Anisotropic Analysis of Nanocrystalline Bismuth Telluride Thin Films Treated by Homogeneous Electron Beam Irradiation

Shohei Kudo1, Saburo Tanaka2, Koji Miyazaki3, Yoshitake Nishi1 and Masayuki Takashiri1,*

1Department of Materials Science, Tokai University, Hiratsuka 259–1292, Japan 2Department of Mechanical Engineering, College of Engineering, Nihon University, Koriyama 963–8642, Japan 3Department of Mechanical and Control Engineering, Kyushu Institute of Technology, Kitakyushu 804–8550, Japan

The in-plane and cross-plane transport properties of nanocrystalline bismuth telluride (Bi2Te3) thin lms were evaluated to analyze their anisotropic behavior. Bi2Te3 thin lms were prepared via radio frequency (RF) magnetron sputtering, followed by a subsequent treatment of thermal annealing and homogeneous electron beam (EB) irradiation at various EB doses. The crystallographic properties of the thin lms were determined by X-ray diffraction (XRD) analysis. It was determined that the crystal orientation (Lotgering factor; F value) of Bi2Te3 thin lms can be controlled by homogeneous EB irradiation treatments, without resulting in crystal growth. The electrical conductivity and Seebeck coef- cient were measured in the in-plane direction of the lms, and the thermal conductivity was measured in the cross-plane direction using the 3ω method. The anisotropic analysis was performed by combining the F value of the thin lms with a simple model based on the transport properties of the basal and lateral planes of single- and poly-crystal Bi2Te3. The electrical and thermal conductivities of the in-plane and cross-plane directions of the EB-irradiated thin lms clearly differed; however, there was no signi cant difference between the Seebeck coef cient values of the two planes. Finally, we determined that the gure of merit, ZT, was enhanced by the homogeneous EB irradiation treatment, and the in- plane ZT value was 25% greater than that of the cross-plane direction. [doi:10.2320/matertrans.M2016295]

(Received August 25, 2016; Accepted December 21, 2016; Published February 3, 2017)

Keywords: anisotropic, Lotgering factor, electron beam irradiation, thermoelectric, bismuth telluride

1. Introduction those in the direction perpendicular to the plane (lateral plane). Thermoelectric materials can potentially be used in ther- Thin- lm technology can potentially be used to improve moelectric power generators because they directly convert the ZT values of modi ed structures, such as stacked lay- thermal energy due to a temperature gradient, into electrical er-,7–9) nanocrystalline-,10–12) nano-porous-,13,14) and energy via the Seebeck effect. Recently, the range of applica- phase-separating lms.15,16) However, it is dif cult to accu- tions of thermoelectric generators has increased because of rately estimate the ZT values of anisotropic thin- lm materi- developments in energy-harvesting technology. Typical ex- als, such as the Bi2Te3 compound. This is because the electri- amples include mobile and wireless electronics.1–3) The ther- cal conductivity and Seebeck coef cient are usually measured moelectric energy conversion ef ciency of such generators in the in-plane direction whereas thermal conductivity is usu- depends on the dimensionless gure of merit, (ZT). This is ally measured in the cross-plane direction, except for the free- 2 17,18) de ned as ZT = σ S T/κtotal, where σ is the electrical conduc- standing lms. Therefore, it is important to analyze the tivity, S is the Seebeck coef cient, T is the absolute tempera- anisotropic thermoelectric properties of Bi2Te3 thin lms to ture, and κtotal is the total thermal conductivity, which com- estimate their in-plane and cross-plane ZT values. prises the electronic thermal conductivity, κe, and the lattice In this study, we prepared n-type Bi2Te3 thin lms via radio thermal conductivity, κl. To improve thermoelectric perfor- frequency (RF) magnetron sputtering. To modify the crystal mance, the power factor, σS 2, should be maximized and the orientation of the thin lms whilst generally maintaining the lattice thermal conductivity should be minimized. crystallite size, we performed thermal annealing. This was Among many types of thermoelectric materials, the bis- followed by a homogeneous electron beam (EB) irradiation muth telluride (Bi2Te3) compound is of great interest because treatment, which is known not to signi cantly enlarge the it exhibits the highest ZT value near room temperature (RT) crystallite size.19,20) X-ray diffraction (XRD) patterns were and its ZT values can potentially be improved by structural used to determine the crystal orientation (Lotgering factor, F modi cation. The Bi2Te3 compound is one of the materials of value) and crystallite size of the thin lms, as well as the the tetradymite family; it has a rhombohedral crystal structure strain induced within them. We measured the in-plane electri- (a = 0.438 nm and c = 3.049 nm) and a space group of cal conductivity and Seebeck coef cient at RT. The cross- 5 ¯ 4) D3d(R3m). The structure of the Bi2Te3 compound is com- plane thermal conductivity was measured at RT using the 3ω monly described as a hexagonal unit cell, in which each method. Finally, we estimated the in-plane and cross-plane charge-neutralized layer consists of ve covalently-bonded ZT values using the measured thermoelectric property values monatomic sheets. The thermoelectric performance of the combined with a simple model to account for the crystal ori- Bi2Te3 compound is characterized by its remarkable anisotro- entation. pic behavior due to the high c-axis to a-axis length ratio of its crystal structure.5,6) The thermoelectric properties of the ma- 2. Experimental Setup terial in the direction parallel to the basal plane are superior to N-type Bi2Te3 thin lms were fabricated on SiO2/Silicon * Corresponding author, E-mail: [email protected] (dimensions: 20 mm × 30 mm × 0.6 mm) substrates via an 514 S. Kudo, S. Tanaka, K. Miyazaki, Y. Nishi and M. Takashiri

RF magnetron sputtering method (Tokuda, CFS-8EP). The was deposited on the sample via EB evaporation, using shad- detailed experimental procedure for the sputtering method is ow masks. The thin aluminum wire had a width of 20 μm and described in our previous report.21) The target consisted of the length of the heater section was 2 mm. We also fabricated high-purity (99.9%) Bi (30 at%)-Te (70 at%) and had a diam- reference samples that did not incorporate the Bi2Te3 thin lm eter of 127 mm (Kojundo Chemical Laboratory Co., Ltd). but were otherwise identical to the primary samples. The ref- The substrate-to-target distance was maintained as 140 mm. erence samples were used to subtract the thermal properties Prior to the lm deposition, the chamber was evacuated to a of the insulation layer. In our previous study, we con rmed pressure of 2.5 × 10−4 Pa, and the substrate temperature was that it was possible to disregard the thermal contact resistance maintained at 200°C. The sputtering was performed in argon values to estimate the thermal conductivity values of the thin gas (99.995%) at a pressure of 1.0 Pa, using an RF power of lms.26) 200 W. To deposit a lm with a uniform thickness, the substrate holder was rotated at 20 rpm. The thickness of the re- 3. Results and Discussion sulting lms was approximately 0.8 μm. To improve the crystallinity and thermoelectric properties 3.1 Structural properties of Bi2Te 3 thin lms of the Bi2Te3 thin lms, we performed thermal annealing us- Figure 1 shows the surface morphology of the Bi2Te3 thin ing an electric furnace. The furnace was lled with an argon lms, captured by SEM. The untreated lm had a very similar (95%) and hydrogen (5%) mixture at atmospheric pressure. The gas ow rate was maintained at 1.0 SLM throughout the annealing process. The temperature was increased to 300°C at a rate of 4 K/min and the samples were annealed at this temperature for 1 h. Following thermal annealing, the samples were cooled to RT naturally in the furnace. The Bi2Te3 thin lms were subjected to homogenous irradiation with an electron-curtain accelerator (Type CB175/15/180L, Iwasaki Electric Group Co., Ltd.) at RT.22–24) In the vacuum, a tungsten lament was used to gen- erate an EB with a voltage of 0.17 MeV and an irradiation dose of 0.43 or 0.86 MGy. The maximum rise in temperature was evaluated to be 25 K at these EB irradiation doses.20) The samples were placed in a vacuum chamber, which had a di- ameter of 24 cm, and were irradiated with the EB through a titanium window. To prevent oxidation, the samples were held in a nitrogen atmosphere with a residual oxygen concentration of less than 0.04%, at 0.1 MPa. The ow rate of the nitrogen gas was 90 SLM. The surface morphology of the Bi2Te3 thin lms was inves- tigated using scanning electron microscopy (SEM; JSM- 6301F, JEOL). The atomic composition was estimated by energy-dispersive X-ray spectroscopy using the SEM equip- ment. The crystallographic properties of the thin lms were evaluated by XRD (Mini Flex II, Rigaku) using the Cu-Kα line (λ = 0.154 nm). The in-plane electrical conductivity, σ, of the Bi2Te3 thin lms was measured at RT using the four-point probe method (RT-70V, Napson) with an accuracy of ±5%. The in-plane Seebeck coef cient, S, of the thin lms was also measured at RT with an accuracy of ±7%. To measure the Seebeck coef - cient, both ends of the thin lm were connected to a heat sink and heater, respectively. The Seebeck coef cient was determined as the ratio of the potential difference along the lm to the temperature difference across it. The in-plane power factor, σS 2, was calculated using the measured electrical conductivity and Seebeck coef cient values. The cross-plane thermal conductivity of the Bi2Te3 thin lms was determined at RT using the 3ω method with an accuracy of ±10%. Details of the thermal conductivity mea- surement and the sample fabrication process for the 3ω method have been described in a previous report.25) To summarize, Fig. 1 Surface morphologies of the Bi2Te3 thin lms, imaged using SEM. The atomic composition is determined by EDX. Photographs (a), (b), and a SiO2 lm (thickness: 500 nm) was deposited on a Bi2Te3 (c) represent the untreated lm, and those treated with EB irradiation dos- thin lm via RF magnetron sputtering. A thin aluminum wire es of 0.43 and 0.86 MGy, respectively. Anisotropic Analysis of Nanocrystalline Bismuth Telluride Thin Films Treated by Homogeneous Electron Beam Irradiation 515 structure to the lm that was treated with an EB irradiation Therefore, we can conclude that the crystal orientation of dose of 0.43 MGy. The stoichiometric atomic composition such thin lms can be controlled by homogeneous EB irradi- (Bi:Te = 40:60) of the lms was determined by EDX, as ation treatments. shown in Figs. 1(a) and 1(b). These lms exhibited a relative- The average crystallite size of the thin lms is shown in ly rough surface, and consisted of a large quantity of granu- Fig. 3(b). The average crystallite size was estimated from the lar-like grains with diameters of less than 100 nm. Moreover, full-width at half-maximum of the (0 1 5) XRD peaks using in Fig. 1(c), the lm that was treated with an EB irradiation Scherrer’s equation. The average crystallite size of all of the dose of 0.83 MGy had a very similar average grain size to that samples was approximately 23 nm, indicating that the crys- of the untreated lm and the lm that was treated with an ir- tals had hardly grown even though the crystallinity and crys- radiation dose of 0.43 MGy. The atomic composition of the tal orientation had been altered by the homogeneous EB irra- lm that was treated with an EB irradiation dose of 0.83 MGy diation treatment. slightly deviated from the stoichiometric composition. The strain values for the thin lms are shown in Fig. 3(c). The XRD patterns of the Bi2Te3 thin lms obtained after In the a-axis direction, for example, the strain can be deter- the various EB irradiation treatments are shown in Fig. 2. In mined as follows: strain (a-axis) = (a − a0)/a0 × 100%, where all the samples, the peaks in the patterns correspond to the “a” is the a-axis lattice constant of our samples, and “a0” is reections of the rhombohedral phase of Bi2Te3 (JCPDS 15- that provided by the standard data (JCPDS 15-0863) for 0863). The strongest crystal peak was (0 1 5), and c-axis-ori- Bi2Te3. The strain induced in the a-axis direction of the un- ented peaks (0 0 l) were also observed for all the samples. The treated lm was 0.09%. On one occasion, the strain decreased XRD peak intensities were enhanced as the EB irradiation at an EB irradiation dose of 0.43 MGy, and subsequently in- dose was increased, indicating that the lms were well crys- creased as the EB irradiation dose was increased to 0.86 MGy. tallized. To further investigate the crystallographic properties of the Bi2Te3 thin lms, we estimated the values for the crystal orientation, average crystallite size, and strain, as shown in Fig. 3. The crystal orientation of the Bi2Te3 thin lms as a function of the EB irradiation dose is shown in Fig. 3(a). The degree of c-axis orientation was evaluated using the Lotger- ing factor, F, which is calculated using eq. (1):27) P P F = − 0 , (1) 1 P − 0 where P0 = ∑I0(0 0 l)/∑I0(h k l) and P = ∑I(0 0 l)/∑I(h k l). I0 and I are the intensities of the peaks in the XRD patterns provided in the JCPDS le (15-0863) and those that were experimentally obtained, respectively. An F value of zero indicates that the crystals exhibit non-orientation and isotropic transport properties; whereas an F value of 1.0 indicates that the crystals are completely oriented in the c-axis direction and exhibit the highest degree of anisotropy. In our experiment, the F value of the untreated thin lm was 0.21. As the EB irradiation dose increased, the F value increased linearly, at- taining a value of 0.28 at an EB irradiation dose of 0.86 MGy.

Fig. 3 (a) Crystal orientation (Lotgering factor, F), (b) average crystallite size, and (c) strain generated in the a-axis and c-axis directions of the Fig. 2 X-ray diffraction patterns of Bi2Te3 thin lms treated with various Bi2Te3 thin lms as a function of the EB irradiation dose, measured using EB irradiation doses. X-ray diffraction peaks. 516 S. Kudo, S. Tanaka, K. Miyazaki, Y. Nishi and M. Takashiri

All the thin lms exhibited positive strain values in the a-axis to be at their lowest. Therefore, the normalized property val- direction, indicating that tensile strains were generated in the ues in the in-plane direction were estimated by dividing the a-axis direction of the thin lms. Moreover, no strain was basal plane property values by the average values. As a result, generated in c-axis direction of the untreated lm. The strain the normalized values in the in-plane direction of the electri- decreased and then reached a value of −0.13% at an EB irra- cal conductivity, Seebeck coef cient, and lattice thermal con- diation dose of 0.43 MGy, indicating that a compressive ductivity were determined as 1.35, 1.00, and 1.19, respective- strain had been induced in the lm. The strain increased as the ly. On the other hand, the normalized property values in the EB irradiation dose was further increased to 0.86 MGy; the cross-plane direction were estimated by dividing the lateral strain had still a negative value, but its magnitude had in- plane property values by the average values. As a result, the creased. Therefore, compressive strain was induced in the normalized values in the cross-plane direction of the electri- c-axis direction of the lm as a result of the EB irradiation cal conductivity, Seebeck coef cient, and lattice thermal con- treatment. Finally, we compared the magnitude of the strain ductivity were determined as 0.29, 0.97, and 0.65, respective- in the a-axis direction with that in the c-axis direction. In the ly. In this model, we assumed that the normalized transport untreated lm, the magnitude of the positive strain in the properties linearly depend on the F value by interpolating the a-axis direction was greater than the magnitude of the nega- properties of poly-crystal Bi2Te3. Here, it is noted that the tive strain in the c-axis direction; therefore, we can conclude transport properties are inuenced by the crystallite size of that the lm was in a state of tensile strain. In contrast, in the the lm.32–34) However, we did not consider the effect of the case of the lm that was irradiated with a dose of 0.43 MGy, crystallite size because the thin lms in this study had very the magnitude of the positive strain in the a-axis direction similar crystallite sizes. Finally, we also assumed that the de- was lower than the magnitude of the negative strain in the gree of anisotropy was not dependent on the crystallite size. c-axis direction; therefore, the lm was in a state of compressive strain. In the case of the lm that was irradiated with an EB irradiation dose of 0.86 MGy, the magnitude of the positive strain in the a-axis direction was almost equal to the magnitude of the negative strain in the c-axis direction, indicating that no strain had generated in the lm.

3.2 Anisotropic analysis of Bi2Te 3 thin lms To estimate the in-plane and cross-plane transport properties of the Bi2Te3 thin lms, we used a simple model based on the transport properties of the basal plane (perpendicular to the c-axis) and lateral plane (parallel to the c-axis) of single- 5,28–31) and poly-crystal Bi2Te3, as presented in Table 1. At an F value of zero, the transport property values of the basal and lateral planes are expected to converge, so the normalized value of the averaged property values is equal to 1.0, as shown in Fig. 4. At an F value of 1.0, the transport property values in Fig. 4 Normalized electrical conductivity, Seebeck coef cient, and lattice thermal conductivity of Bi2Te3 thin lms as a function of the F value, the in-plane direction are expected to be at their highest. At calculated using a simple model based on the transport properties of the the same time, those in the cross-plane direction are expected basal and lateral planes of single- and poly-crystal Bi2Te3.

Table 1 Transport properties of basal and lateral planes of single- and poly-crystal Bi2Te3. The average values are estimated using the properties of both planes. The normalized values are calculated using the values of the basal plane as well as the average values.

31) Single-crystal Bi2Te3 Poly-crystal Bi2Te3 σ S κ σ S κ F l F l [104 S/m] [μV/K] [W/(m·K)] [104 S/m] [μV/K] [W/(m·K)] 8.135) Basal plane : A −227.85) 1.528) 0.50 8.55 −198 0.87 1.0 10.029) (perpendicular to c-axis) −24029) 1.4729) 0.56 1.09 −194 0.80 6.6030) 2.245) Lateral plane : B −206.75) 0.728) 0.50 4.80 −194 0.65 1.0 1.8529) (parallel to c-axis) −24029) 0.8629) 0.56 4.35 −191 0.66 1.5030) 6.175) Average : C −220.85) 1.228) 0.0 7.30 −197 0.80 0.0 7.2829) (2A + B)/3 −24029) 1.2729) 0.0 8.70 −193 0.75 4.9030) 1.325) 1.035) 1.2528) 1.17 1.01 1.08 A/C (normalized) 1.3729) 1.0029) 1.1629) 1.25 1.01 1.07 1.3530) 0.365) 0.945) 0.5828) 0.66 0.98 0.81 B/C (normalized) 0.2529) 1.0029) 0.6829) 0.50 0.99 0.88 0.3130) Anisotropic Analysis of Nanocrystalline Bismuth Telluride Thin Films Treated by Homogeneous Electron Beam Irradiation 517

3.3 Electrical transport properties of Bi2Te 3 thin lms 0.43 MGy, the in-plane and cross-plane electrical conductivi- Figure 5 shows the in-plane and cross-plane electrical ties were 0.8 × 104 and 0.6 × 104 S/m, respectively. For both transport property values (electrical conductivity, Seebeck planes, as the EB irradiation dose was further increased to coef cient, power factor) of the Bi2Te3 thin lms as a func- 0.86 MGy, the electrical conductivity values did not signi - tion of the EB irradiation dose. In Fig. 5(a), the electrical con- cantly change. ductivity was measured in the in-plane direction. The cross- In Fig. 5(b), the Seebeck coef cient was measured in the plane electrical conductivity was calculated by dividing the in-plane direction. The cross-plane Seebeck coef cient was in-plane electrical conductivity by the normalized electrical calculated using the same procedure as that used to calculate conductivity corresponding to the F value of the thin lm, as the electrical conductivity. The in-plane and cross-plane See- depicted in Fig. 4. For example, in the case of the untreated beck coef cient values were almost identical for each EB ir- lm, the measured in-plane electrical conductivity was 1.79 × radiation dose. This is because the Seebeck coef cient is 104 S/m, and the F value was 0.21. The corresponding nor- slightly anisotropic.5) For both planes of the untreated lm, malized electrical conductivities in the in-plane direction and the Seebeck coef cient was determined to be approximately cross-plane direction were 1.08 and 0.83, respectively. There- −45 μV/K. The Seebeck coef cients of both planes signi - fore, the cross-plane electrical conductivity of the untreated cantly improved as a result of the EB irradiation treatment, lm was calculated to be 1.38 × 104 S/m. As the homoge- and reached a value of approximately −150 μV/K at an EB neous EB irradiation treatment was performed, the electrical irradiation dose of 0.43 MGy. This enhancement of the See- conductivity of both planes drastically decreased. This may beck coef cient can be well explained by the Mahan-Sofo be because the carrier concentration decreased by the EB ir- theory.35) This theory points out that the enhancement of See- radiation treatment. When the EB irradiation dose was beck coef cient is caused by the increase in the slope of the density of states (DOS) near the Fermi level. The DOS near the Fermi level is expected to increase by inducing the compressive strain for shrinking the distance between atoms in the unit cells, as presented in Fig. 3(c). As the EB irradiation dose was further increased to 0.86 MGy, the absolute value of the Seebeck coef cient slightly decreased. Based on the results obtained on the electrical conductivity and Seebeck coef cient of the thin lms, we can conclude that the carrier concentration of the thin lms decreased as a result of the EB irradiation treatment. It is known that the carrier concentration depends on the doping density and defect density.36–38) In this case, the defect density affects the carrier concentration because low crystallinity materials have a large quantity of defects at their grain boundaries, and the defects provide un- paired electrons as carriers. When the thin lms were treated with homogeneous EB irradiation, the number of defects at the grain boundaries decreased, leading to a reduction in the carrier concentration. The power factor was estimated by the electrical conductivity and Seebeck coef cient, as shown in Fig. 5(c). The differences between the in-plane and cross-plane power factors were attributed to the differences in the electrical conductivity values because the Seebeck coef cient is slightly isotropic. The in-plane and cross-plane power factors of the untreated lms were less than 40 μW/(m·K2). The maximum power factor value was 194 μW/(m·K2), which was achieved in the in-plane direction of the thin lm that received an EB irradiation treatment of 0.43 MGy. This is because the increase in the absolute value of the Seebeck coef cient resulting from the EB irradiation treatment, contributed to, rather than decreased, the electrical conductivity of the lm. Finally, there were differences between the power factor values obtained for the in-plane and cross-plane directions of the thin lms. In particular, in the case of the lm that was treated with an EB irradiation treatment of 0.43 MGy, the in-plane power factor was 38% higher than the cross-plane power factor. Therefore, we can conclude that it is possible to estimate the anisotropic Fig. 5 (a) Electrical conductivity, (b) Seebeck coef cient, and (c) power electrical transport property values of Bi2Te3 thin lms. factor of the Bi2Te3 thin lms as a function of the EB irradiation dose. The in-plane properties are represented by measured values, and the cross- plane properties are represented by calculated values. 518 S. Kudo, S. Tanaka, K. Miyazaki, Y. Nishi and M. Takashiri

3.4 Thermal transport properties of Bi2Te 3 thin lms mal conductivity of the thin lms. The κl value of both planes Figure 6 shows the in-plane and cross-plane thermal trans- increased following an EB irradiation dose of 0.43 MGy. This port property values (total-, electronic-, and lattice thermal could possibly be attributed to the strains induced in the thin conductivity) of the Bi2Te3 thin lms as a function of the EB lms. In our previous study, we determined that the κl value irradiation dose. The total cross-plane thermal conductivity increased as compressive strains were generated in the thin 39) (κtotal) was measured using the 3ω method. The electronic lms. In this study, the lm that was treated with an EB ir- thermal conductivities (κe) of both planes were estimated radiation dose of 0.43 MGy, was in a state of compressive based on their electrical conductivity and the Wiede- strain, so it exhibited a relatively high κl value. mann-Franz law, with the Lorenz number Ln = 2.44 × −8 2 10 WΩ/K . The cross-plane lattice thermal conductivity 3.5 Dimensionless gure of merit (ZT) of Bi2Te 3 thin (κl) was calculated by subtracting the cross-plane κe value lms from the cross-plane κtotal value. To evaluate the in-plane κl Figure 7 shows the ZT values of the in-plane and cross- value, we employed anisotropic analysis, as shown in Fig. 4. plane directions of the Bi2Te3 thin lms as a function of the For example, for the untreated lm, the estimated cross-plane EB irradiation dose. As the EB irradiation dose was increased, κl value was 0.36 W/(m·K), and the F value was 0.21. The the ZT of both planes increased. The highest ZT values of corresponding normalized lattice thermal conductivities in 0.07 and 0.06, corresponding to the in-plane and cross-plane the in-plane and cross-plane direction were 1.03 and 0.94, directions, respectively, were achieved by the lm that was respectively. Therefore, the in-plane κl value of the untreated treated with an EB irradiation dose of 0.86 MGy because it lm was calculated to be 0.39 W/(m·K). As a result, the in- had relatively low thermal conductivity. There was an 25% plane κtotal was calculated by the addition of the in-plane κe difference between the ZT values of the in-plane and cross- and κl values. For both planes, the κtotal and κl values exhibited plane directions, whereas there was a 38% difference in the a similar trend when plotted as a function of the EB irradia- respective power factors. Therefore, we determined that the tion dose because all the samples had low κe values. There- ZT value was less anisotropic than the power factor. Finally, fore, the κl value is a key parameter for determining the ther- even though the lms in this study exhibited low ZT values compared with those of established Bi2Te3-based alloy thin lms,4) we developed a method for the anisotropic analysis of Bi2Te3-based alloys, and this method can be applied to other anisotropic materials.

4. Conclusions

To perform anisotropic analysis on n-type nanocrystalline Bi2Te3 thin lms, the in-plane and cross-plane transport property values were estimated. The thin lms were prepared via RF magnetron sputtering, followed by a subsequent treatment of thermal annealing and homogeneous EB irradiation at various EB doses. We evaluated the structural properties of the thin lms via SEM observation and XRD analysis. It was determined that the crystal orientation (Lotgering factor; F val- Fig. 6 Total-, electronic-, and lattice thermal conductivity of the Bi2Te3 thin ue) of the thin lms could be controlled by homogeneous EB lms as a function of the EB irradiation dose. These thermal conductivities are estimated by combining the experimentally measured results with irradiation treatments, in which signi cant growth of the the calculated results. crystal grains does not occur. The anisotropic analysis was performed to estimate the in-plane and cross-plane transport property values of Bi2Te3 thin lms using a simple model based on the transport properties of the basal (perpendicular to c-axis) and lateral planes (parallel to c-axis) of single- and poly-crystal Bi2Te3. For this analysis, it was possible to estimate the in-plane transport property values from those of the cross-plane, and vice versa. There was a clear difference between the in-plane and cross-plane electrical conductivities of the samples; however, the Seebeck coef cients of the two planes did not signi cantly differ. As a result, we observed differences between the power factor values of the in-plane and cross-plane directions. In particular, for the lm that was treated with an EB irradiation dose of 0.43 MGy, the in-plane power factor was 38% greater than that of its cross-plane direction. We estimated the thermal conductivity of both planes by categorizing it into three constituents, namely κtotal, κe, and Fig. 7 Dimensionless gure of merit, ZT, of the Bi2Te3 thin lms as a function of the EB irradiation dose. The ZT values were estimated by combin- κl. It was determined that κl predominantly governs the ther- ing the experimentally measured results with the calculated results. mal conductivity of the thin lms. As the EB irradiation dose Anisotropic Analysis of Nanocrystalline Bismuth Telluride Thin Films Treated by Homogeneous Electron Beam Irradiation 519 was increased to 0.43 MGy, there was an increase in the κl and C. Adachi: Appl. Phys. Lett. 98 (2011) 023114. value of both planes owing to the compressive strain induced 14) M. Takashiri, S. Tanaka, H. Hagino and K. Miyazaki: J. Appl. Phys. in the lm. The highest ZT values were achieved by the lm 112 (2012) 084315. 15) K. Agarwal and B.R. Mehta: J. Appl. Phys. 116 (2014) 083518. that was treated with an EB irradiation dose of 0.86 MGy be- 16) M. Takashiri, K. Kurita, H. Hagino, S. Tanaka and K. Miyazaki: J. cause it exhibited relatively low thermal conductivity. The ZT Appl. Phys. 118 (2015) 065301. was less anisotropic than the power factor. Even though the 17) D. Song and G. Chen: Appl. Phys. Lett. 84 (2004) 687–689. ZT values of the lms in this study were low compared with 18) K. Nishimura, H. Wang, T. Fukunaga, K. Kurata and H. Takamatsu: Int. those of established Bi Te -based alloy thin lms, we devel- J. Heat Mass Transfer 95 (2016) 727–734. 2 3 19) M. Takashiri, K. Imai, M. Uyama, H. Hagino, S. Tanaka, K. Miyazaki oped a method for the anisotropic analysis of Bi2Te3 based and Y. Nishi: J. Alloy. Compd. 612 (2014) 98–102. alloys. Furthermore, this method can be applied to other 20) M. Takashiri, K. Imai, M. Uyama, H. Hagino, S. Tanaka, K. Miyazaki anisotropic materials. and Y. Nishi: J. Appl. Phys. 115 (2014) 214311. 21) K. Kusagaya, H. Hagino, S. Tanaka, K. Miyazaki and M. Takashiri: J. Acknowledgments Electron. Mater. 44 (2015) 1632–1636. 22) Y. Nishi, S. Takagi, K. Yasuda and K. Itoh: J. Appl. Phys. 70 (1991) 367–371. This work was partially supported by the Japan Science 23) Y. Nishi, T. Toriyama, K. Oguri, A. Tonegawa and K. Takayama: J. and Technology Agency (JST). The authors wish to thank H. Mater. Res. 16 (2001) 1632–1635. Hagino at Fujikura Ltd., as well as Y. Miyamoto, K. Kusaga- 24) Y. Nishi, H. Sato, K. Iwata and K. Iwata: J. Mater. Res. 24 (2009) ya, N. Hatsuta, and K. Yamauchi at Tokai University for pro- 3503–3509. 25) M. Takashiri, S. Tanaka, K. Miyazaki and H. Tsukamoto: J. Alloy. viding experimental support. Compd. 490 (2010) L44–L47. 26) S. Kudo, H. Hagino, S. Tanaka, K. Miyazaki and M. Takashiri: J. Elec- REFERENCES tron. Mater. 44 (2015) 2021–2025. 27) F.K. Lotgering: J. Inorg. Nucl. Chem. 9 (1959) 113–123. 1) J.A. Paradiso and T. Starner: Pervasive Comput. 4 (2005) 18–27. 28) G. S. Nolas, J. Sharp and H. J. Goldsmid, Thermoelectrics (Springer, 2) N.S. Hudak and G.G. Amatucci: J. Appl. Phys. 103 (2008) 101301. Berlin, 2001) Chap. 5.1, pp. 111–131. 3) Z. Wang, A. Chen, R. Winslow, D. Madan, R.C. Juang, M. Nill, J.W. 29) A. Jacquot, N. Farag, M. Jaegle, M. Bobeth, J. Schmidt, D. Ebling and Evans and P.K. Wright: J. Micromech. Microeng. 22 (2012) 094001. H. Böttner: J. Electron. Mater. 39 (2010) 1861–1868. 4) D. M. Rowe: CRC Handbook of Thermoelectrics, (CRC, Boca Raton, 30) H.J. Goldsmid: J. Appl. Phys. 32 (1961) 2198–2202. 1995) Sec.19, pp. 211–237. 31) T. Kajihara, K. Fukuda, Y. Sato and M. Kikuchi, Proc. 17th Int. Conf. 5) H. Kaibe, Y. Tanaka, M. Sakata and I. Nishida: J. Phys. Chem. Solids on Thermoelectrics (1998) pp. 129–133. 50 (1989) 945–950. 32) M. Takashiri, K. Miyazaki, S. Tanaka, J. Kurosaki, D. Nagai and H. 6) P.J. Taylor, J.R. Maddux, W.A. Jesser and F.D. Rosi: J. Appl. Phys. 85 Tsukamoto: J. Appl. Phys. 104 (2008) 084302. (1999) 7807–7813. 33) C. Liao, Y. Wang and H. Chu: J. Appl. Phys. 104 (2008) 104312. 7) R. Venkatasubramanian, E. Siivola, T. Colpitts and B. O’Quinn: Nature 34) Z. Wang, J. Alaniz, W. Jang, J.E. Garay and C. Dames: Nano Lett. 11 413 (2001) 597–602. (2011) 2206–2213. 8) N. Peranio, O. Eibl and J. Nurnus: J. Appl. Phys. 100 (2006) 114306. 35) G.D. Mahan and J.O. Sofo: Proc. Natl. Acad. Sci. USA 93 (1996) 9) K. Matsuoka, M. Okuhata and M. Takashiri: J. Alloy. Compd. 649 7436–7439. (2015) 721–725. 36) A. Giani, A. Boulouz and F. Pascal-Delannoy: J. Mater. Sci. Lett. 18 10) H. Obara, S. Higomo, M. Ohta, A. Yamamoto, K. Ueno and T. Iida: Jpn. (1999) 541–543. J. Appl. Phys. 48 (2009) 085506. 37) M. Takashiri, K. Miyazaki and H. Tsukamoto: Thin Solid Films 516 11) Y. Zhao, J.S. Dyck, B.M. Hernandez and C. Burda: J. Phys. Chem. C (2008) 6336–6343. 114 (2010) 11607–11613. 38) R. Rostek, V. Sklyarenko and P. Woias: J. Mater. Res. 26 (2011) 1785– 12) M. Takashiri, S. Tanaka and K. Miyazaki: J. Electron. Mater. 43 (2014) 1790. 1881–1889. 39) M. Takashiri, S. Tanaka, H. Hagino and K. Miyazaki: Int. J. Heat Mass 13) M. Kashiwagi, S. Hirata, K. Harada, Y. Zheng, K. Miyazaki, M. Yahiro Transfer 76 (2014) 376–384.