- Email: [email protected]
Growth and structure characterization of epitaxial Bi2Sr2Co2Oy thermoelectric thin films on LaAlO3 (001)
Thin Solid Films 518 (2010) 6829–6832
Contents lists available at ScienceDirect
Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f
Growth and structure characterization of epitaxial Bi2Sr2Co2Oy thermoelectric thin films on LaAlO3 (001) Shufang Wang a,⁎, Liping He a, Dogheche Elhadj b, Jingchun Chen a, Jianglong Wang a, Mingjing Chen a, Wei Yu a, Guangsheng Fu a a b
College of Physics Science and Technology, Hebei University, 071002 Baoding, China Institut Electronique Microélectronique Nanotechnologie IEMN DOAE CNRS UMR 8520, Université de Valenciennes, Le Mont Houy Valenciennes Cedex F-59309, France
a r t i c l e
i n f o
Article history: Received 10 December 2009 Received in revised form 30 March 2010 Accepted 24 June 2010 Available online 1 July 2010 Keywords: Thermoelectric properties Thin films Bi2Sr2Co2Oy Epitaxial growth Microstructure Chemical solution deposition X-ray diffraction Transmission electron microscopy A
a b s t r a c t Epitaxial Bi2Sr2Co2Oy thin films with excellent c-axis and ab-plane alignments have been grown on (001) LaAlO3 substrates by chemical solution deposition using metal acetates as starting materials. Microstructure studies show that the resulting Bi2Sr2Co2Oy films have a well-ordered layer structure with a flat and clear interface with the substrate. Scanning electron microscopy of the films reveals a step–terrace surface structure without any microcracks and pores. At room temperature, the epitaxial Bi2Sr2Co2Oy films exhibit a resistivity of about 2 mΩ cm and a seebeck coefficient of about 115 μV/K comparable to those of single crystals. © 2010 Elsevier B.V. All rights reserved.
1. Introduction The thermoelectric material is a material that can directly convert heat into electricity through seebeck effect and electricity into heat through peltier effect. Its thermoelectric performance is characterized by a dimensionless figure of merit, ZT = S2T / (ρκ), where S is the seebeck coefficient, T the temperature, ρ the electric resistivity and κ the thermal conductivity, respectively. Recently, layered cobalt oxides have attracted many attention because in addition to the good thermoelectric performance, they also have many advantages such as nontoxicity, thermal stability, high oxidation resistance, etc. [1–4]. Among various layered cobalt oxides explored, Bi2Sr2Co2Oy (BSCO) has thought to be one of the promising candidates for hightemperature thermoelectric applications due to its high ZT of ~1.1 at about 1000 K, which is comparable to that of the traditional hightemperature thermoelectric material SiGe alloys [5]. Extensive studies have been carried out on BSCO bulks, and S ~ 130 K and ρ ~ 4–10 mΩ cm have been obtained for single crystals at room temperature [6–12]. For many microscale thermoelectric applications, thin films are highly desirable. In addition, thin-film thermoelectric materials also offer tremendous scope for ZT enhancement by using the supperlattic,
⁎ Corresponding author. E-mail address: [email protected] (S. Wang). 0040-6090/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2010.06.060
quantum-well and two-dimensional electron gas structures [13–15]. However, so far there have been very few reports on thin films due to the difficulty in fabricating high quality thin-film samples which are caused by the incommensurate nature of the BSCO structure and the volatility of Bi element in this material [16,17]. In this paper, we report the growth of epitaxial BSCO thin films by a very simple chemical solution deposition (CSD) technique by using acetates of metal Bi, Sr and Co as starting materials. The resulting BSCO thin films show excellent crystalline quality as well as thermoelectric properties comparable to single crystals.
2. Experiments BSCO thin films were prepared through a CSD method. Briefly, the precursor solution for BSCO was prepared by dissolving the bismuth, strontium and cobalt acetates into propionic acid at the temperature of 50 °C with the cation ratio of Bi:Sr:Co = 1:1:1. The solution with the concentration of 0.05 mol/l was then spin-coated on (001) LaAlO3 single crystal substrates at 3600–4000 rpm for 40 s to form a single layer of BSCO thin film. Each coated layer was heat treated at about 100 °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 final film was then post annealed at temperature of about 850–870 °C for 2 h under flowing high purity oxygen with the pressure higher than 1 atm.
6830
S. Wang et al. / Thin Solid Films 518 (2010) 6829–6832
Fig. 2. (a) SEM surface image of the CSD-derived BSCO film on LaAlO3 (001), which clearly shows a step–terrace surface structure of the film.
microscope (SEM) equipped with an energy-dispersive X-ray spectroscopy (EDX) system. EDX measurements performed on the whole surface of the film revealed a Bi:Sr:Co ratio in the film of 2.06:2.13:2.00, which was very close to the nominal chemical composition of this material. The high-resolution transmission
Fig. 1. (a) XRD θ–2θ scan and (b) φ scan of (1 1 12) peak for a CSD-derived BSCO film on LaAlO3 (001). Substrate peaks are marked with asterisks. (c) The lattice matching relationship between the film and substrate.
The crystal structure of the film was measured using a Philips X'Pert 4-circle diffractometer with CuKα radiation. The surface morphology and the chemical composition of the film were investigated by an XL30 S-FEG field-emission scanning electron
Fig. 3. (a) HRTEM image and (b) the corresponding SEAD pattern of the BSCO/LaAlO3 cross section.
S. Wang et al. / Thin Solid Films 518 (2010) 6829–6832
6831
Fig. 4. EDX measurements of different parts of the BSCO/LaAlO3 cross section. The measured spots on the film and substrates are all very close to the interface. The element Cu detected in the EDX spectra of the substrate comes from the copper mesh used for the TEM sample.
electron microscopy (HRTEM) was performed at 200 kV using a JEOL 2010F microscope with a nominal point resolution of 1.9 Å. The temperature dependence of resistivity was measured by a standard four-probe method over the temperature range of 4.2 K to 300 K. The seebeck coefficient was measured by a steady state mode using a thermal transport option (TTO) in Quantum Design Physical Property Measurement Systems (PPMS) at high vacuum (10− 5 Pa). 3. Results and discussion Fig. 1a presents the X-ray diffraction (XRD) θ–2θ scan of a BSCO thin film on LaAlO3 (001) with the thickness of about 200 nm. Apart from the substrate peak, all peaks in the pattern can be indexed to the (00l) diffractions of BSCO, indicating that a phase-pure film with the c-axis normal to the film surface is obtained. The ω-scan of the (0 0 10) diffraction is very sharp with a full width at half maximum (FWHM) of about 0.3°, further confirming the excellent c-axis alignment of the film on LaAlO3 (001). The ab-plane alignment information of the BSCO films on LaAlO3 (001) was investigated by XRD φ scan. Fig. 1b presents the ϕ scan of the (1 1 12) peak of the film.
Fig. 5. The temperature dependence of resistivity and seebeck coefficient for BSCO thin films on LaAlO3 (001).
It reveals the presence of four sharp peaks with FWHM of only about 0.6, indicating the four-fold symmetry of the BSCO film with perfect ab-plane alignments. This ab-plane epitaxial growth of monoclinic BSCO on cubic LaAlO3 (001) is reasonable considering the lattice parameters of bulk BSCO (a1 = 4.911 Å and b1 = 5.111 Å) are about 3/4 times that of LaAlO3 (a = b = 3.789 Å), as illustrated in Fig. 1c, which leads to a small lattice mismatch of − 2.79% (a-axis) and 1.17% (b-axis) between the film and substrate. The perfect crystalline structure of the BSCO films revealed by XRD measurements, together with the good thermoelectric properties being presented in the latter part of this paper, implies that the CSD process used in this work is promising for high quality BSCO film growth. Fig. 2 shows the SEM surface image of the CSD-derived BSCO thin film on LaAlO3 (001) substrate. A stepped and terraced structure composed of several square-shaped grains can be seen in this image, indicating the monoclinic crystal symmetry of BSCO which is consistent with the XRD measurements. Similar surface structure was also observed in other cobalt oxide epitaxial films such as γ-Na0.7CoO2 and Ca3Co4O9, and it was thought to be related to the twodimensional layer-by-layer growth following the step-flow growth mode [18–20]. In addition, the film is dense and free of any microcracks and pores. This is an obvious improvement for conventional CSD-derived films and we think it can be attributed to the very low concentration of the precursor solution (0.05 mol/l) and the very slow temperature rising and cooling rates (5 °C/min) during the postannealing treatment in the CSD process. Fig. 3a is a cross-sectional HRTEM image of a BSCO film on LaAlO3 (001) substrate. It is clear that the interface is flat and without any visible secondary phases or amorphous phases. The HRTEM image of the film shown in the top part of Fig. 3a reveals a well-ordered layer structure of BSCO stacked along the c-axis. The corresponding selected area electron diffraction (SAED) pattern, shown in Fig. 3b, confirms the epitaxial growth of BSCO film on LaAlO3, evidenced by the distinguished diffraction dots from the film and the substrate. The epitaxial relationships between the film and the substrate deduced from the SAED pattern are consistent with the XRD analysis. We also investigate the interdiffusion between the film and substrate by using EDX equipped on the JEOL-2010F, as shown in Fig. 4. No significant
6832
S. Wang et al. / Thin Solid Films 518 (2010) 6829–6832
interdiffusion is observed between the film and substrate even though the annealing temperature of the CSD process described in this work is taken at high temperature of 850–870 °C. In addition, EDX measurements performed on the different spots over the film cross section show that no obvious composition fluctuation is detected along the film, and the element ratio in the film is similar to that obtained from the film surface. The thermoelectric properties of the CSD-derived epitaxial BSCO films on (0 0 1) LaAlO3 were investigated by measuring the temperature dependence of resistivity and seebeck coefficient of the films by a PPMS. Fig. 5 shows the ab-plane resistivity and seebeck coefficient as a function of temperature for the film. The resistivity ρab of the film shows a metalliclike behavior as temperature decreases down to Tmin ~ 97 K and diverges with further decreasing of the temperature. The resistivity diverging behavior at the low temperatures is similar to that observed in the bulks, which has been attributed to the decrease of the effective carrier number due to a pseudogap formation at low temperature or the magnetic ordering in low temperature [21,22]. At room temperature, the ρab value of the epitaxial BSCO film is around 2 mΩ cm, which is slightly lower than those reported for single crystal bulks (~4–10 mΩ cm) [10–12] and films fabricated by pulsed laser deposition (PLD) technique (~3 mΩ cm) [16]. The very low resistivity of the present CSD-derived BSCO film is most likely due to the reduced grain boundary scattering resulting from the excellent crystallinity with an enlarged grain size when compared with that of the PLD-grown films which are composed of irregularly-shaped grains with a size of 200–300 nm [16]. Another possible reason for the low resistivity could be associated with the oxygen content in the films, and this explanation is also supported by the following seebeck measurement results of the films. Studies on Ca3Co4Oy with similar crystal structure to BSCO revealed that the oxygen content had great effects on the transport properties of this compound and higher oxygen content in Ca3Co4Oy would result in a lower resistivity and a smaller seebeck coefficient [23,24]. In this work, BSCO film samples were obtained by annealing the precursor CSD films under high purity flowing oxygen with the pressure higher than 1 atm, resulting in oxygen rich BSCO films. The ab-plane S–T behavior of the film shown in Fig. 5 is also similar to that of the bulk samples and the positive S values in the whole temperature range studied reveal the hole transport in this material. The room temperature seebeck coefficient S of the films is about 115 μV/K. Although seebeck coefficients in the present epitaxial films are a little lower than that of single crystals (~130 μV/K) [10], larger thermoelectric power factor S2 / ρ is obtained at room temperature owing to lower resistivity in BSCO epitaxial films. 4. Summary In conclusion, we report growth and structure properties of epitaxial BSCO thin films on LaAlO3 (001) fabricated by using a simple
CSD technique. The films have a step–terrace surface structure without any microcracks and pores. Transmission electron microscopy and x-ray diffraction measurements reveal that the CSDderived Bi2Sr2Co2Oy films have well-ordered layer structures with excellent c-axis and ab-plane alignments on (001) LaAlO3 substrate. At 300 K, the epitaxial films show a low resistivity of ~ 2 mΩ cm and a reasonably large seebeck coefficient of ~ 115 μV/K. The results demonstrate that the present CSD method can produce high quality BSCO epitaxial films with thermoelectric properties comparable to single crystals, which suggests they may have great potential applications for thin-film thermoelectric devices. Acknowledgments The authors would like to thank Prof. Winnie Wong-Ng and Dr. Mark D. Vaudin of the National Institute of Standards and Technology, USA for the lattice parameter measurements of bulk samples. This work was partially supported by NSFC of China under Grant No. 10904030, NSFC of Hebei Province under Grant No. A2009000144 and the Key Project of Hebei Education Department under Grant No. ZD200909. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16]
[17] [18] [19] [20] [21] [22] [23] [24]
I. Terasaki, Y. Sasago, K. Uchinokura, Phys. Rev. B 56 (1997) R12685. R.E. Schaak, T. Klimczuk, M.L. Foo, R.J. Cava, Nature 424 (2003) 527. Y. Nagao, I. Terasaki, Phys. Rev. B 76 (2007) 144203. A.C. Masset, C. Michel, A. Maignan, M. Hervieu, O. Toulemonde, F. Studer, B. Raveau, Phys. Rev. B 62 (2000) 166. R. Funahashi, M. Shikano, Appl. Phys. Lett. 81 (2000) 1459. J.-S. Kang, S.W. Han, T. Fujii, I. Terasaki, S.S. Lee, G. Kim, C.G. Olson, H.G. Lee, J.-Y. Kim, B.I. Min, Phys. Rev. B 74 (2006) 205116. Y. Klein, D. Pelloquin, S. Hébert, A. Maignan, J. Appl. Phys. 98 (2005) 013701. F. Chen, K.L. Stokes, R. Funahashi, Appl. Phys. Lett. 81 (2002) 2379. R. Funahashi, I. Matsubara, S. Sodeoka, Appl. Phys. Lett. 76 (2000) 2385. K. Koumoto, I. Terasaki, R. Funahashi, Mater. Res. Soc. Bull. 31 (2006) 206. T. Yamamoto, K. Uchinokura, I. Tsukada, Phys. Rev. B 65 (2002) 184434. X.G. Luo, X.H. Chen, G.Y. Wang, C.H. Wang, X. Li, G. Wu, Y.M. Xiong, Eur. Phys. J. B 49 (2006) 37. R. Venkatasubramanian, E. Siivola, T. Colpitts, B. O'Quinn, Nature 413 (2001) 597. T.C. Harman, P.J. Taylor, M.P. Walsh, B.E. LaForge, Science 297 (2009) 2229. H. Ohta, S. Kim, Y. Mune, T. Mizoguchi, K. Nomura, S. Ohta, T. Nomura, M. Masahiro, H. Hosono, K. Koumoto, Nat. Mater. 6 (2007) 129. S.F. Wang, A. Venimadhav, S.M. Guo, K. Chen, Q. Li, A. Soukiassian, D.G. Schlom, M. B. Katz, X.Q. Pan, W. Wong-Ng, M.D. Vaudin, X.X. Xi, Appl. Phys. Lett. 94 (2009) 022110. M. Hirai, T. Mihara, R. Funahashi, Trans. Mater. Res. Soc. Jpn 31 (2006) 395. K. Sugiura, H. Ohta, K. Nomura, M. Hirano, H. Hosono, K. Koumoto, Appl. Phys. Lett. 89 (2006) 032111. B.J.Y. Son, B.G. Kim, J.H. Cho, Appl. Phys. Lett. 86 (2005) 221918. J.Y. Son, Y.H. Shin, C.S. Park, J. Appl. Phys. Lett. 104 (2008) 033538. T. Itoh, I. Terasaki, Jpn. J. Appl. Phys. 39 (2000) 6658. S. Hebert, S. Lambert, D. Pelloquin, A. Maignan, Phys. Rev. B 64 (2001) 172101. M. Karppinen, H. Fjellvåg, T. Konno, Y. Morita, T. Motohashi, H. Yamauchi, Chem. Mater. 16 (2004) 2790. M. Sano, S. Horii, I. Matsubara, R. Funahashi, M. Shikano, J.-I. Shimoyama, K. Kishio, Jpn. J. Appl. Phys. 42 (2003) L198.
Contents lists available at ScienceDirect
Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f
Growth and structure characterization of epitaxial Bi2Sr2Co2Oy thermoelectric thin films on LaAlO3 (001) Shufang Wang a,⁎, Liping He a, Dogheche Elhadj b, Jingchun Chen a, Jianglong Wang a, Mingjing Chen a, Wei Yu a, Guangsheng Fu a a b
College of Physics Science and Technology, Hebei University, 071002 Baoding, China Institut Electronique Microélectronique Nanotechnologie IEMN DOAE CNRS UMR 8520, Université de Valenciennes, Le Mont Houy Valenciennes Cedex F-59309, France
a r t i c l e
i n f o
Article history: Received 10 December 2009 Received in revised form 30 March 2010 Accepted 24 June 2010 Available online 1 July 2010 Keywords: Thermoelectric properties Thin films Bi2Sr2Co2Oy Epitaxial growth Microstructure Chemical solution deposition X-ray diffraction Transmission electron microscopy A
a b s t r a c t Epitaxial Bi2Sr2Co2Oy thin films with excellent c-axis and ab-plane alignments have been grown on (001) LaAlO3 substrates by chemical solution deposition using metal acetates as starting materials. Microstructure studies show that the resulting Bi2Sr2Co2Oy films have a well-ordered layer structure with a flat and clear interface with the substrate. Scanning electron microscopy of the films reveals a step–terrace surface structure without any microcracks and pores. At room temperature, the epitaxial Bi2Sr2Co2Oy films exhibit a resistivity of about 2 mΩ cm and a seebeck coefficient of about 115 μV/K comparable to those of single crystals. © 2010 Elsevier B.V. All rights reserved.
1. Introduction The thermoelectric material is a material that can directly convert heat into electricity through seebeck effect and electricity into heat through peltier effect. Its thermoelectric performance is characterized by a dimensionless figure of merit, ZT = S2T / (ρκ), where S is the seebeck coefficient, T the temperature, ρ the electric resistivity and κ the thermal conductivity, respectively. Recently, layered cobalt oxides have attracted many attention because in addition to the good thermoelectric performance, they also have many advantages such as nontoxicity, thermal stability, high oxidation resistance, etc. [1–4]. Among various layered cobalt oxides explored, Bi2Sr2Co2Oy (BSCO) has thought to be one of the promising candidates for hightemperature thermoelectric applications due to its high ZT of ~1.1 at about 1000 K, which is comparable to that of the traditional hightemperature thermoelectric material SiGe alloys [5]. Extensive studies have been carried out on BSCO bulks, and S ~ 130 K and ρ ~ 4–10 mΩ cm have been obtained for single crystals at room temperature [6–12]. For many microscale thermoelectric applications, thin films are highly desirable. In addition, thin-film thermoelectric materials also offer tremendous scope for ZT enhancement by using the supperlattic,
⁎ Corresponding author. E-mail address: [email protected] (S. Wang). 0040-6090/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2010.06.060
quantum-well and two-dimensional electron gas structures [13–15]. However, so far there have been very few reports on thin films due to the difficulty in fabricating high quality thin-film samples which are caused by the incommensurate nature of the BSCO structure and the volatility of Bi element in this material [16,17]. In this paper, we report the growth of epitaxial BSCO thin films by a very simple chemical solution deposition (CSD) technique by using acetates of metal Bi, Sr and Co as starting materials. The resulting BSCO thin films show excellent crystalline quality as well as thermoelectric properties comparable to single crystals.
2. Experiments BSCO thin films were prepared through a CSD method. Briefly, the precursor solution for BSCO was prepared by dissolving the bismuth, strontium and cobalt acetates into propionic acid at the temperature of 50 °C with the cation ratio of Bi:Sr:Co = 1:1:1. The solution with the concentration of 0.05 mol/l was then spin-coated on (001) LaAlO3 single crystal substrates at 3600–4000 rpm for 40 s to form a single layer of BSCO thin film. Each coated layer was heat treated at about 100 °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 final film was then post annealed at temperature of about 850–870 °C for 2 h under flowing high purity oxygen with the pressure higher than 1 atm.
6830
S. Wang et al. / Thin Solid Films 518 (2010) 6829–6832
Fig. 2. (a) SEM surface image of the CSD-derived BSCO film on LaAlO3 (001), which clearly shows a step–terrace surface structure of the film.
microscope (SEM) equipped with an energy-dispersive X-ray spectroscopy (EDX) system. EDX measurements performed on the whole surface of the film revealed a Bi:Sr:Co ratio in the film of 2.06:2.13:2.00, which was very close to the nominal chemical composition of this material. The high-resolution transmission
Fig. 1. (a) XRD θ–2θ scan and (b) φ scan of (1 1 12) peak for a CSD-derived BSCO film on LaAlO3 (001). Substrate peaks are marked with asterisks. (c) The lattice matching relationship between the film and substrate.
The crystal structure of the film was measured using a Philips X'Pert 4-circle diffractometer with CuKα radiation. The surface morphology and the chemical composition of the film were investigated by an XL30 S-FEG field-emission scanning electron
Fig. 3. (a) HRTEM image and (b) the corresponding SEAD pattern of the BSCO/LaAlO3 cross section.
S. Wang et al. / Thin Solid Films 518 (2010) 6829–6832
6831
Fig. 4. EDX measurements of different parts of the BSCO/LaAlO3 cross section. The measured spots on the film and substrates are all very close to the interface. The element Cu detected in the EDX spectra of the substrate comes from the copper mesh used for the TEM sample.
electron microscopy (HRTEM) was performed at 200 kV using a JEOL 2010F microscope with a nominal point resolution of 1.9 Å. The temperature dependence of resistivity was measured by a standard four-probe method over the temperature range of 4.2 K to 300 K. The seebeck coefficient was measured by a steady state mode using a thermal transport option (TTO) in Quantum Design Physical Property Measurement Systems (PPMS) at high vacuum (10− 5 Pa). 3. Results and discussion Fig. 1a presents the X-ray diffraction (XRD) θ–2θ scan of a BSCO thin film on LaAlO3 (001) with the thickness of about 200 nm. Apart from the substrate peak, all peaks in the pattern can be indexed to the (00l) diffractions of BSCO, indicating that a phase-pure film with the c-axis normal to the film surface is obtained. The ω-scan of the (0 0 10) diffraction is very sharp with a full width at half maximum (FWHM) of about 0.3°, further confirming the excellent c-axis alignment of the film on LaAlO3 (001). The ab-plane alignment information of the BSCO films on LaAlO3 (001) was investigated by XRD φ scan. Fig. 1b presents the ϕ scan of the (1 1 12) peak of the film.
Fig. 5. The temperature dependence of resistivity and seebeck coefficient for BSCO thin films on LaAlO3 (001).
It reveals the presence of four sharp peaks with FWHM of only about 0.6, indicating the four-fold symmetry of the BSCO film with perfect ab-plane alignments. This ab-plane epitaxial growth of monoclinic BSCO on cubic LaAlO3 (001) is reasonable considering the lattice parameters of bulk BSCO (a1 = 4.911 Å and b1 = 5.111 Å) are about 3/4 times that of LaAlO3 (a = b = 3.789 Å), as illustrated in Fig. 1c, which leads to a small lattice mismatch of − 2.79% (a-axis) and 1.17% (b-axis) between the film and substrate. The perfect crystalline structure of the BSCO films revealed by XRD measurements, together with the good thermoelectric properties being presented in the latter part of this paper, implies that the CSD process used in this work is promising for high quality BSCO film growth. Fig. 2 shows the SEM surface image of the CSD-derived BSCO thin film on LaAlO3 (001) substrate. A stepped and terraced structure composed of several square-shaped grains can be seen in this image, indicating the monoclinic crystal symmetry of BSCO which is consistent with the XRD measurements. Similar surface structure was also observed in other cobalt oxide epitaxial films such as γ-Na0.7CoO2 and Ca3Co4O9, and it was thought to be related to the twodimensional layer-by-layer growth following the step-flow growth mode [18–20]. In addition, the film is dense and free of any microcracks and pores. This is an obvious improvement for conventional CSD-derived films and we think it can be attributed to the very low concentration of the precursor solution (0.05 mol/l) and the very slow temperature rising and cooling rates (5 °C/min) during the postannealing treatment in the CSD process. Fig. 3a is a cross-sectional HRTEM image of a BSCO film on LaAlO3 (001) substrate. It is clear that the interface is flat and without any visible secondary phases or amorphous phases. The HRTEM image of the film shown in the top part of Fig. 3a reveals a well-ordered layer structure of BSCO stacked along the c-axis. The corresponding selected area electron diffraction (SAED) pattern, shown in Fig. 3b, confirms the epitaxial growth of BSCO film on LaAlO3, evidenced by the distinguished diffraction dots from the film and the substrate. The epitaxial relationships between the film and the substrate deduced from the SAED pattern are consistent with the XRD analysis. We also investigate the interdiffusion between the film and substrate by using EDX equipped on the JEOL-2010F, as shown in Fig. 4. No significant
6832
S. Wang et al. / Thin Solid Films 518 (2010) 6829–6832
interdiffusion is observed between the film and substrate even though the annealing temperature of the CSD process described in this work is taken at high temperature of 850–870 °C. In addition, EDX measurements performed on the different spots over the film cross section show that no obvious composition fluctuation is detected along the film, and the element ratio in the film is similar to that obtained from the film surface. The thermoelectric properties of the CSD-derived epitaxial BSCO films on (0 0 1) LaAlO3 were investigated by measuring the temperature dependence of resistivity and seebeck coefficient of the films by a PPMS. Fig. 5 shows the ab-plane resistivity and seebeck coefficient as a function of temperature for the film. The resistivity ρab of the film shows a metalliclike behavior as temperature decreases down to Tmin ~ 97 K and diverges with further decreasing of the temperature. The resistivity diverging behavior at the low temperatures is similar to that observed in the bulks, which has been attributed to the decrease of the effective carrier number due to a pseudogap formation at low temperature or the magnetic ordering in low temperature [21,22]. At room temperature, the ρab value of the epitaxial BSCO film is around 2 mΩ cm, which is slightly lower than those reported for single crystal bulks (~4–10 mΩ cm) [10–12] and films fabricated by pulsed laser deposition (PLD) technique (~3 mΩ cm) [16]. The very low resistivity of the present CSD-derived BSCO film is most likely due to the reduced grain boundary scattering resulting from the excellent crystallinity with an enlarged grain size when compared with that of the PLD-grown films which are composed of irregularly-shaped grains with a size of 200–300 nm [16]. Another possible reason for the low resistivity could be associated with the oxygen content in the films, and this explanation is also supported by the following seebeck measurement results of the films. Studies on Ca3Co4Oy with similar crystal structure to BSCO revealed that the oxygen content had great effects on the transport properties of this compound and higher oxygen content in Ca3Co4Oy would result in a lower resistivity and a smaller seebeck coefficient [23,24]. In this work, BSCO film samples were obtained by annealing the precursor CSD films under high purity flowing oxygen with the pressure higher than 1 atm, resulting in oxygen rich BSCO films. The ab-plane S–T behavior of the film shown in Fig. 5 is also similar to that of the bulk samples and the positive S values in the whole temperature range studied reveal the hole transport in this material. The room temperature seebeck coefficient S of the films is about 115 μV/K. Although seebeck coefficients in the present epitaxial films are a little lower than that of single crystals (~130 μV/K) [10], larger thermoelectric power factor S2 / ρ is obtained at room temperature owing to lower resistivity in BSCO epitaxial films. 4. Summary In conclusion, we report growth and structure properties of epitaxial BSCO thin films on LaAlO3 (001) fabricated by using a simple
CSD technique. The films have a step–terrace surface structure without any microcracks and pores. Transmission electron microscopy and x-ray diffraction measurements reveal that the CSDderived Bi2Sr2Co2Oy films have well-ordered layer structures with excellent c-axis and ab-plane alignments on (001) LaAlO3 substrate. At 300 K, the epitaxial films show a low resistivity of ~ 2 mΩ cm and a reasonably large seebeck coefficient of ~ 115 μV/K. The results demonstrate that the present CSD method can produce high quality BSCO epitaxial films with thermoelectric properties comparable to single crystals, which suggests they may have great potential applications for thin-film thermoelectric devices. Acknowledgments The authors would like to thank Prof. Winnie Wong-Ng and Dr. Mark D. Vaudin of the National Institute of Standards and Technology, USA for the lattice parameter measurements of bulk samples. This work was partially supported by NSFC of China under Grant No. 10904030, NSFC of Hebei Province under Grant No. A2009000144 and the Key Project of Hebei Education Department under Grant No. ZD200909. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16]
[17] [18] [19] [20] [21] [22] [23] [24]
I. Terasaki, Y. Sasago, K. Uchinokura, Phys. Rev. B 56 (1997) R12685. R.E. Schaak, T. Klimczuk, M.L. Foo, R.J. Cava, Nature 424 (2003) 527. Y. Nagao, I. Terasaki, Phys. Rev. B 76 (2007) 144203. A.C. Masset, C. Michel, A. Maignan, M. Hervieu, O. Toulemonde, F. Studer, B. Raveau, Phys. Rev. B 62 (2000) 166. R. Funahashi, M. Shikano, Appl. Phys. Lett. 81 (2000) 1459. J.-S. Kang, S.W. Han, T. Fujii, I. Terasaki, S.S. Lee, G. Kim, C.G. Olson, H.G. Lee, J.-Y. Kim, B.I. Min, Phys. Rev. B 74 (2006) 205116. Y. Klein, D. Pelloquin, S. Hébert, A. Maignan, J. Appl. Phys. 98 (2005) 013701. F. Chen, K.L. Stokes, R. Funahashi, Appl. Phys. Lett. 81 (2002) 2379. R. Funahashi, I. Matsubara, S. Sodeoka, Appl. Phys. Lett. 76 (2000) 2385. K. Koumoto, I. Terasaki, R. Funahashi, Mater. Res. Soc. Bull. 31 (2006) 206. T. Yamamoto, K. Uchinokura, I. Tsukada, Phys. Rev. B 65 (2002) 184434. X.G. Luo, X.H. Chen, G.Y. Wang, C.H. Wang, X. Li, G. Wu, Y.M. Xiong, Eur. Phys. J. B 49 (2006) 37. R. Venkatasubramanian, E. Siivola, T. Colpitts, B. O'Quinn, Nature 413 (2001) 597. T.C. Harman, P.J. Taylor, M.P. Walsh, B.E. LaForge, Science 297 (2009) 2229. H. Ohta, S. Kim, Y. Mune, T. Mizoguchi, K. Nomura, S. Ohta, T. Nomura, M. Masahiro, H. Hosono, K. Koumoto, Nat. Mater. 6 (2007) 129. S.F. Wang, A. Venimadhav, S.M. Guo, K. Chen, Q. Li, A. Soukiassian, D.G. Schlom, M. B. Katz, X.Q. Pan, W. Wong-Ng, M.D. Vaudin, X.X. Xi, Appl. Phys. Lett. 94 (2009) 022110. M. Hirai, T. Mihara, R. Funahashi, Trans. Mater. Res. Soc. Jpn 31 (2006) 395. K. Sugiura, H. Ohta, K. Nomura, M. Hirano, H. Hosono, K. Koumoto, Appl. Phys. Lett. 89 (2006) 032111. B.J.Y. Son, B.G. Kim, J.H. Cho, Appl. Phys. Lett. 86 (2005) 221918. J.Y. Son, Y.H. Shin, C.S. Park, J. Appl. Phys. Lett. 104 (2008) 033538. T. Itoh, I. Terasaki, Jpn. J. Appl. Phys. 39 (2000) 6658. S. Hebert, S. Lambert, D. Pelloquin, A. Maignan, Phys. Rev. B 64 (2001) 172101. M. Karppinen, H. Fjellvåg, T. Konno, Y. Morita, T. Motohashi, H. Yamauchi, Chem. Mater. 16 (2004) 2790. M. Sano, S. Horii, I. Matsubara, R. Funahashi, M. Shikano, J.-I. Shimoyama, K. Kishio, Jpn. J. Appl. Phys. 42 (2003) L198.