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Enhancement of thermoelectric properties in nanocrystalline M–Si thin film composites (M = Cr, Mn)
Journal of Alloys and Compounds 557 (2013) 239–243
Contents lists available at SciVerse ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Enhancement of thermoelectric properties in nanocrystalline M–Si thin film composites (M = Cr, Mn) S.V. Novikov a,⇑, A.T. Burkov a, J. Schumann b a b
A.F. Ioffe Physical-Technical Institute, Sankt-Petersburg 194021, Russia Leibniz Institute for Solid State and Materials Research, Dresden, Germany
a r t i c l e
i n f o
Article history: Received 7 November 2012 Received in revised form 17 December 2012 Accepted 18 December 2012 Available online 27 December 2012 Keywords: Nanocrystalline composite Thermoelectric properties Electric transport
a b s t r a c t We study thermoelectric properties, crystallization kinetics and stability of amorphous and nanocrystalline state in Cr–Si and Mn–Si composite films. The as-deposited amorphous films are transformed by annealing with in situ thermopower and electrical resistivity measurements into nanocrystalline state with continuous monitoring their state. Depending on the initial film composition, the films are transformed during the annealing into single phase or multi-phase nanocrystalline composites. A clear correlation between structural state and thermoelectric properties of the composites is found. Particularly, the thermoelectric power factor of nanocrystalline films is enhanced in comparison to the power factor of polycrystalline films, obtained by high-temperature re-crystallization of the nanocrystalline films, due to strong energy dependent charge carrier scattering. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Thermoelectric efficiency of a material is characterized by 2 dimensionless figure-of-merit: ZT ¼ Sjr, where r is the electrical conductivity, S is the thermoelectric power, and j is the thermal conductivity. These material parameters are dependent on electronic structure and on micro-structure of a material. One common route, which has been exploited to improve the thermoelectric efficiency, is a reduction of lattice thermal conductivity by preparing of finely grained composites. However, in conventional micro-crystalline substances with grain size in micrometer range a decrease of thermal conductivity due to scattering of phonons on grain interfaces is compensated by the decrease of electrical conductivity due to scattering of the charge carriers on the same interfaces, whereas thermopower is hardly affected by this scattering and remains almost the same as in coarse-grained materials. Nanocrystalline (NC) composites provide new opportunities to extend our control of the structure–property correlations in solids [1]. NC materials are polycrystalline substances with grain sizes of up to about 100 nm [2]. The NC state is structurally characterized by ultra-fine grains, and a large volume fraction of associated interfaces. Many properties of NC materials are significantly different from, and frequently superior to, those of their coarse-grained counterparts, and therefore have drawn much attention in recent years. Electronic transport properties of NC-conductors are strongly affected by interaction of conduction electrons with the inter-grain ⇑ Corresponding author. Tel.: +7 812 5159173; fax: +7 812 2971017. E-mail address: [email protected] (S.V. Novikov). 0925-8388/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2012.12.088
interfaces. In NC-conductors the interfaces represent an additional scattering channel for the conduction electrons, resulting in an enhanced resistivity (q = 1/r). An appropriate length scale for electronic transport properties is the mean free path of the charge carriers, l. In NC composites the mean free path of the charge carriers can be comparable or even larger than the grain size, d. When l P d, scattering on the interfaces will give main contribution to the electronic transport. Moreover, under this condition, a reflection of conduction electrons from the interfaces may lead to a partial confinement of charge carriers within nano-grains, thus, essentially modifying the electronic structure of the material. Furthermore, for the typical grain size of about 5–50 nm, the band structure parameters shall be depend on the grain size. Therefore, the crystalline grains in a NC semiconductor can have a variable band gap parameter, depending on their individual size. Hence, the NC-composite can be visualized as an array of hetero-junctions with randomly distributed band gaps. The scattering and confinement of the charge carries in NC grains should lead, of course, to a reduction of electrical conductivity. On the other hand, the transmission probability of the inter-grain interfaces may have a strong dependence on charge carrier energy. This will result in an enhancement of S. Therefore the product S2r S2/q – the power factor of a NC material, can be comparable or even larger than the power factor of corresponding usual polycrystalline sample. In the same time, due to high degree of structural disorder on the length-scale of about 10 nm, the lattice thermal conductivity of a material in NC form should be strongly suppressed in comparison with coarse-grained state. Indeed, experiments with NC materials have shown that thermal conductivity of the compounds with
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grain size of order 10–100 nm can be reduced by almost order of magnitude in comparison with polycrystalline samples [3–6]. Therefore the nanocrystallization is considered as a promising way to improve thermoelectric efficiency. However, there are many obstacles in a practical realization of NC state. Particularly, preparation of bulk NC samples often includes such procedures as grinding, pressing and sintering. Almost unavoidable contamination of NC interfaces from environment during these procedures modifies in an uncontrollable way properties of resulting NC composite. This makes correct interpretation of experimental studies of such composites difficult and, probably, it is one of the reasons for only a modest improvement of ZT which has been achieved in bulk NC samples [7]. We use for preparation of NC samples crystallization from amorphous state. In this method of nanocrystallization, the crystallites and their interfaces are formed via solid state reaction, therefore the NC samples are dense and the internal interfaces are clean. This removes additional factors affecting properties of NC composite and allows a straightforward interpretation of the corresponding experimental data. Transition metal silicides have been considered as promising materials for thermoelectric applications because they are environment friendly, stable at high temperatures, inexpensive, some of them have high thermoelectric efficiency. We found that silicides can be comparatively easy prepared in NC form by crystallization from amorphous state. In this study we investigate in a most unambiguous way the effect of nanocrystallization on electronic transport properties and on thermoelectric performance of transition metal silicides.
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2. Experimental procedures The composite films were produced by the crystallization from amorphous state in the course of annealing of the film with in situ transport measurements. The amorphous films were prepared by magnetron sputtering from composite targets onto unheated substrates. The film thickness was in the range 50–500 nm. Si wafers, thermally oxidized to form a 1 lm layer of SiO2, were used as substrates. The SiO2 layer provides electrical insulation of the film from Si substrate and prevents chemical reactions of the film with the substrate during the annealing procedures. The state of the film composite in the course of its transformation from amorphous to NC composite is controlled by means of the in situ transport measurements – simultaneous measurements of q and S. Standard, DC 4 point configuration was used in the resistivity measurements, while differential method was utilized for thermopower [8]. Transmission Electron Microscopy and X-ray Diffraction were used for structural characterization of the samples [9].
3. Results and discussion Fig. 1 presents temperature dependences of S and q of three Cr– Si film composites: Cr0.35Si0.65, Cr0.33Si0.67 (stoichiometric CrSi2), and Cr0.28Si0.72 with thickness of 180 nm, 100 nm and 52 nm, respectively. We have studied films of different thickness in the range 50–500 nm for every composition and found no essential dependence of the film properties on the thickness. The as-deposited films have amorphous structure. The films were annealed during one cycle with continuously increasing temperature from 300 K to 960 K, and with following cooling to room temperature. The beginning of the crystallization is marked by the sharp increase of S and q in these temperature dependences. The results reveal a general tendency in the dependency of S(T) and q(T), and of annealing behavior of the film composites on Si/Cr ratio. The amorphous state is stable up to crystallization temperature Tcr (Tcr 580 K for Cr–Si films). For a given Cr–Si composite Tcr weakly increases with increasing Si/Cr ratio. According to TEM and X-ray results for these composite films [10], the crystallization proceeds due to increasing number of nanocrystals with average size of about 10–20 nm. At annealing temperatures Tann < 1000 K we have not observed an increase of the average grain size with annealing time or with annealing temperature. In fact, structural investigations show that the mean grain size decreases in the
Fig. 1. Temperature dependences of thermopower (a) and of electrical resistivity of CrxSi1x composite films. Grey symbols – heating; black symbols – cooling.
course of annealing. This is due to the shift of grain size distribution to smaller grain size direction: in partially crystallized system there is no room for formation of large crystallites. The nucleation rate of the nanocrystals decreases with time at any annealing temperature Tcr < Tann < 1000 K. Therefore long-time annealing at high temperatures is needed to achieve maximum nanocrystallization. A sharp increase of both, S and q is observed on first heating at Tcr. However, immediately above Tcr only a small fraction of the amorphous matrix is transformed into NC state. Moreover, according to our earlier results [10], at least for off-stoichiometric Cr–Si composite films with Si excess in comparison with stoichiometric 1:2 ratio, the nanocrystalline CrSi2 phase does not contribute to the total film conductivity until it forms a percolating cluster. The existence of such percolation threshold is confirmed by decrease of resistivity, accompanied by increase of thermopower with annealing time at fixed annealing temperature, presented in Fig. 2. At Tann < 1000, only the volume fraction of NC phase (not size of nanocrystals) increases in the course of the annealing. Therefore the dependences of q and S on annealing time in Fig. 2 represent the dependence of these properties on volume fraction of NC phase. The maximum in dependence of q on annealing time signals the onset of percolation throughout NC cluster. The initial sharp increase of resistivity and thermopower at Tcr results from the scattering of charge carriers in the amorphous matrix on the nanocrystal interfaces. The increase of thermopower indicates that this scattering is strongly energy–dependent. It follows from Fig. 1 that the stoichiometric CrSi2 film has the highest power factor S2/q and, potentially, can be a bases for development of efficient NC composite. Fig. 3 shows S and q of a Mn0.31Si0.69 film. The film has thickness of 496 nm. Fig. 3 demonstrates the film transformation from amorphous to NC state during several annealing cycles, each annealing
S.V. Novikov et al. / Journal of Alloys and Compounds 557 (2013) 239–243
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cycle includes heating the sample from room temperature to annealing temperature Tann, annealing at Tann for 15 h and cooling down to room temperature. The amorphous state is stable up to crystallization temperature Tcr 600 K. At first annealing cycle we also observe a sharp increase of both, S and q. With further annealing, (cycles 2 and 3) at higher Tann, q increases by more than order of magnitude, while comparatively small changes of S are observed. At annealing cycles 4–6, q decreases, its temperature
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Fig. 3. Temperature dependences of thermopower (a) and electrical resistivity (b) of a Mn0.31Si0.69 thin film, shown on different annealing stages. The grey symbols denote data points, measured on heating, the black symbols – on cooling. The numbers indicate the annealing cycles. At the highest temperature Ti (i = 1, 2, 3, 4) of each annealing cycle the sample was annealed for about 15 h with subsequent cooling to room temperature (the cooling curve is shown here only for the last cycle).
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variation qualitatively changes, at the same time S increases considerably. The non-monotonous dependence of resistivity on annealing time (hence – on volume fraction of NC phase) is similar to the behavior observed in Cr–Si films. Our results indicate that such behavior is a common feature for M–Si (M = Cr, Mn, Re) film composites for a broad range of Si/M ratio, including stoichiometric composition [11]. Note, the stoichiometric compounds can be considered during nanocrystallization process as a binary composite of amorphous and NC phases where the ratio of amorphous to nanocrystalline phase changes during annealing. For a binary composite there is an exact result within Effective Medium Approximation (EMA): the thermoelectric parameters (Z, S and q) of a binary composite always lies between the corresponding values for pure components [12], which implies that the dependence of the parameters on the composition should be a monotonous function. Therefore, the non-monotonous dependence of resistivity on its phase composition indicates failure of EMA in case of NC composites. The temperature dependences of the electrical resistivity and thermopower of the composites, depending on the crystallization stage, are shaped by different mechanisms. In the amorphous films up to crystallization temperature Tcr, S(T) and q(T) dependencies are typical for amorphous conductors. The temperature dependencies in partially crystallized films, measured on heating, result from a combination of two mechanisms: first, a change of the volume fractions of crystallized and amorphous phases with increasing exposure to elevated temperatures, and secondly, the intrinsic temperature dependence of the properties of a composite. The intrinsic S(T) and q(T) for a composite with certain ratio between crystalline and amorphous phases can be obtained in measurement on cooling. The S(T) and q(T) dependences in completely crystallized films of the stoichiometric composition are qualitatively similar to those of bulk polycrystalline composites [13]. More detailed consideration of S(T) and q(T) dependences of the composites at different annealing stages will be given in a subsequent paper. In order to examine the effect of nanocrystallization on thermoelectric performance we compare properties of NC and polycrystalline states of the same sample film. Our investigation of structure of thin film composites on different annealing stages yield information about range of stability of NC state. For Cr–Si film composites the NC state is stable below 1000 K: when Tann was below 1000 K we did not observe an increase of the mean grain size in NC compounds for at least 100 h. When annealing temperature is well above 1000 K the mean grain size increases and becomes comparable with the film thickness. Therefore, a Cr–Si film, annealed at Tann below 1000 K for less than about 100 h is considered as NC composite, while annealing the same film at Tann well above 1000 K transforms it into polycrystalline state. In case of Mn–Si composites the range of NC state stability is less certain. Our data indicate that NC state is stable to at least 900 K. For the comparison of NC and polycrystalline states we selected stoichiometric Cr0.33Si0.67 and nearly stoichiometric Mn0.31Si0.69 films. The Cr0.33Si0.67 films were 164 nm thick, while Mn0.31Si0.69 films were 100 nm thick. Prior to these experiments the films were annealed to obtain the NC composite with NC phase content, corresponding to the percolation threshold of the NC phase. This state corresponds to the maximum in the resistivity vs. time dependence, shown in Fig. 2. Then the films were further annealed: at first below 1000 K: 970 K for Cr0.33Si0.67, and two-step annealing at 925 K with following cooling and second heating to 964 K – for Mn0.31Si0.69. And finally the films were annealed well above 1000 K: at 1100 K for Cr0.33Si0.67 and 1050 K for Mn0.31Si0.69. At the latter annealing temperatures, according to structural data, the grains start to grow, rapidly reaching the size of the film thickness. The temperature dependences of thermopower, resistivity and of the power factor S2/q, measured after these annealing steps, are shown for Cr0.33Si0.67 and for Mn0.31Si0.69 on Figs. 4 and 5, respectively. As expected,
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Fig. 4. Temperature dependences of thermopower – top panel, resistivity – middle panel, and power factor – bottom, of stoichiometric Cr0.33Si0.67 film in nanocrystalline and polycrystalline state.
annealing leads to a decrease of the electrical resistivity. The resistivity of the NC films decreases mostly due to increasing effective cross-section of the percolating NC cluster. This mechanism does not imply a change of thermopower. Indeed, there is no essential changes of the thermopower of Mn0.31Si0.69 in the course of two annealing at 925 K and at 964 K, although the resistivity significantly decreases at these annealing steps. The resistivity further decreases with annealing above 1000 K, the mechanism of the decrease is different: it connected with crystal grain growth and with corresponding decrease of the charge carrier scattering on the grain interfaces. The substantial decrease of thermopower, observed during this annealing for both compounds, outweighs the decrease of the resistivity and leads to the decrease of the power factor in the films with increased grain size. 4. Conclusion The nanocrystalline thin films Cr–Si and Mn–Si composites can be prepared by crystallization from amorphous state. The amorphous films were produced by magnetron sputtering onto unheated substrates. Rapid crystallization of the films starts at annealing temperature Tcr 570 K for Cr–Si films and at Tcr 600 K for Mn–Si films. The crystallization temperature of CrxSi1x weakly increases with increasing Si to Cr ratio, however it is hardly dependent on the film thickness (in the 50–500 nm range). The CrxSi1x films, annealed at Tcr < Tann [ 1000 K, crystallize in nanocrystalline state with average grain size 10–20 nm. The NC state is
Fig. 5. Temperature dependences of thermopower – top panel, resistivity – middle panel, and power factor – bottom, of nearly stoichiometric Mn0.31Si0.69 film in nanocrystalline and polycrystalline state. The polycrystalline state is realized after annealing at temperatures above 1000 K.
stable for CrxSi1x composites below 1000 K for at least 100 h. The structure and temperature range of stability of NC state in Mn–Si films has been established less definitely, our data indicate that it is stable at least to 900 K. We found that scattering on the NC interfaces gives large contributions to both, resistivity and thermopower. Thermopower of the NC film composites is significantly larger than thermopower of the same composite in polycrystalline state. This suggests that the scattering of charge carriers on the NC interfaces is strongly energy dependent. The enhancement of thermopower in NC state outweighs the increase of electrical resistivity, therefore, the power factor of NC Cr0.33Si0.67 and Mn0.31Si0.69 composites exceeds the power factor of the same composites in polycrystalline state. Since it is experimentally has been proved that nanocrystallization leads to a considerable suppression of lattice thermal conductivity, the enhancement of power factor in NC composites implies that NC materials have better thermoelectric efficiency in comparison with respective conventional polycrystalline materials. Note, this conclusion is in accord with recent results on enhancement of thermoelectric efficiency of skutterudites subjected to high-pressure torsion – a type of severe plastic deformation, which results in nanocrystallization of material [14].
Acknowledgments This work in a part was supported by Russian Foundation for Basic Research under Grant No. 13-08-00398. SVN gratefully
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acknowledges financial support by Stipendium of the President of the Russian Federation SP-543.2012.1. References [1] M. Kanatzidis, Chem. Mater. 22 (2010) 648–659. [2] C. Surynarayana, Int. Mater. Rev. 40 (1995) 41–64. [3] L. Bertini, K. Billquist, M. Christensen, C. Gatti, L. Holmgren, B. Iversen, M. Mueller, E. Muhammed, G. Noriega, A. Palmqvist, D. Platzek, D.M. Rowe, A. Saramat, C. Stiewe, M. Toprak, S.G. Williams, Y. Zhang, in: Proceedings of 22nd International Conference on Thermoelectrics, IEEE Catalog No. 03TH8726, pp. 48–51. [4] K.T. Wojciechowski, J. Morgiel, in: Proceedings of 22nd International Conference on Thermoelectrics, IEEE Catalog No. 03TH8726, pp. 97–99. [5] B.L. Yu, X.F. Tang, Q.J. Zhang, T.X. Liu, P.F. Luo, J. Wang, in: Proceedings of 22nd International Conference on Thermoelectrics, IEEE Catalog No. 03TH8726, pp. 101–104.
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[6] C. Wan, Y. Wang, N. Wang, W. Norimatsu, M. Kusunoki, K. Koumoto, Sci. Technol. Adv. Mater. 11 (2010). 044306-1–7. [7] B. Poudel, Q. Hao, Y. Ma, Y. Lan, A. Minnich, B. Yu, X. Yan, D. Wang, A. Muto, D. Vashaee, X. Chen, J. Liu, M.S. Dresselhaus, G. Chen, Z. Ren, Science 320 (2008) 634–638. [8] A.T. Burkov, A. Heinrich, P.P. Konstantinov, T. Nakama, K. Yagasaki, Meas. Sci. Technol. 12 (2001) 264–272. [9] W. Pitschke, D. Hofman, J. Schumann, C.A. Kleint, A. Heinrich, A.T. Burkov, J. Appl. Phys. 89 (2001) 3229–3241. [10] A. Burkov, J. Schumann, H. Vinzelberg, T. Nakama, K. Yagasaki, J. Appl. Phys. 95 (2004) 7903–7907. [11] A.T. Burkov, A. Heinrich, C. Gladun, W. Pitschke, J. Schumann, Phys. Rev. B 58 (1998) 9644–9647. [12] D. Bergman, O. Levy, J. Appl. Phys. 70 (1991) 6821–6833. [13] M. Fedorov, V. Zaitsev, Thermoelectrics Handbook: Macro to Nano, CRC Press, Boca Raton, London, New York, pp. 1–19. [14] G. Rogl, Z. Aabdin, E. Schafler, J. Horky, D. Setman, M. Zehetbauer, M. Kriegisch, O. Eibl, A. Grytsiv, E. Bauer, M. Reinecker, W. Schranz, P. Rogl, J. Alloys Comp. 537 (2012) 183–189.
Contents lists available at SciVerse ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Enhancement of thermoelectric properties in nanocrystalline M–Si thin film composites (M = Cr, Mn) S.V. Novikov a,⇑, A.T. Burkov a, J. Schumann b a b
A.F. Ioffe Physical-Technical Institute, Sankt-Petersburg 194021, Russia Leibniz Institute for Solid State and Materials Research, Dresden, Germany
a r t i c l e
i n f o
Article history: Received 7 November 2012 Received in revised form 17 December 2012 Accepted 18 December 2012 Available online 27 December 2012 Keywords: Nanocrystalline composite Thermoelectric properties Electric transport
a b s t r a c t We study thermoelectric properties, crystallization kinetics and stability of amorphous and nanocrystalline state in Cr–Si and Mn–Si composite films. The as-deposited amorphous films are transformed by annealing with in situ thermopower and electrical resistivity measurements into nanocrystalline state with continuous monitoring their state. Depending on the initial film composition, the films are transformed during the annealing into single phase or multi-phase nanocrystalline composites. A clear correlation between structural state and thermoelectric properties of the composites is found. Particularly, the thermoelectric power factor of nanocrystalline films is enhanced in comparison to the power factor of polycrystalline films, obtained by high-temperature re-crystallization of the nanocrystalline films, due to strong energy dependent charge carrier scattering. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Thermoelectric efficiency of a material is characterized by 2 dimensionless figure-of-merit: ZT ¼ Sjr, where r is the electrical conductivity, S is the thermoelectric power, and j is the thermal conductivity. These material parameters are dependent on electronic structure and on micro-structure of a material. One common route, which has been exploited to improve the thermoelectric efficiency, is a reduction of lattice thermal conductivity by preparing of finely grained composites. However, in conventional micro-crystalline substances with grain size in micrometer range a decrease of thermal conductivity due to scattering of phonons on grain interfaces is compensated by the decrease of electrical conductivity due to scattering of the charge carriers on the same interfaces, whereas thermopower is hardly affected by this scattering and remains almost the same as in coarse-grained materials. Nanocrystalline (NC) composites provide new opportunities to extend our control of the structure–property correlations in solids [1]. NC materials are polycrystalline substances with grain sizes of up to about 100 nm [2]. The NC state is structurally characterized by ultra-fine grains, and a large volume fraction of associated interfaces. Many properties of NC materials are significantly different from, and frequently superior to, those of their coarse-grained counterparts, and therefore have drawn much attention in recent years. Electronic transport properties of NC-conductors are strongly affected by interaction of conduction electrons with the inter-grain ⇑ Corresponding author. Tel.: +7 812 5159173; fax: +7 812 2971017. E-mail address: [email protected] (S.V. Novikov). 0925-8388/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2012.12.088
interfaces. In NC-conductors the interfaces represent an additional scattering channel for the conduction electrons, resulting in an enhanced resistivity (q = 1/r). An appropriate length scale for electronic transport properties is the mean free path of the charge carriers, l. In NC composites the mean free path of the charge carriers can be comparable or even larger than the grain size, d. When l P d, scattering on the interfaces will give main contribution to the electronic transport. Moreover, under this condition, a reflection of conduction electrons from the interfaces may lead to a partial confinement of charge carriers within nano-grains, thus, essentially modifying the electronic structure of the material. Furthermore, for the typical grain size of about 5–50 nm, the band structure parameters shall be depend on the grain size. Therefore, the crystalline grains in a NC semiconductor can have a variable band gap parameter, depending on their individual size. Hence, the NC-composite can be visualized as an array of hetero-junctions with randomly distributed band gaps. The scattering and confinement of the charge carries in NC grains should lead, of course, to a reduction of electrical conductivity. On the other hand, the transmission probability of the inter-grain interfaces may have a strong dependence on charge carrier energy. This will result in an enhancement of S. Therefore the product S2r S2/q – the power factor of a NC material, can be comparable or even larger than the power factor of corresponding usual polycrystalline sample. In the same time, due to high degree of structural disorder on the length-scale of about 10 nm, the lattice thermal conductivity of a material in NC form should be strongly suppressed in comparison with coarse-grained state. Indeed, experiments with NC materials have shown that thermal conductivity of the compounds with
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grain size of order 10–100 nm can be reduced by almost order of magnitude in comparison with polycrystalline samples [3–6]. Therefore the nanocrystallization is considered as a promising way to improve thermoelectric efficiency. However, there are many obstacles in a practical realization of NC state. Particularly, preparation of bulk NC samples often includes such procedures as grinding, pressing and sintering. Almost unavoidable contamination of NC interfaces from environment during these procedures modifies in an uncontrollable way properties of resulting NC composite. This makes correct interpretation of experimental studies of such composites difficult and, probably, it is one of the reasons for only a modest improvement of ZT which has been achieved in bulk NC samples [7]. We use for preparation of NC samples crystallization from amorphous state. In this method of nanocrystallization, the crystallites and their interfaces are formed via solid state reaction, therefore the NC samples are dense and the internal interfaces are clean. This removes additional factors affecting properties of NC composite and allows a straightforward interpretation of the corresponding experimental data. Transition metal silicides have been considered as promising materials for thermoelectric applications because they are environment friendly, stable at high temperatures, inexpensive, some of them have high thermoelectric efficiency. We found that silicides can be comparatively easy prepared in NC form by crystallization from amorphous state. In this study we investigate in a most unambiguous way the effect of nanocrystallization on electronic transport properties and on thermoelectric performance of transition metal silicides.
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2. Experimental procedures The composite films were produced by the crystallization from amorphous state in the course of annealing of the film with in situ transport measurements. The amorphous films were prepared by magnetron sputtering from composite targets onto unheated substrates. The film thickness was in the range 50–500 nm. Si wafers, thermally oxidized to form a 1 lm layer of SiO2, were used as substrates. The SiO2 layer provides electrical insulation of the film from Si substrate and prevents chemical reactions of the film with the substrate during the annealing procedures. The state of the film composite in the course of its transformation from amorphous to NC composite is controlled by means of the in situ transport measurements – simultaneous measurements of q and S. Standard, DC 4 point configuration was used in the resistivity measurements, while differential method was utilized for thermopower [8]. Transmission Electron Microscopy and X-ray Diffraction were used for structural characterization of the samples [9].
3. Results and discussion Fig. 1 presents temperature dependences of S and q of three Cr– Si film composites: Cr0.35Si0.65, Cr0.33Si0.67 (stoichiometric CrSi2), and Cr0.28Si0.72 with thickness of 180 nm, 100 nm and 52 nm, respectively. We have studied films of different thickness in the range 50–500 nm for every composition and found no essential dependence of the film properties on the thickness. The as-deposited films have amorphous structure. The films were annealed during one cycle with continuously increasing temperature from 300 K to 960 K, and with following cooling to room temperature. The beginning of the crystallization is marked by the sharp increase of S and q in these temperature dependences. The results reveal a general tendency in the dependency of S(T) and q(T), and of annealing behavior of the film composites on Si/Cr ratio. The amorphous state is stable up to crystallization temperature Tcr (Tcr 580 K for Cr–Si films). For a given Cr–Si composite Tcr weakly increases with increasing Si/Cr ratio. According to TEM and X-ray results for these composite films [10], the crystallization proceeds due to increasing number of nanocrystals with average size of about 10–20 nm. At annealing temperatures Tann < 1000 K we have not observed an increase of the average grain size with annealing time or with annealing temperature. In fact, structural investigations show that the mean grain size decreases in the
Fig. 1. Temperature dependences of thermopower (a) and of electrical resistivity of CrxSi1x composite films. Grey symbols – heating; black symbols – cooling.
course of annealing. This is due to the shift of grain size distribution to smaller grain size direction: in partially crystallized system there is no room for formation of large crystallites. The nucleation rate of the nanocrystals decreases with time at any annealing temperature Tcr < Tann < 1000 K. Therefore long-time annealing at high temperatures is needed to achieve maximum nanocrystallization. A sharp increase of both, S and q is observed on first heating at Tcr. However, immediately above Tcr only a small fraction of the amorphous matrix is transformed into NC state. Moreover, according to our earlier results [10], at least for off-stoichiometric Cr–Si composite films with Si excess in comparison with stoichiometric 1:2 ratio, the nanocrystalline CrSi2 phase does not contribute to the total film conductivity until it forms a percolating cluster. The existence of such percolation threshold is confirmed by decrease of resistivity, accompanied by increase of thermopower with annealing time at fixed annealing temperature, presented in Fig. 2. At Tann < 1000, only the volume fraction of NC phase (not size of nanocrystals) increases in the course of the annealing. Therefore the dependences of q and S on annealing time in Fig. 2 represent the dependence of these properties on volume fraction of NC phase. The maximum in dependence of q on annealing time signals the onset of percolation throughout NC cluster. The initial sharp increase of resistivity and thermopower at Tcr results from the scattering of charge carriers in the amorphous matrix on the nanocrystal interfaces. The increase of thermopower indicates that this scattering is strongly energy–dependent. It follows from Fig. 1 that the stoichiometric CrSi2 film has the highest power factor S2/q and, potentially, can be a bases for development of efficient NC composite. Fig. 3 shows S and q of a Mn0.31Si0.69 film. The film has thickness of 496 nm. Fig. 3 demonstrates the film transformation from amorphous to NC state during several annealing cycles, each annealing
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cycle includes heating the sample from room temperature to annealing temperature Tann, annealing at Tann for 15 h and cooling down to room temperature. The amorphous state is stable up to crystallization temperature Tcr 600 K. At first annealing cycle we also observe a sharp increase of both, S and q. With further annealing, (cycles 2 and 3) at higher Tann, q increases by more than order of magnitude, while comparatively small changes of S are observed. At annealing cycles 4–6, q decreases, its temperature
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Fig. 3. Temperature dependences of thermopower (a) and electrical resistivity (b) of a Mn0.31Si0.69 thin film, shown on different annealing stages. The grey symbols denote data points, measured on heating, the black symbols – on cooling. The numbers indicate the annealing cycles. At the highest temperature Ti (i = 1, 2, 3, 4) of each annealing cycle the sample was annealed for about 15 h with subsequent cooling to room temperature (the cooling curve is shown here only for the last cycle).
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variation qualitatively changes, at the same time S increases considerably. The non-monotonous dependence of resistivity on annealing time (hence – on volume fraction of NC phase) is similar to the behavior observed in Cr–Si films. Our results indicate that such behavior is a common feature for M–Si (M = Cr, Mn, Re) film composites for a broad range of Si/M ratio, including stoichiometric composition [11]. Note, the stoichiometric compounds can be considered during nanocrystallization process as a binary composite of amorphous and NC phases where the ratio of amorphous to nanocrystalline phase changes during annealing. For a binary composite there is an exact result within Effective Medium Approximation (EMA): the thermoelectric parameters (Z, S and q) of a binary composite always lies between the corresponding values for pure components [12], which implies that the dependence of the parameters on the composition should be a monotonous function. Therefore, the non-monotonous dependence of resistivity on its phase composition indicates failure of EMA in case of NC composites. The temperature dependences of the electrical resistivity and thermopower of the composites, depending on the crystallization stage, are shaped by different mechanisms. In the amorphous films up to crystallization temperature Tcr, S(T) and q(T) dependencies are typical for amorphous conductors. The temperature dependencies in partially crystallized films, measured on heating, result from a combination of two mechanisms: first, a change of the volume fractions of crystallized and amorphous phases with increasing exposure to elevated temperatures, and secondly, the intrinsic temperature dependence of the properties of a composite. The intrinsic S(T) and q(T) for a composite with certain ratio between crystalline and amorphous phases can be obtained in measurement on cooling. The S(T) and q(T) dependences in completely crystallized films of the stoichiometric composition are qualitatively similar to those of bulk polycrystalline composites [13]. More detailed consideration of S(T) and q(T) dependences of the composites at different annealing stages will be given in a subsequent paper. In order to examine the effect of nanocrystallization on thermoelectric performance we compare properties of NC and polycrystalline states of the same sample film. Our investigation of structure of thin film composites on different annealing stages yield information about range of stability of NC state. For Cr–Si film composites the NC state is stable below 1000 K: when Tann was below 1000 K we did not observe an increase of the mean grain size in NC compounds for at least 100 h. When annealing temperature is well above 1000 K the mean grain size increases and becomes comparable with the film thickness. Therefore, a Cr–Si film, annealed at Tann below 1000 K for less than about 100 h is considered as NC composite, while annealing the same film at Tann well above 1000 K transforms it into polycrystalline state. In case of Mn–Si composites the range of NC state stability is less certain. Our data indicate that NC state is stable to at least 900 K. For the comparison of NC and polycrystalline states we selected stoichiometric Cr0.33Si0.67 and nearly stoichiometric Mn0.31Si0.69 films. The Cr0.33Si0.67 films were 164 nm thick, while Mn0.31Si0.69 films were 100 nm thick. Prior to these experiments the films were annealed to obtain the NC composite with NC phase content, corresponding to the percolation threshold of the NC phase. This state corresponds to the maximum in the resistivity vs. time dependence, shown in Fig. 2. Then the films were further annealed: at first below 1000 K: 970 K for Cr0.33Si0.67, and two-step annealing at 925 K with following cooling and second heating to 964 K – for Mn0.31Si0.69. And finally the films were annealed well above 1000 K: at 1100 K for Cr0.33Si0.67 and 1050 K for Mn0.31Si0.69. At the latter annealing temperatures, according to structural data, the grains start to grow, rapidly reaching the size of the film thickness. The temperature dependences of thermopower, resistivity and of the power factor S2/q, measured after these annealing steps, are shown for Cr0.33Si0.67 and for Mn0.31Si0.69 on Figs. 4 and 5, respectively. As expected,
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Fig. 4. Temperature dependences of thermopower – top panel, resistivity – middle panel, and power factor – bottom, of stoichiometric Cr0.33Si0.67 film in nanocrystalline and polycrystalline state.
annealing leads to a decrease of the electrical resistivity. The resistivity of the NC films decreases mostly due to increasing effective cross-section of the percolating NC cluster. This mechanism does not imply a change of thermopower. Indeed, there is no essential changes of the thermopower of Mn0.31Si0.69 in the course of two annealing at 925 K and at 964 K, although the resistivity significantly decreases at these annealing steps. The resistivity further decreases with annealing above 1000 K, the mechanism of the decrease is different: it connected with crystal grain growth and with corresponding decrease of the charge carrier scattering on the grain interfaces. The substantial decrease of thermopower, observed during this annealing for both compounds, outweighs the decrease of the resistivity and leads to the decrease of the power factor in the films with increased grain size. 4. Conclusion The nanocrystalline thin films Cr–Si and Mn–Si composites can be prepared by crystallization from amorphous state. The amorphous films were produced by magnetron sputtering onto unheated substrates. Rapid crystallization of the films starts at annealing temperature Tcr 570 K for Cr–Si films and at Tcr 600 K for Mn–Si films. The crystallization temperature of CrxSi1x weakly increases with increasing Si to Cr ratio, however it is hardly dependent on the film thickness (in the 50–500 nm range). The CrxSi1x films, annealed at Tcr < Tann [ 1000 K, crystallize in nanocrystalline state with average grain size 10–20 nm. The NC state is
Fig. 5. Temperature dependences of thermopower – top panel, resistivity – middle panel, and power factor – bottom, of nearly stoichiometric Mn0.31Si0.69 film in nanocrystalline and polycrystalline state. The polycrystalline state is realized after annealing at temperatures above 1000 K.
stable for CrxSi1x composites below 1000 K for at least 100 h. The structure and temperature range of stability of NC state in Mn–Si films has been established less definitely, our data indicate that it is stable at least to 900 K. We found that scattering on the NC interfaces gives large contributions to both, resistivity and thermopower. Thermopower of the NC film composites is significantly larger than thermopower of the same composite in polycrystalline state. This suggests that the scattering of charge carriers on the NC interfaces is strongly energy dependent. The enhancement of thermopower in NC state outweighs the increase of electrical resistivity, therefore, the power factor of NC Cr0.33Si0.67 and Mn0.31Si0.69 composites exceeds the power factor of the same composites in polycrystalline state. Since it is experimentally has been proved that nanocrystallization leads to a considerable suppression of lattice thermal conductivity, the enhancement of power factor in NC composites implies that NC materials have better thermoelectric efficiency in comparison with respective conventional polycrystalline materials. Note, this conclusion is in accord with recent results on enhancement of thermoelectric efficiency of skutterudites subjected to high-pressure torsion – a type of severe plastic deformation, which results in nanocrystallization of material [14].
Acknowledgments This work in a part was supported by Russian Foundation for Basic Research under Grant No. 13-08-00398. SVN gratefully
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