materials Article
Synthesis and Evaluation of Thick Films of Electrochemically Deposited Bi2Te3 and Sb2Te3 Thermoelectric Materials Nguyen Huu Trung 1, *, Kei Sakamoto 2 , Nguyen Van Toan 3 and Takahito Ono 1 1 2 3
*
Graduate School of Engineering, Tohoku University, Sendai 980-8579, Japan;
[email protected] Micro/Nano-Machining Education and Research Center, Tohoku University, Sendai 890-8579, Japan;
[email protected] Microsystem Integration Center (µSIC), Tohoku University, Sendai 980-8579, Japan;
[email protected] Correspondence:
[email protected]; Tel.: +81-22-795-5806
Academic Editor: Christof Schneider Received: 18 November 2016; Accepted: 2 February 2017; Published: 10 February 2017
Abstract: This paper presents the results of the synthesis and evaluation of thick thermoelectric films that may be used for such applications as thermoelectric power generators. Two types of electrochemical deposition methods, constant and pulsed deposition with improved techniques for both N-type bismuth telluride (Bi2 Te3 ) and P-type antimony telluride (Sb2 Te3 ), are performed and compared. As a result, highly oriented Bi2 Te3 and Sb2 Te3 thick films with a bulk-like structure are successfully synthesized with high Seebeck coefficients and low electrical resistivities. Six hundred-micrometer-thick Bi2 Te3 and 500-µm-thick Sb2 Te3 films are obtained. The Seebeck coefficients for the Bi2 Te3 and Sb2 Te3 films are −150 ± 20 and 170 ± 20 µV/K, respectively. Additionally, the electrical resistivity for the Bi2 Te3 is 15 ± 5 µΩm and is 25 ± 5 µΩm for the Sb2 Te3 . The power factors of each thermoelectric material can reach 15 × 10−4 W/mK2 for Bi2 Te3 and 11.2 × 10−4 W/mK2 for Sb2 Te3 . Keywords: thermoelectric materials; electrochemical deposition; annealing effects; thick films; thermoelectric power generators
1. Introduction Among thermoelectric materials, N-type bismuth telluride (Bi2 Te3 ) and P-Type antimony telluride (Sb2 Te3 ) are attractive due to their high performances for thermoelectric power generation applications near room temperature. Thick films of thermoelectric material with a high Seebeck coefficient and low electrical resistivity are highly desired to fabricate high performance micro power generator devices. There are many methods aimed at synthesizing these materials [1–4]. Electrochemical deposition is one of potential methods for thick film deposition [3]. In this method, it is reported that optimized techniques can enable the synthesis of materials with high quality morphology and compactness. One of them is a pulsed deposition method. The advantage of pulsed deposition is first demonstrated in Reference [5]. According to this reference, thick and compact Bi2 Te3 films are deposited at a rate of 50 µm/h using electrolytes with high concentrations of 80 mM Bi3+ and 90 mM Te2− . However, the Seebeck coefficients of synthesized films are very low (~−40 µV/K). Another option is an addition of non-aqueous electrolytes that leads to high ion solubilities. Many non-aqueous additives have been researched for Bi2 Te3 : ethylene glycol [6,7] dimethyl sulfoxide [8,9], ethanol [10], ionic liquids (choline chloride) [11,12], molten salt (AlCl3 –NaCl–KCl) [13], and polyvinyl alcohol [14]. Additionally, the appearance of the Bi2 Te3 soluble anode is proven to enhance a homogeneous composition of deposited films in References [14–16]. In this work, another possibility of a deposition
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of thick and stable thermoelectric films has been demonstrated. The electrolytes are used with a low concentration of cations and ions, resulting in a controlled low deposition rate. The concentrations of Bi3+ and Te2− are 4 mM and 3.6 mM, respectively. Therefore, the amorphous material is easily Materials 2017, 10, 154 17 crystallized during the pulsed deposition. By this method, without the necessity of using2 ofnon-aqueous additives used and with a soluble anode, theofSeebeck coefficients ofinsynthesized Bideposition films 2 Te3 thick a low concentration cations and ions, resulting a controlled low rate. The are more 3+ 2− improved concentrations than that of using high [5], Therefore, and the same as usingmaterial a soluble anode of Bi and Te concentration are 4 mM and 3.6electrolytes mM, respectively. the amorphous is easily crystallized during thethis pulsed deposition.is Bysuccessfully this method, without necessity of using (~−80 µV/K) [15,16]. Additionally, mechanism appliedthefor not only N-type Bi2 Te3 non-aqueous additives and a soluble anode, the Seebeck coefficients of synthesized Bi2Te3 thick films but also P-type Sb2 Te3 thick films. 2.
are more improved than that of using high concentration electrolytes [5], and the same as using a soluble anode (~−80 μV/K) [15,16]. Additionally, this mechanism is successfully applied for not only Experimental N-type Bi2Te3 but also P-type Sb2Te3 thick films.
2.1. Sample2.Preparation Experimental The sample preparation 2.1. Sample Preparation process begins from a 300-µm-thick silicon substrate. Cr-Au films with thicknesses ofThe 10 sample nm and 150 nm, respectively, depositedsilicon on a substrate. silicon substrate preparation process begins fromare a 300-μm-thick Cr-Au filmsby withsputtering. A three-electrode deposition method is used follows: the by 300-µm-thick silicon thicknesseselectrochemical of 10 nm and 150 nm, respectively, are deposited on as a silicon substrate sputtering. A three-electrode is used as follows: the 300-μm-thickmesh siliconis used as substrate with a Cr/Au electrochemical seed layer isdeposition used as method the working electrode, a platinum substrate with a Cr/Au seed layer is used as the working electrode, a platinum mesh is used as a a counter electrode, and Ag/AgCl with a 3 M KCl (Potassium Chloride) solution is used as a reference counter electrode, and Ag/AgCl with a 3 M KCl (Potassium Chloride) solution is used as a reference electrode. electrode. A schematic of deposition system inFigure Figure A schematic of deposition systemisisshown shown in 1. 1.
Figure 1. Electrochemical deposition system.
Figure 1. Electrochemical deposition system. 2.2. Electrochemistry
2.2. Electrochemistry Electrochemical deposition of the BiTe is performed and compared using constant and pulsed waveform methods at room temperature with slow stirring (60 rpm). According to previous
Electrochemical deposition of the BiTe is performed and compared using constant and pulsed discussion, the usage of electrolytes with high concentrations of ions and cations has the advantage waveform in methods at room temperature slowencounters stirring (60 previous high deposition rate. However, the with deposition the rpm). obstacleAccording to grow highto quality crystaldiscussion, Therefore, a low concentration electrolyte has been used to mitigate this negative behavior. The in high the usage films. of electrolytes with high concentrations of ions and cations has the advantage 3+, 3.6 mM HTeO2+, and 1 M HNO3. The solution is prepared electrolyte solution consists of 4 mM Bi deposition rate. However, the deposition encounters the obstacle to grow high quality crystal from the following steps. At first, both Bi2O3 and TeO2, are dissolved in nitric acid. Deionized (DI) films. Therefore, a low concentration electrolyte has been used to mitigate this negative behavior. water is added to both solutions to obtain a 1 M concentration (1 mol/L) of nitric acid at a pH = 0. 3+ 2+ , and 1 M HNO . The solution is The electrolyte solution consists of together. 4 mM Bi mM HTeO Then, both solutions are mixed Using, 3.6 this method, all the oxide components of3Bi and Te are completely dissolved in the Nitric acid is used because can dissolve oxideDeionized prepared from the following steps. Atelectrolyte. first, both Bi2 O TeO dissolved inbismuth nitric acid. 3 and 2 , are it tellurium that H+ acts a working NO3− acts as a(1 counter ion. of Meanwhile, SbTe (DI) waterand is added tooxide bothsosolutions toasobtain a 1ion M and concentration mol/L) nitric acid at a pH = 0. thick films are grown by pulsed electrochemical deposition at room temperature and a stirring speed Then, bothofsolutions are mixed together. Using this method, all the oxide components of Bi and Te are 60 rpm. The electrolyte solution consists of 6 mM Sb3+, 3.6 mM HTeO2+, 0.5 M C4H6O4·5H2O, and completely1 M dissolved thecomponent electrolyte. Nitric used acid. because it can dissolve HNO3. Thein oxide of Sb is inertacid in theisnitride Therefore, tartaric acid isbismuth employedoxide and 3− acts as a counter ion. Meanwhile, SbTe thick to dissolve the Sb 3 and as results in SbO+ and 4H4O 6)2−. tellurium oxide so that H2+Oacts a working ion(C and NO films are grown by pulsed electrochemical deposition at room temperature and a stirring speed of 60 rpm. The electrolyte solution consists of 6 mM Sb3+ , 3.6 mM HTeO2+ , 0.5 M C4 H6 O4 ·5H2 O, and 1 M HNO3. The oxide component of Sb is inert in the nitride acid. Therefore, tartaric acid is employed to dissolve the Sb2 O3 and results in SbO+ and (C4 H4 O6 )2− .
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2.3. Characterization Because a material property evaluation needs to be conducted on an insulating substrate to avoid short circuiting, the synthesized films are peeled from the substrate by epoxy resin and mounted on a glass substrate [17]. The in-plane Seebeck coefficient is measured at room temperature between two points of the film. The temperatures are observed by 50 µm diameter K-type thermocouples. ◦ C to ensure Multiple measurements Materials 2017, 10, 154 are carried out at many temperature differences from 4 to 8 3 of 17 accurate results. The Seebeck coefficient, obtained from the generated voltage as a result of given 2.3. Characterization temperature gradient, is measured for these samples. The electrical resistivity is measured using a four Because a material property evaluation needs to be conducted on anMicroscopy insulating substrate terminal method. Synthesized films are imaged by Scanning Electron (SEM)toequipped avoid short circuiting, the synthesized films are peeled from the substrate by epoxy resin and with Energy Dispersive X-ray spectroscopy (EDX), which is used for composition analysis. These mounted on a glass substrate [17]. The in-plane Seebeck coefficient is measured at room temperature measurements aretwo performed FieldThe Emission Gun are Scanning Electron Microscope Hitachi SU-70 between points of on theafilm. temperatures observed by 50 μm diameter K-type thermocouples. measurements are carried out atof many from 4 to with an (Hitachi, Tokyo, Japan).Multiple X-ray diffraction (XRD) patterns the temperature deposited differences films are recorded 8 °C to ensure accurateD8 results. The Seebeck coefficient, obtained from the generated voltage as kV, a 40 mA, X-ray Diffractometer Bruker(Billerica, MA, USA) using Cu α radiation (λ = 1.5418 Å, 40 result of given temperature gradient, is measured for these samples. The electrical resistivity is ◦ step size 0.02 , 2 s/step, and with a sample rotation 60 rpm). The nanostructures of the sample are measured using a four terminal method. Synthesized films are imaged by Scanning Electron evaluated Microscopy by High Resolution Transmission Microscopy (HRTEM) and Selected Area (SEM) equipped with EnergyElectron Dispersive X-ray spectroscopy (EDX), which is used for Electron Diffractioncomposition (SAED) onanalysis. a JEOL-2100F instrument are (JEOL, Tokyo,onJapan). cross-sectional preparation These measurements performed a FieldThe Emission Gun Scanning Electron Hitachi SU-70 (Hitachi, Tokyo, Japan). X-rayfollowed diffraction by (XRD) of themilling to is performed by aMicroscope mechanical thinning and dimpling method, Ar+patterns ion beam deposited films are recorded with an X-ray Diffractometer Bruker- D8 (Billerica, MA, USA) using Cu make the area transparent to electrons. α radiation (λ = 1.5418 Å, 40 kV, 40 mA, step size 0.02°, 2 s/step, and with a sample rotation 60 rpm). The nanostructures of the sample are evaluated by High Resolution Transmission Electron 3. ResultsMicroscopy and Discussion (HRTEM) and Selected Area Electron Diffraction (SAED) on a JEOL-2100F instrument (JEOL, Tokyo, Japan). The cross-sectional preparation is performed by a mechanical thinning and 3.1. Synthesis of N-Type Bismuth dimpling method, followedTelluride by Ar+ ion beam milling to make the area transparent to electrons.
3.1.1. Voltammetry 3. Results and Discussion The reaction equation for BiTe Telluride is [18]: 3.1. Synthesis of N-Type Bismuth 3.1.1. Voltammetry13H+
+ 18e− + 2BiO+ + 3HTeO2 + ↔ Bi2 Te3 + 8H2 O.
(1)
The reaction equation for BiTe is [18]:
The cyclic voltammetry (CV) presented in Figure 2 is recorded between −1 V and 1 V(1)with a scan 13H+ + 18e− + 2BiO+ + 3HTeO2+ ↔ Bi2Te3 + 8H2O. speed of 10 mV/s. The literature states that, during the deposition process, the first main reduction The cyclic voltammetry (CV) presented in Figure 2 is recorded between −1 V and 1 V with a scan relates to the formation of BiTe films [19,20]. In this work, the first reduction ranges between −0.1 V speed of 10 mV/s. The literature states that, during the deposition process, the first main reduction and 0.1 V. relates During backward three oxidation O1, O2, and O3, appear at 300 to the the formation of scan, BiTe films [19,20]. In this peaks, work, the first reduction ranges between −0.1mV, V 600 mV andrespectively. 0.1 V. During the backward scan, three oxidation peaks,process O1, O2, and appear at the 300 mV, and 800 mV, Peak O3 belongs to the depriving of BiO3, and Te on gold surface. mV andrepresent 800 mV, respectively. Peak O3 to the depriving process Bi and Te on thethe goldCV curve, Peaks O1 600 and O2 an oxidation of belongs the residual elemental Bi of [19,20]. From surface. Peaks O1 and O2 represent an oxidation of the residual elemental Bi [19,20]. From the CV the appropriate potentials for growing BiTe films are determined to be among the first reduction peak curve, the appropriate potentials for growing BiTe films are determined to be among the first occurring reduction at approximately 20 mV. peak occurring at approximately 20 mV.
Figure 2. Cyclic voltammetry for BiTe deposition.
Figure 2. Cyclic voltammetry for BiTe deposition.
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3.1.2. Constant Deposition constant deposition is used to synthesize thick Bi2Te3 films. From the CV analysis, some 3.1.2.First, Constant Deposition Materials 2017, 10, 154 4 of 17 deposition potentials in the first reduction range are examined to evaluate the potential dependence First, constant deposition is used to synthesize Bi2 Te3 films. FromBiTe thestoichiometric CV analysis, on the atomic composition, as shown in Figure 3. Duringthick the deposition process, 3.1.2. Constant Deposition some deposition potentials in the first reduction range are examined to evaluate the potential formation is obtained from the balance of potential-dependent chemical kinetics for the deposition of First, constant deposition is used to synthesize thick Bi 2Te3 films. From the CV analysis, some dependence atomic [19]. composition, as shown in Figure 3. Duringatthe process, both bismuthon andthe tellurium That balanced stoichiometry is obtained the deposition potential where the deposition potentials in the first reduction range are examined to evaluate the potential dependence BiTe is obtained from the is balance of potential-dependent chemical kinetics first stoichiometric reduction peakformation of the electrolyte curve observed, as shown inBiTe Figure 2. Because the on the atomic composition, as shown CV in Figure 3. During the deposition process, stoichiometric for the deposition of both bismuth and tellurium [19]. That balanced stoichiometry is obtained at reduction potential of tellurium is balance greaterofthan that of bismuth, the atomic of tellurium formation is obtained from the potential-dependent chemical kineticscomposition for the deposition of the potential where reduction peakbalanced of the electrolyte CViscurve is observed, asthan shown Figure 2. both bismuththe andfirst tellurium That stoichiometry obtained atis thelarger potential where is more advantageous than that [19]. of bismuth when the applied potential theinthe balanced Because the reduction potential of tellurium is greater than that of bismuth, the atomic composition first reduction peak of the electrolyte CV curve is observed, as shown in Figure 2. Because the stoichiometry. In contrast, if the potential is smaller than that of balanced stoichiometry, more reduction potential of tellurium is greater than that of bismuth, thethe atomic composition of tellurium of tellurium is more advantageous that of bismuth when applied potential is larger to than bismuth atoms will be deposited, asthan shown in Figure 3. From the results, 20 mV is determined be is more advantageous than that of bismuth when the applied potential is larger than the balanced the balanced stoichiometry. In contrast, if the potential is smaller than that of balanced stoichiometry, the moststoichiometry. balanced stoichiometry potential to electrochemically deposit Bi2Te3. At −40 mV, bismuth In contrast, if the potential is smaller than that of balanced stoichiometry, more more bismuth atoms will be deposited, as shown inBi Figure 3. From the results, 20 mV is determined telluride is synthesized in the form ofasBishown 2.3Te2.7in , and 1.8 3.2 appears at 60 mV.is determined bismuth atoms will be deposited, Figure 3.Te From the results, 20 mV to be to be thethemost stoichiometry potential to electrochemically BimV, . At −40 mV, 2 Te3bismuth most balanced balanced stoichiometry potential to electrochemically deposit Bi2deposit Te3. At −40 bismuthtelluride telluride synthesized the of form of2.7Bi , and Bi1.8 Teat3.2 at 60 mV. is is synthesized in theinform Bi2.3Te , and Bi2.7 1.8Te 3.2 appears 60appears mV. 2.3 Te
3. Potential dependence on the atomic composition of BiTe. FigureFigure 3. Potential Potential dependence on on the the atomic Figure 3. dependence atomic composition composition of of BiTe. BiTe.
The deposition potential not only changes the atomic composition of the material but also has a
Thesignificant deposition potential not only only changesFigure theatomic atomic composition thematerial material butalso alsohas hasa effect on the crystal morphology. 4 showscomposition the depositionofof potential dependence of potential not changes the the but the effect surface morphology of the film. The sample synthesized with a potential of −40 mV exhibits a asignificant significant effect on the crystal morphology. Figure 4 shows the deposition potential dependence on the crystal morphology. Figure 4 shows the deposition potential dependence of standing plate-like shape with large grains of approximately 4with μm. When the deposition potential of surface morphology film. The sample synthesized with potential −40 mVexhibits exhibitsa thethe surface morphology of of thethe film. The sample synthesized a apotential ofof−40 mV increases to 20 mV, the grain size decreases (~1 μm) and changes to a granular structure for a astanding standingplate-like plate-likeshape shapewith withlarge largegrains grainsof ofapproximately approximately 44 μm. µm. When When the deposition potential deposition potential of 60 mV. An XRD measurement is also performed to analyze the effect of the increases toto2020mV, thethe grain size size decreases (~1 µm) changes to a granular structure for a deposition increasesdeposition mV, grain decreases (~1and μm) and changes to a granular structure for a potential on the crystal growth orientation. potential 60 mV. An measurement is also performed analyze the effect ofthe theeffect deposition depositionofpotential of XRD 60 mV. An XRD measurement is also to performed to analyze of the potential onpotential the crystal orientation. deposition ongrowth the crystal growth orientation.
(a) Figure 4. Cont.
(a) Figure 4. Cont. Figure 4. Cont.
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(b) (b)
(c) (c) Figure 4. Surface Surface morphology of of the BiTe BiTe deposition potentials: potentials: (a) (a) −40 −40 mV; (b) (b) 20 mV; mV; and and (c) (c) 60 mV. Figure Figure 4. 4. Surfacemorphology morphology ofthe the BiTedeposition deposition potentials: (a) −mV; 40 mV; 20 (b) 20 mV; and60(c)mV. 60 mV.
Figure 55 shows shows the the XRD XRD patterns patterns of of the the deposited deposited bismuth bismuth telluride telluride films, films, which which exhibit exhibit aa Figure Figure 5 shows the XRD patterns of(1010) the deposited bismuth telluride films, which exhibit a polycrystalline polycrystalline structure with (110), and (015) as the prominent diffracted peaks. However, the polycrystalline structure with (110), (1010) and (015) as the prominent diffracted peaks. However, the structure with (110), (1010) and (015) as the prominent diffracted peaks. However, the intensity ratios of intensity ratios of the peaks are not similar for each deposition potential, indicating an effect on the intensity ratios of the peaks are not similar for each deposition potential, indicating an effect on the the peaks are not similar for each deposition potential, indicating an effect on the growth orientation. growth orientation. orientation. growth
Figure 5.5.X-ray X-ray diffraction XRD)pattern patternof ofthe theBiTe BiTefilms filmsfor fordifferent differentdeposited depositedpotentials. potentials. Figure ((XRD) pattern of the BiTe films for different deposited potentials. Figure5. X-raydiffraction diffraction (XRD)
To research research the the dependence dependence of of the the metallic metallic seed layer layer on the the lattice matching matching of the the initial layer, layer, To To research the dependence of the metallic seed seed layer on on the lattice lattice matching of of the initial initial layer, the material material film is is separated from from the AuAu- film interface interface using epoxy epoxy resin [17], [17], as shown shown in Figure Figure 6a. the the material film film is separated separated from the the Au- film film interface using using epoxy resin resin [17], as as shown in in Figure 6a. 6a. Subsequently, the sample is mounted on the glass wafer to perform measurements on the initial layer Subsequently, the onon thethe glass wafer to perform measurements on the layer Subsequently, the sample sampleisismounted mounted glass wafer to perform measurements oninitial the initial of the the film. film. Figure Figure 6b 6b shows the the SEM SEM image image of of the the interface obtained obtained by above above separation. of layer of the film. Figure shows 6b shows the SEM image of theinterface interface obtainedby by above separation. separation. Additionally, the the XRD XRD patterns patterns illustrate illustrate the the difference difference as as the the diffracted diffracted peak peak of of (103) (103) in in the the Additionally, Additionally, the XRD patterns illustrate the difference as the diffracted peak of (103) in the subsequent subsequent thick film is disappeared and a new peak of (101) is found in the initial matching layer, subsequent thick film is disappeared and a new peak of (101) is found in the initial matching layer, as shown shown in in Figure Figure 6c. 6c. It It can can be be concluded concluded that, that, although although the the growth growth orientation orientation is is slightly slightly altered altered as the crystal crystal structure structure is is unchanged. unchanged. the
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thick film is disappeared and a new peak of (101) is found in the initial matching layer, as shown in Figure 6c. It can be concluded that, although the growth orientation is slightly altered the crystal structure is unchanged. Materials 2017, 10, 154 6 of 17
Figure 6. The dependence of the metallic seed layer on the lattice matching of the initial monolayer: Figure 6. The dependence of the metallic seed layer on the lattice matching of the initial monolayer: (a) sample structure after being separated from the metallic seed layer with the monolayer on the top (a) sample structure after being separated from the metallic seed layer with the monolayer on the top surface; (b) surface morphology determined via Scanning Electron Microscopy (SEM) observation; surface; (b) surface morphology determined via Scanning Electron Microscopy (SEM) observation; and (c) XRD pattern of the initial monolayer. and (c) XRD pattern of the initial monolayer.
The electron diffraction pattern and the HRTEM image are shown in Figure 7a,b, respectively. electron diffraction pattern anddiffraction the HRTEM imagethat areoverlap shown along in Figure 7a,b, respectively. TheThe SAED observation illustrates many patterns the concentric circles, The SAED observation illustrates many diffraction patterns that overlap along the concentric circles, indicating grain texturing. It is concluded that the sample consists of polycrystalline nanograins. indicating grain texturing. It is concluded that the sample consists of polycrystalline nanograins. When the deposition is performed for a long time, the oxidation of Bi or the over-deposition of Te When performed for a long thestressed oxidation Bi or the over-deposition ofbecome Te leads leadsthe to adeposition change inisthe stoichiometry andtime, causes orofporous layers. These layers tounstable a changeasinthe thelayer stoichiometry and causes stressed or porous layers. These layers become unstable as thickness increases. For this reason, only a small number of studies have been the layer thickness increases. For this reason, only a small number of studies have been successful in successful in synthesizing thick films of BiTe and SbTe [3,14–16,21]. As shown in Figure 8a, although synthesizing thick film films can of BiTe SbTeusing [3,14–16,21]. As shown in Figure although a 200-µm-thick a 200-μm-thick be and grown the constant method at a8a,high deposition rate of film can be grown using only the constant at a high deposition ratequality of approximately 30 µm/h, approximately 30 μm/h, 20 μm ofmethod the thickness is actually of good (Figure 8b). The thick only µmon ofthe thetop thickness actually of goodorquality (Figure 8b). The filmunderlying layer on the top film20 layer containsisporous structures particles that easily peel thick from the layer. contains porous structures particles that easily peel from the underlying layer. 10-µm-thick layer The 10-μm-thick layer in or Figure 8c exhibits an initial 4-μm-thick layer that has The a compact structure. inUpon Figure 8c exhibits an initial layer that has a to compact structure. deposition, further deposition, the4-µm-thick porous structure begins appear. To solveUpon this further problem, another the porous structure begins appear. To solve this problem, another method and technique musttobe considered when synthesizing a thickmethod film. and technique must be considered when synthesizing a thick film.
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(a) (b) (a) (b) Figure 7. (a) Selected Area Electron Diffraction (SAED) pattern; and (b) High Resolution Transmission Figure 7. (a) Selected Area Electron Diffraction (SAED)pattern; pattern;and and (b)High High Resolution Transmission Figure 7. (a) Selected Area Electron Diffraction (SAED)the Te3 sample.Transmission Electron Microscopy (HRTEM) observation between boundaries(b) of the Bi2Resolution Electron Microscopy (HRTEM) observation between the boundaries of the Bi Te sample. 2 3 sample. 3 Electron Microscopy (HRTEM) observation between the boundaries of the Bi2Te
(a)
(a)
(b)
(b)
(c) Figure 8. Morphology of the thick BiTe film synthesized by the constant potential method: (a) 200-μm-thick film; (b) cross-section view of(c) the 200-μm-thick film consisting of two different layers; and (c) cross-section structure of the initial 10-μm-thick film. Figure 8. Morphology of the thick BiTe film synthesized by the constant potential method:
Figure 8. Morphology of the thick BiTe film synthesized by the constant potential method: (a) 200-μm-thick film; (b) cross-section view of the 200-μm-thick film consisting of two different (a) 200-µm-thick film; (b) cross-section view of the 200-µm-thick film consisting of two different layers; and (c) cross-section structure of the initial 10-μm-thick film. layers; and (c) cross-section structure of the initial 10-µm-thick film.
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3.1.3. Pulsed Deposition 3.1.3. Pulsed Deposition
To synthesize thick Bi2 Te3 films, pulsed electrochemical deposition is performed using a waveform To synthesize thick Bi2Te3 films, pulsed electrochemical deposition is performed using a shown in Figure 9. The applied potentials used in the pulsed deposition alternate between Eon cycles waveform shown in Figure 9. The applied potentials used in the pulsed deposition alternate between and Ioff Ecycles. At the E cycles. cycles, of the potentiostatic mode is adjusted on cycles and Ioff on At the the potential Eon cycles,condition the potential condition of the potentiostatic mode isto grow Bi2 Te3 , adjusted as described in the deposition section.deposition Meanwhile, the working electrode current is to grow Bi2Teconstant 3, as described in the constant section. Meanwhile, the working maintained at 0 current mA during the Ioff at cycles. electrode is maintained 0 mA during the Ioff cycles.
Figure 9. Pulsed deposition waveform. Figure 9. Pulsed deposition waveform.
The working mechanism is as follows. During the deposition cycle with Eon = 20 mV, the
The working mechanism as follows. During the deposition with 20 mV, reduction reduction reaction can be is precisely controlled to grow material andcycle during theEcycle Ioff =the 0, any on =with reactions controlled are prevented. The pulse width and is setduring to 0.1 s for ONwith statusIoff and s forundesired the reactionundesired can be precisely to grow material thethe cycle = 0.2 0, any OFF status. The advantages of the width pulsed deposition overs constant is that cansbalance reactions are prevented. The pulse is set to 0.1 for the deposition ON status andit0.2 for the OFF the loss of ions at the surface of the working electrode that occurs when a constant potential is applied status. The advantages of the pulsed deposition over constant deposition is that it can balance the loss to ionize the electrolyte [22]. In principle, the pulsed deposition is used to ensure the amorphous of ions at the surface of the working electrode that occurs when a constant potential is applied to material deposited during the ON period is in a position to crystallize during the OFF period. ionize the electrolyte [22]. Inmay principle, usedgrowth: to ensure the amorphous Significant improvements be due tothe two pulsed differentdeposition effects of the is crystal new crystals can material deposited during the ON period is in a position to crystallize during the OFF period. grow on the surface as a homogeneous distribution, or atoms and ions can be added to any existing layers. A small surfacemay diffusion and atohigh potential possibly incite the growth Significant improvements be due twoelectrochemical different effects of the crystal growth: newofcrystals new crystals, whereas a high surface diffusion and a small electrochemical potential can promote the can grow on the surface as a homogeneous distribution, or atoms and ions can be added to adhesion of formed atoms to existing layers. During pulsed electrochemical deposition, a better any existing layers. A small surface diffusion and a high electrochemical potential possibly incite supply of material within the solution may result in crystal growth [23]. This is proven in Figures 4b the growth of new crystals, whereas a high surface diffusion and a small electrochemical potential can and 10. The surface of the Bi2Te3 deposited by the pulsed waveform is more uniform and smoother promote thethat adhesion of formed atoms to existing layers. During pulsed electrochemical deposition, than for a constant deposition. Although the surface of the sample prepared by pulsed deposition a bettermethod supply of material within the solution may result in crystal growth [23]. This is proven is dense, it is rough and loose with dendritic growth for the constant deposition method. The 10-μm-thick sample a roughness of 2 μm. This is much in Figures 4b and 10. Thesynthesized surface of by theconstant Bi2 Te3 deposition depositedshows by the pulsed waveform is more uniform larger than the that 300-nm of the sample formed by the pulsed deposition method. The crystal and smoother than forroughness a constant deposition. Although the surface of the sample prepared by of approximately formed by constant is apparently smaller than that of thethe pulse pulsed size deposition method1 μm is dense, it is roughdeposition and loose with dendritic growth for constant deposited crystals (~5 μm). This is related to the diffusion limitations during the constant deposition, deposition method. The 10-µm-thick sample synthesized by constant deposition shows a roughness of since the lateral growth is restricted if enough ions are not supplied to the edges of the crystals. As a 2 µm. This is the much the 300-nm roughness of the sample formed bytothe pulsed deposition result, newlarger crystalsthan will tend to grow on the top of the already formed crystals create a standing method.shape, The crystal size approximately µmsize formed constant deposition is apparently as shown in of Figure 4b. Therefore,1 the of theby crystals becomes narrower. For the filmssmaller deposited using the pulsed method, much are obtained, as shownlimitations in Figure 10.during than that of the pulse deposited crystals (~5 wider µm). crystal This issizes related to the diffusion Again, deposition, the observation of thethe material cross-sections in Figure 11 confirms the constant since lateral growth is shown restricted if enough ionsthis areexplanation. not supplied to The crystals deposited by pulsed deposition are formed uniformly to create a more the edges of the crystals. As a result, the new crystals will tend to grow on the topreinforced of the already compact structure than that from constant deposition. However, the effect of the pulsed deposition formed crystals to create a standing shape, as shown in Figure 4b. Therefore, the size of the crystals method will be limited if the deposition rate is too high during the ON period. A long OFF period is becomes For the films deposited using the pulsed method, much crystal sizes are notnarrower. really effective in this case. Therefore, a millisecond pulsed deposition is wider first presented in obtained, as shown insolve Figure Again, the observation of the cross-sections shown in Reference [23] to this10. issue. The purpose of this method is tomaterial limit the high deposition rate Figure 11 confirms this explanation. The crystals deposited by pulsed deposition are formed uniformly to create a more reinforced compact structure than that from constant deposition. However, the effect of the pulsed deposition method will be limited if the deposition rate is too high during the ON period. A long OFF period is not really effective in this case. Therefore, a millisecond pulsed deposition is first presented in Reference [23] to solve this issue. The purpose of this method is to limit the high
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deposition rate during the ON period. In this work, we propose another approach of low concentration during the ON period. In this work, we propose another approach of low concentration electrolytes. electrolytes. usage soluble anodes can stabilize the deposition, but it may affect during theAdditionally, ONthe period. Inthe thissoluble work,of we propose of low concentration electrolytes. Additionally, usage of anodes can another stabilizeapproach the deposition, but it may affect a low a low concentration of and ionscations. cations. Therefore, electrolytes are renewed frequently avoid Additionally, theions usage ofand soluble anodes can stabilize deposition, but to it avoid may affect atolow concentration of Therefore, electrolytes arethe renewed frequently a significant a significant depletion of low concentrations of the species. concentration of ions and cations. Therefore, electrolytes are renewed frequently to avoid a significant depletion of low concentrations of the species. depletion of low concentrations of the species.
Figure 10. SEM image of Bi2Te3 film surface formed by pulsed deposition.
Figure 10. SEM image of Bi Te film surface formed by pulsed deposition. Figure 10. SEM image of Bi22Te33 film surface formed by pulsed deposition.
Figure 11. Cross-section structure of the film grown by pulsed deposition. Figure 11. Cross-section structure of the film grown by pulsed deposition.
Figure 11. Cross-section structure of the film grown by pulsed deposition. As a result, a film with a thickness of approximately 600 μm and single crystalline bulk-like As a is result, a filmaswith a thickness 600 μmis and single crystalline bulk-like structure achieved, shown in Figureof12.approximately The deposition rate approximately 10 μm/min. An As a result, a film with a thickness of approximately 600 µm and single crystalline bulk-like structure is achieved, as shown in Figure 12. The deposition rate is approximately 10 μm/min. An XRD measurement is performed to analyze the crystal growth orientation of the synthesized films. structure is achieved, shown in The deposition rate is3 approximately 10films. µm/min. XRD The measurement isas performed to Figure analyze the growth orientation ofthick the synthesized X-ray diffraction proves that 12. the crystal pulsed-deposited Bi2Te film possesses a The X-ray diffraction proves that thethe Bi 2Te3intensive thick film possesses a An XRD measurement is performed toas analyze crystal of the synthesized films. Rhombohedral-hexagonal structure, shown in pulsed-deposited Figure 13growth [14]. Theorientation most peak (110) occurs Rhombohedral-hexagonal structure, asthat shown in Figure 13 [14]. The most peak (110) occurs X-ray diffraction proves pulsed-deposited Biintensive thick film possesses atThe 41° of two theta while peaks at (015) and the (103) are eliminated and become unobservable. 2 Te3 almost at 41°result of two theta while peaks (015) and (103) areBieliminated and [19,24,25]. become almost unobservable. This is in agreement withat previous studies of 2Te 3 thin In comparison with a Rhombohedral-hexagonal structure, as shown in Figure 13 [14].films The most intensive peak (110) occurs This result is in agreement with previous studies of Bi 2Te3 thin films [19,24,25]. In comparison with ◦ constant deposited films, the diffracted peak intensity becomes much narrower. This change indicates at 41 of two theta while peaks at (015) and (103) are eliminated and become almost unobservable. constant deposited films, the diffracted peak intensity becomes much narrower. This change indicates significant in the grain size and improved again confirms results fromwith Thisaresult is in increase agreement with previous studies ofcrystallinity. Bi2 Te3 thin This films [19,24,25]. Inthe comparison athe significant increase in the grain size and improved crystallinity. This again confirms the results from SEM observation, which indicates the difference in crystal growth between the constant and constant deposited films, the diffracted peak intensity becomes much narrower. This change indicates the SEM observation, whichItindicates the the difference in crystal betweendeposition the constant and pulsed deposition methods. is clear that Bi2Te3 films growngrowth using constant change a significant increase in the grain size and improved crystallinity. This again confirms the results from pulsed deposition methods. It issingle clear that the Bi2Te 3 films grown using pulsed constant deposition change from a polycrystalline to a (110) crystal-like structure when using deposition. the SEM observation, which indicates thecrystal-like differencestructure in crystalwhen growth between the constant and pulsed fromIn a polycrystalline to a (110) single using pulsed deposition. the conclusions from References [26,27], the figure of merit (ZT) is an anisotropic property. deposition methods. It is from clearReferences that the Bi[26,27], grownofusing constant deposition change from 2 Testructure 3 films In the conclusions the figure (ZT) is an property. The single peak of the (110) highly oriented has the merit highest value ofanisotropic ZT. The (110) highly a polycrystalline to a (110) single crystal-like structure when using pulsed deposition. The single peak of yielded the (110) the highest from valuethis of ZT. The (110) highly oriented structure forhighly thick oriented films are structure one of thehas achievements research. In contrast In the conclusions fromfor References figure of merit from (ZT) this is anresearch. anisotropic property. oriented structure yielded thick films[26,27], are one the of the achievements In contrast
The single peak of the (110) highly oriented structure has the highest value of ZT. The (110) highly
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for thick films are one of the achievements from this research. In to to the constant deposition method, the clear SEAD pattern of the pulsed-deposited sample reveals the constant deposition method, the clear SEAD pattern of the pulsed-deposited sample reveals that it that it constant basically possesses method, amethod, single the crystalline structure with a major Bi2Te3 phase, as shown in to clear pattern of the pulsed-deposited sample reveals tothe the constantdeposition deposition the clear SEAD SEAD pattern pulsed-deposited sample reveals basically possesses athe single crystalline structure with a major Bi2 Tebetween as shown in Figure 14a. 3 phase,two Figure From possesses HRTEM observation in Figure 14b, the distance neighboring faces that basically aa single structure with Bi22Te Te33 phase, phase, as shown shown in thatitit14a. basically possesses single crystalline crystalline structure a major Bi as in From the HRTEM observation in Figure 14b, the distance between two neighboring faces is estimated isFigure estimated to bethe approximately 2.2 Å, which is the14b, same the inter-planar distance between the to Figure 14a. HRTEM in theas distance between two two neighboring faces 14a.From From the HRTEMobservation observation in Figure Figure 14b, between neighboring faces be approximately 2.2 Å, which is the same as the inter-planar distance between the (110) lattice planes. (110) lattice planes. is totobe inter-planar distance distance between betweenthe the isestimated estimated beapproximately approximately2.2 2.2 Å, Å, which which is is the the same as the inter-planar (110) (110)lattice latticeplanes. planes.
Figure 12. SEM image of the cross section of the 600-μm-thick Bi2Te3 film.
Figure 12. 12. SEM image ofofthe of the the600-μm-thick 600-µm-thick 3 film. Figure SEM image thecross crosssection section of Bi2Bi Te23Te film. Figure 12. SEM image of the cross section of the 600-μm-thick Bi2Te3 film.
Figure 13. XRD pattern of the BiTe films with different deposited potentials. Figure 13. XRD pattern of the BiTe films with different deposited potentials.
Figure 13.13. XRD withdifferent differentdeposited deposited potentials. Figure XRDpattern patternof ofthe the BiTe BiTe films films with potentials.
(a) (a) (b) (a) (b) Figure 14. (a) SAED pattern; and (b) HRTEM observation between the boundaries of the Bi2Te3 sample
Figure 14. (a) SAED pattern; and (b) HRTEM observation between the boundaries of the Bi2Te3 sample fabricated using pulsed deposition. fabricated using pulsed deposition. Figure 14. (a) SAED pattern; and (b) HRTEM observation between the boundaries of the Bi2Te3 sample Figure 14. (a) SAED pattern; and (b) HRTEM observation between the boundaries of the Bi2 Te3 sample fabricated using pulsed deposition.
fabricated using pulsed deposition.
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3.1.4. Thermo-Electric Properties Evaluation
electricalresistivity resistivity Seebeck coefficient are evaluated Bi2deposited Te3 films The electrical (ρ)(ρ) andand Seebeck coefficient (S) are(S) evaluated for the Bifor films 2 Te3the deposited by bothThe methods. The Seebeck of coefficient of the samplewith deposited with the constant mode by both methods. Seebeck coefficient the sample deposited the constant mode is observed is observed to be a maximum of −60 μV/K. The pulsed-deposited Bi 2 Te 3 film exhibits a slightly higher to be a maximum of −60 µV/K. The pulsed-deposited Bi2 Te3 film exhibits a slightly higher Seebeck Seebeck coefficient with an of However, −80 μV/K.the However, electrical resistivity of the films coefficient with an average of average −80 µV/K. electricalthe resistivity of the films synthesized by synthesized by pulsed is much improved overvia specimens fabricated The via electrical constant pulsed deposition is muchdeposition improved over specimens fabricated constant deposition. deposition.of The electrical thethe samples prepared the pulsed method exhibit a value resistivity the samplesresistivity preparedofby pulsed methodbyexhibit a value of 20 µΩm. This of is 20 μΩm. This is2.5 approximately 2.5 times lower than 50constant μΩm from the constant method. The approximately times lower than 50 µΩm from the method. The main reason formain this reason for is this is the aforementioned crystal growth structure. In a material containing difference thedifference aforementioned crystal growth structure. In a material containing defective grain defective grain boundaries, charge carriers may beinterfaces scattered between at the interfaces between grains. boundaries, charge carriers may be scattered at the grains. The cross-section The cross-section structure XRD and TEM show that theusing sample structure observations, XRDobservations, and TEM analyses show thatanalyses the sample deposited thedeposited constant using the constant method moreboundaries defects inthan the for grain than forThis theleads pulsed method contains more defectscontains in the grain theboundaries pulsed deposition. to Thisconductivity leads to a low electrical conductivity when using the constant deposition between method. adeposition. low electrical when using the constant deposition method. The relationship The relationship between Seebeck coefficients and electrical conductivities of BiTe SbTe istoa Seebeck coefficients and electrical conductivities of BiTe and SbTe is a complicated issue.and According complicated issue. According to works many behaviors of this relationship have been works reported, many behaviors of thisreported, relationship have been observed. In References [18,20], observed. In References [18,20], the relationship Seebeck coefficients and electrical conductivities the relationship between Seebeck coefficients between and electrical conductivities behaves differently when behaves growth differently when crystals growth is improved by pulsedprocess. deposition or annealing process. crystals is improved by pulsed deposition or annealing In detail, it is shown that In detail,coefficients it is shownincrease that Seebeck coefficients increase while electrical conductivities in Seebeck while electrical conductivities decrease in Reference [20]. decrease In contrast, Reference [20]. Inare contrast, bothdramatically properties are enhanced dramatically in Reference same both properties enhanced in Reference [18]. The same results are[18]. alsoThe reported results are also reported thatand both Seebeck coefficients and electrical conductivities arequality much that both Seebeck coefficients electrical conductivities are much improved due to high improved due tolow highdefect quality crystallinity[23,28]. and low defect concentration [23,28]. crystallinity and concentration Additionally, this paper reports how annealing affects the Seebeck coefficient and electrical the Bi Bi22Te Te33. The resistivity of the The Bi Bi22Te Te33 films films are are annealed annealed at at various temperatures in N22 at ambient ramp rate rate during during the the annealing annealing process process isis22◦°C/min. conditions for 1 h. The heat ramp C/min. Then, the films are analyzed using EDX to confirm that there is no elemental oxygen detected to prevent the formation of oxidation during the annealing process. The result shows that, although annealing can improve the thermal and electrical properties of materials, some annealing temperatures are found to optimize material performance. The The highest highest Seebeck Seebeck coefficient coefficient for the Bi22Te Te33 films films is found at an annealing ◦ approximately 250 250 °C, Figure 15. 15. temperature of approximately C, as shown in Figure
Figure on the the annealing annealing temperature. temperature. Figure 15. 15. Dependence Dependence of of the the Seebeck Seebeck coefficients coefficients on
At a 250 °C annealing temperature, thermoelectric property measurements show that the At a 250 ◦ C annealing temperature, thermoelectric property measurements show that the annealing annealing process can significantly improve Seebeck coefficient and electrical resistivity of the process can significantly improve Seebeck coefficient and electrical resistivity of the materials synthesized materials synthesized from both constant and pulsed deposition methods. The Seebeck coefficients from both constant and pulsed deposition methods. The Seebeck coefficients of the films prepared using of the films prepared using the constant and pulsed deposition methods are evaluated to be improved the constant and pulsed deposition methods are evaluated to be improved by a factor of roughly by a factor of roughly two after the annealing process. The annealed sample is measured to have a two after the annealing process. The annealed sample is measured to have a Seebeck coefficient Seebeck coefficient of −110 μV/K, which is remarkably improved from the −50 μV/K in the as-deposited of −110 µV/K, which is remarkably improved from the −50 µV/K in the as-deposited sample sample fabricated using constant deposition. For pulsed deposition, a similar increase is also observed when the Seebeck coefficient improves from −80 μV/K to −150 μV/K. The electrical resistivity of the sample synthesized using pulsed deposition is slightly improved from 20 μΩm to
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fabricated using constant deposition. For pulsed deposition, a similar increase is also observed when the Seebeck coefficient improves from −80 µV/K to −150 µV/K. The electrical resistivity of the sample synthesized using pulsed deposition is slightly improved from 20 µΩm to 15 µΩm. Meanwhile, the electrical resistivity of the annealed films produced using constant deposition is impressively reduced in comparison with the non-annealed films. This improvement may be attributed Materials 154 12 ofexplained 17 to decreases in2017, the10, defects in the grain boundaries occurring from the annealing process. As in References [21,29], the rearrangement of crystal grains and removal of the defects are also the reasons 15 μΩm. Meanwhile, the electrical resistivity of the annealed films produced using constant for a similar improvement of the Seebeck coefficientwith duethe tonon-annealed the annealing process. The summarized deposition is impressively reduced in comparison films. This improvement results are in Table 1. mayshown be attributed to decreases in the defects in the grain boundaries occurring from the annealing process. As explained in References [21,29], the rearrangement of crystal grains and removal of the Tablefor 1. Effects ofimprovement annealing onofthe 2 Te3 properties. defects are also the reasons a similar theBiSeebeck coefficient due to the annealing process. The summarized results are shown in Table 1. Constant Deposited Bi2 Te3
Pulsed-Deposited Bi2 Te3
◦ C) Table 1. Effects of annealing on the(250 Bi2Te 3 properties. Non-Annealing Annealing Non-Annealing
Seebeck coefficient (±20 µV/K) Electrical resistivity (±5 µΩm) Power Factor (W/mK2 )
−50Constant Deposited −110 Bi2Te3 50 20 Annealing 0.5 ×Non-Annealing 10−4 6 × 10−4 (250 °C) Seebeck coefficient (±20 μV/K) −50 −110 3.2. Synthesis of P-Type Antimony Telluride 50 Electrical resistivity (±5 μΩm) 20 Power Factor (W/mK2) 0.5 × 10−4 6 × 10−4
Annealing (250 ◦ C)
−80 −3150 Pulsed-Deposited Bi2Te 20
Non-Annealing 3.2 × 10−4 −80 20 3.2 × 10−4
15 Annealing 15 × 10−4 (250 °C) −150 15 15 × 10−4
3.2.1. Voltammetry 3.2. Synthesis of P-Type Antimony Telluride
To understand the formation mechanism for SbTe during the deposition process, the cyclic voltammetry of the reactions is studied. The potentials are swept over a range from −1.0 to 1.5 V at 3.2.1. Voltammetry a scan speedToofunderstand 10 mV/s. the According the cyclicfor voltammetry shownprocess, in Figure two main formationtomechanism SbTe during results the deposition the16, cyclic reduction peaks (D1 andreactions D2) and three oxidation peaks (O1, O2, O3) are−1.0 observed. first voltammetry of the is studied. The potentials are swept over and a range from to 1.5 V at The a scan speedpeak, of 10 D1, mV/s.isAccording to approximately the cyclic voltammetry shown correlates in Figure 16,totwo main reduction located at −150results mV, which themain formation peaks (D1 and D2) and three oxidationreaction peaks (O1, O2, and O3) observed. The firstof main of SbTe reduction films, whereas the following reduction appears at −are 450 mV because hydrogen reduction peak, D1, is located at approximately −150 mV, which correlates to the formation of evolution [23,30]. The three oxidation peaks correspond to the process of stripping the Te atoms. SbTe films, whereas the following reduction reaction appears at −450 mV because of hydrogen Therefore, a potential range between −200 mV and 100 mV is determined to be an appropriate potential evolution [23,30]. The three oxidation peaks correspond to the process of stripping the Te atoms. condition for depositing SbTe films on a gold film. The taking place during the deposition Therefore, a potential range between −200 mV and 100reaction mV is determined to be an appropriate process potential is showncondition to be [23]: for depositing SbTe films on a gold film. The reaction taking place during the deposition process is shown to be [23]: + − 3HTeO3HTeO + 2SbO+ + 13HTeO + ↔ Sb Te3 +2O.8H2 O. 2 +18e 2+ +18e− + 2SbO+ + 13HTeO2+2 ↔ Sb2Te32+ 8H
(2)
(2)
Figure 16. Cyclic voltammetry of SbTe deposition.
Figure 16. Cyclic voltammetry of SbTe deposition. The dependence between deposition potentials and atomic composition in the range of suitable from between the CV measurement is also investigated, as composition shown in Figure 17. range The ideal Thepotentials dependence deposition potentials and atomic in the of suitable composition of 40 atomic % Sb and 60 atomic % Te is achieved at a potential of -144 mV. In the SbTe potentials from the CV measurement is also investigated, as shown in Figure 17. The ideal composition formation, antimony is deposited at a higher potential than bismuth and in the region where the
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of 40 atomic % Sb and 60 atomic % Te is achieved at a potential of -144 mV. In the SbTe formation, Materials 2017, 10, 154 13 of 17 antimony is deposited at a higher potential than bismuth and in the region where the tellurium Materials 2017, 10, 154 13 of 17 deposition potential is already large. Therefore, a stronger stoichiometry dependencedependence on the deposition tellurium deposition potential is already large. Therefore, a stronger stoichiometry on potentials than for BiTe is predicted. the deposition potentials than for BiTe is predicted. tellurium deposition potential is already large. Therefore, a stronger stoichiometry dependence on the deposition potentials than for BiTe is predicted.
Figure 17. Potentials dependence on the atomic composition of the Sb2Te3.
Figure 17. Potentials dependence on the atomic composition of the Sb2 Te3 . Figure 17. Potentials dependence on the atomic composition of the Sb2Te3. 3.2.2. Pulsed Deposition
3.2.2. Pulsed Deposition
Sb2Te3 thick films have been grown using pulsed electrochemical deposition at room 3.2.2. Pulsed Deposition Sb been grown pulsed electrochemical deposition at mV room temperature and a have stirring speed of 60using rpm. The pulsed waveform is applied at −144 fortemperature the ON 2 Te3 thick films Sb2Tefor 3 speed thick films have beenduring grownthe using pulsed electrochemical deposition 0.1 s and current OFF period 0.2 s. After one hour, precipitation of and aperiod stirring of a60zero rpm. The pulsed waveform is for applied at −144 mV for the at ONroom period 2−applied 2−the temperature and a stirring speed of 60 rpm. The pulsed waveform is at −144 mV for ON Sb 2 O 3 begins to appear. This phenomenon causes a loss of (C 4 H 4 O 6 ) ions because (C 4 H 4 O 6 ) is the for 0.1 s and a zero current during the OFF period for 0.2 s. After one hour, precipitation of Sb2 O3 2− − is the period for and phenomenon a zeroofcurrent during OFF period for 0.2 s. After one hour, precipitation of result of 0.1 the sdissolution Sb2O3 in tartaric acid. the electrolyte is renewed for 2every hour begins to appear. This causes athe loss ofTherefore, (C ions because (C result 4 H4 O6 ) 4 H4 O6 ) of Finally, 500-μm-thick Sb 2Te3causes film is successfully synthesized with a deposition rate of Sb 2Odeposition. 3 begins to appear. This phenomenon a loss of (C 4H 4O6)2− ions because (C 4H4O6)2− is the of the dissolution of Sb2 O3 in tartaric acid. Therefore, the electrolyte is renewed for every hour of approximately 4 μm/h,of asSb shown Figureacid. 18. The crystal structure is analyzed using XRD for Sbhour 2Te3 result of the dissolution 2O3 inin tartaric Therefore, the electrolyte is renewed for every deposition. Finally, 500-µm-thick Sb2 Te3 film is successfully synthesized with a deposition rate of films deposited by both constant and pulsed depositions at −144 mV. The film synthesized by pulsed of deposition. Finally, 500-μm-thick Sb2Te3 film is successfully synthesized with a deposition rate of approximately 4 µm/h,narrower as shown in Figure 18. The crystal structureconstant is analyzed using XRD for deposition exhibits diffracted intensity that ofisthe method. approximately 4 μm/h, as shown in Figurepeak 18. The crystalthan structure analyzed using XRDHowever, for Sb2Te3 Sb2 Teboth deposited byaboth constant and pulsed at −19. 144Similarly, mV. Theall film synthesized 3 films samples possess polycrystalline structure, as depositions shown in Figure of by the SbTe films deposited by both constant and pulsed depositions at −144 mV. The film synthesized pulsed by pulsed deposition exhibits narrower diffracted peak intensity than that of the constant method. films synthesized at three different potentials near thethan first that mainofreduction peakmethod. exhibit randomly deposition exhibits narrower diffracted peak intensity the constant However, However, both samples possess a polycrystalline structure, as shown in Figure 19. Similarly, all of oriented polycrystalline, as shown in Figure 20. as shown in Figure 19. Similarly, all of the SbTe both samples possess a polycrystalline structure,
the films SbTe synthesized films synthesized three different near thereduction first mainpeak reduction peak exhibit at three at different potentialspotentials near the first main exhibit randomly randomly oriented polycrystalline, as shown in Figure 20. oriented polycrystalline, as shown in Figure 20.
Figure 18. SEM image of the cross section of the 500-μm-thick Sb2Te3 film.
Figure 18. SEM image of the cross section of the 500-μm-thick Sb2Te3 film.
Figure 18. SEM image of the cross section of the 500-µm-thick Sb2 Te3 film.
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Figure 19. XRD patterns of the Sb2Te3 films synthesized by constant and pulsed deposition at −144 mV. Figure 19. XRD XRDpatterns patternsof ofthe theSb Sb2Te filmssynthesized synthesized constant and pulsed deposition −144 2 Te Figure 19. 33 films byby constant and pulsed deposition at at −144 mV.mV.
Figure 20. XRD pattern of the SbTe films at different deposited potentials. Figure 20. XRD different deposited deposited potentials. potentials. XRD pattern pattern of the SbTe films at different
3.2.3. Thermo-Electric Properties Evaluation 3.2.3. Thermo-Electric Properties Evaluation Evaluation 3.2.3. Thermo-Electric Properties The Sb2Te3 films are also annealed to improve the thermal and electrical properties. The The Sb Sb 2Te3 films annealed to improve the and electrical properties. The The areare alsoalso annealed to N improve the thermal and electrical properties. The annealing annealing process is performed for 1 h in 2 atmosphere. Thethermal heat ramp rate of 2 °C/min is conducted 2 Te3 films ◦ annealing process is performed for 1 h inof N2the atmosphere. heat ramp rate of 2 °C/min is conducted process is performed for 1 h inThe N The heat The ramp rate of 2 Seebeck C/min is conducted during the annealing process. effect annealing process on the coefficient forduring Sb2Te3 2 atmosphere. during the annealing process. The effect of the annealing process on the Seebeck coefficient for Sb the annealing process. effect the annealing process onas-deposited the Seebeck Sb coefficient for exhibits Sb2 Te23Teisa3 is different from that of The Bi2Te 3. Theof Seebeck coefficient for the 2Te3 sample is different from that of Bi 2 Te 3 . The Seebeck coefficient for the as-deposited Sb 2 Te 3 sample exhibits different that of Bi2 Te140 Seebeck coefficient for the as-deposited a valuea value offrom approximately μV/K. After the annealing process, thisSbincreases to exhibits approximately 3 . The 2 Te3 sample value of approximately μV/K.the After the isannealing process, this increases to approximately of 140 µV/K. After annealing process, increases approximately µV/K. 170approximately μV/K. However, the 140 electrical resistivity as much this as three timesto better than that170 of the as170 μV/K. However, the electrical resistivity is as much as three times better than that of the asHowever, electrical resistivity is as much as three times betterfrom than 60 thatμΩm of thetoas-deposited films. deposited the films. The electrical resistivity remarkably decreases 20 μΩm after the deposited films. The electrical resistivity from 60 μΩmthe 20high μΩmnumber after the The electrical resistivity remarkably decreases fromresistivity 60decreases µΩm topossibly 20 µΩm after annealing process. annealing process. The large value for the remarkably electrical relates totothe of annealing process. The large value for the electrical resistivity possibly relates to the high number of The largeatvalue theboundaries. electrical resistivity possibly relates to the high number defectsthe at the grain defects the for grain Therefore, the annealing process greatly of affects electrical defects at This the grain boundaries. Therefore, the annealing process affectsThis the boundaries. Therefore, the behavior annealing affects the electrical resistivity. iselectrical aspulsed same resistivity. is as same asprocess for thegreatly constant deposition of Bigreatly 2Te 3 compared to the resistivity. is same as for deposition ofpulsed Bi2Te3 deposition. compared toInthe pulsed behavior asThis forthis theas constant deposition ofthe Bi2 constant Te3 compared to theproduced this case, deposition. In case, thebehavior film’s electrical resistivity in the sample by the constant method deposition. In this case, the film’s electrical resistivity in the sample produced by the constant method the film’s electrical resistivity inby thethe sample produced bythan the constant method is deposition more dramatically is more dramatically decreased annealing process that for the pulsed method is more dramatically decreased by the annealing process than that the pulsed deposition decreased the worse annealing process than that the pulsed method of itsmethod worse because ofby its crystal defects. In for comparison todeposition the for thin Sb 2Te3 because films reported on because of its worse crystal defects. In comparison to the thin Sb 2Te3 films reported on crystal defects. In comparison to the thin Sb Te films reported on previously [23,31], the thick films previously [23,31], the thick films synthesized 2 in 3 this work do not show a significant difference in the previously [23,31], the thick films synthesized inproves this work not show a significant inthin the synthesized in this work do afilms. significant difference inthe the electrical resistivity than forwork the electrical resistivity than fornot theshow thin This thatdo thick films obtained difference in this can electrical resistivity than for the thin films. This proves that the thick films obtained in this work can films. Thissame proves that the as thick films obtained in thisannealing work can temperature have the same films. have the properties thin films. The proper forproperties the Sb2Teas 3 isthin found to have the same properties as thin films. The proper annealing temperature for the Sb 2Te3 is found to be lower than for the Bi2Te3, which is approximately 200 °C, as shown in Figure 21. Summarized be lower for in theTable Bi2Te2.3, which is approximately 200 °C, as shown in Figure 21. Summarized results arethan shown results are shown in Table 2.
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The proper annealing temperature for the Sb2 Te3 is found to be lower than for the Bi2 Te3 , which is approximately 200 ◦ C, as shown in Figure 21. Summarized results are shown in Table 2. Materials 2017, 10, 154 15 of 17
Figure 21. Dependence of the Seebeck coefficients from the Sb2Te3 on the annealing temperature. Figure 21. Dependence of the Seebeck coefficients from the Sb2 Te3 on the annealing temperature. Table 2. The effects of annealing on the Sb2Te3 properties. Table 2. The effects of annealing on the Sb2 Te3 properties.
Seebeck coefficient (±20 μV/K) Electrical resistivity μΩm) Seebeck coefficient (±20(±5 µV/K) 2) Electrical resistivity (±5 µΩm) Power Factor (W/mK Power Factor (W/mK2)
Pulsed-Deposited Sb2Te3 Pulsed-Deposited Sb2 Te3 Non-Annealing Annealing (200 °C) Non-Annealing Annealing (200 130 170 ◦ C) 25 13060 170 60 × 10−4 25× 10−4 2.8 11.2 2.8 × 10−4
11.2 × 10−4
4. Conclusions 4. Conclusions In this report, the syntheses of thick bulk-like thermoelectric Bi2Te3 and Sb2Te3 materials are In this report, the syntheses of as thick bulk-like thermoelectric Bi2 Te3 and Sb demonstrated for such applications micro thermoelectric power generators. N-type Bi2Te3 andare P2 Te3 materials demonstrated for such applications as micro thermoelectric power generators. N-type Bi Te and type Sb2Te3 thick films are obtained. A new usage of low concentration electrolyte is proposed 2 3as an P-type Sb2 Te films are obtained. A new usage of low concentration electrolyte is proposed asThe an approach for the synthesis of materials with high quality morphology and compactness. 3 thick approach theeffects synthesis of materials with high quality morphology andBoth compactness. conditions conditionsforand of the annealing process are also investigated. materials The exhibit a high and effects of the annealing are also investigated. Both materials exhibit a high Seebeck Seebeck coefficient and lowprocess electrical resistivity. The Seebeck coefficient of the synthesized coefficient and materials low electrical resistivity. The Seebeck of the synthesized thermoelectric can reach approximately ±150coefficient μV/K. Electrical resistivities thermoelectric of 15 ± 5 μΩm materials reach ±150 µV/K. Bi Electrical of 15 5 3µΩm 25 ± 5 µΩm and 25 ± 5can μΩm areapproximately obtained for 600-μm-thick 2Te3 andresistivities 500-μm-thick Sb± 2Te films,and respectively. are obtained for 600-µm-thick Bi2 Te3 and 500-µm-thick Sb2 Te3 films, respectively. Acknowledgments: Part of this work is performed in the Micro/Nanomachining Research Education Center Acknowledgments: Part of thisMemorial work is performed in theof Micro/Nanomachining Research (MNC) and Junichi Nishizawa Research Center Tohoku University. This work is Education supported Center in part (MNC) and Junichi Nishizawa Memorial Research Center of Tohoku University. This work is supported in by a Grant-in Aid for Scientific Research from the Japanese Ministry of Education, Culture, Sports, Science, and part by a Grant-in Aid for Scientific Research from the Japanese Ministry of Education, Culture, Sports, Science, Technology. Additionally, this research is partially supported by thebyCenter of Innovation Program from Japan and Technology. Additionally, this research is partially supported the Center of Innovation Program from Science and Technology Agency, JST. JST. Japan Science and Technology Agency, Author designs the the experiment set up; Van Toan to the scientific Author Contributions: Contributions:Kei KeiSakamoto Sakamoto designs experiment setNguyen up; Nguyen Vancontributes Toan contributes to the discussion; Nguyen Huu Trung performs the experiments and writes the manuscript under the supervision of scientific discussion; Nguyen Huu Trung performs the experiments and writes the manuscript under the Takahito Ono. supervision of Takahito Ono. Conflicts of Interest: The authors declare no conflict of interest. Conflicts of Interest: The authors declare no conflict of interest.
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