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Different magnetothermoelectric behavior in Al- and Ga-doped ZnO thin films
Journal of Alloys and Compounds 588 (2014) 370–373
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Different magnetothermoelectric behavior in Al- and Ga-doped ZnO thin films Han Liu, Liang Fang ⇑, Dexiang Tian, Fang Wu, Wanjun Li Department of Applied Physics, Chongqing University, Chongqing 401331, China
a r t i c l e
i n f o
Article history: Received 24 August 2013 Received in revised form 6 November 2013 Accepted 11 November 2013 Available online 21 November 2013 Keywords: Seebeck coefficients Magnetothermoelectric properties ZnO thin films RF magnetron sputtering
a b s t r a c t This essay mainly focuses on the influence of magnetic filed on thermoelectric properties of Al-doped ZnO (AZO) and Ga-doped ZnO (GZO) thin films. The Seebeck coefficients (S) of AZO and GZO films show opposite behavior as a function of the magnetic field intensity (B), which should be attributed to the effect of magnetic field on the effective electron number and potential barrier at grain boundary. For GZO film, with higher electron number, the influence of magnetic field on the effective electron number is the dominant factor, while more potential barriers at grain boundaries exist in AZO film due to the Al–O bond length with a higher mismatch to Zn–O bond length than the Ga–O bond length, so the effect of magnetic field on the potential barriers at grain boundaries dominates in AZO film. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Thermoelectric materials have attracted a great deal of interest for their potential applications in energy conversion and solidstate refrigerating. In typical energy conversion process, nearly 50% of the energy is wasted mostly as heat [1–3]. The waste heat can be directly converted into electric power by the thermoelectric generation and its efficiency is mainly governed by the properties of thermoelectric materials. In such processes, it is common to use temperatures above 500 °C, the poor chemical stability and the brittle nature of conventional materials such as Bi2Te3, PbTe, SiGe have limited these materials to use at low temperatures. Oxides like ZnO are considered as high-temperature thermoelectric materials because of its high thermal stability and excellent oxidation resistance. Despite Al-doped ZnO and Ga-doped ZnO have been known as remarkable thermoelectric materials [4–7], the charge transport mechanism in ZnO has not yet been well understood. Because the thermoelectric phenomenon is governed by the transport mechanisms of electrons, understanding the charge transport mechanism is of great importance for further improvement of ZnO thermoelectric materials [2]. An external magnetic field can influence electrons’ transport, which could be used as an effective way to study on the charge transport mechanism, furthermore on the thermoelectric properties. In this work, we have performed Seebeck coefficient measurement under external magnetic field for both AZO and GZO films. ⇑ Corresponding author. Tel.: +86 23 65678369. E-mail address: [email protected] (L. Fang). 0925-8388/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2013.11.060
It’s observed the dependence of Seebeck coefficients of AZO and GZO films on the magnetic field intensity are opposite. Taking use of the result of Hall measurement and theory about bond length, we discuss the opposite trends and give a reasonable explanation for it from viewpoint of electrons transport mechanism. 2. Experimental details The AZO and GZO thin films were deposited on glass substrates by RF magnetron sputtering (JGP-450). The sputter targets were prepared using the mixed powder of high purity ZnO (99.99%) and X2O3 (99.99%) (X = Al, Ga) with prescribed molar ratio (X/(Zn + X) = 0.03). The sputtering chamber was first evacuated to a base pressure below 8.0 10 4 Pa with a turbo molecular pump, and then filled with Ar (99.99%) up to 2.0 Pa. The sputtering power and time were 100 W and 60 min respectively. The distance between the target and substrate was 70 mm. The crystal structure of the films was analyzed by X-ray diffraction (XRD, MRD) with a MRD-Single Scan diffractometer with Cu Ka radiation (k = 0.154 nm). The surface morphologies of samples were studied by scanning electron microscopy (FEG-SEM Nova 400). The compositions and chemical bond configuration of thin films were examined by X-ray photoelectron spectroscopy (XPS). The electrical properties were measured by Hall measurement system (HMS-3000.Ver 3.5). Seebeck coefficients and Nernst voltage were measured at room temperature (RT) by the thermoelectric measurement system. The external magnetic fields (range from 0 T to 1.1 T) were perpendicularly applied to the films.
3. Results and discussion The XRD pattern in Fig. 1(a) show that AZO and GZO films have a sole detectable diffraction peak, which is nearly consistent with that of the (0 0 2) of standard ZnO crystal plane. It indicates that the films were grown with c-axis orientation perpendicular to the substrate and the doping elements of Al and Ga do not change the wurtzite structure of ZnO. The average grain sizes for AZO and
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Fig. 1. (a) XRD pattern of AZO and GZO films. (b) SEM image of AZO film. (c) SEM image of GZO film.
GZO thin films calculated with the Scherrer formula are 22.7 and 23.4 nm, respectively. The surface morphologies of AZO and GZO films are depicted in Fig. 1(b) and (c). It is found that most of the particles display grain-like morphologies in both films. The GZO film exhibits a denser microstructure and smoother surface compared with AZO film. It can be known the GZO film with a better crystallinity combined with the result of peak intensity in XRD pattern. The XPS spectra were measured for studying the composition and chemical bond configuration of AZO and GZO thin films. The binding energies are calibrated by taking the carbon C1s peak (284.6 eV) as reference in XPS spectroscopy. Fig. 2 shows the X-
(a)
ray photoelectron spectrum of AZO and GZO films. Fig. 2(a) displays the binding energy value of Al2p locating 74.00 eV ± 0.10 eV, which is a characteristic of Al2O3 [8]. From narrow scan XPS spectrum shown in Fig. 2(b), the positions of Ga 2p3/2 and Ga 2p1/2 peaks were found to be at 1144.9 eV ± 0.10 eV and 1117.7 eV ± 0.10 eV, respectively. These peak positions are in good agreement with the previously reported values [9], which suggests that Ga exists as Ga3+ in the GZO film. It can be known that Al3+ and Ga3+ acting as effective donors come into the crystal lattice to substitute for zinc ions respectively. Electrical and thermoelectric properties of the AZO and GZO films are shown in Table 1. It indicates the electrical properties
(b)
Ga2P3/2
Intensity (a.u)
Intensity (a.u)
Al 2p3/2
60
65
70
75
Binding energy (eV)
80
Ga2p1/2
1110
1120
1130
1140
Binding energy (eV)
Fig. 2. XPS narrow scan spectra of (a) Al2p in AZO film and (b) Ga2p in GZO film.
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Table 1 Electrical and thermoelectric properties of AZO and GZO films. Sample (3at.%)
Concentration n (1019 cm 3)
Hall mobility l (cm2 v1 s1)
Resistivity q (103 X cm)
Seebeck coefficient S (l V/K)
Power factor PF (104 W m1 K)
AZO GZO
8.34 16.32
6.32 9.20
11.85 6.54
225.19 77.03
4.28 1.40
of both GZO and AZO films are improved significantly relative to pure ZnO film [10]. Furthermore, the electrical properties of GZO film are better than that of AZO film, especially for the carrier concentration. Firstly, because the element of Al is easy to form aluminum oxide segregation on the surface of sputtering target leading to a decrease in the number of Al atom [11]. Secondly, the average grain size in AZO film is smaller, however the Hall measurement can detect only in-grain carrier concentration, trapped electrons at grain boundary interfaces can be the other reason for the lower carrier concentration. It is observed from Table 1. that the Seebeck coefficients of AZO and GZO films are negative, indicating that the major carriers are electrons. Furthermore, the GZO film with smaller absolute value of Seebeck coefficient (|S|) than that of AZO film is thought to be due to the fact that a higher carrier concentration could cause a reduction in Seebeck coefficient according to Eq. (1) [12]:
S¼
k Nc ln þ A e n
ð1Þ
where S is the Seebeck coefficient, e, k and n are electron charge, Boltzmann constant and the carrier concentration, Nc and A are the density of states and transport constant. The power factor S2/ q given in Table 1. represents the electrical contribution to the overall thermoelectric performance. Compared with GZO film, the higher power factor of AZO is mainly due to the significantly high value of |S|. Electrons transport in the presence of temperature gradient and magnetic field is vividly described in Fig. 3(a). The Seebeck coefficient (S) and Nernst voltage of AZO and GZO films versus magnetic field intensity (B) at RT are shown in Fig. 3(b). It is observed that as magnetic field intensity increases, the |S| value of AZO film decreases whereas the |S| of GZO film increases. We propose that this is mainly attributed to the influence of magnetic field on effective electrons number and grain boundary barrier height [2,13,14]. On one hand, in the presence of external magnetic field (z-axis), electrons transporting along the temperature gradient (x-axis) will deflect to y-axis by Lorentz force, so the number of electrons transporting along x-axis decreases to some extent. According to
the formula (1), it can be known that the decrease of the effective electron number in the x-axis leads to the increase of |S|. On the other hand, the potential barrier height is proportional to the electrons number [15], so with the increase of deflection electrons accumulating at grain boundaries in the y-axis, the potential barrier height in this direction increases, while the potential barrier in the x-axis decreases [12,16]. According to energy filtering effect [15], the decrease of potential barrier makes the electrons with lower energy could pass through potential barriers, leading to the decrease of the average energy of transport electrons. Because the |S| is proportional to the average energy of electrons, the |S| decreases [17]. Based on what we have just discussed in the above paragraph, it can be known that for GZO film, with a higher carrier concentration, the effect of grain boundary potential barriers on carrier is smaller [17], so the increase of |S| may be mainly ascribed to the decrease of effective electrons number caused by the application of external magnetic field. In contrast, more potential barriers exist in AZO film than in GZO film due to that the Al–O bond length (0.27 nm) is more dissimilar to the Zn–O bond length (0.197 nm) than the Ga–O bond length (0.191 nm). In order to further certify more potential barrier existing in AZO than GZO, the Nernst voltage (in y-axis) were measured and depicted in the inset in Fig. 3(b), because it is sensitive to check scattering event [18]. Potential barriers have scattering process to electrons in which potential barriers have a larger obstacle to the ‘‘cold’’ electrons (move from cold side to hot side along x-axis) than ‘‘hot’’ electrons (move from the opposite direction) [18–20], so a large deviation of electron number between two sides along y-axis are generated, leading to an ascent of the Nernst voltage due to the increase of the number of net charge [19]. We deduce that potential barrier could promote the Nernst voltage. The inset in Fig. 3(b) shows that the Nernst voltage of AZO increase rapidly with increasing magnetic field intensity, while that of GZO film rise only sluggishly. According to what we discussed above, this phenomenon is mainly attributed to more potential barriers existing in AZO than GZO. The same conclusion was demonstrated by means of UV–PL emission by Wiff et al. [21]. Therefore, the change of the potential barrier
Fig. 3. (a) The schematic diagram of Seebeck effect and Nernst effect. (b) the Seebeck coefficients and Nernst voltage as a function of magnetic field intensity.
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height dominates in the AZO film and the decrease of |S| of AZO film under magnetic field should be mainly attributed to it. In conclusion, the effect of magnetic field on grain boundary potential barrier and on the effective electrons number dominates in AZO film and GZO film, respectively.
and the Natural Science Foundation of Chongqing (Grant Nos. CSTC 2013jjB0023 and cstc2012gg-gjhz50001) the Fundamental Research Funds for the Central Universities (No. DJZR12138801) and the Sharing Fund of Large-scale Equipment of Chongqing University (Grant Nos. 2012121556, 2012121559 and 2012121560).
4. Conclusions
References
The structural, electrical, thermoelectric and megetothermoelectric properties of AZO and GZO films deposited by RF magnetron sputtering have been investigated. It’s found that both AZO and GZO films are polycrystalline and preferentially oriented in the c-axis. Despite the poor electrical properties, the AZO films have higher absolute values of S than that of GZO film. Furthermore, it’s found that the absolute value of Seebeck coefficients of GZO film show a positive dependence on the magnetic field intensity, while the AZO film shows the opposite behavior. A reasonable explanation for abnormal phenomenon is given from viewpoint of electrons transport mechanism. Because the influence of magnetic field on the effective electron number and on the potential barrier at the grain boundary plays a dominant role in GZO film and AZO film respectively, leading to different electron transport behaviors, different thermoelectrical behaviors is observed in Al- and Ga-doped ZnO films.
[1] T. Tsubota, M. Ohtaki, K. Eguchi, J. Mater. Chem. 8 (1998) 409–412. [2] W.H. Nam, Y.S. Lim, S.M. Choi, W.S. Seo, J.Y. Lee, J. Mater. Chem. 22 (2012) 14633–14638. [3] K. Park, K.K. Kim, J.K. Seong, S.J. Kim, Mater. Lett. 61 (2007) 4759–4762. [4] F. Wu, L. Fang L, Y.J. Pan, Appl. Surf. Sci. 255 (2009) 8855–8859. [5] L. Shi, J. Chen, G. Zhang, B.W. Li, Phys. Lett. A 376 (2012) 978–981. [6] S. Teehan, H. Efstathiadis, P. Haldar, J. Alloy. Comp. 539 (2012) 129–136. [7] K.H. Jung, K.H. Lee, W.S. Seo, Appl. Phys. Lett. 100 (2012) 253902. [8] M. Chen, X. Wang, Y.H. Yu, Z.L. Pei, X.D. Bai, C. Sun, R.F. Huang, L.S. Wen, Appl. Surf. Sci. 158 (2000) 134–140. [9] V. Bhosle, A. Tiwari, J. Narayan, J. Appl. Phys. 100 (2006) 033713. [10] G.Z. Xing, B. Yao, C.X. Cong, T. Yang, Y.P. Xie, B.H. Li, D.Z. Shen, J. Alloys Comp. 457 (2008) 36–41. [11] V. Assuncao, E. Fortunato, A. Marques, H. Aguas, I. Ferreira, M.E.V. Costa, R. Martins, Thin Solid Films 427 (2003) 401–405. [12] Y. Kinemuchi, C. Ito, H. Kaga, T. Aoki, J. Mater. Res. 22 (2007) 1942–1946. [13] S. Bandyopadhyay, G.K. Paul, R. Roy, Mater. Chem. Phys. 74 (2002) 83–91. [14] X.l. Tian, Z.C. Pan, H.C. Zhang, et al., Mater. Lett. 97 (2013) 1–73. [15] K. Kishimoto, T. Masayoshi, K. Tsukamoto, J. Appl. Phys. 92 (2002) 5331– 5339. [16] J.P. Wiff, Y. Kinemuchi, K. Watari, J. Eur. Ceram. Soc. 29 (2009) 1413–1418. [17] J. Martin, L. Wang, L.D. Chen, G.S. Nolas, Phys. Rev. B 79 (2009) 115311. [18] D.L. Young, T.J. Coutts, V.I. Kaydanov, A.S. Gilmore, W.P. Mulligan, J. Vac. Sci. Technol. A 18 (2000) 2978–2985. [19] R.P. Huebener, F. Kober, H.-C. Ri, Phys. C 181 (1991) 345–354. [20] D.L. Young, T.J. Coutts, Rev. Sci. Instrum. 71 (2000) 462–466. [21] J.P. Wiff, Y. Kinemuchi, K. Watari, Mater. Lett. 63 (2009) 2470–2472.
Acknowledgements The research was financially supported by the National Natural Science Foundation of China (Grant Nos. 11074314 and 11304405)
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Different magnetothermoelectric behavior in Al- and Ga-doped ZnO thin films Han Liu, Liang Fang ⇑, Dexiang Tian, Fang Wu, Wanjun Li Department of Applied Physics, Chongqing University, Chongqing 401331, China
a r t i c l e
i n f o
Article history: Received 24 August 2013 Received in revised form 6 November 2013 Accepted 11 November 2013 Available online 21 November 2013 Keywords: Seebeck coefficients Magnetothermoelectric properties ZnO thin films RF magnetron sputtering
a b s t r a c t This essay mainly focuses on the influence of magnetic filed on thermoelectric properties of Al-doped ZnO (AZO) and Ga-doped ZnO (GZO) thin films. The Seebeck coefficients (S) of AZO and GZO films show opposite behavior as a function of the magnetic field intensity (B), which should be attributed to the effect of magnetic field on the effective electron number and potential barrier at grain boundary. For GZO film, with higher electron number, the influence of magnetic field on the effective electron number is the dominant factor, while more potential barriers at grain boundaries exist in AZO film due to the Al–O bond length with a higher mismatch to Zn–O bond length than the Ga–O bond length, so the effect of magnetic field on the potential barriers at grain boundaries dominates in AZO film. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Thermoelectric materials have attracted a great deal of interest for their potential applications in energy conversion and solidstate refrigerating. In typical energy conversion process, nearly 50% of the energy is wasted mostly as heat [1–3]. The waste heat can be directly converted into electric power by the thermoelectric generation and its efficiency is mainly governed by the properties of thermoelectric materials. In such processes, it is common to use temperatures above 500 °C, the poor chemical stability and the brittle nature of conventional materials such as Bi2Te3, PbTe, SiGe have limited these materials to use at low temperatures. Oxides like ZnO are considered as high-temperature thermoelectric materials because of its high thermal stability and excellent oxidation resistance. Despite Al-doped ZnO and Ga-doped ZnO have been known as remarkable thermoelectric materials [4–7], the charge transport mechanism in ZnO has not yet been well understood. Because the thermoelectric phenomenon is governed by the transport mechanisms of electrons, understanding the charge transport mechanism is of great importance for further improvement of ZnO thermoelectric materials [2]. An external magnetic field can influence electrons’ transport, which could be used as an effective way to study on the charge transport mechanism, furthermore on the thermoelectric properties. In this work, we have performed Seebeck coefficient measurement under external magnetic field for both AZO and GZO films. ⇑ Corresponding author. Tel.: +86 23 65678369. E-mail address: [email protected] (L. Fang). 0925-8388/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2013.11.060
It’s observed the dependence of Seebeck coefficients of AZO and GZO films on the magnetic field intensity are opposite. Taking use of the result of Hall measurement and theory about bond length, we discuss the opposite trends and give a reasonable explanation for it from viewpoint of electrons transport mechanism. 2. Experimental details The AZO and GZO thin films were deposited on glass substrates by RF magnetron sputtering (JGP-450). The sputter targets were prepared using the mixed powder of high purity ZnO (99.99%) and X2O3 (99.99%) (X = Al, Ga) with prescribed molar ratio (X/(Zn + X) = 0.03). The sputtering chamber was first evacuated to a base pressure below 8.0 10 4 Pa with a turbo molecular pump, and then filled with Ar (99.99%) up to 2.0 Pa. The sputtering power and time were 100 W and 60 min respectively. The distance between the target and substrate was 70 mm. The crystal structure of the films was analyzed by X-ray diffraction (XRD, MRD) with a MRD-Single Scan diffractometer with Cu Ka radiation (k = 0.154 nm). The surface morphologies of samples were studied by scanning electron microscopy (FEG-SEM Nova 400). The compositions and chemical bond configuration of thin films were examined by X-ray photoelectron spectroscopy (XPS). The electrical properties were measured by Hall measurement system (HMS-3000.Ver 3.5). Seebeck coefficients and Nernst voltage were measured at room temperature (RT) by the thermoelectric measurement system. The external magnetic fields (range from 0 T to 1.1 T) were perpendicularly applied to the films.
3. Results and discussion The XRD pattern in Fig. 1(a) show that AZO and GZO films have a sole detectable diffraction peak, which is nearly consistent with that of the (0 0 2) of standard ZnO crystal plane. It indicates that the films were grown with c-axis orientation perpendicular to the substrate and the doping elements of Al and Ga do not change the wurtzite structure of ZnO. The average grain sizes for AZO and
371
H. Liu et al. / Journal of Alloys and Compounds 588 (2014) 370–373
Fig. 1. (a) XRD pattern of AZO and GZO films. (b) SEM image of AZO film. (c) SEM image of GZO film.
GZO thin films calculated with the Scherrer formula are 22.7 and 23.4 nm, respectively. The surface morphologies of AZO and GZO films are depicted in Fig. 1(b) and (c). It is found that most of the particles display grain-like morphologies in both films. The GZO film exhibits a denser microstructure and smoother surface compared with AZO film. It can be known the GZO film with a better crystallinity combined with the result of peak intensity in XRD pattern. The XPS spectra were measured for studying the composition and chemical bond configuration of AZO and GZO thin films. The binding energies are calibrated by taking the carbon C1s peak (284.6 eV) as reference in XPS spectroscopy. Fig. 2 shows the X-
(a)
ray photoelectron spectrum of AZO and GZO films. Fig. 2(a) displays the binding energy value of Al2p locating 74.00 eV ± 0.10 eV, which is a characteristic of Al2O3 [8]. From narrow scan XPS spectrum shown in Fig. 2(b), the positions of Ga 2p3/2 and Ga 2p1/2 peaks were found to be at 1144.9 eV ± 0.10 eV and 1117.7 eV ± 0.10 eV, respectively. These peak positions are in good agreement with the previously reported values [9], which suggests that Ga exists as Ga3+ in the GZO film. It can be known that Al3+ and Ga3+ acting as effective donors come into the crystal lattice to substitute for zinc ions respectively. Electrical and thermoelectric properties of the AZO and GZO films are shown in Table 1. It indicates the electrical properties
(b)
Ga2P3/2
Intensity (a.u)
Intensity (a.u)
Al 2p3/2
60
65
70
75
Binding energy (eV)
80
Ga2p1/2
1110
1120
1130
1140
Binding energy (eV)
Fig. 2. XPS narrow scan spectra of (a) Al2p in AZO film and (b) Ga2p in GZO film.
1150
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H. Liu et al. / Journal of Alloys and Compounds 588 (2014) 370–373
Table 1 Electrical and thermoelectric properties of AZO and GZO films. Sample (3at.%)
Concentration n (1019 cm 3)
Hall mobility l (cm2 v1 s1)
Resistivity q (103 X cm)
Seebeck coefficient S (l V/K)
Power factor PF (104 W m1 K)
AZO GZO
8.34 16.32
6.32 9.20
11.85 6.54
225.19 77.03
4.28 1.40
of both GZO and AZO films are improved significantly relative to pure ZnO film [10]. Furthermore, the electrical properties of GZO film are better than that of AZO film, especially for the carrier concentration. Firstly, because the element of Al is easy to form aluminum oxide segregation on the surface of sputtering target leading to a decrease in the number of Al atom [11]. Secondly, the average grain size in AZO film is smaller, however the Hall measurement can detect only in-grain carrier concentration, trapped electrons at grain boundary interfaces can be the other reason for the lower carrier concentration. It is observed from Table 1. that the Seebeck coefficients of AZO and GZO films are negative, indicating that the major carriers are electrons. Furthermore, the GZO film with smaller absolute value of Seebeck coefficient (|S|) than that of AZO film is thought to be due to the fact that a higher carrier concentration could cause a reduction in Seebeck coefficient according to Eq. (1) [12]:
S¼
k Nc ln þ A e n
ð1Þ
where S is the Seebeck coefficient, e, k and n are electron charge, Boltzmann constant and the carrier concentration, Nc and A are the density of states and transport constant. The power factor S2/ q given in Table 1. represents the electrical contribution to the overall thermoelectric performance. Compared with GZO film, the higher power factor of AZO is mainly due to the significantly high value of |S|. Electrons transport in the presence of temperature gradient and magnetic field is vividly described in Fig. 3(a). The Seebeck coefficient (S) and Nernst voltage of AZO and GZO films versus magnetic field intensity (B) at RT are shown in Fig. 3(b). It is observed that as magnetic field intensity increases, the |S| value of AZO film decreases whereas the |S| of GZO film increases. We propose that this is mainly attributed to the influence of magnetic field on effective electrons number and grain boundary barrier height [2,13,14]. On one hand, in the presence of external magnetic field (z-axis), electrons transporting along the temperature gradient (x-axis) will deflect to y-axis by Lorentz force, so the number of electrons transporting along x-axis decreases to some extent. According to
the formula (1), it can be known that the decrease of the effective electron number in the x-axis leads to the increase of |S|. On the other hand, the potential barrier height is proportional to the electrons number [15], so with the increase of deflection electrons accumulating at grain boundaries in the y-axis, the potential barrier height in this direction increases, while the potential barrier in the x-axis decreases [12,16]. According to energy filtering effect [15], the decrease of potential barrier makes the electrons with lower energy could pass through potential barriers, leading to the decrease of the average energy of transport electrons. Because the |S| is proportional to the average energy of electrons, the |S| decreases [17]. Based on what we have just discussed in the above paragraph, it can be known that for GZO film, with a higher carrier concentration, the effect of grain boundary potential barriers on carrier is smaller [17], so the increase of |S| may be mainly ascribed to the decrease of effective electrons number caused by the application of external magnetic field. In contrast, more potential barriers exist in AZO film than in GZO film due to that the Al–O bond length (0.27 nm) is more dissimilar to the Zn–O bond length (0.197 nm) than the Ga–O bond length (0.191 nm). In order to further certify more potential barrier existing in AZO than GZO, the Nernst voltage (in y-axis) were measured and depicted in the inset in Fig. 3(b), because it is sensitive to check scattering event [18]. Potential barriers have scattering process to electrons in which potential barriers have a larger obstacle to the ‘‘cold’’ electrons (move from cold side to hot side along x-axis) than ‘‘hot’’ electrons (move from the opposite direction) [18–20], so a large deviation of electron number between two sides along y-axis are generated, leading to an ascent of the Nernst voltage due to the increase of the number of net charge [19]. We deduce that potential barrier could promote the Nernst voltage. The inset in Fig. 3(b) shows that the Nernst voltage of AZO increase rapidly with increasing magnetic field intensity, while that of GZO film rise only sluggishly. According to what we discussed above, this phenomenon is mainly attributed to more potential barriers existing in AZO than GZO. The same conclusion was demonstrated by means of UV–PL emission by Wiff et al. [21]. Therefore, the change of the potential barrier
Fig. 3. (a) The schematic diagram of Seebeck effect and Nernst effect. (b) the Seebeck coefficients and Nernst voltage as a function of magnetic field intensity.
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height dominates in the AZO film and the decrease of |S| of AZO film under magnetic field should be mainly attributed to it. In conclusion, the effect of magnetic field on grain boundary potential barrier and on the effective electrons number dominates in AZO film and GZO film, respectively.
and the Natural Science Foundation of Chongqing (Grant Nos. CSTC 2013jjB0023 and cstc2012gg-gjhz50001) the Fundamental Research Funds for the Central Universities (No. DJZR12138801) and the Sharing Fund of Large-scale Equipment of Chongqing University (Grant Nos. 2012121556, 2012121559 and 2012121560).
4. Conclusions
References
The structural, electrical, thermoelectric and megetothermoelectric properties of AZO and GZO films deposited by RF magnetron sputtering have been investigated. It’s found that both AZO and GZO films are polycrystalline and preferentially oriented in the c-axis. Despite the poor electrical properties, the AZO films have higher absolute values of S than that of GZO film. Furthermore, it’s found that the absolute value of Seebeck coefficients of GZO film show a positive dependence on the magnetic field intensity, while the AZO film shows the opposite behavior. A reasonable explanation for abnormal phenomenon is given from viewpoint of electrons transport mechanism. Because the influence of magnetic field on the effective electron number and on the potential barrier at the grain boundary plays a dominant role in GZO film and AZO film respectively, leading to different electron transport behaviors, different thermoelectrical behaviors is observed in Al- and Ga-doped ZnO films.
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Acknowledgements The research was financially supported by the National Natural Science Foundation of China (Grant Nos. 11074314 and 11304405)