). The atomic selectivity of
EELS enables an atomic column identification thatexactly matches the infinite-layer phase.
By measuring the distance between Cu and O
21

Feature Articles(dCu-O) from Fig. 4(c), we determined the in-planelattice constant of Ca0.96Nd0.04CuO2 to be 0.386 nm.
This is 0.001 nm longer than for CaCuO2 [13]. Similar expansion is found for the Sr0.9La0.1CuO2 system,
where a substitution of 10% of La results in an expansion of 0.002 nm of the in-plane lattice constant [14].
It is intuitively suggested that a critical in-plane lattice constant (dCu-O x 2) for the induction of superconductivity is to be expected. As the ionic size of Ca2+
ions is smaller than that of Sr2+ ions, CaCuO2 has
shorter dCu-O than for SrCuO2. During the course of
our study, we found that a higher concentration of Nd
causes instability of the infinite-layer phase. Consequently, it is not possible to prepare the infinite-layer
phase of Ca1-xNdxCuO2 with a sufficient amount of
Nd to induce a superconducting transition. This result
suggests that lattice constant (the Cu-O bond length
in the CuO2 plane) engineering is important for the
induction of superconductivity in cuprates.
4. Future outlook
Infinite-layer cuprates are very important to understand the physics of high-Tc superconductivity. We
plan to apply our thin film growth method to further
understand the mechanism of high-Tc superconductivity commonly emerging in materials containing
CaCuO2. This method is currently being extended tothe synthesis of superlattices containing CaCuO 2[15].
References
[1]

[2]

A. P. Drozdov, M. I. Eremets, I. A. Troyan, V. Ksenofontov, and S. I.
Shylin, “Conventional Superconductivity at 203 Kelvin at High Pressures in the Sulfur Hydride System,” Nature, Vol. 525, No. 7567, pp.
73–76, 2015.
M. Somayazulu, M. Ahart, A. K. Mishra, Z. M. Geballe, M. Baldini,
Y. Meng, V. V. Struzhkin, and R. J. Hemley, “Evidence for Superconductivity above 260 K in Lanthanum Superhydride at Megabar Pres-NTT Technical Review

Vol. 17 No. 10 Oct. 2019sures,” Phys. Rev. Lett., Vol. 122, No. 2, 027001, 2019.
Y. K. Wakabayashi, Y. Krockenberger, N. Tsujimoto, T. Boykin, S.
Tsuneyuki, Y. Taniyasu, and H. Yamamoto, “Ferromagnetism above
1000 K in a Highly Cation-ordered Double-perovskite Insulator
Sr3OsO6,” Nat. Commun., Vol. 10, No. 1, 535, 2019.
[4] H. Yamamoto, Y. Krockenberger, and M. Naito, “Multi-source MBE
with High-precision Rate Control System as a Synthesis Method sui
generis for Multi-cation Metal Oxides,” Journal of Crystal Growth,Vol. 378, pp. 184–188, 2013.
[5] H. Yamamoto and H. Gotoh, “Overview of Novel Materials Creation
Research at NTT,” NTT Technical Review, Vol. 17, No. 10, pp. 1–6,
2019.
https://www.ntt-review.jp/archive/ntttechnical.php?contents=
ntr201910fa1.html[6] Y. Krockenberger, K. Sakuma, and H. Yamamoto, “Molecular Beam
Epitaxy and Transport Properties of Infinite-layer Sr0.90La0.10CuO2Thin Films,” Appl. Phys. Express, Vol. 5, No. 4, 043101, 2012.
[7] S. Karimoto and M. Naito, “Electron-doped Infinite-layer Thin Films
with Tc over 40 K Grown on DyScO3 Substrates,” Appl. Phys. Lett.,Vol. 84, No. 12, pp. 2136–2138, 2004.
[8] S. Karimoto, K. Ueda, M. Naito, and T. Imai, “Superconducting Thin
Films of Electron-doped Infinite-layer Sr1−xLaxCuO2 Grown by
Molecular Beam Epitaxy,” Physica C Supercond., Vol. 378–381, Part
1, pp. 127–130, 2002.
[9] M. Naito, S. Karimoto, and A. Tsukada, “Epitaxy-stabilized n-type
Superconducting Cuprates,” Supercond. Sci. Technol., Vol. 15, No.
12, 1663, 2002.
[10] L. Maritato, A. Galdi, P. Orgiani, J. W. Harter, J. Schubert, K. M.
Shen, and D. G. Schlom, “Layer-by-layer Shuttered Molecular-beam
Epitaxial Growth of Superconducting Sr1–xLaxCuO2 Thin Films,” J.
Appl. Phys., Vol. 113, No. 5, 053911, 2013.
[11] A. Ikeda, Y. Krockenberger, and H. Yamamoto, “Molecular Beam
Epitaxy of Electron-doped Infinite-layer Ca1-xRxCuO2 Thin Films,”
Phys. Rev. Materials, Vol. 3, No. 6, 064803, 2019.
[12] S. J. Pennycook, C. Li, M. Li, C. Tang, E. Okunishi, M. Varela, Y.-M.
Kim, and J. H. Jang, “Material Structure, Properties, and Dynamics
through Scanning Transmission Electron Microscopy,” J. Anal. Sci.
Technol., Vol. 9, 11, 2018.
[13] Y. Krockenberger, A. Ikeda, K. Kumakura, and H. Yamamoto, “Infinite-layer Phase Formation in the Ca1–xSrxCuO2 System by Reactive
Molecular Beam Epitaxy,” J. Appl. Phys., Vol. 124, No. 7, 073905,
2018.
[14] N. Ikeda, Z. Hiroi, M. Azuma, M. Takano, Y. Bando, and Y. Takeda,“Synthesis and Superconducting Properties of the Infinite-layer Compounds Sr1−xLnxCuO2 (Ln = La, Nd, Sm, Gd),” Physica C Supercond.,
Vol. 210, No. 3–4, pp. 367–372, 1993.
[15] D. Di Castro, M. Salvato, A. Tebano, D. Innocenti, C. Aruta, W. Prellier, O. I. Lebedev, I. Ottaviani, N. B. Brookes, M. Minola, M.
Moretti Sala, C. Mazzoli, P. G. Medaglia, G. Ghiringhelli, L. Braicovich, M. Cirillo, and G. Balestrino, “Occurrence of a High-temperature Superconducting Phase in (CaCuO2)n/(SrTiO3)m Superlattices,”
Phys. Rev. B, Vol. 86, No. 13, 134524, 2012.
[3]22

Feature Articles

Ai Ikeda

Research Scientist, Low-Dimensional Nanomaterials Research Group, NTT Basic ResearchLaboratories.
She received a B.E., M.E., and Ph. D in applied
physics from Tokyo University of Agriculture
and Technology in 2010, 2012, and 2015. She
joined NTT Basic Research Laboratories in2015. She has been researching superconducting
materials in order to push the limits of the variety
of superconducting oxide materials available.

Yoshiharu Krockenberger

Senior Researcher, Low-Dimensional Nanoma­
terials Research Group, NTT Basic ResearchLaboratories.
He received a diploma in physics from theTechnical University of Munich in 2002 and a
Ph.D. in physics from Darmstadt University ofTechnology, Germany, in 2006. His principal
research areas are superconductivity, strongly
correlated electron systems, and thin film
growth. He has been working at NTT since 2010.
He also worked as a research scientist at RIKEN,
Advanced Science Institute, in the cross-correlated materials research group of Prof. Y. Tokura(2006–2010).

NTT Technical Review

Vol. 17 No. 10 Oct. 2019

Yoshitaka Taniyasu

Senior Researcher, Group Leader of LowDimensional Nanomaterials Research Group,
NTT Basic Research Laboratories.
He received a B.E., M.S., and Dr. Eng. in electrical engineering from Chiba University in
1996, 1998, and 2001. He joined NTT Basic
Research Laboratories in 2001. He has been
engaged in wide-bandgap semiconductor
research. He received the Young Scientist Awardat the 2007 Semiconducting and Insulating Materials Conference, the Young Scientist Award of
the International Symposium on Compound
Semiconductors 2011, the Commendation for
Science and Technology by the Minister of Education, Culture, Sports, Science and Technologyof Japan (the Young Scientists’ Prize) in 2011,
and the Japan Society for the Promotion of Science Prize in 2019.

Hideki Yamamoto

Senior Research Scientist, Supervisor, Executive Manager of Materials Science Laboratory,
NTT Basic Research Laboratories.
He received a B.S., M.S., and Ph.D. in chemistry from the University of Tokyo in 1990, 1992,
and 1995. He joined NTT in 1995. His principal
research fields are thin film growth, surface science, and superconductivity. He was a visiting
scholar at the Geballe Laboratory for Advanced
Materials, Stanford University, USA (2004–
2005). He received the 2nd Young Scientist Presentation Award (1997) from the Japan Societyof Applied Physics (JSAP) and the 20th Superconductivity Science and Technology Award(2017) from the Forum of Superconductivity Science and Technology, the Society of Non-traditional Technology. He is a member of JSAP, thePhysical Society of Japan, the Japan Society of
Vacuum and Surface Science, the American
Physical Society, and the Materials ResearchSociety.

23

Feature Articles: Creating Novel Functional Materials

High-quality Atomic-layer Materials
Fabricated by Chemical Vapor
Deposition
Shengnan Wang and Yoshitaka Taniyasu
Abstract

Atomic-layer materials such as graphene and hexagonal boron nitride (h-BN) are promising for theiruse in next-generation optoelectronic devices due to the novel and unique physical phenomena as wellas superior material properties being demonstrated in such materials. Scalable fabrication methods forhigh-quality atomic-layer materials are essential for their industrial applications. We introduce here
chemical vapor deposition growth for high-quality graphene and h-BN.
Keywords: atomic-layer materials, chemical vapor deposition, crystal orientation control

1. Atomic-layer materials
Atomic-layer materials have layered structureswith a thickness of only one or a few atoms (Fig. 1).
Graphene, a representative atomic-layer material,consists of a single layer of carbon (C) atoms
arranged in a two-dimensional hexagonal lattice
structure like a honeycomb, and is therefore an
extremely thin material with a thickness of only one
atom [1]. Graphene shows a semimetallic (or zerogap semiconductor) character with a linearly dispersed energy band, which results in high carrier
mobility (electrical conductivity) and large wavelength-independent light absorption in a wide range
from infrared to visible light. Furthermore, graphene
has the highest thermal conductivity and mechanicalstrength of any kind of material.
In addition to graphene, there are many kinds ofatomic-layer materials including transition metal
dichalcogenides (TMDs), MX2 (where M is a transition metal atom such as Mo (molybdenum) or W
(tungsten), and X is a chalcogen atom such as S (sulfur), Se (selenium), or Te (tellurium). Hexagonalboron nitride (h-BN) is another type of atomic-layermaterial. Monolayer TMDs consist of a single layerof transition metal atoms (M) sandwiched between
two layers of chalcogen atoms (X); thus, the total
NTT Technical Review

Vol. 17 No. 10 Oct. 2019

thickness is three atoms. Some TMDs are semiconductors with bandgaps in the near-infrared to visible
spectral region, and the bandgap energy varies with
the number of stacked layers. TMDs have also uniqueproperties such as spin- and valley-dependent electrical and optical properties.
The material h-BN is composed of boron (B) andnitrogen (N) and has a layered hexagonal lattice
structure similar to graphene. h-BN is an insulator
with a large band gap of about 6 eV and is therefore
used as an insulating layer and a tunnel barrier invarious types of devices made from atomic-layermaterials.
There are numerous atomic-layer materials ranging
from metallic, superconducting, magnetic, semiconducting, to dielectric or insulating depending on theirchemical composition and crystal structure, whichmeans they are promising for next-generation devices
such as ultra-thin flexible, low-power-consumption,
high-speed nanoelectronic, high-efficiency photonic,
and high-sensitivity devices. Thus, the application of
atomic-layer materials will lead to innovative information technology.
2. Fabrication of atomic-layer materials
Various fabrication methods for atomic-layer
24

Feature Articles

Low-power-consumption
high-speed devices

Ultra-thin devices

High-efficiency
optoelectronic devices

B
N

M (Mo, W, …)X (S, Se, Te)C

TMDs (MX2)Grapheneh-BN

B: boron, C: carbon, N: nitrogen, Mo: molybdenum, W: tungsten, S: sulfur, Se: selenium, Te: telluriumh-BN: hexagonal boron nitride
TMD: transition metal dichalcogenide

Fig. 1. Atomic-layer materials and their applications.

Graphite
Graphene(microcrystal) (single layer)
Tape
Substrate

10 µm

Source gas

Cu substrate
10 x 10 cm

Tape

te

tra

bs

Su

Subs

trate

(a) Mechanical exfoliation and transfer(b) CVD growth

Cu: copper

Fig. 2. Methods of fabricating atomic-layer materials.

materials including mechanical exfoliation and thinfilm growth are proposed. In mechanical exfoliation,a monolayer flake is cleaved from a microcrystal byusing adhesive tape and is transferred onto a substrate, as shown in Fig. 2. K. S. Novoselov and A. K.
Geim, Nobel laureates in Physics in 2010, demonstrated mechanical exfoliation for graphene andshowed new phenomena in quantum physics.
Mechanical exfoliation has been used in fabricating
NTT Technical Review

Vol. 17 No. 10 Oct. 2019atomic-layer materials in order to investigate their
physical properties and to demonstrate new functional device applications. However, the size of theatomic-layer material obtained with this method islimited to the size of the microcrystals, typically
about tens of micrometers (μm). This small size is notsuitable for industrial applications.
Chemical vapor deposition (CVD) is a growthmethod for thin films on large-area wafers
25

Feature Articles

RT

1000°C

Temperature rising

Annealing

Cu oxide

1000°CRT

Cu
Graphene growth

Temperature cooling
Ar
H2

CH4
(a) CVD growth process

GrapheneCu

1 cm

Mobility (cm2/Vs)

30,00020,000FET
10 µm

10,000
0
−2

−1
0
1
2
Carrier density (X 1012 cm−2)(b) Millimeter-size large graphene and its electrical property
Ar: argon, CH4: methane, H2: hydrogen
FET: field-effect transistor
RT: room temperature

Fig. 3. Large-size single-crystal graphene growth.

(substrates) via chemical reaction of gas sources. It is
widely used in the semiconductor industry for mass
production of thin films. CVD growth technology iscurrently under development all over the world foratomic-layer materials. Although CVD growth of
atomic-layer materials on large-area substrates has
been demonstrated, the crystal quality is low, mainly
due to defects originating at the boundaries of randomly orientated domains. The domain size is as
small as tens of micrometers, similar to that of
mechanically exfoliated flakes. In this article, we
introduce our CVD technology for large-area singledomain graphene and for crystal orientation control
of h-BN.
3. Large-size single-crystal graphene growth
A schematic diagram of CVD growth of graphene
on a copper (Cu) substrate is shown in Fig. 3(a). The
surface of as-received Cu is naturally oxidized. In a
CVD reactor, the Cu substrate is heated to the growthtemperature at ~1000°C and thermally annealed
under argon (Ar) and/or hydrogen (H 2) flow to
NTT Technical Review

Vol. 17 No. 10 Oct. 2019

remove the natural oxide layer on the Cu surface.
Then methane (CH4) is introduced for graphene
growth. When the growth starts, graphene domains(nuclei) form on the Cu surface. The domain size
increases with growth time. Once the domains
coalesce with nearby domains, the graphene covers
the entire Cu surface. Note that the lower nucleation
density results in a larger domain size. After thegrowth, CH4 is shut off and the sample is cooled toroom temperature.
The domain size can be controlled by the nucleation density, which depends on CVD growth conditions, the substrate material, and its surface condition.
Regarding the surface conditions, the Cu oxide prevents the nucleation of graphene while graphene
preferentially nucleates on the clean Cu surface without the oxide. In the thermal annealing of the substrate, the etching rate of the oxide layer depends on
the ambient gas. For example, the etching rate of the
Cu oxide under H2 gas is higher than under Ar gas. Tovary the coverage of the oxide layer for nucleation
density control, we carried out thermal annealing of
the Cu substrate under Ar gas alone and precisely
26

Feature Articles

adjusted the annealing time as shown in Fig. 3(a).
Consequently, we succeeded in fabricating a millimeter-size graphene single crystal that is about 100
times larger than conventional ones with domain
sizes of approximately tens of micrometers [2](Fig. 3(b)).
Next, we fabricated field-effect transistors in orderto characterize the electrical properties of the graphene. The millimeter-size graphene showed nearly
10 times higher charge mobility than conventional
CVD-grown ones. These results show that nucleation
control by selective growth is a promising way toachieve large-area growth of high-quality graphene
single crystals.
4. Crystal orientation control of h-BN
Similar to graphene, h-BN can be grown on Cu
substrates by CVD. For h-BN growth, ammonia
borane (H6BN) was used as a BN source. The crystal
orientation of thin-film materials generally depends
on the surface structure and symmetry of the substrate. Cu has a face-centered cubic lattice structure.
For example, on Cu (001), (101), and (111) planes,h-BN shows multiple orientations, that is, two or fourorientations due to the crystal rotational symmetry
mismatching between h-BN and Cu. The multipleorientations cause crystal defects formed at theboundaries of domains with different orientations(rotation angles).
The commercial Cu substrate is polycrystalline.
During the thermal annealing process, the Cu substrate is recrystallized by heating. In recrystallization,polycrystalline grains coalesce and become large, andspecific crystal planes of the grains appear on thesubstrate surface. We systematically investigated the
crystal orientation of h-BN domains grown on morethan 100 different Cu crystal planes. For h-BN grownon crystal planes inclined from a Cu (101) plane, as
shown in the microscopic image (Fig. 4(a)), we foundthat all of the triangular h-BN domains are oriented tothe same direction, indicating the formation of singleorientation h-BN [3].
We clarified the mechanisms of the single-orienta-

NTT Technical Review

Vol. 17 No. 10 Oct. 2019tion h-BN formation using theoretical calculations. In
the case of a (101) plane, there are two energeticallystable orientations for h-BN because the (101) plane
has two-fold rotational symmetry. In contrast, for the
inclined (101) plane, because the symmetry is broken, there is only one energetically stable orientation
for h-BN, resulting in single-orientation h-BN formation. This mechanism is applicable to other atomiclayer materials.
To investigate the effect of the crystal orientation on
the electrical properties, we measured the electricalresistance images of the multi- and single-orientationh-BN as shown in Fig. 4(b). The single-orientationh-BN showed higher electrical resistance (darker
contrast) than multi-orientation h-BN. This is because
the single-orientation h-BN has lower defect density
than multi-orientation h-BN.
Finally, we grew h-BN layers that fully covered theCu substrate as shown in Fig. 4(c). For the multiorientation h-BN, multi-layer h-BN domains werepartly formed on single-layer h-BN. In the case of thesingle-orientation h-BN on the other hand, no multilayer h-BN domains were formed, and single-layerh-BN was uniformly formed on the entire Cu surface.
Thus, the single-orientation control is essential toachieve high-quality atomic-layer materials.
5. Future prospects
CVD growth of heterostructures for a variety ofatomic-layer materials is a challenging process torealize novel functional devices. Vertical heterostructures, in which atomic-layer materials are stacked,
have demonstrated recently that material properties
change greatly depending on the crystal orientation(rotation angle) of each layer. Furthermore, lateralheterostructures, in which different atomic-layermaterials are atomically bonded in-plane, can lead tonew physical phenomena originating from onedimensional heterointerfaces and in-plane superlattice structures. We will continue to develop CVD
growth technologies for high-quality heterostructureson large-area substrates for integrated functional
devices.

27

Feature Articles

Multi-orientation h-BN

h-BN

Single-orientation h-BN

Cu

Cu

h-BN

10 µm

10 µm

(a) Microscopic image of h-BN domains (partial coverage)

Cu
Cu

2 M

2 µm

2 M

h-BN

h-BN
2 µm

(b) Electrical resistance images

Multi-layer
Single layer

Single layer
10 µm

10 µm

(c) Microscopic image of h-BN layers (full coverage)

Fig. 4. Crystal orientation control of h-BN.

References
[1]

[2]

H. Hibino, “Graphene Research at NTT,” NTT Technical Review, Vol.
11, No. 8, 2013.
https://www.ntt-review.jp/archive/ntttechnical.php?contents=
ntr201308fa1.html
S. Wang, H. Hibino, S. Suzuki, and H. Yamamoto, “Atmospheric Pres-NTT Technical Review

Vol. 17 No. 10 Oct. 2019[3]

sure Chemical Vapor Deposition Growth of Millimeter-scale Singlecrystalline Graphene on the Copper Surface with a Native Oxide
Layer,” Chem. Mat., Vol. 28, No. 14, pp. 4893–4900, 2016.
S. Wang, A. E. Dearle, M. Maruyama, Y. Ogawa, S. Okada, H. Hibino,
and Y. Taniyasu, “Catalyst-selective Growth of Single-orientation
Hexagonal Boron Nitride toward High-performance Atomically Thin
Electric Barriers,” Adv. Mater., Vol. 31, 1900880, 2019.

28

Feature Articles

Shengnan Wang

Research Scientist, NTT Basic Research Laboratories.
She received a B.S. in light chemical engineering from East China University of Science andTechnology, People’s Republic of China, in 2007
and a Ph.D. in condensed matter physics from the
National Center for Nanoscience and Technology, People’s Republic of China, in 2012. She
joined NTT Basic Research Laboratories in 2012and has been conducting research on atomiclayer materials including graphene and h-BN.
She received the Japan Society of Applied Physics (JSAP) Presentation Award at the 43rd Spring
Meeting of JSAP in 2019.

NTT Technical Review

Vol. 17 No. 10 Oct. 2019

Yoshitaka Taniyasu

Senior Researcher, Group Leader of LowDimensional Nanomaterials Research Group,
NTT Basic Research Laboratories.
He received a B.E., M.S., and Dr. Eng. in electrical engineering from Chiba University in
1996, 1998, and 2001. He joined NTT Basic
Research Laboratories in 2001. He has been
engaged in wide-bandgap semiconductor
research. He received the Young Scientist Awardat the 2007 Semiconducting and Insulating Materials Conference, the Young Scientist Award of
the International Symposium on Compound
Semiconductors 2011, the Commendation for
Science and Technology by the Minister of Education, Culture, Sports, Science and Technologyof Japan (Young Scientists’ Prize) in 2011, andthe Japan Society for the Promotion of Science
Prize in 2019.

29

Feature Articles: Creating Novel Functional Materials

Development of Next-generation
Wide-bandgap Semiconductors
Kazuyuki Hirama, Yoshitaka Taniyasu, Hideki Yamamoto,and Kazuhide Kumakura
Abstract

Cubic boron nitride (c-BN) is a wide-bandgap semiconductor with the highest breakdown field among
semiconductors. It has the potential to dramatically improve efficiency in power devices. Although c-BN
is a metastable material, we have heteroepitaxially grown high-quality c-BN thin films by developing aunique growth technique. We have also used doping to control the electrical conductivity of the c-BN
films. These accomplishments are major steps toward the fabrication of c-BN-based power devices.
Keywords: cubic boron nitride (c-BN), diamond, next-generation wide-bandgap semiconductors

1. Power device applications ofwide-bandgap semiconductors

1000
Si

4H-SiCNTT Technical Review

Vol. 17 No. 10 Oct. 2019

Power loss (arb. unit)

100

Wide-bandgap semiconductors are expected toserve as components of high-efficiency power devices, in which electrical power loss is significantly
lower than in conventional semiconductor-based
power devices. Here, silicon (Si) and gallium arsenide (GaAs) are typical conventional semiconductors. The main functions of power devices include
regulation of direct current (DC) voltage, frequency
conversion of alternating current (AC) voltage, andconversion between AC and DC. For example, in AC
adapters used in personal computers and mobile
phones, power devices convert the voltage from AC
100–200 V to DC ~20 V.
The relationship between power loss and operation
voltage of power devices consisting of various semiconducting materials is shown in Fig. 1. Power loss isdetermined by the on-resistance of power devices; it
becomes smaller with lower on-resistance. Increasing
the carrier concentration and reducing the currentpath length are both effective in reducing the onresistance, and consequently the power loss.
Such conditions are more readily fulfilled when the
breakdown field*1 of the constituent semiconductor ishigher. The high breakdown fields of wide-bandgap
semiconductors, at least one order of magnitude

GaN
AlN

10

GaAs

1
c-BN

0.1

0.01
Diamond
0.001
103

104

105

Operation voltage (V)AIN: aluminum nitride, c-BN: cubic boron nitride, SiC: silicon carbide,
GaN: gallium nitride

Fig. 1. R elationship between power loss and operation
voltage.

*1 Breakdown field: When the electrical field is applied to an insulator or a semiconductor, the current drastically increases above athreshold value of the electrical field (i.e., the breakdown occurs).
The threshold electrical field is defined as the breakdown field.
Values of the inherent breakdown field vary by material.

30

Feature Articles

sp2 bonding

sp3 bonding

N
B

B

N

(a) h-BN(b) c-BN

Fig. 2. Crystal structures of (a) hexagonal BN (h-BN) and (b) c-BN.

higher than those in Si or GaAs, means that the onresistance of the power devices can be reduced bythree orders of magnitude. As a result, wide-bandgap
semiconductor-based power devices can operate
more efficiently with lower power losses. Some widebandgap semiconductor-based power devices are
already in practical use, as exemplified by siliconcarbide (SiC)- or gallium nitride (GaN)-based ones,
including high-efficiency inverters for trains.
At NTT Basic Research Laboratories, we have been
studying next-generation wide-bandgap semiconductors such as cubic boron nitride (c-BN), aluminum
nitride (AlN), and diamond. In the following sections, we describe the growth of c-BN thin films withthe highest breakdown field among semiconductors
and describe how we control their electrical conductivity. The power loss in c-BN-based power devices is
expected to be four orders of magnitude lower thanthose in Si- or GaAs-based ones and at least one orderof magnitude lower than those in SiC- or GaN-based
ones (Fig. 1).
If such high-efficiency c-BN-based power devices
become available, power loss can be significantly
reduced in electrical vehicles, railways, and solarpower and wind-power generation systems. Accordingly, research aimed at developing c-BN-based
power devices is underway, as this approach uses
fewer natural resources and will contribute to constructing a sustainable society on a global scale.

NTT Technical Review

Vol. 17 No. 10 Oct. 2019

2. Growth of c-BN epitaxial films using uniquegrowth technique
Boron nitride (BN) is a compound semiconductor
composed of boron and nitrogen. BN can have several crystal structures depending on the bonding
character and stacking sequence. Representative
crystal structures of BN are shown in Fig. 2. The
thermodynamically stable phase under standard temperature and pressure is sp2-bonded hexagonal BN(h-BN). In contrast, sp3-bonded c-BN is a high-pressure and high-temperature stable phase, that is, ametastable phase under standard temperature andpressure.
To utilize c-BN with a high breakdown field for
power devices, first of all, the growth of phase-pure(i.e., composed of only the sp3-bonded phase) c-BN
films is necessary. Ion irradiation to the growth surface is known to be effective for forming the metastable sp3-bonded c-BN phase. Nevertheless, growthof a phase-pure c-BN thin film has been hampered bypartial formation of the thermodynamically stablesp2-bonded BN phase. The sp2-bonded BN phase has
a disordered h-BN structure and is called turbostratic
BN (t-BN). The inclusion of the t-BN phase prevents
epitaxial growth and renders the resultant BN films
polycrystalline ones.
Despite the above-mentioned difficulties, we havegrown phase-pure c-BN films by developing a uniquegrowth technique, that is, ion-beam-assisted molecular beam epitaxy (MBE) (Fig. 3). Boron (B) atoms
are supplied from an electron beam evaporator, while
nitrogen radicals (N*) are supplied from an RF (radio31

Feature Articles

Substrate heater
Faraday cup

Rate monitor

RHEED
e-gun
RHEED
screen
Si
N

Ar+

B

K-cellRF radical source
Ar+: argon ion, B: boron, N: nitrogen radicals
E-gun: electron gun
K-cell: Knudsen cells

E- gunIon source

RF: radio frequency
RHEED: reflection high-energy electron diffraction

Fig. 3. Schematic of ion-beam-assisted MBE.

*2 FT-IR absorption measurement: An optical measurement method
to characterize specific infrared (IR) absorbance in a material
from the difference in intensities of transmitted and incident
light. The detected signal is converted to the spectrum by Fouriertransformation (FT).

NTT Technical Review

Vol. 17 No. 10 Oct. 2019

sp2

sp3

sp2

Absorbance (arb. unit)

frequency) radical generator. At the same time, thegrowth surface is irradiated with argon ions (Ar+).
The Ar+/B ratio was found to play a crucial role informing the sp3-bonded BN [1, 2]. The substrate isdiamond with a small lattice mismatch with c-BN.
To investigate the effect of the Ar+ irradiation during the BN growth on the bonding characteristics andcrystal structures of the resultant BN films, we firstgrew BN films without and with Ar+ irradiation. The
Fourier transform infrared spectroscopy (FT-IR)
absorption measurement*2 of the samples preparedwithout and with Ar+ irradiation is shown in Fig. 4. In
the absence of Ar+ irradiation, two absorption peaks
were observed at around 800 cm–1 and 1380 cm–1,
which are attributed to sp2-bonded BN. No absorption peak related to sp3-bonded BN was observed.

B + N

B + N+ Ar+

1600

1400

1200

1000

800

600

400

Wavenumber (cm–1)

Fig. 4. FT-IR absorption spectra of BN films.

32

Feature Articles

002
1-11
[001]
[110]

-111
-220

2-20
[-110]

1-1-1

c-BN

-11-1
00-2

[001]
[110]

[-110]

Diamond (001)5 nm

Fig. 5. Cross-sectional TEM image of c-BN film and selective-area electron diffraction pattern of c-BN region.

This means that the sp2-bonded BN film was grown.
In contrast, in the presence of Ar+ irradiation, only
an absorption peak was observed at around 1070
cm-1, which is attributed to the sp3-bonded BN. No
other peaks originating from sp2-bonded BN wereobserved. Therefore, the grown film is phase-pure
sp3-bonded BN. Accordingly, the ion irradiation
allows for a selective formation of the sp3-bonded
BN. We surmise that the kinetic energy and/or
momentum transferred from the Ar+ ions to thegrowth surface may contribute to the preferential formation of the sp3-bonds in BN.
To identify the crystal structure, we performed
cross-sectional TEM (transmission electron microscopy) measurement for the BN film prepared with theAr+ irradiation, which was phase-pure (sp3-bondedBN) judging from the FT-IR measurement (Fig. 5).
The BN film is grown uniformly from the heterointerface (interface between the diamond substrate and theBN film) to the surface. The inset in Fig. 5 shows aselective-area electron diffraction pattern of the BN
film. All the diffraction spots are assigned to those forsingle-crystal c-BN (001) taken along the [110] zone
axis. No other spots or ring patterns attributed to sp2bonded BN are observed. Therefore, we can conclude
that the single-crystal c-BN (001) film is heteroepitaxially grown on the diamond (001) substrate [3].
3. Electrical conductivity control in c-BN films
To fabricate power devices, it is necessary to grown-type and p-type c-BN films and control their electrical conductivity. For c-BN, Si and S (sulfur) serveas donors, while Be (beryllium) serves as an acceptor.
NTT Technical Review

Vol. 17 No. 10 Oct. 2019

We carried out the n-type doping first; Si was supplied as a dopant during the epitaxial growth of thec-BN (001) films by using a K-cell (Knudsen cell).*3
The dependence of carrier concentration on temperature for a Si-doped c-BN film with a Si concentration ([Si]) of 1.2 × 1018 cm–3 is shown in Fig. 6.
The carrier concentration monotonically increases
with the measurement temperature. The carrier concentrations at room temperature (RT) and 500°C arerespectively 2.1 × 1015 cm–3 and 6.0 × 1017 cm–3.
From the fitting with the equation for the charge neutrality condition (Fig. 6, inset), the donor ionizationenergy (ED), donor concentration (ND), and acceptor
concentration (NA) are respectively estimated to be
0.20 eV, 1.1 × 10 18 cm –3, and 4.6 × 10 17 cm –3.
Because ND is almost the same as [Si], most of the
doped Si atoms are considered to be substitutionallyincorporated into boron sites, where they then serveas donors. Remarkably, ED of 0.20 eV is lower thanthose for other next-generation wide-bandgap semiconductors such as AlN and diamond. The low ED in
c-BN is promising for higher carrier concentrationand lower resistivity, suggesting the potential of c-BN
to exceed AlN or diamond, once a c-BN-based powerdevice is achieved.
The dependence of the resistivity on [Si] at RT for
the Si-doped c-BN films is shown in Fig. 7. For comparison, the resistivity of the non-doped c-BN film isalso plotted in Fig. 7. The resistivity of the non-dopedc-BN film is as high as ~108 Ω·cm, while the Si-doped*3 Knudsen cell: An evaporation cell, which can supply a molecularbeam of a material at a precisely controlled evaporation rate bytuning the cell temperature.

33

Feature Articles

109

1019

108

Non-doped c-BN

1018
Resistivity (·cm)

Carrier concentration (cm–3)[Si]: 1.2 × 1018 cm−3

1017

10

16

1015

106
105

n (n + NA)EN= C exp − D
gD
kBT
ND − NA − n

104

ED: 0.20 eV
ND: 1.1 × 1018 cm−3
NA: 4.6 × 1017 cm−3

103
102
1016

1014
0.5

107

1.0

1.5

2.0

2.5

3.0

3.5

1017

1018

1019

1020

[Si] (cm–3)

1000/T (K )–1

Fig. 7. D ependence of resistivity on Si concentration for
Si-doped and non-doped c-BN films.

ED: donor ionization energy
ND: donor concentration
NA: acceptor concentration

Fig. 6. T emperature dependence of carrier concentrationin Si-doped c-BN film.

c-BN film with a [Si] of 1.5 × 1019 cm–3 shows low
resistivity of ~260 Ω·cm. The resistivity decreases as[Si] increases, demonstrating the successful controlof electrical conductivity by varying the dopant concentration, in this case, [Si].
4. Summary
We described the recent progress in research on
c-BN at NTT Basic Research Laboratories. We haveachieved the epitaxial growth of single-crystal c-BN
thin films and control of electrical conductivity inn-type Si-doped c-BN films. These accomplishments

NTT Technical Review

Vol. 17 No. 10 Oct. 2019are important steps toward the fabrication of electron
devices. Further improving crystalline quality andp-type doping will pave the way to achieving highperformance c-BN-based power devices.
References
[1]

[2]

[3]

K. Hirama, Y. Taniyasu, H. Yamamoto, and K. Kumakura, “Heteroepitaxial Growth of Cubic Boron Nitride (c-BN) Thin Films by Ionbeam-assisted MBE,” OYO BUTURI, Vol. 85, No. 4, pp. 306–310,
2016 (in Japanese).
K. Hirama, Y. Taniyasu, S. Karimoto, H. Yamamoto, and K. Kumakura, “Heteroepitaxial Growth of Single-domain Cubic Boron Nitride
Films by Ion-beam-assisted MBE,” Appl. Phys. Exp., Vol. 10, No. 3,035501, 2017.
K. Hirama, Y. Taniyasu, S. Karimoto, Y. Krockenberger, and H. Yamamoto, “Single-crystal Cubic Boron Nitride Thin Films Grown by
Ion-beam-assisted Molecular Beam Epitaxy,” Appl. Phys. Lett., Vol.
104, No. 9, 092113, 2014.

34

Feature Articles

Kazuyuki Hirama

Senior Research Scientist, Thin-Film Materials
Research Group, NTT Basic Research Laboratories.
He received a B.S., M.S., and Ph.D. in electrical engineering from Waseda University, Tokyo,
in 2004, 2006, and 2008. He joined NTT Basic
Research Laboratories in 2009. His current
research interests are the growth of wide-bandgap semiconductors (c-BN, AlN, and diamond)and their device applications. He received theaward for encouragement of research at IUMRSICA (International Union of Materials ResearchSocieties International Conference in Asia) 2014,
outstanding presentation and best poster awards
at Diamond Symposium 2010 and 2012, and the
young scientist presentation award at the 2008
Japan Society of Applied Physics (JSAP) meeting. Dr. Hirama is a member of JSAP.

Yoshitaka Taniyasu

Senior Researcher, Group Leader of LowDimensional Nanomaterials Research Group,
NTT Basic Research Laboratories.
He received a B.E., M.S., and Dr. Eng. in electrical engineering from Chiba University in
1996, 1998, and 2001. He joined NTT Basic
Research Laboratories in 2001. He has been
engaged in wide-bandgap semiconductor
research. He received the Young Scientist Awardat the 2007 Semiconducting and Insulating Materials Conference, the Young Scientist Award of
the International Symposium on Compound
Semiconductors 2011, the Commendation for
Science and Technology by the Minister of Education, Culture, Sports, Science and Technologyof Japan (the Young Scientists’ Prize) in 2011,
and the Japan Society for the Promotion of Science Prize in 2019.

NTT Technical Review

Vol. 17 No. 10 Oct. 2019

Hideki Yamamoto

Senior Research Scientist, Supervisor, Executive Manager of Materials Science Laboratory,
NTT Basic Research Laboratories.
He received a B.S., M.S., and Ph.D. in chemistry from the University of Tokyo in 1990, 1992,
and 1995. He joined NTT in 1995. His principal
research fields are thin film growth, surface science, and superconductivity. He was a visiting
scholar at the Geballe Laboratory for Advanced
Materials, Stanford University, USA (2004–
2005). He received the 2nd Young Scientist Presentation Award (1997) by JSAP and the 20th
Superconductivity Science and Technology
Award (2017) from the Forum of Superconductivity Science and Technology, the Society of
Non-traditional Technology. He is a member of
JSAP, the Physical Society of Japan, the Japan
Society of Vacuum and Surface Science, theAmerican Physical Society, and the Materials
Research Society.

Kazuhide KumakuraGroup Leader of Thin-Film Materials ResearchGroup, Materials Science Laboratory, NTT Basic
Research Laboratories.
He received a B.E., M.E., and Ph.D. in electrical engineering from Hokkaido University in
1993, 1995, and 1998. He joined NTT Basic
Research Laboratories in 1998. His current
research interests include crystal growth of
nitride-based semiconductors and their optoelectronic and magnetic device applications. He is amember of JSAP.

35

Feature Articles: Creating Novel Functional Materials

Crystal Growth of Wurtzite GaP
Nanowires for Solar-water-splitting
Devices
Kouta Tateno and Kazuhide Kumakura
Abstract

Gallium phosphide (GaP) is an environmentally friendly III-V semiconductor, and it has an indirect
bandgap as a stable zincblende structure. However, if controlled nanowire growth is carried out, a wurtzite structure with a direct bandgap can be achieved. We successfully grew wurtzite GaP nanowires without stacking faults by using our original method using chlorine etching and repeated gallium supply. We
present here our results of solar-water splitting using a p-type nanowire photoelectrode, and we explain
the fabrication and properties of a pin-type nanowire solar cell.
Keywords: nanowire, gallium phosphide, solar-water splitting

1. Nanowire solar-water-splitting device
Research and development to harvest solar energythat is inexhaustible and clean has been progressing
as part of efforts to achieve a sustainable society. In
1972, Fujishima et al. reported on water and carbon
dioxide reduction using a semiconductor photoelectrochemical method for the first time [1]. Theirmethod is promising for producing fuel and organic
substances that can be stored and is still being
researched actively.
The redox (reduction oxidation reaction) process ofwater using sunlight (solar-water splitting) is the simplest process because the products are hydrogen andoxygen. For this reaction, the potential difference
between cathodes and anodes in an electrochemicalsystem must be larger than 1.23 V (thermodynamically required). In addition, the materials used forsolar-water splitting should be selected in consideration of low toxicity, abundance on earth, and longstability without corrosion during the reaction. Photocatalysts composed of fine powders have also beenstudied for solar-water splitting, but their solar-tohydrogen conversion efficiency is low at present(about 1–2%). In practical use, an ideal device is to
produce hydrogen and oxygen separately.
NTT Technical Review

Vol. 17 No. 10 Oct. 2019

Our target device considering the above points isshown in Fig. 1 [2]. The transparent tube-like structure contains dense nanowire diodes with both ends
exposed to water. When the nanowire diodes absorb
sunlight, the generated electrons react with hydrogenions at the cathode side of the nanowires (outside thetube) to form hydrogen, and the holes react withwater at the anode side of the nanowires (inside thetube) to form oxygen. Keeping the hydrogen andoxygen separate makes their collection easier.
Here, individual nanowire diodes independently
contribute to light absorption and water splitting.
Even if some nanowires fail, the device performanceof others should not be affected. Since nanowires are
small in diameter, we can grow good heterostructuresusing various materials with a relatively large difference in the lattice constant. Therefore, some semiconductors with different bandgaps can be connectedaxially and absorb a wide wavelength range of sunlight. In principle, such a tube-like structure can be
fabricated by embedding a myriad of nanowire
diodes on the substrate surface with a transparent
resin, exposing the nanowire tips, peeling the resin
film off the substrate, and rounding it.
There are many challenges to achieve this target
device. The growth of nanowires requires a good36

Feature Articles

Sunlight
Membrane

H2

Outside

O2

H2

O2

InsideNanowire diodeHydrogen (H2) and oxygen (O2) are collected
separately.

Fig. 1. Schematic illustration of our target solar-water-splitting device.

structure with uniform length and thickness and highdensity. In addition, techniques to grow different
materials in the axial direction and to control the concentration of dopants are also required. For photoelectrochemical devices, it is necessary to develop aproper coating film with optical transparency andcorrosion protection as well as cocatalysts to promote
chemical reactions at the nanowire ends. The road toachieving the target device is long, but we have madeprogress by growing wurtzite gallium phosphide(GaP) nanowires. We have also performed p- andn-type doping in nanowires and solar-water splitting
using nanowire electrodes, and we have fabricatednanowire solar cells [3, 4]. These achievements are
explained in this article.
2. Wurtzite GaP nanowires
Typical GaP materials are III-V semiconductors
with an indirect bandgap and are not used for highperformance devices because of their poor electronic
conduction and optical absorption characteristics.
However, because GaP is composed of elementsabundantly present on the earth, it is suitable as amaterial for solar cells and photoelectrodes, whichmust be inexpensive and have large areas. Since thebandgap is 2.1 eV, it is possible to perform solarwater splitting without bias if the voltage loss at theelectrode is small. Band structure calculations predicted that wurtzite GaP has a direct bandgap,
although stable GaP normally has a zincblende structure.
NTT Technical Review

Vol. 17 No. 10 Oct. 2019

A research group in Europe successfully carried out
crystal growth of wurtzite GaP nanowires for the firsttime recently, and those nanowires showed a large
increase in photoluminescence compared with zincblende GaP [5].
We also succeeded in carrying out crystal growth ofwurtzite GaP nanowires [4]. The growth was carriedout with the vapor-liquid-solid (VLS) method*1 using
gold nanoparticles as catalysts. First, gold particles
were dispersed on the substrate, and then the sourcegases were supplied at 400–500˚C in the crystal
growth apparatus. As shown in Fig. 2(a), TBCl (tertiarybutylchloride) was used as a chlorine etchingmaterial, and TEGa (triethylgallium) as a group III
source and TBP (tertiarybutylphosphine) as a group
V source were supplied alternately. This techniqueenabled us to successfully grow wurtzite GaP nano­wires without stacking faults.
A transmission electron microscope (TEM) image
and an electron diffraction pattern of one wurtzite
GaP nanowire are shown in Fig. 2(b). The nanowires
were about 500 nm in length, and the diameter of thegold particles was about 40 nm. The TEM image
shows the gold part as hemispherical black material
at the nanowire tip. The electron diffraction pattern*1 VLS method: Vapor-liquid-solid method. A method used for
growing nanowires. It is a crystal growth method in a vapor
phase atmosphere using metal fine particles as a catalyst. For
gold (Au) catalyzed GaP nanowires, gallium (Ga) and phosphide(P) from the gas (vapor) phase are dissolved in Au particles andbecome liquid. Solid crystalline GaP grows from the liquid AuGa-P particle.

37

Feature Articles

Repeat (60 times)

5 nm−1

TEGa
TBP
TBCl
10 s

10 s

(a) Growth sequence

20 nm(b) TEM image & electron diffraction image

TBCl: tertiarybutylchloride
TBP: tertiarybutylphosphine
TEGa: triethylgallium

Fig. 2. Wurtzite GaP nanowire.

indicates a typical wurtzite structure. The TEM
image also shows that the nanowires have smoothsidewalls without stacking faults.
3. Impurity doping in nanowires
In growing the GaP nanowires, we tried to incorporate zinc (Zn), a p-type dopant, and sulfur (S), an
n-type dopant, into the nanowires. The p-type dopant
source was diethylzinc (DEZn), and the n-type dopant source was DTBS (ditertiarybutylsulfide). Graphsof nanowire lengths versus flow rates are shown in
Fig. 3(a, b), and scanning electron microscope(SEM) and TEM images of the nanowires are shownin Fig. 3(c). The lengths of the nanowires changed
with the Zn and S doping. In particular, the length ofthe p-type ones obviously increased as the flow rateof DEZn increased, and they became wider at the bottom. These results show that the variation of thegrowth rate should be taken into account when fabricating devices by performing impurity doping. The
TEM images show that both types of nanowires had
some stacking faults due to doping, but they still had
wurtzite structures.
Impurity and carrier concentrations were evaluatedby secondary ion mass spectrometry (SIMS)*2 measurements and Mott-Schottky plots*3 from electrochemical experiments. The nanowire-grown samples
were embedded in a resin or SiO2 (silicon dioxide)
film by sputter deposition and measured as average
concentrations. It was confirmed that the concentraNTT Technical Review

Vol. 17 No. 10 Oct. 2019tion of impurities and carriers in the nanowires
increased as the flow rates of the dopant sources
increased. A concentration of about 1018 cm–3 was
obtained for both types of doped samples for bothcarrier and impurity. Since the measurement methods
used here provide average information on a large
number of dispersed nanowires, we will continue toconsider a method for measuring individual nano­wires for more accurate measurements.
4. Solar-water splitting using
nanowire electrodes
We grew p- and n-type wurtzite GaP nanowires andconducted photoelectrochemical experiments inorder to achieve solar-water-splitting devices. The
results obtained using a p-type sample as a photocathode electrode are shown in Fig. 4. In Fig. 4(a), we
compare a p-type GaP substrate and p-type GaP
nanowire samples.
We also compared nanowires with and without
platinum (Pt) nanoparticles (as cocatalyst) / titanium*2 SIMS: Secondary ion mass spectrometry is an analytical method.
One spot on a sample is irradiated with an ion beam and thensputtered. The released ionized atoms and molecules are measured by mass spectrometry. The elemental concentration in thedepth direction can be analyzed.
*3 Mott-Schottky plot: In an electrochemical measurement of semiconductor electrodes, carrier concentration can be determinedfrom the slope by plotting the reciprocal of the differential capacitance against the potential. The differential capacitance is determined by impedance measurement.

38

2

2

1.5

1.5

Length (µm)

Length (µm)

Feature Articles

1
0.5
0

0

1
0.5
0

0.05
0.1
0.15
0.2
DTBS flow rate (µmol/min.)0

0.2 0.4 0.6 0.8
1DEZn flow rate (µmol/min.)(b) Zn doping (p type)(a) S doping (n type)SEM imageTEM image

200 nm20 nm

SEM image

200 nmDTBS flow rate = 0.066 mmol/min.

TEM image

20 nm

DEZn flow rate = 0.49 mmol/min.

(c) SEM and TEM images of nanowires

Fig. 3. Impurity doped GaP nanowires.

2

Current density (mA/cm )

0.2
0
−0.2
−0.4

OFF
Light ON

−0.6
p-GaP
w/ Pt/TiO2

−0.8
−1
−0.4

20 nm

w/o Pt/TiO2
−0.2

0

0.2

0.4

Potential versus RHE (V)(a) Linear sweep voltammograms for p-type
GaP substrate, GaP nanowires with andwithout Pt/TiO2, performed underchopped illumination. The scan rate was
1 mV/s. Each ON/OFF time was 50 s.

(b) Scanning TEM image of
GaP nanowires with Pt/TiO2

Pt: platinum
RHE: reversible hydrogen electrodeTiO2: titanium dioxideFig. 4. GaP nanowire photoelectrode.

NTT Technical Review

Vol. 17 No. 10 Oct. 201939

Feature Articles

i
p

1 µm

Current density (mA/cm2)

0.6n(a) SEM image of pin-type GaP
nanowires

0.5
0.4
0.3
0.2
0.1
0

0

0.05 0.1 0.15 0.2 0.25 0.3 0.35
Voltage (V)(b) Measured J-V curve of GaP nanowire
solar cell under AM 1.5G one sun
illumination

Fig. 5. GaP nanowire solar cell.

dioxide (TiO2) protective thin film. The TEM imageof GaP nanowires with Pt/TiO2 is shown in Fig. 4(b),where the TiO2 thin film covers the nanowire uniformly and Pt fine nanoparticles are dispersed. In thephotoelectrochemical experiments, photocurrent wasmeasured by repeating ON/OFF of the light while
scanning the voltage using simulated sunlight (Solar
simulator, AM 1.5G). The electrolyte was a 0.5-M
H2SO4 (sulfuric acid) aqueous solution, and the counter electrode was a Pt plate. Compared with the GaP
substrate, the photocurrent output of the nanowire
samples was found to be large.
We also confirmed that the nanowire sample with
TiO2 dispersed with Pt fine nanoparticles showed nodegradation of nanowires. The density was evaluatedto be 70–110 μm−2, which was relatively high, but thephotocurrent itself was lower than expected due to thefact that the nanowires could not be made longer than
1 μm and the length of nanowires varied. Similar
measurements were carried out on n-type nanowires.
However, degradation due to corrosion was severe,even for samples with the TiO2 protective film. We
therefore need to investigate the optimum protective
film for n-type photoanode electrodes.
5. Nanowire solar cell
Since the final target of the solar-water-splitting
device includes pin-type nanowire photodiodes, wegrew pin-type nanowires and examined the solar cellcharacteristics. An SEM image of the grown pin-type
nanowires is shown in Fig. 5(a), and the photocurrent
voltage plot measured using simulated sunlight after
device fabrication is shown in Fig. 5(b). The device
NTT Technical Review

Vol. 17 No. 10 Oct. 2019was fabricated by depositing 5 nm of alumina on thesample surface using atomic layer deposition, embedding the nanowires in resin for planarization, exposing the nanowire tips, and then forming electrodes on
the back of the substrate and the sample surface. ITO(tin doped indium oxide) was used for the transparent
electrode on the sample surface. The lengths of the p-,i-, and n-segments were designed to be 330, 1300,
and 330 nm, respectively. The p-type segments were
tapered by doping as described above, resulting in a
structure with a large bottom diameter as shown inthe schematic illustration. The measured result was
fitted with a diode model of the following equation.
J = Jsc – (V – Rs J) /Rp – J0 exp(q (V – Rs J)/(n k T) – 1),
where q is the electron charge, k is the Boltzmann
constant, T is the absolute temperature, Jsc is theshort-circuit current density, Rs is the series resistance, Rp is the parallel resistance, J0 is the saturation
current density, and n is the ideal factor. The voltage
at J = 0 is the open-circuit voltage Voc.
The result was Jsc = 0.6 mA/cm2, Voc = 0.32 V, J0 =4.63×10–8 A/cm2, n = 1.33, Rp = 6.5 kΩ·cm, and Rs =40 Ω·cm, and the conversion efficiency of this sample
was 0.11%. This low efficiency is due to the low
nanowire density (9.8×107 cm–2). On a single nano­wire basis, Jsc = 160 mA/cm2. This large photocurrent
is due to the effect of the nanowires collecting theambient light. Therefore, by optimally designing
nanowire solar cells, we can expect to achieve highly
efficient solar cells.

40Feature Articles

6. Future development
Through our experience in fabricating nanowire
photoelectrodes and nanowire solar cells, we wereable to get closer to our target solar-water-splitting
devices using wurtzite GaP nanowires. The nano­wires absorb light efficiently, and a large photocurrent can be expected, but a large open-circuit voltage
of about 2 V, which is sufficient for water splitting, isconsidered to be difficult when using a single pintype nanowire diode.
Therefore, we are planning to form a tandem typenanowire consisting of grown pin-type nanowires inseries. It is also necessary to examine the material of
the protective film applied to prevent corrosion. For
the anode side in particular, we may have to change
the electrolyte to a weak alkaline solution. It is also
necessary to examine the cocatalyst. At present, precious metals such as Pt are widely used as cocatalysts

Kouta Tateno

Senior Research Scientist, NTT Basic ResearchLaboratories.
He received a B.S., M.S., and Ph.D. in chemistry from the University of Tokyo in 1991, 1993,
and 2001. He joined NTT Opto-electronics Laboratories in 1993. His current research interests
include the fabrication and physics of nanostructures using semiconductor nanowires. He is amember of the Japan Society of Applied Physics(JSAP), the Electrochemical Society of Japan,
and the Materials Research Society.

NTT Technical Review

Vol. 17 No. 10 Oct. 2019

due to high efficiency. Pt is a rare element and needs
to be replaced with another material in the future.
Corrosion prevention and cocatalysts are challenging
issues, but we will continue to investigate them.
References
[1]
[2]
[3]

[4]

[5]

A. Fujishima and K. Honda, “Electrochemical Photolysis of Water at
a Semiconductor Electrode,” Nature, Vol. 238, pp. 37–38, 1972.
K. Tateno, Y. Ono, and K. Kumakura, “Photoreactor,” Japanese patentapplication No. 2017-024024.
K. Tateno, Y. Ono, and K. Kumakura, “Photoelectrochemical Properties of Wurtzite Gallium Phosphide Nanowires Grown on GaP(111)B
Substrates,” 2016 International Conference on Solid State Devices
and Materials (SSDM 2016), PS-13-12, Tsukuba, Japan, Sept. 2016.
K. Tateno, G. Zhang, S. Sasaki, M. Takiguchi, and K. Kumakura,“Wurtzite GaP Nanowire Grown by Using Tertiarybutylchloride andUsed to Fabricate Solar Cells,” Jpn. J. Appl. Phys., Vol. 58, No. 1,
015004, 2019.
S. Assali, I. Zardo, S. Plissard, D. Kriegner, M. A. Verheijen, G. Bauer,
A. Meijerink, A. Belabbes, F. Bechstedt, J. E. M. Haverkort, and E. P.
A. M. Bakkers, “Direct Band Gap Wurtzite Gallium Phosphide
Nanowires,” Nano Lett., Vol. 13, pp. 1559–1563, 2013.

Kazuhide KumakuraGroup Leader of Thin-Film Materials ResearchGroup, Materials Science Laboratory, NTT Basic
Research Laboratories.
He received a B.E., M.E., and Ph.D. in electrical engineering from Hokkaido University in
1993, 1995, and 1998. He joined NTT Basic
Research Laboratories in 1998. His current
research interests include crystal growth of
nitride-based semiconductors and their optoelectronic and magnetic device applications. He is amember of JSAP.

41

Regular Articles

High Power and High Efficiency
Operation of Semiconductor Optical
Amplifier Assisted Extended Reach
Electroabsorption Modulated DFB
Laser (AXEL) for Extension ofTransmission Distance
Takahiko Shindo, Shigeru Kanazawa, Mingchen Chen,
Toshihide Yoshimatsu, Yoshitaka Ohiso, Kimikazu Sano,and Hideaki Matsuzaki
Abstract

We have designed a semiconductor optical amplifier integrated with an electroabsorption modulator
integrated distributed feedback laser (EML). We call this device AXEL; it is designed to enhance thepower conversion efficiency and modulated light output power of EMLs. In this study, we investigated
AXELs for both the L-band and O-band wavelength ranges. The results of experiments indicated that
dramatically increased power conversion efficiency and modulated light output power were obtainedwith the fabricated AXELs. For the L-band wavelength range AXEL, a high modulated light output
power of 9 dBm and an extension of 10-Gbit/s transmission distance to 80 km due to low chirp
characteristics of the AXEL were simultaneously demonstrated. In addition, the 25-Gbit/s 80-km
transmission achieved using the O-band wavelength range AXEL along with an avalanche photodiodewas demonstrated for the first time thanks to the significantly increased modulated light output power ofthe AXEL.
Keywords: electroabsorption modulator, distributed feedback laser, semiconductor optical amplifier

1. Introduction
Electroabsorption modulator integrated distributed
feedback (DFB) lasers (EMLs) are employed in many
applications that use intensity-modulation directdetection (IM-DD) schemes because of their highextinction ratio and low chirp characteristics compared with directly modulated lasers (DMLs) [1–3].
To meet the demand for huge data capacity, which isincreasing exponentially due to the growing use of
NTT Technical Review

Vol. 17 No. 10 Oct. 2019

various services and devices, future optical communication systems will require a light source that canoperate at high speed with high output power, andthey will need to transmit over long distances. Therefore, it is very important to enhance the power conversion efficiency of the EML without degrading thetransmission characteristics.
However, there is a limitation on how much themodulated light output of the EML can be enhanced
without significantly increasing the power consumption.
42

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This is because the electroabsorption modulator(EAM) has a large modulation loss, generally around
5 dB. The optical power loss in the EAM becomeslarge as the input power of the DFB laser diode (LD)becomes large in efforts to enhance the modulated
light output. Therefore, it is intrinsically difficult toenhance the modulated light output of an EML without significantly increasing the power consumption.
In other words, the intrinsic power conversion efficiency of the EML is very small compared with thatof DMLs.
To overcome this limitation and efficiently enhance
the modulated light output power, we have proposeda monolithically integrated semiconductor opticalamplifier (SOA) with an EML [4, 5]. The device iscalled an SOA assisted extended reach EML, or
AXEL for short. The SOA and LD in the AXEL havethe same multiple quantum well (MQW) structure so
as to avoid complicating the fabrication process. For
the AXEL, the active region is separated into the LD
and SOA components, and the input power is allocated to each component. Therefore, the SOA can
independently amplify the optical output from the
EAM without being affected by the large modulation
loss in the EAM. As a result, the AXEL can significantly enhance the power conversion efficiency andmodulated light output power.
Previously, these excellent results of the AXEL
were mainly investigated in the C-band (1530–1565nm) wavelength range. The advantages of the AXEL
such as its high efficiency operation and high modulated light output, however, can be applied to all
wavelength ranges. In this study, we fabricated bothL-band (1565–1625 nm) and O-band (1260–1360nm) wavelength range AXELs to extend the transmission distance. We first investigated an AXEL in the
1.57-μm wavelength range to meet the demand for alight source in 10-Gbit/s based next-generation
access networks (i.e., with an extended transmission
distance and increased modulated light output
power). As a result, we simultaneously extended the
10-Gbit/s transmission distance to 80 km andachieved an enhanced modulated light output powerof 9 dBm without significantly increasing power consumption, thanks to the high power conversion efficiency of the AXEL [6, 7].
Next, we demonstrated an AXEL in the 1.3-μm
wavelength range and proposed the concept of the
optical link by using the AXEL and an avalanche
photodiode (APD) receiver. As a result, we successfully demonstrated a 28-Gbit/s 80-km optical link
without any in-line amplifier or device for chromatic
NTT Technical Review

Vol. 17 No. 10 Oct. 2019dispersion compensation [7, 8].
2. Transmission at 10 Gbit/s with L-band AXEL
To take advantage of the high efficiency characteristics of the AXEL, we first investigated the L-band
wavelength range AXEL, which is intended for nextgeneration access networks. The main targets for
future access networks are to achieve an extended
transmission distance and increased power splitting
ratio. In general, a high modulated light output powerof over 9 dBm and a long distance transmission over
40 km are simultaneously required for the lightsource for 10-Gbit/s based access networks. For the
L-band wavelength range in particular, low chirp
characteristics are very important for the light source
because large chromatic dispersion of the opticalfiber severely limits the transmission distance. One
advantage of the AXEL is that the SOA can act not
only as an optical booster but also as a chirp compensator [9–12]. Therefore, the low chirp characteristicsof the AXEL enable us to extend the transmission
distance for the L-band wavelength range.
In this work, an EML having the same LD and
EAM sections as those of the AXEL was fabricatedon the same wafer to evaluate the superior chirp characteristics and high efficiency operation of the AXEL.
Schematic views of (a) a conventional EML and (b)an AXEL are shown in Fig. 1. The respective lengthsof the LD (DFB laser) and EAM sections were 300
and 150 μm. As explained above, the SOA section of
the AXEL has the same MQW structure as that of the
LD section to avoid complicating the fabrication process [4, 5]. Therefore, the AXEL can be fabricatedusing the same fabrication process as that of the conventional EML.
In addition, AXELs with SOA lengths of 50, 100,
and 150 μm were fabricated in order to assess the lasing characteristics. In Fig. 2(a), we show the injection
current-light output (I-L) characteristics of the fabricated EML and the AXELs with various SOA lengths.
In the measurement, a bias voltage Vdc of –2 V wasapplied assuming actual driving conditions. In addition, to simplify the driving method, the LD and SOAof the AXEL were electrically connected and commonly driven by the same power supply [13, 14].
For all SOA lengths, the light output power of the
AXEL increased greatly compared with that of the
EML. In addition, the light output power of the
AXEL became higher as the length of the SOA
became longer. The dependence of the power conversion efficiency on the injection current of the EML
43

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Electrode

Electrode

DFB laser

DFB laser
EAM
EAM

SOA

ActiveregionActiveregion(a)(b)

Fig. 1. Schematic device structure of fabricated (a) conventional EML and (b) AXEL for L-band wavelength range.

@50°C
Vdc = −2 V

Light output Po (mW)

16

0.07

150 µm
100 µmPower conversion efficiency 

20
18
14
12

SOA length:
50 µm

10
8
6

EML

4
2

@50°C
Vdc = −2 V

0.06

150 µm0.05
0.04

SOA length:
50 µm

0.03
0.02

100 µm

EML

0.01
0

0
0

50

100

150

200

250

Injection current Iactive (mA)
(a)

0

50

100

150

200

250

Injection current Iactive (mA)(b)

Fig. 2. (a) I-L characteristics and (b) dependence of power conversion efficiency on injection current of EML and AXEL with
SOA lengths of 50, 100, and 150 μm.

and AXELs with various SOA lengths is plotted inFig. 2(b). It is clearly seen that the power conversion
efficiency also becomes large as the SOA length
becomes longer. The SOA length of 150 μm is preferred for high efficiency operation and high output
power. Therefore, we used the 150-μm-long SOA for
the L-band wavelength range AXEL in this investigation.
As previously described, an additional advantage of
the AXEL for the L-band wavelength range is the
chirp compensation effect of the SOA. To confirm
this advantage, we experimentally compared thechirp characteristics of the AXEL and the conventional EML. The measured dependence of the chirp
NTT Technical Review

Vol. 17 No. 10 Oct. 2019parameter on EAM bias voltage is shown in Fig. 3.
The SOA length of the AXEL was 150 μm in thismeasurement. The fiber peak method was used toevaluate the chirp characteristics [15]. The driving
currents ILD of the EML and AXEL were set to 80 mA
and 120 mA, respectively. The SOA and LD of the
AXEL were electrically connected and commonly
driven. As can be seen, the chirp parameter of the
AXEL was relatively small compared with that of the
EML, especially in the low bias voltage range. In
other words, the chirp parameter of the AXEL
decreased, particularly in the high optical power
range. This indicates that the AXEL can extend thetransmission distance because of its low chirp
44Regular Articles

4
@50°C

EML
ILD = 80 mA

3
Chirp parameter 

2
1
0
−1
−2

AXEL
Iactive = 120 mA

−3
−4
−1.5

−2

−2.5

−3

EAM bias voltage (V)Fig. 3. Dependence of chirp parameter on EAM bias voltage of EML and AXEL.

@50°C

@50°C
10−6

10−6

10−8

40 km

BER

BER

80 km
BtB

10−10
10−12
−26

−24

−22

−20

−18

−16

10−8

80 km

10−10

40 km

10−12
−26

−24

BtB

−22

−20

−18

Received power (dBm)

Received power (dBm)

(a)

(b)

−16

Fig. 4. Measured 10-Gbit/s BER characteristics for (a) EML and (b) AXEL.

characteristics [16].
Then we compared the 10-Gbit/s transmission
characteristics of the AXEL with those of the conventional EML. The measured 10-Gbit/s bit error rate
(BER) performance is shown in Fig. 4 for back-toback (BtB), 40-, and 80-km transmissions. In thesemeasurements, the Vdc and voltage swing Vpp for the
EAM were respectively set to –1.92 and 2.0 V. These
modulation conditions were optimized to minimize
the BER for 40-km transmission for the EML. The
same modulation conditions were used for the AXEL.
The electrical signal input was a 9.953-Gbit/s nonreturn-to-zero (NRZ) pseudorandom binary sequence(PRBS) of 231–1. The ILD of the EML was set to 80
mA.
In contrast, the LD and SOA of the AXEL were
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Vol. 17 No. 10 Oct. 2019

commonly driven, and the total current of the active
region was set to 126 mA. As can be seen in Fig. 4(a),error-free operation was achieved for 40-km transmission with the EML. However, the 80-km transmission with the EML could not be achieved due tolarge chromatic dispersion in the optical fiber. The
estimated total amount of chromatic dispersion of the
80-km single-mode fiber (SMF) for 1570 nm was
1440 ps/nm. In contrast, the transmission distance
was successfully extended to 80 km by using theAXEL, as shown in Fig. 4(b). This extension of thetransmission distance is attributed to the AXEL’s low
chirp characteristics.
The measured eye diagrams for BtB and 40-km
transmission are shown in Fig. 5. As can be seen,clear eye openings of the AXEL and EML were
45

BtB

Light output Po (mW)

5

40 km

20 ps/div

4

AXEL

Fig. 5. Measured eye diagrams for EML and AXEL.

achieved even after 40-km transmission. For the
AXEL, we confirmed that there is no significant distortion of the quality of the eye diagram due to theintegration of the SOA.
The modulated light output power of the EML and
AXEL were also measured. The modulated light output power for the EML was as low as 6.8 dBm when
the injection current Iactive for the LD was set to 80
mA. Even when the Iactive for the LD was 150 mA, themodulated light output power reached only 8.0 dBm.
This result indicates that it is very difficult to achievesufficient light output by using a conventional EML.
In contrast, the modulated light output power of the
AXEL was 9.0 dBm with an Iactive of 126 mA. The
modulated light output power was greatly increased
without an excessive increase in the injection current,
thanks to the high efficiency characteristics of the
AXEL. These results indicate that the AXEL is an
attractive light source even for the L-band wavelength
range due to its significantly increased power conversion efficiency and low chirp characteristics.
3. Transmission at 28 Gbit/s with O-band AXEL
In contrast to the L-band wavelength range, thelarge optical loss in the SMF limits the transmission
distance for the O-band wavelength range because
chromatic dispersion in the SMF is very small compared with that for the L-band. Therefore, anenhancement of the modulated light output by using
the SOA is a breakthrough to overcome the limitationof large optical loss in the SMF for the O-band.
Next, we conducted experiments to demonstrate the
1.3-μm wavelength range AXEL with high-power
NTT Technical Review

Vol. 17 No. 10 Oct. 2019

0.03
AXEL
0.02

3

EML

2

0.01

1
0

EML

@55°C
Vdc = −1.5 V

00
20 40 60 80 100 120 140 160
Injection current Iactive (mA)

Power conversion efficiency Regular Articles

Fig. 6. M easured I-L characteristics and dependence ofpower conversion efficiency on injection current ofO-band EML and AXEL.

conversion efficiency and to extend the transmission
distance of a 28-Gbit/s optical link by using the
AXEL. Again, we also fabricated a conventional
EML that had the same LD and EAM sections as
those of the AXEL by using the same fabrication
process. The length of the LD and EAM were set to
300 and 150 μm, respectively. In addition, the SOA
length of the AXEL was set to 100 μm.
The I-L characteristics of the EML and AXEL at
55°C (solid lines) are shown in Fig. 6. As previously
described, the LD and SOA of the AXEL were electrically connected and commonly driven. In this measurement, Vdc of –1.5 V was applied to the EAM. The
dependence of the power conversion efficiency on theinjection current for the EML and AXEL are also
shown in Fig. 6 (dashed lines). As can be seen, thelight output power of the AXEL was significantly
enhanced compared with that of the EML even for the
O-band wavelength range. The light output power ofthe AXEL is almost two times larger than that of the
EML with an Iactive of 160 mA. In addition, the power
conversion efficiency of the AXEL with Iactive of 160
mA is also two times larger than that of the EML.
To take advantage of the high output power operation of the O-band AXEL, we focused on extending
the 28-Gbit/s optical link to 80 km by using the
AXEL and APD. The experimental setup used for thetransmission is shown in Fig. 7. A standard SMF witha chromatic dispersion of –1.8 ps/nm/km and transmission loss of 0.375 dB/km for the 1.3-μm wavelength range was used for this measurement. We used
an APD receiver optical subassembly (APD-ROSA),
which can detect an NRZ signal of over 25 Gbit/swith a sensitivity of –21.5 dBm [17]. The estimated46Regular Articles

PPG

ED

AXEL

APD-ROSA
Optical fiber: 20 km x 4 = 80 km
Total chromatic dispersion: −144 ps/nm
Total optical power loss: 30 dB

APD-ROSA: APD receiver optical subassembly
PPG: pulse pattern generator
ED: error detector

Fig. 7. Experimental setup for 28-Gbit/s, 80-km transmission.

BtB

Iactive = 160 mA
Vdc = −1.3 V

BER

10−2

10−3

80 km

10−4
10−5
−27
EML

AXEL

Fig. 8. Measured eye diagrams of EML and AXEL.

total optical fiber loss and dispersion were 30 dB and–144 ps/nm for 80 km with a wavelength of 1.3 μm.
Therefore, the modulated light output of the AXEL
should be more than 8.5 dBm in order for it to transmit sufficient light intensity to the APD-ROSA,
which is 80 km ahead.
Next, we compared the 28-Gbit/s transmission
characteristics of the 1.3-μm AXEL with those of theconventional EML. Measured eye diagrams for BtB
and after 80-km transmission with the AXEL and
EML are shown in Fig. 8. For both the AXEL and
EML, an Iactive of 160 mA was applied, and the modulation Vpp and the Vdc were 2.0 and –1.3 V, respectively. The electrical signal we used for the measurement was a 28-Gbit/s NRZ PRBS of 231–1. As aresult, the dynamic extinction ratio (ER) and mask
margin (MM) determined from the measured eye
diagrams were 8.7 dB and 8% for the EML.
Almost the same quality of the eye diagram for the
NTT Technical Review

Vol. 17 No. 10 Oct. 2019

EML

AXEL

Loss limit
FEC limit ~ 5E-5
−25
−23
−21
Average received power (dBm)

FEC: forward error correction

Fig. 9. M easured BER characteristics of the 28-Gbit/s
80-km transmission.

AXEL with an ER of 8.9 dB and MM of 9% was
confirmed. Furthermore, the modulated light output
power of the AXEL and EML was 5.7 and 8.5 dBm,
respectively. We therefore confirmed that the AXEL
successfully enhanced the modulated light output by
about 2.8 dB.
The measured BER characteristics of the AXEL
and EML after 80-km transmission are shown in
Fig. 9. In this measurement, we assumed the RS(Reed Solomon; 528,514) forward error correction(FEC; KR4-FEC) and set a target BER of 5.0E-5. As
can be seen in the figure, we could not achieve thetarget BER of 5.0E-5 by using the EML due to thelimitation of the modulated light output power. In
contrast, the AXEL successfully extended the transmission distance to 80 km with sufficient receiversensitivity. Therefore, we can conclude that the significantly improved power conversion efficiency and47

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modulated light output power of the AXEL enabled
us to extend the 28-Gbit/s NRZ transmission distance.
4. Conclusion
An SOA integrated EML, called an AXEL, wasexperimentally investigated for both the O-band andL-band wavelength ranges in order to extend thetransmission distance. We first investigated a 1.57μm wavelength range AXEL with various SOA
lengths. Both the power conversion efficiency andmodulated light output power of the AXEL with an
SOA length of 150 μm were significantly enhanced
compared with those of the conventional EML thatwas fabricated using the same process.
The low chirp characteristics of the AXEL werealso confirmed compared with those of the conventional EML, which enabled us to extend the transmission distance. The measured BER characteristics of
the AXEL and EML showed that the 10-Gbit/s transmission distance was successfully extended to 80 km
with sufficient light output power of 9 dBm.
Next, the 1.3-μm wavelength AXEL with an SOAlength of 100 μm was also investigated to see whether
it was possible to overcome the limitation of transmission distance due to large optical fiber loss for the
1.3-μm wavelength range. We confirmed that themodulated light output power was increased toalmost twice that of the conventional EML. Furthermore, we experimentally demonstrated a 28-Gbit/s
optical link by using the AXEL and APD-ROSA andachieved 80-km transmission with a BER of less than
5.0E-5. These results indicate that the AXEL has significantly enhanced power conversion efficiency, andit can be applied for various optical networks using
the IM-DD scheme with any wavelength range.
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[2]

T. Fujisawa, K. Takahata, W. Kobayashi, T. Tadokoro, N. Fujiwara, S.
Kanazawa, and F. Kano, “1.3 μm, 50 Gbit/s Electroabsorption
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LAN Applications,” Electron. Lett., Vol. 47, No. 12, pp. 708–710,
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K. Naoe, N. Sasada, Y. Sakuma, K. Motoda, T. Kato, M. Akashi, J.
Shimizu, T. Kitatani, M. Aoki, M. Okayasu, and K. Uomi, “43-Gbit/s
Operation of 1.55-μm Electro-absorption Modulator Integrated DFB
Laser Modules for 2-km Transmission,” Proc. of the 31st European
Conference on Optical Communication (ECOC 2005), Glasgow,
Scotland, UK, Th2.6.4, Sept. 2005.

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W. Kobayashi, M. Arai, T, Yamanaka, N. Fujiwara, T. Fujisawa, T.
Tadokoro, K. Tsuzuki, Y. Kondo, and F. Kano, “Design andFabrication of 10-/40-Gb/s, Uncooled Electroabsorption Modulator
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[4] W. Kobayashi, M. Arai, T. Fujisawa, Y. Shibata, T. Sato, T. Ito, K.
Hasebe, T. Yamanaka, and H. Sanjoh, “40-Gbit/s EADFB Laser
Integrated with Short Cavity SOA Improving Chirp Characteristics
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[5] W. Kobayashi, M. Arai, T. Fujisawa, T. Sato, T. Ito, K. Hasebe, S.
Kanazawa, Y. Ueda, T. Yamanaka, and H. Sanjoh, “Novel Approachfor Chirp and Output Power Compensation Applied to a 40-Gbit/s
EADFB Laser Integrated with a Short SOA,” Opt. Exp., Vol. 23, No.
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[6] T. Shindo, W. Kobayashi, N. Fujiwara, Y. Ohiso, K. Hasebe, H. Ishii,
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[12] K.-C. Chang, S.-W. Shen, M.-C. Hsu, Y.-J. Chiu, C.-C. Wei, and C.-K.
Lee, “Negative-chirped EAM-SOA for Distance-insensitive Optical
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[13] W. Kobayashi, Japan patent JP5823920B (October 16, 2015).
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Ishii, and M. Itoh, “Ultra Low Power Consumption Operation of SOA
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48

Regular Articles

Takahiko Shindo

Research Engineer, NTT Device Innovation
Center.
He received a B.E., M.E., and Ph.D. in electrical and electronic engineering from Tokyo Institute of Technology in 2008, 2010, and 2012. Hereceived a research fellowship for young scientists from the Japan Society for the Promotion of
Science for the years 2010 to 2012. In April
2013, he joined NTT Photonics Laboratories(now NTT Device Innovation Center), where heis researching optical semiconductor devices. Dr.
Shindo is a member of the Institute of Electricaland Electronics Engineers (IEEE) Photonics
Society, the Japan Society of Applied Physics(JSAP), and the Institute of Electronics, Information and Communication Engineers (IEICE).

Shigeru Kanazawa

Senior Research Engineer, NTT Device Innovation Center.
He received a B.E., M.E., and Ph.D. in electronic engineering from Tokyo Institute of Technology in 2005, 2007, and 2016. In April 2007,
he joined NTT Photonics Laboratories (nowNTT Device Innovation Center). He is engaged
in the research and development (R&D) of optical semiconductor devices and integrated devices
for optical communications systems. Dr. Kanazawa is a senior member of IEEE/Photonics Society and a member of JSAP and IEICE.

Mingchen Chen

Researcher, NTT Device Innovation Center.
He received a B.E. and M.E. in electronicengineering from the University of ElectroCommunications, Tokyo, in 2010 and 2012 and a
Ph.D. in engineering from the University ofElectro-Communications in 2015. He joined
NTT Device Technology Laboratories in 2015.
He is engaged in R&D of tunable laser sources.

Yoshitaka Ohiso

Senior Research Engineer, NTT Device Technology Laboratories.
He received a B.S., M.S., and Ph.D. in electrical engineering from Keio University, Kanagawa, in 1989, 1991, and 2003. Since joining NTT
in 1991, he has been researching and developing
optical semiconductor devices such as verticalcavity surface emitting lasers, optical filters andoptical modulators of electroabsorption andMach-Zehnder modulators. He also has been
involved in the epitaxial growth of III-V compound semiconductors using metal organic
chemical vapor deposition. Dr. Ohiso is a member of JSAP and IEICE.

Kimikazu Sano

Project Manager, NTT Device Innovation Center.
He received a B.S., M.S., and Ph.D. in electrical engineering from Waseda University, Tokyo,
in 1994, 1996, and 2004. He joined NTT in 1996.
Since then, he has been engaged in the design andevaluation of high-speed integrated circuits (ICs)and optoelectronic ICs for large-capacity optical
networks. From 2005 to 2006, he was a visiting
researcher at the University of California, Los
Angeles (UCLA), where he researched a microwave/millimeter-wave sensing system. From2012 to 2014, he was with NTT Electronics,
where he developed high-speed analog ICs andpackaged modules for coherent optical systems.
He is currently with NTT Device Innovation
Center, where he manages the development oflasers, photodiodes, analog/digital circuits for
optical metro-access networks. He served as amember of the Technical Program Committee forIEEE Compound Semiconductor IC Symposium(CSICS) from 2008 to 2010.

Hideaki Matsuzaki

Senior Research Engineer, Supervisor, NTT
Device Technology Laboratories.
He received a B.S. and M.S. in physics from
Kyoto University in 1993 and 1995. He joined
NTT’s Atsugi Electrical Communications Laboratories in 1995. He is currently involved in R&D
of compound semiconductor devices, photodiodes, and laser diodes. He is a member of IEICE.

Toshihide Yoshimatsu

Senior Research Engineer, NTT Device Innovation Center.
He received a B.E. and M.E. in applied physics
from Tohoku University, Miyagi, in 1998 and2000. He joined NTT Photonics Laboratories(now NTT Device Innovation Center) in 2000.
He is involved in researching and developing
ultrafast optoelectronic devices. He received theInternational Conference on Solid State Devices
and Materials (SSDM) Paper Award in 2004. He
is a member of IEICE and JSAP.

NTT Technical Review

Vol. 17 No. 10 Oct. 201949

Global Standardization Activities

Surviving in the Digital
Transformation Era; Technical
Trends and Issues from the
Perspective of The Telecommunication
Technology Committee
Mai Kaneko
Abstract

As a standards development organization certified by the Ministry of Internal Affairs and Communications of Japan, The Telecommunication Technology Committee (TTC) has been contributing to standardization activities in the field of information and communication technology by developing and disseminating standards for information and communications networks for more than 30 years. The role of
TTC is changing as the environment surrounding information and communications undergoes dramaticchanges such as the development of digital transformation. This article describes the current status andchallenges of Japanese companies in the global market and explains the latest technological trends andTTC’s initiatives to serve as a reference for future business strategies.
Keywords: digital transformation, SDGs, innovation

1. Introduction
An overview of The Telecommunication Technology Committee (TTC) and some examples of theareas of standardization the TTC handles are
explained in this section.

technology (IT)-related companies, participate.
Members can participate in standardization activities
by participating in working groups (WGs) and canalso participate in seminars and events for free or fora fee. Members actively interact with each otherthrough activities of the WGs and other events.

1.1 TTC overview
The TTC is certified by the Ministry of Internal
Affairs and Communications of Japan to standardize
information and communication technology (ICT).
The purpose of this organization is to contribute tostandardization in the information and telecommunications field and to promote the spread of standardization by creating standards related to informationand telecommunications networks.
TTC is a members-only organization in which 95
companies, including the NTT Group, informationand telecommunications companies, and information

1.2 TTC standardization activities through WGs
The TTC has 18 WGs that are organized in 5 technical fields. This organization is based on the layered
structure of a telecommunications network. The WGsgather information on related technologies and holddiscussions related to standardization. In addition to
the WGs, there are sub-WGs, ad hoc groups, andliaison committees (Fig. 1).
The Telecommunications System Committee of the
Ministry of Internal Affairs and Communications has
decided to refer upstream activities concerning all
study groups (SGs)*1 (excluding SG3 and SG9) and

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Vol. 17 No. 10 Oct. 201950

Global Standardization Activities

Divisions

WGs and other major groups

ICT Applications

Multimedia
Application

Connected
Car

BSG

IoT·
Smart Citye-health

Accessibility

Platforms

AI for Next
Gen. Services

oneM2M

SecurityEnterprise
Network

DLT

Emergency
Communication

Protocols, NWManagement and
QoS/QoE

Signalling

Network
Mgmt.

Numbering
Plan

QoS/QoE

Architecture

Network
VisionMobile NW
Mgmt

3GPP

Network
Softwarization

IoT Area
Network

Transport
Networks
and EMCILE

ClimateChange5G Coordination

Transport, Accessand Area Network

AccessNetwork

OpticalFiber

Soft Error

WGs

Source: TTC website [1]AI: artificial intelligence
BSG: Bridging the Standardization Gap
DLT: distributed ledger technology
EMC: electromagnetic compatibility

5G: fifth-generation mobile communications
ILE: immersive live experience
IoT: Internet of Things
M2M: machine to machine

Major
sub-WGs

Ad-hocgroups

NW: network
QoE: quality of experience
QoS: quality of service
3GPP: 3rd Generation Partnership Project

Fig. 1. Organization of WGs in TTC (as of June 2019).

the Telecommunication Standardization Advisory
Group (TSAG)*2 of the International Telecommunication Union - Telecommunication Standardization
Sector (ITU-T)*3 to TTC. These activities include
deliberating in advance on Japanese contribution letters (proposal documents), drawing up a Japanese
response policy, and, if necessary, submitting proposals to the Telecommunications System Committee. In
addition, the TTC is recognized as a qualified standardization organization that ITU-T can exchange
information with; ITU-T can also use TTC documents as a reference in ITU-T standardization documents, according to ITU-T Recommendations A.5*4
and A.6*5. WGs within the TTC conduct these activities.
1.3 TTC standardization examples
Here, two examples of standardization addressedby TTC in 2018 are introduced. The first example
concerns immersive live experience (ILE), which isrepresented by NTT’s Kirari! technology. ILE
enables real-time transmission of sports events andentertainment performances to remote locations andprovides an ultra-realistic live experience. Service
scenarios and frameworks of ILE were standardized
based on a proposal from Japan.
The second example is an international standard
that defines design, test, and evaluation methods and
NTT Technical Review

Vol. 17 No. 10 Oct. 2019quality standards for countermeasures against soft
errors, which are malfunctions of terrestrial communications equipment caused mainly by cosmic rays. A
soft error is not an error that permanently causes thedevice to fail, but a temporary failure that can berecovered by, for example, restarting the device. It
has recently become possible to measure the effects
of soft errors on telecommunications equipment byusing a compact accelerator-driven neutron source.
Consequently, design and evaluation indices of soft
errors were required. This is an example of Japan’s
proactive proposal that embodies the strategy of thestandard is not to follow; it is to make.

*1 SG: A group of researchers working on a particular topic.
*2 TSAG: A group providing advice on the management and operation of standardization activities in parallel with SGs.
*3 ITU-T: The ITU is a specialized agency of the United Nations
that is responsible for establishing international standards andregulations. ITU-T is the division responsible for the development of standards in the telecommunications field.
*4 Recommendation A.5: Generic procedures for including references to documents of other organizations in ITU-T Recommendations.
*5 Recommendation A.6: Cooperation and exchange of information
between the ITU Telecommunication Standardization Sector andnational and regional standards development organizations.

51

Global Standardization Activities

Standardization activities as part of a business strategy
Ensured compatibility to improve
production efficiency
Cost savings from mass production
Predecessor advantage in early
deployment
Patent license income

Company

Dissemination of technologies
and business models
Enhancement of competitivenessFinding business partners
Building an ecosystem
Marketdevelopment

ProfitSocietyStandardization
activity

User
Safety and security
Accessibility
Variety of options and choices
Usage fee reduction

User
benefits

SDGs

Market fairness andopenness
Smooth trade
Ensured quality,
performance, and safety
Environmental protection

SDGs: Sustainable Development Goals

Fig. 2. Benefits of standardization from a business perspective.

2. Significance of standardization
Here, the benefits and strategic use of standardization in business are explained.
2.1 B enefits of standardization from a businessperspective
Standardization provides many benefits as part of abusiness strategy. By ensuring compatibility and consistency through standardization, companies canimprove production efficiency. Standardization also
leads to cost reductions through mass production andsimplification. Furthermore, the early adoption ofstandardization technology will lead to first-moverprofits and license income. Finally, it is possible tocreate an ecosystem in which technologies and business models become more prevalent, competitivenessincreases, and ultimately all stakeholders benefit.
In this way, the opening of the market promotessocial trade, and the establishment of rules to meetthe environment and standards leads to improvements
in safety and security.
Not only is safety guaranteed for users, but a varietyof companies participate in standardized specifications, and a variety of products are available on themarket, which broadens the range of choices. Standardization is therefore a strategy that benefits all
NTT Technical Review

Vol. 17 No. 10 Oct. 2019

stakeholders (Fig. 2).
2.2 Strategic use of standards
Standards are classified into three categories: dejure, forum, and de facto, and they need to be strategically used for different purposes.
(1) De jure standards
De jure standards are established by formal standards organizations, for example, the ITU. The TTC
is also classified as an organization that develops dejure standards. De jure standards have great impact
on global markets and developing countries, but
because of the involvement of national administrations, deliberation and coordination may take time.
(2) Forum standards
Forum standards can be formed through the effortsof multiple companies or organizations to speed up
the development of standards. Representative examples of such organizations are the Internet Engineering Task Force (IETF)*6, the Institute of Electricaland Electronics Engineers (IEEE)*7, and the World
Wide Web Consortium (W3C)*8. Forum standards*6 IETF: A voluntary organization promoting standardization of Internet technology.
*7 IEEE: An organization that standardizes electrical and electronic
communications specifications.
*8 W3C: A nonprofit organization that standardizes web technology.

52

Global Standardization Activities

To support the business creation of member companies, it is important to engage in a wide rangeof standardization as well as marketing from the user’s point of view based on open innovation.
Change indevelopment system
Closedinnovation(Core management
resources)

Conventional
Technology
development

CurrentOpen
innovation 1.0(One-to-one external
resource
combination)

Standardize

Enterprisecentric
marketingEmerging
technology

Maturity oftechnology

ImportantUser-centric
marketingOpen
innovation 2.0(Resolving many
stakeholders andsocial issues)Standardize
Technology
combination

Source: “Open Innovation White Paper” (2nd Edition) by the New Energy
and Industrial Technology Development Organization (NEDO) andthe Japan Open Innovation Council (JOIC) [2]

Fig. 3. Expanded role of standardization.

are disadvantageous for small businesses and developing countries because standardization is influenced
by power relationships centered on developed companies.
(3) De facto standards
De facto standards, also known as market-driven
standards, are standards that are adopted widely by anindustry and customers. One example is typified by
the Windows operating system.
It is important that standardization is carried out
strategically in consideration of the balance between
the areas of cooperation that are open to the market
through standardization and the areas of competition
that are not standardized in terms of improving competitiveness.
2.3 Standardization as the role expands
In this age where the market has matured and customer needs have diversified, it is more difficult tocreate innovative services, and it is not easy to keep
up with the changes that are accelerating with thedevelopment of ICT. Until now, the worldwide trend
has been to develop technologies that make the bestuse of existing management resources and then to
standardize and commercialize them. However, as
many companies feel that creating services on their
NTT Technical Review

Vol. 17 No. 10 Oct. 2019own is time consuming and costly, they have shiftedto an open innovation system in which they create
services beyond the confines of their company andhave been moving to a user-oriented development
system in which we think about services based onuser issues and the way the services should be. As aresult, the role of the TTC has been shifting fromtechnology to marketing in corporate support (Fig. 3).
3. Environment surrounding informationand communications
This section presents information on various trends
and issues facing Japanese companies today.
3.1 D
 igital transformation and Sustainable
Development Goals (SDGs) required forenterprises
The evolution of technologies such as the Internetof Things, big data, and artificial intelligence (AI),which affect the way society should be (i.e., a modern, well-functioning society), has accelerated crossindustry collaboration, and the transformation of theindustrial structure has been remarkable. The digitaltransformation (DX) is progressing; this refers not
only to the digitization of existing businesses but also53

Global Standardization Activities

Member companies need to address DX and SDG challenges.

Changing
industrial
structure

Collaboration
betweendifferent
industries
Entry fromdifferent
industries
New players

Transforming
business models
through digitization

To achievea sustainable society

DX

SDGs

Evolution of technology
Utilization of new technologies (IoT, big data, AI, etc.)that affect the way society exists in a variety of fields
Digitization in which all kinds of software and things
connect to the Network
Operational improvement tool
Value creation toolSocialchangeHumancenteredsociety
Problemsolving
society
Rise ofdigitalconsumers

Fig. 4. DX and SDGs required for enterprises.

the transformation of business models using digitaltechnology. Achieving the United Nations SDGs has
become a common practice in global markets, andJapanese companies also face an urgent need to promote activities to achieve them (Fig. 4).
3.2 R
 eality of Japanese companies in the global
marketJapanese companies face a difficult situation in the
global market. About 30 years ago, there were 32
Japanese companies in the top 50 in the world market
capitalization ranking, but in 2018, there was only 1,
Toyota. In contrast, the United States had 32 companies in the ranking. IT companies are on top, and therise of Chinese companies is obvious (Table 1).
3.3 T
 rends at the Consumer Electronics Show
(CES) 2019
Samsung from South Korea won numerous Innovation Awards at CES 2019, topping the list of companies with the number of wins. CES is the world’s
largest consumer electronics trade show and is held
annually in Las Vegas, US. Japan had little presenceat the event, including exhibition booths. Many of theexhibits from other countries featured combinationsof existing technologies such as robots and beauty
appliances that were popular in Japan some years
ago, but with sensors and other advances. Innovation
is not only about developing cutting-edge technology
NTT Technical Review

Vol. 17 No. 10 Oct. 2019

but also about combining existing technologies andreworking them in innovative ways.
3.4 Trends in ITU-T participating companies
The membership of ITU-T, which is a representative de jure standardization organization, has
increased significantly in recent years. More than halfof the participating companies are in the traditional
IT communication field, involved in areas such as
quantum communication, digital currency, MVNO
(mobile virtual network operator), and over-the-top(OTT). However, insurance companies and those in
the automobile industry are also participating, and therange of fields represented by the participating companies is expanding. Since 2017, 4 companies inJapan (Canon, Murata Manufacturing, Softbank, andInternet Initiative Japan) have joined ITU-T, in addition to 13 companies in China and 15 in the US.
China leads the number of ITU-T contributions bycountry, with 27%, followed by South Korea with
11%, the United States with 10%, and Japan with 4%.
In China, companies as well as the government
actively participate in standards activities.
3.5 Global responses to SDGs
No Japanese company has been selected as one of
the 50 leading companies in the world that support
SDGs. The overwhelming majority are in the UnitedStates, and half are in Europe. In Asia, companies in
54

Global Standardization Activities

Table 1. Comparison of world market capitalization rankings.
1989

Country
nameType of industry

Country
name2018Type of industry

1

NTT

Japan

Telecommunications

Apple

USA

IT

2

Industrial Bank of Japan

Japan

BankingAmazon.com

USA

IT

3

Sumitomo BankJapan

BankingAlphabet, Inc.

USA

IT

4

Fuji BankJapan

Banking

Microsoft

USA

IT

5

Dai-Ichi Kangyo BankJapan

Banking

Facebook

USA

IT

6

IBM

USA

Computers

Berkshire Hathaway

USA

Investing

7

Mitsubishi BankJapan

Banking

Alibaba Group Holdings

China

IT

8

Exxon

USA

Oil

Tencent Holding

China

IT

9

Tokyo Electric Power Company

Japan

EnergyJ.P. Morgan Chase & Co.

USA

Financial

10

Royal Dutch Shell PLC

UK

Oil

Exxon Mobil

USA

Oil

11

Toyota Motor Corporation

Japan

Automotive

Johnson & Johnson

USA

Healthcare

12

GE

USA

Conglomerate

Visa

USA

Financial

Source: Diamond Weekly, August 25 issue, 2018 [3], prepared by the author

Hong Kong and Singapore are most prominent. By
industry, there are many medical, infrastructure, andchemical manufacturers. As Japan is lagging behind
other countries in SDG efforts, it can be expected thatthe formulation of SDG rules will proceed mainly
through the efforts of Europe and the United States.
4. Key technology and TTC initiatives in 2019
The TTC is constantly monitoring ITU-T trends.
ITU-T is promoting standardization under the slogan
Smart ABC (AI, banking, cities) – Innovative use of
ICT to improve quality of life, service efficiency, andcompetitiveness. There are six topics for standardization: 1) new standards for optical fiber, 2) 5G (fifthgeneration mobile communications), 3) framework
for cooperation between OTT operators and network
operators, 4) quality of service (network quality) andquality of experience (user experience quality) suchas in video streaming, 5) interoperability for personal
health, and 6) key performance indicators for achieving smart cities. TTC is focusing on trends that ITU-T
will discuss in the future such as connected cars,
quantum communications and cryptography, digitalcurrency and blockchains, AI, and Network 2030
beyond 5G, with various activities being carried out
in the TTC WGs (Table 2).

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Vol. 17 No. 10 Oct. 2019

5. Innovation study group for service innovation
To help Japanese companies develop humanresources capable of adapting to the changing times,
the TTC established a practical study group to create
use cases for innovative services using an open innovation approach. The five-part program consists oflearning how to generate the ideas needed in the service creation process, listening to innovative examples from key players in advanced companies thatdevelop and provide services from the user’s perspective, and experiencing open innovation through collaboration with participants from different organizations. The TTC launched this initiative to address thegrowing role of standards in 2018.
6. Conclusion
Japanese companies are falling behind in terms ofboth competitiveness and SDG compatibility in the
global market. It is hoped that the TTC will help todevelop innovative services, expand into global markets, and promote cross-industry catch-up and open
innovation. The TTC website [4] contains more information on standardization and the latest technicalinformation not covered in this article.

55

Global Standardization Activities

Table 2. ITU-T trends and TTC activities.
Development of new business and ICT services in the DX era• Specific examination of automobile communications technology, next-generation
mobility services, and disaster response using cars

MaaS

Connected Car WG

ILE

Multimedia Application WG(ILE-SWG)• Advanced MMT—elemental technology of NTT’s Kirari!—is ultra-realistic media
synchronization technology that makes it possible to realize a variety of video services.
This WG promotes international standardization of ILE.

e-health

Multimedia Application WG(e-health SWG)

• Promoting standardization of work items for remote medical care and ICT health careusing the latest multimedia ICT technology• Downstreaming international standards for safe listening (volume restriction function toprevent hearing disabilities, etc.)

Response to new standardization themes such as quantum communication and AI
Quantum
communication

Security WG

Digital currency
AI• Inviting experts to study groups and holding collaborative seminars with the Information
Processing Society of Japan (IPSJ)• Establishing SWGs and starting reviews

AI for Next Gen.
Services WG• Collecting and analyzing examples of use cases using AI

Contributing to building infrastructures in the future with the Network 2030
Network 2030/5G

Network Vision WG• Discussing future networks such as inter-industry linkage with OTT services and open
API
Contribution to SDGs to solve social issues

SDG

International coordination• To achieve SDGs, we held TTC seminars for executives and the public, and organized
connections between SDG efforts and TTC activities.
• At the TSAG meeting, we showed the mappings between the ITU-T SG issues and the
17 SDG goals and proposed clarification of the rationale. Proposal of a mapping
guideline from Japan is planned.

Smart City

IoT·Smart City WG
OneM2M WG• Analyzing and understanding IoT and smart city use cases, and implementing casesand disseminating them to domestic companies.

API: application programming interface
MaaS: mobility as a service

References[1][2]

Website of TTC, Organization, Working groups,
https://www.ttc.or.jp/e/org/workin-groups
NEDO and JOIC, “Open Innovation White Paper,” 2nd Edition, June2018 (in Japanese).

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Vol. 17 No. 10 Oct. 2019

[3]
[4]

Diamond Weekly, August 25 issue, 2018 (in Japanese).
TTC, https://www.ttc.or.jp/eTrademark notes

All brand names, product names, and company names that appear in thisarticle are trademarks or registered trademarks of their respective owners.

56

Global Standardization Activities

Mai Kaneko

Director, Planning and Strategy, The Telecommunication Technology Committee.
She received a B.S. in mathematics from Tokyo
Woman’s Christian University in 1997 and an
MBA from Yokohama National University in
2014. She joined NTT in 1997 and was in chargeof designing and building large-scale information systems, including one for the National
Museum of Emerging Science and Innovation, asa system engineer in corporate sales. She collaborated with various companies as an alliancestrategist from 2002 to 2006. Among her accomplishments, she devised a new tablet service thatoriginated in Japan and initiated a project in the
product development division in 2009. She wasin charge of training planning and labor management for 100 young technical employees as amanager from 2010 to 2012. She was temporarily transferred to the Center for International
Public Policy Studies (CIPPS), where she
engaged in policy recommendation work. She
returned to NTT EAST in 2015 and worked as asales manager for the Kanagawa area. She took
up her current position in 2018.
Books authored:
Individual Number Card, Pioneering the Future(contributed as a member of Nomura Institute of
Capital Markets Research)
Medical Care and Individual Number Card (coauthor)
Japan’s Growth Strategy Considered in 10 Points(co-author)

NTT Technical Review

Vol. 17 No. 10 Oct. 201957

Practical Field Information about
Telecommunication Technologies
Practical Field Information about Telecommunication Technologies

Introduction to Troubleshooting
Cases Related to Telephone Systems
in Customer Premises
Technical Assistance and Support Center, NTT EAST
Abstract

In this article, we introduce two cases involving problems that occurred in telephone systems. In thefirst case, a connection could not be established even when the customer dialed the correct number. In
the second case, silence suddenly occurred during the connection. We investigated and solved these
problems using various measurement tools and analysis methods. This is the fifty-fourth article in aseries on telecommunication technologies.
Keywords: key telephone system, IP phone, PSTN

1. Introduction
The Network Interface Engineering Group at theTechnical Assistance and Support Center provides
technical support and on-site troubleshooting fordifficult-to-solve problems in Internet protocol (IP)
services such as the FLET’S HIKARI NEXT fiberoptic broadband service and public switched telephone network (PSTN) services such as plain old
telephone services (POTS).
The rapid progression of information and communication technology (ICT) has led to our support ofvarious ICT services. Thus, when we investigate thecause of a problem, we use a variety of measurement
methods and equipment. For IP services, we obtain IP
packets in a network using packet capture tools, andwe analyze those packets using various methods. For
PSTN services, we analyze signals using waveformrecorders and ISDN (integrated services digital network) protocol analyzers. To inspect key telephonesystems, we collect and analyze specific signals in a
cable between the master equipment and terminal
telephone sets using the α line monitoring system (atool developed by the Technical Assistance and SupNTT Technical Review

Vol. 17 No. 10 Oct. 2019

port Center). Two recent cases of problems that
occurred in customer premises are introduced in thisarticle.
2. Introduction to troubleshooting cases incustomer premisesThis section describes in detail the two problems
that occurred in customer premises.
2.1 C ase 1: Connection cannot be establishedeven if customer dials correct number
2.1.1 Overview and investigation method
In this case, the customer’s key telephone system(αGX-M) was connected to five POTS lines. The
customer claimed that a connection could not beestablished even if the correct number was dialed. On
site, the master equipment unit and telephone sets
were replaced by service personnel, but the problem
persisted.
Thus, we conducted an investigation to solve theproblem. This consisted of collecting and analyzing
data when the problem occurred. The configurationof the measurement setup and tools used in the58

Practical Field Information about Telecommunication Technologies

Exchange

5 POTS
lines

GX-M master equipment unit

Star-wiring telephones

SU

4ACOU

Extension
numbers
11/20c) Waveformrecorderb) Collection ofcall historya)  star-wiring
monitoring system

ACOU: analog control unit
SU: star-wiring unit

Fig. 1. Configuration of call analysis system and measurement points.

Table 1. System status at problem occurrence.
Date/time

Outside line

Extension number

Number dialed

Date/Month/2018 at 10:00

2

20

02xx-xx-xxxx

Date/Month/2018 at 10:20

3

20

02yy-yy-yyyy

Date/Month/2018 at 10:21

2

20

02zz-zz-zzzz

investigation are shown in Fig. 1. The red circles indicate the measurement points. We investigated thefollowing using several measurement tools.
a)	Monitoring telephone operation using the αline monitoring systemb)	Collecting call history using a personal computer, that is, the maintenance console of thesystem, and checking the call-connection status
at the time of the problemc)	Checking outgoing-call sequence/selection signals (push button (PB) signals) by monitoring
the voltage between the L1-L2 lines using the
waveform recorder
The customer explained that the problem occurred
three times during the investigation period. After conducting the investigations above, we analyzed thecollected data. The system status at the time the problem occurred is summarized in Table 1.
2.1.2 Results of analyzing collected dataa) α line monitoring system
When the telephone operation was checked at thetime the problem occurred, it was found that the customer was operating the telephone normally by connecting to an outside line using the line key and pressing the dialing buttons corresponding to the destinaNTT Technical Review

Vol. 17 No. 10 Oct. 2019tion telephone number.
b)	Collection of call history via maintenance console
When the call history of the master equipment waschecked, it was found that the number dialed by thecustomer on the telephone was recorded; that is, it
indicated that the customer was operating the telephone correctly.
c)	Voltage monitoring between L1-L2 lines by
waveform recorder
The following information was obtained from the
waveform recorder. The waveforms obtained when
the problem occurred are shown in Fig. 2.
•	The PB signal of the first digit dialed (‘0’) wastransmitted approximately 640 to 790 ms afterconnecting to an outside line (loop closure).
•	The dial tone (DT) was received from theexchange after the PB signal of the first digit had
already been transmitted (‘0’).
•	The DT stopped when the second digit was
dialed (‘2’).
•	It took approximately 800 to 900 ms for the DT
to be received in the customer’s environment.
2.1.3 Cause of problem and countermeasureThis problem occurred because the PB signal of the59Practical Field Information about Telecommunication Technologies

Approx. 800 ms before
receiving DT

100 (V)
Voltage
between
L1-L2 lines(DC)

Approx. 640 ms before transmitting
dialing number
0−100

Obtain outside
line (off-hook)

0.2 (V)
Voltage
between
L1-L2 lines(AC)

100 msPB signal

DT
stop

DT

0

2

0

−0.2

x

AC: alternating current
DC: direct current
DT: dial tone

Fig. 2. Waveforms at time of problem.

Connection
destinationKey
telephonesystemExchange
Off-hook

800–
900 msDialed number

Actual connection

02xx-xx-xxxx

2x-xxxx

02yy-yy-yxxx

2y-yyyy02zz-zz-zzzz

2z-zzzz

640–
790 msDial: 0DT
Dial: 2xxxxx

Ringing signal (IR, etc.)

Ringback tone (RBT)
Dial: xxx

IR: inter ringing

Fig. 3. Calling sequence at time of problem.

first digit dialed (‘0’) was transmitted from the telephone before receiving the DT from the exchange,
preventing that digit from being recognized by theexchange. Furthermore, since the PB signal of thesecond digit dialed (‘2’) (transmitted after receiving
the DT) was processed by the exchange, and giventhat the first digit of a local office number is ‘2,’ aconnection was made to an existing 6-digit local teleNTT Technical Review

Vol. 17 No. 10 Oct. 2019

phone number. The calling sequence is shown in
Fig. 3. This problem was solved by changing the initial value of the guard interval before transmitting thefirst digit of the dialed number from 600 ms to 1000
ms. This can be easily set by using the αGX-M master equipment system data, that is, the 07–47 early
dialing prevention timer for making an outside call.
The waveforms when setting this value to 1200 ms
60

Practical Field Information about Telecommunication Technologies

100 (V)
Voltage
between
L1-L2 lines(DC)

100 msApprox. 1230 ms

0

Obtain outside
line (off-hook)

−100

DT

0.2 (V)
Voltage
between
L1-L2 lines(AC)

PB signal

0

0

−0.2

Fig. 4. Waveforms for guard interval set to 1200 ms.

Company A

Company B

ONU
Telephonesets

HIKARI
DENWA
Office A

ONU
Telephone
sets

OGW

Key
telephonesystemPBX
Analog
line

FAX

FAX

FAX: facsimile
ONU: optical network unit

Fig. 5. Equipment configuration.

are shown in Fig. 4.
2.2 C
 ase 2: Voice suddenly disappears inconnection with specific destination
2.2.1 Overview and investigation method
A customer (Company A) was using the NTT
HIKARI DENWA IP phone service in its key telephone system. The customer claimed that a voicelesscondition occurred during calls with Company B.
Company B, meanwhile, was using the NTT HIKARIDENWA (via OGW (Office Gateway) with Primary
Rate Interface) in its private branch exchange (PBX).
The equipment configuration at both locations is
NTT Technical Review

Vol. 17 No. 10 Oct. 2019shown in Fig. 5. We captured IP packets using packet-capturing equipment near the ONU (optical network unit) at both locations. Then the data at the timeof the problem occurred were analyzed.
2.2.2 Results of analyzing collected data
Six incidents were reported by the customer during
the time the packet-capturing equipment was
installed. The captured data revealed the following
information. These were common to the six incidents.
•	At Company A, the incidents always occurred
when the telephone set was used on extension
number 25.
•	Analysis by Wireshark (local area network
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Practical Field Information about Telecommunication Technologies

Destination: HIKARI DENWA network
Source: OGW

Packet data

Fig. 6. Voice packet data from Company B.

(dB)
−60

Frequency: Approx. 2100 Hz
Level: Approx. –58 dBV

−63

−81
−84
−87
−90

4

500

1000

1500

2000

2500

3000

3500 (Hz)Fig. 7. Results of analyzing frequency components.

analyzer) revealed that voice packet data from
Company B contained “ff,” which indicates complete silence (Fig. 6). This state continued until
the call was disconnected.
•	Analysis of voice packet data indicated that thesound of facsimile (FAX) communications was
mixed in with the voice call from Company A.
•	This mixed-in sound is the called terminal identification (CED) tone (frequency: 2100 Hz),which is the response signal issued on receiving
a FAX (Fig. 7).
2.2.3 Inference of cause and countermeasure
After analyzing the collected data, we inferred that
one of the causes of this problem was the mixing in
NTT Technical Review

Vol. 17 No. 10 Oct. 2019of the FAX CED tone from Company A during thecall. Consequently, on checking the surroundings
near the telephone set at Company A, we found thatthe telephone set having extension number 25 was
installed about 1 m away from the FAX machine, andthe machine was transmitting the sound of FAX communications from the speaker used for monitoring its
operation. Thus, the monitor sound during FAX communications was picked up by the microphone oftelephone number 25. To avoid unintended pickup of
the sound, the volume of the monitor sound during
FAX communications was decreased.
2.2.4 PBX operation check
In addition, we also checked the operation of the62Practical Field Information about Telecommunication Technologies

Company A

Company B
PBX

OGWIP phone

VoIP-MGC
Voice call
CED tone (frequency: 2100 Hz) becomes mixed in during call.

CED
detected

FAX communications(FAX over IP)
Voice communications

Send/receive disabled (silent)Call is cut off since no conversation can be held.

VoIP-MGC: Voice over Internet Protocol Media Gateway Card

Fig. 8. Conditions of problem occurrence.

PBX at Company B. We added a 2100-Hz signal tothe voice communication between Company A andCompany B. As a result, the voice communication
was disabled. Furthermore, on checking the specifications of the PBX equipment, we found that thisproblem would occur under the following conditions,as shown in Fig. 8.
•	The equipment configuration included IP phones(with a Voice over Internet Protocol (VoIP)Media Gateway Card installed) used as extension telephones.
•	The VoIP Media Gateway Card would switch to
the FAX communications mode (FAX over IP)
from the voice communications mode when it
detected a CED tone (2100 Hz).
•	A voiceless condition would occur after the card
switched to FAX communications mode because
the telephone set (i.e., IP phone) was not a FAX
machine.
We confirmed that the equipment configuration at
Company B was the same as that at Company A. Thismeans that there was a possibility that the same problem could occur when Company B was making a call

NTT Technical Review

Vol. 17 No. 10 Oct. 2019with another destination. Therefore, we proposed that
the PBX (VoIP Media Gateway Card) should be set todisable the switching to FAX communications mode
during a call (set FAX over IP to disabled). This proposal was accepted and implemented, and then problem was solved.
3. Conclusion
This article introduced two case studies of problems occurring in telephone systems in customer
premises. The first one described a problem in a key
telephone system caused by the timing of transmission of a dial tone in POTS. The other involved aproblem in which a PBX would have a misoperation
due to picking up a sound in the surrounding area. As
we explained here, we use not only modern IP technologies but also conventional PSTN technologies tosolve problems in the field. The Technical Assistanceand Support Center will continue to provide solutions
to problems in the field by using various tools andmethods in order to ensure safe and secure telecommunication services.

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External Awards
Outstanding Catalyst Depicting Business Assurance
Awareness
Winner: Skynet catalyst program (The member includes Kenichi
Tayama, Shingo Horiuchi, and Chao Wu, NTT Access Network Service Systems Laboratories.)
Date: May 16, 2019
Organization: TM Forum

For “Field Extension in Secret-shared Form and Its Applications to
Efficient Secure Computation.”
Published as: R. Kikuchi, N. Attrapadung, K. Hamada, D. Ikarashi,
A. Ishida, T. Matsuda, Y. Sakai, and J. C. N. Schuldt, “Field Extension in Secret-shared Form and Its Applications to Efficient Secure
Computation,” ACISP 2019, Christchurch, New Zealand, pp. 343–
361, July 2019.

For “Skynet Catalyst proof of concept (PoC).”
The Skynet is a TM Forum catalyst project and it aims for an endto-end service orchestration and monetization on a 5G network slicein hybrid network for large enterprises with multiple global locations.
Its catalyst champions include BT, Chunghwa Telecom, Deutsche
Telekom, Du, NTT Group, Orange, Verizon, Vodafone, TelenetGroup, Telecom Italia, and Telus. Participants include Amartus,
BearingPoint, Ericsson, EXFO, Infosys, and RIFT. At Digital Transformation World 2019, Skynet demonstrated PoC operation of
remote medical services using a network resource management technology prototype.

Best Paper Award
Winner: Yuto Sagae, Takashi Matsui, and Kazuhide Nakajima, NTT
Access Network Service Systems Laboratories
Date: July 10, 2019
Organization: The 24th OptoElectronics and Communications Conference (OECC 2019)Best Paper Award
Winner: Yousuke Takahashi, NTT Network Technology Laboratories; Keisuke Ishibashi, International Christian University; Masayuki
Tsujino, NTT Network Technology Laboratories; Noriaki Kamiyama, Fukuoka University; Kohei Shiomoto, Tokyo City University;
Tatsuya Otoshi, Yuichi Ohsita, and Masayuki Murata, Osaka University
Date: June 6, 2019
Organization: The Institute of Electronics, Information and Communication Engineers (IEICE)
For “Separating Predictable and Unpredictable Flows via Dynamic Flow Mining for Effective Traffic Engineering.”
Published as: Y. Takahashi, K. Ishibashi, M. Tsujino, N. Kamiyama,
K. Shiomoto, T. Otoshi, Y. Ohsita, and M. Murata, “Separating Predictable and Unpredictable Flows via Dynamic Flow Mining for
Effective Traffic Engineering,” IEICE Trans. Commun., Vol. E101-B,
No. 2, pp. 538–547, 2018.
Best Paper Award
Winner: Ryo Kikuchi, NTT Secure Platform Laboratories; Nuttapong Attrapadung, National Institute of Advanced Industrial Scienceand Technology; Koki Hamada and Dai Ikarashi, NTT Secure Platform Laboratories; Ai Ishida, Takahiro Matsuda, Yusuke Sakai, andJacob C. N. Schuldt, National Institute of Advanced Industrial Science and Technology
Date: July 4, 2019
Organization: The 24th Australasian Conference on Information
Security and Privacy (ACISP) 2019

NTT Technical Review

Vol. 17 No. 10 Oct. 2019

For “Ultra-low-XT Multi-core Fiber with Standard 125-μm Cladding for Long-haul Transmission.”
Published as: Y. Sagae, T. Matsui, and K. Nakajima, “Ultra-low-XTMulti-core Fiber with Standard 125-μm Cladding for Long-haul
Transmission,” OECC 2019, Fukuoka, Japan, July 2019.
Best Paper Award
Winner: Sayaka Yagi, Kimio Tsuchikawa, and Kohji Tsuji, NTT
Access Network Service Systems Laboratories
Date: July 18, 2019
Organization: 23rd International Conference Information Visualisation
For “A Heatmap-based Visualization Technique for Finding
Operational Problems.”
Published as: S. Yagi, K. Tsuchikawa, and K. Tsuji, “A Heatmapbased Visualization Technique for Finding Operational Problems,”
23rd International Conference Information Visualisation (IV19) Part II, pp. 110–115, Adelaide, Australia, July 2019.
Technical Committee on Communication Quality Best
Research Award
Winner: Takuto Kimura, NTT Network Technology Laboratories;
Tatsuaki Kimura, Osaka University; Arifumi Matsumoto and Jun
Okamoto, NTT Network Technology Laboratories
Date: July 18, 2019
Organization: IEICE Technical Committee on Communication
Quality
For “Video Engagement Control with Bayesian Optimization.”
Published as: T. Kimura, T. Kimura, A. Matsumoto, and J. Okamoto,“Video Engagement Control with Bayesian Optimization,” IEICE
Tech. Rep., Vol. 118, No. 503, CQ2018-100, pp. 43–48, 2019.

64

Papers Published in Technical Journals andConference Proceedings
Women in Security: Building a Female InfoSec Community in Korea, Japan, and Taiwan
S. Kang, A. Nakajima, and H. Yen
Black Hat USA 2019, Las Vegas, NV, USA, August 2019.
The information security industry has historically been a maledominated field, and today, unfortunately, this situation has notchanged much. Indeed, the survey conducted by Frost & Sullivan in2017 concluded that women represented only 11% of the current
information security workforce. While more women are expected tojoin the information security industry to reach new levels of innovation, still, it is difficult to increase their number because women face

NTT Technical Review

Vol. 17 No. 10 Oct. 2019many obstacles, such as stereotype bias. One way to bridge this gap
is a female community, which can support and motivate women, andproduce more role models. In this talk, we will introduce three representative Asian female communities, Power of XX, CTF for GIRLS,
and HITCON GIRLS, which were established in Korea, Japan, andTaiwan, respectively. We will first explain how we build and maintaineach female community and then go into the details of how we tackled various challenges, such as building a sustainable community andattracting and retaining new members.

65