Q N E R C N E W S F E B R U A R Y 2 0 0 8 | V O L . 4TOKYO INSTITUTE OF TECHNOLOGY

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ERC

NEW

S A Word from the Director

Research at QNERC continues to yield new insight into nanoscale phenomena and to spawn practical, valuable applications for nano-technology. Our research centers on optics and electronics. Our

researchers have been notably successful in translating advances in theo-retical physics into the real-world fabrication of working devices. We lever-age our core strengths in optics and electronics through joint research withcolleagues in other disciplines, including even medical science.

The research findings summarized on the following pages offer repre-sentative glimpses of the work under way at QNERC. All of those findingsunderline two definitive and hugely exciting truths of nanotechnology: (1) that matter and energy in the nano realm interact in ways utterly differentfrom anything observed at larger dimensions and (2) that nanoscale phenom-ena can render service in diverse applications that will drive advances inscience and industry and, thereby, in the quality of life.

Tokyo Tech has equipped QNERC with world-class facilities to make internationally significant contributions to advances in nanotechnology. Our researchers have made the most of our resources in important projectsthat have earned support from sources in government and in industry.

QNERC is a small—just six research groups—institute that handles several large and scientifically important projects. It has been extremely productive in regard to scientific and technological return on investment. I look forward to maintaining that high productivity while expanding QNERC gradually by adding research teams.

Masahiro AsadaDirector, QNERC

www.pe.titech.ac.jp/AsadaLab

The Tokyo Institute of Technology

(Tokyo Tech), Japan’s foremost

university devoted to technological

disciplines, established QNERC in

April 2004. QNERC inherited the R&D

portfolio of its predecessor, Tokyo

Tech’s Research Center for Quantum

Effect Electronics. It translates advances

in nanoscience into practical sugges-

tions for industrial applications.

QNERC director Masahiro Asada, whose research specialty is terahertz devices and ultrahigh-frequency electronics, stands outsidethe laboratory building that houses the institute.

Shigehisa Arai and his colleaguesdemonstrate two membrane dis-

tributed feedback (DFB) lasers thatare promising as low-power lightsources for on-chip and chip-to-chip optical interconnection. One ison a silicon-on-insulator (SOI) sub-strate fitted with a silicon wave-guide structure for integration withsilicon electrical and opticaldevices. The other is an air-bridgestructure for robust fabricationwithout using wafer bonding.

The laser bonded to an SOI sub-strate oscillated under opticallypumped continuous-wave condi-tions. A threshold pump power (Pth)

of 11.3 mW arose at 15°C for a cavity length of 140 µm and for a waveguide length of 500 µm.Although the threshold was higherthan with previously reported membrane DFB lasers, continuous-wave operation occurred at tem-peratures up to 85°C.

Arai and his colleagues fabricat-ed the air-bridge membrane DFBlaser with selective chemical wetetching. The air-bridge membranewidth (Wb) was 20 µm. Stable sin-gle-mode operation (SMSR . 30dB) occurred at temperatures up to 60°C. The thermal resistance (Rth) improved from 23 K/mW to 11

K/mW—one-half the level previous-ly reported with benzocyclobutene-bonded membrane DFB lasers.

Tadashi Okumura1, Takeo Maruyama1,3,Shinichi Sakamoto1, Masaki Kanemaru1,Hideyuki Naitoh1, Mamoru Ohtake1,Nobuhiko Nishiyama2, and Shigehisa Arai1,3

1. Quantum Nanoelectronics Research Center,Tokyo Tech

2. Department of Electrical and ElectronicEngineering, Tokyo Tech

3. CREST, Japan Science and TechnologyAgency

Japanese Journal of Applied Physics 46, pp.L1158–L1160 and L1206–L1208 (2006).

Finding real-world solutions

Dr. Shigehisa Arai Professor, Tokyo Tech

www.pe.titech.ac.jp/AraiLab/index-e.html

Biosensors are invaluable inmedical diagnosis, as in the

disposable biosensors usedwidely by diabetics to check theglucose levels in their blood.They are also useful in monitor-ing indicators of environmentalquality, such as water purity.

Most biosensors work bydetecting fluorescent or mag-netic labels attached to targets.Monitoring labels, however, is a time-consuming and costlymeans of analyzing biomolecu-lar reactions.

Adarsh Sandhu’s researchgroup has demonstrated ameans of using X-ray photo-electron spectroscopy to quan-tify DNA hybridization withoutrelying on labels. The groupmonitored the ratio of nitrogento sulfur signals from thiol-

modified oligonucleotideswhere sequences of HS-dT20, HS-dA20, HS-dT10C10, and HS-dG10A10 served as probesfor the immobilization andhybridization procedures.Immobilizing streptavidin-coated magnetic beads confirmed the hybridization and thus verified the surfaceimmobilization of the thiolatedsamples.

The group conducted all ofthe measurements with a mono-chromatic Al K-alpha X-raysource that had an incident area of 400 micrometers and an incidence angle of 53°. Inconfirming the hybridization, the range of statistical errorwas less than 20%.

Adam Lapicki1, Fumitaka Sakamoto2,and Adarsh Sandhu1

1. Quantum Nanoelectronics ResearchCenter, Tokyo Tech

2. Thermo Electron Corporation

Japanese Journal of Applied Physics 46,no. 2, pp. L49–L52 (2007).

Dr. Adarsh Sandhu Associate Professor, Tokyo Tech

spirit.pe.titech.ac.jp/index-e.html

Monolithically integrated distrib-uted reflector (DR) lasers with

wire-like active regions have attract-ed a great deal of attention in high-speed optical communication sys-tems. Although their ultralow powerconsumption and simplicity of inte-gration had been demonstrated, inte-gration with other active devices hadremained unestablished.

Shigehisa Arai and colleaguesreport deeply etched narrow-grooveisolation and its application; that is, aDR laser integrated with a front-power monitor. The resultant laser ispromising as a high-performanceintegrated light source for high-speed optical data communications.

A deep isolation groove of 3.8 µmoccasioned high electrical isolationresistance of greater than 60 MV,and the optical coupling between the

power monitor and the DR laser was95%. Enabling the high coupling effi-ciency was the 500 nm width of thegroove in BCB polymer, which corre-sponds to a half-wavelength. Theintegrated device had a moderatelylow threshold current (Ith) of 2.5 mA,and it exhibited the property of linearpower monitoring. Shigehisa Araiand colleagues also report sub-mA Ith

with DR lasers, one of which operat-ed at an Ith of 0.8 mA.

Saeed Mahmud Ullah1, Ryo Suemitsu1, SeungHun Lee1, Masato Otake1, NobuhikoNishiyama2, and Shigehisa Arai1, 3

1. Quantum Nanoelectronics Research Center,Tokyo Tech

2. Department of Electrical and ElectronicEngineering, Tokyo Tech

3. CREST, Japan Science and Technology Agency

Japanese Journal of Applied Physics 46, pp.L954–L956 and L1068–L1070 (2007).

Hybridization results showing the high resolu-tion spectra of the N 1s signal for complemen-tary (C) and noncomplementary (N) samples:(a) surface, after immobilization of the thiolat-ed probe molecules; (b) solution, before thesurface immobilization.

The I-L characteristics of the low-thresh-old DR laser.

The lasing characteristics of a membrane DFB laserintegrated with a silicon-on-insulator waveguide.

Monitoring DNA Hybridization by Quantifying NitrogenContent with X-ray Photoelectron Spectroscopy

Integration of Functional Devices with Low ThresholdGaInAsP/InP Distributed Reflector Lasers through a Deeply EtchedNarrow Isolation Groove

GaInAsP/InP Membrane Buried Heterostructure DistributedFeedback Lasers for Optical-Interconnecting Applications

The I-L and photocurrent characteristicsof the integrated device.

The lasing spectra of an air-bridge membrane DFB laser.

Silicon nanowires are promisingmaterials for future nanoelec-

tronic and photonic devices. Theycould also become crucial compo-nents in chemical and biomedicalsensors. Those potential appli-cations have stimulated work on growth technologies for thecontrolled synthesis of siliconnanowires.

An obstacle in the vapor-liquid-solid growth of silicon nanowiresof controlled diameters has beenthe aggregation of gold catalyst athigh temperatures. A researchgroup under the supervision ofShunri Oda reports developing a

low-temperature growth methodfor small- and uniform-diametersilicon nanowires that overcomesthe problem of gold aggregation.The resultant nanowires aredefect free and are promisingmaterials for applications in elec-tronic and optoelectronic devices.

The group found that a lowgrowth temperature of 350°C iscrucial for preserving the size distribution of closely-packed gold droplets (865 nm) during the vapor-liquid-solid nucleation of silicon nanowires of small (12 nm) and uniform (65 nm) diam-eters. It achieved silicon-nanowire

growth at such a low temperaturefor the first time through the low-temperature decomposition ofSi2H6 in a low-pressure chemicalvapor deposition system.

Suppressing gold aggregationpermits high-density siliconnanowire growth at controlleddiameters. The small-diameter silicon nanowires, grown predomi-nantly in (111) direction, are defect

free and have single-crystal cores.Large-diameter silicon nanowires,on the other hand, are prone to ahigh density of defects.

Saeed Akhtar1, Koichi Usami1, YoshishigeTsuchiya1, 2, Hiroshi Mizuta2, 3, and ShunriOda1, 2

1. Quantum Nanoelectronics Research Center,Tokyo Tech

2. Quantum Nanoelectronics Research Centerand Department of Physical Electronics,Tokyo Tech, and SORST, Japan Science andTechnology Agency

3. School of Electronics and ComputerScience, Southampton University

Applied Physics Express 1, p. 014003 (2008)

Researchers have reportedcharge quantum bits (qubits)

composed of 100 nm-diameterdouble quantum dots (DQDs) of along decoherence time of around200 ns. In addition, researchershave measured qubits with sili-con single-electron transistors(SETs), which have ultrahighcharge sensitivity. Scaling downthe dimensions of the siliconDQDs further would presumablyincrease the two-level splittingand multiply the fundamentalqubit frequency accordingly

Yoshiyuki Kawata and his col-leagues have for the first timeintegrated nanocrystalline siliconDQDs into a silicon SET. Theyfabricated the silicon SET withelectron-beam lithography. The

capacitance values for the fabri-cated SET agreed well with theresults of three-dimensionalcapacitance simulations. Thatconfirmed that the silicon SETswere defined by geometricalstructure, not by dopant-inducedtunnel barriers.

The group then deposited 10nm–diameter nanocrystallinedots on a prepared resist pattern.It integrated the dots into the SETby removing unwanted dots bythe lift-off process. The groupemployed single-electron circuitsimulation, using the resultsobtained with the three-dimen-sional capacitance simulations,to analyze the effect of qubitcharge polarization on the SETcurrent.

Yoshiyuki Kawata, Mohammed A. H.Khalafalla, Kouichi Usami, YoshishigeTsuchiya, Hiroshi Mizuta, and ShunriOda

Quantum Nanoelectronics Research Centerand Department of Physical Electronics,Tokyo Tech, and SORST, Japan Science andTechnology Agency

Japanese Journal of Applied Physics 46, p.4386 (2007).

The scanning Hall probe micro-scope permits highly sensitive,

noninvasive, and quantitative map-ping of localized stray fields at thesurfaces of ferromagnetic materi-als. Adarsh Sandhu is a pioneer inthe development of the variable-temperature scanning Hall probemicroscope, which permits mag-netic imaging at nanometer-scalespatial resolution over areas ofhundreds of square micrometers.

A good grasp of the effect ofexternal fields and temperature onthe properties of domain structuresin garnets is crucial in the opto-electronics industry. Optical isola-tors of single-crystal ferromagneticgarnets prevent reflected light fromdamaging the laser sources inscanning Hall probe microscopes.The light can pass through the gar-

nets in only one direction becauseof the Faraday effect.

Sandhu’s group describes vari-able-temperature scanning Hallprobe microscopy for imaging thesurfaces of bismuth-substitutediron garnets (BiTbHo)3Fe5O12 of 50micrometer thickness at tempera-tures between 77°K and 300°K. Thegroup fabricated micro-Hall probes(1.5 µm2) photolithographically from2DEG InGaAs/AlGaAs heterostruc-tures that had an electron densityof 2.2 3 1012 cm-2 and an electronmobility of 20,000 cm2/Vs. Its find-ings were highly consistent withthe compensation temperature forthe material employed. The width of the magnetic domains increasedfrom approximately 7 µm at 300°K to 22 µm at 77°K.

Zaki Primadani1, Adarsh Sandhu1, 2

1. Department of Electrical and ElectronicEngineering

2. Quantum Nanoelectronics Research CenterBoth at Tokyo Tech

Journal of Magnetism and Magnetic Materials310, pp. 2693–2695 (2007).

Visualization ofmagneticdomains at thesurface of iron-garnet film.

A transmission electron microscopic image ofa single-crystal silicon nanowire grown in(111) direction of 8 nm diameter.

Variable-Temperature Scanning Hall Probe Microscopy ofFerromagnetic Garnet Thin Films

A Low-Temperature Growth Method for Small- and Uniform-Diameter Silicon Nanowires

The Integration of Tunnel-Coupled Double NanocrystallineQuantum Dots with a Multiple-Gate Single-Electron Transistor

A scanning electron micrograph of aSET into which nanocrystalline quantumdots have been integrated.

Dr. Shunri Oda Professor, Tokyo Tech

odalab.pe.titech.ac.jp/en/

Temperaturedependence ofmagneticdomains at thesurface of irongarnets.

Cu(InGa)Se2 (CIGS) is a materialused in solar cells to convert

solar energy directly into electrici-ty. A thin CIGS layer absorbs radia-tion from the sun, and it createselectron-hole pairs, resulting in theelectron current.

A group led by Akira Yamadareports having invented a novelmethod for growing high-qualityCIGS film, which is promising inregard to raising energy-conver-sion efficiency in solar cells. In theconventional evaporation method,

the metal precursors impinge onthe substrate surface with thermalenergy. Yamada’s group proposesan active-source method. Itsmethod uses cracked seleniumand ionized gallium, and thoseexternally excited precursorsmigrate and diffuse easily on thegrowing surface, resulting in theenhancement of film quality unac-companied by substrate heating.

The resultant CIGS film exhibitsa large grain size suggestive of high quality. In addition, the

method is a modified version of the conventional evaporationmethod and is suitable for massproduction.

Akira Yamada1, Fanying Meng1, YoshiyukiChiba2, Masahiro Kawamura2, and MakotoKonagai2

1. Quantum Nanoelectronics Research Center2. Department of Physical ElectronicsBoth at Tokyo Tech

Materials Research Society Spring Meeting(April 9–13, San Francisco), SymposiumProceedings 1012, p. 57 (2007).

Dr. Akira Yamada Associate Professor, Tokyo Tech

tkhshi.pe.titech.ac.jp/index_e.html

© 2008 Quantum Nanoelectronics Research Center (QNERC) Tokyo Institute of Technology 2-12-1, Ookayama, Meguro-ku, Tokyo 152-8552, Japan

Finding real-world solutions

Silicon quantum dot superlat-tices are a new absorber mate-

rial for silicon-based solar cells.Shipments of solar cells exceeded2.5 GW worldwide in 2006. Solarcells of single-crystal silicon andof cast silicon accounted, how-ever, for more than 90% of produc-tion. Raising solar cell efficiencyand achieving the absorption ofthe full solar spectrum will dependon progress in controlling the band gap energy, and that is there-fore an exciting challenge forresearchers.

A group led by Akira Yamadareports having developed size-

controlled silicon quantum dotsuperlattices. The structure isapplicable to the absorber layer of thin-film silicon solar cells, andits band gap is controllable bychanging the dot size.

To fabricate the superlattices,the group deposited stoichiometrichydrogenaged a-SiC/silicon-rich a-SiC multilayers onto a quartzsubstrate with plasma chemicalvapor deposition. It then annealedthe multilayers at a temperature of around 900°C. The size of thequantum dots is controllable byadjusting the thickness of the layers. Transmission electron

microscopic observation con-firmed the formation of the siliconquantum dots, and reducing thedot size produced an observableblue shift of the photolumines-cence peak.

This process is a simple andeasy means of enhancing large

modules. The group’s next target is to demonstrate the applicabilityof the superlattices in improvingsolar cell performance and to fab-ricate tandem solar cells based onthe superlattice material.

Yasuyoshi Kurokawa1, Shigeki Tomita1,Shinsuke Miyajima1, Akira Yamada2, andMakoto Konagai1

1. Department of Physical Electronics2. Quantum Nanoelectronics Research CenterBoth at Tokyo Tech

Japanese Journal of Applied Physics 46, no.35, p. L833 (2007).

Contact DetailsFor information about research and education atQNERC:

E-mail sandhu@pe.titech.ac.jpWebsite www.pe.titech.ac.jp/qnerc

Visiting ProfessorsSamit RayProfessor, Indian Institute of Technology, Kharagpur

Ruud E. I. SchroppProfessor, Utrecht University

Affiliated Researchers Kazuhito FuruyaNew concepts and structures for compound semiconductor deviceswww.pe.titech.ac.jp/Furuya-MiyamotoLab/e-index.htm

Makoto KonagaiSemiconductor nanostructure devicessolid.pe.titech.ac.jp

Yasuyuki MiyamotoVertical ultrahigh-speed electron devices and EBL processingwww.pe.titech.ac.jp/Furuya-MiyamotoLab/

Hiroshi MizutaSilicon-based nanoelectronicsodalab.pe.titech.ac.jp

Masahiro WatanabeMetal, insulator, and semiconductor nanostructureswww.pe.titech.ac.jp/WatanabeLab/index-j.html

A Novel Method for Growing High-Quality CIGS Film

Photoluminescence from Silicon Quantum Dots in a SiliconQuantum Dot/Amorphous SiC Superlattice

A surface scanning electronic microscop-ic image of CIGS film grown with crackedselenium.

A cross-sectional transmission electronmicroscopic image of size-controlled siliconquantum dot superlattices.