Composition dependent charge transport and temperature dependent density of state effective mass interpreted by temperature normalized Pisarenko plot in Bi2−xSbxTe3 compounds Composition dependent cha[.]
Composition-dependent charge transport and temperature-dependent density of state effective mass interpreted by temperature-normalized Pisarenko plot in Bi2-xSbxTe3 compounds , Tae-Ho An, Young Soo Lim , Mi Jin Park, Jang-Yeul Tak, Soonil Lee, Hyung Koun Cho, Jun-Young Cho, , , Chan Park , and Won-Seon Seo Citation: APL Mater 4, 104812 (2016); doi: 10.1063/1.4961106 View online: http://dx.doi.org/10.1063/1.4961106 View Table of Contents: http://aip.scitation.org/toc/apm/4/10 Published by the American Institute of Physics APL MATERIALS 4, 104812 (2016) Composition-dependent charge transport and temperature-dependent density of state effective mass interpreted by temperature-normalized Pisarenko plot in Bi2−x Sbx Te3 compounds Tae-Ho An,1,2 Young Soo Lim,3,a Mi Jin Park,1 Jang-Yeul Tak,1,4 Soonil Lee,1 Hyung Koun Cho,4 Jun-Young Cho,2 Chan Park,2,a and Won-Seon Seo1,a Energy and Environmental Materials Division, Korea Institute of Ceramic Engineering and Technology, Jinju 52851, South Korea Department of Materials Science and Engineering, Seoul National University, Seoul 08826, South Korea Department of Materials System Engineering, Pukyong National University, Busan 48547, South Korea School of Advanced Materials Science and Engineering, Sungkyunkwan University, Suwon 16419, South Korea (Received 16 May 2016; accepted August 2016; published online 15 August 2016) Composition-dependent charge transport and temperature-dependent density of state effective mass-dependent Seebeck coefficient were investigated in Bi2−x Sb x Te3 (x = 1.56-1.68) compounds The compounds were prepared by the spark plasma sintering of high-energy ball-milled powder High-temperature Hall measurements revealed that the charge transport in the compounds was governed dominantly by phonon scattering and influenced additionally by alloy scattering depending on the amount of Sb Contrary effects of Sb content on the Seebeck coefficient were discussed in terms of carrier concentration and density of state effective mass, and it was elucidated by temperature-normalized Pisarenko plot for the first time C 2016 Author(s) All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/) [http://dx.doi.org/10.1063/1.4961106] Recently, thermoelectric energy conversion has been explosively investigated for its applications to power generation using waste heat and electronic cooling devices The efficiency of the thermoelectric energy conversion is dictated by the performance of thermoelectric material, and it is determined by dimensionless figure of merit ZT = S 2σT/κ, where S, σ, T, and κ are the Seebeck coefficient, the electrical conductivity, the absolute temperature, and the thermal conductivity, respectively.1–3 Among numerous thermoelectric materials, Bi2Te3 and its alloyed compounds have been mostly commercialized due to their low thermal conductivities, high Seebeck coefficients, and high electrical conductivities near room temperature.4 For the enhancement of ZT in Bi2Te3-based compounds, many approaches mostly based on nanostructuring have been attempted successfully to reduce the thermal conductivity.5–14 However, for the further enhancement of ZT in Bi2–x Sb x Te3 compounds, optimizing the power factor (PF = S 2σ) in the nanostructured compounds is also critical and it can be carried out by the control of the Sb content in the compounds Although there have been numerous reports on the effects of Sb on the thermoelectric properties of Bi2–x Sb x Te3 compounds,15–19 detailed studies on the charge transport mechanism which governing the electrical conductivity, and on the density of state (DOS) effective mass which influencing on the Seebeck coefficient can be hardly found especially at relatively high temperatures Because these properties are strongly correlated to each a Authors to whom correspondence should be addressed Electronic addresses: yslim@pknu.ac.kr; pchan@snu.ac.kr; and wsseo@kicet.re.kr 2166-532X/2016/4(10)/104812/7 4, 104812-1 © Author(s) 2016 104812-2 An et al APL Mater 4, 104812 (2016) other, the simultaneous understanding of the effects of Sb content on the charge transport and also on the DOS effective mass in detail is of great significance In this work, we report the composition-dependent charge transport and temperature-dependent DOS effective mass in Bi2−x Sb x Te3 compounds Although the charge transport in the compound was governed generally by the well-known hole-phonon scattering regardless of the Sb content, the effect of Sb on the additional alloy scattering was elucidated by high-temperature Hall measurement The change in the DOS effective mass depending on both the Sb content and the temperature was interpreted by temperature-normalized Pisarenko plot for the first time, and the contrary effects of the Sb incorporation on the Seebeck coefficient were discussed Bi2−x Sb x Te3 (x = 1.56-1.68) compounds were synthesized by a vacuum melting method Bi (99.999%, N Plus), Sb (99.999%, N Plus), and Te (99.999%, N Plus) were used as starting materials Mixtures of starting materials in evacuated silica tubes were melted at 1273 K for h, annealed at 973 K for h, and quenched in water to obtain a homogeneous Bi2−x Sb x Te3 phase The annealed ingots were pulverized by Spex-mill (Spex Certiprep, 8000M mixer/mill) for 50 in an Ar ambient, and then consolidated by using spark plasma sintering (Well Tech, WT 4000A) under an uniaxial pressure of 60 MPa at 753 K for Phase analyses were performed by using an X-ray diffractometer (XRD, New D8 Advance, Bruker) Microstructural characterizations were also carried out by using a scanning electron microscope (SEM, JSM-6700, JEOL) Electrical conductivities and Seebeck coefficients were measured by using a four-point probe method (TPMS, ZEM-3, ULVAC-RIKO), and temperature-dependent carrier concentrations and mobility were characterized by using a high-temperature Hall measurement system (ResiTest 8300, Toyo Corporation) Both the TPMS and high-temperature Hall measurements were carried out along the perpendicular direction to the pressing axis for the sintering process Fig 1(a) shows XRD patterns of the Bi2−x Sb x Te3 (x = 1.56-1.68) compounds prepared by spark plasma sintering The XRD patterns showed that the compounds consisted of a single phase of Bi2−x Sb x Te3 (PDF #49-1713) without any secondary phase The relative intensities of (006) to (015) peak in the XRD patterns were ∼0.13 in all Bi2−x Sb x Te3 compounds, and they were not strongly dependent on the amount of Sb in the compounds Figs 1(b)-1(e) show the fractured surfaces of the Bi2−x Sb x Te3 compounds Randomly oriented grains with shape anisotropy were observed clearly in all compounds, and structural difference between them could be hardly observed in the micrographs With these XRD and SEM results, it was found that the Sb content does not considerably influence the microstructure of the Bi2−x Sb x Te3 compounds in this experimental range Therefore, the following thermoelectric properties of the compounds, which are critically affected by the microstructure, can be directly compared without further consideration on the additional effects of the microstructure Fig 2(a) shows temperature-dependent hole concentrations of the Bi2−x Sb x Te3 compounds The Hall measurements were carried out within the temperature range from room temperature (RT) to 450 K, and detailed results at RT are summarized in Table I The increase in the hole concentration with increasing Sb content was observed obviously, resulting from the generation of ′ 20–23 antisite defects (SbTe ) Due to the small difference in the electronegativity between Bi (2.02) ′ and Te (2.1), antisite defect of BiTe can be easily produced in Bi2Te3 compound In Bi2−x Sb x Te3 compounds, the formation energy of antisite defects can be reduced further by the decrease in the difference of the electronegativity between Sb (2.05) and Te atoms, leading to the increase ′ in the hole concentration via the formation of SbTe antisite defects It is noteworthy that the hole concentrations in the compounds with a relatively high Sb content (x ≥ 1.64) were not significantly influenced by temperature, demonstrating that they are degenerately doped with the antisite defects However, the compounds with low Sb content exhibited slightly increasing hole concentration with increasing temperature, and this tendency could be more clearly observed in the Bi0.44Sb1.56Te3 compound with the lowest Sb content In a degenerately doped p-type semiconductor where its Fermi level is located below the valence band maxima (VBM), the carrier concentration is independent of temperature up to extrinsic-intrinsic transition temperature.24 Therefore, this result shows that all the Bi2−x Sb x Te3 compounds in this experiment are heavily doped extrinsic semiconductors, and that the large amount of hole originates from the extrinsic antisite defects facilitated by the Sb incorporation 104812-3 An et al APL Mater 4, 104812 (2016) FIG (a) X-ray diffraction patterns and ((b)-(e)) SEM micrographs of the Bi2−x Sb x Te3 compounds (x = 1.56-1.68) Fig 2(b) represents temperature-dependent Seebeck coefficients of the Bi2−x Sb x Te3 compounds Seebeck coefficients decreased with increasing Sb content in the Bi2−x Sb x Te3 compounds, and it resulted from the increase of the hole concentration by the Sb incorporation as discussed above In an extrinsic degenerate semiconductor, Seebeck coefficients can be described by Pisarenko relation in Eq (1),25 S= 8π k 2BT ∗ π 2/3 m ( ) , 3qh2 d 3p (1) where k B is the Boltzmann constant, q is the carrier charge, h is the Plank constant, m∗d is the DOS effective mass, and p is the hole concentration Therefore, the Seebeck coefficient is not only affected by the carrier concentration, but also influenced by the DOS effective mass in the compound To elucidate the complex effects of Sb on the DOS effective mass and related Seebeck coefficient in the Bi2−x Sb x Te3 compounds, we adopted temperature-normalized Pisarenko plot (S/T vs p) as shown in Fig 2(c) In this figure, dashed lines indicate the relation between the temperaturenormalized Seebeck coefficient and hole concentration at a constant DOS effective mass determined by Eq (1), and me is the mass of electron (9.11 × 10−31 kg) The DOS effective mass in the compound increases significantly with increasing Sb content At the lowest temperature of T1 (∼306 K), the effective masses in the Bi2−x Sb x Te3 compounds were 0.691, 0.811, 0.902, and 0.923 me when x = 1.56, 1.60, 1.64, and 1.68, respectively Therefore, the effects of Sb alloying on the Seebeck coefficient should be considered from two different viewpoints The increase in the 104812-4 An et al APL Mater 4, 104812 (2016) FIG Temperature-dependent (a) hole concentrations and (b) Seebeck coefficients (T1: ∼306 K, T6: ∼420 K) of the Bi2−x Sb x Te3 compounds (c) S/T is plotted as a function of hole concentration with estimated DOS effective mass obtained by Pisarenko relation (dashed line: m ∗d /m e = 0.6, 0.7, 0.8, and 0.9), and (d) temperature-dependent DOS effective mass of the compounds hole concentration via the formation of antisite defect weakens the Seebeck coefficient as discussed above, while the increase in the DOS effective mass can make a positive impact upon it These contrary effects of Sb are especially important to the optimization of power factor (PF = S 2σ) in the Bi2−x Sb x Te3 compounds because each parameter in PF is critically dependent on both the carrier concentration and the DOS effective mass It is noteworthy that the DOS effective mass tends to decrease monotonously with the increase in the temperature (see the arrows from T1 to T6) The estimated DOS effective masses are shown in Fig 2(d) This temperature-dependence of the DOS effective mass leads to the weakened Seebeck coefficient at relatively high temperatures, and it is the reason why the Seebeck coefficients are not linearly increasing with the temperature even in the degenerately doped Bi2−x Sb x Te3 compounds (x = 1.64 and 1.68) whose hole concentrations are almost independent of temperature as shown in Fig 2(a) In addition, detailed composition-dependent charge transport behavior in the Bi2−x Sb x Te3 compounds was investigated Figs 3(a) and 3(b) show temperature-dependent electrical conductivities and Hall mobilities of the Bi2−x Sb x Te3 compounds, respectively As the amount of Sb in the compounds increased, the electrical conductivity was significantly improved mostly due to the increase in the hole concentration The effect of Sb content on the mobility was not clear in this figure, TABLE I Charge transport properties of Bi2−x Sb x Te3 compounds at room temperature: electrical conductivity, Seebeck coefficient, hole concentration, Hall mobility, and DOS effective mass obtained by Pisarenko relation at room temperature x σ (S/cm) S (µV/K) P (1019/cm3) µ (cm2/Vs) m ∗d /m e 1.56 1.60 1.64 1.68 803.16 993.19 1092.52 1175.26 211.12 194.64 187.78 177.82 1.74 2.51 3.10 3.48 260.60 239.51 242.35 232.92 0.69 0.81 0.90 0.92 104812-5 An et al APL Mater 4, 104812 (2016) FIG (a) Temperature-dependent electrical conductivities and (b) Hall mobilities of the Bi2−x Sb x Te3 compounds (c) Normalized mobilities as a function of T −3/2 and (d) temperature-dependent power factor of the compounds and it was differently observed at each temperature The electrical conductivities exhibited negative temperature-dependence in all compounds, resulting from the well-known phonon scattering (µ ∝ T −3/2).19,26–28 To clarify the scattering mechanism in detail, normalized mobilities were replotted as a function of T −3/2 in Fig 3(c) The normalized mobilities exhibited linear relationship with T −3/2 in all compounds, indicating that the mobility is dominantly governed by acoustic phonon scattering and/or nonpolar optical phonon scattering.28 However, if the charge transport was dictated solely by the phonon scattering, the extrapolated mobility should be zero when 1/T is zero As shown in this figure, the extrapolated mobility did not converge on the origin point, and the offset from the origin point decreased with increasing Sb content as marked by an arrow This result implies strongly that an additional charge scattering mechanism is also working in the compounds as well as the phonon scattering, and that the additional scattering can be weakened by the Sb incorporation Although the increasing effective mass with increasing Sb content can retard the charge transport, this weakened additional scattering by the Sb incorporation can give beneficial effects on the charge transport Therefore, the effect of Sb content on the mobility could be elucidated by the temperature-dependence of mobility rather than the value of mobility itself For the additional effects in the Bi2−x Sb x Te3 compounds, grain boundary scattering, ionized impurity scattering, and alloy scattering can be suggested.29,30 Firstly, the effects of the grain boundary scattering on the charge transport can be disregarded in this experiment because the structural properties in the compounds were not significantly different to each other as shown in Fig Our observation in Fig 3(c) cannot be explained by the ionized impurity scattering either Because the Sb incorporation increases the antisite defect with the hole concentration, the incorporation of Sb should intensify the additional scattering by the ionized impurity Therefore, the effect of the ionized impurity scattering on the charge transport can be negligible in the experiment On the other hand, because the alloy scattering is due to the locally distorted band structure in the alloyed compounds, it can be alleviated in the Bi2−x Sb x Te3 compounds by the dominant occupation of Sb at Bi-sites.19,31,32 Recently, Hu et al reported the enhancement of mobility by the incorporation of Sb in Bi2−x Sb x Te3 compounds especially when x > 1.60, and they attributed the enhanced mobility to the weakened alloy scattering.19 Although their Bi2−x Sb x Te3 compounds were prepared by hot press and hot deformation methods, their result is quite consistent to ours Therefore, it was manifested 104812-6 An et al APL Mater 4, 104812 (2016) that the charge transport in the Bi2−x Sb x Te3 compounds is dominantly governed by the hole-phonon scattering and partially influenced by the alloy scattering in this experimental condition Furthermore, this alleviated additional alloy scattering with increasing x is the reason why the mobility values in Fig 3(b) were weakly dependent on the increasing effective mass with the Sb content in the Bi2−x Sb x Te3 compounds When we simultaneously consider its metallic charge transport behaviour in Fig 3(a) and the temperature-dependent DOS effective mass in Fig 2(d), the maximum power factor should be achieved at the lowest temperature of T1 (∼306 K) as shown in Fig 3(d) and this behaviour can be one of the reasons why Bi2−x Sb x Te3 compound exhibits the best thermoelectric performance near room temperature In summary, the effects of Sb content on the charge transport and DOS effective mass-related Seebeck coefficient in Bi2−x Sb x Te3 compounds were investigated The increase in the hole concentration with increasing Sb content led to the enhancement of the electrical conductivity The charge transport was dominantly governed by the phonon scattering in all compounds However, the charge transport was additionally affected by the alloy scattering and it could be alleviated by increasing Sb content in the compounds Therefore, it was proven that the incorporation of Sb gives the positive effects on the charge transport properties not only by increasing the carrier concentration, but also by restraining the additional alloy scattering On the other hand, contrary effects of Sb incorporation on the Seebeck coefficient were observed Although the Seebeck coefficient decreased with increasing Sb content due to the drastic generation of hole carriers via the easy formation of antisite defects, the degree of the reduction was limited due to the increase in the DOS effective mass with increasing Sb content Furthermore, temperature-dependent DOS effective masses in Bi2−x Sb x Te compounds were estimated for the first time based on the temperature-normalized Pisarenko plot The temperature-normalized Pisarenko plot not only provides the relation between the carrier concentration and Seebeck coefficient in the compound, but also presents the dependence of the DOS effective mass on temperature which critically affects the Seebeck coefficient It can be generally applicable to other thermoelectric materials system, and it will be helpful for the comprehensive understanding of the temperature-dependent thermoelectric transport properties This research was supported by the Nano·Material Technology Development Program (No 20110030147) and also 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Composition- dependent charge transport and temperature -dependent density of state effective mass -dependent Seebeck coefficient were investigated in Bi2−x Sb x Te3 (x = 1.56-1.68) compounds The compounds