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Annealing temperature influence on electrical properties of ion beam sputtered Bi2Te3 thin films
Journal of Physics and Chemistry of Solids 71 (2010) 1713–1716
Contents lists available at ScienceDirect
Journal of Physics and Chemistry of Solids journal homepage: www.elsevier.com/locate/jpcs
Annealing temperature influence on electrical properties of ion beam sputtered Bi2Te3 thin films Zhuang-hao Zheng, Ping Fan n, Guang-xing Liang, Dong-ping Zhang, Xing-min Cai, Tian-bao Chen College of Physical Science and Technology, Institute of Thin Film Physics and Applications, Shenzhen Key Laboratory of Sensor Technology, Shenzhen University, 518060, China
a r t i c l e in fo
abstract
Article history: Received 10 March 2010 Received in revised form 22 August 2010 Accepted 10 September 2010
Ion beam sputtering process was used to deposit n-type fine-grained Bi2Te3 thin films on BK7 glass substrates at room temperature. In order to enhance the thermoelectric properties, thin films are annealed at the temperatures ranging from 100 to 400 1C. X-ray diffraction (XRD) shows that the films have preferred orientations in the c-axis direction. It is confirmed that grain growth and crystallization along the c-axis are enhanced as the annealing temperature increased. However, broad impurity peaks related to some oxygen traces increase when the annealing temperature reached 400 1C. Thermoelectric properties of Bi2Te3 thin films were investigated at room temperature. The Bi2Te3 thin films, including as-deposited, exhibit the Seebeck coefficients of 90 to 168 mV K 1 and the electrical conductivities of 3.92 102–7.20 102 S cm 1 after annealing. The Bi2Te3 film with a maximum power factor of 1.10 10 3 Wm 1 K 2 is achieved when annealed at 300 1C. As a result, both structural and transport properties have been found to be strongly affected by annealing treatment. It was considered that the annealing conditions reduce the number of potential scattering sites at grain boundaries and defects, thus improving the thermoelectric properties. & 2010 Elsevier Ltd. All rights reserved.
Keywords: A. Thin films D. Crystal structure D. Electrical properties
1. Introduction Thermoelectric techniques based on Peltier and Seebeck coefficient, respectively, are widely employed in power generators, cooling, sensors and optical storage systems [1–5]. V–VI compound semiconductors, such as n-type bismuth telluride (Bi2Te3), are well-established room temperature thermoelectric materials due to its excellent thermoelectric properties [1–7]. Low-dimensional structures of Bi2Te3 can obtain better thermoelectric properties with higher thermoelectric figure of merit (ZT) due to their stronger quantum confinement compared with that of their bulk materials [8–10]. Various processing techniques, including flash evaporation [11], sputtering [12,13], electrochemical deposition [14–16] and chemical-vapor deposition [17] have been used to grow Bi2Te3 thin films. Because of the high vapor pressure of Te, it is difficult to obtain stoichiometric Bi2Te3 thin films. The Seebeck coefficients (a) and resistivities (s) have been worse when the films depart from atoichiometry [18]. Therefore, those techniques often require either long time periods to prepare the starting materials or relatively complicated and expensive equipment to prepare stoichiometric and high crystalline quality thin films. Actually, ion beam sputtering deposition (IBSD) is a very attractive technique for this purpose
n
Corresponding author. E-mail address: [email protected] (P. Fan).
0022-3697/$ - see front matter & 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.jpcs.2010.09.012
since it combines a high deposition rate with great versatility in the deposition of films by adjusting the target composition. Thin films prepared by IBSD have several advantages such as good uniformity and simple setup required for film formation. Note in accordance with these that IBSD could be realized at better vacuum conditions. Actually, IBSD preparing thermoelectric thin films have been rarely reported. P-type antimony telluride (Sb2Te3) with high thermoelectric quality was prepared by IBSD in our previous works [19]. It is an attempt to gain a significant result by IBSD preparing Bi2Te3 thin film. In this paper, ion beam sputtering process was used to deposit Bi2Te3 thermoelectric thin films at room temperature. Instead of using alloy targets, the target was made of fan-shaped Bi–Te. The influences of annealing temperature conditions on thermoelectric properties and structure of films were then investigated.
2. Experimental details Bi2Te3 thin films were deposited on BK7 glass substrates at room temperature by IBSD in argon ambience. The incident angle of the argon ion beam onto the target was 451. The target was made of fan-shaped high purity Bi (99.99%) and Te (99.99%) plates. The Bi/Te proportion was controlled by adjusting the ratio of the corresponding plate areas. The background pressure was 7.0 10 4 Pa and the work pressure was 6.1 10 2 Pa. The substrates were ultrasonically cleaned in acetone and alcohol
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for 10 min, respectively. Prior to film deposition, a 15-min sputter cleaning process was performed to remove native oxides and contaminants on the surfaces of the Bi–Te sputter target. Samples were deposited at room temperature and the deposition time was 60 min. After the deposition processes was complete, the films were then annealed at 100, 200, 300 and 400 1C for 1 h in the vacuum chamber, respectively. The chamber pressure of annealing process was less than 8.0 10 4 Pa. The composition ratios of the thin films were determined using an energy dispersive X-ray spectroscopy microanalysis system (EDS, Hitachi S-3400N(II)). The thickness of the films was obtained using a DEKTAK3 ST surface-profile measurement system. The structure of the films was studied by X-Ray diffraction (XRD) technique (BRUKER-D8-ADVANCE). The electrical properties, including electrical conductivity (s), carrier concentration (p) and carrier mobility (m) of the films, were tested at room temperature by Hall coefficient measurement (ET9000) and the Seebeck coefficient (a) was measured by Seebeck coefficient measurement systems. Power factor PF is defined as PF¼ a2s.
3. Results and discussion Fig. 1 shows the element content of Bi2Te3 thin films as a function of the plate area ratio. It has been found that the stoichiometric Bi2Te3 film was achieved when the plate area ratio of Bi to Te is 1–3.5. The properties of the stoichiometric Bi2Te3 thin films are shown in Table 1. The positive Seebeck coefficient a suggests the film to be p-type. The stoichiometric film has a power factor of 0.58 10 3 Wm 1 K 2 with a conductivity of 7.2 102 S cm 1 and a Seebeck coefficient of 90 mV K 1. The Seebeck coefficient of our samples approaches the value of Bi2Te3 thin films, while the power factor and conductivity are much smaller than those of Bi2Te3 thin films reported by others [12,13].
The poor electrical properties might be due to the poor crystalline quality in the films prepared at room temperature. More samples were prepared at the same condition with the stoichiometric Bi2Te3 sample. These samples were then annealed at 100, 200, 300 and 400 1C for 1 h in the vacuum chamber. Fig. 2 shows the XRD patterns of the films, including as-deposited, as a function of annealing temperature. It can be found that three major diffraction peaks are located at 27.961, 38.021 and 57.441 and they are indexed as the reflection from the (0 1 5), (1 0 1 0) and (0 2 1 0) planes of Bi2Te3. This result demonstrates that a hexagonal structure belonging to the R3m space group of Bi2Te3 thin films is obtained [20,21]. When the annealing temperature increases from 100 to 300 1C, the XRD intensity enhances. This indicates that the grain gets larger and the crystalline quality improved after annealing. It is worth noting that the XRD pattern of the sample annealed at 300 1C. The film annealed at 300 1C obviously has the (0 1 5) preferred phase, which can yield better thermoelectric performance [17–21]. When the annealing temperature reached 400 1C, the intensity of the (0 1 5) peak decreases. From the inset figure in Fig. 2, the range of 2y from 301 to 601, broad impurity peaks related to some oxygen traces (Bi, O) are observed more clearly and numbers of those peaks increase when the annealing temperature is above 300 1C. It arises from the precipitation reaction of the supersaturated Bi–Te alloy. According to the XRD results, high crystalline quality Bi2Te3 thin films are obtained when the annealing temperatures are less than 300 1C. Defects such as interface scattering, etc. are reduced by increasing the crystalline quality and the electrical properties are improved by annealing. However, secondary phases and nonstoichiometries become increasingly dominant and the number of defects in the material increases for the annealing temperature above 300 1C. In addition, the extended defects and the thermal stress will increase at high annealing temperature. It is considered that more defects are generated in the thin film owing to the evaporation of some elements and oxygen traces enhanced at high temperature. Fig. 3 illustrates the Seebeck coefficients, the electrical conductivities, the carrier concentrations and motilities of the Bi2Te3 films, including as-deposited, as a function of annealing temperatures. The Seebeck coefficients of subtraction sign indicate that the Bi2Te3 films have n-type conduction. As shown in Fig. 3(a), it can be found that the Seebeck coefficient increases from 90 to 168 mV K 1 as the temperature increases from
Fig. 1. Plot of the element content of Bi2Te3 thin films as a function of the plate area ratio.
Table 1 Properties of the stoichiometric Bi2Te3 thin film. Films
Plate area ratio Type
a/mV K 1 s/ 102 S cm 1 PF/Wm 1 K 2
As-deposited
Bi:Te¼ 1:3.5
90
N
7.2
0.58
Fig. 2. X-ray diffraction patterns of the films annealed of the as-deposited sample and samples annealed at 100, 200, 300 and 400 1C.
Z.-h. Zheng et al. / Journal of Physics and Chemistry of Solids 71 (2010) 1713–1716
1715
Fig. 3. Plot of the Seebeck coefficients, the electrical conductivities, the carrier concentration and motilities of the Bi2Te3 films, including as-deposited, as a function of annealing temperatures: (a) Seebeck coefficients; (b) the electrical conductivities; (c) the carrier concentration and (d) the carrier motilities.
room temperature to 300 1C. However, the Seebeck coefficient decreases to 141 mV K 1 when the annealing temperature reached 400 1C. It can be found from Fig. 3(b) that the electrical conductivities are in the range of 3.92 102–7.20 102 S cm 1 and has the lowest value while the annealing temperature is 300 1C. Fig. 3(c) shows that the carrier concentration decreases with the increase in annealing temperature and decreases greatly while the temperature reaches 400 1C. As shown in Fig. 3(d), the carrier mobility changes slightly when the annealing temperature is less than 300 1C. However, it is enhanced greatly when the annealing temperature is above 300 1C. The Seebeck coefficient and electrical conductivities can be expressed as follows [22]: ! kB 2ð2pm kB TÞ3=2 S¼ 7 ðr þ 2Þ þ ln ð1Þ e h3 n
s ¼ nem
ð2Þ
Here, kB is Boltazman’s constant, e is the electron charge, r is the scattering factor, m* is effective mass, h is Planck’s constant, n is the carrier concentration, and m is the carrier mobility. The increases of Seebeck coefficient at annealing temperature below 300 1C are due to reduction of the carrier concentration
by decreasing the number of defects, which acted as acceptors [23]. The increase of electrical conductivity is mainly due to the preferred crystal orientation and the reduction of carrier scattering by grain boundaries. The great change of carrier concentration and mobility of the film annealed at 400 1C is due to the grain boundaries scatter increases with crystallinity of the thin films deteriorating and secondly the collisions of charge carriers with the surface becoming significant when the free path decreases while the temperature is too high [21–24]. This phenomenon is consistent with the results of XRD patterns as shown in Fig. 2. The variations of PF in response to the change of annealing temperature are presented in Fig. 3. As shown in Fig. 4, the power factors increase from 0.58 10 3 to 1.10 10 3 Wm 1 K 2 as the annealing temperature increases from room temperature to 400 1C. The sample annealed at 300 1C has a maximum PF value of 1.10 10 3 Wm 1 K 2 with the highest Seebeck coefficient and a moderate electrical conductivity. This implies that annealing can improve the thermoelectric properties of the films due to the improvement of crystalline quality. When the annealing temperature reached 400 1C, the thermoelectric power factor of the thin film deteriorates because the Seebeck coefficient degrades at this high annealing temperature. Compared to various deposition methods for the bismuth–telluride based thin
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that the electrical conductivity has a minimum value of 3.92 102 S cm 1when the annealing temperature is 300 1C. The best film has a maximum power factor of 1.10 10 3 Wm 1 K 2, comparable to the best results of the same material prepared by another sputtering technology. Obviously, adjustment of annealing temperature can improve the structure quality and thermoelectric properties of ion beam sputtered Bi2Te3 thin films. Acknowledgments This work was supported by the Project of the Bureau of Science and Technology, Nanshan District, Shenzhen (No. 2009035), and Open Funding Projects of Shenzhen Key Laboratory of Sensor Technology (Project no.: SST200901). Reference Fig. 4. Plot of power factor of the Bi2Te3 films, including as-deposited, as a function of annealing temperatures.
films, the thermoelectric power factor of ion beam sputtered thin film obtains similar performances (with a maximum power factor of 1.10 10 3 Wm 1 K 2) as those deposited by another technology.
4. Conclusions The n-type Bi2Te3 thin films were fabricated by the IBSD method with different annealing conductions. The structure and thermoelectric properties were investigated. EDS shows that the atomic composition of the films approaches the stoichiometric ratio of 2–3 when the area ratio of Bi plate to Te plate is 1–3.5. XRD shows that the film has a hexagonal structure. After annealing, the crystalline quality increases and the grains of the thin films grow along the c-axis, their natural growth axis [20–24]. However, the crystalline quality becomes worse when the annealing temperature is too high. Seebeck coefficient measurement shows that the Seebeck coefficient of the asdeposited thin film is 90 mV K 1 and increases to 168 mV K 1 after annealing at 300 1C. Hall coefficient measurement reveals
[1] J.P. Heremans, V. Jovovic, E.S. Toberer, A. Saramat, K. Kurosaki, A. Charoenphakdee, S. Yamanaka, G.J. Snyder, Science 321 (2008) 554. [2] L.E. Bell, Science 321 (2008) 1457. [3] L.D. Hicks, M.S. Dresslhaus, Phys. Rev. B 47 (1993) 16631. [4] Y. Jiang, Y.J. Zhu, J. Cryst. Growth 306 (2007) 351. [5] D. Pinisetty, R.V. Devireddy, Acta Mater. 58 (2010) 570. [6] S.K. Lim, M.Y. Kim, T.S. Oh, Thin Solid Films 517 (2009) 4199. [7] F. Xiao, C. Hangarter, B.Y. Yoo, Y.W. Rheem, K.H. Lee, N.V. Myung, Electrochim. Acta 53 (2008) 8103. [8] S. Michel, S. Diliberto, C. Boulanger, N. Stein, J. Cryst. Growth 277 (2005) 274. [9] D.C. Grauer, Y.S. Hor, A.J. Williams, R.J. Cava, Mater. Res. Bull. 44 (2009) 1926. [10] M. Takashiri, T. Shirakawa, K. Miyazaki, H. Tsukamoto, Sens. Actuators, A 138 (2007) 329. [11] M. Takashiri, K. Miyazaki, H. Tsukamoto, Thin Solid Films 516 (2008) 6336. [12] C.N. Liao, T.H. She, Thin Solid Films 515 (2007) 8059. [13] V. Richoux, S. Diliberto, C. Boulanger, J.M. Lecuire, Electrochim. Acta 52 (2007) 3053. [14] W.L. Wang, C.C. Wan, Y.Y. Wang, Electrochim. Acta 52 (2007) 6502. [15] P. Magri, C. Boulangerm, J.M. Lecurie, J. Mater. Chem. 6 (1996) 773. [16] S. Michel, S. Diliberto, C. Boulanger, B. Bolle, J. Cryst. Growth 296 (2006) 227. [17] H. Zou, D.M. Rowe, G. Min, J. Cryst. Growth 222 (2001) 82. [18] A. Boyer, E. Cisse´, Mater. Sci. Eng. B 13 (1992) 103. [19] P. Fan, Z.H. Zheng, G.X. Liang, X.M. Cai, D.P. Zhang, Chin. Phys. Lett. 8 (2010) 087201. [20] A. Bailini, F. Donati, M. Zamboni, V. Russo, M. Passoni, C.S. Cassri, A.L. Bassi, C.E. Bottani, Appl. Surf. Sci. 254 (2007) 1249. [21] J.P. Carmo, L.M. Goncalves, J.H. Correia, Electron. Lett. 45 (2009) 803. [22] G.S. Nolas, J. Sharp, H.J. Goldsmid, in: Thermoelectric, Springer, Berlin, 2001 p. 40. [23] L. Qiu, J. Zhou, X. Cheng, R. Ahuja, J. Phys. Chem. Solids 71 (2010) 1131. [24] A. Mzerd, B. Aboulfarah, A. Giani, A. Boyer, J. Mater. Sci. 41 (2006) 1659.
Contents lists available at ScienceDirect
Journal of Physics and Chemistry of Solids journal homepage: www.elsevier.com/locate/jpcs
Annealing temperature influence on electrical properties of ion beam sputtered Bi2Te3 thin films Zhuang-hao Zheng, Ping Fan n, Guang-xing Liang, Dong-ping Zhang, Xing-min Cai, Tian-bao Chen College of Physical Science and Technology, Institute of Thin Film Physics and Applications, Shenzhen Key Laboratory of Sensor Technology, Shenzhen University, 518060, China
a r t i c l e in fo
abstract
Article history: Received 10 March 2010 Received in revised form 22 August 2010 Accepted 10 September 2010
Ion beam sputtering process was used to deposit n-type fine-grained Bi2Te3 thin films on BK7 glass substrates at room temperature. In order to enhance the thermoelectric properties, thin films are annealed at the temperatures ranging from 100 to 400 1C. X-ray diffraction (XRD) shows that the films have preferred orientations in the c-axis direction. It is confirmed that grain growth and crystallization along the c-axis are enhanced as the annealing temperature increased. However, broad impurity peaks related to some oxygen traces increase when the annealing temperature reached 400 1C. Thermoelectric properties of Bi2Te3 thin films were investigated at room temperature. The Bi2Te3 thin films, including as-deposited, exhibit the Seebeck coefficients of 90 to 168 mV K 1 and the electrical conductivities of 3.92 102–7.20 102 S cm 1 after annealing. The Bi2Te3 film with a maximum power factor of 1.10 10 3 Wm 1 K 2 is achieved when annealed at 300 1C. As a result, both structural and transport properties have been found to be strongly affected by annealing treatment. It was considered that the annealing conditions reduce the number of potential scattering sites at grain boundaries and defects, thus improving the thermoelectric properties. & 2010 Elsevier Ltd. All rights reserved.
Keywords: A. Thin films D. Crystal structure D. Electrical properties
1. Introduction Thermoelectric techniques based on Peltier and Seebeck coefficient, respectively, are widely employed in power generators, cooling, sensors and optical storage systems [1–5]. V–VI compound semiconductors, such as n-type bismuth telluride (Bi2Te3), are well-established room temperature thermoelectric materials due to its excellent thermoelectric properties [1–7]. Low-dimensional structures of Bi2Te3 can obtain better thermoelectric properties with higher thermoelectric figure of merit (ZT) due to their stronger quantum confinement compared with that of their bulk materials [8–10]. Various processing techniques, including flash evaporation [11], sputtering [12,13], electrochemical deposition [14–16] and chemical-vapor deposition [17] have been used to grow Bi2Te3 thin films. Because of the high vapor pressure of Te, it is difficult to obtain stoichiometric Bi2Te3 thin films. The Seebeck coefficients (a) and resistivities (s) have been worse when the films depart from atoichiometry [18]. Therefore, those techniques often require either long time periods to prepare the starting materials or relatively complicated and expensive equipment to prepare stoichiometric and high crystalline quality thin films. Actually, ion beam sputtering deposition (IBSD) is a very attractive technique for this purpose
n
Corresponding author. E-mail address: [email protected] (P. Fan).
0022-3697/$ - see front matter & 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.jpcs.2010.09.012
since it combines a high deposition rate with great versatility in the deposition of films by adjusting the target composition. Thin films prepared by IBSD have several advantages such as good uniformity and simple setup required for film formation. Note in accordance with these that IBSD could be realized at better vacuum conditions. Actually, IBSD preparing thermoelectric thin films have been rarely reported. P-type antimony telluride (Sb2Te3) with high thermoelectric quality was prepared by IBSD in our previous works [19]. It is an attempt to gain a significant result by IBSD preparing Bi2Te3 thin film. In this paper, ion beam sputtering process was used to deposit Bi2Te3 thermoelectric thin films at room temperature. Instead of using alloy targets, the target was made of fan-shaped Bi–Te. The influences of annealing temperature conditions on thermoelectric properties and structure of films were then investigated.
2. Experimental details Bi2Te3 thin films were deposited on BK7 glass substrates at room temperature by IBSD in argon ambience. The incident angle of the argon ion beam onto the target was 451. The target was made of fan-shaped high purity Bi (99.99%) and Te (99.99%) plates. The Bi/Te proportion was controlled by adjusting the ratio of the corresponding plate areas. The background pressure was 7.0 10 4 Pa and the work pressure was 6.1 10 2 Pa. The substrates were ultrasonically cleaned in acetone and alcohol
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Z.-h. Zheng et al. / Journal of Physics and Chemistry of Solids 71 (2010) 1713–1716
for 10 min, respectively. Prior to film deposition, a 15-min sputter cleaning process was performed to remove native oxides and contaminants on the surfaces of the Bi–Te sputter target. Samples were deposited at room temperature and the deposition time was 60 min. After the deposition processes was complete, the films were then annealed at 100, 200, 300 and 400 1C for 1 h in the vacuum chamber, respectively. The chamber pressure of annealing process was less than 8.0 10 4 Pa. The composition ratios of the thin films were determined using an energy dispersive X-ray spectroscopy microanalysis system (EDS, Hitachi S-3400N(II)). The thickness of the films was obtained using a DEKTAK3 ST surface-profile measurement system. The structure of the films was studied by X-Ray diffraction (XRD) technique (BRUKER-D8-ADVANCE). The electrical properties, including electrical conductivity (s), carrier concentration (p) and carrier mobility (m) of the films, were tested at room temperature by Hall coefficient measurement (ET9000) and the Seebeck coefficient (a) was measured by Seebeck coefficient measurement systems. Power factor PF is defined as PF¼ a2s.
3. Results and discussion Fig. 1 shows the element content of Bi2Te3 thin films as a function of the plate area ratio. It has been found that the stoichiometric Bi2Te3 film was achieved when the plate area ratio of Bi to Te is 1–3.5. The properties of the stoichiometric Bi2Te3 thin films are shown in Table 1. The positive Seebeck coefficient a suggests the film to be p-type. The stoichiometric film has a power factor of 0.58 10 3 Wm 1 K 2 with a conductivity of 7.2 102 S cm 1 and a Seebeck coefficient of 90 mV K 1. The Seebeck coefficient of our samples approaches the value of Bi2Te3 thin films, while the power factor and conductivity are much smaller than those of Bi2Te3 thin films reported by others [12,13].
The poor electrical properties might be due to the poor crystalline quality in the films prepared at room temperature. More samples were prepared at the same condition with the stoichiometric Bi2Te3 sample. These samples were then annealed at 100, 200, 300 and 400 1C for 1 h in the vacuum chamber. Fig. 2 shows the XRD patterns of the films, including as-deposited, as a function of annealing temperature. It can be found that three major diffraction peaks are located at 27.961, 38.021 and 57.441 and they are indexed as the reflection from the (0 1 5), (1 0 1 0) and (0 2 1 0) planes of Bi2Te3. This result demonstrates that a hexagonal structure belonging to the R3m space group of Bi2Te3 thin films is obtained [20,21]. When the annealing temperature increases from 100 to 300 1C, the XRD intensity enhances. This indicates that the grain gets larger and the crystalline quality improved after annealing. It is worth noting that the XRD pattern of the sample annealed at 300 1C. The film annealed at 300 1C obviously has the (0 1 5) preferred phase, which can yield better thermoelectric performance [17–21]. When the annealing temperature reached 400 1C, the intensity of the (0 1 5) peak decreases. From the inset figure in Fig. 2, the range of 2y from 301 to 601, broad impurity peaks related to some oxygen traces (Bi, O) are observed more clearly and numbers of those peaks increase when the annealing temperature is above 300 1C. It arises from the precipitation reaction of the supersaturated Bi–Te alloy. According to the XRD results, high crystalline quality Bi2Te3 thin films are obtained when the annealing temperatures are less than 300 1C. Defects such as interface scattering, etc. are reduced by increasing the crystalline quality and the electrical properties are improved by annealing. However, secondary phases and nonstoichiometries become increasingly dominant and the number of defects in the material increases for the annealing temperature above 300 1C. In addition, the extended defects and the thermal stress will increase at high annealing temperature. It is considered that more defects are generated in the thin film owing to the evaporation of some elements and oxygen traces enhanced at high temperature. Fig. 3 illustrates the Seebeck coefficients, the electrical conductivities, the carrier concentrations and motilities of the Bi2Te3 films, including as-deposited, as a function of annealing temperatures. The Seebeck coefficients of subtraction sign indicate that the Bi2Te3 films have n-type conduction. As shown in Fig. 3(a), it can be found that the Seebeck coefficient increases from 90 to 168 mV K 1 as the temperature increases from
Fig. 1. Plot of the element content of Bi2Te3 thin films as a function of the plate area ratio.
Table 1 Properties of the stoichiometric Bi2Te3 thin film. Films
Plate area ratio Type
a/mV K 1 s/ 102 S cm 1 PF/Wm 1 K 2
As-deposited
Bi:Te¼ 1:3.5
90
N
7.2
0.58
Fig. 2. X-ray diffraction patterns of the films annealed of the as-deposited sample and samples annealed at 100, 200, 300 and 400 1C.
Z.-h. Zheng et al. / Journal of Physics and Chemistry of Solids 71 (2010) 1713–1716
1715
Fig. 3. Plot of the Seebeck coefficients, the electrical conductivities, the carrier concentration and motilities of the Bi2Te3 films, including as-deposited, as a function of annealing temperatures: (a) Seebeck coefficients; (b) the electrical conductivities; (c) the carrier concentration and (d) the carrier motilities.
room temperature to 300 1C. However, the Seebeck coefficient decreases to 141 mV K 1 when the annealing temperature reached 400 1C. It can be found from Fig. 3(b) that the electrical conductivities are in the range of 3.92 102–7.20 102 S cm 1 and has the lowest value while the annealing temperature is 300 1C. Fig. 3(c) shows that the carrier concentration decreases with the increase in annealing temperature and decreases greatly while the temperature reaches 400 1C. As shown in Fig. 3(d), the carrier mobility changes slightly when the annealing temperature is less than 300 1C. However, it is enhanced greatly when the annealing temperature is above 300 1C. The Seebeck coefficient and electrical conductivities can be expressed as follows [22]: ! kB 2ð2pm kB TÞ3=2 S¼ 7 ðr þ 2Þ þ ln ð1Þ e h3 n
s ¼ nem
ð2Þ
Here, kB is Boltazman’s constant, e is the electron charge, r is the scattering factor, m* is effective mass, h is Planck’s constant, n is the carrier concentration, and m is the carrier mobility. The increases of Seebeck coefficient at annealing temperature below 300 1C are due to reduction of the carrier concentration
by decreasing the number of defects, which acted as acceptors [23]. The increase of electrical conductivity is mainly due to the preferred crystal orientation and the reduction of carrier scattering by grain boundaries. The great change of carrier concentration and mobility of the film annealed at 400 1C is due to the grain boundaries scatter increases with crystallinity of the thin films deteriorating and secondly the collisions of charge carriers with the surface becoming significant when the free path decreases while the temperature is too high [21–24]. This phenomenon is consistent with the results of XRD patterns as shown in Fig. 2. The variations of PF in response to the change of annealing temperature are presented in Fig. 3. As shown in Fig. 4, the power factors increase from 0.58 10 3 to 1.10 10 3 Wm 1 K 2 as the annealing temperature increases from room temperature to 400 1C. The sample annealed at 300 1C has a maximum PF value of 1.10 10 3 Wm 1 K 2 with the highest Seebeck coefficient and a moderate electrical conductivity. This implies that annealing can improve the thermoelectric properties of the films due to the improvement of crystalline quality. When the annealing temperature reached 400 1C, the thermoelectric power factor of the thin film deteriorates because the Seebeck coefficient degrades at this high annealing temperature. Compared to various deposition methods for the bismuth–telluride based thin
1716
Z.-h. Zheng et al. / Journal of Physics and Chemistry of Solids 71 (2010) 1713–1716
that the electrical conductivity has a minimum value of 3.92 102 S cm 1when the annealing temperature is 300 1C. The best film has a maximum power factor of 1.10 10 3 Wm 1 K 2, comparable to the best results of the same material prepared by another sputtering technology. Obviously, adjustment of annealing temperature can improve the structure quality and thermoelectric properties of ion beam sputtered Bi2Te3 thin films. Acknowledgments This work was supported by the Project of the Bureau of Science and Technology, Nanshan District, Shenzhen (No. 2009035), and Open Funding Projects of Shenzhen Key Laboratory of Sensor Technology (Project no.: SST200901). Reference Fig. 4. Plot of power factor of the Bi2Te3 films, including as-deposited, as a function of annealing temperatures.
films, the thermoelectric power factor of ion beam sputtered thin film obtains similar performances (with a maximum power factor of 1.10 10 3 Wm 1 K 2) as those deposited by another technology.
4. Conclusions The n-type Bi2Te3 thin films were fabricated by the IBSD method with different annealing conductions. The structure and thermoelectric properties were investigated. EDS shows that the atomic composition of the films approaches the stoichiometric ratio of 2–3 when the area ratio of Bi plate to Te plate is 1–3.5. XRD shows that the film has a hexagonal structure. After annealing, the crystalline quality increases and the grains of the thin films grow along the c-axis, their natural growth axis [20–24]. However, the crystalline quality becomes worse when the annealing temperature is too high. Seebeck coefficient measurement shows that the Seebeck coefficient of the asdeposited thin film is 90 mV K 1 and increases to 168 mV K 1 after annealing at 300 1C. Hall coefficient measurement reveals
[1] J.P. Heremans, V. Jovovic, E.S. Toberer, A. Saramat, K. Kurosaki, A. Charoenphakdee, S. Yamanaka, G.J. Snyder, Science 321 (2008) 554. [2] L.E. Bell, Science 321 (2008) 1457. [3] L.D. Hicks, M.S. Dresslhaus, Phys. Rev. B 47 (1993) 16631. [4] Y. Jiang, Y.J. Zhu, J. Cryst. Growth 306 (2007) 351. [5] D. Pinisetty, R.V. Devireddy, Acta Mater. 58 (2010) 570. [6] S.K. Lim, M.Y. Kim, T.S. Oh, Thin Solid Films 517 (2009) 4199. [7] F. Xiao, C. Hangarter, B.Y. Yoo, Y.W. Rheem, K.H. Lee, N.V. Myung, Electrochim. Acta 53 (2008) 8103. [8] S. Michel, S. Diliberto, C. Boulanger, N. Stein, J. Cryst. Growth 277 (2005) 274. [9] D.C. Grauer, Y.S. Hor, A.J. Williams, R.J. Cava, Mater. Res. Bull. 44 (2009) 1926. [10] M. Takashiri, T. Shirakawa, K. Miyazaki, H. Tsukamoto, Sens. Actuators, A 138 (2007) 329. [11] M. Takashiri, K. Miyazaki, H. Tsukamoto, Thin Solid Films 516 (2008) 6336. [12] C.N. Liao, T.H. She, Thin Solid Films 515 (2007) 8059. [13] V. Richoux, S. Diliberto, C. Boulanger, J.M. Lecuire, Electrochim. Acta 52 (2007) 3053. [14] W.L. Wang, C.C. Wan, Y.Y. Wang, Electrochim. Acta 52 (2007) 6502. [15] P. Magri, C. Boulangerm, J.M. Lecurie, J. Mater. Chem. 6 (1996) 773. [16] S. Michel, S. Diliberto, C. Boulanger, B. Bolle, J. Cryst. Growth 296 (2006) 227. [17] H. Zou, D.M. Rowe, G. Min, J. Cryst. Growth 222 (2001) 82. [18] A. Boyer, E. Cisse´, Mater. Sci. Eng. B 13 (1992) 103. [19] P. Fan, Z.H. Zheng, G.X. Liang, X.M. Cai, D.P. Zhang, Chin. Phys. Lett. 8 (2010) 087201. [20] A. Bailini, F. Donati, M. Zamboni, V. Russo, M. Passoni, C.S. Cassri, A.L. Bassi, C.E. Bottani, Appl. Surf. Sci. 254 (2007) 1249. [21] J.P. Carmo, L.M. Goncalves, J.H. Correia, Electron. Lett. 45 (2009) 803. [22] G.S. Nolas, J. Sharp, H.J. Goldsmid, in: Thermoelectric, Springer, Berlin, 2001 p. 40. [23] L. Qiu, J. Zhou, X. Cheng, R. Ahuja, J. Phys. Chem. Solids 71 (2010) 1131. [24] A. Mzerd, B. Aboulfarah, A. Giani, A. Boyer, J. Mater. Sci. 41 (2006) 1659.