Dependence of oxygen content on transverse thermoelectric effect in tilted Bi2Sr2Co2Oy thin films Guoying Yan,1,2,3 Zilong Bai,1 Shufang Wang,1,3,4 Liqing Sun,1 Jianglong Wang,1,3 and Guangsheng Fu1,2,5 1
The College of Physics Science and Technology, Hebei University, Hebei Baoding 071002, China 2 3
School of Information Engineering, Hebei University of Technology, Tianjin 300401, China
Hebei KeyLab of Optic-Electronic Information and Materials, Hebei Baoding 071002, China 4
e-mail: [email protected] 5
e-mail: fugs‐[email protected]
Received 23 April 2014; revised 22 May 2014; accepted 23 May 2014; posted 2 June 2014 (Doc. ID 210608); published 27 June 2014
The transverse thermoelectric (TE) effect has been investigated in c axis tilted Bi2 Sr2 Co2 Oy thin films with different oxygen content. The film samples were fabricated by a chemical solution deposition method annealed under different atmospheres of O2 , air, and N2 , respectively. Open-circuit transverse voltage signals were observed when the surface of the films was heated by a pulsed laser as well as a continuous thermal source. With the increase of oxygen content in the films, the amplitude of the observed voltage signals increased while the response time decreased. The experimental results can be explained by a mechanism involving the transverse TE effect as well as the transport theory of TE materials. © 2014 Optical Society of America OCIS codes: (040.5160) Photodetectors; (310.3840) Materials and process characterization; (310.6845) Thin film devices and applications. http://dx.doi.org/10.1364/AO.53.004211
1. Introduction
Thermoelectric (TE) technology is the direct energy conversion from heat into electricity and has attracted much attention as a renewable energy solution. However, conventional TE devices, based on the longitudinal TE effect with electric charges moving parallel to the heat flow, cannot be scaled down in size much smaller than a millimeter. The transverse TE effect, in which heat and electric energy flow through a material perpendicular to each other, have recently attracted increasing attention for researchers [1–8]. These transverse TE devices could
1559-128X/14/194211-05$15.00/0 © 2014 Optical Society of America
be scaled down to very small sizes and therefore open the door for novel TE applications. The transverse TE effect emerges uniquely in tilted structures with the anisotropic Seebeck coefficient. As shown in Fig. 1(a), when the surface of a tilted sample is heated by light radiations or thermal heaters, a temperature gradient ∇z T will be set up across the sample along its thickness direction (i.e., z axis), resulting in a transverse thermal voltage signal expressed by Vx
l sin2αΔS∇z T; 2
(1)
where l is the heating length of the sample along the x axis, α is the tilting angle of the sample, and 1 July 2014 / Vol. 53, No. 19 / APPLIED OPTICS
4211
by the chemical solution method, and the oxygen content of the films was controlled by annealing the film under different atmosphere of O2, air, and N2 , respectively. It is found that, with increasing oxygen content in the films, the amplitude of the transverse TE voltage signals increase while the response time decreases, which provides valuable insights on the improvement in the performance of this new type TE devices. 2. Experiments
Fig. 1. (a) Schematic illustration of the transverse TE effect measurements, where a, b, c and x, z represent the crystal axis and the spatial axis, respectively. (b) Schematic crystal structure of Bi2 Sr2 Co2 Oy .
ΔS Sab − Sc is the difference of the Seebeck coefficient in the ab plane and along the c axis of the sample [9]. According to this equation, exploring new materials with large anisotropy in a Seebeck coefficient (that is, large ΔS) is crucial for developing this type of novel TE device. Layered cobaltites, including Mx CoO2 (M Na or Ca), Ca3 Co4 O9 , Bi2 Sr2 Co2 Oy , etc., are newly discovered oxide TE materials, and their longitudinal TE effect has been extensively studied [10–14]. The crystal structure of these layered cobaltites consists of the conducting CoO2 layer and the insulating layer, which are alternately stacked along the c axis [Fig. 1(b)]. This layered structure results in highly anisotropic electronic transport properties with the in-plane Seebeck coefficient being much larger than that of the out-of-plane, suggesting that layered cobaltites are also promising candidate for transverse TE applications. Recently, the transverse TE effect has been demonstrated in the c axis tilted thin films of these layered cobaltites by us and other researchers, and the dependence of the transverse TE effect on the film thickness and microstructure has been investigated [2,15]. It is known that the oxygen content in oxide TE materials can greatly influence its Seebeck coefficient, electrical conductivity, and thermal conductivity as well as crystalline quality, which are all key parameters that determine the transverse TE effect. In this work, we reported a systematic study of the dependence of the oxygen content on the transverse TE effect in Bi2 Sr2 Co2 Oy (BSCO) thin films. The film samples were fabricated 4212
APPLIED OPTICS / Vol. 53, No. 19 / 1 July 2014
BSCO thin films with thickness of about 200 nm were prepared on 0° and 10° tilted LaAlO3 (001) substrates through a chemical solution deposition technique, which is described in detail in [16]. As in [16], each coated layer was heat treated at 120°C for 2 min and then 400°C for 30 min in air. The spin coating was repeated to obtain the desired film thickness. The oxygen content in the final films was changed by post-annealing the samples for 2 h under different atmospheres of O2, air, and N2 , respectively. To avoid the formation of large amounts of impurities or second phases in the film annealed in N2 , in this work, all final films were post-annealed at a relatively low temperature of 810°C. The crystalline quality of the films was examined by x-ray diffraction (XRD). The electrical conductivity and Seebeck coefficient were simultaneously measured in air by the standard dc four-probe technique in an LSR-3 measurement system (Linseis, Germany). For the measurements of the transverse TE effect, two indium electrodes separated by 8 mm were symmetrically deposited on the surface of the BSCO films grown on 10° tilted substrates. An aluminum heat sink was attached to the back side of substrates with thermally conductive double-faced adhesive tape. A pulsed XeCl excimer laser (308 nm, 20 ns) with the energy density of 0.5 mJ∕mm2 was used as the heating source. The laser spot was located at the center position between two electrodes, and its area was about 2 mm × 5 mm. The induced voltage signals were recorded using a digital oscilloscope terminated into 1 MΩ (Tektronix, TDS3052). Apart from the laser, a point-like thermal heater was also used as a heating source. The temperature of the heater can be adjusted by changing its input electric power. To ensure uniform heating between two electrodes, a copper foil was attached to the film surface by an ultrathin thermal conducting paste. In this measurement, the induced voltage waveforms were recorded by using a 2700 Keithley source meter. 3. Results and Discussion
Figure 2 shows XRD θ–2θ scan curves of three BSCO films on 0° tilted LaAlO3 substrates annealed in O2 , air, and N2 , respectively. Apart from the substrate peaks, all peaks in films annealed in O2 and air can be indexed to the (00l) diffractions of BSCO, indicating that these two films are pure phase and c axis oriented. While for the film annealed in N2 , additional peaks can be observed besides such peaks
9
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σab(S/cm)
0014 sub.-βκ
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O2 air N2
8
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*
(a)
100
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100
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Fig. 3. In-plane electric conductivity σ ab and Seebeck coefficient Sab of the untilted BSCO films annealed in O2 , air, and N2 , respectively.
1.6x105 N2 air O2
1.2x105
8.0x104
4.0x104
0.0 17.4
17.6
17.8
18.0
18.2
2θ (degree) Fig. 2. (a) XRD θ–2θ scan curves of the untilted BSCO thin films annealed in O2 , air, and N2 , respectively. “” represents a second phase in the film annealed in N2 . (b) Magnified curves of (006) peak for these three films.
originating from the (00l) diffractions of BSCO, which suggests that the obtained BSCO film is c axis oriented grown with a second phase in it. Figure 2(b) presents the magnified curves of (006) peak for these three films. With the increase of oxygen content in the film, the diffraction peak becomes stronger. The result implies that films with more oxygen content have better crystalline quality. The in-plane electrical conductivity σ ab and Seebeck coefficient Sab of these three films are measured at room temperature. As shown in Fig. 3, with the reduction of oxygen content in the films, the electrical conductivity of the film decreases while the Seebeck coefficient increases, which can be explained as the result of the reduction in the number of holes carriers due to the increase of oxygen vacancies in the films, since BSCO is a p-type TE material. In addition, the enhanced carriers scattering from the oxygen vacancies and crystal defects is also responsible for the decrease of the electrical conductivity. It should be mentioned here that in this work the room temperature Sab of the films annealed in O2 is about 102 μV∕K, which is lower than that of the films prepared in [16] (Sab 116 μV∕K). We suggest the following two reasons might be responsible for the difference in Sab . First, the film samples of this work were annealed at 810°C, and the films in [16] were
annealed at 870°C. The different annealing temperature had an effect on the crystalline quality of the samples, leading to the changes of Sab value. Second, the Seebeck coefficient of these two samples was measured by different equipment under different atmosphere conditions, which resulted in different measurement errors and thus a different Sab . Figure 4(a) presents the transverse TE voltage signals of the 10° tilted BSCO films annealed in O2 , air, and N2 , respectively. A 308 nm pulsed laser with the pulse width of 20 ns is used as the heating source. It can be seen that the film annealed in O2 has the largest voltage sensitivity R (R V P ∕E) of about 0.67 V∕mJ; here V p is the amplitude of the induced voltage and E is the laser energy on the films surface. Although this R value is higher than that of SrTiO3, ZnO, La1−x Bx (B Sr, Ca, Pb) MnO3 , etc. [5,17–19], it is still lower than that reported for other layered cobaltites such as Mx CoO2 (M Na or Ca), despite having the similar ΔS [1–4]. This is mainly because BSCO has a smaller electrical conductivity σ than that of Nax CoO2 or Cax CoO2, which will result in a smaller ∇z T when its surface is being heated by the laser irradiation, since the absorption coefficient α of a conductor for the incident light is proportional to its electrical conductivity σ, i.e., α ∝ σ 1∕2 . Figure 4(b) displays the variation of the voltage sensitivity R with the annealing atmosphere for these three tilted BSCO thin films. Film with more oxygen content annealed in O2 shows higher voltage sensitivity R than that of the film with less oxygen annealed in N2 . As seen in XRD measurements, the film annealed in O2 has better crystalline quality than that of the film annealed in N2 . So the c axis alignment degree of the film with more oxygen content must be higher than that of the film with less oxygen content, which will lead to enhanced anisotropy of S, i.e., ΔS, in the films with more oxygen content; even its S value is smaller than that of the film with less oxygen content. Moreover, the light absorption coefficient α of the film with more oxygen content should be larger than that of the film with less oxygen content because of its larger electrical 1 July 2014 / Vol. 53, No. 19 / APPLIED OPTICS
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4 O2 air N2
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Annealing atmosphere Fig. 4. (a) Transverse TE voltage signals of the 10° tilted BSCO films annealed in O2 , air, and N2 , respectively. The heating source is a 308 nm pulsed laser. (b) Variation of voltage sensitivity R and decay time τd of the transverse TE voltage signals with the annealing atmosphere.
conductivity. In this case, more light power will be used for heating the film surface, leading to an increased temperature gradient ∇z T along the thickness of the sample. According to Eq. (1), all these effects can increase the amplitude of the output transverse voltage V x , thus the voltage sensitivity R of the transverse TE effect. Besides the higher voltage sensitivity R, the transverse TE voltage signal of films with more oxygen content shows faster response time than that of the films with less oxygen content. As seen in Fig. 4(a), the response time of the voltage signal consists of one sharp rising time τr and another gradual decay time τd . All films show a similar τr, indicating that the time of establishing the transverse TE field is nearly the same when those films are heated by the 308 nm pulsed irradiation. However, the decay time τd of these three films, which is related to the time of thermal relaxation, increases with the reduction oxygen content of the films. In this work, the film thickness of BSCO is larger than that of the light penetration depth of the film at λ 308 nm. In this case, according to the thermal transfer model of the transverse TE effect, the TE field generated in the tilted film after heating by light (λ 308 nm) will decay mainly by thermal diffusion within the film, i.e., τd ∝ d2 ∕κ where κ is the thermal conductivity 4214
APPLIED OPTICS / Vol. 53, No. 19 / 1 July 2014
and d is the film thickness [14,20,21]. Therefore, the slower decay time τd observed in BSCO film with less oxygen content annealed in N2 can be attributed to the smaller thermal conductivity κ of the film. In this work, although we did not obtain the thermal conductivity κ of these three films due the difficulty in measuring κ of thin film samples, we can expect that the BSCO film with less oxygen content would have a smaller κ than that of the film with more oxygen content. This is due to the strong phonon scattering at oxygen vacancies and crystal defects of the film. In addition, the reduction of the contribution of the carrier thermal conductivity κ e to κ (κ κe κph , κ ph is the phonon thermal conductivity) also leads to a decreased κ in the film with less oxygen content because that κe is linked to the electrical conductivity σ by Wiedemann–Franz’s law, κ e L0 Tσ. Instead of directly heating the surface of these three BSCO films with a laser source, copper foil was placed onto the film surface and then heated with a point-like thermal heater. Open-circuit voltage signals are also clearly observed when the surface of these films is heated by the heater, and the magnitude of the induced voltage is found to increase with the temperature of the heater. The observation of continuous electric-potential differences in case of thermal heating experiments furthermore proves the TE nature of the reported effect in this work. Figure 5 shows that the film with more oxygen content has larger voltage sensitivity than that of film with less oxygen. For example, for films annealed in O2 , air, and N2 , the magnitude of the observed transverse TE voltage is about 172, 92, and 15 μV respectively. This result further demonstrates that the transverse TE effect of BSCO films can be greatly improved by controlling the oxygen content of the films. It should to be noted here that the rising time τr and decay time τd shown in Fig. 5 are on the order of second, which are much longer than those shown in Fig. 4(a). As we described in the former part 200
150
Voltage ( µV)
(a)
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50 O2 air N2
0 0
30
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Time (s) Fig. 5. Transverse TE voltage signals of the three tilted BSCO films by using a point-like thermal heater as the heating source. Inset is the schematic illustration of the measurements.
of this paper, the rising time τr of a transverse TE signal of a tilted thin film reflects how fast the temperature gradient ∇z T is established along the thickness of the thin film while the decay time τd is related to the thermal relaxation of the thin film [8]. For the case of Fig. 4(a), both the establishment of ∇z T and the thermal relaxation of the thin film are all very fast due to the transient temperature response of the 308 nm pulsed laser heating. While for the case of Fig. 5, these two processes are quite slow because it will take a long time for thermal equilibrium to be reached due to the continuous heating of the thermal heater. 4. Conclusion
In conclusion, the transverse TE effect has been investigated in c axis tilted BSCO thin films with different oxygen content. All films show an open-circuit transverse TE voltage signal when their surface is heated by a pulsed laser or a thermal heater. The voltage signal of films with more oxygen content exhibits higher voltage sensitivity and faster response time. To test the repeatability of our results, we measured several sets of film samples fabricated in the same conditions, and almost consistent results were obtained. It is suggested that the oxygen content in BSCO plays an important role in determining the crystalline quality of the film sample as well as its electronic and thermal properties. The results are useful for fabrication of light/thermal detectors or power generators based on the transverse TE effect. This project was supported by the National Natural Science Foundation of China (nos. 51372064, 11205046), the Nature Science Foundation for Distinguished Young Scholars of Hebei Province, China (no. 2013201249), and the Science and Technology Research Projects of Colleges and Universities in Hebei Province (no. QN20131040). References 1. K. Takahashi, T. Kanno, A. Sakai, H. Adachi, and Y. Yamada, “Influence of interband transition on the laser-induced voltage in thermoelectric CaxCoO2 thin film,” Phys. Rev. B 83, 115107 (2011). 2. K. Takahashi, T. Kanno, A. Sakai, H. Adachi, and Y. Yamada, “Gigantic transverse voltage induced via off-diagonal thermoelectric effect in CaxCoO2 thin films,” Appl. Phys. Lett. 97, 021906 (2010). 3. K. Takahashi, T. Kanno, A. Sakai, H. Adachi, and Y. Yamada, “Light-induced off-diagonal thermoelectric effect via indirect optical heating of incline oriented CaxCoO2 thin film,” Appl. Phys. Lett. 100, 181907 (2012). 4. S. F. Wang, S. S. Chen, F. Q. Liu, G. Y. Yan, J. C. Chen, H. L. Li, J. L. Wang, W. Yu, and G. S. Fu, “Laser-induced voltage effects in c-axis inclined NaxCoO2 thin films,” Appl. Surf. Sci. 258, 7330–7333 (2012).
5. K. Zhao, K. J. Jin, Y. H. Huang, S. Q. Zhao, H. B. Lü, M. He, Z. H. Chen, Y. L. Zhou, and G. Z. Yang, “Ultraviolet fastresponse photoelectric effect in tilted orientation SrTiO3 single crystals,” Appl. Phys. Lett. 89, 173507 (2006). 6. Y. Qin, T. Zhao, H. H. Zhang, B. Wang, P. X. Zhang, and J. F. Yang, “Large transverse thermoelectric voltage effect in incline-oriented SrTi1-xNbxO3 films with cubic structure,” Appl. Phys. Lett. 102, 253901 (2013). 7. L. Yu, Y. Wang, P. X. Zhang, and H. U. Habermeie, “Ultrafast transverse thermoelectric response in c-axis inclined epitaxial La0.5Sr0.5CoO3 thin films,” Phys. Status Solidi RRL 7, 180–183 (2013). 8. Y. Qin, T. Zhao, B. Wang, P. X. Zhang, and J. F. Yang, “Development of transverse thermoelectric voltage effect in artificial SrTiO3/SrTi1-xNbxO3 epitaxial multilayer films with inclineoriented sublayers,” Cryst. Eng. Comm. 16, 5345–5351 (2014). 9. H. Lengfellner, G. Kremb, A. Schnellbögl, J. Betz, K. F. Renk, and W. Prettl, “Giant voltages upon surface heating in normal YBa2Cu3O7−δ films suggesting an atomic layer thermopile,” Appl. Phys. Lett. 60, 501–503 (1992). 10. I. Terasaki, Y. Sasago, and K. Uchinokura, “Large thermoelectric power in NaCo2O4 single crystals,” Phys. Rev. B 56, R12685 (1997). 11. Y. N. Huang, B. C. Zhao, S. Lin, W. H. Song, and Y. P. Sun, “Renormalized bands and low-temperature colossal thermopower induced by Ir doping in Ca3Co4O9 system,” J. Appl. Phys. 114, 093709 (2013). 12. J. Ravichandran, A. K. Yadav, W. Siemons, M. A. McGuire, V. Wu, A. Vailionis, A. Majumdar, and R. Ramesh, “Size effects on thermoelectricity in a strongly correlated oxide,” Phys. Rev. B 85, 085112 (2012). 13. S. F. Wang, Z. L. Bai, H. F. Wang, Q. Lü, J. L. Wang, and G. S. Fu, “High temperature thermoelectric properties of Bi2Sr2Co2Oy/Ag composites,” J. Alloys Compd. 554, 254–257 (2013). 14. N. V. Nong, N. Pryds, S. Linderoth, and M. Ohtaki, “Enhancement of the thermoelectric performance of p-type layered oxide Ca3Co4O9+δ through heavy doping and metallic nanoinclusions,” Adv. Mater. 23, 2484–2490 (2011). 15. G. Y. Yan, S. F. Wang, S. S. Chen, F. Q. Liu, Z. L. Bai, J. L. Wang, W. Yu, and G. S. Fu, “The effect of microstructure on the laser-induced transverse voltage in Pb-doped Bi2Sr2Co2Oy thin films on tilted substrates,” Appl. Phys. A 111, 1203–1206 (2013). 16. S. F. Wang, Z. C. Zhang, L. P. He, M. J. Chen, W. Yu, and G. S. Fu, “Epitaxial growth and transport properties of Bi2Sr2Co2Oy thin films by metal organic deposition,” Appl. Phys. Lett. 94, 162108 (2009). 17. X. F. Zhou, H. Zhang, Q. M. Chen, J. Shang, and P. X. Zhang, “Effects of annealing atmosphere on thermoelectric signals from ZnO films,” Thin Solid Films 519, 3026–3028 (2011). 18. K. Zhao, M. He, G. Z. Liu, and H. B. Lu, “Photo-induced voltage characteristics of La0.9Sr0.1MnO3 films epitaxially grown on vicinal SrTiO3 (001) substrates,” J. Phys. D 40, 5703–5706 (2007). 19. P. X. Zhang, C. Wang, S. L. Tan, H. Zhang, and H.-U. Habermeier, “Improving the performance of thermoelectric devices by doping Ag in LaPbMnO3 thin films,” J. Cryst. Growth 310, 2732–2737 (2008). 20. S. Zeuner, W. Prettl, and H. Lengfellner, “Fast thermoelectric response of normal state YBa2Cu3O7−δ films,” Appl. Phys. Lett. 66, 1833–1835 (1995). 21. Th. Zahner, R. Förg, and H. Lengfellner, “Transverse thermoelectric response of a tilted metallic multilayer structure,” Appl. Phys. Lett. 73, 1364–1366 (1998).
1 July 2014 / Vol. 53, No. 19 / APPLIED OPTICS
4215
The College of Physics Science and Technology, Hebei University, Hebei Baoding 071002, China 2 3
School of Information Engineering, Hebei University of Technology, Tianjin 300401, China
Hebei KeyLab of Optic-Electronic Information and Materials, Hebei Baoding 071002, China 4
e-mail: [email protected] 5
e-mail: fugs‐[email protected]
Received 23 April 2014; revised 22 May 2014; accepted 23 May 2014; posted 2 June 2014 (Doc. ID 210608); published 27 June 2014
The transverse thermoelectric (TE) effect has been investigated in c axis tilted Bi2 Sr2 Co2 Oy thin films with different oxygen content. The film samples were fabricated by a chemical solution deposition method annealed under different atmospheres of O2 , air, and N2 , respectively. Open-circuit transverse voltage signals were observed when the surface of the films was heated by a pulsed laser as well as a continuous thermal source. With the increase of oxygen content in the films, the amplitude of the observed voltage signals increased while the response time decreased. The experimental results can be explained by a mechanism involving the transverse TE effect as well as the transport theory of TE materials. © 2014 Optical Society of America OCIS codes: (040.5160) Photodetectors; (310.3840) Materials and process characterization; (310.6845) Thin film devices and applications. http://dx.doi.org/10.1364/AO.53.004211
1. Introduction
Thermoelectric (TE) technology is the direct energy conversion from heat into electricity and has attracted much attention as a renewable energy solution. However, conventional TE devices, based on the longitudinal TE effect with electric charges moving parallel to the heat flow, cannot be scaled down in size much smaller than a millimeter. The transverse TE effect, in which heat and electric energy flow through a material perpendicular to each other, have recently attracted increasing attention for researchers [1–8]. These transverse TE devices could
1559-128X/14/194211-05$15.00/0 © 2014 Optical Society of America
be scaled down to very small sizes and therefore open the door for novel TE applications. The transverse TE effect emerges uniquely in tilted structures with the anisotropic Seebeck coefficient. As shown in Fig. 1(a), when the surface of a tilted sample is heated by light radiations or thermal heaters, a temperature gradient ∇z T will be set up across the sample along its thickness direction (i.e., z axis), resulting in a transverse thermal voltage signal expressed by Vx
l sin2αΔS∇z T; 2
(1)
where l is the heating length of the sample along the x axis, α is the tilting angle of the sample, and 1 July 2014 / Vol. 53, No. 19 / APPLIED OPTICS
4211
by the chemical solution method, and the oxygen content of the films was controlled by annealing the film under different atmosphere of O2, air, and N2 , respectively. It is found that, with increasing oxygen content in the films, the amplitude of the transverse TE voltage signals increase while the response time decreases, which provides valuable insights on the improvement in the performance of this new type TE devices. 2. Experiments
Fig. 1. (a) Schematic illustration of the transverse TE effect measurements, where a, b, c and x, z represent the crystal axis and the spatial axis, respectively. (b) Schematic crystal structure of Bi2 Sr2 Co2 Oy .
ΔS Sab − Sc is the difference of the Seebeck coefficient in the ab plane and along the c axis of the sample [9]. According to this equation, exploring new materials with large anisotropy in a Seebeck coefficient (that is, large ΔS) is crucial for developing this type of novel TE device. Layered cobaltites, including Mx CoO2 (M Na or Ca), Ca3 Co4 O9 , Bi2 Sr2 Co2 Oy , etc., are newly discovered oxide TE materials, and their longitudinal TE effect has been extensively studied [10–14]. The crystal structure of these layered cobaltites consists of the conducting CoO2 layer and the insulating layer, which are alternately stacked along the c axis [Fig. 1(b)]. This layered structure results in highly anisotropic electronic transport properties with the in-plane Seebeck coefficient being much larger than that of the out-of-plane, suggesting that layered cobaltites are also promising candidate for transverse TE applications. Recently, the transverse TE effect has been demonstrated in the c axis tilted thin films of these layered cobaltites by us and other researchers, and the dependence of the transverse TE effect on the film thickness and microstructure has been investigated [2,15]. It is known that the oxygen content in oxide TE materials can greatly influence its Seebeck coefficient, electrical conductivity, and thermal conductivity as well as crystalline quality, which are all key parameters that determine the transverse TE effect. In this work, we reported a systematic study of the dependence of the oxygen content on the transverse TE effect in Bi2 Sr2 Co2 Oy (BSCO) thin films. The film samples were fabricated 4212
APPLIED OPTICS / Vol. 53, No. 19 / 1 July 2014
BSCO thin films with thickness of about 200 nm were prepared on 0° and 10° tilted LaAlO3 (001) substrates through a chemical solution deposition technique, which is described in detail in [16]. As in [16], each coated layer was heat treated at 120°C for 2 min and then 400°C for 30 min in air. The spin coating was repeated to obtain the desired film thickness. The oxygen content in the final films was changed by post-annealing the samples for 2 h under different atmospheres of O2, air, and N2 , respectively. To avoid the formation of large amounts of impurities or second phases in the film annealed in N2 , in this work, all final films were post-annealed at a relatively low temperature of 810°C. The crystalline quality of the films was examined by x-ray diffraction (XRD). The electrical conductivity and Seebeck coefficient were simultaneously measured in air by the standard dc four-probe technique in an LSR-3 measurement system (Linseis, Germany). For the measurements of the transverse TE effect, two indium electrodes separated by 8 mm were symmetrically deposited on the surface of the BSCO films grown on 10° tilted substrates. An aluminum heat sink was attached to the back side of substrates with thermally conductive double-faced adhesive tape. A pulsed XeCl excimer laser (308 nm, 20 ns) with the energy density of 0.5 mJ∕mm2 was used as the heating source. The laser spot was located at the center position between two electrodes, and its area was about 2 mm × 5 mm. The induced voltage signals were recorded using a digital oscilloscope terminated into 1 MΩ (Tektronix, TDS3052). Apart from the laser, a point-like thermal heater was also used as a heating source. The temperature of the heater can be adjusted by changing its input electric power. To ensure uniform heating between two electrodes, a copper foil was attached to the film surface by an ultrathin thermal conducting paste. In this measurement, the induced voltage waveforms were recorded by using a 2700 Keithley source meter. 3. Results and Discussion
Figure 2 shows XRD θ–2θ scan curves of three BSCO films on 0° tilted LaAlO3 substrates annealed in O2 , air, and N2 , respectively. Apart from the substrate peaks, all peaks in films annealed in O2 and air can be indexed to the (00l) diffractions of BSCO, indicating that these two films are pure phase and c axis oriented. While for the film annealed in N2 , additional peaks can be observed besides such peaks
9
N2 LAO(002)
σab(S/cm)
0014 sub.-βκ
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Fig. 3. In-plane electric conductivity σ ab and Seebeck coefficient Sab of the untilted BSCO films annealed in O2 , air, and N2 , respectively.
1.6x105 N2 air O2
1.2x105
8.0x104
4.0x104
0.0 17.4
17.6
17.8
18.0
18.2
2θ (degree) Fig. 2. (a) XRD θ–2θ scan curves of the untilted BSCO thin films annealed in O2 , air, and N2 , respectively. “” represents a second phase in the film annealed in N2 . (b) Magnified curves of (006) peak for these three films.
originating from the (00l) diffractions of BSCO, which suggests that the obtained BSCO film is c axis oriented grown with a second phase in it. Figure 2(b) presents the magnified curves of (006) peak for these three films. With the increase of oxygen content in the film, the diffraction peak becomes stronger. The result implies that films with more oxygen content have better crystalline quality. The in-plane electrical conductivity σ ab and Seebeck coefficient Sab of these three films are measured at room temperature. As shown in Fig. 3, with the reduction of oxygen content in the films, the electrical conductivity of the film decreases while the Seebeck coefficient increases, which can be explained as the result of the reduction in the number of holes carriers due to the increase of oxygen vacancies in the films, since BSCO is a p-type TE material. In addition, the enhanced carriers scattering from the oxygen vacancies and crystal defects is also responsible for the decrease of the electrical conductivity. It should be mentioned here that in this work the room temperature Sab of the films annealed in O2 is about 102 μV∕K, which is lower than that of the films prepared in [16] (Sab 116 μV∕K). We suggest the following two reasons might be responsible for the difference in Sab . First, the film samples of this work were annealed at 810°C, and the films in [16] were
annealed at 870°C. The different annealing temperature had an effect on the crystalline quality of the samples, leading to the changes of Sab value. Second, the Seebeck coefficient of these two samples was measured by different equipment under different atmosphere conditions, which resulted in different measurement errors and thus a different Sab . Figure 4(a) presents the transverse TE voltage signals of the 10° tilted BSCO films annealed in O2 , air, and N2 , respectively. A 308 nm pulsed laser with the pulse width of 20 ns is used as the heating source. It can be seen that the film annealed in O2 has the largest voltage sensitivity R (R V P ∕E) of about 0.67 V∕mJ; here V p is the amplitude of the induced voltage and E is the laser energy on the films surface. Although this R value is higher than that of SrTiO3, ZnO, La1−x Bx (B Sr, Ca, Pb) MnO3 , etc. [5,17–19], it is still lower than that reported for other layered cobaltites such as Mx CoO2 (M Na or Ca), despite having the similar ΔS [1–4]. This is mainly because BSCO has a smaller electrical conductivity σ than that of Nax CoO2 or Cax CoO2, which will result in a smaller ∇z T when its surface is being heated by the laser irradiation, since the absorption coefficient α of a conductor for the incident light is proportional to its electrical conductivity σ, i.e., α ∝ σ 1∕2 . Figure 4(b) displays the variation of the voltage sensitivity R with the annealing atmosphere for these three tilted BSCO thin films. Film with more oxygen content annealed in O2 shows higher voltage sensitivity R than that of the film with less oxygen annealed in N2 . As seen in XRD measurements, the film annealed in O2 has better crystalline quality than that of the film annealed in N2 . So the c axis alignment degree of the film with more oxygen content must be higher than that of the film with less oxygen content, which will lead to enhanced anisotropy of S, i.e., ΔS, in the films with more oxygen content; even its S value is smaller than that of the film with less oxygen content. Moreover, the light absorption coefficient α of the film with more oxygen content should be larger than that of the film with less oxygen content because of its larger electrical 1 July 2014 / Vol. 53, No. 19 / APPLIED OPTICS
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4 O2 air N2
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Annealing atmosphere Fig. 4. (a) Transverse TE voltage signals of the 10° tilted BSCO films annealed in O2 , air, and N2 , respectively. The heating source is a 308 nm pulsed laser. (b) Variation of voltage sensitivity R and decay time τd of the transverse TE voltage signals with the annealing atmosphere.
conductivity. In this case, more light power will be used for heating the film surface, leading to an increased temperature gradient ∇z T along the thickness of the sample. According to Eq. (1), all these effects can increase the amplitude of the output transverse voltage V x , thus the voltage sensitivity R of the transverse TE effect. Besides the higher voltage sensitivity R, the transverse TE voltage signal of films with more oxygen content shows faster response time than that of the films with less oxygen content. As seen in Fig. 4(a), the response time of the voltage signal consists of one sharp rising time τr and another gradual decay time τd . All films show a similar τr, indicating that the time of establishing the transverse TE field is nearly the same when those films are heated by the 308 nm pulsed irradiation. However, the decay time τd of these three films, which is related to the time of thermal relaxation, increases with the reduction oxygen content of the films. In this work, the film thickness of BSCO is larger than that of the light penetration depth of the film at λ 308 nm. In this case, according to the thermal transfer model of the transverse TE effect, the TE field generated in the tilted film after heating by light (λ 308 nm) will decay mainly by thermal diffusion within the film, i.e., τd ∝ d2 ∕κ where κ is the thermal conductivity 4214
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and d is the film thickness [14,20,21]. Therefore, the slower decay time τd observed in BSCO film with less oxygen content annealed in N2 can be attributed to the smaller thermal conductivity κ of the film. In this work, although we did not obtain the thermal conductivity κ of these three films due the difficulty in measuring κ of thin film samples, we can expect that the BSCO film with less oxygen content would have a smaller κ than that of the film with more oxygen content. This is due to the strong phonon scattering at oxygen vacancies and crystal defects of the film. In addition, the reduction of the contribution of the carrier thermal conductivity κ e to κ (κ κe κph , κ ph is the phonon thermal conductivity) also leads to a decreased κ in the film with less oxygen content because that κe is linked to the electrical conductivity σ by Wiedemann–Franz’s law, κ e L0 Tσ. Instead of directly heating the surface of these three BSCO films with a laser source, copper foil was placed onto the film surface and then heated with a point-like thermal heater. Open-circuit voltage signals are also clearly observed when the surface of these films is heated by the heater, and the magnitude of the induced voltage is found to increase with the temperature of the heater. The observation of continuous electric-potential differences in case of thermal heating experiments furthermore proves the TE nature of the reported effect in this work. Figure 5 shows that the film with more oxygen content has larger voltage sensitivity than that of film with less oxygen. For example, for films annealed in O2 , air, and N2 , the magnitude of the observed transverse TE voltage is about 172, 92, and 15 μV respectively. This result further demonstrates that the transverse TE effect of BSCO films can be greatly improved by controlling the oxygen content of the films. It should to be noted here that the rising time τr and decay time τd shown in Fig. 5 are on the order of second, which are much longer than those shown in Fig. 4(a). As we described in the former part 200
150
Voltage ( µV)
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50 O2 air N2
0 0
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Time (s) Fig. 5. Transverse TE voltage signals of the three tilted BSCO films by using a point-like thermal heater as the heating source. Inset is the schematic illustration of the measurements.
of this paper, the rising time τr of a transverse TE signal of a tilted thin film reflects how fast the temperature gradient ∇z T is established along the thickness of the thin film while the decay time τd is related to the thermal relaxation of the thin film [8]. For the case of Fig. 4(a), both the establishment of ∇z T and the thermal relaxation of the thin film are all very fast due to the transient temperature response of the 308 nm pulsed laser heating. While for the case of Fig. 5, these two processes are quite slow because it will take a long time for thermal equilibrium to be reached due to the continuous heating of the thermal heater. 4. Conclusion
In conclusion, the transverse TE effect has been investigated in c axis tilted BSCO thin films with different oxygen content. All films show an open-circuit transverse TE voltage signal when their surface is heated by a pulsed laser or a thermal heater. The voltage signal of films with more oxygen content exhibits higher voltage sensitivity and faster response time. To test the repeatability of our results, we measured several sets of film samples fabricated in the same conditions, and almost consistent results were obtained. It is suggested that the oxygen content in BSCO plays an important role in determining the crystalline quality of the film sample as well as its electronic and thermal properties. The results are useful for fabrication of light/thermal detectors or power generators based on the transverse TE effect. This project was supported by the National Natural Science Foundation of China (nos. 51372064, 11205046), the Nature Science Foundation for Distinguished Young Scholars of Hebei Province, China (no. 2013201249), and the Science and Technology Research Projects of Colleges and Universities in Hebei Province (no. QN20131040). References 1. K. Takahashi, T. Kanno, A. Sakai, H. Adachi, and Y. Yamada, “Influence of interband transition on the laser-induced voltage in thermoelectric CaxCoO2 thin film,” Phys. Rev. B 83, 115107 (2011). 2. K. Takahashi, T. Kanno, A. Sakai, H. Adachi, and Y. Yamada, “Gigantic transverse voltage induced via off-diagonal thermoelectric effect in CaxCoO2 thin films,” Appl. Phys. Lett. 97, 021906 (2010). 3. K. Takahashi, T. Kanno, A. Sakai, H. Adachi, and Y. Yamada, “Light-induced off-diagonal thermoelectric effect via indirect optical heating of incline oriented CaxCoO2 thin film,” Appl. Phys. Lett. 100, 181907 (2012). 4. S. F. Wang, S. S. Chen, F. Q. Liu, G. Y. Yan, J. C. Chen, H. L. Li, J. L. Wang, W. Yu, and G. S. Fu, “Laser-induced voltage effects in c-axis inclined NaxCoO2 thin films,” Appl. Surf. Sci. 258, 7330–7333 (2012).
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