Enhanced Thermoelectric Properties of Sb₂Te₃ Thin Films by In Doping

Abstract

Flexible Sb₂Te₃-based thermoelectric (TE) materials are promising candidates for fabricating energy devices that power wearable electronics and sensors. Enhancing the TE properties of Sb₂Te₃ thin films represents a significant scientific investigation. In this work, a thermal diffusion method is applied to prepare the In-doped Sb₂Te₃ thin film. In doping can lead to a high Seebeck coefficient of ~137.04 μV K⁻¹ as well as moderate electrical conductivity. As a result, the high power factor of ~18.22 μW cm⁻¹ K⁻² at 303 K is achieved. Moreover, In doping could reduce the thermal conductivity owing to the increase in phonon scattering. Finally, the high ZT values of ~0.47 at room temperature (303 K) and ~0.6 at 453 K are obtained. This indicates that In doping is a highly promising and effective approach to improving the TE performance of Sb₂Te₃ thin films.

1. Introduction

With the growing demand for providing the power for wearable micro-electronic devices or the Internet of Things, high-performance micro-scale power generators are in increasingly high demand [1,2,3,4]. Thermoelectric (TE) materials have been extensively studied for their ability to convert thermal energy into electrical energy [5,6]. TE devices offer a sustainable, reliable power supply with efficient energy harvesting from complex geometric surfaces, including the human body [7]. The efficiency of the TE devices typically depends on the figure of merit (ZT) value, ZT = S²σT/κ, where S, σ, T, and κ (κ = κ_l + κ_e) signify Seebeck coefficient, electrical conductivity, absolution temperature, and total thermal conductivity (κ_e and κ_l are electronic thermal conductivity and lattice thermal conductivity), respectively. Obviously, a higher ZT value can be attained through an increase in S²σ and a decrease in κ, where S²σ represents the power factor (PF) [8,9]. And an optimized S²σ can be obtained by tuning carrier concentration. Meanwhile, a low κ_l can be achieved by optimizing structural designs.

Thin-film TE materials are currently a research hotspot due to their greater potential for powering wearable electronics devices. Among many the state-of-the-art TE materials, p-type Sb₂Te₃ has attracted much attention owing to its excellent TE performance around room temperature. Several deposition technologies to enhance TE performance have been investigated, such as magnetron sputtering [10], vacuum thermal evaporation [11,12], pulsed laser deposition (PLD) [13], molecular beam epitaxy [14], screen printing [15], and electrochemical deposition method [16]. Vieira et al. [17] deposited Sb₂Te₃ thin film on flexible (polyimide, PI) substrate by a thermal evaporation method, and the S²σ of ~12.0 μW cm⁻¹ K⁻² at 298 K was achieved. Wu et al. [18] used a co-sputtering technology to obtain Sb₂Te₃/Te thin film with a S²σ of ~11.2 μW cm⁻¹ K⁻² at room temperature. Kong et al. [19] carried out a magnetron sputtering method to prepare a Sb₂Te₃ flexible thin film with an optimal S²σ of 17.5 μW cm⁻¹ K⁻² at room temperature. Shang et al. [20] achieved a S²σ of 12.4 μW cm⁻¹ K⁻² at 300 K in flexible Bi_0.5Sb_1.5Te₃-based films and further improved the S²σ of 23.2 μW cm⁻¹ K⁻² at 300 K using a facile approach that involves introducing the Te and Sb₂Te₃ nanoinclusions [21]. The intrinsic TE performance of Sb₂Te₃ is insufficient to meet practical application requirements, and further enhancing the thermoelectric properties is necessary.

Doping has been scientifically proven to be an effective method for enhancing the TE properties of materials by manipulating carrier concentration to regulate electrical conductivity and the Seebeck coefficient. Argemi et al. [22] achieved a high S²σ of 18.7 μW cm⁻¹ K⁻² for Ag_3.9Sb_33.6Te_62.5 by optimizing Ag doping. With co-doping of Ag/In in Sb₂Te₃, enlarged E_g was observed, which yielded a ZT of ~1 at 673 K [23]. Hu et al. [24] increased carrier concentration by coordinating the ratio of Bi/Sb and obtained a high ZT of ~1.3 at 373 K in Bi_0.3Sb_1.7Te₃. Additionally, In has also been frequently considered as a doping element in Sb₂Te₃, which may lead to intrinsic vacancies with the high density at cation sites owing to the lattice mismatch between indium (In³⁺) (In³⁺ is supposed to substitute Sb³⁺) and tellurium ions (Te²⁻) [25,26]. Therefore, TE performance should be enhanced via phonon-vacancy scattering, which plays a substantial role in reducing thermal conductivity [27,28,29]. In summary, In is a promising doping element for improving the TE properties of the Sb₂Te₃ thin film.

In this work, we aim at successfully preparing Sb₂Te₃ thin film using a simple deposition method that facilitates rapid and simple deposition on large, flexible surfaces. The thin film composition could be easily adjusted by controlling the thickness of each film. It will lay the foundation for the following doping research. In order to further enhance the thermoelectric properties of intrinsic thin films and explore more in-depth mechanisms, the effect of In doping on the properties of p-type Sb₂Te₃ flexible TE thin film has been investigated. Through the analysis of the microstructure of the prepared thin films and the combined theoretical calculation analysis, it is proved that the In doping can eventually lead to achieving enhanced thermoelectric properties. Especially the ZT value of ~0.6 at 453 K was obtained in In-doped Sb₂Te₃, which is approximately 1.5 times higher than that of undoped Sb₂Te₃ thin film. This indicates In doping is an effective and promising method to improve the TE properties of Sb₂Te₃ thin films.

2. Materials and Methods

A flexible polyimide substrate with a low thermal conductivity of 0.12 W K⁻¹ m⁻¹ was selected and directed at the deposition of Sb and Te thin films. The substrates were cleaned in an ultrasonic bath for 10 min with absolute ethyl alcohol, then for 10 min with deionized water. The Sb was prepared via direct current (DC) magnetron sputtering, while the Te and In were deposited using radio frequency (RF) magnetron sputtering. The parameters of deposition for Te thin films were as follows: a sputtering power of 20 W during 42 min, an argon flow of 40 sccm, and a working pressure of 0.7 Pa. The deposition parameters of In-doped Sb thin films were as follows: Sb and In sputtering power of 20 W and 10 W, Sb and In sputtering time of 23 min and 3 min, respectively, and the working pressure of 0.7 Pa. In-doped Sb₂Te₃ thin film was prepared by thermal diffusion at 623 K. Based on In doping, the samples are marked as In _{0 at.%} and In _{0.98 at.%}.

Crystal structural formations were detected and analyzed by X-ray diffraction (XRD, D/max 2500, Rigaku Corporation, Tokyo, Japan) with the θ-2θ mode using the incident angle of 20–80° (Cu/Kα, 40 kV). Raman scattering measurements were implemented by the Micro-Raman Spectroscopy System (RENISHAW inVia, Gloucestershire, UK). X-ray photoelectron spectroscopy (XPS, Thermo ESCALAB 250XI, Waltham, MA, US) was carried out to investigate the chemical valence states of elements. Surface and cross-sectional morphology were performed by scanning electron microscopy (SEM, Zeiss supra 55, Carl Zeiss, Oberkochen, Germany), while chemical content analysis was analyzed by energy dispersive spectroscopy (EDS, Quantax, Bruker, Karlsruhe, Germany). The σ and S were measured by the TE performance measurement system (Netzsch SBA458, Netzsch, Shanghai, China) at a temperature of 303–453 K. The room temperature carrier concentration (n) and mobility (μ) were characterized using the Hall measurement system (HL5500PC, Nanometrics, Milpitas, CA, USA). The thicknesses of the thin films were tested by the surface-profile measurement system (Bruker Dektak XT, Bruker, Karlsruhe, Germany). The values of thermal conductivity κ were measured by the transient electrothermal technique [30]. The errors of the parameters were as follows: S, σ, and κ are 7%, 7%, and 10%, respectively.

3. Results and Discussion

Figure 1a,d display the SEM surface morphology of In _{0 at.%} and In _{0.98 at.%} thin films. Uniform and denseness of the film surface can be observed for In _{0 at.%} thin film, and there are no obvious void defects. The In _{0.98 at.%} thin film shows a clearly quite rough surface, which is mightily attributed to the internal stress resulting from In doping. The cross-sectional images in Figure 1c,d present that the In doping can significantly increase the grain size. The EDS mapping of the sample In _{0.98 at.%} shown in Figure 1e–h demonstrates the homogenous distribution of all the constituting elements. The EDS results are shown in

Table 1. The chemical content, thicknesses, µ and n of undoped and In-doped Sb₂Te₃ thin films.

Sample	Sb (at.%)	Te (at.%)	In (at.%)	Thickness (nm)	µ (cm2 V−1 s−1)	n (×1019 cm−3)
Undoped	38.90	61.10	0	680	240.72	3.2
In-doped	41.63	57.39	0.98	700	126.14	4.8

. The corresponding thicknesses of the In _{0 at.%} and In _{0.98 at.%} thin films were 680 and 700 nm, respectively. The thickness of ~700 nm for In _{0.98 at.%} thin films is owing to the large grain size, as can be seen in Figure 1d. The atomic ratio of Sb/Te for the In _{0 at.%} tin film is close to 2:3, acquired by regulating the thickness of Sb and Te thin films. The proportion of In-doped samples changes due to the influence of In doping elements.

Figure 2a exhibits the XRD patterns of the In _{0 at.%} and In _{0.98 at.%} thin films, which can be essentially indexed to the standard peaks of Sb₂Te₃ (PDF #15-0874). The (101) peak of Te can be suspected in In _{0 at.%} thin film; however, it is not really visible in In _{0.98 at.%} thin film. Therefore, In doping appears to inhibit Te precipitation in the Sb₂Te₃ thin film. Room-temperature Raman scattering measurements were executed using the Micro-Raman Spectroscopy System (Figure 2b). As supposed, there are five atoms per unit cell in Sb₂Te₃ for two-dimensional, thus each wavevector K exhibits 15 phonon modes. Theoretical analysis for K = 0 is the following: Γ_Raman = 2A_1g + 2E_g [31,32]. There are three Raman-sensitive phonon vibration modes for the Sb₂Te₃ single crystal in the range of 55 to 210 cm⁻¹. Here, three typical Sb₂Te₃ Raman modes are visible, located at 70.0 cm⁻¹, 111.3 cm⁻¹, and 165.9 cm⁻¹, which are in accordance with the A¹_1g, E²_g, and A²_1g, respectively. For the In-doping sample, the Raman shift has slightly shifted, possibly due to the In-doping.

To further analyze the microstructures of the Sb₂Te₃ thin film, the chemical valence was carried out by XPS, as shown in Figure 3. The spectra of the Sb 3d, Te 3d, and In 3d were obtained, as shown in Figure 3a, which uses the C1s as the calibration reference at 284.8 eV. The binding energies were obviously detected, which correspond to the energy levels of Sb, Te, and In. As shown in Figure 3b, the binding energies at 529.88 eV and 537.28 eV are consistent with Sb 3d_5/2 and Sb 3d_3/2, confirming the Sb³⁺ valence state. In Figure 3c, the binding energies at 582.08 eV and 571.68 eV are consistent with the Te 3d_3/2 and Te 3d_5/2, confirming the Te²⁻ valence states. Some impurities, including an oxygen peak, were observed due to the absence of plasma etching to remove the surface impurities. The peak of O 1s at ∼531.78 eV in Figure 3b corresponds to Sb₂O₃, while the peak of O 1s at ∼587.68 eV in Figure 3c refers to TeO₃ [33]. The binding energy of In 3d_5/2 is located at ~443.58 eV and In 3d_3/2 is located at ~451.28 eV, confirming the In³⁺ valence state (Figure 3d). It indicated that In was introduced into the cationic sites of Sb₂Te₃, which may replace Sb.

Figure 4a–c shows the temperature dependence of TE performance (σ, S, and S²σ) in In _{0 at.%} and In _{0.98 at.%} thin films. Both σ decreased with increasing temperature, as shown in Figure 4a. Correspondingly, the maximum room-temperature σ is achieved for the In _{0 at.%} and In _{0.98 at.%} thin films. The σ of the In _{0.98 at.%} thin film is lower than that of the In _{0 at.%} thin film, as shown in Figure 4a. To further understand the electric performance evaluation, Hall performance was investigated in Table 1. The µ values of In _{0.98 at.%} thin film are lower than those of In _{0 at.%}. As can be seen, the high σ of In _{0 at.%} is attributed to the high µ. The effective mass (m*) and deformation potential (E_def) in In _{0 at.%} and In _{0.98 at.%} thin film calculations are presented in Figure 4d. The m* and E_def are calculated via a single parabolic band (SPB) model based on density-functional-theory [34]. The high µ is mainly due to the low E_def. Figure 4b shows the comparison of S of In _{0 at.%} and In _{0.98 at.%} thin film. The S increased with increasing temperature for In _{0 at.%} and In _{0.98 at.%} thin films. And S of In _{0.98 at.%} thin film is higher than that of In _{0 at.%} thin film. The maximum S value of ~137.04 μV K⁻¹ is achieved in In _{0.98 at.%} at 303 K. The high S of In _{0.98 at.%} thin film is mainly attributed to the high m*, but regardless of the high n as shown in Table 1. Figure 4c presents the S²σ as a function of temperature in In _{0 at.%} and In _{0.98 at.%} thin films. The S²σ decreased with increasing temperature. The higher S²σ of In _{0.98 at.%} thin film is achieved compared with In _{0 at.%} thin film. The highest S²σ of 18.22 μW cm⁻¹ K⁻² at 303 K is achieved due to the high S and a moderate σ of In _{0.98 at.%} thin film. Additionally, the fitted n-dependent S²σ was calculated with the single parabolic band model and experimental S²σ at room temperature as shown in Figure 4d. It is indicated that the high S²σ should be mainly attributed to the tuning n close to the optimal level (~1 × 10¹⁹ cm⁻³ to ~1 × 10²⁰ cm⁻³).

To further investigate the influence of the In-doping effect on the TE properties of the thin films, the room temperature (303 K) thermal conductivity (κ) was measured. Figure 5a presents a schematic illustration of measuring the thermal conductivity κ of a Sb₂Te₃-based thin film. The measurement method of κ is the transient electrothermal technique [35,36]. The temperature evolution history can be used to calculate the thermal diffusivity of In _{0 at.%} and In _{0.98 at.%} thin films [35,37]. And the κ of 1.18 W m⁻¹ K⁻¹ for In _{0.98 at.%} thin film is obtained, which is lower than that of In _{0 at.%} thin film. It is indicated In doping could decrease the κ of the Sb₂Te₃ thin film, as shown in Figure 5b. The In doping could create more phonon scattering centers in the lattice, leading to a decrease in κ. The doping of In may also introduce more lattice defects or grain boundaries, which could cause a decrease in thermal conductivity. Finally, the dependence of temperature on the ZT value is also shown in Figure 5b. The ZT values of ~0.47 at room temperature (303 K) and ~0.6 at 453 K are obtained in In _{0.98 at.%} thin film, which is higher than that of undoped thin film. As a result, In doping has great potential for improving TE performance due to the high S²σ and low κ.

4. Conclusions

In summary, In _{0 at.%} and In _{0.98 at.%} Sb₂Te₃ thin films were successfully fabricated by an innovative thermally induced diffusion reaction. In doping could increase the Seebeck coefficient to ~137.04 μV K⁻¹, due to the high m*. Meanwhile, tuning n optimizes it to an optimal level. As a result, it leads to a high power factor of ~18.22 μW cm⁻¹ K⁻² at 303 K. Correspondingly, the thermal conductivity of In _{0.98 at.%} thin film is 1.18 W m⁻¹ K⁻¹ owing to the increase of phonon scattering and lattice or grain boundary defects. Consequently, a high ZT value of ~0.47 at room temperature (303 K) and ~0.6 at 453 K is achieved due to the high power factor and low thermal conductivity. This work indicates In doping can enhance the thermoelectric properties of the Sb₂Te₃−based thin film.