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Article

Nearly Perfect Crystal Orientation of Nanocrystalline Bismuth Telluride Thin Films Deposited by Pressure-Gradient Sputtering and Their Thermal Transport Properties

by
Tetsuya Takizawa
1,
Keisuke Kaneko
1,
Yoshiyuki Shinozaki
1,
Takumi Kaneko
2,
Hiroshi Murotani
2 and
Masayuki Takashiri
1,*
1
Department of Materials Science, Tokai University, 4-1-1 Kitakaname, Hiratsuka 259-1292, Japan
2
Department of Human and Information Science, Tokai University, 4-1-1 Kitakaname, Hiratsuka 259-1292, Japan
*
Author to whom correspondence should be addressed.
Coatings 2025, 15(11), 1234; https://doi.org/10.3390/coatings15111234
Submission received: 29 September 2025 / Revised: 17 October 2025 / Accepted: 19 October 2025 / Published: 22 October 2025
(This article belongs to the Collection Feature Paper Collection in Thin Films)
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Versions Notes

Abstract

Bismuth telluride (Bi2Te3) is a thermoelectric material that exhibits excellent thermoelectric properties primarily because of its low thermal conductivity. The ideal structure of Bi2Te3 contains nanocrystals with a high crystal orientation. However, achieving both nanocrystallization and a high crystal orientation is challenging. Furthermore, experimental analyses of thermal transport properties, namely the sound velocity, lattice thermal conductivity, and phonon mean free path (MFP) are limited. In this study, Bi2Te3 thin films were deposited using pressure-gradient sputtering (PGS), and their thermal transport properties were determined. These films exhibited a crystallite size of 23.0 nm and an F value of 0.97, indicating a nearly perfect crystal orientation. The average sound velocity of 2046 m/s, in-plane lattice thermal conductivity of 0.66 W/(m·K), and phonon MFP of 0.37 nm were determined using nanoindentation, the 3ω method, and a combination of both of these methods, respectively. The dimensionless figures of merit of the Bi2Te3 thin films were 1.3 × 10−1 and 1.0 × 10−1 in the in-plane and cross-plane directions, respectively. The PGS system is useful for the fabrication of high quality thermoelectric materials, and the analysis method that combines the 3ω method and nanoindentation provides a detailed estimation of their thermal transport properties.

1. Introduction

Thermoelectric materials possess high potential for recovering unused thermal energy and for application in solid-state cooling devices, and are expected to contribute to environmentally conscious energy technology [1,2,3,4,5]. The performance of thermoelectric materials is evaluated by the dimensionless figure of merit, ZT, expressed as ZT = σS2T/κ, where σ, S, T, and κ denote the electrical conductivity, Seebeck coefficient, absolute temperature, and thermal conductivity, respectively. Bismuth-telluride (Bi2Te3)-based alloys have been studied since the 1950s and commercialized as thermoelectric materials with the highest ZT values near 300 K [6,7,8,9,10]. These alloys are the only group of materials that can achieve ZT > 1.0 near 300 K, establishing themselves as the thermoelectric materials closest to realistic applications [11,12,13,14,15].
Recently, controlling phonon scattering by introducing nanostructures has attracted considerable attention as a means of further improving thermoelectric performance [16,17,18,19,20]. Nanocrystallization, which is particularly easy in thin films, significantly increases phonon scattering at lattice defects, grain boundaries, and interlayer interfaces and considerably reduces the thermal conductivity [21,22,23,24,25,26]. Additionally, optimizing the phonon scattering mechanism suppresses the thermal conductivity while maintaining electron transport properties.
To understand phonon scattering in thermoelectric materials in detail, the thermal transport properties, including the lattice thermal conductivity, sound velocity, and phonon mean free path (MFP), must be measured [27,28,29]. However, methods for experimentally measuring all three thermal transport properties are limited [30,31]. Therefore, we developed a measurement method that determines the phonon MFP by combining thermal conductivity measurements including the 3ω and infrared radiation thermometer methods [32,33,34,35] and nanoindentation, where the sound velocity was determined from the Young’s and shear moduli [36,37,38].
Several deposition methods have been used to prepare nanostructured thin films including sputtering, vacuum evaporation, and electrodeposition [39,40,41,42,43,44,45]. Sputtering is the most widely used because it produces a variety of nanostructures with a uniform film thickness. Furthermore, pressure-gradient sputtering (PGS) can be used to obtain high-quality thin films with a high deposition rate due to its low plasma damage and high plasma density [46,47,48].
In this study, Bi2Te3 thin films were deposited by PGS. After analyzing the structural and thermoelectric properties of the films, the thermal transport properties of the thin films were measured using a combination of the 3ω method and nanoindentation. The resulting thin films exhibited nanocrystalline structures with a nearly perfect crystal orientation. Phonon scattering in the nanostructured thin films was evaluated based on the experimental results. The findings of this study are expected to enhance the performance of various thermoelectric materials and establish guidelines for nanoscale thermal designs.

2. Experimental Procedure

Bi2Te3 thin films were deposited using a radio frequency (RF) magnetron PGS system (Kenix, Himeji, Japan), as shown in Figure 1. The system comprised a vacuum chamber, which included a substrate holder that acted as an anode and a sputtering target that was placed on permanent magnets and worked as a cathode connected to a matching box and an RF power supply. The main feature of the PGS system is that the sputtering gas is injected between the cathode and the shield cover, and the gas is evacuated from a port near the substrate holder using a turbomolecular pump and a rotary pump. These structures generate a high-density plasma near the target by applying RF power and create a pressure gradient between the substrate and target. The sputtered particles derived from the target material travel to the substrate, maintaining long mean free paths (MFPs) by passing through the low-pressure region.
To produce thin films in which the stoichiometric ratios of Bi and Te were as close as possible, the composition of the sputtering target was adjusted to 28 at.% Bi and 72 at.% Te. This is because the vapor pressure of Te is higher than that of Bi. The thin films were deposited on polished alumina (Al2O3) substrates (AO-2525, Furuuchi Chemical, Tokyo, Japan) (25 × 25 mm2 and 1.0 mm thick). The distance between the substrate and target was maintained at 200 mm. When the background pressure reached 1 × 10−4 Pa, Ar gas (>99.9% purity) was introduced into the chamber at a flow rate of 20 sccm while maintaining a process pressure of 0.6 Pa (measured near the substrate holder). The substrates were not heated, and an input power of 30 W was applied to the cathode during the deposition time of 1.5 h. After film deposition, post-thermal annealing was performed to improve the crystallinity. The substrate was placed in a furnace at 573 K in a gas mixture of 95% Ar and 5% H2 introduced at atmospheric pressure and annealed for 1 h.
The film thicknesses were determined using a Dektak XT stylus profiler (Bruker, Billerica, MA, USA). The atomic compositions of the annealed thin films were analyzed using an electron probe microanalyzer (EPMA-8050G, Shimadzu, Kyoto, Japan). The surface morphologies and roughness of the thin films were analyzed by field-emission scanning electron microscopy (FE-SEM; JSM-7100F, JEOL, Akishima, Japan) and white light interferometry (BW-S507, Nikon, Tokyo, Japan), respectively. The crystallographic properties of the thin films were evaluated by X-ray diffraction (XRD; Mini Flex II, Rigaku, Akishima, Japan) using Cu–Kα radiation (λ = 0.154 nm at 2θ range of 7–80°). Rietveld refinement was used to evaluate the average crystallite sizes and crystal orientations of the thin films from their XRD peaks and patterns.
The sound velocity of the Bi2Te3 thin films was determined from the elastic moduli measured using a nanoindenter (ENTNEXUS; ELIONIX, Hachioji, Japan) operating in continuous stiffness mode (CSM) at approximately 300 K [49,50]. The indentation depth was set to one-tenth of the film thickness. Detailed measurements and calculations are provided in the Supplementary Materials (Figure S1) [51]. The thermal conductivity κ of the films was measured using the 3ω method and is provided in the Supplementary Materials (Figure S2) [51].
The in-plane electrical conductivity σ of the Bi2Te3 thin films at approximately 300 K was calculated using the electrical resistance measured by a four-point probe method (RT-70V, Napson, Tokyo, Japan) and the film thickness. The electrical resistance was measured four times per sample, and the average value was calculated. To measure the in-plane Seebeck coefficient S at approximately 300 K, we followed the basic measurement procedure employed previously [52]. Briefly, one end of the film was connected to a heat sink, while the other was connected to a heater. Two 0.1-mm-diameter K-type thermocouples were pressed on the center of the thin films, with a distance of 13 mm between them. The temperature difference between the thermocouples was varied from 1 K to 4 K, and the Seebeck voltage was recorded every 1 K (temperature reader: GR-3500, KEYENCE, Osaka, Japan; digital multimeter: R6561, ADVANTEST, Tokyo, Japan). The Seebeck coefficient was measured four times for each sample, and the average was calculated. The in-plane power factor, represented by σS2, was calculated at approximately 300 K using the measured electrical conductivity and Seebeck coefficient. The in-plane dimensionless figure of merit ZT, represented by σS2T/κ, was determined at approximately 300 K using the power factor, thermal conductivity, and absolute temperature.

3. Results and Discussion

3.1. Structural Properties of Bi2Te3 Thin Films

The thicknesses and atomic compositions of the thin films are listed in Table 1. The film thickness was 0.6 μm, and the deposition rate was calculated as 6.7 nm/min. The atomic compositions of the thin films were slightly deviated from the stoichiometric proportion of Bi and Te, namely 40.0 at.% and 60.0 at.%, respectively. The SEM image of the Bi2Te3 thin film is shown in Figure 2a. The film did not exhibit clear crystal grains or grain boundaries. Instead, the thin film exhibited a relatively smooth surface, except for the steps caused by polishing scratches on the substrate. For comparison, an SEM image and XRD pattern of Bi2Te3 thin film deposited by the conventional sputtering method is shown in the Supplementary Materials (Figure S3), which exhibited fine crystal grains, a relatively rough surface, and random crystal orientation [53]. The image of the white light interferometry is shown in Figure 2b. The 3D (Sa) and 2D (Ra) surface roughness were estimated to be 30 and 20 nm, respectively. Therefore, the PGS method yielded thin films with smooth surfaces. Although the exact mechanism has not yet been clarified, a possible explanation is as follows. In the PGS method, the low gas pressure reduces the number of collisions between sputtered particles in the area between the target and substrate, thereby suppressing grain growth. Consequently, small crystal grains are deposited on the substrate, producing a smooth film surface [54,55].
The XRD patterns of the Bi2Te3 thin films are shown in Figure 3. Most of the peaks observed in the XRD patterns of all Bi2Te3 thin films were indexed to the standard diffraction pattern of Bi2Te3 (JCPDS 15-0863) [56]; however, these shifted to higher angles. This phenomenon indicates that the lattice parameters decreased due to the deviation from the stoichiometric ratio. Specifically, the atomic composition of Bi, which has a larger atomic radius, decreased, whereas that of Te, which has a smaller atomic radius, increased. This is supported by the EPMA analysis of the atomic compositions (Table 1). The crystal structures of the Bi2Te3 thin films were analyzed in greater detail via Rietveld analysis of the XRD patterns, and the crystallite size and crystal orientation were evaluated. As shown in Table 2, the crystallite size of the Bi2Te3 thin film, as calculated using the full width at half maximum (FWHM) of the highest peaks of the film and Scherrer equation [57], was 23.0 nm. The crystal orientation was evaluated using the Lotgering factor F, which was calculated using Equation (1) [58,59].
F = P − P 0 1 − P 0 ,
where P = ΣI(00l)/ΣI(hkl) and P0 = ΣI0(00l)/ΣI0(hkl). I0 and I represent the peak intensities in the XRD patterns of Bi2Te3 (JCPDS 15-0863) and of the experimentally obtained XRD patterns, respectively. An F value of zero indicates no crystal orientation (random orientation), whereas an F value of 1.0 indicates complete orientation along the c-axis. The F value of the Bi2Te3 thin film was 0.97, which indicated an almost perfect crystal orientation along the c-axis. Therefore, nanocrystalline Bi2Te3 thin films with nearly perfect crystal orientations were obtained using the PGS system. The nearly perfect crystal orientation in the Bi2Te3 thin films obtained using the PGS system can be attributed to the large distance between the thin-film growth surface on the substrate and the high-density plasma generated directly above the target in the PGS system. This results in less damage to the thin-film surface from plasma exposure compared with conventional sputtering methods. The resulting thin-film surface has fewer defects, and the small sputtered particles migrate and settle at stable sites on the thin-film surface, leading to a high orientation of the thin film [60].
The mechanical properties of the Bi2Te3 thin film are listed in Table 3. The shear modulus (G) was calculated using Equation (2):
G = E 2 ( 1 + ν ) ,
where E and ν are the Young’s modulus and Poisson’s ratio, respectively. The literature value of 0.23 for the Poisson’s ratio of Bi2Te3 thin films was used [61]. The Young’s and shear moduli of the Bi2Te3 thin films were 67 GPa and 27 GPa, respectively. The longitudinal (vL) and transverse (vT) sound velocities were calculated using Equations (3) and (4), respectively.
υ L = E / ρ ,
υ T = G / ρ ,
where ρ denotes the mass density, which was assigned as 7.74 g/cm3 [62,63]. The average sound velocity (vave) was obtained using Equation (5):
1 υ a v e 3 = 1 3 1 υ L 3 + 2 υ T 3 .
The longitudinal, transverse, and average sound velocities of the Bi2Te3 thin film were 2929, 1862, and 2046 m/s, respectively.

3.2. Thermal Transport Properties of Bi2Te3 Thin Films

Table 4 lists the thermal conductivities of the Bi2Te3 thin films. As the total thermal conductivity (κtot) obtained using the 3ω method is the sum of the lattice (κlat) and electronic (κele) thermal conductivities, κlat is determined by subtracting κele from κtot. The 3ω method measures the κtot in the cross-plane direction (⊥) because the film thickness (0.6 μm) is shorter than the width of the aluminum wire (46 μm) obtained using the 3ω method [64,65]. The value of κtot (⊥) was 0.31 W/(m∙K), and κele was calculated using the Wiedemann–Franz law as expressed in Equation (6) [66].
κ e l e = L σ T .
Here, L, σ, and T denote the Lorentz number, electrical conductivity, and absolute temperature, respectively. The Lorentz numbers were calculated using Equation (7): [67,68]
L = 1.5 + e x p − S 116 ,
where S is the absolute value of the measured Seebeck coefficient. Therefore, from the Wiedemann–Franz law and measured in-plane electrical conductivity, κele in the in-plane direction (//) was determined as κele (//) = 2.0 × 10−3 W/(m∙K). Based on the property that the electrical conductivity of the Bi2Te3 single crystal in the c-axis direction is 37% of that in the a,b-axis direction, κele (⊥) was calculated as 7.4 × 10−4 W/(m∙K) [53,69]. This assumption is reasonable because the crystals of the thin film in this study were nearly perfectly oriented along the c-axis. Thus, the value of κlat (⊥) determined by subtracting κele (⊥) from κtot (⊥) was 0.31 W/(m∙K). κlat (//) was calculated as 0.66 W/(m∙K) because the lattice thermal conductivity of the Bi2Te3 single crystal in the a,b-axis direction was 2.1 times higher than that in the c-axis direction [53,70]. Finally, κtot (//) was calculated as 0.66 W/(m∙K), which was the same as κlat (//) because κele (//) was significantly lower than κtot (//).
Table 5 summarizes the thermal transport properties of the Bi2Te3 thin films. The phonon MFP (Λ) was obtained from vave and κlat (//) using Equation (8).
Λ = 3 κ l υ a v e C ,
where C is the specific heat, which was assigned a value of 165 J/(kg·K) [71]. The Λ of the Bi2Te3 thin film was obtained as 0.37 nm.

3.3. Thermoelectric Properties of Bi2Te3 Thin Films

Table 6 summarizes the in-plane and cross-plane thermoelectric properties of the Bi2Te3 thin films measured at approximately 300 K using the PGS. Due to the significant difficulties in measuring the Seebeck coefficient in the cross-plane direction, we assumed that the Seebeck coefficients of the Bi2Te3 thin films in the in-plane and cross-plane directions were the same due to their anisotropic characteristics [72]. Therefore, the measured and assumed Seebeck coefficients in the in-plane and cross-plane directions was −106 μV/K, respectively. The electrical conductivity measured in the in-plane direction was 254 S/cm. Because of the assumption that the electrical conductivity of the Bi2Te3 single crystal in the c-axis direction was 37% of that in the a,b-axis direction, the electrical conductivity in the cross-plane direction was estimated to be 94 S/cm. The thermal conductivities in the in-plane and cross-plane directions (Table 4) were 0.66 and 0.31 W/(m∙K), respectively. Finally, the ZT values in the in-plane and cross-plane directions were determined as 1.3 × 10−1 and 1.0 × 10−1, respectively.

4. Conclusions

Although nanocrystallization and high crystal orientation of thermoelectric materials are essential for improving their performance, their simultaneous realization is challenging. In this study, Bi2Te3 was selected as the thermoelectric material because of its high performance near 300 K, and Bi2Te3 thin films were deposited using PGS. The resulting Bi2Te3 thin films exhibited a crystallite size of 23.0 nm and an F value of 0.97, indicating a nearly perfect crystal orientation. In addition, the thermal transport properties of Bi2Te3 thin films, namely the sound velocity, lattice thermal conductivity, and phonon MFP, were successfully estimated. The Bi2Te3 thin films exhibited a sound velocity of 2046 m/s, lattice thermal conductivity in the in-plane direction of 0.66 W/(m·K), and phonon MFP of 0.37 nm, which were determined using the 3ω method, nanoindentation, and their combination, respectively. ZT was estimated to be 1.3 × 10−1 and 1.0 × 10−1 in the in-plane and cross-plane directions, respectively. The PGS system is useful for the fabrication of high-quality thermoelectric materials, and the analytical method that combines the 3ω method and nanoindentation provides a detailed estimation of their thermal transport properties.

Supplementary Materials

The following information can be downloaded from https://www.mdpi.com/article/10.3390/coatings15111234/s1: Figure S1: Principle diagram of the nanoindentation method, where the reference [51] is cited in the Supplementary Materials. Figure S2: Principle diagram of the 3ω method, where the reference [51] is cited in the Supplementary Materials. Figure S3: FE-SEM image and XRD pattern of the Bi2Te3 thin film deposited using the conventional RF magnetron sputtering system, where the reference [53] is cited in the Supplementary Materials.

Author Contributions

Conceptualization, M.T.; Methodology, T.T., K.K., Y.S., T.K. and H.M.; Investigation, T.T., K.K., Y.S., T.K. and H.M.; Writing—original draft, T.T., K.K., Y.S. and M.T.; Supervision, M.T.; Project administration, M.T.; Funding acquisition, M.T. All authors have read and agreed to the published version of the manuscript.

Funding

This study was partially supported by the JSPS KAKENHI (grant number 22H04953).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Data are contained within the article or Supplementary Materials.

Acknowledgments

All authors thank K. Yonezawa at Kenix for providing the PGS system, and M. Morikawa, Y. Oda, S. Yonezawa, and T. Komori at Tokai University for their experimental support.

Conflicts of Interest

The authors declare no conflicts of interest.

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Coatings 15 01234 g001
Figure 1. Schematic diagram of the PGS system.
Figure 1. Schematic diagram of the PGS system.
Coatings 15 01234 g001
Coatings 15 01234 g002
Figure 2. (a) Surface morphology and (b) roughness of the Bi2Te3 thin film analyzed by FE-SEM and white light interferometry, respectively.
Figure 2. (a) Surface morphology and (b) roughness of the Bi2Te3 thin film analyzed by FE-SEM and white light interferometry, respectively.
Coatings 15 01234 g002
Coatings 15 01234 g003
Figure 3. XRD pattern of Bi2Te3 thin film.
Figure 3. XRD pattern of Bi2Te3 thin film.
Coatings 15 01234 g003
Table 1. Film thickness and atomic composition of the Bi2Te3 thin films deposited using PGS.
Table 1. Film thickness and atomic composition of the Bi2Te3 thin films deposited using PGS.
Thickness
[μm]		Atomic Composition
Bismuth [at.%]Tellurium [at.%]
Bi2Te3 thin film0.637.862.2
Table 2. Crystallographic properties of the Bi2Te3 thin film deposited using the PGS system.
Table 2. Crystallographic properties of the Bi2Te3 thin film deposited using the PGS system.
Crystallite Size [nm]F Value
Bi2Te3 thin film23.00.97
Table 3. Elastic modulus and sound velocity of the Bi2Te3 thin films deposited using PGS.
Table 3. Elastic modulus and sound velocity of the Bi2Te3 thin films deposited using PGS.
Elastic Modulus [GPa]Sound Velocity [m/s]
Young’sShearLongitudinalTransverseAverage
Bi2Te3 thin film67.027.0292018622046
Table 4. Thermal conductivities of the Bi2Te3 thin films deposited using PGS.
Table 4. Thermal conductivities of the Bi2Te3 thin films deposited using PGS.
Thermal Conductivity [W/(m·K)]
κtot (⊥)κtot (//)κele (⊥)κele (//)κlat (⊥)κlat (//)
Bi2Te3 thin film0.310.667.4 × 10−42.0 × 10−30.310.66
Table 5. Thermal transport properties of the Bi2Te3 thin films deposited using PGS.
Table 5. Thermal transport properties of the Bi2Te3 thin films deposited using PGS.
vave
[m/s]		κlat (//)
[W/(m·K)]		Λ
[nm]
Bi2Te3 thin film20460.660.37
Table 6. Thermoelectric properties of the Bi2Te3 thin films deposited using PGS.
Table 6. Thermoelectric properties of the Bi2Te3 thin films deposited using PGS.
DirectionS
[μV/K]		σ
[S/cm]		κtot
[W/(m·K)]		ZT
at 300 K
Bi2Te3 thin filmIn-plane−1062540.661.3 × 10−1
Cross-plane−106 *940.311.0 × 10−1
* The Seebeck coefficients in the in- and cross-plane directions were assumed to be the same.
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Takizawa, T.; Kaneko, K.; Shinozaki, Y.; Kaneko, T.; Murotani, H.; Takashiri, M. Nearly Perfect Crystal Orientation of Nanocrystalline Bismuth Telluride Thin Films Deposited by Pressure-Gradient Sputtering and Their Thermal Transport Properties. Coatings 2025, 15, 1234. https://doi.org/10.3390/coatings15111234

AMA Style

Takizawa T, Kaneko K, Shinozaki Y, Kaneko T, Murotani H, Takashiri M. Nearly Perfect Crystal Orientation of Nanocrystalline Bismuth Telluride Thin Films Deposited by Pressure-Gradient Sputtering and Their Thermal Transport Properties. Coatings. 2025; 15(11):1234. https://doi.org/10.3390/coatings15111234

Chicago/Turabian Style

Takizawa, Tetsuya, Keisuke Kaneko, Yoshiyuki Shinozaki, Takumi Kaneko, Hiroshi Murotani, and Masayuki Takashiri. 2025. "Nearly Perfect Crystal Orientation of Nanocrystalline Bismuth Telluride Thin Films Deposited by Pressure-Gradient Sputtering and Their Thermal Transport Properties" Coatings 15, no. 11: 1234. https://doi.org/10.3390/coatings15111234

APA Style

Takizawa, T., Kaneko, K., Shinozaki, Y., Kaneko, T., Murotani, H., & Takashiri, M. (2025). Nearly Perfect Crystal Orientation of Nanocrystalline Bismuth Telluride Thin Films Deposited by Pressure-Gradient Sputtering and Their Thermal Transport Properties. Coatings, 15(11), 1234. https://doi.org/10.3390/coatings15111234

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