Japanese Journal of AppliedPhysics

     

PROGRESS REVIEW

Silicon-based low-dimensional materials forthermal conductivity suppression: recent advancesand new strategies to high thermoelectricefficiencyTo cite this article: Huajun Lai et al 2021 Jpn. J. Appl. Phys. 60 SA0803

 

View the article online for updates and enhancements.

You may also likeMeasurement of electrical resistance ofthermoelectric materials with atemperature gradient using instant load-voltage analysisHyeon-Gu Jeon, Jae Yong Song,Byungsung O et al.

-

Capturing anharmonic and anisotropicnatures in the thermotics and mechanicsof Bi2Te3 thermoelectric material throughan accurate and efficient potentialBen Huang, Guodong Li, Xuqiu Yang et al.

-

Determination of Contact Resistivity by theCox and Strack Method for Metal Contactsto Bulk Bismuth Antimony TellurideR. P. Gupta, J. B. White, O. D. Iyore et al.

-

Recent citationsDimensionality effects in highperformancethermoelectric materials: Computationaland experimental progress in energyharvesting applicationsDeobrat Singh and Rajeev Ahuja

-

This content was downloaded from IP address 65.21.228.167 on 31/10/2021 at 22:09

Silicon-based low-dimensional materials for thermal conductivity suppression:recent advances and new strategies to high thermoelectric efficiency

Huajun Lai1,2, Ying Peng1,2, Jie Gao1, Masashi Kurosawa3,4 , Osamu Nakatsuka3,5 , Tsunehiro Takeuchi2,6, andLei Miao1,7*

1Guangxi Key Laboratory of Information Material, School of Material Science and Engineering, Guilin University of Electronic Technology, Guilin 541004,People’s Republic of China2Research Center for Smart Energy Technology, Toyota Technological Institute, Nagoya 468-8511, Japan3Department of Materials Physics, Graduate School of Engineering, Nagoya University, Nagoya 464-8603, Japan4PRESTO, Japan Science and Technology Agency, 4-1-8, Honcho, Kawaguchi, Saitama 332-0012, Japan5Institute of Materials and Systems for Sustainability, Nagoya University, Nagoya 464-8601, Japan6Institute of Materials Innovation, Nagoya University, Nagoya 464-8603, Japan7Department of Materials Science and Engineering, Faculty of Engineering, Shibaura Institute of Technology, Tokyo 135-8548, Japan*E-mail: miaolei@guet.edu.cn

Received June 10, 2020; revised July 20, 2020; accepted September 23, 2020; published online October 19, 2020

Thermoelectric (TE) materials can convert any kind of heat into electricity through the Seebeck effect. Harvesting body heat and generatingelectricity by TE wearable devices can provide a convenient charge service for electrical equipment, even in the case of emergency or disaster. Asa high-temperature excellent TE material, silicon also exhibits promising room temperature (RT) potential for wearable TE devices due to its safeand mature production line for the semiconductor industry. Aiming to search for solutions for reducing thermal conductivity (k), this reviewsummarizes the low-dimensional strategies for reducing k based on nanostructural classification, thus enhancing zT at RT, and it also looks intothe prospect of wearable application. Following in the footsteps of nanostack (NS), nanowire (NW), nanoporous (NP) and nanocomposite (NC) Si-based TE materials research, we found that the thermal conductivity has been well controlled and that harmonious regulation of the power factor(PF) with k will be the future direction. © 2020 The Japan Society of Applied Physics

1. Introduction

The commercial launch of the 5th generation wirelesssystems (5G) is promoting further development of human–machine interaction and the Internet of Things (IoT), whichmakes a “mobile internet of everything” no longer distant inthe near future. As one of the wireless sensor network (WSN)application terminals with the most significant potential,portable wearable wellness monitoring gadgets for contact-less and long-play use are subject to a new wave ofrequirements triggered by the current worldwide spread ofthe COVID-19 epidemic. However, the life cycle of theenergy source has become an Achilles’ heel that limits thedevelopment of wearable devices. In order to get moreextension of the lifetime, there are currently three reliablesolutions.1) The first is to design a more durable energystorage part as a power source. Currently, commonly-usedenergy storage devices include lithium batteries, fuel cells,and supercapacitors; although their performance has beensignificantly enhanced, they still cannot meet the long-termbattery usage time. The second is to reduce the powerconsumption of wearable devices. By improving the inte-grated circuit inside the sensor and the routing algorithm ofthe WSN, the power consumption of the gadgets should bereduced to a certain extent, but the wireless feature largelylimits the minimum power consumption. The third solution isto harvest sustainable energy in ambient for the powersupply. In contrast to batteries, energy harvesting technologynot only avoids environmental pollution caused by wastebatteries, but also provides power for theoretically permanentbatteries, which greatly increases the operational duration ofwearable devices.Compared to other alternative energy sources that function

around our bodies, i.e. mechanical energy, radio wave energyand light, body heat in a general person becomes the most

renewable energy source for wearable devices as it is almostimpervious to environment, geographical climate or season.Thermoelectric (TE) phenomena, which involve the conver-sion between temperature and electric potential through theSeebeck effect and the Peltier effect in an efficient tempera-ture gradient, are expected to play an essential role inwearable practicality via operational advantages such as along life cycle, high reliability, no maintenance and novibration. The evaluation of TE performance is a dimension-less figure of merit, which is expressed as below.

s k k k k= = + +zT T PF TS 1l c B2 · · ( ) ( )/ /

Here, T is absolute temperature, S is the Seebeck coefficient,σ is electrical conductivity, s=PF S2 is the power factor(PF) and k is the total thermal conductivity that consists oflattice thermal conductivity (kl), carrier conductivity (kc) andbipolar diffusion thermal conductivity (kB), respectively.In the past two decades, many TE materials for application

at room temperature (RT) have been investigated.2) Organicseries were explored for wearability due to their inherentflexibility; typical examples are polymer3–7) and carbon-based materials,8–10) but they are overshadowed by therelatively high zT of inorganic materials at RT—severalexhibit a high zT value exceeding ∼1.1, such as BiSbTe,11,12)

Bi0.4Sb1.59Ge0.01Te3,13) CuInTe2+Bi0.4Sb1.6Te3

14) andMg0.97Zn0.03Ag0.9Sb0.95.

15) Nevertheless, two critical short-comings limit their application on wearability. On the onehand, most of them are rare in lithosphere and toxic to thehuman body, which is not desirable in terms of their massapplication on skin. On the other hand, the volume ofconvectional bulk materials cannot satisfy the demand ofthe increasing miniaturization of wearable terminals.As known in the most widely-used semiconductor mate-

rials, silicon has recently again attracted attention in TEmaterials, not only due to the advantages of its abundancy

© 2020 The Japan Society of Applied PhysicsSA0803-1

Japanese Journal of Applied Physics 60, SA0803 (2021) PROGRESS REVIEWhttps://doi.org/10.35848/1347-4065/abbb69

and non-toxicity, but also because of the control level of itsultra-high purity, the stability of its oxide, the chemicaldiffusion barrier during device fabrication and its easy integra-tion in electronic equipment. However, Si is active only in high-temperature applications, i.e. Si has been the preferred choicefor radioisotope TE generators (RTG),16) but has received littleattention at RT due to its modest zT, which is hindered by anoverlarge k. In recent years, the vigorous advance of a low-dimensionality strategy has shown exciting effects in theenhancement of zT and the restriction ofk in Si-based materials,and confined material volume combined with advanced inte-grated circuit techniques make it more convenient from rawmaterials to practical integration—these exploit a new path forSi to the blue ocean of wearable energy harvesting.On the related development line, Dresselhaus et al.17)

revealed the promising direction of low-dimensional TEmaterials. From that time to now, various TE low-dimen-sional materials have emerged which have excellent proper-ties. As an outstanding semiconductor material, recently,several brilliant summaries about this potential material havebeen proposed. In 2015, Narducci et al.18) expounded thefunction of nano-precipitates in the TE performance of Si-based bulk and film materials. Nozariasbmarz et al.19)

summarized the bulk TE materials of metal silicide in detailup to 2017. In 2018, Gadea et al.20) reviewed the advancedmicrostructure of Si-based TE materials in order to present anapplicated-oriented prospect. Nakamura21) elaborated on thedramatic reduction of k in the precise controlling of a Si-based epitaxy. In 2019, the systematic sum-up by He et al.22)

exhibited the TE behavior about the nano-bulk structure of Siand SiGe alloys as well as low-dimensional Si; additionally,TE properties of silicon scrap reclaimed from wafer manu-facture were introduced. In the same year, Tanusilp andKurosaki23) briefly reviewed the Si-based TE materials whichsynthesized by a natural nano-structuring method, and then arapid-solidification melt-spinning (MS) technique was intro-duced—ultimately a new ytterbium silicide was presented.Recently, reviews of tuning phonons by employing nanos-tructuration have been investigated, which exhibited a dra-matic effect in controlling k.24,25) To emphasize the con-tribution of low-dimensionality in enhancing the TE behaviorof Si-based materials by suppressing k, this review willcomprehensively summarize the low-dimensional approachfor reducing k and enhancing zT at RT and looking into theprospect of their wearable application.This review consists of four sections. In part 2, multi-

farious low-dimensional strategies for reducing-k-orientedare presented in accordance with views of morphologies andcomposition. In part 3, we comprehensively discuss variouslow-dimensional morphologies of Si-based TE materials,such as the nanocomposite (NC), nanostack (NS), nanowire(NW) and nanoporous (NP). In this section, experimentalresults would be focused on and the related principle will bediscussed, if necessary. Finally, part 4 predicts prospectiveintegrated solutions of low-dimensional Si-based devicesbased on their features, and the challenge and outlook willbe presented.

2. Low-dimensional strategies of reducing-k-oriented

Excessively high k (∼150 WmK−1 at 300 K26)) restricts theadvancing of Si-based materials in the field of

thermoelectricity, even though their power factor (PF) couldbe as enormous as ∼5000 μWmK−2.27) At RT atmosphere,the thermal conductivity of a single-carrier semiconductorcould be expressed as k k k= + ,l c therein, based onWiedemann–Franz law, carrier thermal conductivity (kc)could be defined as:28)

k s= L T. 2c ( )

Here, L is the Lorentz constant. In the semiconductor, latticevibration contributes to the vast majority of the thermaltransmission, hence the regulation of kl is the key means foroptimizing k, especially for Si-based materials. According tothe Boltzmann transport equation, lattice thermal conduc-tivity (kl) could be defined as:25)

åk = C v L1

33l

ss s s

qq q q

,, , , . ( )

Here, s expresses polarization, q is the wave vector, C is thespecific heat per phonon mode, v is the phonon velocity andL is the mean free path (MFP) of phonon scattering, which isdetermined by a diverse phonon scattering mechanism.Synthesizing new silicide materials with a heavier element,which can provide random point-scattering sources to scatterhigh-frequency phonons, could effectively reduce17–22) Thetypical representative is the SiGe alloy that already has beensuccessfully used in an auxiliary power source in the field ofdeep space exploration. Other silicon compounds, namely Fe-Si, Cr-Si, Mn-Si, Mg-Si, Ru-Si, and Re-Si, have beeninvestigated and are active in medium-high temperature bulkapplications.29,19)

However, further decreasing kl via such an approach isdeficient because point scattering is specific for short-wavephonons, and the coupling relationship of S and σ alsorestricts the reinforcement of zT. S and σ show an oppositetendency according to the change of carrier concentration;augmenting of S ordinarily leads to a drop of σ, andincreasing of σ directly increases k .c

17) Low-dimensionalstructuration30–32) was predicted to be able to decouple theinner correlation of S and σ. When material dimension goesfrom three-dimensions (3D, conventional bulk materials) totwo-dimensions (2D, thin films), one-dimension (1D, nano-wires, NWs) or zero-dimensions (0D, quantum dots, QDs),the density of the electron energy state (DOS) could becomenoticeably different,33) as shown in Fig. 1(a). In the case ofthe lower-dimensional, the DOS would be raised moresharply near the Fermi level in contrast with 3D materials,thus independent manipulation of S and σ becomes easier.What is more, in a low-dimensional system, phonon trans-mitting would be accompanied by dimensional confinementand diverse interface scattering/reflection, which hierarchi-cally blocks phonons in different frequencies and achievesmore effective suppression of k ,l as clearly shown inFig. 1(b). Besides, the microstructure could also purposefullycontrol the behavior of the carrier and k ,c so that the overall kcan be more comprehensively reduced.The first low-dimensional morphology to be realized was

the superlattice (SL), which demonstrated the enhancementof zT in PbTe/Pb1-xEux quantum wells accordant with thetheoretical prediction,34) and then similar enhancement wasalso realized in the Si/Ge SL.35) From then on, the explora-tion of low dimensions was divided into two directions:

© 2020 The Japan Society of Applied PhysicsSA0803-2

Jpn. J. Appl. Phys. 60, SA0803 (2021) PROGRESS REVIEW

design of the advanced NS heterostructure and furtherreduction of the dimension. In the former, highly controlledSi-based NS TE thin film was promoted by precise depositionand the epitaxy technique; arrays of ordered nanodots wereformed among the stacked layers, which provided diversesource centers and interfaces for phonon scattering. Such anSL QD (SLQD) was one of the effective methods to suppressphonon transportation without restricting carriers arrestinglydue to the different MFPs of phonon and carrier, as shown inFig. 2(a). In another way, lower-dimensional NW, whichshaped as a higher aspect ratio and diameter of ∼100 nm, waspursued in TE territory. The MFP of the phonon could belimited in confined wire morphology, as shown in Fig. 2(b).In such a case, the ultra-high lattice k ,l which is blocking Si-series TE materials was decreased to the limitation ofamorphous Si and the zT value increased dramatically atRT. Although the mechanism for decreasing k is not clear atpresent, several possible interpretations were explicated:22)

(1) more frequent boundary scattering, (2) changing ofphonon dispersion relationship, and (3) quantized transporta-tion of the phonon induced by the decreasing feature size.However, the inherent shortcoming of mechanical strength inNWs always impedes the integration and application, eventhough there is considerable zT value at RT. For conqueringthese shortcomings while maintaining the advantages ofNWs, porous morphology thin film is a compromise strategy,as shown in Fig. 2(c). The sidewall of highly dense in holesinduces more phonon backscattering, which suppresses keffectively. Particularly, calculation demonstrated that thephonon would be trapped behind the hole since the size is

smaller than the MFP of the phonon, which is called thenecking effect.36) However, nanoporous morphology is al-ways accompanied by a relatively low σ due to the hollowed-out mesh access and most of them require high precisionpreparative techniques. Recently, Si-based NC thin filmexhibited k close to amorphous Si due to hierarchical low-dimensional scattering sources for the phonon, as shown inFig. 2(d). In addition to phonon manipulation, ingeniousseparated phases could regulate carrier behavior purposefullyand the relatively simple preparation technology brings con-venience to integrated applications. Up to now, correspondingSi-based low-dimensional materials with a colossal PF (largeto orders of magnitude from 104 to 107 μWmK−2)37–42) havebeen revealed, which give us a glimpse of the considerable TEperformance and wide application prospect, even though theydid not have an adequate theoretical explanation and effectiverepeatability.There are various low-dimensional strategies for suppres-

sing k of Si-based materials in TE applications; they caninclude but are not limited to the following. First, top-downetching or bottom-up growing for directly forming materialsof low-dimensional morphology, i.e. NWs and NP. Second,periodically epitaxial growth for precisely growing specialheterostructure, i.e. nanostacked (NS). Third, vapor deposi-tion for preparing a thin film of specific composition orheterostructure, and/or combined with a transient thermalprocess for precipitating a low-dimensional phase, i.e. NC,such as columnar, QDs, and other separated phases.Therefore, in order to reveal the state-of-the-art strategies

legibly, we will trace from the view of morphology

(a) (b)

Fig. 1. (Color online) (a) DOS for 3D, 2D, 1D and 0D materials. Adapted from Ref. 33. (b) Hierarchical phonon scattering in low-dimensional materials:point scattering for high-frequency phonons, grain interface for medial frequency phonons, and NC interface for low-frequency phonons.

© 2020 The Japan Society of Applied PhysicsSA0803-3

Jpn. J. Appl. Phys. 60, SA0803 (2021) PROGRESS REVIEW

manipulation, namely NC, NS, NW, and NP, to show thedevelopment of low-dimensional Si-based TE materials inrecent decades.

3. Recent development of low-dimensional Si-basedTE materials

3.1. NCProverbially, quite high thermal conductivity is the biggestbarrier in the pathway to the progress of Si-based TE materials.Means of NC, which is commonly deemed to exist in at leasttwo phases and one or more of them are nanostructured,43) isconsidered to have the capability to scatter phonons by multi-scale scattering sources. George et al.44) already verified thatan ultra-thin amorphous aluminum layer at the surface of asilicon thin film could scatter phonons more effectively thanonly roughening the surface, as shown in Fig. 3(a). About∼1 nm of an amorphous aluminum layer reduced k from 28 to20 WmK−1 and 35 to 23 WmK−1 in boron and phosphorus-doping single-crystalline Si membranes, respectively. Thereason for this can be explained by arguing that amorphousaluminum may not only diffuse phonons but also absorb theenergy of the wave packet and capture or return this energy inthe form of high-frequency lattice ripples. Controlling theamorphousness is another way of decreasing k.Banerjee et al.45) employed low-pressure chemical vapor

deposition (LPCVD) and arsenic ion implantation for highamorphousness Si thin film. The appropriate temperature of

the thermal process, which was below its growth temperature(525 °C), increased the amount of well-ordered tetrahedral sitesin α-Si and probably supplied electronically kinetic impuritysites. Such a scenario made σ as high as 1556 Sm−1 in such lowcrystallinity. Simultaneously, k and S were kept parallel withamorphous Si, which were 1.5 WmK−1 and 1450 μVK−1,respectively, thus the zT value was as high as 0.64 at RT.

Fig. 2. (Color online) Schematics of low-dimensional strategies in Si-based TE materials: (a) NS, (b) NW (c) NP and (d) NC. Partially adapted from Refs. 20and 75.

(a)

(b) (c)

Fig. 3. (Color online) Transmission electron microscope (TEM) pictures of(a) cross-sectional Si membrane with an ultra-thin Al layer. Reproduced fromRef. 44. (b) Grains in annealed polycrystalline Si thin film of 100 nmthickness. Reproduced from Ref. 46. (c) Hierarchical NC made by sintering aNW array. Reproduced from Ref. 47.

© 2020 The Japan Society of Applied PhysicsSA0803-4

Jpn. J. Appl. Phys. 60, SA0803 (2021) PROGRESS REVIEW

Valalaki et al.46) discovered the potential relationship ofthe thickness and grain size in Si thin film after annealing.They directly confirmed the corresponding grain size, whichwas achieved from different thicknesses through TEMimages, and obtained an inverse tendency between filmthickness and grain size. The thinner thickness correspondedto the smaller grains and also the lower k, which wasdecreased from 48 WmK−1 of 500 nm to 9.4 WmK−1 of100 nm, as shown in Fig. 3(b). Despite the best zT in thiswork being only 0.0332, the relationship was a considerablereference for controlling the microstructure.Recently, Kashiwagi et al.47) realized a hierarchical NC by a

plasma-activated sintering Si NW array. The raw NW array,which contained porous NWs and silver precipitated grains (theby-product of metal-assisted chemical etching), was collapsedafter pressured sintering and formed a unique morphology of amulti-level scale compound that consisted of nanoscale wire-grains, pores, and silver particles, as shown in Fig. 3(c).Besides, softened interfaces of heterogenous grains weredemonstrated for crucial impact in reducing ofk, which reachedthe lowest of ∼1.48 WmK−1 in this research. Although thedense boundaries led to a suppressed σ (∼26.5 S cm−1), theextraordinary zT value of 0.3 was realized at RT.In recent decades, transition metals, e.g. Mo, Ni, Fe, and

Mn, have been paid sufficient attention in terms of beingcombined with Si or SiGe for enhancing their TE behaviors.Ohishi and Uchida et al. focused on phase separation in thecompound of Mo/Ni and a Si-based matrix for thermoelec-tricity. A thin film of Ni-Si composition with boron andphosphorus doping was exhibited.48) A rapid thermal processseparated the NiSi2 phase as well as the planar defect, asshown in Fig. 4(a). Annealing at high temperature

(1100∼ 1200 °C) not only activated carriers but also reducedthe mid-gap for increasing S. Furthermore, the planer defectserved as a scattering center and suppressed k to 5.4 and 4.4WmK−1 at N- and P-type, respectively. Although k issignificantly lower than that of bulk Si, the resistivity stillunsatisfied because of the electron scattering mechanismprobably caused by the effect of the potential barrier of Nisilicide.49) Immediately after, they used silicon on insulator(SOI) and silicon on quartz (SOQ) as substrates for indicatingthe effects of different substrates on Ni-Si phase separationand the corresponding TE performance.50) Studies shown thatSOI and SOQ substrates generate more planar defects andtwin defects in the film after annealing at 1200°C, which actas phonon scattering centers. The twin defects directly lead tolower k (2.8 WmK−1) than previous samples. Furthermore,compared with their previous work, the directional growth ofSOI and SOQ substrate films resulted in larger σ; finally zT ofP- and N-type are 0.24 and 0.1 at RT, respectively.Kurosaki et al.51) controlled k by introducing thermal

boundary resistance (TBR) into thermal transportation. Awell-designed amorphous SL of MnSiγ/Si1-xGex beforeannealing promoted proper grain size and correspondingTBR among granular interfaces; such a case could result ink of 1.2 WmK−1 at minimum.52) Additionally, the char-acterization of synchrotron hard X-ray photoemission spec-troscopy (HAXPES) showed that it is possible to adjust thechemical bond of MnSiγ to change the proportion of the Mnand Si by increasing the ratio of Ge, namely the adding of theGe-reduced Si ratio in MnSiγ, which resulted in an excesscarrier of hole and increased carrier concentration.For more extensive applications, such as flexible devices

for portability, researchers have explored the contribution of

(a)(b)

(c) (d)

Fig. 4. (Color online) TEM picture of (a) Ni-Si composite thin film without doping; planer defects are circled by the solid line. Reproduced from Ref. 48.(b) SiGe/Al thin film after annealing at 873 K. Reproduced from Ref. 53. (c) Twin boundaries in SiGe thin film after Al-induced layer exchange (ALILE).Reproduced from Ref. 56. (d) Scanning electron microscope (SEM) picture of surface morphology of ternary SiGeSn after annealing at different temperatures.Reproduced from Ref. 57.

© 2020 The Japan Society of Applied PhysicsSA0803-5

Jpn. J. Appl. Phys. 60, SA0803 (2021) PROGRESS REVIEW

(a)

(b)

(c)

Fig. 5. (Color online) TEM pictures of columnar morphology: (a) P-type SiGe thin film. Reproduced from Ref. 58. (b) N-type SiGe thin film after pre- andpost-annealing. The scale bar is 300 nm. Reproduced from Ref. 59. (c) P-type SiGe thin film. Reproduced from Ref. 60.

(a)(b)

(c) (d)

Fig. 6. (Color online) TEM pictures of (a) and (b) SiGe thin film with Au doping. Reproduced from Refs. 37 and 63, respectively. (c) β-FeSi2 QDs inepitaxial Si thin film. The black arrow shows the interface between the film and the substrates. Reproduced from Ref. 65. (d) Mo-Si nanodot thin film.Reproduced from Ref. 66, respectively.

© 2020 The Japan Society of Applied PhysicsSA0803-6

Jpn. J. Appl. Phys. 60, SA0803 (2021) PROGRESS REVIEW

metal-induced crystallization (MIC), typically aluminum, toreduce the heat treatment temperature of Si series materialswhile improving their TE properties for flexible substrates.Lindorf et al.53,54) studied the crystallized effect of Al toSiGe. They adopted an alternant multilayer of the SiGe/Alstructure for controlling the crystallization of SiGe. Resultsrevealed the changing of transport properties via in-situmeasurement but did not go deep into analysis of k.Nevertheless, the density of nanodot boundaries and thepossibility of heterogeneous SixGe1-x phases55) after an-nealing [Fig. 4(b)] have the potential to restrict k.Afterward, ALILE was applied to the development of

flexible TE film by Kusano et al.,56) which further reducedthe temperature of heat treatment in SiGe. Layers ofamorphous SiGe and Al were interchanged after annealing.The convenient process substantially decreased the thermalbudget in thin-film growing (below 400 °C) so that it can beused on flexible substrates with a low tolerance in tempera-ture, such as polyimide (PI). Eventually, they achieved a PFvalue of 190 μWmK−2 on the flexible PI substrate at RTwithout performance deterioration significantly in thebending range of 0–120°. In particular, twin defects[Fig. 4(c)] analogical to Ni-Si assembly50) were also found,which was reasonable for enhancing phonon scattering anddecreasing k.Peng et al.57) introduced Sn for manipulating TE properties

of SiGe thin film. As shown in Fig. 4(d), instantaneous hightemperature caused Sn to precipitate as a melting phase andmixed tissue of poly crystallite SiGe, amorphous SiGe and Snprecipitated composite was obtained, which induced a con-siderable low k of 1.1 WmK−1 after annealing above1100 °C. Meanwhile, Sn doping not only induced crystal ofSiGe, but made Hall mobility increased with the increasing ofcarrier concentration, which was reversed with the conven-tional rule. Such a case led to a relatively high PF of1130 μWmK−2, and zT of 0.31 were achieved at RT.Columnar precipitated phases perpendicular to the horizontal

substrate were investigated in Si-based thin film. Such vertical-SL-like morphology contributes to reduce k of the in-planedirection due to the enhancement of phonon scattering atcolumned grain boundaries. Takashiri et al.58) revealed columnarSi0.8Ge0.2 thin film, as shown in Fig. 5(a). Vertical graininterfaces hindered phonon transportation and contributed to kof 2.9 WmK−1 in the cross-sectional direction, which wasreasonable to achieving much lowerk in the horizontal direction.This anisotropy has been confirmed by Lu et al.,59,60) both

N-type and P-type SiGe thin films with pillar grains displayedanisotropy of k between the in-plane and cross-sectionaldirection, as exposed in Figs. 5(b) and 5(c), respectively. Thein-plane k, which was 2 and 2.3 WmK−1 respectively, wasabout twice lower than that of the cross-sectional directiondue to pillar sidewall scattering in N- and P-type materials.Most particularly, the phosphorus-doping sample exhibitedlower k due to relatively violent phonon-defect scattering andmore dopant segregation in grain boundaries. Furthermore,precipitation of the dopant reduced the energy barriers in thegrain interfaces and contributed to the increase of carriermobility. Ultimately, the zT of 0.16 and 0.2 was obtained inN-type and P-type materials at RT, respectively.Similar morphology but a different scattering mechanism

was achieved in hydrogenated microcrystalline Si (μc-Si:H)

thin film by Acosta et al.61,62) Thermal annealing did notcause an obvious variation in crystalline size and volumefraction but k dropped from 3.2 to 1.5 WmK−1 (P-type),which was dominantly attributed to carrier-phonon scatteringcaused by increased carrier concentration after annealing.However, energy barriers in grain interfaces induced thereduction of global carrier concentration and restrained σ to acertain degree; zT of 0.06 was achieved at RT.SiGe with Au-doping thin film once depicted a colossal PF

in orders of magnitude of 105∼ 106 μWmK−2,38–40) but thedetailed mechanism is still unclear. Takiguchi et al.37)

deemed the proper size [∼6 nm, Fig. 6(a)] of nano QDs,which precipitated after the collapse of a pre-deposited SLvia annealing, strongly contributed to the anomalously hugePF. However, it seems unconvincing that they only ascribedthe anomalous enhancement of S and σ to a quantum sizeeffect without enough experimental repeatability and evidenttheoretical analysis. Nishino et al.63) tried to repeat thisextremely large PF. As shown in Fig. 6(b), although a similargrain size (5–10 nm) was achieved, the maximum S value of502 μVK−1 was far smaller than that of the previousmaximum S of 2000 μVK−1 and exhibited an inconspicuouscorrelation in Au-content. Nevertheless, they verified that themorphology of QD crystal dramatically restricted k to aminimum ∼0.72 WmK−1, which exhibited the advantage ofQDs in hindering phonon transmission.Nakamura et al.64) depicted a novel phonon scattering

phenomena by means of wrapping monocrystalline Si QDsby an ultra-thin amorphous SiO2 layer. Reduction of k abovethe amorphous Si limit owed to confined phonon transporta-tion, which was different from conventional phonon scat-tering. The unique effect caused k to be suppressed to∼0.92 WmK−1, while exhibiting σ of ∼190 S cm−1 due toaccordant orientation in each monocrystalline Si QD, whichdemonstrated the promising prospect of pure Si.Sakane et al.65) revealed hierarchical phonon scattering of

FeSi2 QDs in suppressing k. Two phases of iron silicide QDswere formed in a Si matrix by solid phase epitaxy (SPE)under different temperature and exhibited almost the same PF(∼2650 μWmK−2 at RT), QDs of α-FeSi2 with a uniformsize of ∼5–20 nm was weaker in restricting phonons than thatof β-FeSi2 [Fig. 6(c)], which distributed in a widelyhierarchical size of ∼5–120 nm. Benefiting from the hier-archical phonon scattering, k of the β-FeSi2 sample wastwice lower than that of α-FeSi2 and eventually exhibited zTof ∼0.1 at RT (estimated from curve tendency).In addition, Uchida and Ohishi et al.66,67) precipitated high

serried nanodot phases of MoxSiy from a Si-based matrix, asshown in Fig. 6(d). Density interfaces contributed to sup-pressing k to 2.2 WmK−1 and it was reasonable to achievemore drops if hierarchical size inclusions were involved.However, abounding interface defects not only induced highresistivity (0.17–9W m· at RT), but fixed the Fermi level dueto the mid-gap state, and then deteriorated S to ∼60 μVK−1.Nevertheless, the morphology of dense nanodots, whose sizewas as tiny as 8–10 nm, has a reference for quantum effectmorphology, and it is rational to improve S and σ compre-hensively by employing additional impurity doping, such asion implantation.In addition to suppressing k by scattering phonons in dense

boundaries, various evidence indicates that the 0D-QD

© 2020 The Japan Society of Applied PhysicsSA0803-7

Jpn. J. Appl. Phys. 60, SA0803 (2021) PROGRESS REVIEW

system seems to have a special effect on independentregulating coupled S and σ. However, the detailed me-chanism of the boost for TE properties has not yet beenfully exposed.3.2. NSA NS consists of periodic low-dimensional stacks of differentmaterials in the order of 100 nm. After the achievement ofenhancing zT by a quantum well,35,68) a more efficientheterostructure was pursued via further reducing the dimen-sion of the material. Samarelli et al.69) showed the effect ofthe well width on k and zT in a Ge/SiGe NS heterostructure.Shallower quantum wells with smooth interfaces promoteddiffuse phonon scattering and contributed to lower k, whichapproached a maximum zT of 0.135 at RT. They firstlyintegrated a SL to a TE module by employing micromanu-facturing, which took a practical step forward. Furtherreduction of k was expected by introducing diverse bound-aries for phonon scattering. Conventional SLQDs are almostmade by the lattice mismatch and demonstrated the capacityon the considerable zT of ∼1.6 at RT.70)

In Si-based SLQDs, Chang et al.71) exhibited an architec-ture of composite stack dome-like QDs (CQDs) with Si as thebuffer layer, as shown in Fig. 7(a). The high-density Si/Geinterfaces and local alloying in the 3-fold CQD of Ge/Si/Ge/Si/Ge promoted k of the vertical direction to diverge from the

Debye T3 law at the low-temperature region (under 200 K),suggesting that certain quantum effects occurred in thetransmission of phonons in addition to phonon scattering ofCQD interfaces for proper size. Nadtochiy et al.72) exploredthis stacked dome-structure to gauge TE behavior bycombining experiment and mathematical calculations. Atthe range of 50–300 K, interactive effects of phonon scat-tering and carrier energy filtering at the inner boundaries of Siand Ge layers in the nano-dome were known as the mainfactor for S enhancement, which showed numerical S above1 mVK−1 at RT. It is worth noting that the CQD thin filmexhibited N-type quality without any doping probablybecause of the potential well for the hole carrier in thevalence band in this dome-like CQD architecture. If theinterlayer details, such as doping concentration and thickness,are optimized, the strategy would have a promising TEbehavior.Besides the apophysis heterostructure, Savelli et al.73)

prepared an N-type Si/Ge SL within regular sphere Ti silicideQDs embedding, as shown in Fig. 7(b). The embedding QDsdecreased cross-sectional k from 8.5 to 6.8 WmK−1 inmonocrystalline samples but remain almost unchanged(∼4.4 WmK−1) in polycrystalline ones; this was attributedto the dominated boundaries scattering. In addition, energy-dispersive X-ray spectroscopy (EDS) shows dopant of

(a) (b)

(c) (d)

Fig. 7. TEM pictures of (a) Si/Ge stack film of dome-like QDs. Reproduced from Ref. 71. (b) SiGe SL with Ti NDs embedding. Reproduced from Ref. 73.(c) Highly controlling stack morphology consisting of Ge QDs sandwiched between ultra-thin SiO2 layers. Reproduced from Ref. 75. (d) Phosphorusimplanted stack morphology with stacking faults (SFs); black arrows indicate Ge QDs near SiO2 interfaces. Reproduced from Ref. 76.

© 2020 The Japan Society of Applied PhysicsSA0803-8

Jpn. J. Appl. Phys. 60, SA0803 (2021) PROGRESS REVIEW

phosphorus congregated in QDs because chemical affinitybetween phosphorus and titanium might induce the highercarrier concentration in QDs for modulation doping.Furthermore, it is possible to control the potential barriers byaccumulating scattering and carrier-trapping at the boundaries,thereby adjusting S. The PF was increased about two times viaembedding QDs in mono and polycrystalline NS samples.Analogously, a highly controlled Si-based SLQD was

introduced by Yamasaka et al.74,75) Sphere Ge QDs weregrown epitaxially by MBE in each Si layer, which separatedby ultra-thin SiO2 layers, as shown in Fig. 7(c). Adjusting thesize and shape of Ge QDs governed the phonon scatteringprocess, and channel Si layers could control carrier transpor-tation. The controlling of the curvature radius and shape ofQD interfaces, which governed the probability and directionof phonon scattering, contributed to the large interfacialthermal resistance. k of ∼1.2 WmK−1 was achieved insamples of 8 nm-size QDs. Transport properties wereanalyzed in subsequent research for independent tuning ofphonon and carrier scattering for the purpose of practicalapplication.76) After doping by ion implantation, the analo-gous SLQD architecture still retained [Fig. 7(d)] and ex-hibited lower k compared to the non-doped sample, whichwas attributed to the widened wavelength range of thescattered phonon.Recently, several types of QD, such as Ge and β-FeSi2,

were employed for control carrier and phonon scattering77)

comprehensively. Hierarchical inclusions, namely QDs, de-fects and implanted impurities, were emphasized for hin-dering phonon transportation, which suppressed k below2 WmK−1. Meanwhile, high-density bundle defects acted asan energy barrier for energy filtering and enhanced S and PF.

In addition, band splitting induced by the strain of latticemismatch contributed to increased carrier mobility andachieved a higher PF and zT value at RT. With the assistanceof highly controllable epitaxy, precisely designing andfabricating the architecture of the NS is expected to achievepromising TE materials in the future.3.3. NWSi-based NW exhibits a larger aspect ratio and diameter of∼100 nm; in such a case, confinement of phonon transporta-tion induces k of single-crystalline Si below the amorphouslimit. Hochbaum et al.78) realized a large enhancement of zTby reducing k substantially. The morphology of dimensionalconfinement in ∼50 nm diameter NWs, which led to a zTvalue of 0.6 at RT, reduced k to a phonon-glass-like value of1.6 WmK−1 with less reduction of electrical transportproperties.The promising perspective aroused researchers’ interests;

Lee et al.79) incorporated Ge into SiGe NWs for preventingthe impact of a high-frequency phonon by introducing pointscattering. NWs were grown in good crystallinity by thevapor-liquid-solid (VLS) method and measured S, σ and kconcurrently, as shown in Fig. 8(a). The Ge ratio from 0.06 to0.86 was investigated and Si0.73Ge0.27 showed the best zT of∼0.2 with k of ∼1.2 WmK−1 at RT. It seems that therelatively low S of −100 μVK−1 was the main factor to limitits enhancement, which might be caused by the suppressionof mobility in the confined SiGe wires.As k has been suppressed to the limitation of amorphous

Si, further enhancement of zT requires the elevation of thePF, which related two coupled parameters of S and s.Benefitting from a field effect transistor (FET), novel fieldeffect TE NWs were investigated by Curtin et al.80) (110)

(a) (b) (c)

(d) (e)

Fig. 8. (Color online) TEM pictures of (a) SiGe NW places in the simultaneously-measuring system. Reproduced from Ref. 79. (b) Field-effect modulationNW system consists of SiO2 gate and Al gate metal. Reproduced from Ref. 80. (c) Modulation doping NW. Inset schematic shows the structure of δ-dopingNW. Inset high-resolution TEM picture shows layered NW. Reproduced from Ref. 81. (d) Cross-section of SEM view of the etching NW array; inset shows thetop-view. Reproduced from Ref. 87. (e) SL NWs of SiGe/Si unit; scale bar is 500 nm. Reproduced from Ref. 88.

© 2020 The Japan Society of Applied PhysicsSA0803-9

Jpn. J. Appl. Phys. 60, SA0803 (2021) PROGRESS REVIEW

oriented monocrystalline silicon pith was wrapped by asilicon dioxide layer for insulation and the outside wassurrounded by an Al metal layer, as shown in Fig. 8(b).The three-gated NWs would exhibit high charge density inthe surface of the silicon core due to the adjacent Al gate.Band bending of the Fermi level was significantly induced bybias voltage due to the Al gate, then made non-uniformdistribution of S and s because of the different charge densityacross the NWs. Thus, the gate voltage modulated thetransport properties; at a gate voltage range from 0 to 7 V,the maximum PF of ∼2300 μVmK−2 was obtained while thegate voltage was ∼7 V at RT.Modulation doping has been applied to NWs by Zhuge et

al.81) The NW consisted of an inner core and outer shell, asshown in Fig. 8(c). The heavier-dopant shell and inner corewere attested to produce a concentration difference in theradial direction, and carrier diffusion would occur from shellto core, which mitigated impurity scattering without overlydecreasing mobility. Furthermore, the structure of the largerconcentration difference was optimized by δ-doping, whichassembled by a thinner shell with heavier doping andintrinsic Si core, as shown in the inset of Fig. 8(c). Theenhancement of the PF by modulation doping was feasible inlayered NWs; a PF of ∼2000 μVmK−2 was achieved at RT.Additionally, a Si porous NW array was adopted for

further decreasing k and increasing the applicability of theNW. Zhang et al.82) fabricated large-area arrays of roughlyporous Si NWs in an array/wafer/array structure by metalassist chemical etching (MACE); the array of high-porosityNWs suppressed k. to 1.68 WmK−1 while keeping S as acrystalline Si wafer (307 μVK−1 at RT). The hybrid structureconducted zT up to the highest in contrast with previous NWarrays.83–86)

Up to now, integrating raw materials to practical deviceshas gradually become the focus of researchers’ consideration—namely the convenience and feasibility of mass implemen-tation. The bottom-up growth method is definitely moreefficient by metal catalyzing in the precursor solution,

(a) (b)

(c)

(d)

Fig. 9. SEM pictures of (a) nano-HS thin film; scale bar is 1 μm.Reproduced from Ref. 36. (b) SiGe porous thin film of ∼436 nm diametergrown on an anodized aluminum-oxide (AAO) substrate. Reproduced fromRef. 90. (c) Porous Si etched by electrochemical etching in a current densityof 50 mA cm−2. Reproduced from Ref. 91. (d) Surface morphology ofsegregated Si and Ge after annealing in excessive temperature. Reproducedfrom Ref. 93.

(a) (b)

Fig. 10. (Color online) Porous Si thin film: (a) k of different pitch size compared to samples of non-holey and amorphous SiO2. (b) zT of 55 nm pitch andnon-holey samples. (a) and (b) are reproduced from Ref. 36.

© 2020 The Japan Society of Applied PhysicsSA0803-10

Jpn. J. Appl. Phys. 60, SA0803 (2021) PROGRESS REVIEW

however, it is difficult to practically integrate due tomechanical stability. Thus, Lee et al.87) fabricated a verticalNW array by inductively coupled plasma reactive ion etching(ICP-RIE) from a top to down direction, as shown inFig. 8(d). A regular geometry shape and array spacing wereobtained by etching in the top-down method, which showedhigh manipulation. In the practical-oriented fabrication, theytheoretically demonstrated that the roughness of the NWsurface and implanted dopant all contributed to reduce kdramatically due to surface and impurity scattering, respec-tively.Finally, a TE module was integrated by corresponding NW

arrays, which showed an open voltage of 216.8 mV cm−2 andpower of 3.74 μWcm−2 at a temperature difference of 180 K.Furthermore, a SL NW88) is a potential alternative strategyand expected for better TE performance by involved sizeeffects, as shown in Fig. 8(e).3.4. NPPorous Si-based materials induce strong phonon scatteringbetween dense boundaries of nanoscale void holes andexhibited a quite low k of ∼0.8 WmK−1 as early as1997.89) In general, those thin films with morphologies ofnanoscale hole or mesh belong to this category. Tang et al.36)

fabricated holey silicon (HS) mesh by using deep reactive ionetching (DRIE), as shown in Fig. 9(a). k was greatlydecreased by the porous structure, and approached amor-phous SiO2 when the hole size decreased, as shown inFig. 10(a). Simulation testified that a phonon was trappedbehind the cavities and locally induced an adverse

temperature gradient along the transfer direction, namelythe necking effect, which was a difference in contrast withthe NW system. It is worth noting that the neck size in thenecking effect was smaller than the phonon MFP, which islarger than that of the electron (1–10 nm in doping of∼1E19 cm−3), thus the impact of the PF was negligible. kof the 55 nm pitch sample approached ∼1.7 WmK−1 and azT of ∼0.4 was achieved at RT, as shown in Fig. 10(b).Similarly, Perez-Taborda et al.90) put forward a nano-mesh

of Si80Ge20 by using a holey substrate, as shown in Fig. 9(b).Mesh film was duplicated from a porous AAO template andan ultra-low k of ∼0.55 WmK−1 was obtained in a diameterof ∼31 nm. The exact mechanism for such an experimentallylow k was still not clear; it was speculatively ascribed to thereduction of sound velocity or the necking effect. A zT of0.08 was obtained in a ∼300 nm diameter sample at RT.To simplify fabrication, handier techniques were at-

tempted, such as electrochemical etching and transientannealing. Martín-Palma et al.91) and Boor et al.92) synthe-sized tridimensional porous Si by electrochemical etching, asshown in Fig. 9(c). Such a process largely simplifies thepreparation of porous Si. The fabricated etching current-controlled the feature size, such as porosity and crystal size,to become analogous with the phonon MFP and takeeffective phonon scattering. Xie et al.93) came up with aneconomic synthesis of porous SiGe thin film by transientannealing. After plasma deposition, raw films were treatedthermally by a long-wave pulsed infrared laser, and a poroussurface morphology with segregation of Ge hemispheres was

Fig. 11. (Color online) zT and k of recent advanced low-dimensional Si-based TE materials of NC, NWs, NP, SL and SLQDs at RT (300 K). The upper partis P-type and the under part is N-type.

© 2020 The Japan Society of Applied PhysicsSA0803-11

Jpn. J. Appl. Phys. 60, SA0803 (2021) PROGRESS REVIEW

obtained, as shown in Fig. 9(d). Local heating of an ultra-high temperature-induced not only efficient activation of thedopant but also produced the special mesh surface whichexposed a dramatically low k of 0.84 WmK−1. Later, high-temperature analysis was carried out.94,95) The simplificationand low-cost process reveals the key direction for futureresearch.

4. Summary, challenge, and outlook for TE devices

We reviewed the results of Si-based low-dimensional TEmaterials in the past decade upon the theoretical predictionthat low-dimensionality has great potential to improve zT viadecreasing k. Figure 11 shows zT@RT and k in corre-sponding materials that are discussed above; one can see thematerials with a relatively high zT benefit from a relativelylow k. Low-dimensional handling successfully restrained kclose to the amorphous Si limit, which is much lower thanthat of Si-based bulk materials. Particularly for NW, forexample, the maximum value of zT is 0.6 at RT.78)

Mechanical-sturdier NP also exhibits extremely low k anda zT of 0.436) for maxima, however, precision controllingtechniques of the micropore, such as photolithography andmask etching, make it unsuitable for mass application.Furthermore, carrier conduction is confined in the porouspathway, which would cause an overlarge inner resistance inintegrated devices.The NS verifies the effect in hierarchical phonon scattering

and is expected to independently manipulate S, σ and k, evenwhen the specific mechanism is still unclear. By comparison,

the NC, with such zT values of 0.3–0.6 at RT,45,47,57) issuitable for practical-oriented application due to the relativelysimple manufacturing process and excellent TE properties.More details are shown in Table I for handy consultation,inquiry, and reference.Nowadays, wearable terminals are increasingly miniatur-

ized and sustainable; the uniquely tiny volume, environmen-tally friendly, and integrated circuit compatibility of low-dimensional Si-based materials offer a strong competitivesolution for a wearable power supply. Thus, the design andintegration of TE devices play a crucial role in human body-heat harvesting systems. Yan et al.1) summarized micro TEgenerators (μ-TEGs), but most of them are conventionalbulk-oriented structures, such as the sandwich, in-plane, ormixed structure μ-TEG, and the majority of devices are madeby Si or SiGe without any other low-dimensional strategy. Tohighlight the advantages of low-dimensional Si-based mate-rials, device-oriented material synthesis needs to be the focusof attention in the future. Actually, low-dimensional Si-basedmaterials are usually integrated to in-plane structure devices,which are more suitable for refrigerated devices,96) but it isdifficult to establish an effective temperature gradient com-pared to the sandwich structure, which is generally integratedby bulk materials. Dávila et al.97–99) introduced an improvedin-plane module for NWs, as shown in Fig. 12(a); thesuspended center region acts as the cold side while con-necting with a surrounding area of hot side by NW TEmaterials. Such a design optimized the heat flux route andmade NW, which is fragile but excellent in performance, able

Table I. Morphology, composite, and properties of published Si-based TE materials.

Year Morphology Materials W mK−1 uV K−1 S cm−1 uW mK−2 zT at 300 K Researchers

2019 NC-QDs FeSi 8 −175 865 2650 0.1 Sakane et al.65)

2019 NC SiGeMn 1.7 129 370 615 0.108 Kurosaki et al.51)

2019 NC Si 1.48 747.8 26.5 1480 0.3 Kashiwagi et al.47)

2019 NC-columnar uc-Si:H 1.5 252 47.6 300 0.06 Acosta et al.61)

2019 NC Si-Al P:20 P:168 P:1000 P:2822 P:0.042 George et al.44)

N:23 N:85 N:1370 N:990 N:0.0132019 NC Si0.864Ge0.108Sn0.028 1.1 162.5 428 1130 0.31 Peng et al.57)

2019 SLQDs SiGe — 2000 0.001 0.4 — Nadtochiy et al.72)

2019 SLQDs SiFe 5 −180 840 2721 0.165 Sakane et al.77)

2018 NC-columnar uc-Si: H — −136 112.5 208 — Acosta et al.62)

2018 NC Si 1.5 −1450 15 3258 0.64 Banerjee et al.45)

2018 NC Si40Ge60 — 140 100 190 — Kusano et al.56)

2018 NC-QDs SiGeAu 0.72 — — 400 0.17 Nishino et al.63)

2018 NP SiGe 0.84 133.7 74.2 126 0.045 Xie et al.93)

2016 NC-columnar Si70Ge30 2 −150 474 1067 0.16 Lu et al.59)

2016 NC NiSi20 P:2.3 P:130 P:1111 P:1877 P:0.24 Uchida et al.50)

N:3.3 N:-220 N:222 N:1075 N:0.12016 NC Si 9.4 237 185 1039 0.033 Valalaki et al.46)

2016 NP Si80Ge20 0.55 −692.5 3.07 147 0.08 Perez-Taborda et al.90)

2016 SLQDs SiGe 3.6 220 162 784 0.065 Yamasaka et al.76)

2015 NC-columnar Si74Ge26 2.3 200 388 1552 0.2 Lu et al.60)

2015 NC NiSi20 3.6 120 990 1425 0.13 Ohishi et al.49)

2015 NWs Si 1.68 382 131 1911 0.34 Zhang et al.82)

2015 SLQDs SiGeTi 6.8 −166 400 1102 0.05 Savelli et al.73)

2014 NWs Si — 203 486 2000 — Zhuge et al.81)

2014 SL Ge/SiGe 13.4 279.5 772 6021 0.135 Samarelli et al.66)

2013 NC NiSi20 P:4.4 P:132 P:1086 P:1900 P:0.13 Uchida et al.48)

N:5.4 N:-150 N:445 N:1000 N:0.062013 NWs Si — −291 268 2280 — Curtin et al.80)

2012 NWs Si73Ge27 1.2 −110 750 908 0.22 Lee et al.79)

2010 NP Si 1.73 267 320 2280 0.4 Tang et al.36)

2008 NWs Si 1.6 237 593 3300 0.6 Hochbaum et al.78)

2006 NC-columnar Si80Ge20 2.9 221 30.9 151 0.016 Takashiri et al.58)

© 2020 The Japan Society of Applied PhysicsSA0803-12

Jpn. J. Appl. Phys. 60, SA0803 (2021) PROGRESS REVIEW

to be applied to wearable applications. For the flexibilityapplication, Choi et al.100) set prepared triangular Si NW coupleson a plastic flexible substrate, as shown in Figs. 12(b)–12(c).Generally, raw membranous TE materials are common; Nanet al.101) exhibited out-plane coil TE couples that transformedfrom in-plane thin films, as shown in Fig. 12(d). They proposeda universal solution of device integration for film-like Si-basedmaterials, which contains most of the low-dimensional Si-basedmaterials during the recent decade. Spiral leg pairs withencapsulation established matching thermal impendence betweenthe device and convectional cooling surface for increasing theproportion of heat flow through TE materials in legs instead ofparasitic heat. Significant flexibility and an optimized heat flowpath made the wearable prospect of Si-based material promising.Furthermore, they combined experiments and finite elementanalysis (FEA) to predict output power in such an integrationstrategy; the relationship between the zT value of differentmaterials and the output power in the spiral module are presentedin Fig. 13. According to the zT values in Fig. 11, the modelforecasted that microwatt-scale output power was possible insuch a module by employing corresponding materials (assumethe other type of material is matched). It means that Si-basedlow-dimensional materials have great practical potential forsupplying energy to portable wearable gadgets and WSN nodes,whose energy consumption is a magnitude of μW.102)

Undoubtedly, low-dimensional manipulation leads to pro-ductive results for suppressing k and optimizes zT to a certain

degree. However, there is still a lot of space for the advancingof TE performance due to the modest zT value in contrastwith other conventional RT TE materials.2) Introducing ahierarchical NC is important for all-scale phonon scatteringand it is also reasonable to control carrier behavior fordecoupling S and σ concurrently. Particularly, simultaneouslyregulating the PF and k for high zT is still a big challenge dueto their coupling relations. How to harmoniously combineband engineering, the convergence of electronic bands, theenergy filtering effect, and quantum confinement with low kstrategies needs novel theory and experimental identification.In addition, for the view of integration, it is difficult to

fabricate efficient devices by employing excellent materialsthat are mentioned above, mainly because the film depositionprocess depends on the substrates; most of them arecommercial single-crystalline Si wafer, whose k is too highto get an effective temperature gradient in devices. Even totransfer thin film to other carriers during integration, the costswill increase and product yields will be limited. Thus, asuitable substrate or reliable dissection techniques should beincluded in future design.In view of the inner resistance of a device that is integrated

by low-dimensional Si-based materials—in addition to thecontact resistance during electrodes preparing, an overlargeresistance in legs, which is caused by an ultra-small cross-sectional area, would be presented throughout the device, evenif intrinsic resistivity of the material is low enough. Therefore,

(a)(b)

(c)

(d)

Fig. 12. (Color online) (a) Prototype of Si NW-based TE module. Reproduced from Ref. 99. (b) and (c) Sketch and entity of flexible Si NW TE module.Reproduced from Ref. 100. (d) Cross-sectional sketch of flexible TE module converted from 2D thin film to 3D helical coil. Referred from Ref. 101.

© 2020 The Japan Society of Applied PhysicsSA0803-13

Jpn. J. Appl. Phys. 60, SA0803 (2021) PROGRESS REVIEW

increasing the effective cross-sectional area in units of legswithout restricting their substantial TE performance should beconcentrated on during material preparation.Finally, some of the advantaged contents in low-dimen-

sional architecture should be referred in bulk materials whichis already industrialized. For instance, the strategy that SiNW arrays were plasma sintered for preparing hierarchicalmorphology material,47) which is a combination of cross-dimensional technologies that would promote the general-ization of low-dimensionality to conventional bulk materials.

Acknowledgments

This work was partly supported by the National NaturalScience Foundation of China (Grant No.51772056,No.51961011, No.52061009), the Guangxi Natural ScienceFoundation of China (Grant No.2019GXNSFAA245039,No.2017GXNSFFA198015), the Open Foundation of theGuangxi Key Laboratory of Processing for Non-ferrousMetals and Featured Materials, Guangxi University (GrantNo.2020GXYSOF11), and PRESTO (Grant No.JPMJPR15R2) and CREST (Grant No. JPMJCR19Q5) fromthe JST in Japan.

ORCID iDs

Masashi Kurosawa https://orcid.org/0000-0001-9151-0820Osamu Nakatsuka https://orcid.org/0000-0002-5198-0737Lei Miao https://orcid.org/0000-0002-2281-2689

1) J. Yan, X. Liao, D. Yan, and Y. Chen, J. Microelectromech. Syst. 27, 1(2018).

2) Z. Soleimani, S. Zoras, B. Ceranic, S. Shahzad, and Y. Cui, Sustain. EnergyTechnol. Assess. 37, 100604 (2020).

3) X. Wang, A. K. K. Kyaw, C. Yin, F. Wang, Q. Zhu, T. Tang, P. I. Yee, andJ. Xu, RSC Adv. 8, 18334 (2018).

4) L. Zhang, T. Goto, I. Imae, Y. Sakurai, and Y. Harima, J. Polym. Sci., PartB: Polym. Phys. 55, 524 (2017).

5) I. H. Jung, C. T. Hong, U.-H. Lee, Y. H. Kang, K.-S. Jang, and S. Y. Cho,Sci. Rep. 7, 44704 (2017).

6) Z. Fan, P. Li, D. Du, and J. Ouyang, Adv. Energy Mater. 7, 1602116 (2017).7) S. N. Patel, A. M. Glaudell, D. Kiefer, and M. L. Chabinyc, ACS Macro

Lett. 5, 268 (2016).8) W. Zhou et al., Nat. Commun. 8, 14886 (2017).9) C. J. An, Y. H. Kang, H. Song, Y. Jeong, and S. Y. Cho, J. Mater. Chem. A

5, 15631 (2017).10) C. Cho, K. L. Wallace, P. Tzeng, J.-H. Hsu, C. Yu, and J. C. Grunlan, Adv.

Energy Mater. 6, 1502168 (2016).11) S. Duan, N. Man, J. Xu, Q. Wu, G. Liu, X. Tan, H. Shao, K. Guo, X. Yang,

and J. Jiang, J. Mater. Chem. A 7, 9241 (2019).12) R. Deng, X. Su, Z. Zheng, W. Liu, Y. Yan, Q. Zhang, V. P. Dravid,

C. Uher, M. G. Kanatzidis, and X. Tang, Sci. Adv. 4, eaar5606 (2018).13) Y. S. Wang, L. L. Huang, C. Zhu, J. Zhang, D. Li, H. X. Xin,

M. H. Danish, and X. Y. Qin, Scr. Mater. 154, 118 (2018).14) Y. S. Wang, L. L. Huang, D. Li, J. Zhang, and X. Y. Qin, J. Alloys Compd.

758, 72 (2018).15) Y. Zheng, C. Liu, L. Miao, C. Li, R. Huang, J. Gao, X. Wang, J. Chen,

Y. Zhou, and E. Nishibori, Nano Energy 59, 311 (2019).16) J. Yang and T. Caillat, MRS Bull. 31, 224 (2006).17) M. S. Dresselhaus, G. Chen, M. Y. Tang, R. G. Yang, H. Lee, D. Z. Wang,

Z. F. Ren, J.-P. Fleurial, and P. Gogna, Adv. Mater. 19, 1043 (2007).18) D. Narducci, S. Frabboni, and X. Zianni, J. Mater. Chem. C 3, 12176 (2015).19) A. Nozariasbmarz et al., Jpn. J. Appl. Phys. 56, 05DA04 (2017).20) G. Gadea, M. Pacios, Á. Morata, and A. Tarancón, J. Phys. D: Appl. Phys.

51, 423001 (2018).21) Y. Nakamura, Sci. Technol. Adv. Mater. 19, 31 (2018).22) R. He et al., J. Materiomics 5, 15 (2019).23) S. Tanusilp and K. Kurosaki, Materials 12, 1943 (2019).24) M. Nomura, J. Shiomi, T. Shiga, and R. Anufriev, Jpn. J. Appl. Phys. 57,

080101 (2018).25) T. Hori and J. Shiomi, Sci. Technol. Adv. Mater. 20, 10 (2019).26) C. J. Glassbrenner and G. A. Slack, Phys. Rev. 134, A1058 (1964).27) G. H. Zhu et al., Phys. Rev. Lett. 102, 196803 (2009).28) A. Bejan and A. D. Kraus, Heat Transfer Handbook (Wiley, New York,

2003).29) M. I. Fedorov and G. N. Isachenko, Jpn. J. Appl. Phys. 54, 07JA05 (2015).30) L. D. Hicks, T. C. Harman, and M. S. Dresselhaus, Appl. Phys. Lett. 63,

3230 (1993).31) L. D. Hicks and M. S. Dresselhaus, Phys. Rev. B 47, 16631 (1993).

Fig. 13. (Color online) Predictive relationship of output power and zT value for the TE module by FEM simulation.101)

© 2020 The Japan Society of Applied PhysicsSA0803-14

Jpn. J. Appl. Phys. 60, SA0803 (2021) PROGRESS REVIEW

32) L. D. Hicks and M. S. Dresselhaus, Phys. Rev. B 47, 12727 (1993).33) J. Mao, Z. Liu, and Z. Ren, npj Quant. Mater. 1, 16028 (2016).34) L. D. Hicks, T. C. Harman, X. Sun, and M. S. Dresselhaus, Phys. Rev. B

53, R10493 (1996).35) T. Koga, S. B. Cronin, M. S. Dresselhaus, J. L. Liu, and K. L. Wang, Appl.

Phys. Lett. 77, 1490 (2000).36) J. Tang, H.-T. Wang, D. H. Lee, M. Fardy, Z. Huo, T. P. Russell, and

P. Yang, Nano Lett. 10, 4279 (2010).37) H. Takiguchi, M. Aono, and Y. Okamoto, Jpn. J. Appl. Phys. 50, 041301

(2011).38) Y. Okamoto, H. Uchino, T. Kawahara, and J. Morimoto, Jpn. J. Appl.

Phys. 38, L945 (1999).39) H. Uchino, Y. Okamoto, T. Kawahara, and J. Morimoto, Jpn. J. Appl.

Phys. 39, 1675 (2000).40) M. Hamabe, H. Takahashi, S. Yamaguchi, T. Komine, T. Eura,

H. Okumura, Y. Okamoto, and J. Morimoto, Jpn. J. Appl. Phys. 42, 6779(2003).

41) H. Takiguchi, A. Matoba, K. Sasaki, Y. Okamoto, H. Miyazaki, andJ. Morimoto, Mater. Trans. 51, 878 (2010).

42) J. Ha, B. Jeon, C. Yoon, and G. Yoon, Appl. Phys. Lett. 113, 173901(2018).

43) A. Nozariasbmarz, F. Suarez, J. H. Dycus, M. J. Cabral, J. M. LeBeau,M. C. Öztürk, and D. Vashaee, Nano Energy 67, 104265 (2020).

44) A. George, R. Yanagisawa, R. Anufriev, J. He, N. Yoshie, N. Tsujii,Q. Guo, T. Mori, S. Volz, and M. Nomura, ACS Appl. Mater. Interfaces11, 12027 (2019).

45) D. Banerjee, Ö. Vallin, K. M. Samani, S. Majee, S.-L. Zhang, J. Liu, andZ.-B. Zhang, Nano Energy 44, 89 (2018).

46) K. Valalaki, N. Vouroutzis, and A. G. Nassiopoulou, J. Phys. D: Appl.Phys. 49, 315104 (2016).

47) M. Kashiwagi, Y. Liao, S. Ju, A. Miura, S. Konishi, T. Shiga, T. Kodama,and J. Shiomi, ACS Appl. Energy Mater. 2, 7083 (2019).

48) N. Uchida, T. Tada, Y. Ohishi, Y. Miyazaki, K. Kurosaki, andS. Yamanaka, J. Appl. Phys. 114, 134311 (2013).

49) Y. Ohishi, Y. Miyazaki, H. Muta, K. Kurosaki, S. Yamanaka, N. Uchida,and T. Tada, J. Elec Materi 44, 2074 (2015).

50) N. Uchida, Y. Ohishi, Y. Miyazaki, K. Kurosaki, S. Yamanaka, andT. Tada, Mater. Trans. 57, 1076 (2016).

51) Y. Kurosaki, S. Yabuuchi, D. Takamatsu, A. Nambu, and J. Hayakawa,Materialia 7, 100374 (2019).

52) Y. Kurosaki, S. Yabuuchi, A. Nishide, N. Fukatani, and J. Hayakawa,Appl. Phys. Lett. 113, 013904 (2018).

53) M. Lindorf, H. Rohrmann, G. Span, S. Raoux, J. Jordan-Sweet, andM. Albrecht, J. Appl. Phys. 120, 205304 (2016).

54) M. Lindorf, H. Rohrmann, G. Span, and M. Albrecht, J. Elec Materi 45,1730 (2016).

55) Z. Zamanipour, E. Salahinejad, P. Norouzzadeh, J. S. Krasinski, L. Tayebi,and D. Vashaee, J. Appl. Phys. 114, 023705 (2013).

56) K. Kusano, A. Yamamoto, M. Nakata, T. Suemasu, and K. Toko, ACSAppl. Energy Materacsaem.8b00899 (2018).

57) Y. Peng, L. Miao, J. Gao, C. Liu, M. Kurosawa, O. Nakatsuka, andS. Zaima, Sci. Rep. 9, 14342 (2019).

58) M. Takashiri, T. Borca-Tasciuc, A. Jacquot, K. Miyazaki, and G. Chen, J.Appl. Phys. 100, 054315 (2006).

59) J. Lu, R. Guo, and B. Huang, Appl. Phys. Lett. 108, 141903 (2016).60) J. Lu, R. Guo, W. Dai, and B. Huang, Nanoscale 7, 7331 (2015).61) E. Acosta, V. Smirnov, P. S. B. Szabo, J. Buckman, and N. S. Bennett, J.

Electron. Mater. 48, 2085 (2019).62) E. Acosta, N. M. Wight, V. Smirnov, J. Buckman, and N. S. Bennett, J.

Electron. Mater. 47, 3077 (2018).63) S. Nishino, S. Ekino, M. Inukai, M. Omprakash, M. Adachi, M. Kiyama,

Y. Yamamoto, and T. Takeuchi, J. Electron. Mater. 47, 3267 (2018).64) Y. Nakamura et al., Nano Energy 12, 845 (2015).65) S. Sakane, T. Ishibe, T. Taniguchi, T. Hinakawa, R. Hosoda, K. Mizuta,

M. M. Alam, K. Sawano, and Y. Nakamura, Jpn. J. Appl. Phys. 59, SFFB01(2019).

66) N. Uchida, Y. Ohishi, K. Kurosaki, S. Yamanaka, T. Tada, andT. Kanayama, MRS Proc. 1456, mrss12 (2013).

67) Y. Ohishi, K. Kurosaki, T. Suzuki, H. Muta, S. Yamanaka, N. Uchida,T. Tada, and T. Kanayama, Thin Solid Films 534, 238 (2013).

68) T. Koga, X. Sun, S. B. Cronin, and M. S. Dresselhaus, Appl. Phys. Lett. 75,2438 (1999).

69) A. Samarelli et al., Solid-State Electron. 98, 70 (2014).70) T. C. Harman, M. P. Walsh, B. E. Laforge, and G. W. Turner, J. Electron.

Mater. 34, L19 (2005).71) H.-T. Chang, C.-C. Wang, J.-C. Hsu, M.-T. Hung, P.-W. Li, and

S.-W. Lee, Appl. Phys. Lett. 102, 101902 (2013).72) A. Nadtochiy, V. Kuryliuk, V. Strelchuk, O. Korotchenkov, P.-W. Li, and

S.-W. Lee, Sci. Rep. 9, 16335 (2019).73) G. Savelli, S. Silveira Stein, G. Bernard-Granger, P. Faucherand,

L. Montès, S. Dilhaire, and G. Pernot, Nanotechnology 26, 275605 (2015).74) S. Yamasaka, Y. Nakamura, T. Ueda, S. Takeuchi, Y. Yamamoto, S. Arai,

T. Tanji, N. Tanaka, and A. Sakai, J. Electron. Mater. 44, 2015 (2015).75) S. Yamasaka, Y. Nakamura, T. Ueda, S. Takeuchi, and A. Sakai, Sci. Rep.

5, 14490 (2015).76) S. Yamasaka, K. Watanabe, S. Sakane, S. Takeuchi, A. Sakai, K. Sawano,

and Y. Nakamura, Sci. Rep. 6, 22838 (2016).77) S. Sakane, T. Ishibe, T. Taniguchi, N. Naruse, Y. Mera, T. Fujita,

M. M. Alam, K. Sawano, N. Mori, and Y. Nakamura, Mater. Today Energy13, 56 (2019).

78) A. I. Hochbaum, R. Chen, R. D. Delgado, W. Liang, E. C. Garnett,M. Najarian, A. Majumdar, and P. Yang, Nature 451, 163 (2008).

79) E. K. Lee et al., Nano Lett. 12, 2918 (2012).80) B. M. Curtin, E. A. Codecido, S. Krämer, and J. E. Bowers, Nano Lett. 13,

5503 (2013).81) F. Zhuge et al., J. Am. Chem. Soc. 136, 14100 (2014).82) T. Zhang, S. Wu, J. Xu, R. Zheng, and G. Cheng, Nano Energy 13, 433

(2015).83) A. R. Abramson, W. C. Kim, S. T. Huxtable, H. Yan, Y. Wu,

A. Majumdar, C.-L. Tien, and P. Yang, J. Microelectromech. Syst. 13, 505(2004).

84) B. X. W. Khouri and K. Fobelets, IEEE Electron Device Lett. 35, 596(2014).

85) B. M. Curtin, E. W. Fang, and J. E. Bowers, J. Electron. Mater. 41, 887(2012).

86) Y. Li, K. Buddharaju, B. C. Tinh, N. Singh, and S. J. Lee, IEEE ElectronDevice Lett. 33, 715 (2012).

87) S. Lee, K. Kim, D.-H. Kang, M. Meyyappan, and C.-K. Baek, Nano Lett.19, 747 (2019).

88) Y. Wu, R. Fan, and P. Yang, Nano Lett. 2, 83 (2002).89) G. Gesele, J. Linsmeier, V. Drach, J. Fricke, and R. Arens-Fischer, J. Phys.

D: Appl. Phys. 30, 2911 (1997).90) J. A. Perez-Taborda, M. Muñoz Rojo, J. Maiz, N. Neophytou, and

M. Martin-Gonzalez, Sci. Rep. 6, 32778 (2016).91) R. J. Martín-Palma, H. Cabrera, B. Martín-Adrados, D. Korte, E. Pérez-

Cappe, Y. Mosqueda, M. A. Frutis, and E. Danguillecourt, Mater. Res.Express 5, 015004 (2018).

92) J. Boor, D. S. Kim, X. Ao, M. Becker, N. F. Hinsche, I. Mertig, P. Zahn,and V. Schmidt, Appl. Phys. A 107, 789 (2012).

93) K. Xie, K. Mork, J. T. Held, K. A. Mkhoyan, U. Kortshagen, andM. C. Gupta, J. Appl. Phys. 123, 094301 (2018).

94) K. Xie, K. Mork, U. Kortshagen, and M. C. Gupta, AIP Advances 9,015227 (2019).

95) K. Xie and M. C. Gupta, J. Alloys Compd. 820, 153182 (2020).96) Y. Su, J. Lu, D. Villaroman, D. Li, and B. Huang, Nano Energy 48, 202

(2018).97) D. Dávila, A. Tarancón, C. Calaza, M. Salleras, M. Fernández-Regúlez,

A. S. Paulo, and L. Fonseca, J. Electron. Mater. 42, 1918 (2013).98) D. Dávila et al., J. Electron. Mater. 40, 851 (2011).99) D. Dávila, A. Tarancón, M. Fernández-Regúlez, C. Calaza, M. Salleras,

A. S. Paulo, and L. Fonseca, J. Micromech. Microeng. 21, 104007 (2011).100) J. Choi, K. Cho, and S. Kim, Adv. Energy Mater. 7, 1602138

(2017).101) K. Nan et al., Sci. Adv. 4, eaau5849 (2018).102) A. R. M. Siddique, S. Mahmud, and B. V. Heyst, Renew. Sustain. Energy

Rev. 73, 730 (2017).

© 2020 The Japan Society of Applied PhysicsSA0803-15

Jpn. J. Appl. Phys. 60, SA0803 (2021) PROGRESS REVIEW