Effect of substrates on thermoelectric properties of Ag–Sb–Te thin films within the temperature annealing

Journal Pre-proof Effect of Substrates on Thermoelectric Properties of Ag-Sb-Te Thin Films within the Temperature Annealing

Natchanun Prainetr, Athorn Vora-ud, Somporn Thaowonkaew, Mati Horprathum, Pennapa Muthitamongkol, Tosawat Seetawan PII:

S0921-4526(19)30854-3

DOI:

https://doi.org/10.1016/j.physb.2019.411977

Reference:

PHYSB 411977

To appear in:

Physica B: Physics of Condensed Matter

Received Date:

13 September 2019

Accepted Date:

31 December 2019

Please cite this article as: Natchanun Prainetr, Athorn Vora-ud, Somporn Thaowonkaew, Mati Horprathum, Pennapa Muthitamongkol, Tosawat Seetawan, Effect of Substrates on Thermoelectric Properties of Ag-Sb-Te Thin Films within the Temperature Annealing, Physica B: Physics of Condensed Matter (2019), https://doi.org/10.1016/j.physb.2019.411977

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Effect of Substrates on Thermoelectric Properties of Ag-Sb-Te Thin Films within the Temperature Annealing Natchanun Prainetr1, Athorn Vora-ud1,2,3*, Somporn Thaowonkaew1,2,3, Mati Horprathum4, Pennapa Muthitamongkol5, and Tosawat Seetawan1,2,3† 1Program

of Physics, Faculty of Science and Technology, Sakon Nakhon Rajabhat University, 680 Nittayo Road, Mueang District, Sakon Nakhon 47000, Thailand

2Thin

Films Research Laboratory, Center of Excellence on Alternative Energy, Research and Development Institution, Sakon Nakhon Rajabhat University, 680 Nittayo Road, Mueang District, Sakon Nakhon 47000, Thailand

3Thailand

Center of Excellence in Physics, Ministry of Higher Education, Science, Research and Innovation, 328 Si Ayutthaya Road, Bangkok 10400, Thailand

4National

Electronics and Computer Technology Center, National Science and Technology Development Agency, Pathumthani 12120, Thailand

5National

Metal and Materials Technology Center, National Science and Technology Development Agency,

114 Thailand Science Park, Pahonyothin Road, Klong Nueng, Khlong Luang, Pathumthani 12120, Thailand

Abstract The Ag-Sb-Te (AST) thermoelectric thin film was prepared on soda-lime glass and SiO2 1-µm/Si-wafer substrates by DC magnetron sputtering method at the room temperature (RT). As-deposited thin film samples on both substrate were annealed in temperature range 573773 K for 30 min within argon (Ar) atmosphere. The crystal structure, morphology, composition, electrical resistivity and Seebeck coefficient of as-deposited and annealed thin films are investigated. The results demonstrated that the effect of substrates was influenced the crystallinity, morphology and thermoelectric properties of the AST thin films within *

Corresponding author. E–mail address: [email protected], Tel.&Fax: +664-274-4319

†

Corresponding author. E–mail address: [email protected], Tel.&Fax: +664-274-4319

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annealing temperature. Highlighted, the AST thin film on SiO2/Si-wafer substrate showed the highest power factor of 0.84 mW m1 K2 while the AST thin film on soda-lime glass substrate was around 0.053 mW m1 K2 at annealing temperature 773 K.

Keywords: thermoelectric thin films; silver-antimony-telluride (AST) thin films; SiO2/Siwafer substrate; soda-lime glass substrate; magnetron sputtering method

1. Introduction Nowadays, thermoelectric (TE) materials have been offered to convert of low-grade waste heat energy into electrical power application such as; vehicle mobility and accessories [1], wearable electronics, biometric sensors, and autonomous robots [2-5] which based on the TE thin film [6]. Actually, thermoelectric materials that rely on high performance were assessed with the dimensionless figure of merit (ZT) as followed;

ZT 

S 2T





( PF )T



,

(1)

where S, ρ, κ and T are Seebeck coefficient, electrical resistivity, thermal conductivity, and absolute temperature, respectively. Then, S2/ρ is defined as the power factor (PF). Hence, the

ZT could be enhanced by raising the S2/ρ and then decreased of the thermal conductivity. It is well-known that the power factor plays an important in enhancing the output power of the heat to electric conversion rather than thermal conductivity. Examples of TE materials with high performance such as; Ag-Ge2Sb2Te5 thin film (PF ⁓ 5.83 mW m1 K2) [7], AgPbSbTe alloys (bulk, ZT ⁓ 2.20) [8], and SnSe alloys (bulk, ZT⁓ 2.60) [9-10], which are alloy based materials. Especially, AgSbTe2 p-type material has been received much more attention to very interesting for the thermoelectric devices within high ZT around 1.59 at 673 K [6] and 1.55 at 533 K [11]. This material was promised to TE material since the early 1960s [12],

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which obtained from the solid solution between Ag2Te and Sb2Te3 [13-15]. Moreover, the AgSbTe2 compound is low thermal conductivity about of 0.60 – 0.70 W m1 K1 [16-18] and large Seebeck coefficient to be much more promised to the candidate for high-efficiency of ptype TE applications. Here, we reported the influence of substrate (soda-lime glass versus SiO2/Si wafer) on thermoelectric properties of Ag-Sb-Te thin films. For preparing the thin film samples focused on DC magnetron sputtering technique. This technique is well known that the thermoelectric properties can be improved due to stronger quantum confinement effect, a mismatch between film and substrate as well as control crystallinity [19].

2. Experimental detail The AST thin film samples were deposited on soda-lime glass and SiO2 1-µm/Siwafer substrates by DC magnetron sputtering system using the Ag : Sb : Te; 33.33% : 33.33% : 33.33% ratio target (99.99% purity, ULVAC (Thailand) Ltd.) for 59.00 mm of diameter and 3.00 mm of thickness. The conditions of thin film preparation were showed in Table 1. Before deposition process, all substrates floating were cleaned by ultrasonic washer within acetone, methanol and deionized water each for 30 min, and then loaded on a substrate holder in deposit chamber with a conventional position (upper side). The distance between the substrate holder and the sputtering cathode (ds-t) was fixed at 60 mm with a direct toward the substrate (lower side). The deposition processing and condition are base pressure below 4.00 × 103 Pa, the Ar flow rate of 30 sccm, working pressure about of 2.67 Pa, and sputtering power for AST target around 40 W. The deposition time fixed at 1 min to be controlled the film thickness of 100 nm with the ds-t variation to obtain of the deposition rate around 100 nm/min. The base pressure as seemed low vacuum which induces the residual oxygen probably incorporated during the film deposition. However, as-deposited thin films will be annealed at temperature

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573 K to 773 K under Ar atmosphere at pressure 40 Pa, each for 30 min to be phase transformation and removed oxygen probably contaminated. The characterization of thin film composed of the crystal structure, morphology and atomic composition of thin films were investigated by X-ray diffractometor (XRD; XRD6100, Simadzu), field emission scanning electron microscope (FE-SEM; SU8030, Hitachi), energy dispersive X-ray spectroscopy (EDX; SU8030, Hitachi), respectively. Finally, thin film thermoelectric measurement at room temperature for the electrical resistivity and Seebeck coefficient were measured by ZEM-3 method (ZEM-3, Advance Riko) under the He atmosphere. In addition, electrical properties were measured by Hall Effect method (HMS-3000, Ecopia) to be obtained the carrier concentration and mobility values to compare the electrical resistivity as measured from ZEM-3 method. Moreover, the power factor could be calculated from the Seebeck coefficient and electrical resistivity values.

3. Results and Discussion The crystallinity data of AST thin films on difference substrate were analyzed from XRD result as shown in Fig. 1. Figure 1 (a) and (b) showed the XRD pattern of AST thin films deposited on soda-lime glass and SiO2 1-µm/Si-wafer substrates, respectively. Both asdeposited thin films displayed the amorphous phase and then became crystalline phase after the temperature annealed. All annealed thin film showed the phase mixing with homologous phase structure for the cubic structure of AgSbTe2 (PDF# 015-0540) to diffraction reflections of (200), (220) and (222) planes and rhombohedra structure of Ag2Te (PDF# 015-0874) according to (0111) and (110) planes. In addition, as annealed at 723 K and 773 K, thin film has been the displayed impurities peaks of Te phase because phase disorder affected [20]. From the XRD results, the substrates and annealing temperature also affected on the grain size (D), and lattice strain (ε) as shown in Fig. 1 (c) and (d), respectively, which

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analyzed via PCXRD software Ver. 7 rel. 001 (Shimadzu) based on Debye-Scherrer’s formula and Hall’s method equation: 2   (  meas   ref2 )1/ 2 ,

D



K , and  cos 

 4 tan 

,

(2) (3)

(4)

where  meas ,  ref , K ,  , and  are the measured full width at half maximum (FWHM), reference FWHM, shape factor (0.9), wavelength of CuKα radiation, and diffraction angle, respectively. Figure 2 showed the SEM images to be analyzed the surface morphology of AST thin films on soda-lime glass and SiO2 substrates as more differenced. The AST thin films on soda-lime substrates were more crystal growth but the AST thin films on SiO2 glass substrates were exhibited high smoothness and uniformity. Strong difference in the morphology of films sputtered on soda-lime glass and SiO2/Si-wafer substrates can be connected with different energy of bombarding Ar ions and condensing sputtered atoms probably due to their different of resistance and thermal shock. In addition, The AST thin films on soda-lime substrates have been tended of the porosity to be increased at high temperature annealing. These results corresponded to the lattice strain value of AST thin films on soda-lime glass and SiO2 substrates as difference. Moreover, the difference of substrate has been affected by the composition of thin films as displayed in Fig. 3. Figure 3 showed the atomic composition percentage of AST thin film on soda-lime glass and SiO2 substrates as analyzed by EDX technique. The result found that the Ag element of both thin films is same within decreased continue with annealing temperature increasing. While the Sb and Te elements of both thin films are seemed the value switching due to the difference of

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surface interphase of substrates. Which, the atomic composition is more effected to electrical and thermoelectric properties, as discussed in the next section. Electrical properties of AST thin films on soda-lime glass and SiO2 substrates compose of concentration and mobility were shown in Fig. 4. In Fig. 4 (a), the concentration on soda-lime glass substrate of AST thin films scratchy increased from 12.60  1020 cm3 at annealed temperature 573 K to 17.90  1020 cm3 at annealed temperature 673 K. While, the mobility on soda-lime glass substrate of the thin films serially decreases with annealing temperature increase which reduced about of 21%. The highest mobility is 5.24 cm2 V3 s1 at annealed 723 K and lowest to 4.05 cm2 V3 s1 at annealed temperature 773 K due to the effect of separate grains boundary increasing and transmute to disorders of crystal. In Fig. 4 (b), the concentration on SiO2 substrate of AST thin films continuous increase from 30.70  1020 cm3 at annealed temperature 573 K to 110  1020 cm3 at annealed temperature 773 K while mobility of thin films firstly increases then gradually decreases due to there had smallest grains size and without space on the surface of the film. The electrical resistivity, Seebeck coefficient and power factor of AST thin films on soda-lime glass and SiO2/Si-wafer substrates with annealed at 573 K to 773 K, are shown in Fig. 5 (a), (b) and (c), respectively. In comparison, the electrical resistivity of thin films for both substrates found that the electrical resistivity of the thin film on soda-lime glass substrate higher than that of the electrical resistivity of thin film on SiO2/Si-wafer substrate at all temperature. While, the Seebeck coefficient of AST thin films on both substrates have positive, which indicates that they are p-type semiconducting material within decreased with annealing temperature increasing. The reduction of Seebeck coefficient for thin films when annealing temperatures increases due to changes in their film surface. In addition, the Seebeck coefficient and electrical resistivity values are calculated to obtain the power factor. The power factor of AST thin films on both substrates are same as showed the power factor

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around 0.10 – 0.15 mW m–1 K–2 at annealed temperature range 573 – 673 K. But after annealed temperature 673 K, the power factor of AST thin film on SiO2/Si-wafer substrate has been slightly increased with the temperature annealing increasing, while the power factor of AST thin films on soda-lime glass substrate was seemed consistence. At annealed temperature 773 K, the power factor of AST thin film on SiO2/Si-wafer substrate had the value more than AST thin film on soda-lime glass substrate as approximately two orders of magnitude. As these results of power factor on SiO2/Si-wafer substrate seem to increases agreeing with the concentration and electrical resistivity due to increasing carriers scattering in lattice structure [21].

4. Conclusion The thin film thermoelectric properties of AST thin films as deposited by DC magnetron sputtering method were reported base on the influence of soda-lime glass and SiO2/Si wafer substrates through the temperature annealing. Strong difference results within the difference substrate have been affected to morphology, atomic composition as well as electrical and thermoelectric properties of AST thin films. At temperature annealing 573 – 673 K, the power factor of AST thin films on both substrates are same within around 0.10 – 0.15 mW m–1 K–2. While the AST thin films deposited on SiO2/Si-wafer substrate had highest power factor more than soda-lime glass substrate around two times of magnitude at annealed temperature 773 K (0.84 mW m1 K2 for SiO2/Si-wafer substrate and 0.053 mW m1 K2 for soda-lime glass substrate).

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Acknowledgments This work was financially supported by National Research Council of Thailand (NRCT), Thailand Research Fund (TRF) Research Career Development Grant, RSA (RSA6180070), and the TRF-MRG Young Scientific Researcher (Grant No. MRG6180007).

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List Figures and Table Figure 1 XRD patterns of AST thin films on (a) soda-lime glass substrate and (b) SiO2 /Siwafer substrate, and (c) grain size and (d) lattice strain between difference substrate at annealing temperature functions. Figure 2 Comparisons of SEM images of AST thin films on (a) soda-lime glass substrate and (b) SiO2/Si-wafer substrate as annealing temperature functions. Figure 3 Atomic compositions of AST thin films on (a) soda-lime glass and (b) SiO2/Si-wafer substrates. Figure 4 The concentration and mobility of AST thin films on (a) soda-lime glass and (b) SiO2/Si-wafer substrates. Figure 5 Thermoelectric properties of AST thin films on soda-lime glass and SiO2/Si-wafer substrates (a) electrical resistivity, (b) Seebeck coefficient and (c) power factor. Table 1 The conditions of thin film preparation.

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Conflict of Interest form The authors declare that there is no conflict of interests regarding the publication of this manuscript. I hereby declare the contributions of the authors to the original manuscript, as the following: Natchanun Prainetr, Athorn Vora-ud, Somporn Thaowonkaew, Mati Horprathum, Pennapa Muthitamongkol, and Tosawat Seetawan designed this experiments and the experiments were carried out by Athorn Vora-ud and Toswat Seetawan have analyzed the results and discussed the manuscript during the preparation. All authors discussed the results and implications and commented on the manuscript at all.

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Credit Author Statement The authors declare that the credit author statement of the authors to the original manuscript, as the following: 1. Assist. Prof. Natchanun Prainetr (20%) 2. Dr. Athorn Vora-ud (20%) 3. Mr. Somporn Thaowonkaew (10%) 4. Dr. Mati Horprathum (20), 5. Miss Pennapa Muthitamongkol (10) 6. Prof. Dr. Tosawat Seetawan (20%)