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[Yen-Ju Wu](https://orcid.org/0000-0003-2647-3407), Shih-Chieh Hsu, Ya-Cheng Lin, [Yibin Xu](https://orcid.org/0000-0001-8600-8748), Tung-Han Chuang, Sheng-Chi Chen

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[Study on thermoelectric property optimization of mixed-phase bismuth telluride thin films deposited by co-evaporation process](https://mdr.nims.go.jp/datasets/a77c0743-003c-4c21-97c5-1d51e2da9972)

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Study on thermoelectric property optimization of mixed-phase bismuth telluride thin films deposited by co-evaporation processContents lists available at ScienceDirectSurface & Coatings Technologyjournal homepage: www.elsevier.com/locate/surfcoatStudy on thermoelectric property optimization of mixed-phase bismuthtelluride thin films deposited by co-evaporation processYen-Ju Wua,1, Shih-Chieh Hsub,1, Ya-Cheng Linc, Yibin Xua, Tung-Han Chuangd,⁎,Sheng-Chi Chenc,e,⁎a Center for Materials Research by Information Integration (CMI2), Research and Services Division of Materials Data and Integrated System (MaDIS), National Institute forMaterials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki 305-0044, JapanbDepartment of Chemical and Materials Engineering, Tamkang University, New Taipei City 251, Taiwanc Department of Materials Engineering and Center for Plasma and Thin Film Technologies, Ming Chi University of Technology, Taipei 243, Taiwand Institute of Materials Science and Engineering, National Taiwan University, Taipei 106, Taiwane College of Engineering, Chang Gung University, Taoyuan 333, TaiwanA R T I C L E I N F OKeywords:Bi2Te3Thermoelectric propertiesCo-evaporationThin filmsMixed phasesA B S T R A C TThe development of Bi2Te3 thin films has huge potential in the pursuit of efficient thermoelectric micro/na-nodevices due to their high Seebeck coefficient, high electrical conductivity and low thermal conductivity. Theoptimization of experimental parameters of Bi-Te thin films produced by co-evaporation will be investigated inthis study. Co-evaporation is a low cost, easy-to-control process which can be used for high throughput and isscalable. We found that an optimal Te/Bi ratio of 1.5 with good thermoelectric properties can be directly syn-thesized by Bi and Bi2Te3 co-evaporation. Compared to the conventional Bi/Te co-evaporation process, hightemperature annealing or substrate heating is not necessary for the process mentioned in this paper, which is adesirable feature when using polymer-based substrates, organic/inorganic hybrid thermoelectric generators, andflexible devices since they have relatively low tolerance to heat. The optimized Bi2Te3 thin films, which aremixed phases of Bi2Te3, Bi3Te4 and Te, possess high carrier concentration (6.65× 1020 cm−3), low electricalresistivity (3.17×10−3 Ωcm), and extremely low thermal conductivity (0.59W/mK) at room temperature on asmooth surface (roughness <5.5 nm) and are achieved by adjusting the deposition rate of Bi and Bi2Te3. Thecorrelation between the structures of mixed phases, electrical and thermal properties will be discussed in detail.1. IntroductionThermoelectric materials require a high figure-of-merit ZT[ZT=(S2σ/κ)T] for high efficiency and performance; i.e. the materialsneed to exhibit high Seebeck coefficient (S), high electrical conductivity(σ) and low thermal conductivity (κ). Bi2Te3, a narrow-gap semi-conductor of 0.15 eV with a rhombohedral unit cell of space group R3̅m[1], is a well-established thermoelectric material because of its goodthermoelectric properties (ZT≈ 1). The unit cell is composed of fiveatomic layers of Te1-Bi1-Te2-Bi1-Te1, which stacked by Van der Waalsalong the c-axis [1,2]. This structural characterization can effectivelyhinder the phonon transport in the materials.Thin films technology is desirable in the production of efficient in-tegrated thermoelectric devices especially for thermoelectric micro-generation. In order to scale the bulk of thermoelectric devices down tothe nano/micron-size scale, several deposition techniques have beenreported in literature for the fabrication of Bi2Te3 thin films; for ex-ample, metal-organic chemical vapor deposition [3–5], co-sputtering[6–8], evaporation [9,10] and electrochemical deposition [11]. Tanet al. reported the improvement of thermoelectric properties of n-Bi2Se0.5Te2.5 and p-(Bi0.5Sb0.5)2Te3 pillar array films produced by ion-beam-assisted sputtering [12,13], They used this method to fabricate n-type Bi2Te3/ZrB2 superlattice films in order to further enhance the ZT[14]. Pires et al. also prepared Bi-Te thin films by adjusting the ionbeam sputtering and analyzing the beam voltage impact on the ther-moelectric performance [15]. Zhou et al. deposited and optimizedthermoelectric properties of the n-type Pb-doped Bi2Te3 thin films byradio frequency magnetron sputtering [16]. Lal et al. reported on theoptimization of annealing conditions for p-type BiSbTe thin films usingpulse electrodeposition [11]. Sudarshan et al. noted the change fromhttps://doi.org/10.1016/j.surfcoat.2020.125694Received 9 January 2020; Received in revised form 27 February 2020; Accepted 25 March 2020⁎ Corresponding authors.E-mail addresses: tunghan@ntu.edu.tw (T.-H. Chuang), chensc@mail.mcut.edu.tw (S.-C. Chen).1 These authors contributed equally to this work.Surface & Coatings Technology 394 (2020) 125694Available online 26 March 20200257-8972/ © 2020 Published by Elsevier B.V.Thttp://www.sciencedirect.com/science/journal/02578972https://www.elsevier.com/locate/surfcoathttps://doi.org/10.1016/j.surfcoat.2020.125694https://doi.org/10.1016/j.surfcoat.2020.125694mailto:tunghan@ntu.edu.twmailto:chensc@mail.mcut.edu.twhttps://doi.org/10.1016/j.surfcoat.2020.125694http://crossmark.crossref.org/dialog/?doi=10.1016/j.surfcoat.2020.125694&domain=pdfsemiconductor to metallic behavior of e-beam evaporated Bi2Te3 thinfilms through vacuum annealing [17]. Goncalves et al. deposited Bi2Te3thin films by the thermal co-evaporation of Te and Bi targets [9]. Thecontributions on the control of the compositions and phases are re-ported as well via various techniques. Nuthongkum et al. investigatedthe structures effect in BixTey by changing the sputtering pressures viaRF magnetron sputtering [18]. Concepcion et al. controlled the epi-taxial growth of various Bi-Te phases by tuning the Nitrogen gas flowvia physical vapor transport [2]. The temperature and the Bi/Te ratioare used to control various Bi-Te systems via metal-organic vapor-phaseepitaxy by Kuznetsov et al. [19] Attila et al. reported a phase transitionfrom Bi4Te3 to Bi2Te3 by tuning the Bi/Te ratio of molecular beamepitaxy [20]. Detailed analyses of thermoelectric thin-films can befound elsewhere [21–23].In the present work the thermal evaporation method was chosendue to its scalability, relative simplicity and ease of control. We foundthe direct evaporation of a single Bi2Te3 target results in thin films thatare predominantly Te instead of the desired Bi2Te3 thin films due to thelarge difference in the vapor pressures of Bi and Te. Goncalves et al. alsoreported similar results of a compositional gradient through the thick-ness of films produced by single-target evaporation [9]. Therefore, weused Bi2Te3 and Bi as the two targets in the film preparation using co-evaporation. From the literatures of Bi2Te3 thin film fabrication in-cluding the abovementioned methods, high-temperature heat treatmentduring deposition or annealing is necessary to achieve good thermo-electric properties. To the best of our knowledge, little attention hasbeen paid to low-temperature fabrication, which is important forwearable or flexible thermoelectric devices. These devices usually havelow tolerance to high temperatures. We were able to produce Bi2Te3thin films possessing good thermoelectric properties through co-eva-poration of Bi and Bi2Te3, with no further heat treatment necessary.This simple and easy procedure is attractive in the preparation of un-ique integrated thermoelectric microdevices. Moreover, the optimizedparameters as well as insights into the relationship between mixedphases and film properties was also investigated.Fig. 1. XRD of bismuth telluride thin films deposited with various Bi-deposition rate of 1.5 (blue), 2.0 (red) and 2.5 (black) Å/s. Top are thin films with 250 °Cannealing and the bottom are as-deposited thin films. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version ofthis article.)Y.-J. Wu, et al. Surface & Coatings Technology 394 (2020) 12569422. Experimental procedureThe Bi2Te3 thin films were deposited on Corning 1737F glass and Sisubstrates via co-evaporation by Bi and Bi2Te3 targets in a pure Ar at-mosphere with flow ratio of 5 sccm at ambient temperature. The sub-strates were cleaned using deionized water and acetone in an ultrasoniccleaner for 15min prior to deposition. The background pressure andworking pressure for the evaporation were <6×10−6 Torr and1.5×10−4 Torr, respectively. The rotation of the substrate holder waskept at 10 rpm. The deposition rate of Bi2Te3 was fixed at 3 Å/s and Biwas in the range of 1.5 to 2.5 Å/s. Some of the resulting films wereannealed at 250 °C in an Ar atmosphere for 1 h for the purpose ofcomparison with as-deposited thin films.The crystal structure of the various Bi2Te3 thin films was examinedby X-ray diffraction (XRD) using Cu–Kα radiation. The surfacemorphologies and roughness were analyzed by an atomic forceFig. 2. The SEM, EDS of Bi and AFM analysis are shown in three rows from top to bottom, respectively. The samples deposited via various Bi-deposition rate of 1.5 Å/s (a, d, g), 2.0 Å/s (b, e, h) and 3 Å/s (c, f, i) are listed from left to right. The scale bars of SEM and EDS are the same at the right bottom in each SEM images.Fig. 3. (a) EPMA composition analysis and (b) electrical analysis of bismuth telluride thin films deposited with various Bi-deposition rate of 1.5, 2.0 and 2.5 Å/s. Thesolid line in (b) is represented as as-deposited thin films and dashed line are represented as 250 °C-annealed thin films.Y.-J. Wu, et al. Surface & Coatings Technology 394 (2020) 1256943microscope (AFM) and Scanning Electron Microscope (SEM). The che-mical composition and Bi distribution of the Bi2Te3 thin films wasanalyzed using energy disperse spectrometry (EDS) and electron probeX-ray microanalyzer (EPMA). The thicknesses of the films were mea-sured by α-step. The electrical properties were measured by the four-point probe and Hall measurement system. For the thermal measure-ment, the Bi2Te3 thin films can directly perform as the heat transducer.The thermal conductivity along the cross-plane was characterized bythe frequency-domain thermoreflectance (FDTR) [24], where threepositions in each sample were measured at four different frequencies atroom temperature.3. Results and discussionThe XRD results of bismuth telluride thin films deposited via co-evaporation at various Bi-deposition rates are shown in Fig. 1. The re-sults from as-deposited thin films (the peak intensity was multiplied by4.5) and 250 °C-annealed thin films are shown in the bottom and top,respectively. The crystallinity of all samples was improved while thephase composition has no obvious tendency after the annealing. TheTable 1The carrier concentration, mobility, resistivity, conductivity, roughness, thermal resistance, and (cross-plane) thermal conductivity of bismuth telluride thin filmsdeposited via co-evaporation with various Bi-deposition rate. The sample 1 to 3 are as-deposited thin films and samples 3–6 are the annealed thin films.Sample Bidepositionrate (Å/s)Bi2Te3depositionrate (Å/s)Annealing (°C) Carrierconcentration(cm−3)Mobility(cm2 V−1 s−1)Resistivity(Ωcm)Conductivity(Scm−1)Roughness(nm)ThermalResistance (10−9m2K/W), ⊥Thermalconductivity(W/mK), ⊥1 1.5 3 2.36E+20 7.58 3.81E−03 2.62E+02 1.42 272.6±5.45 0.7262 2 3 6.65E+20 2.05 3.17E−03 3.15E+02 2.14 471.3±14.65 0.5863 2.5 3 5.96E+20 12.17 9.69E−04 1.03E+03 5.19 337.1±7.48 1.0204 1.5 3 250 4.16E+19 50.54 3.27E−03 3.07E+02 1.42 254.8±6.29 0.7775 2 3 250 2.69E+20 2.92 8.23E−03 1.21E+02 2.14 507.1±8.93 0.5446 2.5 3 250 2.79E+19 13.06 1.72E−02 5.81E+01 5.19 257.4±7.89 1.336Fig. 4. XRD of bismuth telluride thin films with Bi-deposition rate of 2.0 Å/s of various thickness.Y.-J. Wu, et al. Surface & Coatings Technology 394 (2020) 1256944dominant XRD peak in most samples can be characterized as beingBi2Te3 phase. When the Bi-deposition rate is increased, the mixedphases of as-deposited thin films change from lower Bi/Te phase(Bi3Te4, Te; Bi2Te3, Bi3Te4), to higher Bi/Te phase (Bi2Te, Bi3Te4).Usually, the most stable mixed phases will remain after deposition. Thisindicates that the most stable mixed phases will change and are stronglyaffected by the Bi-deposition rate which provides different Bi/Te ionratios during deposition. Fig. 2 shows the SEM, EDS of Bi and AFManalyses of the films. The particle size of the film surface increases fromaround 32 nm at 1.5 Å/s to 48 nm and 64 nm as the Bi-deposition rateincreases to 2.0 and 2.5 Å/s respectively, as shown in Fig. 2(a), (b) and(c). The Bi distribution also increases corresponding to the increase ofthe Bi-deposition rate as shown in Fig. 2(d), (e) and (f). The filmroughness is <5.5 nm in all samples, as characterized by AFM. Theroughness of the three samples are 1.42 nm, 2.14 nm, and 5.19 nm,respectively, as shown in Fig. 2(g), (h) and (i).The Bi/Te atomic ratio increases upon increasing the Bi-depositionrate as shown in Fig. 3(a). The sample deposited at a Bi-deposition rateof 2.0 Å/s has an atomic ratio close to Bi2Te3. However, the crystallinestructures are mixed phases composed of Bi2Te3, Bi3Te4, and Te asshown in Fig. 1. This implies that the structures of deposited thin filmsdo not only rely on the atomic ratio of Bi/Te but are also dependentupon the morphology, especially for the mixed phase thin films. Fromthe carrier concentration, mobility and resistivity results in Fig. 3(b),we can see the as-deposited thin films have similar or even lowerelectrical resistivity than the annealed thin films. Hall measurementshows all samples possess n-type conduction. The carrier concentrationdecreases significantly while the mobility does not change as much forthe samples of 2.0 Å/s and 2.5 Å/s after annealing. The conductivity (σ)can be calculated using σ= 1/R=neμe, where the σ is electrical con-ductivity, R is electrical resistivity, n is carrier concentration, and μe iselectrical mobility. The electrical conductivity of the as-deposited filmsincreases as the Bi-deposition rate rises, which correlates to the increasein particle size (as shown in the SEM results in Fig. 2) and the com-position change. Nevertheless, the electrical conductivity does not im-prove after annealing, which could be attributable to the restriction ofgrain growth by the mixed phases. The calculated grain size from thedominant peak of XRD is around 12.6–13.7 nm in the as-depositedfilms, and 12.4–16.6 nm in the annealed films, in reasonable agreementwith the restriction of grain size after annealing.The details of measured electrical and thermal properties of thebismuth telluride thin films deposited at various Bi-deposition rates arelisted in Table 1. The results indicate that as-deposited films canachieve similar thermoelectric properties compared with the annealedfilms. The thin film with a Bi-deposition rate of 2.0 Å/s has the lowestcross-plane thermal conductivity of 0.586W/mK, which is quite lowcompared to other Bi-Te based thin films [12–14,16,25,26]. Moreover,the thermal conductivity changes least after annealing which might beattributable to the limitation of grain growth by the multi-phases co-existing in the films. The as-deposited thin film (sample 2) with a Bi-deposition rate of 2.0 Å/s is the optimal sample due to its very lowthermal conductivity and good electrical conductivity. Our results showthat good thermoelectric properties can be achieved without the needfor annealing or substrate heating via Bi and Bi2Te3 co-evaporation.This film is composed of mixed phases of Bi2Te3, Bi3Te4, and Te, whichindicates that phonon transport can be effectively hindered by the in-terfaces of the various phases. In this study, the Bi2Te3, Bi3Te4 and Tecan be tuned through the deposition rates of Bi and Bi2Te3: As the Bideposition rate rises, the Bi2Te and Bi3Te4 phases increase; As the Bideposition rate drops, the Te and Bi2Te3 phases increase. Goto et al. alsoproposed a low thermal conductivity at around 0.5W/mK in Bi-Te filmswhich are composed of multi-phases (Bi4Te3, Bi2Te and Bi3Te7) bycombinatorial sputter coating technology [27], in reasonable agree-ment with our results. The phonon scattering at the interfaces betweendissimilar phases including the crystallographic mis-orientation, dif-ferential acoustic scattering and the grain size effect on thermal con-ductivity in polycrystalline materials contribute to the low thermalconductivity [28]. The discussion on thermal conductivity of multi-phase materials that incorporates equation derivation can be foundelsewhere [29,30].In order to understand the effects of the film thickness on thestructure and transport properties, films with a Bi-deposition rate of2.0 Å/s were deposited into various thicknesses from 32.5 nm to276 nm for XRD analysis as shown in Fig. 4. With the exception of the32.5 nm film, the dominant XRD peak of all other films can be char-acterized as Bi2Te3 phase. The thinnest film (32.5 nm) also has lowcrystallinity while the other films have clear crystallinity peaks. Uponincreasing the film thickness, the mixed phases change from Bi2Te3, Te,Bi phases to Bi2Te3, Bi3Te4, Te phases. When the thickness rises above110 nm, the composition of the mixed phases does not change. Themeasured electrical properties of these bismuth telluride films versusvarious thicknesses can be seen in Fig. 5. The carrier concentration hasan inflection point at a thickness of 100 nm and mobility and resistivityshow no significant change (2 to 7 cm2 V−1 s−1; 0.001 to 0.01 Ωcm)Fig. 5. Electrical analysis of bismuth telluride thin films with Bi-deposition rateof 2.0 Å/s versus various thickness.Fig. 6. Comparison of the ratio of electrical conductivity and thermal con-ductivity (σ/κ) with various reported Bi-Te based materials, Pb-dope Bi2Te3[16], Ge0.93Bi0.07Te1.005I0.03 [26], n-type Bi2Te3 [25], n-Bi2Se0.5Te2.5 [12],(Bi0.5Sb0.5)2Te3 [13], Bi2Te3/ZrB2 [14], and our bismuth telluride thin filmsdeposited with various Bi-deposition rate of 1.5, 2.0 and 2.5 Å/s.Y.-J. Wu, et al. Surface & Coatings Technology 394 (2020) 1256945above 100 nm. The best electrical conductivity of 1.03× 103 Scm−1 isachieved at 118 nm. This indicates that the optimal thin films posses-sing low thermal conductivity and good electrical conductivity can beachieved at a Bi2Te3-deposition rate of 3.0 Å/s and Bi-deposition rate of2.0 Å/s with a thickness above 100 nm.The ratio of electrical conductivity to thermal conductivity (σ/κ) isan important factor in evaluating the thermoelectric performance. Theσ/κ ratio of various reported Bi-Te based materials are compared withour as-deposited films in Fig. 6. Without annealing treatment, the re-ported σ/κ values are quite low at around 8.6×103 KV−2. However,the σ/κ of our as-deposited films is the highest, even higher than thereported annealed thin films such as Bi2Te3/ZrB2 superlattice thin films[14]. We have developed a simple and annealing-free method to pro-duce mixed-phase bismuth telluride thin films for thermoelectric ap-plications. This room temperature annealing-free process is beneficialand desirable for wearable devices which usually have an inability tohandle high temperatures.4. ConclusionIntegrated flexible thermoelectric microdevices usually have lowmelting point and low tolerance to heat. We developed a low cost,annealing-free co-evaporation method for producing bismuth telluridethin films using Bi and Bi2Te3 targets. Compared to the usual Bi/Te co-evaporation methods, which require heat treatment during depositionor further annealing, the proposed method in the present study isbeneficial for wearable thermoelectric microdevices. We optimized thethermoelectric properties of the bismuth telluride thin films by ad-justing the Bi and Bi2Te3 deposition rates. We found that the Te/Biratio, thickness, and composition of mixed phases all affect the ther-moelectric properties. The mixed phases of the optimized films arecomposed of Bi2Te3, Bi3Te4, and Te and have Te/Bi atomic ratio of 1.5.The interfaces of various phases in the thin films can effectively hinderphonon transport and contribute to the low thermal conductivity. Theoptimized films, which are achieved at a Bi2Te3-deposition rate of3.0 Å/s and Bi-deposition rate of 2.0 Å/s with a thickness above100 nm, possess smooth surface of roughness <5.5 nm, extremely lowthermal conductivity of 0.59W/mK and low electrical resistivity of3.17×10−3 Ωcm.CRediT authorship contribution statementYen-Ju Wu: Investigation. Shih-Chieh Hsu: Formal analysis. Ya-Cheng Lin: Investigation. Yibin Xu: Resources. Tung-Han Chuang:Conceptualization, Methodology. Sheng-Chi Chen: Conceptualization,Methodology.Declaration of competing interestThe authors declare that they have no known competing financialinterests or personal relationships that could have appeared to influ-ence the work reported in this paper.AcknowledgmentWe gratefully acknowledge the “Materials research by InformationIntegration” Initiative (MI2I) project of the Support Program forStarting Up Innovation Hub from Japan Science and TechnologyAgency (JST) and the Ministry of Science and Technology of Taiwan(No. 107-2221-E-131-036). We also thank Prof. H. C. Lin and Mr. C. Y.Kao of the Instrumentation Center, National Taiwan University for theirassistance with EPMA experiments.References[1] J.R. Sootsman, D.Y. Chung, M.G. Kanatzidis, New and old concepts in thermoelectricmaterials, Angew. Chem. Int. Ed. 48 (2009) 8616–8639.[2] O. Concepcion, M. Galvan-Arellano, V. Torres-Costa, A. Climent-Font, D. Bahena,M. Manso Silvan, A. 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Study on thermoelectric property optimization of mixed-phase bismuth telluride thin films deposited by co-evaporation process Introduction Experimental procedure Results and discussion Conclusion CRediT authorship contribution statement Declaration of competing interest Acknowledgment References