（様式 20）成果報告書

TSUBAME 共同利用 平成２８年度 学術利用 成果報告書

利用課題名 ナノ構造界面における熱輸送特性の分子シミュレーション
英文： Thermal transport properties at nanostructured interfaces
塩見淳一郎
Junichiro Shiomi
物質・材料研究機構 情報統合型物質・材料研究拠点
Center for Materials Research by Information Integration, National Institute for Materials Science
http://www.nims.go.jp/MII-I/en/
邦文抄録 近年，材料科学に対する情報科学の応用（マテリアルズ・インフォマティクス，MI）が試みられており，材料の開発・設計の加速が期待されている．本研究では MI を用いたナノ構造の設計を目的に，原子グリーン関数とベイズ最適化を用いシリコン・ゲルマニウム界面の最適構造を探索した．その結果，全候補構造数に対し数%の計算回数で最適構造を発見することに成功し，非周期的な超格子構造により低熱伝導が実現できることを明らかにした．本結果により，材料の熱輸送特性制御を目的にしたナノ構造体設計に MI の技術が有効であることが示され，今後のさらなる発展が期待される．
英文抄録 Materials informatics (MI) is a new interdisciplinary research to provide efficient tools
to accelerate the material discovery and design. In this work, we developed a new MI method
combining atomistic Green’s function approach and Bayesian optimization, which realizes highly
efficient design of nanostructures with optimal thermal transport across Si-Si and Si-Ge interfaces.
The optimal structures can be obtained with calculating only a few percent of the entire candidates.
The obtained optimal aperiodic superlattice interfacial structures are non-intuitive and impacting.
The present work shows the effectiveness and advantage of MI in designing nanostructures tocontrol heat conduction, which can be extended to other structures.

Keywords: Boundary thermal transport, Atomic Green’s function, Bayesian optimization, Machinelearning(MI)

背景と目的 / Background
Designing nanostructures for phonon transport
has

been

achallenging

workduetothemethod

combining

atomistic

Green’sfunction approach and Bayesian optimization
thatrealizes

highlyefficientdesignof

multi-freedom of choice (size, shape, material,nanostructures with optimal thermal transport,etc.) as well as coupled effect such as constructive

as shown in Fig. 1.

and destructive phonon interference. Informatics,
the fourth paradigm of science in addition totheory, simulation, and experiment, is gaining agreat

attention

inthegeneralandinterdisciplinary fields of physics as a powerfulapproach to search for optimal models, structures,and materials. In this research, we are trying todesign nanostructures for phonon transport via
materials informatics.

Figure 1: Schematics of the materials informatics
method概要 / Overview
We have developed a new materials informatics

combing

AtomisticGreen’s(AGF) and Bayesian optimization.

function（様式 20）成果報告書
As a case study, we have applied the developed
methodtodesign

thethatminimize

nanostructures

Si/Ge-compositeormaximize

interfacial thermal conductance (ITC) across Si-Si
and Si-Ge interfaces which are important andrealistic for instance in thermoelectrics. We
formulate two optimization problems: the firstpart is optimization of relatively small interfacial
region accounting for full degrees of freedom (asshown in Fig. 2) to demonstrate the validity andcapability of the current method, and the second

Figurepart move on to the optimization of larger

optimization. (a)-(d) Optimal structures with theinterfacial

superlatticemaximum and minimum interfacial thermalstructures, which is possible to do experimentconductance. (e), (f) The 10 optimization runs

measurement.

with different initial choices of candidates, whereregion

withlayered

2:

Interfacial

Si/Ge

alloy

structure

the insets show the probability distributions of
ITC

結果および考察 / Results and Discussion
The optimal structures as shown in Fig. 2

obtainedfromcalculationsofall

thecandidates.

(a)-(d) were obtained by calculating only a few
percentofstructures,

Another merit of MI lies in possibility to explore

considerably saving the computational resources.

new physics in the course of understanding itsIn

output.

addition,

thetotalthecandidate

obtained

structures

are

By

performing

furthersystematic

non-intuitive and impacting. The validity andanalyses, we identified that the small thermal

capability of the method are demonstrated byconductanceidentifying the thin interfacial structures withoriginates from their degrees of freedom tothe optimal Si/Ge configurations among all themutual-adoptively balance the two competing

possible candidates. Based on the finding that theeffects (as shown in Fig. 3): as the layer thickness

interfacial structures with minimum ITC take ainform of aperiodic superlattice, we extended theFabry–Pérot interference increases, and the rate

search to thicker structures (up to 8.69 nm), andof reflection at the layer-interfaces decreases.

identified non-intuitive structures whose ITCs

Aperiodic superlattice with spatial variation inare significantly smaller than those of the optimal

the layer thickness has a degree of freedom toperiodic superlattices.

realize optimal balance between the above twoinsuperlattice

theaperiodic

increases,

thesuperlattices

impactofcompeting mechanism. Furthermore, the spatial
variation enables weakening the impact ofconstructive phonon interference in relative tothat of destructive interference.

（様式 20）成果報告書

Figure 3: (a), (b) Interfacial thermal conductanceversus the layer thickness and number ofinterfaces. (c), (d) Interfacial thermal conductanceversus number of interfaces for cases of 14-unit
layer (UL) superlattice with equal Si/Ge layer
number and 10-UL superlattice with variable
Si/Ge fraction.

まとめ、今後の課題 / Summary and Future Plans
In conclusion, we have developed a framework
combining

atomisticGreen’s

Bayesian

optimization

functionanddesign

thetonanostructure for phonon transport with highefficiency.

Theeffectivenesspresentandworkshows

advantageofthematerial

informatics in designing nanostructures to control
heat conduction. We believe that the developednovel

method,nanostructurewhich
design

canof

be

any

appliedtomaterials

inprinciple, would have a broad and impacting
appeal to the general scientific and engineeringcommunities, as well as to the general public. Inthefollowing

work,

wewilltry

toapply

informatics method to design more complicatedstructuresindifferent

fieldssuchasthermoelectric, radiation, spintronic devices, etc.
References
S. Ju, T. Shiga, L. Feng, Z. Hou, K. Tsuda, J.
Shiomi, Physical Review X 7, 021024, 2017.