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Improving the performance of thermoelectric devices by doping Ag in LaPbMnO3 thin films
ARTICLE IN PRESS
Journal of Crystal Growth 310 (2008) 2732–2737 www.elsevier.com/locate/jcrysgro
Improving the performance of thermoelectric devices by doping Ag in LaPbMnO3 thin films P.X. Zhanga,c, C. Wangb,, S.L. Tana, H. Zhanga,c, H.-U. Habermeierc a
Institute of Advanced Materials for Photoelcetrons, Kunming University of Science and Technology, Kunming 650051, China Research Institute of Engineering and Technology, Yunnan University, No.2 North Cuihu road, Kunming, Yunnan 650091, China c Max-Planck-Institut fu¨r Festko¨rperforschung, Stuttgart D-70569, Germany
b
Received 12 October 2007; accepted 23 January 2008 Communicated by D.P. Norton Available online 1 February 2008
Abstract A series of Ag-doped La0.6Pb0.4MnO3 thin films were grown on vicinal cut substrates by pulsed laser deposition (PLD). Laser-induced thermoelectric voltages (LITV) had been observed in these films, and these LITV signals had been demonstrated to originate from the anisotropic Seebeck effect. By doping Ag to an optimum value, it was found that the peak values (UP) of the LITV signals were maximized, and the full-width at half-maximum (t) of the response curves of LITV were minimized at the same time. The figure of merit (Fm) of the device used as photodetector is greatly improved by doping Ag in La0.6Pb0.4MnO3 thin films. The possible reason for these improvements had been well discussed. r 2008 Elsevier B.V. All rights reserved. PACS: 77.84.Dy; 81.20.Fw; 85.50.n Keywords: A1. Doping; A1. Radiation; A2. Pulsed laser deposition; B1. Manganites; B3. Thermoelectric devices
1. Introduction
formula based on the anisotropic thermoelectric model:
The experiment of laser-induced voltage (LIV) effect, in which the YBa2Cu3O7d (YBCO) thin film were used as a photosensor, was performed firstly by C.L. Chang et al. [1]. Later, it was proved that the LIV signals were induced by the thermoelectric anisotropy in the high Courier-temperature (TC) superconductor oxides. When the incident pulsed laser radiation has been absorbed by these films, a temperature difference generates between the top and the bottom thin layers of the film as the top surface of the films is heated instantly. This temperature gradient produces an electric voltage through the thermoelectric or Seebeck effect. Once the films are grown on vicinal cut substrates, the amplitude of the laser-induced thermoelectric voltages (LITV) signals becomes even larger. H. Lengfellner et al. [2] described the phenomenon quantitatively by deriving a
Ux ¼
Corresponding author. Tel.: +86 871 5031869; fax: +86 871 5037399.
E-mail address: [email protected] (C. Wang). 0022-0248/$ - see front matter r 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2008.01.044
l ðS ab S c Þ sinð2aÞDT 2d
(1)
where Ux is the induced voltage in the x-direction, l and d are the length and thickness of the films, Sab and Sc are the Seebeck tensor components in the ab plane and c direction, respectively, a is the tilting angle of the substrate, and DT is the temperature difference between the top and bottom parts of the film. This formula explained most of the experimental observations, especially the tilting angle dependence of the induced voltage, which has become the unique criteria for identifying the thermoelectric effect from other light-induced effect. Therefore, Lengfellner et al. subsequently named these materials as atomic layer thermopile (ALT) materials, and the signals as LITV. Since then several high-temperature superconductor (HTSC) oxides, such as doped YBCO, Bi2Sr2CaCu2O8, Tl2Ba2Ca Cu2O8, etc., have been well studied and the induced voltage signals in those materials have been demonstrated to be
ARTICLE IN PRESS P.X. Zhang et al. / Journal of Crystal Growth 310 (2008) 2732–2737
a similar LITV effect [3–7]. Based on these studies, light detection devices with good performance have been designed, which exhibit some superior characteristics over traditional bolometers and photon-counting devices. Most obvious advantages are that the device can function over a broad spectral range and at the same time has a fast time response [8–11]. Recently, Habermeier et al. observed a similar effect in La0.67Ca0.33MnO3 thin films grown on vicinal cut substrates [12,13]. This discovery indicates that the ALT property consists not only in the HTSC oxide material group, but also other oxide materials. On the other hand, lanthanum manganites are interesting for their colossal magnetoresistance (CMR) properties, and have attracted a lot of attention both from the viewpoint of basic physics as well as potential applications. The discovery also provided new insight for the understanding of the mechanisms of CMR, since it is related to transportation of carriers. The observation of LITV signals in these materials puts forward a number of questions from a basic physics point of view. Firstly, there should be very small anisotropy in these thin films due to their cubic (or quasicubic) nature in crystallography. Secondly, the measurements were performed at room temperature, where La0.67Ca0.33MnO3 is in the paramagnetic phase, so there should be no magnetic anisotropy as well. However, further studies revealed that there are a number of manganites and other perovskites which demonstrate the same effect, and the initial small LITV signals can be enhanced by ion substitution or changing the doping level as well as the lattice mismatch between the substrates and the films. Considering that for practical applications, the influence of the thin film materials on the device performance, especially the time response of the device, is rather important. We have also derived a detailed formula based on a plane heat source and thermal diffusion model [9,14]: UðtÞ ¼
2 2 a0 El sinð2aÞ pffiffiffiffiffiffiffiffi ðSab S c Þðed =4Dt ed =4Dt Þ 4drc0 pDt
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a fixed system. Based on the description in Eq. (2), it is obvious that the thermal diffusivity (D), which is proportional to thermal conductivity k, must be reduced to meet the expectation of increasing the peak value of a LITV voltage. However, this reduction will also increase the time constant, which can be defined as the full-width at halfmaximum (t) of the response curve of the time-dependent induced voltages. Considering the importance for device to exhibit as a fast time response and a high sensitivity, one can define a device figure of merit (Fm) for this type of photodetector devices: F m ¼ U P =t
(3)
where UP and t are the peak value of the induced voltage and the full-width at half-maximum of the response curve of the time-dependent LITV signals, respectively. Therefore, synthesizing and searching for more materials with high LITV sensitivity and at the same time the fast time response, namely higher Fm value of thermoelectric device, is the purpose of this work. In this work, we have prepared Ag-doped La0.6Pb0.4MnO3 (Ag-LPMO) thin films by means of pulsed laser deposition (PLD). The LITV signals have been observed in these films and well investigated. By optimizing the doping level of Ag, the performance and the Fm value of the LITV devices were improved substantially. 2. Experimental procedure Ag-LPMO ceramic disk, used as the targets in the PLD experiments, was synthesized by conventional solid reaction or coprecipitation method [19]. Fig. 1 shows the X-ray diffraction (XRD) pattern of these ceramics. The XRD curves demonstrated that the targets are of the perovskite structure and single phase. The calculated lattice parameters show that the c-axis lattice parameter was firstly
(2)
where U(t) is the time-dependent induced voltage. The a0, r, d, c, and D are the absorption coefficient, density of the film material, penetration depth of light, specific heat, and thermal diffusivity of the film material, respectively. The E denotes the energy of the incident-pulsed laser light. Using this formula, one can select a material and design devices to detect pulsed laser radiation with high sensitivity and fast time response at required wavelengths. These devices will function at room temperature, and without any applied bias. These advantages make such devices very promising, both for ultraviolet (UV) and infrared spectra. To improve the performance of the devices, however, it was found that the sensitivity and the time response are some times in contradiction. It is well-known that the thermoelectric material will exhibit high performance if the figure of merit (ZT) is high. Where Z=Ss/k, a smaller thermal conductivity k, larger conductivity s will lead to higher ZT value [15–18], while the Seebeck coefficient S is constant in
Fig. 1. The XRD pattern of different Ag-doped La0.6Pb0.4MnO3 ceramics used as targets for fabricating thin films by pulsed laser deposition.
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reduced and then increased again with increasing Agdoping level from 0.0 to 10 wt%. The substrates are SrTiO3 (STO) and LaAlO3 (LAO) single crystal cut with different tilting angles to the c-axes of the crystals. The PLD experiments were performed by using a KrF excimer-laser of 248 nm in wavelength (l) and 20 ns in pulse width. An optimized laser fluency of 1.6 J/cm2 and repeat of 5 Hz were utilized. For all the used Ag-LPMO thin films, the substrate temperature and the pressure of flowing oxygen were precisely kept, respectively, at 780 1C and 0.4 mbar during the deposition, respectively. The growing process was halted when each film thickness was up to 80 nm. Finally, all the samples were annealed in situ for 1 h at 530 1C in an oxygen pressure of 1.0 bar. More detailed growth method can be found in the literature [20,21]. Three groups of Ag-LPMO film samples were prepared. The first group of samples was deposited with the identical deposition conditions on the STO and LAO substrates with the same tilting angle of 151. In this way, a substrate with larger LITV signals was chosen for the next experiments. With the aim to check the tilting angle factor on the induced voltages, the second group of samples was prepared on the LAO substrates but with different tilting angles. The third group of samples was prepared with different Ag doping levels from 0 to 10 wt% on the LAO substrates cut with 151 off the c-axes. Fig. 2 shows a typical XRD pattern of deposited AgLPMO film on LAO (151) substrate. The films were grown
Fig. 3. Time-dependent LITV signals obtained from Ag–La0.6Pb0.4MnO3 films grown on LaAlO3 (151) and SrTiO3 (151) substrates, respectively. Inset shows the planform of the film samples used in the LITV experiments.
epitaxial like on the substrates with high quality. The asgrown films were fabricated to 1 3 mm2 stripes by using the normal photolithography technique. The Pt electrodes were fixed at both ends of the stripe, and the middle 1 1 mm2 region was used as the sensitive area for light detection. The planform of the samples used in the LITV experiments has been shown in the inset of Fig. 3. To obtain the LITV signals, a pulsed excimer laser with wavelength of 248 nm and the pulse duration of 20 ns has been used to irradiate the sensitive area of Ag-LPMO stripe. The LITV signals were detected by using an oscilloscope with input impedance of 1 MO. Each final LITV signal and the corresponding pulsed laser energy (Epulse) were taken from the average results of 1000 times measurement, and the average LITV signals and Epulse values were recorded synchronously by the oscilloscope and a laser energy-meter [11], respectively. The typical four-point configuration was performed to measure the resistivity and the conductivity of the AgLPMO thin films. In this measurement, a current resource is used to provide the invariable current of 1 mA.
3. Results and discussion
Fig. 2. The typical XRD patterns of single-crystal substrate LaAlO3 (a) and the Ag-doped La0.6Pb0.4MnO3 thin film (b) grown on it.
Three groups of experiments have been performed corresponding to the three groups of samples. The substrate’s effect on the LITV signals has been shown in Fig. 3. The same Ag-LPMO (Ag-4 wt%-doped) films have been grown on the STO (151) and LAO (151) substrates, respectively. In this optical experiment, the energy density of the incident laser was kept at 0.13 mJ/mm2. It is evident that the LITV signals from LAO are larger than that from STO. Similar result has also been observed in the Ag-LPMO films grown on the two tilting 101 STO and
ARTICLE IN PRESS P.X. Zhang et al. / Journal of Crystal Growth 310 (2008) 2732–2737
LAO substrates. Therefore, the LAO substrate was chosen for the later experiments. As that has been mentioned, the relationship between the induced voltage and substrate tilting angles is critical for judging whether the induced voltage should be attributed to the thermoelectric effect. We have performed a series of LITV experiments of Ag-LPMO films grown on the LAO substrates with different tilting angles from 51 to 201. All the Ag-LPMO (Ag-4 wt%-doped) films were prepared under the same conditions and LITV signals were measured at the incident energy density. It is clearly shown in Fig. 4 that the larger the tilting angles, the stronger the induced voltages, which support our expectation that the induced voltages are due to thermoelectric effect [9]. The third experiment was to check the influence of the Agdoping level on the performance of LITV devices when used as a light detector. Special attention is paid on two measured parameters: the peak value UP of LITV signals and the time response t of the LITV time curves. All the measurements were conducted at room temperature. The Ag-doping level influence on the performances has been shown in Fig. 5. As the observation in Fig. 3, the LITV signals of the films grown on LAO are larger than that on STO substrates. Since all the other experimental conditions are the same, one has to conclude that the lattice parameter of the substrates plays an important role in this case. The small lattice parameter in the ab plane of the LAO substrate (a ¼ b ¼ 0.379 nm) causes the grown Ag-LPMO films, for which the lattice parameter is larger (a ¼ b ¼ 0.389 nm) to be compressively strained. On the other hand, the lattice parameters of STO are a ¼ b ¼ 0.390 nm, so the grown films of Ag-LPMO should be tensilely strained in the ab plane, with small c-axis parameter. The transportation of electrons or hopping polarons will be easier in the compressed plane than in the enlarged c-direction. The distortion of a small Mn–O–Mn bond length in the ab
Fig. 4. Time-dependent LITV curves obtained from films grown on LaAlO3 substrates with their vicinal cut angles of 51, 101, 151, and 201, respectively.
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Fig. 5. Time dependent LITV signals obtained from different Ag-doped La0.6Pb0.4MnO3 thin films grown on LaAlO3 substrates with tilting angle of 151.
Table 1 The measured peak values UP of laser induced voltage, time response t, and the calculated figure of merit of the Ag(4 wt%)-LPMO/LAO thermoelectric devices with different tilting angles of LaAlO3 substrates Tilting angle (deg)
UP (mV)
t (ns)
Fm
5 10 15 20
17 38 85 120
108 145 178 203
0.16 0.26 0.48 0.59
plane in addition to a large c-axis lattice parameter leads to large anisotropy of the conductivity as well as the Seebeck components, hence large LITV signals, which we shall prove is due to the thermoelectric anisotropic effect. In order to test that the induced voltages are due to the anisotropic Seebeck effect, we prepared Ag-4 wt% doped LPMO thin films on 51, 101, 151, and 201 vicinal cut LAO substrates and measured the induced voltages with pulsed laser excitation from these different films. It is clear from Fig. 4 that larger the tilting angle, the higher the LITV signals. This UP -substrate tilting angle sin(2a) proportionality demonstrates that the induced voltages originate from the thermoelectric effect, since all the other light-induced effects do not show such tilting angle dependence. This conclusion is also in agreement with the results obtained from the first set of experiments, in which the signals from LAO are larger than that from films grown on STO due to stress-induced Seebeck coefficient anisotropy. With the increase in the tilting angle, another interesting observation from this experiment is that not only the peak voltage but also the time response t increases. This effect has not been observed before in YBCO or in other similar experiments. Table 1 lists the measured UP, t and the calculated Fm values at different tilting angles. Microscopically the surface of a vicinal cut substrate is composed
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of steps like a ladder. It is known that growth of highquality samples on vicinal cut substrates is difficult due to these steps. Haagen et al. have observed nanostructure stripes in YBCO films grown on vicinal cut STO by highresolution atomic force microscope (AFM), similar results in manganite thin films has also been reported [13]. These nanostructured stripes grow along the fixed crystal orientations, thereby introducing anisotropy, which is observed both in resistivity and in flux pinning [22,23]. It is conceivable that a similar nanostructure may also exist in the case of Ag-LPMO thin films. Films grown on substrates with higher tilting angles demonstrate stripe structures of smaller width and higher interface density. Therefore, the electric conductivity is decreased in the direction, in which the substrate was cut vicinal. At the same time, the increased interface causes a reduction of the thermal conductivity due to phonon scattering at the interfaces of these nanostructures. Small thermal conductivity enhances ZT of the film material and leads to a larger LITV signal, and a longer time response, according to Eq. (2). Although t is larger at higher tilting angles, the Fm value of thermoelectric device enhances about fourfold with the increase of tilted angle from 51 to 201, as shown in Table 1. Therefore, using the large tilting angle substrates is beneficial for device application. However, it would also be a challenge to grow high quality films on substrates with tilting angles 4201. This limitation means that some other way to improve the device performance is needed. The influence of various Ag-doping levels on the LITV signals has been shown in Fig. 5. Experiments demonstrated that the induced-voltage peak value UP increases at first with increasing Ag-doping level, reaching a maximum at 4–8 wt%, and then decreases. On the other hand, the time response t changes in opposite manner, reducing first and then increasing. According to Eq. (3), one can calculate the Fm value of each device. Table 2 lists the measured UP, t, and the calculated Fm value at different Ag-doping levels. It is found that the optimal Ag-doping for high Fm value is in the doping range of 4–6 wt%. It is also evident that Fm value can be enhanced several times by manipulating the doping level of Ag in LPMO thin films. There is much discussion on the enhancement of the ZT of thermoelectric materials, since this is the key parameter
Table 2 The measured conductivity s, peak values UP of LITV signals, time response t, and the calculated figure of merit of Ag-LPMO/LAO (151) thermoelectric device with different Ag doping level in the films Ag doping (wt%)
d (S/cm)
UP (mV)
t (ns)
Fm
0 2 4 6 8 10
313.8 567.7 1302.1 2125.9 901.9 749.4
25 35 43 38 48 20
140 106 80 65 100 126
0.18 0.33 0.53 0.58 0.48 0.16
for applications. Several new concepts to obtain materials with ZT higher than 1 have been reported [15,16]. The central issue on how to enhance ZT is to form materials with ‘‘phonon glass and electron crystal’’ behavior, namely with high electric conductivity and low thermal conductivity. Doping with monovalent ions, Ag, either in the lattice or at grain boundary-like interfaces always enhances electronic conductivity, while inhomogeneity due to the doping increases the centers of phonon scattering. The Ag-doped manganite formula can be written as La3þ 1x 1þ 3þ 4þ Pb2þ ðxyÞ Agy Mn1ðxþyÞ Mnxþy O3 for small amounts of substitution. With increasing Ag1+, more Mn4+ are formed, which results in a high density of carriers, hence a high electrical conductivity and an enhanced double exchange interaction. However, over doping of Ag1+ leads to a reduction of Mn3+–O–Mn4+ pairs, which will suppress the double exchange and enhance the superexchange. Therefore, after reaching a maximum, the electrical conductivity will be reduced. This is in agreement with the experimental results: the electrical conductivity of the films increases with small Ag-doping, and after reaching a maximum reduces with further doping. The Seebeck coefficient in polycrystalline La0.7Sr0.3x AgxMnO3 pellets have been studied recently [24]. It was found that the Seebeck coefficient at room temperature becomes negative by doping with Ag, and further increasing doping leads to higher coefficients. In our case, the anisotropy of the Seebeck coefficient (SabSc) is essential, and it is reasonable to suppose that a larger Seebeck coefficient coexists with larger anisotropy, therefore with a larger LITV peak value. On the other hand, the doping of Ag causes a reduction of thermal conductivity due to phonon scattering either at grain boundary-like interfaces or nano-scale inhomogeneity. It has been argued that in a magnetic manganite, the thermal conductivity is composed of three parts: phonons, electrons and magnons (or spin waves). At room temperature, the contribution of thermal conductivity from electrons (and/or holes) is negligible, while that from magnons becomes important below or near the ferromagnetic transition TC (or TN). However, the estimated upper limit of magnon contribution is about 10% [25,26]. Therefore, the dominant part of the thermal conductivity is from the contribution of phonons. Doping with Ag into the lattice or at the grain boundary introduced scattering centers for phonons, and leads to a reduction in thermal conductivity. Therefore, it can be concluded that the effects of Ag doping, enhanced electric conductivity, and the reduction of thermal conductivity, are all favorable to improve the performance of thermoelectric devices. 4. Conclusion The laser-induced thermoelectric voltages have been observed in Ag-doped La0.6Pb0.4MnO3 thin films grown on vicinal cut substrates. It has been confirmed that this is a thermoelectric effect, due to the anisotropic Seebeck tensor
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in the materials. By optimizing the doping level of Ag (in the range of 4–6 wt%), the peak value UP and the time response t of the time-dependent induced voltage curves reach a maximum and minimum, respectively. Therefore, the figure of merit of the device has been enhanced several times. This is due to an increase in electrical conductivity and Seebeck coefficients, while a reduction in thermal conductivity at the same time. Doping with Ag has been demonstrated to be an effective way of improving the performance of atomic layer thermopile devices, and may also be used for increasing ZT of other thermoelectric materials. Acknowledgments The authors thank the people in Max-Planck Institute (FKF) Stuttgart for supporting the chance of the study and intercommunion. This work was financially supported by the National Science Foundation of China (G10274026) and the Science Foundation of Yunnan province (1999E0003Z). One of the authors, C. Wang, would like to thank the support from the Cultivated Foundation for ‘‘Academic Cadreman’’ of Yunnan University. References [1] C.L. Chang, A. Kleinhammes, W.G. Moulton, L.R. Testardi, Phys. Rev. B 41 (1990) 11564. [2] H. Lengfellner, G. Kremb, A. Schnellbo¨gl, J. Betz, K.F. Renk, W. Prettl, Appl. Phys. Lett. 60 (1992) 501. [3] H.S. Kwok, J.P. Zheng, Q.Y. Ying, R. Rao, Appl. Phys. Lett. 54 (1989) 2473. [4] T. Zahner, R. Schreiner, R. Stierstorfer, O. Kus, S.T. Li, R. Roessler, J.D. Pedarnig, D. Bauerle, H. Lengfellner, Europhys. Lett. 40 (1997) 673.
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[5] L.R. Testardi, Appl. Phys. Lett. 64 (1994) 2347. [6] H.-U. Habermeier, N. Jisrawi, G. Ja¨ger-Waldau, Inst. Phys. Conf. Ser. 148 (1995) 1023. [7] X.H. Li, Doctorial Thesis, Institute of Physics, Chinese Academic of Sciences, 2001, p. 57. [8] T. Zahner, R. Schreiner, S. Reindle, T. Schauer, A. Penzkofer, H. Lengfellner, Physica C 313 (1999) 37. [9] P.X. Zhang, W.K. Lee, G.Y. Zhang, Appl. Phys. Lett. 81 (2002) 4026. [10] K. Zhao, Physica B 373 (2006) 72. [11] P.X. Zhang, C. Wang, G.Y. Zhang, Opt. Laser Technol. 36 (2004) 341. [12] H.-U. Habermeier, X.H. Li, P.X. Zhang, B. Leibold, Solid State Commun. 110 (1999) 473. [13] X.H. Li, H.-U. Habermeier, P.X. Zhang, J. Magn. Magn. Mater. 211 (2000) 232. [14] C. Wang, P.X. Zhang, G.Y. Zhang, Acta Sin. Phys. 53 (2004) 1727. [15] R. Venkatasubramanian, E. Siivola, T. Colpitts, B.O. Quinn, Nature 413 (2001) 597. [16] T.C. Harman, P. Taylor, M.P. Walsh, B.E. LaForge, Science 297 (2002) 2229. [17] H. Boettner, G. Chen, R. Venkatasubramanian, MRS Bull. 31 (2006) 211. [18] D. Hicks, M.S. Dresselhaus, Phys. Rev. B47 (1993) 12727. [19] S.L. Tan, Doctorial Thesis, Kunming University of Science and Technology, 2006, p. 39 (in Chinese). [20] S. Soltan, J. Albrecht, H.-U. Habermeier, Phys. Rev. B70 (2004) 144517. [21] D. Dijkamp, T. Venkatesan, X.D. Wu, Appl. Phys. Lett. 51 (1987) 619. [22] T. Haage, J. Zegenhagen, H.-U. Habermeier, M. Cardona, Phys. Rev. Lett. 80 (1998) 4225. [23] P.X. Zhang, U. Sticher, B. Leibold, H.-U. Habermeier, Physica C 282–287 (1997) 2551. [24] S. Bhattacharya, S. Pal, R.K. Mukherjee, B.K. Chaudhuri, S. Neeleshwar, J. Magn. Magn. Mater. 269 (2004) 359. [25] S. Roy, Y.Q. Guo, S. Venkatesh, N.J. Ali, Phys. Condens. Matter 13 (2001) 9547. [26] M. Battabyal, T.K. Dey, J. Phys: Condens. Matter 18 (2006) 493.
Journal of Crystal Growth 310 (2008) 2732–2737 www.elsevier.com/locate/jcrysgro
Improving the performance of thermoelectric devices by doping Ag in LaPbMnO3 thin films P.X. Zhanga,c, C. Wangb,, S.L. Tana, H. Zhanga,c, H.-U. Habermeierc a
Institute of Advanced Materials for Photoelcetrons, Kunming University of Science and Technology, Kunming 650051, China Research Institute of Engineering and Technology, Yunnan University, No.2 North Cuihu road, Kunming, Yunnan 650091, China c Max-Planck-Institut fu¨r Festko¨rperforschung, Stuttgart D-70569, Germany
b
Received 12 October 2007; accepted 23 January 2008 Communicated by D.P. Norton Available online 1 February 2008
Abstract A series of Ag-doped La0.6Pb0.4MnO3 thin films were grown on vicinal cut substrates by pulsed laser deposition (PLD). Laser-induced thermoelectric voltages (LITV) had been observed in these films, and these LITV signals had been demonstrated to originate from the anisotropic Seebeck effect. By doping Ag to an optimum value, it was found that the peak values (UP) of the LITV signals were maximized, and the full-width at half-maximum (t) of the response curves of LITV were minimized at the same time. The figure of merit (Fm) of the device used as photodetector is greatly improved by doping Ag in La0.6Pb0.4MnO3 thin films. The possible reason for these improvements had been well discussed. r 2008 Elsevier B.V. All rights reserved. PACS: 77.84.Dy; 81.20.Fw; 85.50.n Keywords: A1. Doping; A1. Radiation; A2. Pulsed laser deposition; B1. Manganites; B3. Thermoelectric devices
1. Introduction
formula based on the anisotropic thermoelectric model:
The experiment of laser-induced voltage (LIV) effect, in which the YBa2Cu3O7d (YBCO) thin film were used as a photosensor, was performed firstly by C.L. Chang et al. [1]. Later, it was proved that the LIV signals were induced by the thermoelectric anisotropy in the high Courier-temperature (TC) superconductor oxides. When the incident pulsed laser radiation has been absorbed by these films, a temperature difference generates between the top and the bottom thin layers of the film as the top surface of the films is heated instantly. This temperature gradient produces an electric voltage through the thermoelectric or Seebeck effect. Once the films are grown on vicinal cut substrates, the amplitude of the laser-induced thermoelectric voltages (LITV) signals becomes even larger. H. Lengfellner et al. [2] described the phenomenon quantitatively by deriving a
Ux ¼
Corresponding author. Tel.: +86 871 5031869; fax: +86 871 5037399.
E-mail address: [email protected] (C. Wang). 0022-0248/$ - see front matter r 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2008.01.044
l ðS ab S c Þ sinð2aÞDT 2d
(1)
where Ux is the induced voltage in the x-direction, l and d are the length and thickness of the films, Sab and Sc are the Seebeck tensor components in the ab plane and c direction, respectively, a is the tilting angle of the substrate, and DT is the temperature difference between the top and bottom parts of the film. This formula explained most of the experimental observations, especially the tilting angle dependence of the induced voltage, which has become the unique criteria for identifying the thermoelectric effect from other light-induced effect. Therefore, Lengfellner et al. subsequently named these materials as atomic layer thermopile (ALT) materials, and the signals as LITV. Since then several high-temperature superconductor (HTSC) oxides, such as doped YBCO, Bi2Sr2CaCu2O8, Tl2Ba2Ca Cu2O8, etc., have been well studied and the induced voltage signals in those materials have been demonstrated to be
ARTICLE IN PRESS P.X. Zhang et al. / Journal of Crystal Growth 310 (2008) 2732–2737
a similar LITV effect [3–7]. Based on these studies, light detection devices with good performance have been designed, which exhibit some superior characteristics over traditional bolometers and photon-counting devices. Most obvious advantages are that the device can function over a broad spectral range and at the same time has a fast time response [8–11]. Recently, Habermeier et al. observed a similar effect in La0.67Ca0.33MnO3 thin films grown on vicinal cut substrates [12,13]. This discovery indicates that the ALT property consists not only in the HTSC oxide material group, but also other oxide materials. On the other hand, lanthanum manganites are interesting for their colossal magnetoresistance (CMR) properties, and have attracted a lot of attention both from the viewpoint of basic physics as well as potential applications. The discovery also provided new insight for the understanding of the mechanisms of CMR, since it is related to transportation of carriers. The observation of LITV signals in these materials puts forward a number of questions from a basic physics point of view. Firstly, there should be very small anisotropy in these thin films due to their cubic (or quasicubic) nature in crystallography. Secondly, the measurements were performed at room temperature, where La0.67Ca0.33MnO3 is in the paramagnetic phase, so there should be no magnetic anisotropy as well. However, further studies revealed that there are a number of manganites and other perovskites which demonstrate the same effect, and the initial small LITV signals can be enhanced by ion substitution or changing the doping level as well as the lattice mismatch between the substrates and the films. Considering that for practical applications, the influence of the thin film materials on the device performance, especially the time response of the device, is rather important. We have also derived a detailed formula based on a plane heat source and thermal diffusion model [9,14]: UðtÞ ¼
2 2 a0 El sinð2aÞ pffiffiffiffiffiffiffiffi ðSab S c Þðed =4Dt ed =4Dt Þ 4drc0 pDt
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a fixed system. Based on the description in Eq. (2), it is obvious that the thermal diffusivity (D), which is proportional to thermal conductivity k, must be reduced to meet the expectation of increasing the peak value of a LITV voltage. However, this reduction will also increase the time constant, which can be defined as the full-width at halfmaximum (t) of the response curve of the time-dependent induced voltages. Considering the importance for device to exhibit as a fast time response and a high sensitivity, one can define a device figure of merit (Fm) for this type of photodetector devices: F m ¼ U P =t
(3)
where UP and t are the peak value of the induced voltage and the full-width at half-maximum of the response curve of the time-dependent LITV signals, respectively. Therefore, synthesizing and searching for more materials with high LITV sensitivity and at the same time the fast time response, namely higher Fm value of thermoelectric device, is the purpose of this work. In this work, we have prepared Ag-doped La0.6Pb0.4MnO3 (Ag-LPMO) thin films by means of pulsed laser deposition (PLD). The LITV signals have been observed in these films and well investigated. By optimizing the doping level of Ag, the performance and the Fm value of the LITV devices were improved substantially. 2. Experimental procedure Ag-LPMO ceramic disk, used as the targets in the PLD experiments, was synthesized by conventional solid reaction or coprecipitation method [19]. Fig. 1 shows the X-ray diffraction (XRD) pattern of these ceramics. The XRD curves demonstrated that the targets are of the perovskite structure and single phase. The calculated lattice parameters show that the c-axis lattice parameter was firstly
(2)
where U(t) is the time-dependent induced voltage. The a0, r, d, c, and D are the absorption coefficient, density of the film material, penetration depth of light, specific heat, and thermal diffusivity of the film material, respectively. The E denotes the energy of the incident-pulsed laser light. Using this formula, one can select a material and design devices to detect pulsed laser radiation with high sensitivity and fast time response at required wavelengths. These devices will function at room temperature, and without any applied bias. These advantages make such devices very promising, both for ultraviolet (UV) and infrared spectra. To improve the performance of the devices, however, it was found that the sensitivity and the time response are some times in contradiction. It is well-known that the thermoelectric material will exhibit high performance if the figure of merit (ZT) is high. Where Z=Ss/k, a smaller thermal conductivity k, larger conductivity s will lead to higher ZT value [15–18], while the Seebeck coefficient S is constant in
Fig. 1. The XRD pattern of different Ag-doped La0.6Pb0.4MnO3 ceramics used as targets for fabricating thin films by pulsed laser deposition.
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reduced and then increased again with increasing Agdoping level from 0.0 to 10 wt%. The substrates are SrTiO3 (STO) and LaAlO3 (LAO) single crystal cut with different tilting angles to the c-axes of the crystals. The PLD experiments were performed by using a KrF excimer-laser of 248 nm in wavelength (l) and 20 ns in pulse width. An optimized laser fluency of 1.6 J/cm2 and repeat of 5 Hz were utilized. For all the used Ag-LPMO thin films, the substrate temperature and the pressure of flowing oxygen were precisely kept, respectively, at 780 1C and 0.4 mbar during the deposition, respectively. The growing process was halted when each film thickness was up to 80 nm. Finally, all the samples were annealed in situ for 1 h at 530 1C in an oxygen pressure of 1.0 bar. More detailed growth method can be found in the literature [20,21]. Three groups of Ag-LPMO film samples were prepared. The first group of samples was deposited with the identical deposition conditions on the STO and LAO substrates with the same tilting angle of 151. In this way, a substrate with larger LITV signals was chosen for the next experiments. With the aim to check the tilting angle factor on the induced voltages, the second group of samples was prepared on the LAO substrates but with different tilting angles. The third group of samples was prepared with different Ag doping levels from 0 to 10 wt% on the LAO substrates cut with 151 off the c-axes. Fig. 2 shows a typical XRD pattern of deposited AgLPMO film on LAO (151) substrate. The films were grown
Fig. 3. Time-dependent LITV signals obtained from Ag–La0.6Pb0.4MnO3 films grown on LaAlO3 (151) and SrTiO3 (151) substrates, respectively. Inset shows the planform of the film samples used in the LITV experiments.
epitaxial like on the substrates with high quality. The asgrown films were fabricated to 1 3 mm2 stripes by using the normal photolithography technique. The Pt electrodes were fixed at both ends of the stripe, and the middle 1 1 mm2 region was used as the sensitive area for light detection. The planform of the samples used in the LITV experiments has been shown in the inset of Fig. 3. To obtain the LITV signals, a pulsed excimer laser with wavelength of 248 nm and the pulse duration of 20 ns has been used to irradiate the sensitive area of Ag-LPMO stripe. The LITV signals were detected by using an oscilloscope with input impedance of 1 MO. Each final LITV signal and the corresponding pulsed laser energy (Epulse) were taken from the average results of 1000 times measurement, and the average LITV signals and Epulse values were recorded synchronously by the oscilloscope and a laser energy-meter [11], respectively. The typical four-point configuration was performed to measure the resistivity and the conductivity of the AgLPMO thin films. In this measurement, a current resource is used to provide the invariable current of 1 mA.
3. Results and discussion
Fig. 2. The typical XRD patterns of single-crystal substrate LaAlO3 (a) and the Ag-doped La0.6Pb0.4MnO3 thin film (b) grown on it.
Three groups of experiments have been performed corresponding to the three groups of samples. The substrate’s effect on the LITV signals has been shown in Fig. 3. The same Ag-LPMO (Ag-4 wt%-doped) films have been grown on the STO (151) and LAO (151) substrates, respectively. In this optical experiment, the energy density of the incident laser was kept at 0.13 mJ/mm2. It is evident that the LITV signals from LAO are larger than that from STO. Similar result has also been observed in the Ag-LPMO films grown on the two tilting 101 STO and
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LAO substrates. Therefore, the LAO substrate was chosen for the later experiments. As that has been mentioned, the relationship between the induced voltage and substrate tilting angles is critical for judging whether the induced voltage should be attributed to the thermoelectric effect. We have performed a series of LITV experiments of Ag-LPMO films grown on the LAO substrates with different tilting angles from 51 to 201. All the Ag-LPMO (Ag-4 wt%-doped) films were prepared under the same conditions and LITV signals were measured at the incident energy density. It is clearly shown in Fig. 4 that the larger the tilting angles, the stronger the induced voltages, which support our expectation that the induced voltages are due to thermoelectric effect [9]. The third experiment was to check the influence of the Agdoping level on the performance of LITV devices when used as a light detector. Special attention is paid on two measured parameters: the peak value UP of LITV signals and the time response t of the LITV time curves. All the measurements were conducted at room temperature. The Ag-doping level influence on the performances has been shown in Fig. 5. As the observation in Fig. 3, the LITV signals of the films grown on LAO are larger than that on STO substrates. Since all the other experimental conditions are the same, one has to conclude that the lattice parameter of the substrates plays an important role in this case. The small lattice parameter in the ab plane of the LAO substrate (a ¼ b ¼ 0.379 nm) causes the grown Ag-LPMO films, for which the lattice parameter is larger (a ¼ b ¼ 0.389 nm) to be compressively strained. On the other hand, the lattice parameters of STO are a ¼ b ¼ 0.390 nm, so the grown films of Ag-LPMO should be tensilely strained in the ab plane, with small c-axis parameter. The transportation of electrons or hopping polarons will be easier in the compressed plane than in the enlarged c-direction. The distortion of a small Mn–O–Mn bond length in the ab
Fig. 4. Time-dependent LITV curves obtained from films grown on LaAlO3 substrates with their vicinal cut angles of 51, 101, 151, and 201, respectively.
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Fig. 5. Time dependent LITV signals obtained from different Ag-doped La0.6Pb0.4MnO3 thin films grown on LaAlO3 substrates with tilting angle of 151.
Table 1 The measured peak values UP of laser induced voltage, time response t, and the calculated figure of merit of the Ag(4 wt%)-LPMO/LAO thermoelectric devices with different tilting angles of LaAlO3 substrates Tilting angle (deg)
UP (mV)
t (ns)
Fm
5 10 15 20
17 38 85 120
108 145 178 203
0.16 0.26 0.48 0.59
plane in addition to a large c-axis lattice parameter leads to large anisotropy of the conductivity as well as the Seebeck components, hence large LITV signals, which we shall prove is due to the thermoelectric anisotropic effect. In order to test that the induced voltages are due to the anisotropic Seebeck effect, we prepared Ag-4 wt% doped LPMO thin films on 51, 101, 151, and 201 vicinal cut LAO substrates and measured the induced voltages with pulsed laser excitation from these different films. It is clear from Fig. 4 that larger the tilting angle, the higher the LITV signals. This UP -substrate tilting angle sin(2a) proportionality demonstrates that the induced voltages originate from the thermoelectric effect, since all the other light-induced effects do not show such tilting angle dependence. This conclusion is also in agreement with the results obtained from the first set of experiments, in which the signals from LAO are larger than that from films grown on STO due to stress-induced Seebeck coefficient anisotropy. With the increase in the tilting angle, another interesting observation from this experiment is that not only the peak voltage but also the time response t increases. This effect has not been observed before in YBCO or in other similar experiments. Table 1 lists the measured UP, t and the calculated Fm values at different tilting angles. Microscopically the surface of a vicinal cut substrate is composed
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of steps like a ladder. It is known that growth of highquality samples on vicinal cut substrates is difficult due to these steps. Haagen et al. have observed nanostructure stripes in YBCO films grown on vicinal cut STO by highresolution atomic force microscope (AFM), similar results in manganite thin films has also been reported [13]. These nanostructured stripes grow along the fixed crystal orientations, thereby introducing anisotropy, which is observed both in resistivity and in flux pinning [22,23]. It is conceivable that a similar nanostructure may also exist in the case of Ag-LPMO thin films. Films grown on substrates with higher tilting angles demonstrate stripe structures of smaller width and higher interface density. Therefore, the electric conductivity is decreased in the direction, in which the substrate was cut vicinal. At the same time, the increased interface causes a reduction of the thermal conductivity due to phonon scattering at the interfaces of these nanostructures. Small thermal conductivity enhances ZT of the film material and leads to a larger LITV signal, and a longer time response, according to Eq. (2). Although t is larger at higher tilting angles, the Fm value of thermoelectric device enhances about fourfold with the increase of tilted angle from 51 to 201, as shown in Table 1. Therefore, using the large tilting angle substrates is beneficial for device application. However, it would also be a challenge to grow high quality films on substrates with tilting angles 4201. This limitation means that some other way to improve the device performance is needed. The influence of various Ag-doping levels on the LITV signals has been shown in Fig. 5. Experiments demonstrated that the induced-voltage peak value UP increases at first with increasing Ag-doping level, reaching a maximum at 4–8 wt%, and then decreases. On the other hand, the time response t changes in opposite manner, reducing first and then increasing. According to Eq. (3), one can calculate the Fm value of each device. Table 2 lists the measured UP, t, and the calculated Fm value at different Ag-doping levels. It is found that the optimal Ag-doping for high Fm value is in the doping range of 4–6 wt%. It is also evident that Fm value can be enhanced several times by manipulating the doping level of Ag in LPMO thin films. There is much discussion on the enhancement of the ZT of thermoelectric materials, since this is the key parameter
Table 2 The measured conductivity s, peak values UP of LITV signals, time response t, and the calculated figure of merit of Ag-LPMO/LAO (151) thermoelectric device with different Ag doping level in the films Ag doping (wt%)
d (S/cm)
UP (mV)
t (ns)
Fm
0 2 4 6 8 10
313.8 567.7 1302.1 2125.9 901.9 749.4
25 35 43 38 48 20
140 106 80 65 100 126
0.18 0.33 0.53 0.58 0.48 0.16
for applications. Several new concepts to obtain materials with ZT higher than 1 have been reported [15,16]. The central issue on how to enhance ZT is to form materials with ‘‘phonon glass and electron crystal’’ behavior, namely with high electric conductivity and low thermal conductivity. Doping with monovalent ions, Ag, either in the lattice or at grain boundary-like interfaces always enhances electronic conductivity, while inhomogeneity due to the doping increases the centers of phonon scattering. The Ag-doped manganite formula can be written as La3þ 1x 1þ 3þ 4þ Pb2þ ðxyÞ Agy Mn1ðxþyÞ Mnxþy O3 for small amounts of substitution. With increasing Ag1+, more Mn4+ are formed, which results in a high density of carriers, hence a high electrical conductivity and an enhanced double exchange interaction. However, over doping of Ag1+ leads to a reduction of Mn3+–O–Mn4+ pairs, which will suppress the double exchange and enhance the superexchange. Therefore, after reaching a maximum, the electrical conductivity will be reduced. This is in agreement with the experimental results: the electrical conductivity of the films increases with small Ag-doping, and after reaching a maximum reduces with further doping. The Seebeck coefficient in polycrystalline La0.7Sr0.3x AgxMnO3 pellets have been studied recently [24]. It was found that the Seebeck coefficient at room temperature becomes negative by doping with Ag, and further increasing doping leads to higher coefficients. In our case, the anisotropy of the Seebeck coefficient (SabSc) is essential, and it is reasonable to suppose that a larger Seebeck coefficient coexists with larger anisotropy, therefore with a larger LITV peak value. On the other hand, the doping of Ag causes a reduction of thermal conductivity due to phonon scattering either at grain boundary-like interfaces or nano-scale inhomogeneity. It has been argued that in a magnetic manganite, the thermal conductivity is composed of three parts: phonons, electrons and magnons (or spin waves). At room temperature, the contribution of thermal conductivity from electrons (and/or holes) is negligible, while that from magnons becomes important below or near the ferromagnetic transition TC (or TN). However, the estimated upper limit of magnon contribution is about 10% [25,26]. Therefore, the dominant part of the thermal conductivity is from the contribution of phonons. Doping with Ag into the lattice or at the grain boundary introduced scattering centers for phonons, and leads to a reduction in thermal conductivity. Therefore, it can be concluded that the effects of Ag doping, enhanced electric conductivity, and the reduction of thermal conductivity, are all favorable to improve the performance of thermoelectric devices. 4. Conclusion The laser-induced thermoelectric voltages have been observed in Ag-doped La0.6Pb0.4MnO3 thin films grown on vicinal cut substrates. It has been confirmed that this is a thermoelectric effect, due to the anisotropic Seebeck tensor
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in the materials. By optimizing the doping level of Ag (in the range of 4–6 wt%), the peak value UP and the time response t of the time-dependent induced voltage curves reach a maximum and minimum, respectively. Therefore, the figure of merit of the device has been enhanced several times. This is due to an increase in electrical conductivity and Seebeck coefficients, while a reduction in thermal conductivity at the same time. Doping with Ag has been demonstrated to be an effective way of improving the performance of atomic layer thermopile devices, and may also be used for increasing ZT of other thermoelectric materials. Acknowledgments The authors thank the people in Max-Planck Institute (FKF) Stuttgart for supporting the chance of the study and intercommunion. This work was financially supported by the National Science Foundation of China (G10274026) and the Science Foundation of Yunnan province (1999E0003Z). One of the authors, C. Wang, would like to thank the support from the Cultivated Foundation for ‘‘Academic Cadreman’’ of Yunnan University. References [1] C.L. Chang, A. Kleinhammes, W.G. Moulton, L.R. Testardi, Phys. Rev. B 41 (1990) 11564. [2] H. Lengfellner, G. Kremb, A. Schnellbo¨gl, J. Betz, K.F. Renk, W. Prettl, Appl. Phys. Lett. 60 (1992) 501. [3] H.S. Kwok, J.P. Zheng, Q.Y. Ying, R. Rao, Appl. Phys. Lett. 54 (1989) 2473. [4] T. Zahner, R. Schreiner, R. Stierstorfer, O. Kus, S.T. Li, R. Roessler, J.D. Pedarnig, D. Bauerle, H. Lengfellner, Europhys. Lett. 40 (1997) 673.
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