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Thermoelectric Power Factor Enhancement by Pulsed Plasma Engineering in Magnetron Sputtering Induced Ge2Sb2Te5 Thin Films Manish Kumar, Athorn Vora-ud, Tosawat Seetawan, and Jeon Geon Han ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00717 • Publication Date (Web): 11 Jul 2018 Downloaded from http://pubs.acs.org on July 12, 2018

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ACS Applied Energy Materials

Thermoelectric Power Factor Enhancement by Pulsed Plasma Engineering in Magnetron Sputtering Induced Ge2Sb2Te5 Thin Films Manish Kumar,a,b* Athorn Vora-ud,a,c† Tosawat Seetawan,c Jeon Geon Hana a

Center for Advanced Plasma Surface Technology, NU-SKKU Joint Institute for Plasma-Nano Materials, School of

Advanced Materials Science and Engineering, Sungkyunkwan University, Suwon, 440-746, Korea. b

Center for Advanced Materials, Organisation for Science Innovations and Research, Bah, 283104, India.

c

Program of Physics, Faculty of Science and Technology, Sakon Nakhon Rajabhat University, Sakon Nakhon, 47000,

Thailand.

E -mail: *[email protected], [email protected]. †[email protected]. Abstract Precise control over microstructure and composition is desired prerequisite for the performance enhancement of thermoelectric materials. In conventional magnetron plasma sputtering synthesis, composition control is challenging when the sputtering-target is composed by different elements. Here, the potential of pulsed power utilization is demonstrated for compositional control of Ge2Sb2Te5 thin films via pulse -reversal time and -frequency engineering in pulsed DC-magnetron sputtering process. When annealed at 400 oC for 1 h in vacuum conditions, amorphous thin films (of 200 nm thickness, deposited on glass substrate) crystallize in to face centered cubic phase with average nanocrystallite size ~10 nm. Power density enhancement to 5.56 W/cm2 at low pulse reversal time induces maximum process throughput as 450 nm/min. Increase in either of pulse frequency or pulse reversal time decreases the discharge voltage and plasma density. As a consequence, kinetic energy of ions and ionization of plasma species are sequentially controlled in order to improve the stoichiometry of film and eventually; the electronic transport. The optimization of pulse plasma engineering yields maximum thermoelectric power factor value as 1.35 µWcm−1K−2 with process throughput more than 300 nm/min. The obtained values are promising for applications in the automobile and microelectronics industry. Keywords: Ge2Sb2Te5; pulsed plasma; power factor; surface properties; thin films. ACS Paragon Plus Environment

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1. Introduction Thermoelectric (TE) materials are promising for the demands of environmental friendly energy conversion.1-3 Typical applications of TE devices comprise areas of power generation through TE generators, solid-state refrigeration through micro-coolers, smart power management and sensors for automobile industry. TE materials combine thermal, electrical and typically, also semiconductor properties to convert heat into electricity or electrical power directly into cooling. The efficiency of energy conversion depends on the unit-less quantity; figure of merit (ZT) which is defined as;4 =

××

=

×

(1)

where, S is the Seebeck coefficient, σ the electrical conductivity, κ the total thermal conductivity (often expressed as the sum of the lattice thermal conductivity and the carrier thermal conductivity), T the absolute temperature and PF the power factor. Hence, high performance TE materials should possess high PF values and low κ values. Ge-Sb-Te alloy system exhibits low κ values (typically less than 1 Wm-1K-1) owing to its structural features; heavy constituent elements in large unit-cells and relatively weak Van der Waals bonding between their slabs. Both of these structural characteristics lead to strong phonon scattering.5-7 Furthermore, interesting electronic and phase-change properties of Ge-Sb-Te compounds find applications in optical and electrical data storage.8-10 As far TE applications are concerned, these compounds can promise great potential if PF values are sufficiently large. The majority of the efforts for TE performance improvement choose strategies out of; the reduction of lattice part of κ by introducing rattler atoms in cage-like structures, embedding nanoparticles in the host matrix, or nanostructuring in more conventional materials.2,3 However, optimizing one often degrades the value of the other. Synergic benefits in TE performance improvements can be possible by the smart integration of these two approaches through optimum structural engineering. The structural control on TE material depends on the synthesis conditions and used thermal treatments. For the synthesis of TE thin films, magnetron sputtering based synthesis techniques are frequently used.11-13 Using radio frequency (RF) magnetron sputtering, PF values was found to be close to 1 µWcm-1K-2 in 500 nm-thick Ge2Sb2Te5 films.11 In DC magnetron sputtering, PF values was ACS Paragon Plus Environment

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found as 0.563 µWcm-1K-2 in 25 nm thick Ge2Sb2Te5 films.13 The process throughput is usually enhanced by increasing the power density. However, at higher power density, control of elemental composition, ion's kinetic energy induced surface defects, and arcing in plasma emerge as critical concerns. These issues often limit the performance of TE thin films. Pulsed magnetron sputtering process is one of the significant development as it can reduce/eliminate arcing during the plasma process at higher power densities owing to their fundamental behavior of reverse-voltage pulsing.14-16 In addition, the pulsed DC power can also increase the degree of the ionization of sputtered atoms in plasma.15,16 Integrating the potential of pulsed power operation and smart design of magnetron processing, we had developed different kind of thin films17-20 for targeted applications. The present work is planned in the background of our earlier theoretical21 and experimental work22,23 on Ge-Sb-Te clusters and thin films. Using the molecular orbital and dynamical simulations, we have shown the variation of TE properties of Ge-Sb-Te model clusters in different compositions and crystalline phases.21 Theoretical simulations also provided enhancement in S values as well as in PF values for the increase in temperature from 300 K–700 K.21 Experimentally, we developed Ge2Sb2Te5 thin films using pulsed DC magnetron sputtering process, and found that the films exhibited a metastable cubic phase after the annealing.22 In this plasma process, electron temperature was controlled by using pulsed DC plasma power, and the resulting structural disorder of the cubic crystalline phase enhanced S values to 190.8 µVK−1, as supported by molecular orbital calculations.22 Further, optimization of film thickness, power density, working pressure and annealing temperature provided the highest PF values 0.75 x10-4 Wm-1K-2.23 This work is an extension to improve the PF values of Ge2Sb2Te5 thin films by sequential control on pulse frequency (fp) and pulse reversal time (tpr). The effect of fp and tpr on properties of thin film is also important from the perspective of deeper insight of pulse-DC plasmas; which is sparsely reported.

2. Experimental details Thin films were deposited by magnetron sputtering system, as schematically shown in Fig. 1(a), using Ge2Sb2Te5 target (diameter: 5.08 cm, thickness: 0.4 cm, procured from Biz-material). The ACS Paragon Plus Environment

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vertically mounted sputtering target was powered by pulsed-DC power supply (PinnacleTM Plus, Advance energy), which can deliver maximum power up to 10 kW. Before deposition, soda lime glass substrates were cleaned, and loaded on a vertical substrate holder in front of sputtering target at distance of 5 cm for obtaining the most optimal thickness uniformity. The pulsed-DC power was varied from 30 W to 115 W corresponding to 1.48 to 5.67 W/cm2 power density. In all conditions, base pressure was fixed at 2.5×10−5 Torr and working pressure was fixed at 5 ×10−3 Torr using Ar gas flow rate as 40 sccm. At fixed pulsed-DC power, fp was varied from 30 kHz to 70 kHz (at fixed tpr: 3 µs) and tpr was varied from 1µs to 5 µs (at fixed fp: 50 kHz). After the deposition, all the films are identically annealed at 400 oC for 1 h in vacuum condition. The film thickness was investigated by surface profiler (Alpha-step, KLA Tencor). Phase identification of thin films was carried out using X-ray diffraction technique (Bruker, Discover D8). Surface morphology and atomic composition of the thin films were observed by using field emission scanning electron microscope (FESEM) and energy dispersive X-ray spectroscopy (JEOL JSM6500F). Electron beam of 15 keV energy was used and images of micrographs were captured at 100,000 x magnification. Electrical properties were measured at room temperature by a four-point probe of the van der Pauw method using a Hall measurements system (ECOPIA HMS-3000). For estimation of PF, S was measured by our own developed apparatus. The design and details of this set up has been reported in our previous work.23

3. Results and discussion The film thickness after annealing has been reduced ~10-15%, hence the initial thicknesses are controlled in such a manner that post-annealing film thickness should remain same (200 nm ±5 nm). The dependences of deposition rate on variation of; power density, fp and tpr are displayed as (b), (c) and (d), respectively in Fig. 1. It is evident in Fig. 1(b) that deposition rate increases with the power density, which can be justified due to plasma density enhancement. The maximum deposition rate is observed to be 450 nm per min for the power density 5.67 W/cm2. Note that power density can not be further increased because of starting of micro-arcing. For keeping it safe, power density is fixed at ACS Paragon Plus Environment

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4.93 W/cm2 for further experiments. When the tpr is varied (at fixed fp 50 kHz), the deposition rate is minimised at tpr as 3 µs, as shown in Fig. 1(c). This can be due to plasma properties reversal in terms of ionization. For further insight, discharge characteristics of plasmas are monitored. Increase in tpr leads to the decrease in discharge voltage and almost linear increase in the current values. When the fp is increased from 30 kHz to 70 kHz (at fixed tpr 3 µs), deposition rate is decreased almost linearly, as shown in Fig. 1(d). The manual with power supply provided the relation of tpr, fp and duty cycle factor (η);

= 1

(1)

For tpr variation in 0.4-5 µs range, the maximum duty cycle increases linearly from 15% to 35% on increasing fp values from 30 kHz to 70 kHz. The trends of voltage and current are similar to that with

Figure 1

(a) Schematic diagram of pulsed DC- magnetron plasma sputtering system for

the deposition of Ge2Sb2Te5 thin films. Deposition rate variation as a function of power density (b) pulse reversal time (b) and pulse frequency (d). Discharge characteristics of plasmas are shown in (c) and (d)[symbols used: solid sphere- deposition rate, square- voltage, circlecurrent].

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tpr; however the variation is not as steep as with the tpr. These results demonstrate that tpr as well as fp both results in decrease of voltage which is a measure of kinetic energies of ionic species in plasma. Further, sufficient reduction in discharge voltage still provides fare values of deposition rate. The equation (1) indicates that at higher tpr, the η will decrease when the fp is kept constant. Lower η means low voltage drawing from the power supply which shall be compensated by the increase in current. Since, fixed power was supplied to the sputtering target, increase in current is justified with the decrease in voltage at high tpr.

Figure 2

XRD patterns of annealed Ge2Sb2Te5 thin films prepared under varying (a) pulse reversal time and (b) pulse frequency. Corresponding variation in average crystallite size and lattice strain are shown under varying (c) pulse reversal time and (d) pulse frequency [symbols used: solid sphere- average crystallite size, solid square- lattice strain].

As-deposited films are amorphous and crystallize when annealed at 400 oC. X-ray diffraction (XRD) patterns of annealed films are shown in Fig. 2, as a function of (a) tpr and (b) fp. Diffraction ACS Paragon Plus Environment

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patterns exhibit Ge2Sb2Te5 cubic structure (225/Fm-3m) with prominent (111), (200), (220) and (222) diffraction peaks; in agreement to PDF number 054-0484.24 Lattice strain (ε) and average crystallite size (D) are estimated by Hall's equation and Debye-Scherer's formula using full width at half maxima of (200) diffraction peak: D=

Kλ β cos θ

(2)

β

(3)

and ε=

4 tan θ

where, K, λ, β and θ are the shape factor (0.9), the wavelength of Cu-Kα radiation (1.54060 Å), FWHM and diffraction angle, respectively. The variation in lattice strain and average crystallite size is plotted in Figs. 2(c) and (d) to illustrate the effect of tpr and fp variation, respectively. The lattice strain values are relatively small and average crystallite size varies from 8.38 nm to 12.95 nm for tpr variation, whereas from 10.96 nm to 13.18 nm for fp variation. Such smaller crystallite size is important to minimize the thermal transport. The maximum of average crystallite size (with tpr variation) can be associated to the minimum of deposition rate as shown in Fig. 1(c). On the other hand, crystalline growth only continues up to 40 kHz on reduction of fp. These results give an important support for the governing role of pulse parameters on early nucleation of clusters during the deposition. In the appropriate conditions of deposition rate minimization, available energy is distributed to lesser number of depositing species, therefore having higher average energy values for the nucleation and growth. The identical post-deposition thermal treatment induces crystallite size variation according to size-separated cluster deposits. Figure 3 shows the variation in surface morphology and elemental composition, studied by field emission scanning electron microscopy (FESEM). Micrographs (a), (b) and (c) correspond to the annealed films prepared under tpr - fp conditions; 1 µs - 50 kHz, 3 µs - 50 kHz and 3 µs - 40 kHz, respectively. All micrographs exhibit high uniformity and crack-free dense deposition. The grainy structures at the surface hints the post-annealing crystallization of the films, however marked


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differences couldn't be established due to very small size of crystallites. The elemental information is obtained through energy dispersive X-ray spectroscopic (EDX) results corresponding to the films prepared with pulse parameters; (d) 1 µs, 50 kHz, (e) 3 µs, 50 kHz and (f) 3 µs, 40 kHz. EDX results confirm the presence of major elements of target (Ge, Sb and Te) and minor elements of substrates. The FESEM images corresponding to power density variation are also given in supporting information.

Figure 3

Top-view SEM images of annealed Ge2Sb2Te5 thin films prepared at 4.93 W/cm2 power density and varying pulse parameters; (a) 1 µs, 50 kHz, (b) 3 µs, 50 kHz, (c) 3 µs, 40 kHz. EDX patterns are given corresponding to the films prepared with pulse parameters; (d) 1 µs, 50 kHz, (e) 3 µs, 50 kHz, (f) 3 µs, 40 kHz.

Irrespective of the used power density, surfaces are found sufficiently uniform, crack free, densely packed and perfect adherent to the substrate. Composition variation as a function of power density and selective pulse parameters (for best optimization of tpr and fp) is shown in Table 1. It is found that enhancement in power density leads to enhancement in Te majorly on the expense of Sb. Such Te excess can lead to the detrimental Te segregation; usually observed by phase-change stress in Ge2Sb2Te5 materials.25,26 It is apparent that pulse engineering through optimization of tpr and fp improves the compositional stoichiometry and avoids Te segregation.


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Table 1 Power

Elemental composition of thin films by varying the pulse parameters density Pulse frequency

Pulse

reversal Ge

Sb

Te

(W/cm2)

(kHz)

time (µs)

(%)

(%)

(%)

3.45

50

1

19.99

22.77

57.24

4.19

50

1

18.16

22.54

59.3

4.93

50

1

18.21

12.89

68.9

5.67

50

1

18.57

16.32

65.11

5.67

50

3

27.01

22.63

50.35

5.67

40

3

27.21

22.83

49.95

Figure 4 shows the variation in electrical (a, b) and TE properties (c, d) of thin films as a function of tpr and fp. Electrical properties exhibit typical degenerate semiconductor like characteristics. Despite high values of carrier concentration, relatively lower values of carrier mobility limit the effective reduction in resistivity of the films. However, the lower values of mobility come as an advantage as they reduce the mean free path of carriers,13 and therefore grain boundary scattering as well as surface scattering of carriers are significantly avoided. S and PF values are maximised as 282.3 µVK-1 and 1.35 µWcm–1K–2 at tpr as 3 µs followed by reducing fp as 40 kHz. The maximum values of S as well as PF can be correlated to the maximum average crystallite size in the films, as shown by XRD results. These obtained values are compared with other reported Ge-Sb-Te compounds with similar/close compositions in Table 2. The comparison exhibits a descent advance achieved in present case, as the achieved power factor is ~35% increment in comparison to the best reported values in films (with thickness of 2.5 times to those in present case), and almost twice in comparison to films of similar thickness.


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Figure 4

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The variation of electrical properties (carrier concentration, mobility and resistivity) as function of (a) pulse reversal time and (b) pulse frequency [symbols used: solid sphereresistivity, square- carrier concentration, circle- mobility]. The variation of Seebeck coefficients and thermoelectric power factor as function of (c) pulse reversal time and (d) pulse frequency [symbols used: square- Seebeck coefficient, circle- power factor].

For an idealized non-equilibrium collisionless, Maxwellian, and unmagnetized plasma, the electron temperature is a parameter in the Maxwellian velocity distribution function of electrons. For such ideal case, the mean free paths of collision of all particles are larger than the Debye length and electron temperature (Te) is much higher than those of ions and neutral atoms. However, in case of non-thermal plasmas, the velocity distribution function is not Maxwellian. The term Te is then related to the mean electron energy of the non-thermal distribution function: Te = 2/3 Emean. Nevertheless, using this calculated temperature in Maxwellian electron velocity distribution function would be misleading if e.g. electron collision excitation or ionization rates are calculated based on this distribution function. Experimentally, a relatively easy approach to qualitatively discuss about the electron temperature is through the variation of Ar II/Ar I intensity ratio as a function of experimental ACS Paragon Plus Environment

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Table 2 No

Comparison of power factor values of Ge-Sb-Te compounds with similar compositions.

Composition

Material Form

S (µVK-1)

Conductivity

Power

factor Reference

(S cm-1)

(µW cm-1 K-2)

1

Ge2Sb2Te5

Bulk

31

2400

2.306

27

2

Ge2Sb2Te5

Bulk

30.6

3420

3.202

28

3

Ge2Sb2Te5

Bulk

38-42

1500-3700

2.5-7.5

29

4

Ge2Sb2Te5

Thin Film (25 20-200

50-3000

0.563

13

250

0.9

11

9-21

0.8

30

21.27

0.77

22

21.05

0.75

23

16.9

1.35

Present

nm thickness) 5

Ge2Sb2Te5

Thin Film (500 60 nm thickness)

6

Ge3Sb2Te6

Thin Film

190-300

7

Ge2Sb2Te5

Thin Film (200 190.8 nm)

8

Ge2Sb2Te5


9

Ge2Sb2Te5


work

parameters. We have earlier shown that through the variation of Ar II/Ar I, information about the plasma induced energy transfer can be qualitatively deduced,22,23 and correlated to the structural changes of the material. In present case, the optimization of pulse frequencies and pulse reversal time synergically control the duty cycle and ionization of plasma species in order to enhance Te which subsequently induce films with better alloying and high crystallinity in Ge2Sb2Te5 thin films. The variation in S and PF can be correlated to the average crystallite size dependence on tpr and fp. It can be further noted that PF values are largely governed by the variation of S values due to its square dependence. The observed significant improvement in PF values of Ge2Sb2Te5 thin films ACS Paragon Plus Environment

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associated with high values of process throughput (>300 nm/min) may provide interesting TE material option for industries. We anticipate that further stoichiometric control (for addressing Sb deficiencies issue) may further increase the potential of Ge2Sb2Te5 thin films as promising TE material.

4. Conclusion By sequential pulse parameters engineering of tpr and fp (in optimized power density, working pressure and thermal annealing conditions), better control on duty cycle and ionization of plasma species is achieved in pulsed DC magnetron sputtering process for Ge2Sb2Te5 thin films. The synergic control over crystallite size and elemental composition provided process throughput more than 300 nm/min and thermoelectric power factor value as 1.35 µWcm−1K−2. The obtained values can be promising for applications as smart sensors, micro-coolers, and low-power generators in the automobile and microelectronics industry. SUPPORTING INFORMATION See supporting information for additional FESEM images and EDX patterns for thin films prepared with varying power density. ACKNOWLEDGEMENTS This study was supported by R&D Program of ‘Plasma Advanced Technology for Agriculture and Food (Plasma Farming)’ through the National Fusion Research Institute of Korea (NFRI) funded by Government funds, the Global Development Research Center GRDC, a program of the Ministry of Science, ICT and Future Planning (MSIP, Grant No. 2011-0031643, 2nd stage 3rd year), the Korea Institute for Advancement of Technology (KIAT) funded by the Ministry of Trade, Industry & Energy (MOTIE, Grant No. N0000590) and Thailand Research Fund through the RGJ Advanced Program 2017 (RAP60K0020).


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(20) Javid, A.; Kumar, M.; Yoon, S.; Lee, J.H.; Han, J.G. Size-Controlled Growth and Antibacterial Mechanism for Cu:C Nanocomposite Thin Films. Phys. Chem. Chem. Phys. 2017, 19, 237-244. (21) Vora-ud, A.; Rittiruam, M.; Kumar, M.; Seetawan, T.; Han, J.G. Molecular Simulation for Thermoelectric Properties of c-Axis Oriented Hexagonal GeSbTe Model Clusters. Mater. Des. 2016, 89, 957-963. (22) Kumar, M.; Vora-ud, A.; Seetawan, T.; Han, J.G. Enhancement in Thermoelectric Properties of Cubic Ge2Sb2Te5 Thin Films by Introducing Structural Disorder. Energy Tech. 2016, 4, 375379. (23) Kumar, M.; Vora-ud, A.; Seetawan, T.; Han, J.H. Study of Pulsed-DC Sputtering Induced Ge2Sb2Te5 Thin Films using Facile Thermoelectric Measurement. Mater. Des. 2016, 98, 254261. (24) Matsunaga, T.; Yamada, N. A Study of Highly Symmetrical Crystal Structures, Commonly Seen in High-Speed Phase-Change Materials, Using Synchrotron Radiation. Jpn. J. Appl. Phys. 2002, 41, 1674-1678. (25) Lanhorst, M.H.R.; Ketelaars B.W.S.M.M.; Wolters, R.A.M. Low-Cost and Nanoscale Nonvolatile Memory Concept for Future Silicon Chips. Nat. Mater. 2005, 4, 347-352. (26) Krusinn-Elbaum, L.; Cabral Jr., C.; Chen, K.N.; Copel, M.; Abraham, D.W.; Reuter, K.B.; Rossnagel, S.M.; Bruley, J.; Deline, V.R. Evidence for Segregation of Te in Ge2Sb2Te5 Films: Effect on the “Phase-Change” Stress. Appl. Phys. Lett. 2007, 90, 141902. (27) Shelimova, L.E.; Karpinskii, O.G.; Konstantinov, P.P.; Kretova, M.A.; Avilov, E.S.; Zemskov, V.S. Composition and Properties of Layered Compounds in the GeTe–Sb2Te3 System. Inorg. Mater. 2001, 37,342-348. (28) Konstantinov, P.P.; Shelimova, L.E.; Avilov, E.S.; Kretiva, M.A.; Zerskov, V.S. Thermoelectric Properties of nGeTe · mSb2Te3 Layered Compounds. Inorg. Mater. 2001, 37, 662-668. (29) Yan, F.; Zhu, T.; Zhao, X.; Dong, S. Microstructures and Thermoelectric Properties of GeSbTe Based Layered Compounds. Appl. Phys. A 2007, 88, 425. ACS Paragon Plus Environment

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(30) Sittner, E.-R.; Siegert, K.S.; Jost, P.; Schlockermann, C.; Lange, F.R.L.; Wuttig, M. (GeTe)x– (Sb2Te3)1–x Phase‐Change Thin Films as Potential Thermoelectric Materials. Phys. Stat. Sol. A 2013, 210, 147-152.


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Figure 1 (a) Schematic diagram of pulsed DC- magnetron plasma sputtering system for the deposition of Ge2Sb2Te5 thin films. Deposition rate variation as a function of power density (b) pulse reversal time (b) and pulse frequency (d). Discharge characteristics of plasmas are shown in (c) and (d)[symbols used: solid sphere- deposition rate, square- voltage, circlecurrent]. 250x153mm (300 x 300 DPI)


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Figure 2 XRD patterns of annealed Ge2Sb2Te5 thin films prepared under varying (a) pulse reversal time and (b) pulse frequency. Corresponding variation in average crystallite size and lattice strain are shown under varying (c) pulse reversal time and (d) pulse frequency [symbols used: solid sphere- average crystallite size, solid square- lattice strain]. 252x175mm (300 x 300 DPI)



Figure 3 Top-view SEM images of annealed Ge2Sb2Te5 thin films prepared at 4.93 W/cm2 power density and varying pulse parameters; (a) 1 µs, 50 kHz, (b) 3 µs, 50 kHz, (c) 3 µs, 40 kHz. EDX patterns are given corresponding to the films prepared with pulse parameters; (d) 1 µs, 50 kHz, (e) 3 µs, 50 kHz, (f) 3 µs, 40 kHz. 122x53mm (300 x 300 DPI)


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Figure 4 The variation of electrical properties (carrier concentration, mobility and resistivity) as function of (a) pulse reversal time and (b) pulse frequency [symbols used: solid sphereresistivity, square- carrier concentration, circle- mobility]. The variation of Seebeck coefficients and thermoelectric power factor as function of (c) pulse reversal time and (d) pulse frequency [symbols used: square- Seebeck coefficient, circle- power factor]. 250x157mm (300 x 300 DPI)



Table 1 Elemental composition of thin films by varying the pulse parameters 169x73mm (300 x 300 DPI)


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Table 2 Comparison of power factor values of Ge-Sb-Te compounds with similar compositions. 166x80mm (300 x 300 DPI)


Thermoelectric Power Factor Enhancement by ... - ACS Publications

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