Effects of annealing temperature on the structural, mechanical and electrical properties of flexible bismuth telluride thin films prepared by high-pressure RF magnetron sputtering^*

[Bi]:[Te] of bismuth telluride thin films at different annealing temperatures were analyzed by EDS and the Te content (%) is shown in figure 1. From the EDS result, the Te content of the as-deposited film was about 64 at.% (Te rich). The reason that as-deposited film is Te rich can be explained by the effect of the high deposition pressure.

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At a high deposition pressure, the density of gas in the deposited system will be relatively high. Ar⁺ will interrupt the path of Bi atoms, which are bigger than Te atoms (Bi  =  143 pm, Te  =  123 pm [19]), making them to fall down to the substrate and causing the composition of Bi atoms to be less than the composition of the target (Bi₂Te₃). When the films were annealed at the temperature range of 250–400 °C, Te atoms evaporated during the annealing process and caused a content of decreasing Te. According to the crystal structure of Bi₂Te₃, it consists of five-layer units of Bi and Te hexagonal planes, stacked along the c-axis with atomic order Te(1)–Bi–Te(2)–Bi–Te(1) and the layer Te(1)–Te(1),that are weakly bonded together by the Van der Waals force [20]. In conclusion, with the increase of the annealing temperature, the volatile Te elements occur during the annealing process [15].

XRD patterns of as-deposited and different annealed films are shown in figure 2. From figure 2(a), the XRD results indicate that the as-deposited film can be attributed to the rhombohedral Bi₂Te₃ structure (JCPDS 15-0863) and the Te structure found at the (1 1 2) plane (JCPDS 44-0925). Figures 2(b) and (c) are XRD patterns of the films annealed at 250 °C and 300 °C, respectively. It was seen that their intensities and peak positions had obviously changed. It is clear that the peak of the (1 1 2) plane in the Te phase reduces at high temperatures. This result indicated that Te atoms decrease as the annealing temperature increases. When the films were annealed at the temperature of 350 °C and 400 °C (figures 2(d) and (e)) a hexagonal BiTe structure was found (JCPDS 75-1095) with the preferred orientation of (1 0 4). It reveals that the annealing process enhances the crystallinity and changes the crystal structure of the films [9, 10, 15]. The sharpest intensity peaks of the films were also found at an annealing temperature up to 400 °C, as seen in figure 2(e). The crystal size of the films was calculated using the Debye–Scherrer equation

$$\begin{eqnarray}&&D=\frac{k\lambda}{\beta \cos \theta},\end{eqnarray} \tag{ 1 }$$

where k  =  0.9, λ is the wavelength of Cu-Kα  =  0.154 nm, β is the full-width-half-maximum, and θ is the diffraction angle. The results show that the crystal sizes increased from 3.24 to 39.64 nm with the increase in the annealing temperature.

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Raman spectra of bismuth telluride films were acquired in a back scattering configuration, equipped with a frequency-double Ng: YAG 532 nm laser, as shown in figure 3. Raman spectra of as-deposited films were found at Raman wavenumbers at 102, 117 and 133 cm⁻¹. The wavenumbers at 102 and 133 cm⁻¹ were namely $E_{\text{g}}^{2}$ and $A_{1\text{g}}^{2}$, aligned with a Bi₂Te₃ vibration patterns mode reported by Russo et al [21], while the wavenumber at 117 cm⁻¹ referred to A_1g of the Te vibration pattern. In addition, the Raman spectra trended to shift towards the lower wavenumber (blue shift) when the annealing temperature was increased. Analysis of the Raman spectra of bismuth telluride films was supported by EDS and XRD measurements.

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Figures 4(a)–(e) and 5(a)–(e) show the surface and cross-section SEM images of as-deposited and annealed films at temperatures of 250, 300, 350 and 400 °C, respectively. The bismuth telluride films annealed at temperatures of 250 and 300 °C (figures 4(b) and (c)) had smoother surfaces than the as-deposited film (figure 4(a)) as well as the films annealed at 350 and 400 °C (figures 4(d) and (e)). Therefore, this result indicated that the temperature range from 250 to 300 °C was available for surface improvement. The cross-sectional images revealed the thickness of the films in the range of 1.50 to 1.65 µm. As compared with the cross sections in figures 5(a)–(c), it is clear that the films annealed at 250 and 300 °C have higher density than the as-deposited film. For high annealing temperatures above 350 °C, films have a lower density than the as-deposited films, due to the re-evaporation of Te in the films. From SEM results, the increase in roughness and the reduction indensity of the films were found at annealing temperatures above 350 °C.

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The hardness of the as-deposited and annealed films measured at room temperature are shown in figure 6. The hardness of the as-deposited film is 1.89 GPa and it increases to 2.30 GPa at the annealing temperature of 300 °C. However, with films annealed at 350 to 400 °C, the hardness decreased to 1.58 and 1.10 GPa, respectively. This result corresponds to the density of film as seen from the cross-section images (figure 5). The hardness of Bi₂Te₃ depends on the conditions and deposition process. The hardness of Bi₂Te₃ prepared sputtering process has the value in arange between 1.46–2.91 GPa, and that will be even higher when using a PLD technique when itcan go up to 2.92–4.02 GPa [17]. Our result will be important for understanding the structural and nano-mechanical properties of bismuth telluride thin films prepared by a sputtering process.

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The Seebeck coefficient (S) and carrier concentration (n) at room temperature were measured and the results are shown in figure 7. The S of all the samples exhibits negative values, indicating an n-type semiconductor of which electrons are the majority of the carriers. The film annealed at a temperature of 300 °C has the highest Seebeck coefficient of 71.41 µV K⁻¹ and has the lowest carrier concentration of 2.27  ×  10¹⁹ cm⁻³.

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In general, the carrier concentration depends on defects in the films. When the films were annealed at temperatures of 250 to 300 °C, the Te concentration decreased due to volatilization and caused the generation of Te vacancy ($V_{\text{Te}}^{\bullet \bullet}$). Subsequently, the antisite defect ($\text{Bi}_{\text{Te}}^{\prime}$) and Bi vacancy ($V_{\text{Bi}}^{\prime\prime \prime}$) are produced as the Bi atom enters Te vacancies. This process can be described as [22]:

$$\begin{eqnarray}&&\text{B}{{\text{i}}_{2}}\text{T}{{\text{e}}_{3}}=2\text{Bi}_{\text{Te}}^{\prime}+2V_{\text{Bi}}^{\prime\prime \prime}+V_{\text{Te}}^{\bullet \bullet}+\left(\frac{3}{2}\right)\text{T}{{\text{e}}_{2\left(\text{g}\right)}}\uparrow +\,8\overset{\centerdot}{{h}}\,,\end{eqnarray} \tag{ 2 }$$

where $\overset{\centerdot}{{h}}\,$ is the produced hole and ↑ denotes the volatilization of Te. From equation (2), we can see that the hole is generated by the volatilization of Te, and the carrier concentration of n-type Bi₂Te₃ can be reduced by the compensation of holes. On the contrary, the films annealed at temperatures of 350–400 °C exhibited an increase of carrier concentration from 2.43  ×  10²⁰ to 6.15  ×  10²⁰ cm⁻³, respectively. It caused the structure to change from Bi₂Te₃ to BiTe. This result also corresponds to the research of Caha et al [23] which revealed that the free carrier concentration in BiTe is about 20 times larger than Bi₂Te₃. In addition, the S is decreased while the carrier concentration is increased. According to the simple theory for nearly free electrons [13, 24], the S can be shown by the following equation

$$\begin{eqnarray}&&S=\frac{8{{\pi}^{2}}k_{\text{B}}^{2}}{3e{{h}^{2}}}{{m}^{\ast}}T{{\left(\frac{\pi}{3n}\right)}^{2/3}}(1+R),\end{eqnarray} \tag{ 3 }$$

where k_B and h are Boltzman and Planck constants, respectively. m^*, n and T are the effective mass of the carriers, carrier concentration and absolute temperature, respectively. Finally, R is a function of scattering. From equation (3) we conclude that S is directly proportional to T and inversely proportional to n^2/3.

From figure 8, the crystal size and mobility of as-deposited and annealed films were calculated and measured at room temperature. With the increase in annealing temperature, the mobility and crystal size also increased. Large crystal sizes have a long effective mean free path of carriers and cause a reduction ingrain boundary and scattering center. The bismuth telluride film annealed at a temperature of 400 °C has the highest mobility of 34.03 cm² V⁻¹ s⁻¹, and this was relatively close to the research of Zheng et al [13].

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Figure 9 shows the Seebeck coefficient of bismuth telluride films as a function of measured temperature from 50 to 300 °C. The highest Seebeck coefficient (S) was observed on the film annealed at 300 °C. According to equation (3), S directly depends on the measuring temperature (T) and inversely depends on the carrier concentration (n). As the temperature increases, it not only affects the increase of S but it also increases the carrier concentration. The increase of S as the measuring temperature increased was found for the as-deposited film and annealed films at 350 and 400 °C. The films had a high carrier concentration in the order of 10²⁰ cm⁻³. The increase of temperature slightly affected the carrier density. However, the films annealed at 250 and 300 °C with a lower carrier density had a similar Seebeck coefficient as the measuring temperature increased. This may cause a greater increase in carrier concentration and result in a reduction of the Seebeck coefficient.

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Figure 10 shows the electrical conductivity (σ) of bismuth telluride annealed at different temperatures. The results show that σ of the films annealed at 350–400 °C is higher than the as-deposited and annealed films at 250–300 °C. The increase in electrical conductivity is due to the enhancement of the crystal size and defects in the films that led to increased mobility and carrier concentration. The film annealed at 400 °C had the highest electrical conductivity of 50  ×  10⁴ S m⁻¹ at the measured temperature of 200 °C.

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Figure 11 shows the power factor (PF) of bismuth telluride films annealed at different temperatures. PF is a property of material able to generate more energy from temperature difference; it can be calculated by using the Seebeck coefficient and electrical conductivity (S²σ) under a given temperature. It was found that as-deposited films and those annealed at temperatures below 300 °C have a power factor in the range of 2.57  ×  10⁻⁴ W m⁻¹K⁻². This value is close to the research of Huang et al [10] who reported that Bi₂Te₃ thin film annealed at 300 °C has the highest power factor of 4  ×  10⁻⁴ W m⁻¹K⁻². The film annealed at 350 °C has a PF value of 7.49  ×  10⁻⁴ W m⁻¹K⁻² at the measuring temperature of 300 °C. The film annealed at 400 °C has a maximum PF value of 11.45  ×  10⁻⁴ W m⁻¹K⁻² with the highest electrical conductivity even though it had the lowest Seebeck coefficient. Compared to various deposition methods for bismuth telluride base thin film, the PF value of high pressure RF magnetron sputtered thin film obtained similar performances to those deposited by another technology. This implies that high pressure RF magnetron techniques following an annealing process can be used to prepare high performance bismuth telluride thin film.

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