NTT Technical Review
August 2015 Vol. 13 No. 8
Feature Articles: Frontier Research on Low-dimensional
Semiconductor Physics
Ultimate Control of Electronic Properties in Low-dimensional Semiconductors
High-speed Single-electron Transfer toward High-accuracy Current Standards
Noise in Nanometer-scale Electronic Devices
Gate Tuning of Direct/Indirect Optical Transitions in Silicon
Microscopic Probing of Crystalline Electrons Using Magnetic Resonance
Semiconductor Quantum Structures with Single-atom Precision
Creating a Topological Insulator Using Semiconductor Heterostructures

Regular Articles
Two-mode Squeezing in an Electromechanical Resonator
Improving User Capacity and Disaster Recovery Time in IP Telephone
Service Systems
Development of High-capacity Protocol for M2M Services and Its Application
to a Pallet Management System

Global Standardization Activities
Reports of 4th ITU-T Review Committee and 1st FG-DFS (Digital Financial
Services) Meetings

Practical Field Information about Telecommunication
Technologies
Case Studies of Faults and Countermeasures in Access Telephone Office

External Awards/Papers Published in Technical Journals
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Proceedings

Feature Articles: Frontier Research on Lowdimensional Semiconductor Physics

Ultimate Control of Electronic
Properties in Low-dimensional
Semiconductors
Akira Fujiwara
Abstract

Recent progress in semiconductor technology has enabled the development of structures that are
scaled down to the nanometer level with high precision by using highly sophisticated nanofabricationand crystal growth techniques. Confining electrons to such low-dimensional nanostructures makes it
possible to achieve precise control of electrons as particles as well as waves. The Feature Articles in thisissue review our recent research activities involving low-dimensional semiconductor devices that will beapplied to achieve innovative electronics in future science and information and communication technologies.
Keywords: low-dimensional semiconductor, single-electron device, quantum device

1. Introduction
Nowadays, we are surrounded by many electronic
information processing tools such as personal computers (PCs) and smartphones that quickly processenormous amounts of information delivered throughhigh-speed networks all over the world. All of theseinformation and communication technology (ICT)elements have become vital lifelines to support ourdaily life and business. One of the key technologies
for sustainable development of our ICT society issemiconductor technology. A semiconductor is amaterial that has electrical conductivity properties
between those of conductors (metals) and insulators.
They include silicon used for large-scale integrated(LSI) circuits, compound semiconductors used forlaser diodes in optical fiber telecommunications, andnitride semiconductors developed for white lightemitting diodes (LEDs). One of the great features ofsemiconductors is that they can switch electric current or emit light at a high energy-efficiency level aswell as aid in the downsizing of components.
Remarkable progress has been made in semiconductor technology in the last several decades throughcomprehensive research on high-quality material

growth, device fabrication, and device physics aimed
at improving design and performance. If we take alook at the history of the LSI circuit chip, whichworks similarly to the central processing unit, or
brain, of a PC, we can see that improvements in performance have been achieved year-by-year by packaging a larger number of transistors whose sizes havebeen miniaturized using micro- and nano-fabrication
techniques. The number of transistors per chip currently exceeds 1 billion, which is on the same orderas the world population.
However, while performance has grown according
to this simple scaling rule, the power consumption of
the chip has also increased significantly and isbecoming a serious problem in terms of the environmental impact and energy costs. As a result, there arerising technical demands and expectations for lowpower devices and circuits that operate on novel principles and concepts. Semiconductors, on the other
hand, have attracted much attention as a material forbasic research on solid-state physics. Various structures and devices that are artificially fabricated using
advanced nanofabrication processes have provided an
excellent platform to advance the frontiers of physics.
Therefore, one can say that semiconductor research is
NTT Technical Review

Feature Articles

a field where science and technology are most beautifully united. In this respect, if we can control new
physical phenomena that manifest in novel semiconductor structures, we will be able to create new electronics that achieve the ultimate functionality andperformance, which are totally different from thoseof simple current switches and light-emission devices. This has motivated us in the Physical Science
Laboratory of NTT Basic Research Laboratories to
conduct research on semiconductors to explore theirapplications and new physical properties in order toachieve future innovation. The main results of ourrecent activities are reviewed in these Feature Articles.
2. Electronic properties of low-dimensional
semiconductors and their ultimate control
One of the key players in the functioning of semiconductor devices is an electron that moves aroundfreely in the semiconductors. For example, the mostfundamental function of transistors in LSI circuits isto switch electric current by controlling the flow ofelectrons in semiconductors. This function is often
considered similar to stopping and starting the flowof water. However, this is approximately true only if
the size of the semiconductors is large enough. It is
now well known in quantum mechanics that an electron—which is an elementary particle with a minimum electric charge of approximately 1.6 × 10−19
Coulombs—behaves as a wave with the de Broglie
wavelength. When the electron is confined in nanometer-scale structures artificially constructed throughcrystal growth of semiconductor heterostructures
and/or nanofabrication, which are often called lowdimensional semiconductors, its quantum mechanical nature appears as a wave. Furthermore, even as aparticle, electrons behave very differently since electrons confined in a small space have a large repulsive
Coulomb force between them due to their negative
charge, although the amount of the charge is small.
This is in contrast to the case when electrons movearound freely in a larger space. We can utilize such an
electron-electron interaction to manipulate and control individual electrons. In addition, electrons haveinternal states called spin as well as the corresponding magnetic moments. Spin has been applied practically in magnetic memory constructed of ferroelectric
metals, but it has also attracted attention recently as apotential information carrier in new information processes. This is because the use of the flow of spin, thatis, spin current, could prevent energy dissipation andVol. 13 No. 8 Aug. 2015

heat generation, which are inevitable when electron
current—the flow of charge—is used for information
processing.
As described above, electrons in low-dimensional
semiconductors show a variety of unique properties
based on quantum mechanics, electron-electron interaction, spin, and their combined phenomena. Suchproperties are spawning research subjects from bothpure physics and engineering for future science andtechnology. Typical structures of low-dimensional
semiconductors are often categorized into quantum
wells (two-dimensional planar structures), quantum
wires (one-dimensional linear structures), and quantum dots (structures confined in all directions, such as
cubes and spheres, thereby called zero-dimensional
structures).
A zero-dimensional structure, in which electrons
can be regarded as particles but are dominated by theelectron-electron Coulomb interaction, is sometimes
called a single-electron island because we can store
electrons in it one by one. The research on lowdimensional semiconductors has a long history, andmany studies have been done on their physics as well
as device applications such as single-electron devices. Recently, technical progress achieved in both
high-quality crystal growth and sophisticated nanodevice fabrication has been opening up the possibilityof the ultimate control of electrons. The article“High-speed Single-electron Transfer toward Highaccuracy Current Standards” [1] describes fast andaccurate control of electrons as particles achieved
with silicon nanowire devices. One of the targets ofthis study is to achieve electric current standards for
metrology. If we can generate accurate current basedon the clocked transfer of single electrons, it may bepossible to complete the so-called metrological quantum triangle of electric standards. The current standards remain to be developed as the final element inaddition to the already existing quantum resistanceand voltage standards based on semiconductor devices and superconductor devices, respectively.
The article “Noise in Nanometer-scale Electronic
Devices” [2] describes thermal noise in ultimately
miniaturized electronic devices, which is investigatedbased on the resolution of single electrons. A silicon
nanotransistor with ultra-high charge sensitivity isutilized to read out the voltage noise in an ultra-small
dynamic random access memory (DRAM) and isused to measure the quantized values of fluctuating
voltages resulting from the thermal fluctuation in thenumber of electrons in the charge node of the DRAM.
We analyze the thermal noise and determine how itFeature Articles

deviates from that expected from the conventional
electric circuit model in the tiny device.
The efforts to make silicon light emissive are introduced in “Gate Tuning of Direct/Indirect Optical
Transitions in Silicon” [3]. It is well known that silicon is categorized into a material with an indirect
optical transition and has low efficiency in terms oflight emission. What is strikingly unique about thisresearch is that the efficiency of the light emission iselectrically increased by strengthening the direct
optical transition. In fact, the reason we are able to do
this is not yet clear, but the results suggest that theelectron wave is strongly scattered in our quantum
well structures with a specially prepared silicon/silicon dioxide interface. This is really amazing because
it proves that there are still unexplored electronic
properties in silicon, which already has a long historyof research.
The studies above focus on silicon, but gallium
arsenide (GaAs) is the focus in “Microscopic Probingof Crystalline Electrons Using Magnetic Resonance”[4]. In this work, we observe for the first time thecrystallization of electrons in a two-dimensional system in a high magnetic field, which is similar to the
crystal of a solid-state material in which atoms are
regularly allocated in space. This takes place because
electrons tend to avoid each other and find theirplaces in an adequately spaced arrangement. In termsof physics, this corresponds to the situation where theenergy of the electron-electron interaction is higher
than that of the kinetic motion of electrons. Usinghigh quality film grown based on our crystal growthtechnology was essential in confirming this clean
electron system in two dimensions.
In “Semiconductor Quantum Structures with Single-atom Precision” [5], a unique method for fabricating low-dimensional semiconductors is introduced. It is quite different from the standard technology employing lithography (often called the top-down
approach) and is used to construct an ultra-small
structure by placing atoms one by one (the bottom-up
approach). When a compound semiconductor is used
in which the wave-like nature of electrons is prominent, the shape of the electron wave confined in aquantum-dot structure is clearly observed. Thismethod enables us to control the shape of the electronvery precisely since our structure is accurately built
using the atom as a building block.
The last article, “Creating a Topological Insulator
Using Semiconductor Heterostructures” [6], describes
the investigation of a topological insulator, a subjectof great interest in solid-state physics. Topology isoriginally a term used in mathematics, but a topological insulator refers to a unique material whose
surfaces or edges are electrically conductive, although
there are insulators in the internal parts. It is theoretically predicted that by using such a low-dimensional
electron system at the edge of the material, we cangenerate a flow of magnetic moments of electron spin(called spin current) without any flow of electrons as
charges. If this is achieved, it may lead to innovativetechniques enabling information to be transmittedwith almost no energy dissipation.
3. Future prospects
As indicated above, the research on semiconductors
is continuously driven by both scientific and technical interests and is approaching a level where precise
control of electrons and atoms will become a reality.
These goals have been elusive until now. We believethat we will be able to create innovate semiconductor
devices that will surpass existing devices in performance and function by further improving the controllability of electronic properties in low-dimensional
semiconductors created through our advanced fabrication and crystal growth technology. The PhysicalScience Laboratory at NTT Basic Research Laboratories thus continues to conduct research to develop
novel electronics for the future information technology society such as highly accurate and highly sensitive electronics and novel information transmitting
and processing devices based on new quantum
mechanical phenomena.
References
[1]

[2]

[3]

[4]

[5]

G. Yamahata, K. Nishiguchi, and A. Fujiwara, “High-speed Singleelectron Transfer toward High-accuracy Current Standards,” NTT
Technical Review, Vol. 13, No. 8, 2015.
https://www.ntt-review.jp/archive/ntttechnical.php?contents=ntr2015
08fa2.html
K. Nishiguchi and A. Fujiwara, “Noise in Nanometer-scale Electronic
Devices,” NTT Technical Review, Vol. 13, No. 8, 2015.
https://www.ntt-review.jp/archive/ntttechnical.php?contents=ntr2015
08fa3.html
J. Noborisaka, K. Nishiguchi, and A. Fujiwara, “Gate Tuning of
Direct/Indirect Optical Transitions in Silicon,” NTT Technical
Review, Vol. 13, No. 8, 2015.
https://www.ntt-review.jp/archive/ntttechnical.php?contents=ntr2015
08fa4.html
K. Muraki and T. D. Rhone, “Microscopic Probing of Crystalline
Electrons Using Magnetic Resonance,” NTT Technical Review, Vol.
13, No. 8, 2015.
https://www.ntt-review.jp/archive/ntttechnical.php?contents=ntr2015
08fa5.html
K. Kanisawa and S. Fölsch, “Semiconductor Quantum Structures with
Single-atom Precision,” NTT Technical Review, Vol. 13, No. 8,
2015.

NTT Technical Review

Feature Articles[6]

https://www.ntt-review.jp/archive/ntttechnical.php?contents=ntr2015
08fa6.html
K. Suzuki and K. Onomitsu, “Creating a Topological Insulator Using
Semiconductor Heterostructures,” NTT Technical Review, Vol. 13,
No. 8, 2015.
https://www.ntt-review.jp/archive/ntttechnical.php?contents=ntr2015
08fa7.html

Akira Fujiwara

Senior Distinguished Researcher, Senior Manager of Physical Science Laboratory and Group
Leader of Nanodevices Research Group, NTT
Basic Research Laboratories.
He received his B.S., M.S., and Ph.D. inapplied physics from the University of Tokyo in
1989, 1991, and 1994, respectively. He joined
NTT in 1994 and has been engaged in researchon silicon nanostructures and their application to
nanodevices and single-electron devices. He wasa guest researcher at the National Institute ofStandards and Technology (NIST), Gaithersburg,
MD, USA, in 2003–2004. He was a director of
the Japanese Society of Applied Physics (JSAP)in 2010–2011 and a visiting professor at Hokkaido University in 2013. He received the International Conference on Solid State Devices andMaterials (SSDM) Young Researcher Award in
1998, the SSDM Paper Award in 1999, and JJAP
(Japanese Journal of Applied Physics) Paper
Awards in 2003, 2006, and 2013. He was awarded the Young Scientist Award from the Ministryof Education, Culture, Sports, Science and Technology in 2006. He was supported by the funding
program for Next Generation World-Leading
Researchers (NEXT Program), JSPS (Japan
Society of Promotion of Science) during 2011–2014. He is a member of JSAP and a senior
member of IEEE (Institute of Electrical andElectronics Engineers).

Vol. 13 No. 8 Aug. 2015



Feature Articles: Frontier Research on Lowdimensional Semiconductor Physics

High-speed Single-electron Transfer
toward High-accuracy Current
Standards
Gento Yamahata, Katsuhiko Nishiguchi,
and Akira Fujiwara
Abstract

Single-electron transfer is a technique for manipulating single electrons to generate an accurate electric current, which is expected to be used as a new current standard. Realization of the current standard
is important for performing quantum metrology triangle experiments, with which we can confirm theaccuracy of fundamental physical constants, and it could be a contributing factor in redefining theampere. NTT Basic Research Laboratories has been studying single-electron transfer using silicon transistors. In this article, we discuss high-speed single-electron transfer via a small island electricallyformed using silicon transistors and via a trap level in silicon.
Keywords: single-electron transfer, silicon transistor, current standard

1. Single-electron transfer and current
standards
Single-electron (SE) transfer is a technique for conveying electrons one by one in synchronization witha clock signal. It has been extensively studied using
solid devices mainly fabricated from metals or semiconductors. An accurate current flow generated by
SE transfer is expected to be used as a new current
standard, which corresponds to a measure of electric
current. Realization of the current standard could leadto the redefinition of the ampere (Fig. 1(a)), which
has recently attracted much attention. The definitionof the ampere is presently referenced to a force
produced by the same current flowing through two
infinite-length conductors in a vacuum, and it istherefore difficult to achieve the exact conditions
described in the definition in actual experiments. In apractical sense, the definition has the effect of fixing
the value of the vacuum permeability (with a unit ofm·kg·s −2·A −2). In addition, because the ampere
depends on the units of length, mass, and time, theaccuracy of the ampere is strongly affected by thechange in the weight of the international prototype
kilogram*1, which is the only artifact among the standards and defines mass.
To eliminate such uncertainty originating from theartifact, the abolition of the international prototype
kilogram was proposed in 2011 (and will be carriedout after 2018), and, along with the abolition, theampere will also be redefined. In the redefinition, theampere is set by fixing the numerical value of theelementary charge e. SE transfer can generate accurate current with a value of e × f simply using an input
clock signal with frequency f, which can be obtainedwith the highest accuracy among all standards. It istherefore expected to be used as an ultimate current
standard that enables the ampere to be directly set
with a single device.
In addition, an important milestone for achieving
the new current standard is the quantum metrology
triangle experiment (Fig. 1(b)). In this experiment,*1	 International prototype kilogram: This is an alloy of platinum andiridium, which is now used as the definition of mass. After theabolition of the international prototype kilogram, the kilogram
will be set by fixing the Planck constant.

NTT Technical Review

Feature Articles

ice
ev
rd
fe
ns

n

tra

so

E

vo

(S
d
ar
nd
sta
rre
Cu

V = RKI

cd

Boltzmann constant

d

nt

ar

nd

Quantum Hall
resistance standard

Current
k

s ta

h

ge

lta

Planck constantV = f/KJ

I = ef

I
K

ph

c

kg

m

f

se

mol

Speed of light

Frequency
Jo

A)

Hyperfine splitting frequency of 133Cs
Re
v
d
133
( Cs)hfs
th efineit
Avogadro constantElementary am ion
pe ocharge
re f
NA
e
S

V
VoltageKcd
e : elementary charge
K J : Josephson constant
R K: von Klitzing constant

Luminous efficacy of
monochromatic radiation
Fixed value

Base unit(a) Proposed redefinition of the International
System of Units (SI)(b) Quantum metrology triangle

Fig. 1. Application to current standards.

the current generated from a current standard using
SE transfer is compared with that generated from a
combination of the quantum Hall resistance standard*2 and the Josephson voltage standard*3, which
are now used in the International System of Units(SI). If SE transfer with sufficiently high accuracy
can be achieved, the accuracy of the von Klitzing
constant RK = h/e2, where h is the Planck constant,and the Josephson constant KJ = 2e/h would be verifiable through the experiment, which could lead to therealization of more accurate electrical standards.
For application to the current standard, the device
should be able to generate a wide range of current.
However, because e is a tiny value (~1.6 × 10−19 C),
many electrons must be carried to obtain a large current level. For example, high-speed SE transfer witha gigahertz transfer frequency is necessary to performthe quantum metrology triangle experiments withhigh accuracy. In addition, since the accuracy of thepresent electrical standards is about 10−8, the accuracy of SE transfer should be below 10−8. SE transfer
has so far been studied using various systems, but
devices that can simultaneously satisfy both the speed
and accuracy requirements have not yet been developed.
At NTT Basic Research Laboratories, we have beenVol. 13 No. 8 Aug. 2015

studying SE transfer using silicon devices as part ofefforts to achieve high-speed and high-accuracy SEtransfer. With silicon, we should be able to stably
fabricate devices that can be used for the current standard because we can use well-established device
fabrication techniques accumulated through developments in the semiconductor industry. In this article,
we introduce tunable-barrier SE transfer devices,which can be operated at high speed.

*2 Quantum Hall resistance standard: When a two-dimensional electron system is placed under a low temperature and a high magnetic field, Hall resistances are quantized due to the quantum Hall
effect. The quantum Hall resistance standard is a standard using
the value of the quantized Hall resistance, which is the product ofthe von Klitzing constant (RK = h/e2) and the reciprocal of the integer number.
*3 Josephson voltage standard: When a high-frequency signal withfrequency f is applied to a structure having an insulator (or a normal conductor) sandwiched with two superconductors, the output
voltages are quantized due to the alternating-current Josephson
effect. The Josephson voltage standard is a standard using thevalue of the quantized voltage, which is the integer multiple of the
product of f and the reciprocal of the Josephson constant (KJ =2e/h).

Feature Articles(b) SEM image of device

in

G2
e−

SE island

A

G1

Emission
SE
e−

G1

e−

Dra

Sou
rce

Trap level

Trap level
G1

Upper gate (UG)edioxidVG2nSilicoVUG

G2

200 nmVG1

SE island

Silicon dioxide
Frequency fSilicon wire

Silicon(a) Schematic of device

SourceCaptureTrap level

e−

e
e

G2
Drain

−

−

Electron

SE island addition energy(c) SE transfer mechanism

Fig. 2. Silicon tunable-barrier SE transfer device.

2. Silicon tunable-barrier SE transfer devices:transfer via an SE island
A schematic and an SEM (scanning electron microscope) image of the silicon tunable-barrier SE transfer device are respectively shown in Figs. 2(a) and2(b). We used electron beam lithography to fabricate
silicon transistors that have double-layer n+-doped
polycrystalline-silicon gate electrodes on a siliconwire with a width of a few dozen nanometers. Eachlower gate (G1, G2) induces a potential barrier in thesilicon wire, leading to the formation of a small
region between G1 and G2 (hereafter called an SE
island) in which electrons are confined. The uppergate (UG) is used to modulate the electron potential
in the SE island.
To perform SE transfer via the SE island, we apply
high-frequency signal VG1 with frequency f to G1,
with fixed negative voltage V G2 applied to G2(Fig. 2(a)). In this case, the barrier under G1 is periodically modulated, with a fixed barrier formed underG2, as shown in the electron potential diagram in
Fig. 2(c). Since G1 and the SE island are capacitively
coupled, the potential in the SE island rises as the barrier under G1 rises. As a result, electrons captured by
the SE island from the source when the barrier under
G1 is low are eventually emitted to the drain over thebarrier under G2. When an SE is transferred eachcycle, the output current level is ef. To capture an
accurate number of SEs in the SE island, the electron
addition energy*4 in the SE island must be muchlarger than the thermal fluctuation energy determinedby temperature. This condition can be achieved byfabricating nanometer-scale devices. In addition,
when the resistance determined by the potential barrier shape during the capture of electrons is small, thedelay time determined by the product of the resistance and the island capacitance is short, resulting infast SE transfer. In the tunable-barrier SE transfer,since the height of the potential barrier during thecapture of electrons can be low, it is possible toachieve high-speed operation by reducing the resistance of the barrier.
A typical result of high-speed SE transfer using thetunable-barrier transfer device is shown in Fig. 3(a)[1]. By increasing the voltage applied to the UG(VUG), we lower the electron potential of the SE
island and change the number of electrons captured in
the SE island. In the flat plateau regions (with current
levels of 1ef, 2ef, and 3ef ), an integer number of electrons is accurately transferred. In addition, we demonstrated high-speed single-hole transfer by fabricating a p-type source and p-type drain [2] (Fig. 3(b)). If
radiative recombination of simultaneously transferred
electrons and holes can be efficiently induced, it will
be possible to realize a single-photon source with anaccurate period. Furthermore, we combined the tunable-barrier SE transfer device with an SE-resolution
charge sensor and evaluated the transfer accuracy bydetecting the number of transferred SEs. We have so
far reported that the error rate of the SE transfer is*4 Electron addition energy: This is the energy required to add one
electron to a certain region. The value is mainly determined by the
charging energy originating from electrostatic repulsion of electrons and the quantum-mechanical energy originating from confinement of electrons.

NTT Technical Review

f = 2.3 GHz
T = 20 K

1.0

3

0.8

2

0.6
0.4

1

0.2
0

13

14

15 16
VUG (V)(a) High-speed SE transfer

f = 1 GHzT = 17 K

0.25
0.20

1

0.15
0.10
0.05
0

0

17

2

0.30

−1.0

−0.9 −0.8
VUG (V)−0.7

Transfer current (ef )1.2Transfer current (nA)4

1.4Transfer current (ef )Transfer current (nA)

Feature Articles

0(b) High-speed single-hole transfer

Fig. 3. Transfer via an SE island.

SE island

Trap level

2

1

1
0

−0.6

−0.4

−0.2

0

0

0.4
0.3
0.2
0.1
0

0

1

2

3

4

0.8

f = 3.5 GHz
T = 17 K

0.6

1

0.4
0.2
0

−0.40

−0.30

−0.20

VUG (V)

Frequency (GHz)VUG (V)(a) Comparison between
trap level and SE island(b) Frequency dependence(c) High-speed SE transfer
via a trap level

Transfer current (ef )2Experiment
Theory (ef )0.5Transfer current (nA)

3

4
3

1.0

0.6
Transfer current (nA)54
f = 10 MHz
T = 17 K

Transfer current (ef )Transfer current (pA)

60

Fig. 4. SE transfer via a single trap level.

about 10−4 at about 100 MHz and that it is possible toreduce it to about 10−8 by optimizing the operating
conditions [3].
3. Silicon tunable-barrier SE transfer devices:transfer via a trap level
In the SE transfer via the SE island, increasing theelectron addition energy by scaling down the device
size leads to improved transfer accuracy, but device
miniaturization beyond the limitation of semiconductor nanofabrication technology is not easy. In contrast, using a naturally existing trap level with anextremely fine confinement area of less than 10 nm,
which is difficult to artificially fabricate, can lead tohigh-accuracy operation due to its large electron
addition energy. Among the silicon SE transfer devicVol. 13 No. 8 Aug. 2015

es like the one shown in Fig. 2(a), we selected a
device that has a single trap level under the right edgeof G1 and measured the transfer current via the traplevel [4]. The trap level most likely originates from a
trap at the interface between silicon and silicon dioxide. Although the voltages applied to G1 and G2 aresimilar to those for the transfer via the SE island, inthe trap-mediated transfer, an SE is captured by thetrap level from the source when the barrier under G1
is low. Then, when the barrier under G1 is high, thecaptured SE is emitted to the drain.
Current plateaus of the SE transfer at a frequency of
10 MHz are shown in Fig. 4(a). The 1ef plateau
originating from the trap-mediated transfer is muchwider than the other plateaus originating from the SEisland-mediated transfer, which indicates that the
trap-mediated transfer is more accurate. To performFeature Articles

high-speed operation, the SE capture and emission
shown in Fig. 2(c) must be sufficiently fast. Detailed
measurements reveal that fast capture and emission
can be achieved by lowering the barrier height underG1 during the capture phase and by applying a strong
electric field at the trap level during the emission
phase. This can be achieved when the high-frequency
signal has a large amplitude. Under this condition, thevalue of the 1ef plateau increases as frequency fincreases, as shown in Fig. 4(b), and we achieve highspeed operation at 3.5 GHz (Fig. 4(c)). Moreover,high-resolution measurements of the transfer current
at 3.5 GHz reveal that the transfer error rate is below
the level (~10−3) that can be measured using a commercial current meter. In addition, we found theoretically that 1-GHz operation with an error rate of below
10−8 is possible.

the trap-mediated transfer, which is important forrealizing universal standards. However, because theaccuracy of the island-mediated transfer might belower than that of the trap-mediated transfer, a temperature below 10 K is necessary. In the future, it will
be necessary to evaluate the accuracy of both types oftransfer at high speed by measuring the number oftransferred electrons with a high-resolution charge
sensor. In addition to evaluating the accuracy in moredetail, we plan to conduct quantum metrology triangle experiments; we also want to apply the device tothe current standards. Furthermore, we expect that thedevice will be able to be used for information processing based on SE manipulation. In this respect,
one of the long-term goals is to achieve low-power
consumption information processing by integrating
the devices.

4. Future work

References

The SE transfer via the SE island and that via thesingle trap level, which we have described in thisarticle, have both advantages and disadvantages. For
the trap-mediated transfer, sufficiently high-speed
and high-accuracy operation is expected, but it is difficult to improve the device yield because the trap
position is random. One promising way to solve thisproblem is to use impurity doping with a positioncontrol technique. In contrast, the device yield of theisland-mediated transfer should be better than that of



[1]

[2]
[3]
[4]

A. Fujiwara, K. Nishiguchi, and Y. Ono, “Nanoampere Charge Pumpby Single-electron Ratchet Using Silicon Nanowire Metal-oxidesemiconductor Field-effect Transistor,” Appl. Phys. Lett., Vol. 92,
No. 4, 042102, 2008.
G. Yamahata, T. Karasawa, and A. Fujiwara, “Gigahertz Single-hole
Transfer in Si Tunable-barrier Pumps,” Appl. Phys. Lett., Vol. 106,
No. 2, 023112, 2015.
G. Yamahata, K. Nishiguchi, and A. Fujiwara, “Accuracy Evaluationand Mechanism Crossover of Single-electron Transfer in Si Tunablebarrier Turnstiles,” Phys. Rev. B, Vol. 89, No. 16, 165302, 2014.
G. Yamahata, K. Nishiguchi, and A. Fujiwara, “Gigahertz Single-trap
Electron Pumps in Silicon,” Nat. Commun., Vol. 5, 5038, 2014.

NTT Technical Review

Feature Articles

Gento Yamahata

Research Scientist, Nanodevices ResearchGroup, Physical Science Laboratory, NTT Basic
Research Laboratories.
He received his B.E., M.E., and Dr. Eng. inelectrical engineering from Tokyo Institute ofTechnology in 2005, 2007, and 2009, respectively. In 2009–2010, he was a visiting researcher
at Harvard University, MA, USA. He joined NTT
Basic Research Laboratories in 2010. Since then,his research interests have focused on singleelectron manipulation and detection in silicon
nanometer-scale devices. He is a member of theJapan Society of Applied Physics (JSAP).

Katsuhiko Nishiguchi

Senior Research Scientist, Distinguished
Researcher, Nanodevices Group, Physical Science Laboratory, NTT Basic Research Laboratories.
He received his B.E., M.E., and Ph.D. in electrical engineering from Tokyo Institute of Technology in 1998, 2000, and 2002, respectively.
Since joining NTT Basic Research Laboratories
in 2002, he has been researching the physics andtechnology of Si nanometer-scale devices for LSI
applications with low power consumption andnew functions. He was an invited researcher at
the National Center for Scientific Research(CNRS), Lille, France, in September 2008 andalso a guest researcher at Delft University ofTechnology, Delft, the Netherlands in 2012–2013. He received the IUPAP (International
Union of Pure and Applied Physics) Young
Author Best Paper Award at the International
Conference on the Physics of Semiconductors
2000, the Graduate Student Award (Silver) at theMaterials Research Society 2000 Fall Meeting,
the Young Scientist Award at the JSAP Spring
Meeting in 2000, the JSAP Outstanding Paper
Award 2013, and the Commendation for Scienceand Technology by the Minister of Education,
Culture, Sports, Science and Technology ofJapan (Young Scientists’ Prize) in 2013. He is amember of the Institute of Electrical and Electronics Engineers (IEEE) and JSAP.

Vol. 13 No. 8 Aug. 2015

Akira Fujiwara

Senior Distinguished Researcher, Senior Manager of Physical Science Laboratory and Group
Leader of Nanodevices Research Group, NTT
Basic Research Laboratories.
He received his B.S., M.S., and Ph.D. inapplied physics from the University of Tokyo in
1989, 1991, and 1994, respectively. He joined
NTT in 1994 and has been engaged in researchon silicon nanostructures and their application to
nanodevices and single-electron devices. He wasa guest researcher at the National Institute ofStandards and Technology (NIST), Gaithersburg,
MD, USA, in 2003–2004. He was a director of
JSAP in 2010–2011 and a visiting professor at
Hokkaido University in 2013. He received theInternational Conference on Solid State Devices
and Materials (SSDM) Young Researcher Awardin 1998, the SSDM Paper Award in 1999, andJJAP (Japanese Journal of Applied Physics)
Paper Awards in 2003, 2006, and 2013. He wasawarded the Young Scientist Award from the
Ministry of Education, Culture, Sports, Scienceand Technology in 2006. He was supported by
the funding program for Next Generation WorldLeading Researchers (NEXT Program), JSPS
(Japan Society of Promotion of Science) during
2011–2014. He is a member of JSAP and a senior
member of IEEE.

Feature Articles: Frontier Research on Lowdimensional Semiconductor Physics

Noise in Nanometer-scale Electronic
Devices
Katsuhiko Nishiguchi and Akira Fujiwara
Abstract

Downsizing electronic devices in integrated circuits increases the noise-related degradation of circuit
performance, and thus, it is becoming more important to analyze noise with single-electron resolution.
We discuss here the use of a nanometer-scale transistor and capacitor in analyzing thermal noise, one of
the most fundamental types of noise in electronic devices, with single-electron resolution. When thecapacitor size is substantially reduced, the well-known model of thermal noise is no longer valid, andvoltage noise is squeezed.
Keywords: noise, transistor, single electron

1. Introduction
Integrated circuits comprising silicon transistors
and other electronic devices are extremely importantcomponents in our information technology society.
The transistor, one of the most well-known electricaldevices, controls the flow of electricity, that is, electrons, for data processing in the integrated circuits.
The performance of such integrated circuits has
advanced with the downsizing of transistors; forexample, the 14-nm generation of transistors will bereached in 2015. However, downsizing also increases
the noise-related degradation of circuit performance,
and therefore, a microscopic analysis of noise and thedevelopment of countermeasures to such noise havebecome more important than ever before. Indeed,random telegraph noise originating from single-electron trapping in a transistor becomes prominent insmall transistors and gives rise to errors in memory
circuits. Shot noise originating from the discrete
nature of an electron is also expected to become aserious issue in high-speed integrated circuits. Therefore, the downscaling of the transistors increases not
only their performance but also the importance ofunderstanding noise microscopically, ultimately withsingle-electron resolution, in small transistors as wellas in other electronic devices.
In this article, we discuss the analysis of thermalnoise, one of the most fundamental types of noise inelectronic devices, with single-electron resolution
using dynamic random access memory (DRAM)
composed of a nanometer-scale transistor and capacitor [1]. Because the analysis was done by using a
DRAM, one of the most common devices, we believethat our results are relevant to all electronic devices.
2. Techniques for analyzing thermal noise
One of the simplest and most well-known techniques for analyzing thermal noise is to measure voltage noise in a circuit composed of one resistor andone capacitor, as shown in Fig. 1. In this circuit, amassive number of electrons enter and exit the capacitor through the resistor in a random manner with theassistance of thermal energy. This random motion ofelectrons is thermal noise, and it exists absolutely ata finite temperature. When voltage noise at a node
between the resistor and capacitor is measured withan oscilloscope or voltage meter, we can obtain certain information. For example, we can get a histogram of voltage-noise amplitude based on a Gaussian
distribution, whose average is voltage V applied to theresistor. We can also get the variance Vvar2 of the distribution given by kBT/C, where kB is the Boltzmann’sconstant, T is temperature, and C is the capacitance ofthe capacitor. The fact that Vvar2 is proportional to T
NTT Technical Review

Frequency

Feature Articles

R
V
C
Oscilloscope

Voltage noiseat capacitor

V

OutputC

26
Output (nA)

Transistor forTransistor forTransistor for transferring electrons capacitor
transferring electrons charge amplifier
Transistor forcharge amplifier

−3
−2
−1
0
1
2
3
4

25
24

100 nm(a) Equivalent circuit(b) Scanning electron microscope image0

Fluctuation ofe numberFig. 1. Conventional technique to measure thermal noise.

5
10Time (s)(c) Output signal from
amplifier transistor

Fig. 2. Measurements of thermal noise with single-electron resolution.

means that the random motion of electrons originatesfrom thermal energy. In contrast, Vvar2 increases as C
decreases, regardless of the resistor. Therefore, thedownsizing of electronic devices in circuits such as
memory and analog circuits reduces C and thusincreases noise, which leads to an undesirable hindrance of device downsizing. These features can be
explained by the well-known Johnson-Nyquist model
proposed in 1928 [2, 3].
In our technique for analyzing thermal noise withsingle-electron resolution, we use instead of a resistor
a nanometer-scale transistor for transferring the electrons to the capacitor, as shown in Fig. 2(a), because
electrical control of the resistance of the transistor
helps us to monitor the motion of individual electrons
precisely in real time. In a common circuit, time intervals for the electrons to enter and exit the capacitor
are too short for the electrons to be monitored: current
I of 1 mA corresponds to the time interval of 1.6 ×
10−16 (=e/I) seconds, which cannot be measured byany measurement system. However, increasing theresistance of the transistor when it transfers electrons
prolongs the interval so that the electron motion canbe monitored in real time. These mechanisms and thestructure are the same as in a DRAM.
Vol. 13 No. 8 Aug. 2015

Since the charge of an electron is too small (1.6 ×
10−19 C) to be detected by any charge sensor, the tiny
signal is amplified by using another transistor [4]:The transistor for the charge amplifier has anextremely small channel (~10 nm) and is integrated
with the capacitor, as shown in Fig. 2(b). Consequently, the electrons in the capacitor can be detected
precisely with single-electron resolution even atroom temperature. This success is supported by wellestablished fabrication techniques for silicon transistors.
Using these features, we can monitor the random
motion of individual electrons entering and exiting
the capacitor through the transistor in real time, as
shown in Fig. 2(c): An output signal from the chargeamplifier transistor changes among discrete values,
and one gap between these discrete values corresponds to the charge from one electron. To slow down
the electron motion so that it can be monitored, theresistance of the transistor used for the electron transfer is adjusted to be around 1020 Ω. This extremely
high resistance, which cannot be achieved in conventional resistors and transistors, is possible thanks to
our high-quality nanometer-scale transistor.

Voltage noise Vvar2 (10−4 V2)

Probability

Feature Articles

0.2
0.1

0.0

−6 −3
0
3
6
Fluctuation in number of electrons

kBT/C

2

1
Experimental results
0

100

200

300

Temperature (K)(b) Vvar2 vs. temperature(a) Histogram of fluctuation in
number of electrons in acapacitor at room temperature

Fig. 3. Thermal noise with single-electron resolution.

Probability

1.0

60 K

0.5

0.0

−1 0 1
Fluctuation in number of electrons

Voltage noise Vvar2 (10−4 V2)

10

Experimental results
1

kBT/C
Expected value

0.11(a) Histogram of fluctuation in numberof electrons in capacitor at 60 K

60 K

10
C (aF)(b) Vvar2 vs. C

Fig. 4. Thermal noise when charging energy is larger than thermal energy.

3. Thermal noise with single-electron resolution
In analyzing the thermal noise with single-electron
resolution, we first discuss the fluctuation, that is, thedeviation from the average, of the number of electrons in the capacitor. Because single-electron injection into the capacitor increases the voltage of thecapacitor by e/C (= dV), the fluctuation of the numberof electrons in the capacitor can be converted into a
voltage fluctuation, that is, voltage noise, at thecapacitor. The dV can be evaluated from the change
in the output signal originating from one electron, as
shown in Fig. 2(c) [4].
A histogram of the number of electrons in thecapacitor is shown in Fig. 3(a). The histogram follows a Gaussian distribution (solid line), as in thecase shown in Fig. 1. This histogram gives another

piece of information: dV2 multiplied by the varianceof the distribution shown in Fig. 3(a) corresponds toV var2, and this V var2 follows k BT/C, as shown in
Fig. 3(b), which is the same as the case where thermal noise originates from the fluctuation of a massive
number of electrons, as explained above and as shownin Fig. 1. Consequently, we can conclude that thewell-known Johnson-Nyquist model for thermal
noise is adaptable to the random motion of single
electrons.
However, when charging energy EC (= e2/2C) for
injecting one electron into the capacitor is larger thanthermal energy kBT, the Johnson-Nyquist model is nolonger valid. A histogram of the number of electrons
in the capacitor at EC > kBT is shown in Fig. 4(a).
Since the distribution (solid line) becomes sharper,
and the available number of electrons becomes
NTT Technical Review

Feature Articles

smaller than in the case of EC < kBT (Fig. 3(a)), thediscrepancy between the experimental results (bars)
and expected values (solid line) becomes larger in theform of quantization errors in digital circuits. Indeed,
Vvar2 (1.08 × 10−4 V2) evaluated from the variance ofthe experimental distribution shown in Fig. 4(a) issmaller than kBT/C (1.38 × 10−4 V2).
This deviation from the Johnson-Nyquist model issimilar to the case of electromagnetic radiation froma black body. Energy of the electromagnetic radiationis quantized by multiples of hν, where h is Planck’s
constant and ν is the frequency of the electromagnetic radiation. Therefore, when a higher ν makes hνlarger than thermal energy kBT, the thermal energy
cannot assist the emission of electromagnetic waves
from the black body. This idea, based on energy
quantization proposed by Max Planck, overcomes theultraviolet catastrophe in the low-frequency region of
the spectrum of black-body radiation.
Our results for electrons can also be explained
qualitatively by the same model as that for the blackbody radiation. Energy for injecting N electrons into
the capacitor is given by (Ne)2/2C = N2EC and quantized because e is a unit charge. Therefore, as in thecase of black-body radiation, when EC is comparable
or larger than kBT, the thermal energy cannot assist
electrons in entering and exiting the capacitor, which
suppresses electron random motion and thus makes
Vvar2 smaller than the thermal energy.
However, we can observe a unique feature in electron motion unlike in the case of black-body radiation. The average of the number of electrons in thecapacitor is given by CV/e, where V is the voltage
applied to the transistor as shown in Fig. 2(a). However, for example, when the average number is 0.5,
the number of electrons in the capacitor switches
between 0 and 1 with the same probability because anelectron cannot be divided in half. As a result, thevariance of the electron number becomes larger than
0.25, regardless of EC and kBT. In this sense, the fluctuation in the number of electrons depends on the
average number of electrons. When the average number is an integer, the fluctuation becomes minimum;
when a fractional part of the average number is 0.5,
the fluctuation becomes maximum. The change involtage noise Vvar2 at the capacitor as a function of C
is shown in Fig. 4(b). When EC < kBT at larger C, Vvar2
follows kBT/C (open circles) and increases as C

Vol. 13 No. 8 Aug. 2015

decreases. When EC > kBT at smaller C, Vvar2 deviatesfrom kBT/C, and the reduction in C increases the discrepancy between Vvar2 and kBT/C, as indicated by theshaded area depicting the possible values of Vvar2. Itshould be noted that the minimum border of theshaded area in Fig. 4(b) decreases with C, behavior
that is opposite to that of the conventional kBTC-limited case depicted by the open circles. In the conventional case valid at EC < kBT, the reduction in C
increases noise, which is a serious issue that hinders
the downsizing of electronic devices. However, thecase where E > kBT at much smaller C is preferable to
the reduction in C due to noise reduction, which
accelerates the downsizing of electronic devices.
4. Conclusion
As the downsizing of electronic devices accelerates, thermal noise will continue to deviate from its
well-known behavior. However, this does not meanthat any new phenomena will appear. Instead, itmeans that all of the results observed for single electrons follow well-established thermodynamics, forexample, a Boltzmann distribution, and are thus valid
for all electronic devices. Therefore, we believe that
our results are important in the downsizing of devices
to achieve a reduction in the number of electrons in
the device. Indeed, using nanometer-scale transistors,
we have developed memory and data information
circuits that use one electron as one bit of information. In these circuits, errors caused by thermal noise
represent one of the most serious issues. However, awider view is that the minimum energy consumed forcomputation by data information circuits is governedby the thermal energy. Therefore, the analysis of thermal noise is very important for realizing low-powerconsumption circuits.
References
[1]
[2]
[3]
[4]

K. Nishiguchi, Y. Ono, and A. Fujiwara, “Single-electron Thermal
Noise,” Nanotechnology, Vol. 25, No. 27, p. 275201, 2014.
J. B. Johnson, “Thermal Agitation of Electricity in Conductors,” Phys.
Rev., Vol. 32, No. 1, p. 97, 1928.
H. Nyquist, “Thermal Agitation of Electric Charge in Conductor,”Phys. Rev., Vol. 32, No. 1, p. 110, 1928.
K. Nishiguchi, C. Koechlin, Y. Ono, A. Fujiwara, H. Inokawa, and H.
Yamaguchi, “Single-electron-resolution Electrometer Based on Fieldeffect Transistor,” Jpn. J. Appl. Phys., Vol. 47, No. 11R, p. 8305,
2008.



Feature Articles

Katsuhiko Nishiguchi

Senior Research Scientist, Distinguished
Researcher, Nanodevices Group, Physical Science Laboratory, NTT Basic Research Laboratories.
He received his B.E., M.E., and Ph.D. in electrical engineering from Tokyo Institute of Technology in 1998, 2000, and 2002, respectively.
Since joining NTT Basic Research Laboratories
in 2002, he has been engaged in research on thephysics and technology of Si nanometer-scale
devices for LSI applications with low power
consumption and new functions. He was aninvited researcher at the National Center for Scientific Research (CNRS) Lille, France, in September 2008 and also a guest researcher at Delft
University of Technology, Delft, the Netherlands
in 2012–2013. He received the IUPAP (International Union of Pure and Applied Physics) Young
Author Best Paper Award at the International
Conference on the Physics of Semiconductors
2000, the Graduate Student Award (Silver) at theMaterials Research Society 2000 Fall Meeting,
the Young Scientist Award at the Japan Society ofApplied Physics (JSAP) Spring Meeting in 2000,
the JSAP Outstanding Paper Award 2013, and theCommendation for Science and Technology by
the Minister of Education, Culture, Sports, Science and Technology of Japan (Young Scientists’Prize) in 2013. He is a member of the Institute ofElectrical and Electronics Engineers (IEEE) and
JSAP.



Akira Fujiwara

Senior Distinguished Researcher, Senior Manager of Physical Science Laboratory and Group
Leader of Nanodevices Research Group, NTT
Basic Research Laboratories.
He received his B.S., M.S., and Ph.D. inapplied physics from the University of Tokyo in
1989, 1991, and 1994, respectively. He joined
NTT in 1994 and has been engaged in researchon silicon nanostructures and their application to
nanodevices and single-electron devices. He wasa guest researcher at the National Institute ofStandards and Technology (NIST), Gaithersburg,
MD, USA, in 2003–2004. He was a director of
JSAP in 2010–2011 and a visiting professor at
Hokkaido University in 2013. He received theInternational Conference on Solid State Devices
and Materials (SSDM) Young Researcher Awardin 1998, the SSDM Paper Award in 1999, andJJAP (Japanese Journal of Applied Physics)
Paper Awards in 2003, 2006, and 2013. He wasawarded the Young Scientist Award from the
Ministry of Education, Culture, Sports, Scienceand Technology in 2006. He was supported by
the funding program for Next Generation WorldLeading Researchers (NEXT Program), JSPS
(Japan Society of Promotion of Science) during
2011–2014. He is a member of JSAP and a senior
member of IEEE.

NTT Technical Review

Feature Articles: Frontier Research on Lowdimensional Semiconductor Physics

Gate Tuning of Direct/Indirect
Optical Transitions in Silicon
Jinichiro Noborisaka, Katsuhiko Nishiguchi,
and Akira Fujiwara
Abstract

Silicon is one of the most important semiconductor materials in microelectronics and is known as a
typical indirect bandgap semiconductor. In these semiconductors, the minimal energy state in the conduction band and the maximal energy state in the valence band appear at different momenta. This makes
it difficult to obtain efficient light emission. In this article, we present a means of electrically tuning thedirect/indirect optical transitions using a specially prepared Si/SiO2 interface, where anomalously large
valley splitting appears. This tunability is achieved by utilizing the close relationship between valley
splitting and a direct optical transition in silicon and its proportional relationship with respect to a gateelectric field.
Keywords: quantum well, valley splitting, light-emitting devices

1. Introduction
The continued miniaturization of silicon integrated
circuits has considerably improved the performanceof electronic information processors including computers and cell phones over the last decades. Similarly, optical communication technologies have
improved significantly in both communication speed
and capacity thanks to the development of compact
and high-performance solid laser sources using compound semiconductors such as GaAs and InP. However, the recent remarkable increase in the amount of
information requires further improvement in the performance of information processors. This is expected
to involve the integration of optical components withelectronic integrated circuits in order to meet thechallenges presented by the limited bandwidth andpower consumption of conventional metal lines usedin processors and interconnects.
Silicon photonics is a promising technology thatenables both high speed and mass-information processing with low power consumption. By combining
high-performance photonic building blocks such as
waveguides, resonators, and modulators fabricated insilicon with complementary metal-oxide semicon

ductor (CMOS) technologies on the same chip, somecompanies have been able to release commercial
products that achieve ultrahigh speed optical communications. However, the light source of such systems is made from exotic semiconductors (GaAs,
InP) that are incompatible with silicon processing.
This is because implementing a silicon-based lightsource is still a significant challenge due to its indirect bandgap nature, which is a fundamental propertyof bulk silicon.
Therefore, laser components in the system are independently fabricated using an exotic semiconductor
and then assembled in a silicon photonics system,
which makes it difficult to reduce manufacturingcosts. Having a silicon-based light source would
drastically reduce costs and also increase performance thanks to the massive investment and existingknowledge of silicon CMOS technologies over the
past decades, and therefore, the development of an
efficient silicon-based light source is long-awaited.
The study of silicon-based light sources goes backa long way. In the 1990s, many reports were published on methods to make silicon emit light using
low-dimensional structures such as porous silicon
and nanocrystals [1], where electrons are typically
NTT Technical Review

Feature Articles

[100]
A

EnergyB[001]

Energy[010]

Anti-bonding
orbitalValley splitting

A, B

Bonding orbital

B

A

Conduction band (CB)
Indirecttransition

+kz0

−kz0

Confinement(Size shrinkage)

Eg
Bulk

Wave vector k: [001]Heavy hole band (HH)

Wave function

Light hole band (LH)(a) Band structures of bulk silicon: Dispersion
relationship between energy andwave vector. (Upper) Equi-energy surfaceof CB in momentum space

Quantum wells

Co e–ikz0 znCn e–ikzn z(b) Valley splitting due to quantum confinement andeigenfunctions in the systems

Fig. 1. Band structures of silicon.

confined in a small space on the scale of a few nanometers. Most fundamental ideas on making siliconemit light are identical and involve confining an electron in a small space. This makes the electron’s wave
function sharp in real space, and thus, a wave function is dispersed in momentum space, which in turnleads to a direct optical transition in silicon, which is
known as a quasi-direct optical transition. In bothporous silicon and nanocrystals, strong light emission
can be seen even with the naked human eye due to thestrong electron confinement. However, an ideal lightsource for silicon photonics requires certain properties: high stability in both wavelength and power,
adjustability in emission wavelength, compatibility
with the present silicon manufacturing process, andhigh efficiency in light emission. However, these silicon light sources have some problems in that they arephysically unstable and difficult to inject current into
due to their extremely small size and low-dimensional structure. It is also difficult to adjust the emission wavelength because of a lack of controllable
parameters.
We are working to solve these problems by applying electric tuning of direct/indirect optical transitions using a specially prepared Si/SiO2 interface,where anomalously large valley splitting appears.
This electric tuning is a proof of concept demonstration that is based on the close relationship between
valley splitting and a direct optical transition in siliVol. 13 No. 8 Aug. 2015

con and its proportional relationship with respect to agate electric field. Our silicon light-emitting devices
are driven by current injection and fabricated using
fully CMOS-compatible processes. They exhibit
remarkably stable and reproducible optical characteristics thanks to the use of well-established silicon
CMOS processes.
2. Electronic band and valley structures insiliconElectronic band structures in semiconductors are
uniquely determined by the constituent elements of
the lattice. Silicon has a diamond-type structure aswell as an indirect bandgap, where the minimalenergy states of the conduction band and the maximalenergy state of the valence band are misaligned in
momentum space. (Hereafter in this article, we regard
a momentum as having the same meaning as wavenumber (k) because it has a linear relationship withthe wavenumber.) Because the electron’s minimalenergy state in the conduction band appears on [001]momentum axes near the X-point and there are threemutual axes, silicon has in total six energy-equivalent
minima in its conduction band (Fig. 1(a)). Theseenergy minima are called valleys because the states
are first occupied by electrons with an increasing
density, as if rain were pooling in valleys. The electrons in each valley can occupy the same space thanksFeature Articles[110]

Front gate (FG)

SiO2 (FOX)Silicon quantum well (QW)
Buried oxide (BOX)

n

tsol

n or pBack gate (BG)

Front gate voltage: VFG (V)

[001]

6
Onset of current
5
conduction
4
3
2
1
0
−1
−2
−3
Valley splitting−4−5−100−80−60−40 −20 0 20 40 60 80 100120

Back gate voltage: VBG (V)(a) Cross-sectional view of silicon light-emitting device(b) Second derivative of drain current Id with respect to VFG

Fig. 2. Electrical measurement of valley splitting.

to their different momenta even if they have the sameenergy (they are called degenerate). Since bulk crystal has a translational symmetry, the eigenstate of theelectron in the system can be explained by a plane
wave with a specific wavenumber (e−ik0·r), where thewavenumber k0 gives the minimal energy of the conduction band in the dispersion relation. This is known
as the Bloch theorem.
Suppose that spatial confinement normal to that of
the [001] axes is introduced, and a quantum well
(QW) is formed. Then the six-fold degenerate states
split into two- and four-fold degenerate ones due tothe anisotropy of electron effective mass, and accordingly, two valleys (labeled A and B in Fig. 1) lying
along the confining axis become the ground states
because they have heavier effective mass along the[001] axis. The eigenfunctions in the system are not aplane wave but a wave packet confined in the quantum well, as shown in Fig. 1(b). If we perform Fourier transform with respect to the wave packet, it
becomes clear that the electrons are largely scatteredin momentum space due to the confinement because
it involves higher components of a Fourier coefficient. As the degree of confinement increases, further
splitting (called valley splitting) occurs, where thetwo-fold degenerate valley states split, and thus,
degeneracy is fully lifted. This splitting can happen
when the Fourier coefficient that has a wave numberthat is nearly equal to the inverse value of the lattice
constant (kz0~2π/a0) is finite. The resulting coupledstates are called bonding and anti-bonding states,
where the eigenfunctions are hybridized states of the+kz0 and −kz0 valley wave functions.
The valley splitting is tuned by the size of the QW,
electric fields, or magnetic fields. In silicon metal

oxide semiconductor field effect transistors (MOSFETs), it can be tuned by gate electric fields. The
typical size of valley splitting in general MOSFETs
with gate oxides formed by the standard thermal oxidation process is at most several milli-electron volts,
whereas NTT Basic Research Laboratories hasreported large valley splitting up to several tens of
milli-electron volts. The large valley splitting isobserved in MOSFETs fabricated on a separation-by
implantation-of-oxygen (SIMOX) wafer employing
high-temperature thermal treatment [2], where
SIMOX is one of the fabrication methods for a silicon
on insulator (SOI), and a top silicon layer is separated
by a buried oxide (BOX) formed by ion implantationsof oxygen.
The devices used in the present work are thin SOIMOSFETs with a double-layer gate fabricated on a[001] SIMOX wafer employing high-temperaturethermal treatment (Fig. 2). The front gate (FG) isconstructed using polycrystalline silicon with a gate
oxide (FOX) formed by conventional thermal oxidation. The substrate is used as a back gate (BG). Bothn- and p-type contacts are formed to inject the carriers
into the QW sandwiched between the FOX and theBOX. Both FG and BG voltages (VFG, VBG) areapplied in order to control the distribution of bothelectrons and holes in the QW.
When positive VFG and negative VBG are applied,electrons are squeezed into the FOX interface. Since
the size of valley splitting is quite small at the interface, only the onset of current conduction can bedetected as a white line in Fig. 2(b). In contrast, when
the polarities of both gates are mutually replaced,
electrons are squeezed into the BOX interface, andthe white line gradually splits. This splitting is just
NTT Technical Review

Feature Articles

the valley splitting that we discuss in the following
sections. Although the physical origin of the large
valley splitting remains unclear and is still under
debate, the notable points here are that the electrons’
wave functions are strongly dispersed in momentum
space when the electrons are distributed at the BOX
interface, and the size of valley splitting is tunable bygate electric fields.
3. Relationship between valley splitting anddipole transitions
Next, we examine an application of large valley
splitting to light emission for indirect semiconductors. Light emission is known to occur when an electron relaxes from an initial higher energy state to a
final lower one and loses its energy as a photon. Since
photons do not carry a significant momentum, andboth the energy and momentum are preserved in any
physical process, the momentum of an electron in theinitial higher state is preserved in pure direct opticaltransition. In contrast, in bulk silicon, the minimalenergy states of the conduction and valence bands
have different respective momenta. This means thatsuch a transition rarely occurs in bulk silicon. Despite
this, weak light emission is observed in bulk silicon
thanks to the aid of phonons. This process, which iscalled an indirect optical transition, can be completed
only when an electron absorbs or emits a phonon withthe momentum kz0 and also emits a photon with theenergy corresponding to the value of the phonon
energy (hω) subtracted from the bandgap of silicon(Eg). Although this process occurs more frequently
than the pure direct optical transition in indirect semiconductors, another non-radiative process such as
Auger or free-carrier absorption quickly recombines
an electron, and therefore, the photon emission efficiency is quite low in indirect semiconductors. This is
why silicon is inappropriate for light-emitting devices.
With finite valley splitting, the wave function ofelectrons in the QW is composed of a broad range ofcomponents for a Fourier coefficient, as discussed in
the previous section. If the amplitude of 2kz0 components does not have a zero value, then a direct opticaltransition can occur. This is because the variation of
the confining potential formed by the gate electricfield is much gentler than that at a steep Si/SiO2 interface, so the wave function confined by the gate electric field has a smooth profile. The profile of such asmooth wave function gently varies as the gate electric field increases, where the amplitude of the kz0
Vol. 13 No. 8 Aug. 2015

component follows that of 2k z0. Therefore, it isexpected that large valley splitting will lead to significant direct optical transitions.
4. Electric tuning of direct/indirect opticaltransitions
We then examined light emission using finite valley
splitting. In this experiment, light emission wasachieved by electroluminescence (EL), where bothcarriers were electrons and holes and were injected
into the channel. We used n- and p-type contacts as arespective source and drain. We applied forward bias
and injected carriers into the QW; then we tuned bothgate biases VFG and VBG so as to control the distribution of the electron and hole wave functions. EL
spectra as a function of VBG are shown in Fig. 3(a),where the sizes of valley splitting are adjusted by tuning both gates (FG and BG). When we applied positive VFG and negative VBG, a transverse optical (TO)phonon-mediated indirect optical transition dominated the spectra. This transition is the main radiative
recombination process in bulk silicon. In contrast,
when the polarities of both gates were mutuallyreplaced, a higher energy peak gradually developedwith an increasing V BG. The energy separation
between TO and the higher energy peak are approximately 59 meV. The energy separation agrees withthe energy of the TO phonon with a momentum atvalley minimum (kz0). When we take into consideration these peak energies with the energy separation,
the higher energy peak is assigned as a non-phonon(NP) direct optical transition, where the electron in
the conduction band minima directly recombineswith the hole in the valence band without aid of thephonon. The intensities of the NP peak increase withan increasing |VBG| and they greatly increase for apositive VBG. Eventually, the intensity of the NP peak
becomes larger than that of the TO phonon-mediatedindirect optical transition.
The efficiency of light emission is plotted inFig. 3(b) taking into account the current flowing in
the QW that becomes small as valley splitting
increases. We assumed a condition close to the bulk
silicon by applying zero gate voltages to both the FG
and BG, and using a thicker QW. We define this as thebulk condition. The efficiency of the NP direct opticaltransition is 16 times better than that of the TO phonon-mediated indirect optical transition in the bulk
condition. Weak NP direct optical transitions are
known to occur even in bulk silicon thanks to an unintentionally doped slight amount of impurities that

NP VBG = 60 V

Directtransition(NP)

Indirecttransition(TO)
HH (heavy hole)

VBG = 0 V

Eg

5Indirecttransition
(TO)

TO
NP

tSOI = 6 nm

TO
NP

tSOI = 4.3 nm

tSOI = 6 nm

4CB
TO phonon

VBG = −60 V
0.95 1.00 1.05 1.10 1.15 1.20 1.25 1.30
Photon energy (eV)

16
14
12
10
8
6
4
2
0

1000

tSOI = 4.3 nm

750

3
2

500

1

250

0

HH (heavy hole)

NP/TO/(NP/TO)Bulk

Valley splittingCB
TO phonon

NP/TO

EL intensity (arb. unit)

Valley splitting
increase
TO

Normalized intensity (arb. unit)

Feature Articles

0−80 −60 −40 −20 0
20 40
Back gate voltage: VBG (V)6080(b) Emission efficiency (top) and the ratio of direct
to indirect transition (NP/TO) (bottom)(a) Electroluminescence (EL) spectra

Fig. 3. Optical measurement.

bind electrons around them and disperse the electrons’ wave function. We have found that the optimal
efficiency of the NP optical transition was 800 times
greater than the NP direct optical transition in thebulk condition [3]. Furthermore, the ratio of NP/TO
is tuned by the gate electric field. This is the world’s
first demonstration of electric tuning of direct/indirect optical transition in silicon by controlling the sizeof valley splitting at the Si/SiO2 interface.
5. Conclusion
We reported electric tuning of direct optical transitions up to 800 times greater than that in bulk silicon.
However, the efficiency of such a direct optical transition is much weaker than that in GaAs or InP, and of

course, this is not sufficient for implementing silicon
light-emitting devices. Therefore, we will examine
hybrid approaches that combine elements such as
impurities and photonic crystals in order to improve
the light emission efficiency.
References
[1]
[2]
[3]

L. T. Canham, “Silicon Quantum Wire Array Fabrication by Electrochemical and Chemical Dissolution of Wafers,” Appl. Phys. Lett., Vol.
57, 1046, 1990.
K. Takashina, Y. Ono, A. Fujiwara, Y. Takahashi, and Y. Hirayama,“Valley Polarization in Si(100) at Zero Magnetic Field,” Phys. Rev.
Lett., Vol. 96, 236801, 2006.
J. Noborisaka, K. Nishiguchi, and A. Fujiwara, “Electric Tuning of
Direct-indirect Optical Transitions in Silicon,” Sci. Rep., Vol. 4, 6950,
2014.

NTT Technical Review

Feature Articles

Jinichiro Noborisaka

Lead Researcher, Nanodevices ResearchGroup, Physical Science Laboratory, NTT Basic
Research Laboratories.
He received his M.S. and Ph.D. in engineeringfrom Hokkaido University in 2005 and 2008,
respectively. He joined NTT Basic ResearchLaboratories in 2008 and has been studying
nanoscale devices and low-dimensional semiconductor physics. He is a member of the Japan
Society of Applied Physics (JSAP).

Katsuhiko Nishiguchi

Senior Research Scientist, Distinguished
Researcher, Nanodevices Group, Physical Science Laboratory, NTT Basic Research Laboratories.
He received his B.E., M.E., and Ph.D. in electrical engineering from Tokyo Institute of Technology in 1998, 2000, and 2002, respectively.
Since joining NTT Basic Research Laboratories
in 2002, he has been engaged in research on thephysics and technology of Si nanometer-scale
devices for LSI applications with low power
consumption and new functions. He was aninvited researcher at the National Center for Scientific Research (CNRS) Lille, France, in September 2008 and also a guest researcher at Delft
University of Technology, Delft, the Netherlands
in 2012–2013. He received the IUPAP (International Union of Pure and Applied Physics) Young
Author Best Paper Award at the International
Conference on the Physics of Semiconductors
2000, the Graduate Student Award (Silver) at theMaterials Research Society 2000 Fall Meeting,
the Young Scientist Award at the JSAP Spring
Meeting in 2000, the JSAP Outstanding Paper
Award 2013, and the Commendation for Scienceand Technology by the Minister of Education,
Culture, Sports, Science and Technology ofJapan (Young Scientists’ Prize) in 2013. He is amember of the Institute of Electrical and Electronics Engineers (IEEE) and JSAP.

Vol. 13 No. 8 Aug. 2015

Akira Fujiwara

Senior Distinguished Researcher, Senior Manager of Physical Science Laboratory and Group
Leader of Nanodevices Research Group, NTT
Basic Research Laboratories.
He received his B.S., M.S., and Ph.D. inapplied physics from the University of Tokyo in
1989, 1991, and 1994, respectively. He joined
NTT in 1994 and has been engaged in researchon silicon nanostructures and their application to
nanodevices and single-electron devices. He wasa guest researcher at the National Institute ofStandards and Technology (NIST), Gaithersburg,
MD, USA, in 2003–2004. He was a director of
JSAP in 2010–2011 and a visiting professor at
Hokkaido University in 2013. He received theInternational Conference on Solid State Devices
and Materials (SSDM) Young Researcher Awardin 1998, SSDM Paper Award in 1999, and JJAP
(Japanese Journal of Applied Physics) Paper
Awards in 2003, 2006, and 2013. He was awarded the Young Scientist Award from the Ministryof Education, Culture, Sports, Science and Technology in 2006. He was supported by the funding
program for Next Generation World-Leading
Researchers (NEXT Program), JSPS (Japan
Society of Promotion of Science) during 2011–2014. He is a member of JSAP and a senior
member of IEEE.

Feature Articles: Frontier Research on Lowdimensional Semiconductor Physics

Microscopic Probing of Crystalline
Electrons Using Magnetic ResonanceKoji Muraki and Trevor David Rhone
Abstract

Electrons in semiconductors are often referred to as free electrons, meaning that they can move freely.
They are also spatially extended like a wave. On the other hand, electrons are negatively charged particles and thus repel each other. Theory predicts that at low temperatures and in high magnetic fields,
electrons—in an effort to maintain a distance from one another—organize themselves. They form aregular array like the atoms in a crystal, a state known as the Wigner crystal. This article describes
experiments that exploit nuclear magnetic resonance (NMR) to probe the microscopic structure of a
Wigner crystal. These experiments demonstrate the capability of NMR to resolve the spatial variation ofelectron waves on the nanometer scale.
Keywords: two-dimensional electron system, low-temperature physics, nuclear magnetic resonance

1. Two-dimensional electron system
in a semiconductor
Metal-oxide-semiconductor field-effect transistors,
which form the basis of today’s electronics, operateby controlling the motion of charge carriers (electrons, holes) induced at the interface between silicon(a semiconductor) and silicon dioxide (an insulator).
The electrons (or holes) carry electrical current bymoving along the interface in response to the voltage
applied between the source and drain electrodes.
Since these charge carriers are confined near theinterface by the electric field perpendicular to it, they
can only move in the plane of the interface. Such asystem is often referred to as a two-dimensional electron gas. As the term gas suggests, electrons in semiconductors can be considered to move independently
between inter-particle collisions. This situation is
analogous to that of electrons in a vacuum tube traveling from the cathode to the anode without collisions.
However, in the solid state, the mobility of electrons
depends greatly on the material and the concentrationof impurities. Mobility is a quantity that describes theability of electrons to move through a medium in
response to an electric field.
As a result of the continual miniaturization of elec

tronic devices, the distance electrons travel in a transistor (the gate length) is now on the order of tens ofnanometers. Research on new materials with highcarrier mobility (for example, graphene or silicon
germanium) is under way. This is aimed at achieving
high-speed operation of electronic devices—a situation that draws parallels to Olympic sprinters on the
100-meter track.
2. Two-dimensional electron system at low
temperature and high magnetic field
At room temperature, where electronic devices
operate, the mobility of electrons is governed primarily by lattice vibrations. At low temperatures, reduced
lattice vibrations generally lead to an increase inmobility. A heterostructure comprising GaAs andAlxGa1-xAs is a system in which the highest electron
mobility is attained at low temperatures. The electron
mobility of the sample used in this study reaches
107 cm2/Vs at a cryogenic temperature. This implies
that the mean free path of electrons is as long as
100 µm. Such ultrahigh mobility requires the purity of
the host semiconductor crystal to be exceedingly high;the residual impurity concentration in a material with107 cm2/Vs electron mobility is about 1014 cm−3
NTT Technical Review

Feature Articles(~2 ppb).
Under conditions of high material purity and low
temperature, electrons are able to move freely without being scattered by impurities or lattice vibrations.
On the other hand, electrons are negatively charged
particles and thus repel each other. Nevertheless,since electrons occupy low-energy states with longwavelengths, they tend to be spatially spread, so therepulsive interaction does not usually produce visible
effects.
In contrast, when a two-dimensional electron system is subjected to a strong perpendicular magnetic
field, under the influence of the Lorentz force, theelectrons make a circular motion (cyclotron motion)with the radius becoming even smaller than the interelectron distance (for a sufficiently strong field). It is
widely believed that in such cases, electrons—in aneffort to maintain a distance from one other—organize themselves into a regular array, like the atoms ina crystal. Such a state is referred to as a Wigner crystal, named after the physicist who first predicted it.
Thus far, experimental studies on Wigner crystals
have relied on rather indirect information such as
microwave absorption.
3. Nuclear magnetic resonance (NMR)
NMR is a phenomenon in which nuclei in a magnetic field absorb electromagnetic radiation at a frequency specific to the nuclear species. It is widelyused as a highly sophisticated method of spectroscopic analysis in various fields including chemistry,
physics, and the medical sciences. Its principle isbased on the property of nuclei (referred to as nuclearspin) that make them behave like tiny magnets. The
resonance occurs at a frequency determined by themagnetic field strength and the nuclear species. In
addition, the resonance frequency undergoes a tiny
shift whose magnitude reflects the details of thechemical bonds and the state of the electrons surrounding the nuclei. Useful information can beobtained by measuring this tiny frequency shift(Knight shift). The reason the resonance of nuclei isaffected by the surrounding electrons is that electrons
also have the property (referred to as electron spin, or
simply, spin) that makes them behave like tiny magnets. Electron spin can point up or down with respect
to the external magnetic field. When the numbers of
up-spin and down-spin electrons are unequal, theimbalance acts on the nuclei as an effective magnetic
field, which shifts their resonance frequency. In
NMR, measuring this Knight shift allows one toVol. 13 No. 8 Aug. 2015

obtain information on the electrons in contact with
the nuclear spins.
4. Resistively detected NMR
In standard NMR, a pick-up coil is used to detectsignals produced by nuclear spins processing in an
external magnetic field. However, the low sensitivityof the inductive detection makes the application of
the standard NMR technique to a single-layer twodimensional electron system unfeasible. Resistively
detected NMR (RD-NMR) provides a way to overcome the issues of sensitivity, thereby enabling us toperform NMR on a two-dimensional electron system[1]. In RD-NMR, we measure the change in the electrical resistance of the sample that occurs when thefrequency of the applied radio-frequency (rf) wave
matches the resonance frequency of the nuclei. It also
allows us to selectively detect the signal from thosenuclear spins in contact with the two-dimensional
electron system.
The structure of the GaAs/AlxGa1-xAs (x = 0.25)
quantum well sample used in this study is schematically shown in Fig. 1(a). The sample was grown by
molecular beam epitaxy. Electrons are supplied fromthe Si delta doping layer in the upper AlxGa1-xAs barrier to the GaAs quantum well. The electron density
ne is varied using the n+-GaAs substrate as a backgate. The measurement setup is shown in Fig. 1(b).
The sample was processed into a 100-µm-wide Hallbar, mounted on a chip carrier, wrapped with a coilused for applying rf radiation, and cooled to 0.01 K ina dilution refrigerator. The resistance of the sample
was measured using a lock-in technique with alternating current (72 Hz) in a magnetic field applied
perpendicular to the sample.
The energy level diagram of electrons in a magnetic field is schematically shown in Fig. 2. When atwo-dimensional electron system is subjected to astrong perpendicular magnetic field B, the cyclotron
motion of electrons is quantized, and their energy
spectrum splits into a set of equally spaced discretelevels (Landau levels) designated by the orbital quantum number N (= 0, 1, 2, ...). Each Landau level isfurther split into spin-up (↑) and spin-down (↓) levels. The number of electrons that each spin-split Landau level can accommodate per unit area is given bynϕ = eB/h, where h is Planck’s constant and e is theelementary charge. The filling factor is a quantity that
represents the number of occupied (spin-split) Landau levels, and is given by ν = ne/nϕ = nh/eB. As thefigure shows, a filling factor of 2.2 (2.1) means thatFeature Articles(a)

Si delta doping
AlGaAs

GaAs quantum well

2DEG
AlAs/GaAs superlattice barriern +-GaAs substrate(back gate)(b)

Magnetic fieldrf generator

V

Fig. 1. (a) Cross-sectional view of sample and (b) measurement setup for resistively detected NMR.

EnergyLandau levels

Filling factor = 2.2

Filling factor = 2.1

Density of states(No magnetic field) (Strong magnetic field)

Fig. 2. Energy level diagram of two-dimensional electron system in a magnetic field.

both spin branches of the lowest Landau level are
fully occupied, and the lower spin branch of the firstexcited Landau level is 20% (10%) occupied.
The resonance spectra of 75As taken at a magnetic
field of 6.4 T are shown in Fig. 3(a). For these measurements, the filling factor is tuned in the range from
2.20 to 2.33 using the back gate. The dashed line in
the figure indicates the resonance frequency of 75As
nuclei observed when the quantum well is depleted ofelectrons. The NMR spectra taken in the presence ofelectrons are shifted to lower frequencies. The asymmetric line shape of the spectra with a tail on the highfrequency side reflects the variation of the probability
density of electrons along the direction normal to thequantum well (Fig. 3(b)). That is, 75As nuclear spinsnear the center of the GaAs well and those near the
AlxGa1-xAs barriers experience different effective
magnetic fields from the electrons. Simulations thattake into account the variation of the electron probability density along the direction normal to the quantum well, shown by the solid lines in Fig. 3(a), reproduce the experimentally observed spectra.
The resonance spectrum of 75As taken at the samemagnetic field of 6.4 T, but with the filling factor
NTT Technical Review

Feature Articles

Probability densityof electrons0(b)Filling factor
2.33

2.30

2.27
2.23

2.20

46.38

46.39

46.40

46.41

Frequency (MHz)Experiment
Numerical fit

20
As nuclear spins
10
0
−10
−20

Resistively detected NMR
signal intensity

Position along the direction
normal to the quantum well (nm)Resistively detected NMR signal intensity(a)Frequency shift

0

Fig. 3. (a) Resistively detected NMR spectra of 75As at filling factor 2.20–2.33 and (b) effects of finite thickness of
two-dimensional electron system on the NMR lineshape.

reduced to 2.1, is shown in Fig. 4(a). (Here, the horizontal axis is taken to be the frequency shift withrespect to the zero Knight shift bare resonance frequency.) Obviously, the measured spectrum does notmatch the simulation shown in Fig. 4(b), which isbased on the assumption that the electrons are uniformly distributed in the plane of the quantum well.
In contrast, simulations carried out assuming that theelectrons in the first excited Landau level form a
Wigner crystal (solid line in Fig. 4(a)) reproduce theexperimentally measured spectrum very well.
The spatial distribution of the probability density ofelectrons used for the above simulation is shown in
Fig. 4(c). Each of the mounds arranged in a triangularlattice corresponds to a single electron. As this figure
shows, at a filling factor of 2.1, the inter-electron distance becomes greater than the spatial extent of theelectron wave. Consequently, it becomes energetically more favorable for the electrons to be in the
crystalline state. When a Wigner crystal is formed,
the probability density of electrons is no longer uniform in the plane of the quantum well, as Fig. 4(d)shows. Regions appear where the local probability
density is higher or lower than that expected for theVol. 13 No. 8 Aug. 2015homogeneous system. In the spectrum shown in
Fig. 4(a), the tail on the low-frequency side corresponds to the top of the mounds where the electronprobability density is highest and the peak on the highfrequency side to the valleys where the probability
density is lowest. The dip at the center of the mounds
in Fig. 4(c) is a characteristic feature of the first excited
Landau level wave function. The line shape of themeasured NMR spectrum captures such detailsof the microscopic structure of the electron waves.
5. Conclusion
The results presented here demonstrate that NMR
is a powerful means for probing the spatial variationof electron waves in a semiconductor. In particular, itoffers a significant advantage of spatial resolution
provided by nuclear spins, allowing one to obtainmicroscopic information about the electron wave on
the nanometer scale. This opens the way to studies ofother exotic phases with nontrivial spatial order. It
also enables us to quantify the spatial inhomogeneityof electron density induced by randomly distributed
impurities. This will be a useful technique forFeature Articles

Experiment

(a)

(c)

Resistively detected NMR
signal intensity

Fitting assuming the presence of a Wigner crystal

m

0n

10

−15

−10

−5

0

5

Frequency shift (kHz)

Relative probability
density of electrons(d)Resistively detected NMR
signal intensity

(b)

−15

−10

−5

0

Lateral position in quantum well (nm)
−100

−50

0

50

100

3
2
1
0
As nuclear spins
The dashed line represents the probability
density expected when the electrons are
uniformly distributed.

5Frequency shift (kHz)The dashed line represents the calculation without
the effects of inhomogeneous broadening.

Fig. 4. (a) Resistively detected NMR spectra of 75As at filling factor 2.1, (b) simulation assuming that electrons are
distributed uniformly in the two-dimensional plane, (c) probability density of electrons in a crystalline state(simulation), and (d) profile of probability density of electrons in a crystalline state.

characterizing electronic devices facing nanometerscale miniaturization, where eliminating device-todevice variability is a critical issue.
These experiments were performed as a joint
research effort between NTT and the Japan Scienceand Technology Agency (JST).
Koji Muraki

Senior Research Scientist (Distinguished
Researcher) and Group Leader of the Quantum
Solid State Physics Research Group, NTT Basic
Research Laboratories.
He received his B.E., M.E., and Ph.D. inapplied physics from the University of Tokyo in
1989, 1991, and 1994, respectively. He joined
NTT Basic Research Laboratories in 1994. From2001 to 2002, he was a visiting researcher at the
Max Planck Institute for Solid State Research,
Stuttgart, Germany. His research interests arefocused on many-body effects in low-dimensional semiconductor structures. He is a memberof the Physical Society of Japan and the Japan
Society of Applied Physics.



Reference[1]

L. Tiemann, T. D. Rhone, N. Shibata, and K. Muraki, “NMR Profilingof Quantum Electron Solids in High Magnetic Fields,” Nature Physics, Vol. 10, No. 9, pp. 648–652, 2014.

Trevor David Rhone

Research Associate (Postdoctoral Research
Fellow), Quantum Solid State Physics ResearchGroup, NTT Basic Research Laboratories.
He received his B.A. from Macalester College,
MN, USA, in 2005, and his M.A., M.Phil., andPh.D. in physics from Columbia University, NY,
USA, in 2007, 2008, and 2012, respectively. He
joined the Quantum Solid State Physics ResearchGroup, NTT Basic Research Laboratories, in2012. From 2012 to 2013, he was a postdoctoral
fellow at JST. His research interests are focusedon correlated electronic phases in low-dimensional semiconductor structures. He is a memberof the American Physical Society, the National
Society of Black Physicists, and the New York
Academy of Sciences.

NTT Technical Review

Feature Articles: Frontier Research on Lowdimensional Semiconductor Physics

Semiconductor Quantum Structureswith Single-atom Precision
Kiyoshi Kanisawa and Stefan Fölsch
Abstract

Atom manipulation is a technology to assemble microstructures atom-by-atom by controlling thenumber of component atoms and their configuration. Combining this technology with the epitaxial
growth of high quality semiconductor crystals makes it possible to uniformly manufacture and integratequantum structures with single-atom precision. This technological innovation is expected to enable thenext generation of electronics to overcome the limits of conventional silicon technology. In this article,
atom manipulation of quantum structures at a compound semiconductor surface is reviewed, and a typical example is described—the fabrication of an ultimately precise quantum dot and quantum dot molecules.
Keywords: atom manipulation, STM, quantum structure

1. Introduction
We have access to enormous amounts of information on the Internet these days. We can casuallyobserve street views of arbitrary places almost anywhere on Earth. In the bygone era of telephone cards
and message dial services, such a ubiquitous andinteractive service was only a dream. Microfabrication is one of the technologies that has played a leading role in making this dream come true in our daily
lives.
Semiconductor microfabrication technology has
steadily progressed following Moore’s law, which
predicts that the number of transistors in an integrated
circuit (IC) will double approximately every two
years. However, this performance improvement of
ICs is expected to come to an end in the near future
because of quantum mechanical leakage current
manifested by the miniaturization. Extensive researchhas been ongoing to find a way to overcome thislimitation, and some studies have explored novel
electric circuits that exploit evident quantum mechanical phenomena. To achieve such quantum mechanical electric circuits, microstructures called quantum
structures are used in order to take advantage of thequantum mechanical behavior of electrons. A nano

structure*1, one such microstructure, is considered to
be a fundamental structure in which quantum behavior becomes the driving mechanism of novel electric
circuits. The kinetics of a quantum shows a wave-particle duality, a concept that consists of the propertiesof a particle (kinetic momentum p = mv, where m andv are the mass and the velocity of the quantum) characterized by Newton’s equation of motion, and theproperties of a wave (wavelength λ) characterized by
the Schrödinger equation. This λ is called the de Broglie wavelength*2 and is estimated as λ = h / mv (h:Planck constant). The de Broglie wavelength of anelectron in a semiconductor is roughly a few tens ofnanometers (1 nm = 10−9 m).
In contrast, the distance between the nearest-neighbor atoms in solids is of the order of 0.25 nm, whichcorresponds to about 1% of λ. The impact of a microfabrication error becomes more critical as the*1	 Nanostructure: A nanostructure is a microscopic structure formed
or processed to have an exact size smaller than 100 nm in length,
width, or height. In a wide sense, an object that includes a microscopic structure fabricated with these geometric specifications isreferred to as a nanostructure.
*2 de Broglie wavelength: As light propagates as a wave, the motionof matter also behaves as a wave in the quantum mechanical picture. The de Broglie wavelength is the wavelength of the matter
wave.

NTT Technical Review

Feature Articles

Microstructures

Lithography
300 nmL

Self-organization
10 nm

Deposited atom (ion)
1 nm

R

1 nm

−V

InAs

−V

Band discontinuity ofheterostructures

Coulomb potential

GaAs

External gate voltage

GaAs

Quantum confinement

Quantized
levelElectronSize error

5–10%

10–20%

0%

Positioning error

Nanometer scale

Nanometer to micrometer
scale

Nanometer to micrometer scaleor larger

Fig. 1. C omparison of nanostructures constructed using conventional microfabrication techniqueswith focused atom deposition.

manufactured device becomes smaller. This meansthat the electronic states in quantum structures arehighly sensitive to a structural fabrication error on theorder of a few atoms. The present error status of
microfabricated nanostructures is shown in Fig. 1. It
is clear that conventional microfabrication methods
such as lithography and self-organization result inunavoidable variations comparable to or larger than
the required precision in size and positioning. The
effects of the inevitable processing errors make thedevice characteristics considerably inaccurate andproblematic. Thus, to precisely control quantum
mechanical electrons, microfabrication technology
for quantum structures needs to have a degree of
accuracy with the precision of a natural atom, whichis the fundamental unit of matter.
In natural atoms, the Coulomb potential caused by
the nucleus determines the quantum states of confined electrons. Since the potential form is determined by the atomic number, the electronic states are
identical in each atom. Therefore, in the deposition
process of atoms targeting a specific position on asolid crystal surface, the size error of the confinement
structure by atoms becomes zero (Fig. 1), except forthe positioning errors induced by focusing accuracy
Vol. 13 No. 8 Aug. 2015

and thermodynamic uncertainty [1]. Atom manipulation is a method of flexible positioning of atoms on asurface. The practicability was demonstrated [2] soonafter the invention of the scanning tunneling microscope (STM) [3]. If the positioning uncertainty of the
focused atom beam process can be corrected by atom
manipulation after the deposition process, we cansolve the positioning error problem by utilizing thesize-error-free feature of natural atoms. In otherwords, atom manipulation is a key microfabrication
technique with ultimate precision.
2. Experimental method
As a template for the atom manipulation, we used a(111)A-oriented surface of indium arsenide (InAs)
crystal. InAs is a compound semiconductor composed of indium (In) and arsenic (As). It is one of themost important materials in high-speed electronic
and infrared photonic devices. A high-quality InAs
single-crystal thin film was grown on the (111)A-oriented substrate with atomically controlled thickness
by using the crystal growth technique molecularbeam epitaxy (MBE), as shown in Fig. 2(a). Epitaxy
refers to single-crystal growth with one particularFeature Articles

Vacuum pump

Vacuum chamber(MBE chamber)
Display

Vacuum (< 10−8 Pa)
HeaterA

Substrate

Phosphorus
screen(RHEED)Electrongun

LN2
shroudShutter

Tip

Bias voltage

Feedback
control
Scan

High-purity
material

In

Tunnel current

As

Atom

Substrate
Molecular beam sources (Knudsen cells)(a) Molecular beam epitaxy (MBE) equipment(b) Scanning tunneling microscope (STM)

NTT

InAs

PDI
Amorphous Transportation
As layer(JapanGermany)
InAs

Substrate

Substrate
Substrate
Deposition of
MBE growth
protective layer

Amorphous
As layer
InAs
Substrate

InAs

InAs

Substrate
Substrate
Evaporation ofAtom manipulation
protective layer

LN2: liquid nitrogen
RHEED: reflection high-energy electron diffraction

Fig. 2. Collaborative semiconductor crystal growth and atom manipulation.

crystallographic orientation on a substrate. MBE is avapor growth method, which performs the epitaxy by
exposing the substrate to beams of atomic or molecular gases of high purity materials to the substrateunder an ultra-high vacuum of 10−8 Pa or less. The
MBE method can grow semiconductor crystals withvery high purity and crystallinity. After the growth,
the grown InAs surface was covered by a protective
film (amorphous As). Then the sample was sent fromNTT to the Paul-Drude-Institut für Festkörperelektronik (PDI) in Germany.
At PDI, the protective film was removed in an ultrahigh vacuum to recover the clean (111)A surface, andthe sample was loaded into an STM operated at asample temperature of 5 K. An STM is an electron
microscope in which an atomically sharpened conductive needle (tip) is scanned above the conductive
sample surface. The tip probes a tunneling current by


applying a voltage difference between the tip and thesample surface at a distance of 1 nm or less, as shownin Fig. 2(b). Through signal processing of the measured tunneling current, the STM visualizes the local
electronic states to observe the surface atoms.
NTT and PDI have collaboratively developed atom
manipulation at semiconductor surfaces using a lowtemperature STM as a new approach to surpass conventional nanostructure fabrication technologies. Ourachievements demonstrate that an international collaborative research project that combines individual
skills and experience is effective for advancing science and technology.
3. Atom manipulation on semiconductor
surfaces
Surface atoms are arranged periodically on a
NTT Technical Review

Feature Articles

Hollow (fixing site)

1 cm

In vacancy (stable adsorptive site)
1 nm

Array model of fixing sites

In atom(white portion)
STM image of InAs(111)A surface(a) Surface structure of substrate crystal used for atom manipulation
1 cm
Pick-up1

1 nm
Substrate

1

tip
0

2

V

3

2
3

Transfer

Transfer
4

4

5

5

Put-down
6(b) Concept of atom manipulation and models of atomic rows

Tip

substrate0V6(c) Atom manipulation process and STM images of atomic rows

Fig. 3. Atom manipulation to construct In atomic rows using STM.

semiconductor surface. Therefore, stable adsorptive
sites of adsorbates also form periodic arrays. By
using this periodicity, we can use the crystal substrate
as a template to precisely control the structure formation by placing individual atoms at specific welldefined adsorptive sites. After the protective layer isremoved, the (111)A surface recovers intrinsic hollow sites caused by periodic vacancies of surface In
atoms at a distance of 0.857 nm. This surface structure is equivalent to that at the MBE-grown surface,
as shown in Fig. 3(a). These hollow sites work as
stable adsorptive sites.
The concept of atom manipulation and some models of atomic chains are shown in Fig. 3(b). In theatom manipulation process, the STM tip is used topick up target atoms from the surface one by one andput them down in the designated position as the building blocks to form well-tailored nanostructures ofatoms. The manipulated In adatoms are those thatremain at the surface after the MBE growth [4]. The
pick-up process and the put-down process are controlled by the applied sample bias voltage V.
At the experimental substrate temperature of 5 K,
Vol. 13 No. 8 Aug. 2015

thermal diffusion of adatoms at the sample surface issuppressed, and individual manipulated atoms are
fixed stably at the positioned sites. We manufactured
atomic chains (1 ≤ N (the number of atoms) ≤ 6) byarranging each In atom one-by-one in a line at the(111)A surface, as shown in Fig. 3(c) [5]. Precise andreproducible atomic configurations were possible
because of the periodic and symmetric arrangementof stable adsorptive sites.
In addition to enabling microfabrication of quantum devices with single-atom precision, atom manipulation is advantageous in producing novel assemblies of integrated atoms, which do not exist in nature
and are also not possible by means of chemical reaction. This is a very attractive subject that warrants
further study. We expect that atom manipulation on
semiconductor substrates will provide a novel aspect
to our understanding of the physical and chemical
properties of materials.

Feature Articles

Height
high

1 nm
Atom
Electron
STM image of atomic row

Surface electron accumulation layer

Confinement
barrierQuantized
level

low

Fermi level

Confinement barrier

Quantized
level

In ionElectronCore

Quantized level

Electron energy(a) QD (concept)

Electron existence probability

Electron existence probability

Fermi level−N = 6
Quantized
level

5K

−200 −1000Sample bias voltage (mV)(b) Ultra-precise QD (N = 6)

Fig. 4. Quantum dot (QD) structure, energy diagram, and density of states spectrum.

4. Quantum structures with ultimate precision
4.1 Ultra-precise quantum dots
The current atom manipulation technology enables
us to flexibly implement quantum dots (QDs) withproperties identical to those of natural atoms at thesemiconductor substrate surface with single-atomprecision of position and configuration. At the InAs
surface, the indium (In) adatom is self-ionized to
become a +1 charged ion that releases an electron to
the substrate. Thus, a row of such ions behaves as acore, which confines electronic states at the semiconductor surface to the induced local Coulomb potential. This potential is well-defined, like that of a natural atom. These adatom-assembled structures work as
ultra-precise QDs with excellent uniform properties.
A QD is a microscopic structure that confines electrons in a narrow, nanometer-scale space with length,
width, and height all comparable to or smaller thanthe de Broglie wavelength of free electrons (1–100nm), making quantum mechanical effects evident.
The concept of the QD is shown in Fig. 4(a). The QD
is an essential component of electronic and opticalsemiconductor devices and has been extensively
studied in the information technology field as a key
element for developing a quantum computer. The
electrons confined to a QD obey quantum mechanicalstatistics and form discrete energy levels similar to
those of natural atoms. Moreover, QDs reproduce
intrinsic properties of natural atoms such as the electron shell structure and Hund’s rules [6]. Thus, asemiconductor QD is often called an artificial atom
(which is not to be confused with an artificial element).
The experimental results for a QD with N = 6 areshown in Fig. 4(b). Electrons are attracted to the coreof ionized adatom rows. The spectral peak indicatesthat the free electronic states at the semiconductor
NTT Technical Review

Feature Articles

Single QD

QD dimer
2 nm

Heighthigh

STM image

Electron existence
probability(experiment)low

Splitting by
quantum
coupling–114 mVlow

Ground state

dl/dV–97 mV

Antibonding state–167 mV

Bonding statehigh(a) QD dimer

STM image

Electron existence probability (experiment)

2 nm–262 mV–112 mV

Ground state

Excited state

Symmetrically tailored case

Height
low

dl/dV
high

low
Broken symmetry
lifts degeneracy.

high–246 mV–124 mV–108 mV

Displaced along white arrowby one atomic space(b) QD trimer

Fig. 5. Artificial molecules of ultra-precise QDs.

substrate are confined, and a quantized level isobtained below the Fermi level*3. The discrete peak
spectra of QDs (6 ≤ N ≤ 25) were characterized
experimentally [7].
4.2 Integration of ultra-precise quantum dots
An artificial molecule, a microscopic structure
made of artificial atoms, shows properties similar to
those of a natural molecule. When two ultra-precise
QDs (N = 6) are coupled as shown in Fig. 5(a),molecular electronic states appear as a result of theinteraction between individual quantized states in the
QDs [6]. The observed states correspond to a bonding
state and an antibonding state, similar to a naturalVol. 13 No. 8 Aug. 2015

dimer molecule. Here, the electron existence probability maps (spatial distributions of the local densityof states) were visualized by measuring the differential conductance (dI / dV) signal of the tunneling current I at a given sample bias voltage V. When exactly
the same artificial molecule structures were fabricated, the standard deviation of the splitting energy
between the bonding and antibonding levels was
about 1 meV. This means that the fluctuation of thequantization characteristics is equivalent to the*3 Fermi level: The Fermi level is an energy position at which thequantum mechanical electron distribution function has exactly
50% probability of occupancy by an electron.

Feature Articles

unavoidable thermal fluctuation limit at the substratetemperature of 5 K (3.5k BT / 2 = 0.8 meV; k B:
Boltzmann constant). This confirms that the reproducibility of quantization characteristics is excellent.
It should be emphasized that it would not be possible to substantially demonstrate the characteristics of
the ultra-precise quantum structures fabricated using
the atom manipulation technique if there were any
fluctuations due to defects or impurities. We wereable to experimentally confirm the excellent fabrication accuracy by atom manipulation only because thequality of the MBE-grown semiconductor crystal was
sufficiently high. This indicates that very minutesemiconductor quantum structures require the synergistic effect of combining the extremely high processing accuracy at atomic-scale precision and thehigh crystallinity of the semiconductor crystal. The
availability of such ultra-precise quantum structures
enables us to manufacture ultimate quantum devices
with atomic-level reproducibility like an array ofquantum bits with uniform functions, which have notyet been achievable because of statistical errors instructural fluctuations. Furthermore, integration andcontrol of these precise nanostructures will help us todevelop a quantum computer and the next generationof technology to overcome the limits of conventional
silicon technology.
When three ultra-precise QDs (N = 6) are combined
as shown in Fig. 5(b), they form a nanostructure corresponding to a QD trimer molecule [6]. When one of
the component QDs was displaced at a distance of
one adsorptive site (0.857 nm), and thereby the rotational symmetry was broken, tunneling spectroscopy
measurements revealed that the doubly degenerated
excited level split into two individual levels. This
infers that the electronic structure of the symmetric
QD trimer consists of molecular levels, as is expectedfrom a triatomic molecule of threefold symmetry. It
also shows that an atomic-level error of the quantum
structure has a crucial effect on the quantum characteristics.
From the viewpoint of device integration, this QD
trimer is regarded as a semiconductor structure madeof three QDs integrated in a 10 × 10 nm square area.
If the degree of integration is estimated by assumingthat one QD corresponds to one transistor, the degree
is about 1000 times as high as that of conventionalICs, though it is limited to a local area. This indicatesthat the QD trimer molecule in Fig. 5(b) can be considered a mock-up of an ultimately integrated semiconductor microstructure. The thermal de Broglie
wavelength λD (defined by λD = h / 2πmkBT ) givesa good estimate of the maximum length scale of thequantum mechanical features in the surroundings at
temperature T. For semiconductor electrons with thetypical effective mass m* = 0.1m0 (m0: the rest massof an electron in vacuum), λD is 105 nm at the substrate temperature (T = 5 K). At room temperature (T= 300 K), λD is 13.6 nm. This suggests that nanostructures comparable in size to the QD trimer shown in
Fig. 5(b) will operate as functional elements of quantum devices even at room temperature. This remarkable fact greatly encourages advances in nanostructure research.
The design and analysis of the artificial atoms andmolecules were performed by doing theoretical firstprinciples calculations at the Naval Research Laboratory (NRL) in the USA in collaboration with PDI.
This theoretical analysis enabled us to confirm thatthe confined states in QDs originate from the intrinsic
electronic states of the semiconductor substrate, notfrom the atomic orbitals of In adatoms.
5. Future perspectives
At present, STM-based atom manipulation is thesole technology for fabricating ultra-precise quantum
structures with respect to position and configuration.
We expect that the present achievements will open
the door to developing new electronics technology by
combining atomic and molecular electronics withsemiconductor thin film technology. However, thethroughput of atom manipulation, namely the efficiency of manufacturing nanostructures per unit time,is inferior to that of conventional lithography technologies [8]. When this drawback is overcome andthe fabrication and characterization of quantum structures with atomic precision become possible at thesemiconductor substrate surface, we will be a giant
step closer toward realizing a new IC technology
through the fusion of wafer-level semiconductor
technology and atomic and molecular electronics. By
exploring novel properties of many integrated atoms
and their interaction with semiconductor heterostructures, we plan to develop architectures for quantum
computers and high-performance semiconductor
devices composed of well-defined semiconductor
nanostructures with robust fidelity. Further study willbring many benefits to a broad range of science andtechnology fields.
References[1]

T. Shinada, S. Okamoto, T. Kobayashi, and I. Ohdomari, “Enhancing

NTT Technical Review

Feature Articles

[2]
[3]
[4]
[5]

Semiconductor Device Performance Using Ordered Dopant Arrays,”
Nature, Vol. 437, No. 7062, pp. 1128−1131, 2005.
D. M. Eigler and E. K. Schweizer, “Positioning Single Atoms with a
Scanning Tunnelling Microscope,” Nature, Vol. 344, No. 6266, pp.
524−526, 1990.
G. Binning, H. Rohrer, Ch. Gerber, and E. Weibel, “Surface Studies
by Scanning Tunneling Microscopy,” Phys. Rev. Lett., Vol. 49, No. 1,
pp. 57–61, 1982.
K. Kanisawa and T. Fujisawa, “Mechanism of Electron Accumulation
Layer Formation at the MBE-grown InAs(111)A Surface,” Hyomen
Kagaku, Vol. 29, No. 12, pp. 747–757, 2008 (in Japanese).
S. Fölsch, J. Yang, C. Nacci, and K. Kanisawa, “Atom-By-AtomKiyoshi Kanisawa

Senior Research Scientist, Quantum SolidState Physics Research Group, Physical Science
Laboratory, NTT Basic Research Laboratories.
He received his B.S., M.S., and Dr.Sci. in physics from Tokyo University of Science in 1986,
1988, and 1998, respectively. He joined NTT LSILaboratories in 1988. He has been conducting
research on semiconductor surface science at
NTT Basic Research Laboratories since 1995.
His current research interest is the physics ofsemiconductor surfaces and epitaxial growth. He
is a member of the Physical Society of Japan(JPS), the Japan Society of Applied Physics(JSAP), the Surface Science Society of Japan(SSSJ), and the American Vacuum Society(AVS).

Vol. 13 No. 8 Aug. 2015

[6]

[7]
[8]

Quantum State Control in Adatom Chains on a Semiconductor,” Phys.
Rev. Lett., Vol. 103, No. 9, pp. 096104-1–4, 2009.
S. Tarucha, D. G. Austing, T. Honda, R. J. van der Hage, and L. P.
Kouwenhoven, “Shell Filling and Spin Effects in a Few Electron
Quantum Dot,” Phys. Rev. Lett., Vol. 77, No. 17, pp. 3613–3616,
1996.
S. Fölsch, J. Martínez-Blanco, J. Yang, K. Kanisawa, and S. C. Erwin,“Quantum Dots with Single-atom Precision,” Nature Nanotechnol.,Vol. 9, No. 7, pp. 505–508, 2014.
R. Garcia, A. W. Knoll, and E. Riedo, “Advanced Scanning Probe
Lithography,” Nature Nanotechnol., Vol. 9, No. 8, pp. 577–587,
2014.

Stefan Fölsch

Senior Scientist, Leader of the Low-temperature STM Research Group, Paul-Drude-Institut
für Festkörperelektronik (PDI).
He received his B.S. and Ph.D. from the University of Hannover in 1987 and 1991, respectively. He was a research associate at NTT from1992 to 1994 and at the Free University of Berlin
from 1994 to 2002. He received Habilitation(academic qualification) at the Free University of
Berlin in 2004. He joined PDI in 2002. His current research interests are surface physics-related
aspects of atom and molecule manipulation by
STM. He is a member of the Deutsche Physikalische Gesellschaft (DPG, German PhysicalSociety).

Feature Articles: Frontier Research on Lowdimensional Semiconductor Physics

Creating a Topological Insulator
Using Semiconductor
Heterostructures
Kyoichi Suzuki and Koji Onomitsu
Abstract

Topological insulators are new states of matter that cannot be classified into any existing categories ofmaterials. They have attracted much attention for their potential use in electronic and quantum computing devices because of their specific electron transport properties. In this article, we describe how wecreated an artificial topological insulator using a heterostructure comprising common semiconductormaterials.
Keywords: semiconductor heterostructure, topological insulator, edge channel

1. Introduction
We have recently seen a new movement in materials science to classify materials from the viewpoint of
topology. In the process, new states of matter havebeen found, which cannot be classified into existing
categories of materials. One such state is the topological insulator (TI), which has an insulating phasewith a topology different from that of normal insulators. Mathematically, a state cannot continuouslytransit between different topologies and is therefore
necessarily broken at the boundary (Fig. 1). In ananalogous way, the insulating state in TIs cannot continue to the normal insulating state outside, and it istherefore necessary to break the insulating states. As
a result, a conductive channel arises at the boundaryof a TI. This topologically protected conductive channel is expected to have low power consumption or
specific transport properties, and thus, TIs haveattracted a great deal of attention as a new type ofelectronic material.
Several TIs have been confirmed experimentally,for example, Bi2Se3 and HgTe. However, these materials are not common in industry, and specific knowhow is necessary to achieve their crystal growth. They
have a distinct energy band structure by nature, which
is what realizes a TI (discussed later). In contrast, we

created an artificial TI by using a semiconductor heterostructure comprising indium arsenide (InAs) andgallium antimonide (GaSb) [1], which are commonlyused industrially. Taking advantage of the highly
developed semiconductor technology will make itpossible to carry out detailed clarifications and to
implement various device applications of TIs.
2. Topological insulator
When atoms are condensed and crystallized, theelectron orbits for each atom are hybridized. As aresult, electronic states called energy bands areformed due to the periodicities of the crystal. Theenergy band accumulates electrons from the lower
energy side. When one energy band is completely
filled by the electrons and the upper energy band isempty, the state of the matter becomes an insulator(Fig. 2). The filled energy band is called the valenceband, and the upper empty energy band is called the
conduction band. The gap between these energy
bands is called the band gap. This is the situation in
an intrinsic semiconductor. At low temperatures,
thermal excitation of electrons from the valence bandto the conduction band is suppressed. This state
results in an insulator.
In TIs, the conduction and valence bands overlap
NTT Technical Review

Feature Articles

Conduction
band

Empty
electronic
statesEnergyEnergyFig. 1. Topologically different states.

Band gap
Valence
band

Filledelectronic
statesConduction
Conduction
Reconstruction
band
band
Overlap
Band gap
Valence
Valenceband
band
Inner bulk(a) Insulator (intrinsic semiconductor)

Conduction
band
Valenceband
Surface/Edge(b) Topological insulator

Fig. 2. Energy band structures.

Insulating region

Edge channel

Fig. 3. 2D TI and edge channel.

energetically. The wave functions in each band are
hybridized based on the relativistic interaction. As aresult, the energy gap is reopened and the conduction
and valence bands are reconstructed. When the reconstructed valence band is completely filled with electrons and the conduction band is empty, an insulator
state exists. However, the topology of this state is different from that outside. Therefore, the band gap
should be closed at the boundary. Electronic states
connecting the valence and conduction bands appear asa conductive channel (red and blue lines in Fig. 2(b)).
For a three-dimensional (3D) TI, this conductive
channel corresponds to the two-dimensional (2D)
surface state. For a 2D TI, this channel corresponds tothe edge channel. Singular electronic transport is
expected due to the restriction of electron scattering.
We have been investigating 2D TIs whose edge
Vol. 13 No. 8 Aug. 2015

channel is composed of counter-propagating electron
currents with opposite spin directions, which arecalled helical spin currents (Fig. 3). The topologicallyrobust protection of the edge channel prevents theback scattering of electrons between counter-propagating currents in the edge channel with spin flip. As
a result, dissipationless electron transport, that is,quantized conductance, should be achieved. In practice, a value very close to the quantized conductancehas been observed in an HgTe/HgCdTe heterostructure, which is a representative 2D TI. In addition, atthe junction between the helical spin current and thesuperconductor, a curious quasiparticle state called
the Majorana fermion, which is simultaneously aparticle and its antiparticle, is expected to be found.
The Majorana fermion is promising as a quantum bit
for fault-tolerant quantum computing.
AlGaSb

Conduction
band

AlGaSbGaSb
Band gap

Band
gap
InAs

EnergyFeature Articles

Conduction
band of InAs

TI
Valence bandof GaSb

Valence band(a) Energy band structure(b) Construction of TI band structure

Ti/Au Gate metal
AuGeNi electrode

Gate voltage, VG

Fig. 4. InAs/GaSb heterostructure.

AI2O3
GaSb

Molecular
beam epitaxy

AI0.7Ga0.3Sb
2 µm
InAs: 12 nm
GaSb: 10 nm
AI0.7Ga0.3Sb
GaAs substrate(a) Cross-sectional
microscopic image

Atomic layer
depositedgate insulator(b) Layer structure

6

Channel layer
AISb/GaSb,
GaAs/AIAs
superlattice
insulating layer

AuGeNielectrode

5

1

4
2

3

Ti/Au gate metal(c) Lithography patternFig. 5. InAs/GaSb heterostructure sample.

As described above, 2D TIs are promising for use
in low-power-consumption devices, quantum computing, and spintronic devices.
3. InAs/GaSb heterostructure
Both InAs and GaSb are common and widely used
semiconductors. When the heterojunction is made,the conduction band in InAs and the valence band in
GaSb overlap energetically. After the layer thicknesses are optimized and the InAs/GaSb heterostructure is sandwiched between AlGaSb barrier layers,
the energy band structure of TIs is artificially realized
through the hybridization of the adjacent conduction
and valence bands (Fig. 4). This heterostructure
becomes a 2D TI because of the layer structure.
The structure of our sample is shown in Fig. 5. We
grew the semiconductor layers using molecular beam
epitaxy, which enables us to control each layer thickness to one atomic layer. We used photolithography
and etching to fabricate a small device pattern, as
shown in Fig. 5(c). Then, six electrodes were formedby evaporating AuGeNi alloy. An Al2O3 gate insulating layer was deposited by atomic layer deposition,
and a Ti/Au top gate was evaporated. The electron
density can be controlled by applying voltage
between the gate and the InAs/GaSb channel layer.
The TI is realized when the valence band is completely filled with electrons and the conduction bandis empty.
4. Low-temperature measurement
Similar to common insulators and intrinsic semiconductors, the insulating phase in a TI is broken by
thermal electron excitation from the valence band tothe conduction band. Therefore, to obtain the distinct
transport properties of a TI, an extremely low
NTT Technical Review

Feature Articles

He-3 gas

Thermally
insulating
Low-pressureliquid He-4
at 1.3 K forliquefying
He-3

Low-pressureliquid He-3 at
0.25 K

SampleFig. 6. He-3 refrigerator.

Conductance between
adjacent electrodes
e 2/h ~~ 1/(25.8 kΩ)

6

5

1

6
4

2

3

5

1

4
2

3

V

V

R14,23 = h/(2e 2)

R16,34 = h/(6e 2)(a) Longitudinal resistance(b) Nonlocal resistanceFig. 7. Equivalent circuits for 2D TI with six electrodes.

temperature is necessary. Here, we used a He-3 (anisotope of helium) refrigerator (Fig. 6). In this refrigerator, He-3 gas is liquefied by low-pressure liquid
He-4 (common helium), whose temperature is about
1.3 K. Then the pressure of the liquid He-3 is reduced.
As a result, the lowest possible temperature of 0.25 K
is obtained.
5. Confirmation of TI
In the 2D TI, the inner bulk part is insulating. The
Vol. 13 No. 8 Aug. 2015

electron transport takes place through the edge channel between adjacent electrodes. Ideally, the conductance between the adjacent electrodes should bequantized conductance, e 2/h, and the resistanceshould be its inverse, h/e2 ~ 25.8 kΩ. The equivalentcircuit models of the sample with six electrodes
are shown in Fig. 7. Based on a simple resistancecalculation, longitudinal resistance R14,23 = h/(2e2) ~12.9 kΩ and, for example, nonlocal resistance R16,34= h/(6e2) ~ 4.3 kΩ are expected. In practice, however,electron scattering occurs for various reasons.
Feature Articles

100

0.25 Kh/(2e 2)
10

R14,23 (kΩ)

R14,23 (kΩ)

100h/(2e 2)
10
6

5

2

3

4

1

1

1.0

1.2

1.4

1.6

1.8

1

V23
0

1

2

3

4

−1

VG (V)

1/T (K )(a) Longitudinal resistance(b) Dependence on temperature oflongitudinal resistance peakFig. 8. Longitudinal resistance and its dependence on temperature.

Therefore, the conductance between the adjacentelectrodes deviates from the quantized value.
Longitudinal resistance (R14,23) experimentallyobtained as a function of the gate voltage (VG) isshown in Fig. 8. The electron density is low on the
low VG side, and there are unoccupied electron states(holes) in the valence band. The holes act as carriers
with a positive charge and contribute to the electronictransport. Therefore, the inner bulk region is conductive, and R14,23 becomes low. In contrast, on the highVG side, electrons partially accumulate in the conduction band and act as conductive electrons, which also
lowers R14,23. Around VG = 1.25 V, the valence bandis completely filled with electrons and the conduction
band is empty. There are no conductive electrons orholes in the inner bulk region. Therefore, R 14,23
increases. The peak resistance of R14,23 is about 20 kΩ.
If the sample were an intrinsic semiconductor, theresistance peak would become infinite. The finite
resistance indicates that the sample has a conductive
channel, and this resistance peak value is not far fromthat expected as quantized transport, h/(2e2). Thissuggests that the sample becomes a TI at this VG.
The dependence of the R14,23 peak on temperature
is shown in Fig. 8(b), where the horizontal axis is theinverted temperature. On the high-temperature side
(left side) the resistance peak is low. This is because
the thermally excited electrons from the valence bandto the conduction band and simultaneously generated
holes in the valence band contribute to the electronictransport in the inner bulk region. The estimatedenergy gap from the slope in the high-temperature
region is 0.18 meV (~ 2.1 K). On the low-temperature

side, the peak resistance approaches h/(2e2).
In an attempt to obtain clear evidence for the TI, wemeasured nonlocal resistances. R16,23 and R65,23 as afunction of VG are shown in Fig. 9(a). Here, Rij,kl (=Vkl/Iij) indicates that electrodes i and j are used for
current (Iij) injection and ejection, and electrodes k
and l are used for voltage (Vkl) measurement. Nonlocal resistances R16,34 and R65,34 are shown in Fig. 9(b).
The resistance peaks deviate from the expected valueh/(6e2) in the quantized transport. We could not confirm the realization of a TI from these results.
Next, the nonlocal resistance ratios (voltage ratios
between the adjacent electrode pairs, V23/V34) areplotted in Fig. 9(c). For the pink curve, the current is
injected into electrode 1 and ejected from electrode 6.
For the purple one, it is injected into 3 and ejectedfrom 4. Around the resistance peak, these two curves
completely agree with each other independently of
the current injection/ejection path. We observed suchagreement of the nonlocal resistance ratios between
the adjacent voltage electrode pairs for all combinations of electrode arrangements. This indicates thatthe transport occurs only between adjacent electrodes; that is, the inner bulk region is insulating, andthe transport is governed by the edge channel. Here,
we can confirm the realization of a TI in the InAs/
GaSb heterostructure. When the temperature isincreased to 4.3 K, the agreement of the nonlocal
resistance ratios disappears, as shown in Fig. 9(d).
This indicates that the thermally excited carriers contribute to the transport in the inner bulk region.

NTT Technical Review

Feature Articles

10

10

R16,23

R16,341

5 h/(6e 2)

6 5
2 3

4

R65,34
R ,34 (kΩ)

R ,23 (kΩ)

R65,231

5 h/(6e 2)

6 5
2 3

V23

V34

0.25 K
0

1.0

1.2

1.4
1.6
VG (V)

0.25 K01.8(a) Nonlocal resistance R ,23

5

3

6 5
2 3

I34

4

V23 V342

1.4
1.6
VG (V)

1.8

I16
I65

4

1
0

1.2

5

V23/V34

V23/V3411.0(b) Nonlocal resistance R ,34

I16

4

4

3
2
1

0.25 K
1.0

1.2

1.4
1.6
VG (V)

01.8(c) Nonlocal resistance ratios(0.25 K)

4.3 K
1.0

1.2

1.4
1.6
VG (V)

1.8(d) Nonlocal resistance ratios(4.3 K)

Fig. 9. Nonlocal resistances and their ratios.

6. Future prospects
Both InAs and GaSb can achieve good electronic
connections with ferromagnetics or superconductors.
This is expected to lead to their application for spintronic or quantum information devices. In addition,
our artificial TIs created using heterostructures
between non-topological materials create the possi-

Vol. 13 No. 8 Aug. 2015

bility of combining various materials, including insulators or metals, to achieve new TIs.
Reference[1]

K. Suzuki, Y. Harada, K. Onomitsu, and K. Muraki, “Edge Channel
Transport in the InAs/GaSb Topological Insulating Phase,” Phys. Rev.
B, Vol.87, No. 23, 235311, 2013.

Feature Articles

Kyoichi Suzuki

Senior Research Scientist, Quantum Solid
State Physics Research Group, Physical Science
Laboratory, NTT Basic Research Laboratories.
He received his B.E., M.E., and Ph.D. inapplied physics from Tohoku University, Miyagi,
in 1991, 1993, and 2004, respectively. He joined
NTT Basic Research Laboratories in 1993. He
has studied magneto-transport, electronic, andoptical properties in semiconductor low-dimensional structures. He is a member of the PhysicalSociety of Japan (PSJ) and the Japan Society ofApplied Physics (JSAP). In 2008, he received the
JSAP Outstanding Paper Award.



Koji Onomitsu

Senior Research Scientist, Low-Dimensional
Nanomaterials Research Group, Materials Science Laboratory, NTT Basic Research Laboratories.
He received a B.E. and M.E. in electrical engineering from Aoyama Gakuin University, Tokyo,
in 1998 and 2000, respectively, and a Ph.D. inelectrical engineering from Waseda University,
Tokyo, in 2004. He joined NTT Basic ResearchLaboratories in 2006. His current interests are
crystal growth and device applications of twodimensional layered materials such as MoSe2.
He is a member of JSAP.

NTT Technical Review

Regular Articles

Two-mode Squeezing in an
Electromechanical Resonator
Imran Mahboob, Hajime Okamoto,and Hiroshi Yamaguchi
Abstract

A mechanical resonator integrated with piezoelectric transducers enables mechanical nonlinearities tobe dynamically engineered to emulate non-degenerate parametric down-conversion. In this configuration, millions of phonons are simultaneously generated in pairs in two macroscopic vibration modes,
which results in the amplification of their motion by more than 20 dB. Mechanical two-mode squeezed
states are also created in parallel, which exhibit fluctuations 5 dB below the thermal level of their constituent modes and they harbor correlations between the modes that become perfect as their amplification
is increased. This remarkable observation of correlations between two massive phonon ensembles establishes the means to create an entangled macroscopic mechanical system at the single phonon level.
Keywords: electromechanical resonator, parametric down-conversion, squeezed-states, phonons

1. Introduction
Electromechanical systems consist of a single
degree of mechanical freedom hosting a spectrally
pure resonance that is embedded in an electricaltransduction circuit, and they have emerged as a versatile platform for a range of technological applications and to study fundamental science [1, 2]. For
instance, their small inertial mass and high quality
resonance have enabled the development of ultra-precise sensors that can detect a single electron spin andeven the mass of a proton leading to a new class of
medicinal diagnostic technology [3–5]. The electricaltransduction circuit can also enable the underlying
potential energy landscape of the mechanical resonator to be dynamically engineered, yielding a range of
nonlinear motional dynamics that can be harnessed
for both information storage and processing, whichbrings forth the concept of mechanical computation
offering both ultra-low power consumption and thecapacity for unprecedented parallel data processing[6–9].
Most tantalizingly, a high frequency mechanicalresonator operated at sufficiently low temperatures

can even condense into its quantum ground state,
where its low mass and spectral purity yield quantum
zero point fluctuations that are large enough to beobserved, enabling a macroscopic quantum system to
be studied [10–12]. In this instance, although themechanical resonator is composed of billions ofatoms, only the phonons sustained by the fundamental resonance mode are quantized, and it corresponds
to a tangible vibration of the mechanical element.
This phonon picture, namely the collective excitationof atoms in the fundamental vibration mode, enables
concepts from atomic molecular optical (AMO)
physics developed for photons, that is, a quanta ofelectromagnetic radiation, to be exploited. Indeed, itis the laser cooling techniques pioneered in AMO
physics that have been utilized most successfully tocool the macroscopic mechanical oscillator so that on
average, its fundamental mode is occupied by muchless than one phonon [13, 14].
Although the notion of a macroscopic mechanicalresonator composed of billions of atoms being considered only in terms of the number of phonons sustained by its fundamental mode might seem counterintuitive, this paradigm has successfully been exploited
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(a)

(b)
Idler

Nonlinearχ(2) crystal

ωiSignal

ωii
(c)

ω1

ωi

Pump
ωs
ωp
ki

ks

θ

θ

ωi

(d)

ω1
ω2

kp

Fig. 1. (a) Schematic illustration of the spontaneous parametric down-conversion process used to generate non-classicallight where the signal and idler photons are entangled in a two-mode squeezed state. This process occurs at theexpense of the pump photons where the signal and idler photons conserve both momentum kp = ks + ki, where k is
the wavevector, and energy ωp = ωs + ωi. The nonlinear crystals typically used are BBO (beta-barium borate) for TypeII down-conversion (where the signal and idler photons have perpendicular polarization) and KDP (potassium dihydrogen phosphate) for type I down-conversion (where the signal and idler photons have parallel polarization). (b)Schematic depiction of a microwave cavity composed from a superconducting metal (orange) and terminated by a
SQUID with two Josephson junctions (blue) that can sustain electromagnetic standing waves, for instance ωi and ωii,
which are coupled via the SQUID. (c) Cavity electromechanical system with a microwave resonator of frequency ωiconsisting of a spiral inductor and a capacitor and a mechanical resonator of frequency ω1 which forms a compliablecapacitor. Light and sound in these two subsystems are nonlinearly coupled via the oscillating electrical energy
stored in the capacitor. (d) Phonon-cavity electromechanical system consisting of two localized mechanical vibration
modes ω1 and ω2 that are coupled via strain. In all three systems, if the respective flux penetrating the SQUID, theelectric field stored in the capacitor, or the mechanical strain are sinusoidally pumped at the sum frequency of theirelectromagnetic and/or mechanical modes, signal and idler photons and/or phonons are generated that conserve
energy but the momentum conservation constraint is relaxed due to their stationary wave nature.

to demonstrate the signature feature of photonics,
namely lasing but with the localized phonons in themechanical resonator [15–18]. Consequently, theability to cool a solid-state macroscopic mechanicalresonator into its ground state opens up the possibility of generating an all-mechanical macroscopic
entangled state [10, 19], which would enable foundational aspects of quantum mechanics to be queriedsuch as the nature of the quantum to classical divide
and the absence of quantum phenomena in our everyday classical world [20, 21].
2. Two-mode squeezed states
Entangled photons in the guise of two-mode
squeezed states were among the first non-classicalstates of light to be generated in the lab and nowadays
are routinely siphoned from spontaneous parametricVol. 13 No. 8 Aug. 2015

down-conversion [22]. This typically involves a χ(2)nonlinear crystal that is exposed to a strong laser
pump beam whose dielectric polarization responds
nonlinearly to the pump’s electric field. In this process, a pair of photons—the signal and the idler—are
generated at the expense of the pump photons, whichconserve both energy and momentum, as depicted inFig. 1(a). Remarkably, the quantum fluctuations in
the amplitude and phase of the generated photon pairare correlated, as encapsulated by the Einstein,
Podolsky, and Rosen (EPR) paradox, where theirindividual fluctuations are amplified, while their relative fluctuations are reduced below the vacuum noiselevel [23–26]. The EPR paradox recast in the Bellinequality has been routinely violated via the entangled signal and idler photons whose fluctuations
remain correlated even when they are separated overdistances of kilometers [27, 28]. The non-classicalRegular Articles

light generated from spontaneous parametric downconversion has subsequently engendered a whole
host of quantum-enabled technologies including
quantum communication [29], optical quantum computing [30], quantum teleportation [31], quantum
enhanced measurements and metrology [32, 33].
This unprecedented success has led to two-mode
squeezed states being translated to other parts of theelectromagnetic spectrum, for instance microwaves
utilizing superconducting circuits [34]. Specifically, asuperconducting resonator terminated with a nonlinear element such as a superconducting quantum interference device (SQUID), as generically depicted inFig. 1(b), can enable parametric down-conversion[35, 36]. Pumping the SQUID with flux in these socalled Josephson parametric amplifiers (JPAs) yields
a nonlinear variation in the boundary conditions of
the microwave resonator which creates a frequency
modulation in the standing waves it hosts. If the pumpfrequency coincides with the sum of two modes (say
ωi and ωii as depicted in Fig. 1(b)) in the microwave
resonator, it non-degenerately amplifies their vacuum
noise, generating signal and idler photons in a process reminiscent to that described above where thesephotons are entangled [37, 38]. Indeed, JPA-likedevices have now opened the door to many of theconcepts pioneered with optical two-mode squeezed
states but in an on-chip microwave circuit that could
even be yoked into a quantum computer.
A variation on these microwave circuits has recently emerged in the form of cavity electromechanicalsystems where the cavity, namely the microwave
resonator, is composed of an inductor and capacitor
in series, and the mechanical resonator forms amechanically compliant element of the parallel plate
capacitor, thus capacitively coupling the mechanics
to the microwaves as generically depicted in Fig. 1(c)[39–41]. Electric fields in the forms of microwaves
can be pumped into this hybrid circuit via a capacitively coupled transmission line that nonlinearly
modulates the energy stored in the capacitor andhence the frequencies of the mechanics and the cavity. If the pump coincides with the sum frequency ofthe subsystems (i.e. ωi and ω1 as depicted in Fig.
1(c)), it behaves like a non-degenerate parametricamplifier and it creates photons and phonons in pairsthat are correlated in a two-mode squeezed state [42,43]. This remarkable demonstration illustrates thattwo vastly dissimilar systems, namely light andsound, can be entangled in a macroscopic context,
providing a new regime in which quantum physics
can be explored [14].


The cavity electromechanical system detailed inFig. 1(c) straddles the two extremes of light andsound, and it naturally suggests the possibility of apurely mechanical analogue of a two-mode squeezed
state in a phonon-cavity electromechanical system as
depicted in Fig. 1(d). As in the microwave case, themechanical resonator can also sustain multiple
modes, say ω1 and ω2, as shown in Fig. 1(d), which if
nonlinearly coupled, could yield non-degenerateparametric amplification. In contrast to the microwave circuit that needs to be engineered with a nonlinear element, namely the SQUID, to enable themodes to couple, the modes in the mechanical resonator can dispersively couple naturally via the strain
induced from the motion of a given mode that modifies the restoring potential, that is, the spring constantand hence, the natural frequency of the other modes[44]. Consequently, if the strain in the mechanicalelement is sinusoidally pumped, it will nonlinearly
modulate the frequencies of all the localized modes,
and once this modulation coincides with a pair ofmodes, it will non-degenerately amplify their motion
and in the process generate a two-mode squeezed
state [45]. Critically, however, an entanglement will
only be generated if the constituent modes are initialized in their quantum ground state [10–12], but theability to harness this interaction, even if the modes inquestion are thermalized, would lay a pivotal marker
on the road to generating an all-mechanical macroscopic entanglement [46].
3. Electromechanical resonator
Although degenerate parametric amplification wasdemonstrated in the early nineties, it could not be
used to disentangle correlations between the signal
and idler phonons due to their having identical frequencies and being spatially localized to the samemode in the mechanical resonator [47]. Alternatively,
a non-degenerate variant of this process in the firsttwo modes of a beam resonator yielded only a modest
phonon generation rate, which was insufficient tosignificantly amplify the thermomechanical fluctuations before they were dissipated, resulting in statistically insignificant correlations [45].
To that end, the electromechanical resonator shownin Fig. 2(a) was developed, which consists of two
mechanical elements that are strongly coupled via theexaggerated overhangs between them [48]. This
results in the two lowest-order vibration modes
shown in Figs. 2(b) and 2(c) that are extracted from afinite element calculation and henceforth labeled
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(a)

(b)

(c)

12

180

8
6
4
2
0

244800 245600 246400 261600 262400

Frequency (Hz)

(f)

120

8

60

16

240

Theory
12

180

120

8

4

60

4

0
0
0.00 0.05 0.10 0.15 0.20 0.25
Pump amplitude (Vrms)

0

0

25

Gain

Experiment∆ω/2π (Hz)Displacement spectral density(10−13 mHz−0.5)1016∆ω/2π (Hz)

(e)
240

Gain

(d)

0
75

50
Λ (106)

Fig. 2. (a) Scanning electron micrograph of the electromechanical resonator. Each mechanical element is 80 µm long, 20µm wide, 800 nm thick, and is integrated with two piezoelectric transducers formed from a doped GaAs layer located
300 nm below the surface (blue) and a gold gate electrode (yellow). (b) and (c) The electromechanical resonatorsustains symmetric (red) and asymmetric (blue) vibration modes composed from both mechanical elements that arestrongly coupled via the two large overhangs between them. (d) Thermomechanical vibrations of both modes whendriven by the Langevin force (black) and demodulated in a spectrum analyzer. Activating the non-degenerate parametric down-conversion via the piezoelectric transducers on the left mechanical element from 0 to 0.24 Vrms in 0.03
Vrms increments results in the thermomechanical fluctuations of both modes being amplified. (e) The corresponding
amplification gain and spectral power bandwidth (∆ω) with pump amplitude increments of 5 mVrms. (f) This experimental response can be faithfully reproduced by numerically solving the Hamiltonian in eq. (1).

symmetric (S) and asymmetric (A), which are closely
spaced in frequency. In addition, the spatial profilesof both modes have a large overlap yielding greater
strain-mediated dispersive coupling between them.
This combination of enhanced dispersive coupling
and a smaller frequency separation, almost tending
towards a degenerate configuration, is designed toyield larger non-degenerate parametric amplification
when the strain in this system is modulated at the sum
frequency of both modes.
The piezoelectric effect is utilized in order to modulate the strain in the mechanical modes. By design,
the electromechanical system is fabricated from gallium arsenide (GaAs), which is piezoelectrically
active [6, 49]. The piezoelectric transducer is formed
from a GaAs conducting layer and a gold gate elecVol. 13 No. 8 Aug. 2015

trode sandwiching an insulating GaAs layer, which is
integrated directly into the mechanical element, asshown in Fig. 2(a). Application of an electric field
across the transducer generates in-plane strain thatcan nonlinearly modulate the spring constants andhence, the frequencies of the modes supported by themechanical system [6].
The Hamiltonian for the electromechanical systemin this configuration can then be expressed as:
H=A(P / 2m + m w Q / 2)+ΛQ Q cos(w t). (1)

n=S

2
n

n

2 2
n n n

S

A

p

Here, P and Q are the canonical momentum and conjugate position of the symmetric and asymmetricmodes with mass m and natural frequency ωn, where
the summation expresses their kinetic and potentialRegular Articles

energies. The last term describes the piezoelectrically
activated non-degenerate parametric down-conversion with amplitude Λ and frequency wp = wS + wA,
that is, at the sum frequency of the modes in question.
Ostensibly, this Hamiltonian is classical, which
superficially suggests the unavailability of two-mode
squeezing. However, the last term is analogous to† †0 n (ab + a b ) for non-degenerate parametricdown-conversion in cavity electromechanical systems
in the frame rotating at the pump frequency, where a,b and a†, b† are the annihilation and creation operators
for the mechanical and microwave resonators, respectively, and 0 is the intrinsic coupling rate between
them that is enhanced by n pump photons [41–43].
Consequently, Λ encapsulates the intrinsic coupling
rate between the symmetric and asymmetric modes
that is enhanced by the piezoelectric pump, which
results in phonon pairs being simultaneously generated in them [50]. Remarkably, these phonons shouldstill be correlated, even if the two modes are thermalized and far from their quantum ground state, as longas their generation rate exceeds the rate at which they
are dissipated from both modes [46].
4. Results
4.1 Non-degenerate parametric amplification
To ascertain if this latter expectation is satisfied, thethermomechanical fluctuations of both modes arespectrally measured via optical interferometry with a
3-µW helium neon laser probing the right element of
the electromechanical system at room temperatureand in a high vacuum [46]. This measurement reveals
the modes with natural frequencies wS / 2π ≈ 246 kHz
and wA / 2π ≈ 262 kHz with quality factors Qn = wn /∆wn of 1300 and 2200, respectively, from their spectral power bandwidth ∆wn, and it corresponds to bothmodes sustaining > 107 phonons. Next non-degenerate parametric down-conversion is activated, and thethermal fluctuations from both modes are monitored.
The results of this measurement shown in Fig. 2(d)indicate that as the piezoelectric pump voltage isincreased, the thermomechanical fluctuations of bothmodes are enhanced. This amplification can be referenced to the bare thermal fluctuations, and it yields
gains of more than 20 dB that are accompanied by anarrowing of the power spectral bandwidth from bothmodes, as detailed in Fig. 2(e). At the largest pump
amplitudes (>0.275 Vrms) ∆wn → 0, resulting in bothmodes undergoing non-degenerate parametric resonance [51, 52].
To confirm these experimental observations, theequations of motion for both modes are extracted
from the Hamiltonian in eq. (1) and reformulated intheir rotating frames with Qn = Xn cos(wnt) + Ynsin(wnt), where Xn and Yn are the slowly varying inphase and quadrature components. The resultant
equations are then numerically solved, as detailed ina previous study [46], and they faithfully reproduce
the experimental response as shown in Fig. 2(f), thus
verifying that the amplification can be ascribed to
non-degenerate parametric down-conversion.
The temporal dynamics of this amplification canalso be acquired by mixing the output from the optical interferometer with local oscillators locked onto
the resonances of both modes and then demodulated
in a phase sensitive detector (PSD). The PSD samples
the random displacement fluctuations of the mechanical modes driven by the thermal Langevin force at a
rate of 50 ms over a period of 300 s. This yields fourtime series for the in-phase and quadrature components of both modes, enabling their phase portraits tobe constructed as shown in Figs. 3(a) and 3(b). Thismeasurement reveals that the thermal fluctuations ofboth modes are random and uncorrelated as evidenced by their circular distribution in phase space,indicating all vibration phases are equally likely (redand blue points in Figs. 3(a) and 3(b)). Repeating thismeasurement with a pump amplitude of 0.25 Vrms
confirms that the fluctuations in both modes areenhanced via the non-degenerate parametric downconversion while retaining their random nature;
namely, this amplification is phase insensitive [53].
The observed amplification arises from phonons
generated in both modes from the same process;hence, their fluctuations should be correlated. To that
end, the cross quadratures are constructed in phasespace from the in-phase component of the symmetricmode and the quadrature component of the asymmetric mode and vice versa, as shown in Figs. 3(c) and
3(d). This unveils squeezed distributions where a particular phase orientation is amplified while the perpendicular phase is de-amplified. The narrowness of
this distribution implies the existence of correlations
between the symmetric and asymmetric modes (if nocorrelations between the modes existed, the crossquadratures would yield circular and therefore random distributions) and is the signature feature of a
two-mode squeezed state [54, 55]. However, in orderto quantitatively verify the existence of correlations
between the modes, two criteria need to be satisfied.
First, the two-mode squeezed distributions should
exhibit smaller fluctuations than the bare distributions; otherwise, the correlations would be washed
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(b)

0.06

0.06

0.03

0.03
YA (V)YS (V)

(a)

0.00

−0.03

−0.06
−0.06

0.00

−0.03

−0.03

(c)

0.00
XS (V)

0.06

0.03

−0.06
−0.06

−0.03

0.00
XA (V)

0.03

0.06

−0.03

0.00
XA (V)

0.03

0.06

(d)

0.06

0.06
−

y

0.03

0.03
YS (V)YA (V)

+

x

0.00

−0.03

−0.06
−0.06

0.00

−0.03

−0.03

0.00
XS (V)

0.03

0.06

−0.06
−0.06

Fig. 3. (a) and (b) Bare thermal fluctuations of symmetric (red points) and asymmetric (blue points) modes respectively
acquired temporally on-resonance and projected in phase space via the in-phase (X) and quadrature (Y) axes.
Activating the non-degenerate parametric down-conversion with a pump amplitude of 0.25 Vrms amplifies thefluctuations of both modes while preserving their phase (black points). (c) and (d) The cross quadratures of thepumped measurement XS: YA and XA: YS respectively reconstructed in phase space (black points) reveal two-mode
squeezed states have been generated that exhibit fluctuations in a particular phase which are below the bare thermal
fluctuations from both modes (red and blue points), while fluctuations in the orthogonal phase are amplified.

out by the thermomechanical noise from both modes.
Second, the correlations should be statistically evident from the temporal data used to construct thephase portraits.
4.2 Squeezing below the thermal noise
The former condition can in fact be visibly confirmed by examining the data in Figs. 3(c) and 3(d),which indicate that the de-amplified phases encompass a narrower distribution than the bare thermal
motion from both modes. Quantitatively, new axes x+
and y− are introduced, as shown in the inset to Fig.
3(c), onto which the counts in the squeezed distributions are projected, as shown in Figs. 4(b) and 4(c)[46]. This reveals Gaussian profiles with a zero mean
displacement, as expected from random fluctuations
Vol. 13 No. 8 Aug. 2015

driven by the thermal Langevin force, whose full
width at half maximum can be used to extract theirstandard deviation σ. This analysis is repeated as afunction of non-zero pump amplitude for the crossquadratures XS: YA and XA: YS, where the standard
deviations for both the amplified and de-amplified
phases corresponding to the x+ and y− axes are determined as shown in Fig. 4(a). Naturally, as the pump
amplitude is increased, the fluctuations along the x+
axis are amplified, while concurrently they are
reduced along the y− axis where entropy is conservedin this process. The gain can then be extracted bynormalizing with the narrowest standard deviationfrom the bare thermal distribution (corresponding to
the quadrature component of the asymmetric mode).
While this slightly overestimates the amplification, itRegular Articles

(b)
200

0.018
0.016
0.014

5.715
5.08
4.445

0.012

3.81

0.01

3.175

0.008

2.54

40

0.006

1.905

0

0.004

Counts

160

Gain

σ (V)

(a)

x+

−0.04 −0.02 0.00 0.02 0.04
x+ (V)

200
160
Counts

1

0.002

80

(c)

1.27

y−

120

120
80

0.635
40

0.00

0.05

0.10

0.15

0.20

0.25
0

Pump amplitude (Vrms)−0.04 −0.02 0.00 0.02 0.04
y− (V)

Fig. 4. (a) Standard deviation (σ) of the fluctuation distribution from the two-mode squeezed states in phase space when
projected onto the rotated axes detailed in Fig. 3(c) as a function of pump amplitude. The amplification along the x+
axis is concurrently accompanied with a de-amplification along the y− axis. The red (blue) points correspond to the
XS: YA (XA: YS) cross-quadrature reconstruction, and the solid black lines denote the ideal theoretical response
extracted from the Hamiltonian in eq. (1). Also shown is the gain normalized to the narrowest bare thermal distribution
highlighted by the dashed red line. (b) and (c) The phase portrait from XA: YS with a pump amplitude of 0.2 Vrms
projected onto the rotated x+ and y− axes reveals Gaussian distributions (points), and the corresponding leastsquares Gaussian fit (line) enables their standard deviation to be extracted as quantified above.

is consistent with the spectral response detailed inFig. 2(e). However, this normalizing reference underestimates the de-amplification, yielding a conservative 5-dB suppression of the mechanical fluctuations
below the thermal level where further reduction is
inhibited by noise from the piezotransducers at large
pump voltages [46].
To confirm these observations, the thermal Langevin force, in the equations of motion extracted fromthe Hamiltonian in eq. (1), is decomposed into inphase and quadrature components [16, 45, 46]. This
leads to a new set of composite variables for thesqueezed distributions given by XS ± YA and XA ± YS
that naturally correspond to the rotated axes x+ and y−
introduced in Fig. 3(c) [37, 46]. The pump-induced
gain in the fluctuations of these composite variables
can then be extracted from the Langevin correlation
function, yielding the solid lines in Fig. 4(a). This
verifies that the noise reduction below the thermal
level arises from the non-degenerate parametricdown-conversion.


4.3 Correlation coefficient
The statistical correlations between the time series
for the in-phase and quadrature components fromboth modes can be analyzed via the absolute correlation coefficient |cov(ZiZi) /σ Ziσ Zi|, where Zi ∈ {XS, YS,
XA, YA} and the numerator describes the covariance.
Analyzing the bare thermal fluctuations (shown by
the red and blue points in Figs. 3(a) and 3(b)) enables
a correlation coefficient matrix to be constructed,
yielding 16 permutations. However, only 10 combinations are relevant due to their symmetry, as shownin Fig. 5(a). This reveals that the diagonal elements
corresponding to the auto-correlations of Zi yield acoefficient of 1, which indicates that they are perfectly correlated with themselves, as one would
expect. However, all the off-diagonal elements yielda coefficient of 0, indicating an absence of correlations; for instance |cov(XSYS) /σ XSσ YS| ≈ 0, which isunsurprising, as this maps the circular distribution in
Fig. 3(a) corresponding to the random uncorrelated
fluctuations driven by the thermal Langevin force.
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(a)

(b)

1.0

t

0.6

ela

0.4

0.4

rr
Co

e la

t

0.6

rr
Co

0.8

e
co
ion

ff.

0.8

e
co
ion

ff.

1.0

0.2
0.0

0.2
0.0

Y

Xs

A

Y
s

s

Xs

YA

Xs

YA

X

A

XA

Y

XA

A

Ys

X

Ys

Y

Xs

A

(d)

(c)

1.0

ff
oe

.

0.8

t

0.6

ela

0.4

rr
Co

ela

0.4

rr
Co

0.8

c
ion

0.6

t

c
ion

ff
oe

.

1.0

0.2
0.0

Y

Xs

A

A

Y

s

XA

Y
s

s

YA

Xs

Xs

YA

Y

Xs
A

A

Y

XA

0.0

X

X

Y

s

0.2

Fig. 5. (a)–(d) Correlation coefficient matrix as a function of pump amplitude from 0, 0.1, 0.2, and 0.25 Vrms, respectively.
The diagonal elements (black) correspond to auto-correlations, which naturally yield a coefficient of 1. The off-diagonal elements from XS: YS and XA: YA yield a coefficient of 0, which corresponds to the uncorrelated random fluctuations corresponding to the circular distributions in phase space as shown in Figs. 3(a), 3(b) and are independent of
the pump amplitude. In contrast, the off-diagonal elements from the two-mode squeezed states XS: YA (red) and XA:
YS (blue) converge towards a coefficient of 1 as the pump amplitude is increased, implying that the two mechanicalvibration modes have become perfectly correlated via the simultaneous generation of signal and idler phonons inpairs from the non-degenerate parametric down-conversion.

Repeating this analysis as a function of pump amplitude ranging from 0.1, 0.2, and 0.25 Vrms as shown in
Figs. 5(b)–(d) indicates that the auto-correlations in
the diagonal elements remain perfect. However, theoff-diagonal elements corresponding to the crossquadrature phase portraits in Figs. 3(c) and 3(d),namely, XS: YA and XA: YS, emerge and converge
towards 1, which implies that the two modes havebecome perfectly correlated from the simultaneous
generation of phonon pairs via non-degenerate paraVol. 13 No. 8 Aug. 2015

metric down-conversion.
5. Implications and outlook
Although both mechanical modes sustain more than
10 million phonons at room temperature, the simultaneous generation of phonon pairs from the nondegenerate parametric down-conversion occurs at a
rate that renders their fluctuations indistinguishable.
Thus, remarkably, these thermalized macroscopicRegular Articles

mechanical vibration modes become perfectly
entwined in a two-mode squeezed state. Ultimately, if
the modes can be operated with phonon populationsof much less than one, then these correlations will
manifest themselves in a macroscopic all-mechanicalentanglement [10, 42, 56]. Access to such a state
would be extremely tantalizing, as it would provide
an invaluable platform to investigate the absence ofquantum mechanical phenomena in our everyday
classical world [20].
From a technological point of view, these results
herald the emergence of quantum optics to phononics, and thus, concepts such as quantum cryptography
and optical quantum computing could potentially beharnessed with sound in a microchip. More immediately, the possibility of even greater squeezing
becomes available via more strongly coupled modes
in conjunction with a more efficient piezoelectric
pump [57]. This enhanced squeezing would not onlyoffer detectors that could operate below the quantum
limit, yielding unprecedented sensitivities for metrological applications [33] and fundamental science[58], but it could even be utilized to create room temperature entanglements, thus bringing quantum soundinto a more technologically accessible regime [59].
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Using Superconducting Circuits,” Nature, Vol. 474, No. 7353, pp.
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NTT Technical Review

Regular Articles

noise Squeezing via a Josephson-parametric Amplifier,” Phys. Rev.
Lett., Vol. 60, No. 9, pp. 764–767, 1998.
[36] M. A. Castellanos-Beltran, K. D. Irwin, G. C. Hilton, L. R. Vale, andK. W. Lehnert, “Amplification and Squeezing of Quantum Noise witha Tunable Josephson Metamaterial,” Nature Phys., Vol. 4, No. 12, pp.
929–931, 2008.
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Filipp, and A. Wallraff, “Observation of Two-mode Squeezing in theMicrowave Frequency Domain,” Phys. Rev. Lett., Vol. 107, No. 11, p.
113601, 2011.
[38] E. Flurin, N. Roch, F. Mallet, M. H. Devoret, and B. Huard, “Generating Entangled Microwave Radiation over Two Transmission Lines,”
Phys. Rev. Lett., Vol. 109, No. 18, p. 183901, 2012.
[39] C. A. Regal, J. D. Teufel, and K. W. Lehnert, “Measuring Nanomechanical Motion with a Microwave Cavity Interferometer,” Nature
Phys., Vol. 4, No. 7, pp. 555–560, 2008.
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and K. C. Schwab, “Preparation and Detection of a Mechanical Resonator Near the Ground State of Motion,” Nature, Vol. 463, No. 7277,
pp. 72–75, 2009.
[41] M. Aspelmeyer, T. J. Kippenberg, and F. Marquardt, “Cavity Optomechanics,” Rev. Mod. Phys., Vol. 86, No. 4, pp. 1391–1452, 2014.
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[44] H. J. R. Westra, M. Poot, H. S. J. van der Zant, and W. J. Venstra,“Nonlinear Modal Interactions in Clamped-Clamped MechanicalResonators,” Phys. Rev. Lett., Vol. 105, No. 11, p. 117205, 2010.
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180501, 2010.

10

Regular Articles

Imran Mahboob

Senior Research Scientist, Distinguished
Researcher, Hybrid Nanostructure Physics
Research Group, NTT Basic Research Laboratories.
He received an MPhys. in theoretical physics
from the University of Sheffield, UK, in 2001
and a Ph.D. in physics from the University of
Warwick, UK, in 2004. He joined NTT Basic
Research Laboratories in 2005 and has been
researching electromechanical systems to harness their nonlinear dynamics to develop phonon-based information technologies. He is amember of the Institute of Physics (IoP) and theAmerican Physical Society (APS).

Hajime Okamoto

Senior Research Scientist, Hybrid Nanostructure Physics Research Group, NTT Basic
Research Laboratories.
He received a B.E., M.E., and a Ph.D. in materials science and engineering from Waseda University, Tokyo, in 1998, 2000, and 2004, respectively. He joined NTT Basic Research Laboratories in 2004 to research and develop semiconductor opto/electro-mechanical devices. He received
an Outstanding Paper Award from the Japan
Society of Applied Physics (JSAP) (2010),
SSDM2009 Paper Award (2010), and the Young
Scientists’ Prize (2014) from the Minister of
Education, Culture, Sports, Science and Technology (MEXT). He is a member of JSAP.

11

Hiroshi Yamaguchi

Executive Manager of Quantum and Nano
Device Research, Group Leader of Hybrid Nanostructure Physics Research Group, Senior Distinguished Researcher, NTT Basic Research Laboratories.
He received a B.E. and a M.S. in physics and a
Ph.D. in engineering from Osaka University in
1984, 1986 and 1993, respectively. He joined
NTT Basic Research Laboratories in 1986 andstudied compound semiconductor surfaces using
electron diffraction and scanning tunnelingmicroscopy. His current interests are micro/
nanomechanical devices using semiconductor
heterostructures. He was a visiting research fellow at Imperial College, University of London,
UK, during 1995–1996 and a visiting researcher
at the Paul Drude Institute, Germany, in 2003. He
has been a guest professor at Tohoku University
since 2006 and has served on more than 40 committees of academic societies and international
conferences. He has received numerous awards
including a JSAP Fellowship (2013), Commendation for Science and Technology by MEXT
(2013), Inoue Prize for Science (2012), IoP Fellowship (2011), SSDM2009 Paper Award (2010),
MNC2008 Outstanding Paper Award (2009), andPaper Awards from JSAP (1989, 2004, 2010). He
is a member of JSAP, the Physical Society ofJapan, IoP, APS, and the Institute of Electricaland Electronics Engineers.

NTT Technical Review

Regular Articles

Improving User Capacity andDisaster Recovery Time in IP
Telephone Service Systems
Hiroshi Shibata, Kouki Minamida, Hiroshi Miyao,
Takashi Nambu, and Toru Takahashi
Abstract

The NTT Group is working on improving Internet protocol (IP) telephone networks and solving
certain problems associated with them. Specifically, efforts are underway to improve the accommodation
rate, reduce network operating costs when the number of network users increases, and reduce service
recovery time when a network is damaged because of a natural disaster. To resolve these problems, wedeveloped a system architecture that simplifies the operation of IP telephone networks and uses network
equipment much more efficiently. This is possible with a subscriber data management server. We
describe here our new network architecture, the mechanism to solve these problems, and the effect it hason IP telephone networks.
Keywords: IP telephone, disaster recovery, call session control function server, home subscriber server

1. Introduction
Essential social infrastructures such as electric
power systems, water systems, and communication
systems were destroyed over a huge area after theGreat East Japan Earthquake struck on March 11,
2011, and these systems could not be used for a longtime after that. Specifically, 29,000 mobile communications base stations and 1.9 million fixed telephonelines could not be used. It took about three days torecover telephone services provided by the NTT
Group [1]. As a social infrastructure provider, theNTT Group learned from this experience that network operators must ensure consistent availability ofservices and early recovery of services even afterwide-area disasters.
The devices used for Internet protocol (IP) telephone service range from smartphones to conventional fixed telephones. Therefore, IP telephone services must accommodate frequent terminal location
registrations and high-frequency use of each terminal.


We describe here the new IP telephone networkarchitecture that will improve the flexibility of subscriber accommodation, and we explain the results ofa study on how the new network architecture reduces
the time to recover from a disaster.
2. Characteristics of and problems with IP
telephone network architecture
Currently, the NTT Group offers a variety of IP
telephone services such as Hikari Denwa and the 050
number voice over IP (VoIP) service. The IP Multimedia Subsystem (IMS) architecture [2–4] is partially adopted in these IP telephone services.
The IP telephone service was initially available for
fixed telephones via VoIP routers for private use.
Office use, such as the pilot telephone number service, also began in conventional fixed telephone networks. Therefore, we adopted a network architecturethat efficiently performs a call control process; thecorrespondence between the call control server (thecall session control function (CSCF)) and the user
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Regular Articles

Routing
table
CSCF

(1)
Call

Routing
table
User
profile(2)
Routing

CSCF

Routing
table
User
profileCSCF

Core network

User
profile(3)
Call

Access network*Each CSCF has active/standby systems.

Fig. 1. Old IP telephone network architecture.

(telephone number) is fixed, and the CSCF is
designed to accommodate each number range, thesame as for the traditional public switched telephone
network (PSTN). The CSCF holds the subscriber data
(service contract information and authentication
information) related to the telephone number to beaccommodated [5]. Routing information to identify
the CSCF that accommodates the number range ismanaged by each CSCF. Therefore, the CSCF thatreceives a call request from a terminal (indicated as(1) in Fig. 1) determines the destination CSCF that
accommodates the terminal by using its own routing
information (2), and transfers the call request fromthe terminal to the destination CSCF (3). In addition,
to maintain reliability and non-interrupted service
during system changes, each CSCF adopts an active
and standby (ACT/SBY) dual system as a redundant
system.
In extending the system to cope with an increase in
the number of users and the amount of traffic, it isconventionally assumed that there is a proportional
relationship between users and traffic; therefore,
resources are designed after calculating the load fromthe traffic rate and average holding time to install
additional equipment in accordance with the increasein traffic. However, because softphone users, forexample, include many non-active users* who install
applications on their terminal but do not use themfrequently, there are cases in which the number ofusers is large, but the load in the IP telephone network
is low. Therefore, the CPU (central processing unit)
working rate is low at the maximum amount of subscriber data to be accommodated, and vice versa, so
equipment resources cannot be fully used. As a result,Vol. 13 No. 8 Aug. 2015

equipment utilization efficiency is low compared tothe older IP telephone network (Problem 1).
For load-balancing, however, we need to change therouting information of each CSCF to identify thedestination CSCF of incoming users, which is a significant operational burden (Problem 2).
In addition, with the current redundant configuration, both ACT/SBY systems could be damaged in
the event of a large-scale disaster, so there is a problem concerning the availability of the social infrastructure (recovery time for both system failures)(Problem 3). We explain the details of this problem in
the next section.
Our solution to these problems is to separate thesubscriber data and routing information from the
CSCF. The home subscriber server (HSS) manages
subscriber data and routing information, so the relationship between telephone numbers and the CSCF is
flexible. Therefore, the CSCF-HSS separation architecture can be adopted. It should be noted that upon
separation, we take into account the impact on services specific to the conventional fixed telephone
networks such as pilot telephone number and callpick-up services (Problem 4).
3. Issues regarding service recovery time after
natural disasterWe now explain the details of Problem 3. NTT Network Service Systems Laboratories has been working*

A non-active user is an IP telephone service subscriber who has
not used an IP telephone recently for a given period (for example,
a month). The other subscribers are active users.

Regular Articles

CSCF-HSS
separated architecture
On-demand
download
HSS
Routing
table

User
profile(3) Downloads
user profile(2)
Demand

CSCF

(4)
Call

CSCF

(5)
RoutingCore network

CSCF

User
profilecache

Reduction ofoperating time

Automatically updatesrouting table
when downloaded
to CSCF(1) Register location

Access network

Fig. 2. CSCF-HSS separated architecture.

to improve reliability during serious disasters bybuilding on knowledge and experience gained in theaftermath of the 2011 Japan earthquake. With theconventional network configuration, a simultaneous
failure of redundant configuration systems must be
assumed after a serious disaster because the subscriber data contained in the CSCF are fixed. Therefore, we need to rebuild equivalent devices.
Specifically, the following procedure is required.
• Replace afflicted CSCF hardware.
• Restore operating system (OS) and applications.
• Restore subscriber data.
It takes several days to complete this procedure,
which is itself an issue. (Note that the recovery timedepends on the manpower and equipment available
for the recovery work.)
4. CSCF-HSS separated architecture
We introduce here the CSCF-HSS separated architecture to solve the above-mentioned problems.
4.1 E
 ffective use of server resources for
managing users
In the CSCF-HSS separated architecture (Fig. 2),
HSSs manage the master data of user profiles, and the
CSCF downloads user profile data on demand fromthe HSSs when the user equipment registers its loca

tion information via an IP telephone. After that, theCSCF retains the user profile data as a cache during
the validity period of the location information. With
this mechanism, the CSCF does not retain the user
profiles for non-active users who have not used an IP
telephone for a certain period of time. Therefore, theCSCF can manage more user profiles of active users
because it does not consume as much memory as theold architecture. Thus, we can use the CSCF’s
resources effectively to solve Problem 1.
4.2 Automatic updating of routing information
To update the routing information in order to detect
the CSCF that manages a certain user when user profile data are moved from one CSCF to another, we
introduce a mechanism in which an HSS automatically updates the routing table when a CSCF downloads user profile data from the HSS (Fig. 2). Therefore, it is not necessary for each CSCF to have arouting table because the CSCFs can detect the destination CSCF that manages the user profile data.
Therefore, service operators do not need to update therouting information, and we can solve Problem 2.
4.3 M
 aintaining communication quality for
fixed telephone service
We provide fixed IP telephone services for businesses and user-grouping services such as pilot
NTT Technical Review

Regular Articles

Pushes groupuser profiles whensubmitting service order

HSS
Routing
table

CSCF

(1)
Call

(2)
Routing

User
profileCSCF
Core network

CSCF

Groupeduser profile
cache

(3)
Call

Rapid
response
Not necessary to
download all group user
profiles when receiving call

Access network

All group
users

Fig. 3. Rapid response by pushing all group user profiles to CSCFs.

telephone number or call pick-up services. In these
services, we need to update the cache of user profile
data in CSCFs when the master data are updated in
the HSS by submitting or changing the service order.
In the IMS standard models, a CSCF downloads user
profile data from an HSS on demand when receiving
a call; however, this incurs delays due to the downloading of all group member profile data. Thus, communication quality decreases.
Therefore, we extended an HSS’s function to pushuser profile data to CSCFs defined in the IMS standard [6]. Our HSS simultaneously pushes all user
profile data in a group. In our new architecture,
CSCFs do not need to download all group member
profile data on demand and can reduce the number oftimes the HSSs need to be accessed because the
CSCFs already have all profile data in the group dueto the extended push function and can respond rapidly. Thus, the communication quality of user-grouping services is maintained (Fig. 3) and we can solve
Problem 4.
5. New recovery procedure and effectivenessWe solve Problem 3 by creating recovery procedures for the new architecture in which HSSs are
separated from CSCFs. We introduce two new recovery procedures. First, we describe the recovery procedure (pattern 1), in which the time to restore user
profile data is reduced compared to the old procedure.
Vol. 13 No. 8 Aug. 2015

Second, we describe the procedure (pattern 2) inwhich we use server resources of other surviving
CSCFs to recover the failed CSCFs. We describe
these procedures and their effectiveness below.
Pattern 1: Other surviving CSCFs do NOT haveenough unused server resources.
(1) Recovery procedure
Step 1: Replace completely failed CSCF hardware
with new hardware.
Step 2: Restore the OS and application software.
After step 2 is completed, the rebuilt CSCFs canserve users that were prevented from using IP telephone services due to failed CSCFs by downloadinguser profile data on demand from the HSSs (Fig. 4).
(2) EffectivenessWe can reduce the recovery time by skipping
unnecessary steps such as restoring user profile datain the rebuilt CSCFs.
Pattern 2: Other surviving CSCFs HAVE enoughunused server resources and can serve users supported by failed CSCFs.
(1) Recovery procedure
Step 1: Reassign the affected users to other surviving CSCFs instead of the failed CSCFs.
Specifically, we change the settings in the HSSs sothat the surviving CSCFs serve the affected users
instead of the failed CSCFs. After that, the routing
tables in the HSS are updated automatically, so we doRegular Articles

Completely
failedRecovery procedure(1) Replaces CSCF hardware

HSS
Routing
table(2) Restores OS/software

User
profileReplaced
CSCF

CSCF
CSCF

CSCF

CSCF

e
ReplacOn-demand
downloadCore network

Not necessary to
restore user profile data

Access network

Reduction ofrecovery time

Unserved
users

Fig. 4. CSCF recovery procedure in new architecture (pattern 1).

Recovery procedureSignificant reductionof recovery time(1) Reassigns other CSCFs
HSS

Automatically updatesrouting table

Routing
table

On-demand
download
CSCF

User
profileCSCF

Changes settings toreassign other CSCFs

CSCF

Not necessary tobuild new CSCFs

Completely
failedCore network

Access network

Unserved users

Fig. 5. CSCF recovery procedure in new architecture (pattern 2).

NOT need to change the routing tables of all
surviving CSCFs (Fig. 5).
(2) EffectivenessWe can recover IP telephone services for affected
users without building new CSCFs to replace thefailed CSCFs, so the disaster recovery time drastically decreases.
The difference between the old and new IP telephone network architectures with respect to recovery
time when redundant CSCFs fail completely is shownin Fig. 6. Specifically, recovery is very fast in pattern
2 because we can recover the services within several
hours. It takes several days to recover services withthe old architecture. Thus, we can solve Problem 3.
NTT Technical Review

Regular Articles

More than 2 days

Old architecture(2) Restores
OS/software(1) Replaces CSCF hardware[Pattern 1](3) Restoresuser profile

Disaster recovery timeMore Than 1.5–2 days
(1) Replaces CSCF hardware(2) Restores
OS/software

New architecture[Pattern 2]
Severalhours(1) Reassigns
other CSCFs

Fig. 6. Improved disaster recovery time in new architecture.

6. Future work
We plan to work on developing a much faster recovery method and will therefore investigate a recovery
procedure that involves not only servers such as HSSs
and CSCFs but also user equipment, transport network elements, and business support systems.

[2]
[3]
[4]
[5]

References[1]

Ministry of Internal Affairs and Communications, “Part 1: Information and Communications in the Aftermath of the Great East Japan

Vol. 13 No. 8 Aug. 2015

[6]

Earthquake,” in The 2011 White Paper on Information and Communications in Japan.
http://www.soumu.go.jp/johotsusintokei/whitepaper/eng/WP2011/
part1.pdf
3GPP, “TS 23.228: IP Multimedia Subsystem (IMS); Stage 2 (Release7),” 2007.
ETSI/TISPAN, “TS 182.006: IP Multimedia Subsystem (IMS); Stage
2 description,” 2008.
ETSI/TISPAN, “ES 282 001: NGN Functional Architecture,” 2009.
M. Miyasaka, N. Horikome, and K. Kishida, “Bandwidth Management and Control Technology in the NGN,” NTT Technical Review,Vol. 7, No. 7, Jul. 2009.
https://www.ntt-review.jp/archive/ntttechnical.php?contents=ntr2009
07sf5.html
3GPP, “TS 29.229: Cx and Dx Interfaces based on the Diameter Protocol; Protocol details (Release 5),” 2007.



Regular Articles

Hiroshi Shibata

Takashi Nambu

Research Engineer, NTT Network Service
Systems Laboratories.
He received his B.S. and M.S. from Osaka
University in 1994 and 1996, respectively. He
joined NTT Network Service Systems Laboratories in 1996. His current R&D area is software
engineering. He is a member of the Institute ofElectronics, Information and Communication
Engineers (IEICE).

Senior Research Engineer, NTT Network Service Systems Laboratories.
He received his B.E. and M.E. from the University of Tokyo in 1998 and 2000, respectively.
He joined NTT Network Service Systems Laboratories in 2000 and engaged in VoIP systemdevelopment. He is now engaged in R&D on
improving software quality and productivity.

Kouki Minamida

Toru Takahashi

Research Engineer, NTT Network Service
Systems Laboratories.
He received his B.S. and M.S. from the University of Tokyo in 1992 and 1994, respectively. He
joined NTT in 1994. His current R&D area issoftware engineering. He is a member of IEICE.

Senior Research Engineer, Supervisor, NTT
Network Service Systems Laboratories.
He received his B.E. from Tokyo University of
Science in 1991 and joined NTT Communication
Switching Laboratories the same year. His current R&D area is session control technologies.
He is a member of IEICE.

Hiroshi Miyao

Senior Research Engineer, NTT Network Service Systems Laboratories.
He received his B.S. in physics from Ibaraki
University in 1988 and his M.S. in physics from
Hokkaido University in 1990. He joined NTT
Transmission Systems Laboratories in 1990. His
current R&D area is software engineering.



NTT Technical Review

Regular Articles

Development of High-capacity
Protocol for M2M Services and Its
Application to a Pallet Management
SystemNobuaki Mochizuki, Masashi Shimizu, Kenji Suzuki,
Kohei Mizuno, Akihiro Yamagishi, Mitsuru Harada,and Shuichi Yoshino
Abstract

NTT Network Innovation Laboratories has developed a high-capacity protocol for the 920-MHz bandin order to enhance the effectiveness and accelerate the adoption of machine-to-machine (M2M)
services. This protocol can detect the movement of hundreds of terminals and manage more than 10,000
terminals. The protocol is applied to physical distribution systems in which very large inventories need
to be controlled. It offers efficient management of loading/unloading activities. The protocol wascommercialized as a pallet management system in fiscal year 2015. In this article, we describe theprotocol and explain how we verified its performance in tests conducted in an actual goods distribution
center.
Keywords: M2M, high-capacity, physical distribution

1. Introduction
Machine to machine (M2M) communication systems have recently been attracting a lot of attention.
The concept of the Internet of Things (IoT) involves
connecting everything around us to the Internet. The
information that comes from these multiple components will be collected and analyzed in a network.
This information will then be utilized as big data andis expected to lead to the creation of new value services [1]. In addition, the infrastructure for M2M
services is being improved. For example, the Association of Radio Industries and Businesses (ARIB)
issued standard STD-T108 [2] for 920-MHz band use
in 2012.
We have developed a high-capacity protocol that
supports ARIB STD-T108. As a precursor of IoT/
M2M, we applied the protocol to a pallet manage

ment system and completed initial trials.
2. M2M service requirements
M2M services create different wireless loads fromthose of personal communication devices such as cellular phones. Most M2M service terminals cannot
move independently and thus remain stationary forlong periods. M2M systems must therefore be
designed in consideration of these attributes. The
typical communication environment desired forM2M services is depicted in Fig. 1. We assume theenvironment has the following characteristics.
(1)	Wide area: Even if the wireless environment isinadequate, a terminal cannot be moved easily.
For example, the access point and terminal
may not have a direct line of sight. Thus, communication must be established even in
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Changes inpropagation
environment

High-density
environmentLong-time use

Home electronics
3–7 years

Building blocking
radio waves

Movement ofmany terminals

Simultaneoustransit

Wide variety
of use places
Wide areaconnection

High capacity

Metering devices
10 yearsSocial infrastructure
10+ years

Long battery life

Fig. 1. Communication attributes desired for IoT/M2M service.

difficult circumstances.
(2)	High capacity: It is assumed that many terminals are concentrated at one access point orthat they transmit simultaneously after a disaster. Therefore, it is necessary to be able tocommunicate with many terminals.
(3)	Long battery life: It is assumed that a terminal
has no AC (alternating current) power supply
and that some terminals will be installed in thesocial infrastructure, for example, on bridges
and water pipes, and that their operation will
be managed without changing the battery.
Therefore, a long battery life must be achieved
while maintaining the low power consumption.
Various wireless systems have been developed forM2M services in consideration of these requirements.
The characteristics of wireless systems that can beapplied to M2M services are shown in Fig. 2. The
IMS (Industrial, Scientific and Medical) bands are invery heavy use, for example, for passive radio frequency identification (RFID) tags, Bluetooth*, andIEEE (Institute of Electrical and Electronics Engineers) 802.15.4/4g [3]/4e [4], the standards specifying the physical layer for low-rate wireless personalarea networks.
Passive RFID tags have a short transmission distance, about 1 m, but they require no batteries. Thus,
the usage scenario is to apply them as a substitute forbarcodes in managing the logistics of goods. The
Vol. 13 No. 8 Aug. 2015

Bluetooth transmission distance is about 10 m, andhigher communication speeds are possible comparedwith other wireless systems used for M2M services,
although the number of connected terminals is limited to 10. Bluetooth LE (low energy) specializes inlow power consumption. Usage scenarios include
applying it in wearable and healthcare related devices. IEEE 802.15.4/4g/4e specify a transmission speedof about 100 kbit/s, a maximum transmission distance of 1 km, and a capacity of about 1000 terminals.
Unfortunately, the battery life is only about one year.
Thus, the usage scenarios include metering devices,
as stated in Wi-SUN (Wireless Smart Utility Network) alliance standards.
In contrast, the dotted line in Fig. 2 delineates therequirements of the pallet management system
assumed here. This system must accommodate tensof thousands of pallets at a depot with only a few
access points. Thus, the wireless distance is hundredsof meters, and the system capacity is more than
10,000 terminals. Moreover, the battery lifetime
should be at least 10 years, roughly the lifetime of apallet. In addition, rapid recognition of the loading
and unloading of hundreds of pallets carried by trucks
must be achieved, as indicated in Fig. 3. A new innovative wireless system is necessary to meet theserequirements.

*

Bluetooth is a registered trademark of Bluetooth SIG Inc.

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Transmission distance
1 km

100 m
IEEE 802.15.4/4g/4e
10 m
Bluetooth
1m

Physical
distribution
requirements

Passive tags
10
100

1 day
1 month
1 year1000
Capacity(Number ofunits)

10 years

10,000Battery life**Operation life with AA battery

Fig. 2. Wireless system characteristics.

Requirement 1
Efficient management ofloading and unloading

Compatible

Requirement 2
High capacity: more than
10,000 units

Requirement 3
Long battery life

Fig. 3. Requirements of pallet management system.

3. High-capacity protocol technology
Our solution is a new high-capacity protocol thatcan connect tens of thousands of terminals and manage the movement of hundreds of terminals. Moreover, it enables a battery lifetime that can exceed 10
years. The protocol is outlined in Fig. 4.
The terminal has two states, a resting state and a

movement state. The resting state indicates that theterminal is stationary. The movement state is triggered by the terminal’s vibration sensor and indicatesthat the pallet is being transferred by a forklift or
truck. The terminal transmits beacon packets consisting of a terminal ID, sequence number, and state
information of the terminal. The transmission intervalof the beacon packet is controlled by the state of the
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Transmit

Transmit

Transmit

Transmit
Quickidentification

Receive
Movement state(Movement channel)

StaticShortinterval

Shortinterval
Transmit

Transmit

Receive
Resting state(Information channel)

ReceiveLonginterval

High capacity

Vibration

Fig. 4. Outline of high-capacity protocol.

terminal. The beacon transmission interval is set very
short in order to quickly identify the movement state.
The resting state uses long time periods when transmitting beacon packets in order to accommodate
many terminals.
In addition, the movement and resting states use
different frequencies. For this reason, even if many
resting state terminals are present, collision of beacon
packets is minimized, and rapid recognition of terminals in the movement state becomes possible.
The protocol can be applied to a variety of M2M
services with the use of different or additional sensors. For example, if the temperature exceeds or falls
below a set threshold, rapid notification is sent. Furthermore, notification of sudden changes in waterlevel, of a river, for example, can be sent quickly.
Thus, our approach supports various M2M services
by changing the sensor(s) and the beacon transmission interval.
4. Evaluation of pallet management system inpractical use
We developed equipment that hosts the aforementioned high-capacity protocol. The key specificationsof the wireless equipment are listed in Table 1. The
frequency band, transmission power, and other radio
specifications are based on STD-T108, the ARIB
standard specifications for the 920-MHz band. The
transmission speed and modulation method are basedon the IEEE 802.15.4g specifications. The accesspoint can handle multiple frequencies through the useof multiple wireless modules and antennas. The terminal is equipped with components including a wireless module, built-in antenna, vibration sensor, and abattery and is roughly the same size as a pack of cigarettes, so it is easy to mount on a pallet.
Vol. 13 No. 8 Aug. 2015

Table 1. Main specifications of wireless equipment.
ItemSpecification

Frequency

920-MHz band

Transmission power

20 mW

Transmission speed

50 kbit/s

Modulation method

Frequency-shift keying

Equipment size

AP: 280 × 280 × 130.5 mm
Terminal: 130 × 40 × 25 mm

Electric power supply

AP: AC power
Terminal: battery

AP: access point

We applied the protocol in a pallet management
system and conducted tests at a depot of UPR Corporation, a collaborative partner. The system configuration is depicted in Fig. 5. Several terminals were
installed on pallets, and the other terminals were putin the depot to replicate the resting state. The accesspoint was placed in the depot and was able to connect
to the terminals in all locations and at the depot
entrance. The access point transferred the beacon
packets to a cloud server for pallet management.
Through these tests, we confirmed that the prototype system was able to accommodate more than
10,000 terminals. In addition, we confirmed that thereception probability reached 100% within 3 minutes
when the terminal was in a movement state andwithin 100 m from the access point. Note that the
number of terminals that one access point handles canbe increased by enlarging the beacon packet interval
in a resting state.
As a result of these successful tests, commercial
operation of a pallet management system using thisprotocol was commenced at UPR Corporation in
April 2015 [5, 6].
Regular Articles

Cloud server for
pallet management
systemInternet

Depot

Accesspoint
Efficient management ofloading and unloading

Periodic and long-term
inventory management

Wireless terminal
Truck

PalletPalletFig. 5. System configuration of pallet management system.

5. Conclusion
In the future, we will work on advancing ourresearch to achieve higher capacity and lower power
consumption. Our goal is to achieve an M2M service
that offers higher capacity and longer battery life.

[2]

[3]
[4]
[5]

References[1]World Economic Forum: Personal Data: The Emergence of a New
Asset Class, Jan. 2011.

[6]

ARIB STD-T108: 920MHz-band Telemeter, Telecontrol and Data
Transmission Radio Equipment, Feb. 2012.
http://www.arib.or.jp/english/html/overview/doc/1-STD-T108v1_
0.pdf
IEEE Std 802.15.4gTM-2012: Part 15.4-Low-Rate Wireless Personal
Area Networks (LR-WPANs), Apr. 2012.
IEEE Std 802.15.4eTM-2012: Part 15.4-Low-Rate Wireless Personal
Area Networks (LR-WPANs), Apr. 2012.
Press release issued by UPR Corporation, Sept. 2014 (in Japanese).
http://www.upr-net.co.jp/pdf/news_20140908_02.pdf
Press release issued by NTT Electronics, Sept. 2014 (in Japanese).
http://www.ntt-electronics.com/new/information/2014/9/upr-rfidsmart-pallet.html

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Nobuaki Mochizuki

Senior Research Engineer, Wireless Systems
Innovation Laboratory, NTT Network Innovation
Laboratories.
He received his B.E. and M.E. from Tohoku
University, Miyagi, in 1992 and 1994, respectively. He joined NTT in 1994 and engaged in
R&D of a modulation/demodulation scheme fora broadband 5-GHz wireless access system andsynchronization scheme for coded orthogonalfrequency-division multiplexing. He is currently
developing M2M wireless access systems. He isa member of the Institute of Electronics, Information and Communication Engineers (IEICE).

Masashi Shimizu

Senior Research Engineer, Supervisor, Wireless Systems Innovation Laboratory, NTT Network Innovation Laboratories.
He received his B.E. and M.E. in mechanicalengineering from Keio University, Yokohama, in1986 and 1988, respectively. He joined NTT in1988 and began researching pointing control for
deployable space antennas and surface error
compensation through feed distribution control.
His recent interests include active RFID and its
applications. He is a member of IEICE.

Kenji Suzuki

Senior Research Engineer, Wireless Systems
Innovation Laboratory, NTT Network Innovation
Laboratories.
He received his B.E. and M.E. in electrical andelectronic engineering from Tokyo Institute ofTechnology in 1999 and 2001, respectively. In
2001, he joined NTT Telecommunications Energy Laboratories, where he worked on a wirelesstransceiver LSI architecture for low power dissipation. His interests include analog and RFIC
(radio frequency integrated circuit) design for
wireless communications. He is currently developing M2M systems. He is a member of IEICE
and IEEE.

Akihiro Yamagishi

Senior Research Engineer, Wireless Systems
Innovation Laboratory, NTT Network Innovation
Laboratories.
He received a B.E. and M.E. in electrical engineering from Toyama University in 1986 and1988, respectively, and a Ph.D. in electrical engineering from Tohoku University, Miyagi, in
2005. In 1988, he joined NTT LSI Laboratories,
where he engaged in R&D of frequency synthesizer ICs. From 2000 to 2004, he studied 1-Voperation CMOS (complementary metal-oxidesemiconductor) RF circuits for wireless systems.
He is currently studying a wireless terminal forM2M communication services. He is a memberof the Institute of Image Information and Television Engineers of Japan.

Mitsuru Harada

Senior Research Engineer, Supervisor, Group
Leader of Wireless Communication Circuits
Research Group, Smart Devices Laboratory,
NTT Microsystem Integration Laboratories.
He received a B.S. and M.S. in physics from
Tsukuba University, Ibaraki, in 1985 and 1987,
and a Ph.D. in electronic engineering fromTohoku University, Miyagi, in 2003. In 1990, hejoined NTT Atsugi Electrical Communication
Laboratories, where he engaged in research on
thin-film semiconductor-on-insulator devices.
Since 1997, he has been researching high-speed/
low-power CMOS circuits for wireless terminals.

Shuichi Yoshino

Senior Research Engineer, Supervisor, Wireless Systems Innovation Laboratory, NTT Network Innovation Laboratories.
He received his B.E. and M.E. in mechanicalengineering from Kanazawa University, Ishikawa, in 1990 and 1992, respectively. He joined
NTT in 1992 and worked on satellite Internet
system and wireless networking technology
development. He is currently engaged in R&D on
wireless technology for M2M wireless accessservices.

Kohei Mizuno

Senior Research Engineer, Wireless Systems
Innovation Laboratory, NTT Network Innovation
Laboratories.
He received his B.E. and M.E. in electricalengineering from Keio University, Kanagawa, in1997 and 1999, respectively. In 1999, he joined
NTT Network Innovation Laboratories, where heworked on a wireless multihop network, active
RFID tag, and home information and communication technology network. He is currently working on the design of 5G and beyond networks. He
is a member of IEICE and IEEE.

Vol. 13 No. 8 Aug. 2015



Global Standardization Activities

Reports of 4th ITU-T Review
Committee and 1st FG-DFS (Digital
Financial Services) Meetings
Hideo Imanaka
Abstract

Agreement was reached in the ITU-T (International Telecommunication Union, Telecommunication
Standardization Sector) to establish a Review Committee (RevCom) to discuss a new standardization
structure for the next study period starting in 2017. This agreement occurred at the WTSA (World Telecommunication Standardization Assembly) held in 2012. Additionally, it was agreed to establish a new
Focus Group on Digital Financial Services (FG-DFS) at the TSAG (Telecommunication Standardization
Advisory Group) meeting held in June 2014. This article reports on the key topics discussed at two meetings of these committees, the fourth meeting of RevCom, held in Tunis, Tunisia, in January 2015, andthe first meeting of FG-DFS, held in Geneva, Switzerland, in December 2014.
Keywords: ITU-T, Review Committee, digital financial services

1. Introduction
At the World Telecommunication Standardization
Assembly (WTSA) held in Dubai, UAE, in November 2012, it was agreed to establish a Review Committee (RevCom) in order to discuss restructuring theInternational Telecommunication Union, Telecommunication Standardization Sector (ITU-T) standardization structure in order to carry out more efficient
standardization activities. The RevCom has held four
meetings so far, and the discussion topics haveincluded reconstruction ideas to implement startingin the year 2017. The fourth meeting hosted by Tunis
Telecom was held January 19–21, 2015, and included
a discussion on the proposal of a Standardization
Strategy Team.
The establishment of a new Focus Group on Digital
Financial Services (FG-DFS) was also agreed as a
new work item in ITU-T. This was proposed by theBill & Melinda Gates Foundation, which is a memberof ITU-T, in the USA at the Telecommunication Standards Advisory Group (TSAG) meeting in June 2014.
The first meeting was held in Geneva on December 5,
2014, in which the direction of study and the working
structure were discussed.
This article presents an overview of both the fourth
RevCom and first FG-DFS meetings.
2. Fourth ITU-T RevCom meeting
2.1 Brief meeting summary
The fourth RevCom meeting was attended by 84
participants—including those participating remotely—from 15 countries. From Japan, five people
including Mr. Yoichi Maeda, the chairman of RevCom and CEO of the Telecommunication Technology
Committee (TTC), and other members from the MIC(Ministry of Internal Affairs and Communications),
NICT (National Institute of Information and Communications Technology), NEC, and NTT (the authorof this article) attended on site. Additionally, several
people including the Study Group 3 (SG3) chairman(from KDDI) and the SG16 chairman (from Mitsubishi Electric) joined in remotely via an e-meetingsystem. The venue was located in the Gammarth area,
a seaside resort about 20 km north of Tunis.
An ITU-T TSAG rapporteur meeting, which
focused on a discussion of collaboration issues
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between ITU-T and organizations inside/outside
ITU, was held after the RevCom meeting.
2.2 Discussion on ITU-T reconstruction
The structuring and management team of RevCom
is listed in Fig. 1. Mr. Maeda from TTC serves aschairman of RevCom, and there are six vice chairmen
from six ITU regions. Issues on organizational
restructuring in ITU-T were discussed in the rapporteur group under RevCom. The major results of themeeting are summarized as follows.
(1)	Establishment of Standardization Strategy
Team
Japan proposed establishing a new group called aStandardization Strategy Team to develop standardization strategies for future ITU-T activities such as
standardization areas and timings. The strategies
should be developed by taking into consideration the
expectations from industries throughout the world, as
well as future market and technology trends. This
team would be led by the ITU-T director, and themembers are expected to be the chairmen of SGs and
FGs in ITU-T and key persons from industry.
The USA and France argued to clarify the relationship between TSAG and the proposed team, andGermany brought up a membership issue, which
involved whether or not non-ITU-T members would
be able to attend meetings of the proposed team. As aresult of the discussion, the importance of developing
standardization strategies was recognized, and theestablishment of a strategic team was agreed, alongwith an inclusion policy and regulations. The discussion period and concrete framework were discussed
further at the subsequent RevCom meeting held in
June 2015.
(2) Establishment of vertical Study Group
Korea proposed to establish a new SG called a vertical SG, which would study new standardization
issues that are not easily allocated to existing SGs.
Some examples of new issues were: ITS (intelligent
transport systems), IoT (Internet of Things), and 5G(fifth-generation) mobile networks. The main topic in
the discussion was whether the vertical SG was actually necessary, since ITS is already discussed in Collaboration on ITS Communication Standards meetings with the ISO (International Organization forStandardization), and because the IoT-JCA (Joint
Coordination Activity) in ITU-T has been studying
IoT-related issues for several years. As a result, thediscussion on the proposal of a vertical SG will continue at the next TSAG meeting.

Vol. 13 No. 8 Aug. 2015

ITU-T Review Committee (RevCom)
Chairman: Yoichi Maeda (TTC, Japan)
Vice chairmen:
Musab Abdullah (Bahrain government)
Reiner Liebler (German government)
Jim Macfie (Microsoft, Canada)
Fabien Mbeng Ekogha (Gabon government)
Albert Nalbandian (Armenian government)
Kishik Park (ETRI, Korea)
Secretary: Bilel Jamoussi (TSB)
Assistant: Lara Al-Mnini (TSB)

Rapporteur Group on ITU-T Restructuring
Rapporteur: Musab Abdullah(Bahrain government)
TSB: Telecommunication Standardization Bureau

Fig. 1. Management team of ITU-T RevCom.

(3) SG reconstruction
There are currently 10 SGs in ITU-T, and a UK
contribution proposed a discussion on merging someSGs, specifically, merging SG2 and SG3, and SG9and SG16. Because the study items differ in each SG,
it was suggested at the meeting that the measurement
criteria of activities be examined. The proposal wasbasically agreed, and this topic will be studied further.
2.3 Discussion of working method
On the basis of the report of the CTO (Chief Technology Officers’) meeting, it was agreed to make awork plan for working items such as IoT and SDN(software-defined networking) in RevCom. In addition, each ITU-T SG chairman reported their recent
activities.
2.4 Future plans
At WTSA-16 to be held at the end of 2016, therestructuring of ITU-T SGs will be discussed. To
make proposals for SG reconstruction from TSAG to
WTSA-16, it is necessary to summarize the outcomeof RevCom meetings by mid-2016. The next RevCom meeting may be the last chance to bring up new
proposals, so it is important to contribute to therestructuring discussion to achieve efficient standardization activities for Japanese industry.

Global Standardization Activities

3. First ITU-T FG-DFS meeting
3.1 Overview
The first meeting of FG-DFS focused on a discussion about digital financial services and was held in
Geneva on December 5, 2014; additionally, a workshop on digital financial services was held the previous day. These meetings were attended by around 100
persons from 30 countries, including several Africancountries.

TSAG was selected to be the parent group, and theduration of the FG will be two years.
The management team includes Mr. Sacha Polverini from the Bill & Melinda Gates Foundation, who isserving as the chairman of the FG-DFS, and 12 vicechairmen, listed in Table 1, who were elected at thefirst meeting. In addition, four Working Groups(WGs) were agreed to be established, and 2–4 persons were appointed as the leader and sub-leader foreach WG.

3.2 Background of establishing FG-DFS
The digital financial services studied in FG-DFS
include micro-finance in developing countries,
mobile payments using cell phones, and financial
inclusion, which improve financial services in developing countries. Some areas of digital financial services were also studied in TC 68 (Technical Committee 68) in ISO and JTC 1 (Joint Technical Committee
1) in ISO/IEC (International Electrotechnical Commission). This FG was established with the support ofmany countries, mainly developing countries, inorder to clarify the telecommunication functions
required to securely transfer money electronically allover the world, including developing countries, andto rapidly standardize telecommunication technologies related to financial services to promote its utilization in developing countries in ITU-T. Because thescope of FG-DFS may intersect with several SGs,

3.3 Overview of DFS workshop
The day before the first FG-DFS meeting, theworkshop on digital financial services was held; it
was chaired by the FG-DFS chairman [1]. At theworkshop, 22 presentations were given on topics suchas digital financial services, mobile payment services,and financial inclusion, primarily from African andEuropean countries.
The Minister of Posts and Telecommunications ofSomalia, H.E Dr. Mohamed Ibrahim, and the Director-General of the Communications Authority of
Kenya, Mr. Francis Wangusi, delivered keynote
speeches. In Somalia, the demand is strong for digitalfinancial services using mobile devices such as cell
phones, and there is great interest among Somalians
in the concept of mobile money. However, Dr. Ibrahim pointed out that they needed a legal frameworkfor providing such services. In Kenya, mobile money

Table 1. WG structure and management of FG-DFS.
Objectives

Leaders of WG (Vice chairmen of FG-DFS)DFS Ecosystem

WGs

To identify and understand key
elements of a digital financial
services ecosystem that enables
financial inclusion- Carol Coye Benson, Glenbrook Partners (USA)- Yury Grin, Intervale (USA)- Bruno Antunes, UNCTAD (UN)- Zhao Ping, China Telecom (China)

Technology,
Innovation &Competition

To identify evolving technologies
and market dynamics related todigital financial services, and toassess the implications forfinancial inclusion- Leon Perlman, Lethan Consulting (USA)- Madeleine Scherb, Health and Environment
Program (Switzerland)Interoperability

To assess models and policy
frameworks to achieveinteroperable systems, products
and networks, and to develop aframework that promotes
financial inclusion- Thomas Lammer, The World Bank Group (USA)- Mark McCullagh, Learo Consulting (UK)- Yétondji Houyetognon, ARCEP (Benin)

Consumer Experience& Protection

To define minimum requirementsfor quality of service for reliable
digital transactions and dataprotection safeguards in order tobuild consumer confidence- Sumit Jamuar, KYCTrust Limited (UK)
- Bedoui Adel, Tunisie Télécom (Tunisia)- Nicola O’Reilly, Consumers International (UK)* The above list is as of December 5, 2014.



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Global Standardization Activities

services were initiated in 2007, and 64% of the population now use the M-Pesa service, a kind of mobile
money service that handles US$23 billion annually.
Kenya is the leading country in terms of total amountof transactions in mobile money and micro-finance in
the African region.
3.4 Discussion topics of first FG-DFS meeting(1) Terms of ReferenceTerms of Reference, ToR, was discussed, and the
major opinions were as follows:•	It is necessary to clarify the terminology anddefinitions for mobile money, mobile finance,
digital money, electronic money, and digitalfinance.
•	The digital financial service will have a new
financial structure and a new distribution model.
•	Although the administration area of the central
bank and telecommunication regulation officediffers in each government, it is necessary toestablish cooperation between the central bankand telecommunication regulation office. In
addition to the services provided by banks,
related financial services provided by insurancecompanies and non-bank organizations are also
involved in digital financial services.
•	One of the objectives was to include use cases for
women; thus, although the concept is mainly
focused on the poor, the existence of a gender
gap was acknowledged.
•	Security and availability are required as the standards for basic digital financial services. In addition, it is also necessary to consider ensuring
accessibility, minimizing the initial cost, andreducing transaction costs.
(2) WG structure
The FG chairman proposed establishing WGs, andit was agreed to establish four WGs and to nominate
12 leaders as a result of the discussion. The WG structure is indicated in Table 1. Several opinions wereraised in the discussion; for example, one member
said it would be hard to participate if several WGs
were running in parallel. However, because the discussion themes of each WG are different, and theinterests of participants may also differ, it was confirmed that holding several WGs in parallel would not
be a problem. Another opinion was that terminology
should be unified at an early stage, and it was agreed
to define the terminology at an early stage in each
WG.
(3) Discussion in each WG
FG participants were divided into four WGs, and aVol. 13 No. 8 Aug. 2015

one-hour brainstorming session was held in each WG
to define the main issues that should be discussed.
The discussion results were presented orally, and no
output documents were produced.
3.5 Subsequent work of FG-DFS
The second FG meeting was held in Washington,
D.C., USA, in April of 2015, the same timing of thespring conference of the World Bank.
4. Conclusion
4.1 ITU-T RevCom
The task force of the TTC Global Collaboration
Advisory Group (GCAG), led by the author of thisarticle, handles Japan’s policies and contributions for
RevCom and TSAG. To achieve an efficient ITU-T
standardization structure for Japanese industry, it isnecessary to proactively contribute to RevCom bycontinuously aggregating domestic opinions in thetask force and by collaborating with foreign countries.
4.2 ITU-T FG-DFS
There are now two directions for international standards of digital financial services; the first is achieving universal and easy deployment in developing
countries. Standardization of Japanese mobile wallet
services would make traveling more convenient forforeigners who come to Japan and for Japanese travelers who visit foreign countries. The second direction is that international standards require a safer andmore economical way to deploy mobile payment systems such as M-Pesa in Kenya. Utilizing FG-DFS
may not only be effective for promoting Japanesesolutions to the world, but also for clarifying the roleof Japanese companies in achieving financial inclusion in developing countries.
4.3 C
 onsideration of organization for new study
itemsThis article briefly reported on the current status of
RevCom and FG-DFS. In the near future, international standardization activities for required functions
will be more important for non-telecom industries
that utilize the telecommunication infrastructure(vertical industries), for purposes such as digitalfinancial services. ITU standards have played an
important role in the rapid growth of information andcommunication technology (ICT). Currently, ICT has
been entering a second stage; that is, ICT supports
vertical industries, which are indicated as “OO×ICT.”Global Standardization Activities

Contributing to RevCom is expected to lead toachieving an efficient standardization structure toenable progress to be made in anything supported by
ICT.

Reference[1]

ITU Workshop on Digital Financial Services and Financial Inclusion,
http://www.itu.int/en/ITU-T/Workshops-and-Seminars/ifds/Pages/
Programme.aspx

Hideo Imanaka

Senior Manager, NTT R&D Planning Department*.
He received his B.E., M.E., and Ph.D. in electrical engineering from Mie University in 1985,
1987, and 2001, respectively. After joining NTT
Telecommunication Network Laboratories in
1987, he conducted research on a fiber optic
access network architecture and network operation process reengineering methods. From 1996
to 2003, he worked on enterprise resource planning (ERP) systems integration as a consultant in
the Solutions Business Division of NTT Communications. Since 2004, he has been involved in
NGN (Next Generation Network) standardization work at ITU-T. He was the Rapporteur of
Question 1 of SG13 from 2007 to 2010. He has
also played an active role in IP (Internet protocol)
TV standardization work at ITU-T. He is currently in charge of standardization strategies in
the NTT Group. He received the ITU-AJ Award
from the ITU Association of Japan in 2009. He isa member of the Institute of Electronics, Information and Communication Engineers and theSociety of Instrument and Control Engineers.
* He moved to NTT Advanced Technology Corporation in April 2015.



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

Case Studies of Faults andCountermeasures in Access Telephone
OfficeAbstract

This article reports on recent faults in optical access communication facilities that occurred in a telephone office. We review (1) a communication facilities problem caused by removal of a LAN (local area
network) cable, (2) an accidental cable breakage that occurred while fixing cables on a distribution
frame, and (3) a service interruption that occurred due to deterioration of an optical branch module used
for testing optical lines. Cases (1) and (2) involved breakdowns that occurred during construction,
whereas case (3) was an unforeseen breakdown related to long-term deterioration. We also present theappropriate countermeasures to these faults that were applied in order to maintain continuous operationof the communication facilities. This article is the thirtieth in a bimonthly series on practical field information on telecommunication technologies. This month’s contribution is from the Access EngineeringGroup, Technical Assistance and Support Center, Maintenance and Service Operations Department,
Network Business Headquarters, NTT EAST.
Keywords: access communication facilities, fault cases, fault countermeasures

1. Introduction

2. Case studies

Fiber-to-the-home (FTTH) systems support today’s
information society and are capable of transmitting
vast amounts of data. They are primarily designedwith PON (passive optical network) topology [1].
When services supplied through the FTTH systems
cease because of communication failures or maintenance work related to network operation, a large
number of users are disadvantaged.
The Technical Assistance and Support Center at
NTT EAST grapples with faults that occur in access
communication facilities that support the FTTH systems, and for which solutions are difficult, and develops countermeasures to the faults to improve thequality of communication facilities. Previous reports
centered on actual fault cases of optical communication facilities consisting of outdoor offices and customer buildings [2–4]. This article presents recent
faults in access communication facilities that occurred
at a telephone office.

2.1 Case 1: Fault caused by cable removalThis case concerns damage to functioning cables
that occurred while removing disused cables in a telephone office.
(1) Fault description
Multiple damage traces were discovered on theouter sheaths of optical cables and local area network(LAN) cables installed on an aerial rack. Althoughcommunication services had not yet been interrupted
at the time of the discovery, the damage was extensive
enough that the interior of the cables was exposed.
(2) Verification results (Replication experiment)
The fault location and the state of cable damage areshown in Fig. 1. The burred lines indicate the damage, and as shown, the burrs are all oriented in thesame direction. In addition, each mark corresponds toan arc-shaped groove about equal in width to thediameter of the existing LAN cables. In light of theabove, we inferred that the cause of the fault was anaccident involving friction generated when removing

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Practical Field Information about Telecommunication Technologies

Aerial rack

Optical cable

LAN cable

All burrs oriented in the same direction

Fig. 1. Location of fault and cable damage.

Optical cable

LAN cable

LAN cablealcab

Optical cable

tic
ble

ca

LAN

Op

le

(b) Replication experiment sample

Continuous rubbing at 90°
angle in downward direction(a) Replication experiment system(c) Fault sample

Fig. 2. Experiment to replicate cable damage.

LAN cables from the aerial rack. We therefore conducted a replication experiment in which we madetwo cables rub up against each other. Specifically, we
prepared an optical cable and a LAN cable the sameas those in use, and as shown in Fig. 2(a), we pulled
the LAN cable over the optical cable that was fixed inplace in an attempt to simulate actual cable removal
work. We found that a crack began to form on theouter sheath of the fixed cable after pulling the LAN
cable about 2 m and that the tension member of thecable came to be exposed after pulling the cable 10–
20 m (Fig. 2(b)). We confirmed that this damage wassimilar to that of the original fault (Fig. 2(c)). Similar
results were obtained for a LAN cable (Fig. 2(b)).
(3) Countermeasure
In view of the experimental results presented above,
we surmised that this fault was caused by friction
generated on an optical cable or LAN cable whileremoving another LAN cable. We therefore considered that the use of a lubricant or anti-friction sheet atthe point where the cables come in contact and generVol. 13 No. 8 Aug. 2015

ate friction would be an effective countermeasure to
the friction that can accompany cable laying/removal
work.
2.2 Case 2: Fault at CTF-cable binding location
This case concerns a cut in an optical cable fixed ina cable termination frame (CTF) and the resulting
break in communications.
(1) Fault description
A deep cut was found in a double-core optical cablelaid between a CTF and on-premise equipment near
the point where the cable was bound with a binding
strap to other cables at the lower end of the CTF(Fig. 3). No reasons as to why the cable should bedamaged near the lower end of the CTF could befound, and there was no place where the fault location
could have easily come into contact with the floor.
(2) Verification results (Replication experiment)
On dismantling the damaged cable and inspecting
its interior, we found that constituent optical cords
had been cut and that the tension-member coveringPractical Field Information about Telecommunication Technologies

CTF

Cable-breaklocation

Location ofcable problem

Fig. 3. Location of cable problem and image of cable damage.

CTF upper side

Front view

Rear view

Side viewFig. 4. Damage to outer sheath of optical cable.

CTF upper side

Front view

Rear view

Side view

Image of experimental
reenactment

Fig. 5. Verification of reenactment results.

was damaged (Fig. 4). Consequently, we suspectedthat the nippers and cable clippers frequently used in
fixing and removing on-premise cables might be theproblem, so we examined what form of damage suchtools could cause. We found that the fracture causedby nippers resembled the actual fault more closelythan that caused by cable clippers. The replication
setup using nippers and the damage formed in thisexperimental reenactment are shown in Fig. 5.
Through this experiment, we found that orienting theblade tips of the nippers toward the cables when cutting the X-shaped binding strap could damage a cable

as deeply as its internal optical cords. Then, on examining the fiber cross sections at the damaged location
using an electronic microscope, we found that they
resembled those of the fault sample as shown in
Fig. 6.
(3) Countermeasure
On the basis of our observations of the damaged
cable and the results of the replication experiment, we
surmised that the tips of the nippers had penetrated
the cable’s outer sheath when cutting the binding
strap and had reached the optical-cord jacket, and thatthe optical fiber was sliced by the lateral pressure
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Practical Field Information about Telecommunication Technologies

CTF upper side

Fiber jacket

CTF lower side

Optical fiber

Fiber jacket

Optical fiber(a) Cross sections of fault sample
CTF upper side

Fiber jacket

CTF lower side

Optical fiber

Fiber jacket

Optical fiber(b) Cross sections of test sample

Fig. 6. Comparison of fiber jacket and cross sections of optical fiber between fault sample and test sample.

Optical branch module
Fault
consultation
reportService interruption
Deterioration in
uplink bit error rate
OSS

Fault line

8SP

4SP

OLT

ONU 1

FTM

ONU 2
ONU 3

Available line
AURORA
OLT: optical line terminal
ONU: optical network unit
OSS: operation support system

Fig. 7. Facility configuration and optical core switching.

applied at that time. We considered that the maincause of this accident was a working environment
with tight spaces and blind spots. It is therefore
important that maintenance personnel make a conscious effort to orient blade tips away from cables
when using nippers. In addition, cutting the binding
strap from the backside of a fixing location could also
be an effective measure to prevent faults of this type.
2.3 Case 3: Fault in optical branch moduleThis case concerns a fault in an optical branch module used for injecting test signals along an opticalpath.
Vol. 13 No. 8 Aug. 2015

(1) Fault description
A report of a service interruption was received froma customer using the FLET’S Hikari optical broadband service. On troubleshooting the fault, an
increase in loss along the outside optical path that
included an optical branch module was suspected, but
the fault location could not be isolated. Although thelocation of the original fault had still not been clarified, it was eventually decided to switch the fault line
reported as being faulty to a circuit on another opticalbranch module. This action effectively restored services to the customer (Fig. 7).

Practical Field Information about Telecommunication Technologies

Optical branch module
Port AOTDR

In-service fibers
Port B
4MT

4-fiber ribbon

Port D

Secondconnector
Dummy fiber

2 km

Non-reflection terminal
Port C(a) Communication path between ports A and B

Optical branch module
Port APort B

SecondNon-reflection
connector
terminal

In-service fibers
4MT

Port D

2 km

4-fiber ribbon
Non-reflection terminal

Port C

Dummy fiber

OTDR(b) Probe path between ports C and B

14

14

12

12

Transmission loss (dB)

Transmission loss (dB)

Fig. 8. Transmission loss measurement system.

10
8
6
4
2
0
1.3

1.5
1.6
1.4
Wavelength (µm)1.7(a) Actual faulty module: Between ports A-B: Between ports C-B

Specifiedvalue exists

10
8
6
4
2
0

1.3

1.5
1.6
1.4
Wavelength (µm)1.7(b) Normal module for comparison: Specified values between ports A-B: Specified values between ports C-B

Fig. 9. Measurement results.

(2) Verification results (On-site inspection)
Because the optical branch module targeted forinspection was accommodating other core fibers inuse, we proceeded to inspect their current state in thefiber termination module (FTM) rack. First, on visually inspecting the module from the outside, we
found no obvious problems in the rack or connector
section. Next, we measured transmission loss
between ports A-B related to the second connector
cited in the fault report and also between ports C-B
using an optical time-domain reflectometer (OTDR),
as shown in Fig. 8. Transmission loss at the second
connector of the faulty module and that of a normal

one as reference are shown in Fig. 9. The transmission loss of the faulty module satisfied the specified
values at standard wavelengths (1.31, 1.55, and 1.65µm) between ports C-B but greatly exceeded thespecified values between ports A-B. Furthermore, incomparison with the normal product examined in Fig.
9(b), it was found that the difference in transmission
loss between port-interval A-B and port-interval C-B
in the faulty module was smaller.
(3) Inference of fault location
The on-premise optical branch module uses awavelength-independent optical coupler that has an
optical transmission loss difference of about 6 dB
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between port-intervals A-B and C-B, as shown in
Fig. 9(b). However, this difference is only about 1 dBin the faulty module, as shown in Fig. 9(a), so we
assumed that a fault exists for some reason at thesecond connector of the module. We were not able toisolate the fault in this optical branch module throughfault maintenance work at the site during this investigation. We are therefore recommending that an
OTDR test connecting a dummy fiber to port A of the
optical branch module be conducted the next time asimilar event occurs. In this way, we should be able toclarify the transmission loss between ports A-B fromthe difference in levels of backscattering light fromthe dummy fiber and outside path. This difference
could then be used as a criterion for assessing problems in the module.

therefore expect maintenance operations for telephone offices to be just as diverse. The Technical
Assistance and Support Center is committed toresponding to all requests for technical consultation
and assistance and to disseminating practical telecommunication technologies while leveraging theknowledge and experience gained to date from themaintenance of diverse facilities.
References
[1]
[2]

[3]

3. Conclusion
In this article, we introduced case studies of faults
that occurred in a telephone office and explained theircountermeasures. Optical communications services
come in many forms, and the facilities used to support them are likewise many and varied. We canVol. 13 No. 8 Aug. 2015

[4]

ITU-T Recommendation G.983.1, “Broadband optical access systems
based on Passive Optical Networks (PON),” 1998.
NTT EAST, “Case Studies of Faults and Countermeasures in a Passive Optical Network System,” NTT Technical Review, Vol. 10, No. 7,
2012.
https://www.ntt-review.jp/archive/ntttechnical.php?contents=ntr2012
07fa2.html
NTT EAST, “Fault Cases and Countermeasures for Field Assembly
Connectors in Optical Access Facilities,” NTT Technical Review, Vol.
9, No. 7, 2011.
https://www.ntt-review.jp/archive/ntttechnical.php?contents=ntr2011
07fa2.html
NTT EAST, “Failure Cases in Optical Access Facilities (Fiber Breakage in Optical Cabinets) and Countermeasures,” NTT Technical
Review, Vol. 11, No. 4, 2013.
https://www.ntt-review.jp/archive/ntttechnical.php?contents=ntr2013
04pf1.html

External Awards
The 37th Awaya Prize Young Researcher Award
Winner: Daichi Kitamura, Sokendai (The Graduate University for
Advanced Studies); Nobutaka Ono, National Institute of Informatics;
Hiroshi Sawada, NTT Service Evolution Laboratories; Hirokazu
Kameoka, NTT Communication Science Laboratories; and Hiroshi
Saruwatari, the University of Tokyo
Date: March 17, 2015
Organization: The Acoustical Society of Japan (ASJ)

Interop 2015 Best of Show Award (SDI Special Prize)
Winner: O3 project*

For “Efficient Multichannel Nonnegative Matrix Factorization
with Rank-1 Spatial Model.”Published as: D. Kitamura, N. Ono, H. Sawada, H. Kameoka, and H.
Saruwatari, “Efficient Multichannel Nonnegative Matrix Factorization with Rank-1 Spatial Model,” Proc. of ASJ Autumn Meeting, pp.
579–582, Sept. 2014.

For “SDN/OpenFlow Software Switch Lagopus.”
Published as: O3 project, “SDN/OpenFlow Software Switch Lagopus,” Proc. of Interop Tokyo 2015, Chiba, Japan, Jun. 2015.

ITU-AJ Accomplishment Award
Winner: Makoto Murakami, NTT Network Service Systems Laboratories
Date: May 15, 2015
Organization: The ITU Association of Japan (ITU-AJ)
For his contribution to implementation of regulatory revisions andthe creation of a handbook and tutorial relating to a group of recommendations covering optical transport network technology in the
Telecommunication Standardization Sector of International Telecommunication Union (ITU-T) Study Group 15 (SG15). As head of
the Japanese delegation, he is committed to resolving the international controversy over MPLS-TP (multiprotocol label switchingtransport profile) standardization by leading the way in a WTSA
(World Telecommunication Standardization Assembly) joint proposal with other member states in APT (Asia-Pacific Telecommunity). He also contributed as a liaison officer in settling a dispute over
specifications between IEC (International Electrotechnical Commission) and SG15. In addition, he participated in the ITU Focus Groupon disaster relief systems as editor and contributed to finalizing deliverables.

*	NTT researchers involved are Hirokazu Takahashi, Yoshihiro Nakajima, Hitoshi Masutani, Takeshi Kinoshita, Tomoya Hibi, Sakiko
Kawai, Atsushi Yamamoto, and Atsushi Taniguchi, NTT Network
Innovation Laboratories.

Date: June 10, 2015
Organization: Interop Tokyo Steering Committee

The Meritorious Award on Radio presented by the Chairman of the Board of ARIB
Winner: Masashi Shimizu, NTT Network Innovation Laboratories;
Yoshiya Sakata, UPR Corporation; Takumi Watanabe, NTT Electronics Corporation; and Akihiko Taniya, Nagano Japan Radio Co., Ltd.
Date: June 16, 2015
Organization: Association of Radio Industries and Businesses(ARIB)For development of a logistics pallet management system using
active radio-frequency identification techniques.
TTC Distinguished Service Award
Winner: Takefumi Yamazaki, NTT Service Evolution Laboratories
Date: June 22, 2015
Organization: Telecommunication Technology Committee (TTC)
For his contribution to the promotion of standardization of home
network systems.
TTC Distinguished Service Award
Winner: Makoto Murakami, NTT Network Service Systems Laboratories
Date: June 22, 2015
Organization: TTC
For his contribution to the promotion of standardization of internetwork transmission.

NTT Technical Review

Papers Published in Technical Journals andConference Proceedings
Large-scale Collection and Analysis of Personal Questionanswer Pairs for Conversational Agents
H. Sugiyama, T. Meguro, R. Higashinaka, and Y. Minami
Proc. of IVA 2014 (the 14th International Conference on Intelligent Virtual Agents), pp. 420–433, Boston, MA, USA, August 2014.
In conversation, a speaker sometimes asks questions that relate toanother speaker’s detailed personality, such as his/her favorite foods
and sports. This behavior also appears in conversations with conversational agents; therefore, agents should be developed that can
respond to such questions. In previous agents, this was achieved bycreating question-answer pairs defined manually. However, when asmall number of persons create the pairs, we cannot know what typesof questions are frequently asked. Therefore, such essential questionanswer pairs for conversational agents are possibly overlooked. This
study analyzes a large number of question-answer pairs for six
personae created by many question-generators, with one answer-generator for each persona. A comparison with questions appearing inconversations between humans shows that 50.2% of the questions
were contained in our question-answer pairs, and the coverage rate
was almost saturated with the 20 recruited question-generators.

Material Transmission Loss Modeling for Indoor Propagation Modeling
M. Inomata, T. Ogawa, and S. Yoshino
Proc. of IEEE PIMRC 2014 (the 25th International Symposium on
Personal Indoor and Mobile Radio Communications), pp. 800–804,
Washington, DC, USA, September 2014.
We are studying a method to minimize mutual radio wave interference by coordinating wireless home network access points (APs). To
control the radio wave range so as to reduce interference with neighboring terminals or APs, precise techniques to estimate range areimportant. A commonly used method to estimate radio wave reachingranges is site surveys, where the user or operator measures propagation characteristics. However, the problem with this method is that alarge burden is placed on the user because it is necessary to densely
collect the radio data over the entire area to precisely estimate theradio wave reaching ranges, especially in indoor radio propagation
environments where there is considerable propagation variability. We
therefore propose a method that uses sparse data to estimate material
parameters for precisely modeling indoor radio propagation. Experimental evaluations demonstrated it decreased the number of data
items needing to be collected.

Open-domain Utterance Generation Using Phrase Pairs
Based on Dependency Relations
H. Sugiyama, T. Meguro, R. Higashinaka, and Y. Minami
Proc. of SLT 2014 (2014 IEEE Spoken Language Technology
Workshop), pp. 60–65, South Lake Tahoe, NV, USA, December
2014.
The development of open-domain conversational systems remains
difficult since user utterances vary too widely for such systems to
respond appropriately. To address this issue, previous research has
retrieved sentences from the web as system utterances by applying
shallow sentence matching with user utterances. However, since theVol. 13 No. 8 Aug. 2015

retrieved sentences include the inherent contexts of the document inwhich the sentences originally appeared, the retrieved sentences may
possibly contain information that is irrelevant to user utterances. We
propose combining two strongly related semantic units (phrase pairs
with dependency relations) to create a system utterance. Here, thefirst semantic unit is the one found in the user utterance, and thesecond semantic unit is the one that has a dependency relation withthe first one in a large text corpus. This way, we can guarantee thatthe generated utterance is related to the input user utterance. Our
experiments, which examine the appropriateness of response sentences, show that our proposed method significantly outperforms
other retrieval and rule-based approaches.

Comparison of Photo Degradation Behavior of LDPE
Using Accelerated Weathering Instruments
T. Miwa, Y. Takeshita, Y. Akage, M. Watanabe, M. Takaya, and T.
SawadaZairyo-to-Kankyo, Vol. 64, pp. 139–144, April 2015.
Samples of low-density polyethylene (LDPE) were photodegradedusing accelerated weathering instruments and outdoor exposure. The
physical properties and chemical structures of the photodegradedsamples were studied through a tensile test, gel chromatography, andinfrared spectroscopy.
The molecular weight distribution of a photodegraded sample byusing a fluorescent UV lamp at a high black panel temperature (80°C)
was more similar to that of an outdoor-degraded sample than that ofother artificially photodegraded samples by using a Xenon lamp at
the standard black panel temperature (63°C). It is estimated that
accelerated weathering tests at a high sample temperature could
accelerate cross-linking more than chain scission, consequently recreating molecule-enlargement similar to the outdoor-degraded sample.

Path Loss Model for the 2 to 37 GHz Band in Street Microcell Environments
M. Inomata, W. Yamada, M. Sasaki, M. Mizoguchi, K. Kitao, and
T. Imai
IEICE Communications Express, Vol. 4, No. 5, pp. 149–154, May
2015.
Path loss characteristics are analyzed on the basis of measurement
results obtained using the 2 to 37 GHz band in street microcell environments. By taking dependencies on frequency and distance fromtransmitter to intersection into account, the proposed model candecrease the root mean square error of prediction results to within
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A Tracing Technique for Understanding the Behavior of
Large-scale Distributed Systems
Y. Bando
Proc. of LinuxCon Japan, Tokyo, Japan, June 2015.
Debugging or troubleshooting large-scale distributed systems isdifficult due to its complexity; a single request may trigger thePapers Published in Technical Journals and Conference Proceedings

execution of hundreds of components running in parallel on manydifferent machines. To help developers or operators gain deeper
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implemented it in Eventlet, an RPC library widely used in OpenStackprojects, and also started discussing it with the Eventlet community
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In this talk, I demonstrated the tracking of swift, OpenStack object
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Variable-length Lossy Source Code Using a Constrainedrandom-number Generator
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IEEE Transactions on Information Theory, Vol. 61, No. 6, pp.
3574–3592, June 2015.
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to construct a stochastic encoder. The proof of the theorem is basedon the balanced-coloring property of an ensemble of functions. Sincean ensemble of systematic sparse matrices has this property, we canconstruct a tractable code for a memoryless source. Some algorithms
for implementing the code are introduced and compared by simulation.

Smooth Motion Parallax Autostereoscopic 3D Display
Using Linear Blending of Viewing Zones
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Proc. of the SID (Society for Information Display) Display Week
2015, pp. 983–986, San Jose, CA, USA, June 2015.
A new autostereoscopic three-dimensional (3D) display is proposed. Using only a small number of projectors, it produces smoothand exact motion parallax by applying the visual effects of dual edge
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