- Email: [email protected]
Investigation of Mg2(Si,Sn) thin films for integrated thermoelectric devices
Journal of Alloys and Compounds 649 (2015) 573e578
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Investigation of Mg2(Si,Sn) thin films for integrated thermoelectric devices phane Be chu a, b, Ce dric de Vaulx d, Codrin Prahoveanu a, b, c, Ana Lacoste a, b, *, Ste Kamel Azzouz d, Laetitia Laversenne b, c, e, ** LPSC, Universit e Grenoble-Alpes, CNRS/IN2P3, 53 rue des Martyrs, 38026 Grenoble, France Univ. Grenoble Alpes, F-38000 Grenoble, France c CNRS, Inst NEEL, F-38000 Grenoble, France d Valeo Thermal Systems, 8 rue Louis Lormand, BP 517 La Verri ere, 78321 Le Mesnil Saint Denis Cedex, France e School of Engineering and Applied Sciences, Harvard University, Cambridge, MA, USA a
b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 9 October 2014 Received in revised form 12 June 2015 Accepted 4 July 2015 Available online 7 July 2015
The ongoing miniaturization of thermoelectric (TE) modules requires scaling down to thin films of TE materials with high efficiency. Moreover, thin film-based integrated devices contain interfaces (between TE materials and dielectric or conductive components) that must show chemical and mechanical stability in the whole range of the operating temperature. In search for such materials, thin films of Mg2(Si,Sn) solid solutions have been deposited by microwave plasma-assisted co-sputtering method with a fine control over their composition. Three types of substrates were chosen (SiO2/Si, glass and Ni substrates) to examine their potential use as insulators and electrodes in a miniaturized thermoelectric module based on Mg2(Si,Sn). The electrical conductivity and thermo-mechanical properties, as well as the thermal stability, of the thin films have been investigated in the intermediate range of temperature (300e700 K). It is shown that the deposition process, as well as the substrates on which the films are grown, determine the subsequent adherence of the films. Also, the metastability of the Mg2Si0.4Sn0.6 solid solution for small variations in composition (possibly bordering the edge of the miscibility gap in the phase diagram) has been observed, which can lead to a separation into 2 phases during the first annealing treatment at intermediate temperatures. © 2015 Published by Elsevier B.V.
Keywords: Thermoelectric materials Mg2(Si,Sn) Thin films Microwave plasma co-sputtering Thermal stability Film-substrate reactivity
1. Introduction Within the continuous development of different technologies dealing with power generation, a significant emphasis can be found in the field of thermoelectricity [1e5]. Many thermoelectric (TE) materials have been studied, using the figure of merit ZT as criterion for establishing their efficiency. Among these, the Mg2(Si,Sn) solid solutions stand out as promising TE materials not only due to their ZT which has been reported to have surpassed unity after doping [6e11] with values comparable to that of state-of-the-art materials [12,13], but also on account of the abundance of the
Grenoble-Alpes, CNRS/IN2P3, 53 rue * Corresponding author. LPSC, Universite des Martyrs, 38026 Grenoble, France. ** Corresponding author. CNRS, Inst NEEL, F-38000 Grenoble, France. E-mail addresses: [email protected] (A. Lacoste), laetitia.laversenne@ neel.cnrs.fr (L. Laversenne). http://dx.doi.org/10.1016/j.jallcom.2015.07.043 0925-8388/© 2015 Published by Elsevier B.V.
constituent elements and their environmentally friendly feature. Also, by appropriately doping these solid solutions it can result to both n-type and p-type TE materials that can subsequently be implemented in TE modules [14]. The potential of these solid solutions arises from the relatively low thermal conductivity due to the enhanced point defect phonon scattering and strain fluctuations stemmed from the great difference in atomic mass between Si and Sn [15]. The power factor can be improved as well on account of the increase of the Seebeck coefficient determined by the degeneracy of the conduction band minima characteristic to the Mg2(Si,Sn) solid solutions and the possibility to fine-tune it by controlling the Sn content [8]. After a number of studies reported on these solid solutions with different stoichiometries, it was established that the best TE properties correspond to the materials of composition Mg2SixSn1x with x between 0.35 and 0.6 in the temperature range of 500e850 K [8,16,17]. These solid solutions were doped with Sb, known to improve the TE properties of the ternary materials by increasing the
574
C. Prahoveanu et al. / Journal of Alloys and Compounds 649 (2015) 573e578
carrier concentration inside the solid solution and therefore the power factor, up to 4.2 103 WK2 m1 for bulk materials [8]. Thus, promising results and high values of the figure of merit (ZT ¼ 1.3) [8] for Mg2(Si,Sn) solid solutions have been obtained for bulk materials. However, their transport properties still need to be examined thoroughly at different stoichiometries as Mg2Si1xSnx is not thermodynamically stable in the expected range of operating temperature. Actually, despite some disputes about its limits, literature data agree on the existence of a miscibility gap within the pseudo binary Mg2SieMg2Sn phase diagram [18e21]. Scaling down to thin films may be advantageous because of the control over the microstructure and texture which may have a beneficial effect on transport properties of the material [22]. Furthermore, the ongoing miniaturization of TE modules for various applications [23,24] makes the synthesis of TE thin films and their integration to such modules a technological objective. The moderately elevated temperature domain for which the Mg2(Si,Sn) solid solutions were reported to have high figures of merit, imposes for power generation a geometry of the TE module in which the inplane transport properties of the thin films are exploited. Such geometries have been proposed in the 60s for monolithic miniaturized thermoelectric generators [14]. They are built up of stacks of two different thermoelectric layers (p and n), each layer separated from the next by an electrically insulating layer and interconnected in series at both ends of the device. Such monolithic devices require the thermoelectric elements and the dielectric materials to be chemically and mechanically compatible. Moreover, even if the bearing surface is lower for the interface between the thermoelectric material and the electrode, this interface needs also to be stable in temperature. The reported work allocated to the investigation as thin films of Mg2Si [25e32], Mg2Sn [33] and the ternary solid solutions [34,35] are mainly related to their synthesis. To our knowledge, only two articles report on the investigation of the TE properties of these materials as thin films. Mg2(Si,Sn) solid solutions were previously investigated as thin films for Si contents between 0.4 and 0.6 [34,35]. It was reported that the power factor of these thin films doesn't match that of bulk materials, with values lower than 3 104 WK2 m1, while the possibility for improvement remained open. In this paper, we investigate crystalline thin films of Mg2(Si,Sn) solid solutions deposited by microwave plasma-assisted co-sputtering. The chosen stoichiometry was close to that of Mg2Si0.4Sn0.6, being the most promising Mg2(Si,Sn) solid solution composition based on the TE performances that were previously reported. The films have been grown on different substrates, SiO2/Si and glass which are considered as feasible insulating layers and Ni as a potential electrode for a miniaturized TE module. In order to investigate the reactivity at the film-insulator and film-electrode interfaces in the working temperature range, the structural and chemical stability of the film as well as the electrical conductivity have been examined at intermediate temperatures, while the Seebeck coefficient was investigated at ambient temperature.
of each material with the targets positioned diametrically opposed on the target holder is performed to ensure a homogeneous deposition of each element on the substrate. This control over the sputtering process permits a fine tuning of the composition of the resulting thin films. The microstructure of the film can also be modified by controlling the flux of sputtered atoms and their energy. The former is controlled by the microwave power and the gas pressure, while the latter is determined not only by target voltage, but also by the couple of pressure-distance between the targets and the substrate. Furthermore, the temperature of the substrate can be adjusted during the deposition by use of a temperature-controlled circulating bath. The parameters used during the deposition process include a base pressure of 106 Torr, 1 mTorr of working Ar pressure, 2 kW of microwave power. The substrate, which was left at floating potential with its temperature either at 300 K or 450 K, was at a distance of 12 cm from the targets. The composition of the solid solution was controlled by fine-tuning the bias applied to each target. The chemical composition of the thin films was ascertained by EDX (energy dispersive x-ray spectroscopy) and EPMA (electron probe micro-analyzer), with an error lower than 1 wt%. The crystal structure investigations were performed using XRD (x-ray diffraction in reflection mode, l ¼ 1.54 Å or 1.79 Å, angular resolution of 0.2 ) and the morphology of the cross section was characterized by FE-SEM (field-emission scanning electron microscope, EHT ¼ 3 kV). The dependence of the thin films electrical conductivity with temperature was determined from the resistivity measurements performed using the 4-point probe method in the temperature range 300e700 K with a lab-contrived set-up (the heating and cooling rates set to 10 K/min). The measurements were done in an inert atmosphere (He) to avert the oxidation of the samples at high temperatures. The Seebeck coefficient was measured at room temperature with a non-commercial device with an estimated accuracy of 5% [37]. A temperature gradient was applied along the plane of the film, ensured by a heating cartridge, while the electrical and thermal contacts were done by mechanical pressure applied to the surface of the sample. Additionally, in order to verify the thermal stability of the thin films, prolonged thermal treatments at temperatures up to 850 K were carried out in an oven with heating and cooling rates of 5 K/min in sealed ampoules under Ar atmosphere. 3. Results and discussions 3.1. Synthesis of Mg2(Si,Sn) solid solution thin films Mg2(Si,Sn) thin films with a thickness of ~4 mm have been deposited on SiO2 (500 nm)/Si, glass and Ni substrates at room temperature (RT) and at high temperature (HT ¼ 450 K). The composition of the films was regulated by the deposition parameters that characterize the PVD process used (specifically, adjusting the bias applied to the targets). In Table 1 are shown the
2. Experimental details Mg2(Si,Sn) thin films have been deposited on SiO2/Si, glass and Ni substrates at different temperatures, using the setup described in greater detail in Refs. [33,34]. The plasma is ignited in an Ar atmosphere by 20 dipolar plasma sources circularly arranged and supplied by microwave generators with a power of 2 kW, evenly distributed to the sources through power dividers [36]. The deposition process is performed through the co-sputtering of all three types of targets (Mg, Si and Sn, with 99.99% purity) by applying a negative dc bias, separately, to each of them. The dual co-sputtering
Table 1 EPMA measurements of the composition of Mg2(Si,Sn) thin films deposited for different sets of bias applied to the targets (relative values given e VMg,Si,Sn/VSn). Mg/Si/Sn relative bias values
Composition
Si content (relative to Sn)
1.94/2.22/1.00 1.75/2.00/1.00 1.67/1.90/1.00 1.56/1.96/1.00 1.59/1.91/1.00 1.82/1.90/1.00
Mg2.04Si0.42Sn0.53 Mg2.03Si0.43Sn0.55 Mg2.00Si0.42Sn0.58 Mg1.98Si0.41Sn0.61 Mg1.99Si0.4Sn0.61 Mg2.02Si0.37Sn0.61
0.44 0.44 0.42 0.40 0.40 0.38
C. Prahoveanu et al. / Journal of Alloys and Compounds 649 (2015) 573e578
compositions determined by EPMA measurements of Mg2(Si,Sn) thin films obtained for different sets of values for the bias applied to each target. These results highlight the advantage of this deposition method in contrast with the widely used magnetron sputtering, due to a better fine tuning of the composition as outcome of the independent control over the process parameters. In the following, the samples described were deposited using the sets of bias values from Table 1, in order to grow thin films of composition close to a stoichiometric Mg2Si0.4Sn0.6. In the third column are presented the corresponding Si content values in the material with respect solely to the Sn content and the results of this work will be discussed with reference to this ratio. Based on the chosen experimental conditions (2 kW power applied to the plasma sources and 1 mTorr of gas pressure) a relatively low deposition rate of 30 nm/min was obtained. In Fig. 1 are shown the cross-sectional SEM images of two depositions performed at RT and HT. In the whole range of substrate temperature that we investigated, the as-deposited films showed no cracks, nor delamination. The film deposited at RT (Fig. 1a) has a relatively dense columnar microstructure with submicron in-plane feature sizes. By increasing the deposition temperature (450 K), it results to an even denser columnar morphology (Fig. 1b) and to highly crystalline films, due to the surplus of energy allocated to the atoms at the surface of the substrate. This translates to an enhancement of the surface diffusion and mobility of the adatoms which directly affects the resulting structure of the film. The X-ray diffraction patterns of the films deposited on SiO2/Si substrates at RT and HT are shown in Fig. 2. All the peaks are
575
Fig. 2. X-ray diffraction patterns of Mg2(Si,Sn) thin films deposited on SiO2/Si substrate at RT and HT (450 K).
accounted for, corresponding to the ternary solid solution and to the Si substrate. Within the angular resolution of the diffractometer, the RT as-deposited films appear as corresponding to a single phased Mg2(Si,Sn) solid solution with an antifluorite structure (space group Fm-3m). Depositing the films at higher temperature leads to their growth in either of two crystal structures although the deposition parameters are supposedly the same for all films. The first polymorph corresponds to the regular antifluorite structure of the solid solution. On the other hand, the second structure might be described as an hexagonal symmetry related to the P63/m space group previously reported not for solid solutions, but for Mg2Sn and Mg2Si synthesized in high pressure and high temperature conditions [38e40]. Additionally, the two polymorphs may also coexist in the same film, as it can be noticed in Fig. 2. The formation of metastable polymorph is relatively common in thin films deposited by PAPVD [41]. Considering the films with an antifluorite structure deposited at RT and HT, the two diffraction patterns testify a difference in the crystallinity of the two samples. As opposed to the film grown at RT, the film deposited at HT shows no preferential orientation, meaning that the higher deposition temperature inhibits the texturing of the film. Also, the broadening of the (111) peak with increasing deposition temperature testifies a decrease of the grain size inside the film. 3.2. Thermal behavior of thin films deposited on SiO2/Si, glass and Ni substrates
Fig. 1. SEM cross-sectional images of Mg2(Si,Sn) thin films deposited on SiO2/Si substrate at (a) RT and (b) HT (450 K).
After post-deposition thermal treatment at 700 K for 2 h, the film deposited on SiO2/Si substrate at RT suffers almost total delamination. This behavior at high temperature treatments might be put on account of the difference in the coefficients of thermal expansion (CTE) between the film and the SiO2/Si substrate. The CTE of SiO2 is widely known to be 0.5 106 K1; however the value for Mg2(Si,Sn) solid solutions has not been yet specifically determined. Nevertheless, the CTE are known for Mg2Si (11.5 106 K1 [42]) and Mg2Sn (9.9 106 K1 [42]), as bulk materials, from where it can be deduced that there is an order of magnitude separating the values of SiO2 and Mg2(Si,Sn). This CTE mismatch might generate compressive stress in the film upon annealing. The HT deposition resulted in a great amelioration of the adherence between the film and the SiO2 substrate after the annealing treatment, though the detachment of the film is still visible now at the interface between the oxide layer and Si. This
576
C. Prahoveanu et al. / Journal of Alloys and Compounds 649 (2015) 573e578
detachment does not affect the entire film and thus it was not delaminated from the substrate. The increase of the substrate temperature might have decreased either the intrinsic stresses arose during the growth of the film or the thermal stress in the subsequent thermal treatments by reducing the difference between the annealing temperature and the deposition temperature [43]. Moreover, we investigated the mechanical stability of the film at high temperatures, on substrates with higher CTE, respectively glass (dielectric, CTEglass ¼ 9 106 K1) and Ni (conductive, CTENi ¼ 13 106 K1). The film grown on glass at HT shows no adherence issues with respect to the substrate after thermal treatment TT at 700 K and it no longer fractures either. Same results have been validated for the Ni substrate. These results show that the fracturing of the film at high temperatures is not an intrinsic property, but strictly an effect of the CTE mismatch between the film and the substrate. Furthermore, according to the post-annealing structural characterization of the films, a separation of the solid solution into two cubic phases is visible for samples with both under- and overstoichiometric Mg content and with Si content between 0.40 and 0.43 (deposited either at RT or HT). In the case of the as-deposited hexagonal phase, the separation is preceded by a phase transformation (exemplified for the Ni substrate in Fig. 3). The two resulting phases have lattice parameters of 6.59 Å and, respectively, 6.71 Å, indicating the co-existence of two solid solutions with different Si content. This shows that the first TT acts as annealing, stabilizing the structure of the film. On the other hand, the phase separation confirms the metastability of the deposited films prior to the thermal treatment for the indicated compositions. However, for samples with Si content of 0.38, the film remains as single-phased after TT at 653 K as depicted in Fig. 4 (the noticed shift of the peak position between as-deposited and after TT thin films is due to the stress relaxation that follows the annealing). The results obtained on the stability and metastability of the material come in agreement with the existence of a miscibility gap in the Mg2SieMg2Sn pseudo-binary phase diagram, although the actual edges of the gap are highly disputed [18e21]. Nevertheless, the stability of the cubic phase for a Si content lower than 0.40 and the phase separation observed for higher content suggest that the former composition lies outside the miscibility gap in the phase diagram, while the latter is within. As for the reactivity, XRD analysis performed after a 40 h TT at 700 K of the Mg2(Si,Sn) film has revealed reactivity with Ni
Fig. 3. X-ray diffraction patterns of Mg2(Si,Sn) thin films deposited on Ni substrate at HT (450 K) before and after thermal treatment at 700 K for 40 h.
Fig. 4. X-ray diffraction patterns of Mg2(Si,Sn) thin films deposited on borosilicate glass at HT (450 K) with a Si content of 0.38, before and after thermal treatment at 653 K for 168 h.
substrate concurred by Bragg peaks assigned to MgNi2Sn alloy. Interdiffusion due to interfacial reactivity has also been observed by SEM on films deposited on SiO2/Si substrates heat treated for extended time. Conversely, films deposited on borosilicate glass do not show any signs of reactivity and maintain their thermal stability for extended TT of 40 h and 168 h at 653 K. The diffraction patterns of the films (for any Si content) after the aforementioned TT reveal no changes compared to the annealed sample. The SEM image appended as Fig. 5 presents the interface between the thin film and the borosilicate glass after TT 168 h at 653 K. Unfortunately EDX mapping is not conclusive to perform elemental investigation of the interface due to the spatial resolution which is at the scale of the thickness of the film. However the morphology of the interface concurs for the absence of reactivity. After some inquiries in the matter, it has been established that the highest temperature for which the Mg2(Si,Sn) thin films maintain their thermal stability is 653 K when no reaction occurs with the substrate. For higher temperature, the films suffer total decomposition into Sn, Si and Sn-rich Mg2(Si,Sn) phases, which is in agreement with recent studies on the stability of Mg2(Si,Sn) solid solutions at intermediate temperatures in bulk materials [44,45]. However, for previously mentioned thin films, the decomposition has been determined only for samples with Si content between 0.40 and 0.43. Further investigations need to be made for samples with lower Si content to validate whether they are more stable for
Fig. 5. ESB-SEM cross-sectional image of film-substrate (borosilicate glass) interface after thermal treatment at 653 K for 168 h.
C. Prahoveanu et al. / Journal of Alloys and Compounds 649 (2015) 573e578
higher temperatures due to the composition that is hypothesized to be outside the miscibility gap.
3.3. Transport properties The electrical conductivity measurement shows that after the first thermal cycle, the following cycles are reproducible and the electrical conductivity follows the same dependence with temperature both during heating and cooling (Fig. 6). The temperature dependence obtained for the electrical conductivity is specific to semiconductors and with values (104 U1 m1) comparable to those reported for bulk materials [16]. The difference noticed between the first heating and cooling cycles can be translated by the microstructure changes and stress relaxation that occur during the first annealing. Measurements of the Seebeck coefficient have been performed at ambient temperature on thermally treated Mg2(Si,Sn) thin films (single-phased and two-phased). The values obtained range between 110e170 mV/K, suggesting a p-type behavior of the film. This result is in stark contrast with most of what was reported on Mg2(Si,Sn) solid solutions as bulk materials [7], which is indicative of n-type materials. Moreover, by comparing the absolute values between the two, the Seebeck coefficient of the film seems to be much lower than for bulk materials (400e600 mV/K at RT). On account of the possible effect of Mg content on the transport properties of the material, the measurements were performed for both under-stoichiometric and over-stoichiometric thin films (with values of the Mg content between 66.3 and 67.3 at%). However, the Seebeck coefficient has remained the same in all cases, still suggestive of a p-type material. On the other hand, the results presented here are similar to those previously reported on Mg2(Si,Sn) thin films [35]. Further investigation, including microstructural characterization and theoretical modeling of the transport properties, is needed to establish how does a switch of majority carriers occurs, i.e. the change from an n-type to a p-type material. Overall, there appears to be no difference in electrical conductivity between single-phased and two-phased thin films, but a slight increase of the Seebeck coefficient (from 110 mV/K to 170 mV/ K) was observed for the single-phased film. However, this may also be attributed to the composition closer to a Si content of 0.35, for which the overlapping of the two conduction bands (corresponding to Mg2Si and Mg2Sn) was previously reported [8]. Therefore, the impact of the structure (single-phased or two-phased) on the thermoelectric properties (including thermal conductivity) still needs to be assessed.
577
4. Conclusions Thin films of Mg2(Si,Sn) solid solutions have been deposited on SiO2/Si, glass and Ni substrates at different temperatures by microwave plasma-assisted co-sputtering. The depositions resulted in films with an antifluorite and, possibly, a hexagonal crystal structure. Subsequent thermal treatments showed that the films are sensitive to the composition. If the Si content lies between 0.40 and 0.43, the films go through a structural stabilization process or phase transition, leading to the separation into two cubic phases, suggesting that the as-deposited films are grown in a metastable phase. If however, the Si content is lower, in this case 0.38, the films do not go through any phase separation, marking probably one edge of the miscibility gap in the phase diagram. The use of multiple substrates during the depositions emphasized the significance of the CTE difference between the Mg2(Si,Sn) solid solutions and the substrates over the adherence properties of the films after thermal treatments at intermediate temperatures (700 K). However, the deposition temperature also proved to be effective in the amelioration of the film mechanical stability, relieving the stress inside the films during their growth. Extended heat treatments revealed the reactivity between the Mg2(Si,Sn) solid solutions and the Ni and SiO2/Si substrates, which implies that their use in a miniaturized Mg2(Si,Sn)- based TE device would necessitate an additional diffusion barrier. Conversely, borosilicate glass shows both a high mechanical and chemical inertness up to the temperature where Mg2(Si,Sn) solid solutions show stability. The electrical properties of the films were investigated, showing values of the electrical conductivities similar to that of undoped bulk materials. The Seebeck coefficient of the films has also been determined at room temperature. As opposed to previous reports on bulk materials, a lower value was found and suggestive of a ptype behavior, but is comparable to that obtained in a previous work on thin films. The thermal stability of the presented Mg2(Si,Sn) thin films has been validated upto 653 K. For the films which suffer phase separation, this temperature marks the highest value for which they maintain stability and do not start to decompose in phases of constituent elements. However, it still rests to further inquiry whether the single-phased thin films are more stable at higher temperatures and how their transport properties (Seebeck coefficient and thermal conductivity) evolve with temperature and in comparison with two-phased thin films. Acknowledgments The authors acknowledge the help of Daniel Bourgault and de ric Gay for the Seebeck coefficient and, respectively, the Fre gion electrical conductivity measurements. L.L. acknowledges Re ^ ne-Alpes for its financial support within the framework of Rho CMIRA Explora-Pro grant 14.004457. References
Fig. 6. Electrical conductivity dependence with temperature of a Mg2(Si,Sn) thin film deposited on glass substrate at HT (450 K): first and second heatingecooling cycles.
[1] B.C. Sales, Critical overview of recent approaches to improved thermoelectric materials, Int. J. Appl. Ceram. Technol. 4 (2007) 291e296. [2] G.J. Snyder, E.S. Toberer, Complex thermoelectric materials, Nat. Mater. 7 (2008) 105e114. [3] A.V. Dmitriev, I.P. Zvyagin, Current trends in the physics of thermoelectric materials, Phys. Usp 53 (2010) 789e803. [4] T.M. Tritt, Thermoelectric phenomena, materials, and applications, in: D.R. Clarke, P. Fratzl (Eds.), Annual Review of Materials Research, Annual Reviews, 2011, pp. 433e448. Palo Alto. [5] H. Alam, S. Ramakrishna, A review on the enhancement of figure of merit from bulk to nano-thermoelectric materials, Nano Energy 2 (2013) 190e212. [6] W. Liu, Q. Zhang, X.F. Tang, H. Li, J. Sharp, Thermoelectric properties of Sbdoped Mg2Si0.3Sn0.7, J. Electron. Mater. 40 (2011) 1062e1066. [7] Q. Zhang, J. He, T.J. Zhu, S.N. Zhang, X.B. Zhao, T.M. Tritt, High figures of merit
578
[8]
[9]
[10]
[11]
[12]
[13] [14] [15]
[16]
[17]
[18]
[19] [20] [21]
[22]
[23]
[24]
[25] [26]
[27]
C. Prahoveanu et al. / Journal of Alloys and Compounds 649 (2015) 573e578 and natural nanostructures in Mg2Si0.4Sn0.6 based thermoelectric materials, Appl. Phys. Lett. 93 (2008) 102109. W. Liu, X.J. Tan, K. Yin, H.J. Liu, X.F. Tang, J. Shi, Q.J. Zhang, C. Uher, Convergence of conduction bands as a means of enhancing thermoelectric performance of n-type Mg2Si1xSnx solid solutions, Phys. Rev. Lett. 108 (2012) 166601. Y. Isoda, T. Nagai, H. Fujiu, Y. Imai, Y. Shinohara, Thermoelectric properties of Sb-doped Mg2Si0.5Sn0.5, in: ICT'06: XXV International Conference on Thermoelectrics, Proceedings, IEEE, New York, 2006, pp. 406e410. W.J. Luo, M.J. Yang, F. Chen, Q. Shen, H.Y. Jiang, L.M. Zhang, Preparation and thermoelectric properties of Bi-doped Mg2Si0.8Sn0.2 compound, Mater. Trans. 51 (2010) 288e291. W. Liu, X.F. Tang, J. Sharp, Low-temperature solid state reaction synthesis and thermoelectric properties of high-performance and low-cost Sb-doped Mg2Si0.6Sn0.4, J. Phys. D Appl. Phys. 43 (2010) 085406. J.P. Heremans, V. Jovovic, E.S. Toberer, A. Saramat, K. Kurosaki, A. Charoenphakdee, S. Yamanaka, G.J. Snyder, Enhancement of thermoelectric efficiency in PbTe by distortion of the electronic density of states, Science 321 (2008) 554e557. D.M. Rowe, CRC Thermoelectrics Handbook: Macro to Nano, 2006. W. Thorpe, Manufacture of Thermo-electric Generators, Ferranti Ltd, United States, 1969. V.K. Zaitsev, M.I. Fedorov, E.A. Gurieva, I.S. Eremin, P.P. Konstantinov, A.Y. Samunin, M.V. Vedernikov, Highly effective Mg2Si1xSnx thermoelectrics, Phys. Rev. B 74 (2006) 045207. W. Liu, X.F. Tang, H. Li, K. Yin, J. Sharp, X.Y. Zhou, C. Uher, Enhanced thermoelectric properties of n-type Mg2.16(Si0.4Sn0.6)(1y)Sby due to nano-sized Sn-rich precipitates and an optimized electron concentration, J. Mater. Chem. 22 (2012) 13653e13661. Q. Zhang, H. Yin, X.B. Zhao, J. He, X.H. Ji, T.J. Zhu, T.M. Tritt, Thermoelectric properties of n-type Mg2Si0.6ySbySn0.4 compounds, Phys. Status Solidi A 205 (2008) 1657e1661. E.N. Nikitin, E.N. Tkalenko, V.K. Zaitsev, A.I. Zaslavskii, A.K. Kuznetsov, A study of the phase diagram for the Mg2Si-Mg2Sn system and the properties of certain of its solid solutions, J. Inorg. Mater. 4 (1968) 1656e1659. S. Muntyanu, E.B. Sokolov, E.S. Makarov, Study of the Mg2SneMg2Si system, J. Inorg. Mater. 2 (1966) 870e875. € bner, R. Schmid-Fetzer, Phase formation in MgeSneSi and A. Kozlov, J. Gro MgeSneSieCa alloys, J. Alloy. Compd. 509 (2011) 3326e3337. tres nano et microstructuraux sur P. Bellanger, Etude de l'influence des parame te s thermoe lectriques des siliciures de magne sium Mg2(Si,Sn) de les proprie type-n in, Bordeaux, 2014. J.H. Sui, J. Li, J.Q. He, Y.L. Pei, D. Berardan, H.J. Wu, N. Dragoe, W. Cai, L.D. Zhao, Texturation boosts the thermoelectric performance of BiCuSeO oxyselenides, Energy Environ. Sci. 6 (2013) 2916e2920. I. Chowdhury, R. Prasher, K. Lofgreen, G. Chrysler, S. Narasimhan, R. Mahajan, D. Koester, R. Alley, R. Venkatasubramanian, On-chip cooling by superlatticebased thin-film thermoelectrics, Nat. Nanotechnol. 4 (2009) 235e238. S. van Herwaarden, Sensor applications of thermoelectric thin films, in: Proceedings ICT '97. XVI International Conference on Thermoelectrics, 1997, pp. 47e55. I. Kogut, M.C. Record, Magnesium silicide thin film formation by reactive diffusion, Thin Solid Films 522 (2012) 149e158. A. Vantomme, G. Langouche, J.E. Mahan, J.P. Becker, Growth mechanism and optical properties of semiconducting Mg2Si thin films, Microelectron. Eng. 50 (2000) 237e242. M.T. Hosseinnejad, M. Ghoranneviss, G.R. Etaati, F. Shahgoli, Fabrication of
[28]
[29]
[30]
[31]
[32]
[33]
[34]
[35]
[36]
[37]
[38]
[39] [40]
[41]
[42] [43] [44]
[45]
magnesium silicide thin films by pulsed ion beam ablation in a 1.6 kJ plasma focus device, Vacuum 94 (2013) 57e63. Q.Q. Xiao, Q.A. Xie, K.J. Zhao, Z.Q. Yu, Effect of annealing atmosphere on the Mg2Si film growth deposited by magnetron sputtering, in: X. Yi, L. Mi (Eds.), Materials and Manufacturing Technology, Pts 1 and 2, Trans Tech Publications Ltd, Stafa-Zurich, 2010, pp. 290e294. J.E. Mahan, A. Vantomme, G. Langouche, J.P. Becker, Semiconducting Mg2Si thin films prepared by molecular-beam epitaxy, Phys. Rev. B 54 (1996) 16965e16971. S.W. Song, K. Striebel, E. Cairns, Electrochemical studies of the Mg2Si thin films prepared with pulsed laser deposition, in: G.A. Nazri, E. Takeuchi, R. Koetz, B. Scrosati (Eds.), Batteries and Supercapacitors, Electrochemical Society Inc, Pennington, 2003, pp. 262e267. T. Serikawa, M. Henmi, K. Kondoh, Microstructure and Mg concentration of Mg-Si thin film deposited by ion beam sputtering on glass substrate, J. Vac. Sci. Technol. A 22 (2004) 1971e1974. H. Yu, Q. Xie, Q. Chen, Effects of annealing on the formation of Mg2Si film prepared by resistive thermal evaporation method, J. Mater. Sci. Mater. Electron. 24 (2013) 3768e3775. s, T.T. Vinh, D. Fruchart, N. Skryabina, H. Le-Quoc, A. Lacoste, E.K. Hlil, A. Be Thin films of thermoelectric compound Mg2Sn deposited by co-sputtering assisted by multi-dipolar microwave plasma, J. Alloy. Compd. 509 (2011) 9906e9911. chu, A. Be s, D. Bourgault, D. Fruchart, Deposition of H. Le-Quoc, A. Lacoste, S. Be thin films of Mg2Si1xSnx solid solution by plasma-assisted co-sputtering, J. Alloy. Compd. 538 (2012) 73e78. chu, S. Populoh, A. Weidenkaff, A. Lacoste, Thermoelectric H. Le-Quoc, S. Be properties of thin films of Sb doped Mg2Si1xSnx solid solutions, J. Alloy. Compd. 546 (2013) 138e144. D. Vempaire, S. Miraglia, A. Sulpice, L. Ortega, E.K. Hlil, D. Fruchart, J. Pelletier, Plasma-based ion implantation: a valuable industrial route for the elaboration of innovative materials, Surf. Coat. Technol. 186 (2004) 245e247. D. Bourgault, C.G. Garampon, N. Caillault, L. Carbone, J.A. Aymami, Thermoelectric properties of n-type Bi2Te2.7Se0.3 and p-type Bi0.5Sb1.5Te3 thin films deposited by direct current magnetron sputtering, Thin Solid Films 516 (2008) 8579e8583. G.A. Roberts, E.J. Cairns, J.A. Reimer, An electrochemical and XRD study of lithium insertion into mechanically alloyed magnesium stannide, J. Electrochem. Soc. 150 (2003) A912eA916. G. Urretavizcaya, G.O. Meyer, Metastable hexagonal Mg2Sn obtained by mechanical alloying, J. Alloy. Compd. 339 (2002) 211e215. J. Hao, B. Zou, P.W. Zhu, C.X. Gao, Y.W. Li, D. Liu, K. Wang, W.W. Lei, Q.L. Cui, G.T. Zou, In situ X-ray observation of phase transitions in Mg2Si under high pressure, Solid State Commun. 149 (2009) 689e692. chu, A. Be s, L. Laversenne, MgH2 thin H. Le-Quoc, A. Lacoste, S. Miraglia, S. Be films deposited by one-step reactive plasma sputtering, Int. J. Hydrogen Energy 39 (2014) 17718e17725. L. Berger, CRC Handbook of Chemistry and Physics, 87th ed., 2006. K. Holmberg, A. Matthews, Coatings Tribology: Properties, Mechanisms, Techniques and Applications in Surface Engineering, Elsevier Science, 2009. J. Bourgeois, J. Tobola, B. Wiendlocha, L. Chaput, P. Zwolenski, D. Berthebaud, F. Gascoin, Q. Recour, H. Scherrer, Study of electron, phonon and crystal stability versus thermoelectric properties in Mg2X (X ¼ Si, Sn) compounds and their alloys, Funct. Mater. Lett. 06 (2013) 1340005. M. Sondergaard, M. Christensen, K.A. Borup, H. Yin, B.B. Iversen, Thermal stability and thermoelectric properties of Mg2Si0.4Sn0.6 and Mg2Si0.6Sn0.4, J. Mater. Sci. 48 (2013) 2002e2008.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Investigation of Mg2(Si,Sn) thin films for integrated thermoelectric devices phane Be chu a, b, Ce dric de Vaulx d, Codrin Prahoveanu a, b, c, Ana Lacoste a, b, *, Ste Kamel Azzouz d, Laetitia Laversenne b, c, e, ** LPSC, Universit e Grenoble-Alpes, CNRS/IN2P3, 53 rue des Martyrs, 38026 Grenoble, France Univ. Grenoble Alpes, F-38000 Grenoble, France c CNRS, Inst NEEL, F-38000 Grenoble, France d Valeo Thermal Systems, 8 rue Louis Lormand, BP 517 La Verri ere, 78321 Le Mesnil Saint Denis Cedex, France e School of Engineering and Applied Sciences, Harvard University, Cambridge, MA, USA a
b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 9 October 2014 Received in revised form 12 June 2015 Accepted 4 July 2015 Available online 7 July 2015
The ongoing miniaturization of thermoelectric (TE) modules requires scaling down to thin films of TE materials with high efficiency. Moreover, thin film-based integrated devices contain interfaces (between TE materials and dielectric or conductive components) that must show chemical and mechanical stability in the whole range of the operating temperature. In search for such materials, thin films of Mg2(Si,Sn) solid solutions have been deposited by microwave plasma-assisted co-sputtering method with a fine control over their composition. Three types of substrates were chosen (SiO2/Si, glass and Ni substrates) to examine their potential use as insulators and electrodes in a miniaturized thermoelectric module based on Mg2(Si,Sn). The electrical conductivity and thermo-mechanical properties, as well as the thermal stability, of the thin films have been investigated in the intermediate range of temperature (300e700 K). It is shown that the deposition process, as well as the substrates on which the films are grown, determine the subsequent adherence of the films. Also, the metastability of the Mg2Si0.4Sn0.6 solid solution for small variations in composition (possibly bordering the edge of the miscibility gap in the phase diagram) has been observed, which can lead to a separation into 2 phases during the first annealing treatment at intermediate temperatures. © 2015 Published by Elsevier B.V.
Keywords: Thermoelectric materials Mg2(Si,Sn) Thin films Microwave plasma co-sputtering Thermal stability Film-substrate reactivity
1. Introduction Within the continuous development of different technologies dealing with power generation, a significant emphasis can be found in the field of thermoelectricity [1e5]. Many thermoelectric (TE) materials have been studied, using the figure of merit ZT as criterion for establishing their efficiency. Among these, the Mg2(Si,Sn) solid solutions stand out as promising TE materials not only due to their ZT which has been reported to have surpassed unity after doping [6e11] with values comparable to that of state-of-the-art materials [12,13], but also on account of the abundance of the
Grenoble-Alpes, CNRS/IN2P3, 53 rue * Corresponding author. LPSC, Universite des Martyrs, 38026 Grenoble, France. ** Corresponding author. CNRS, Inst NEEL, F-38000 Grenoble, France. E-mail addresses: [email protected] (A. Lacoste), laetitia.laversenne@ neel.cnrs.fr (L. Laversenne). http://dx.doi.org/10.1016/j.jallcom.2015.07.043 0925-8388/© 2015 Published by Elsevier B.V.
constituent elements and their environmentally friendly feature. Also, by appropriately doping these solid solutions it can result to both n-type and p-type TE materials that can subsequently be implemented in TE modules [14]. The potential of these solid solutions arises from the relatively low thermal conductivity due to the enhanced point defect phonon scattering and strain fluctuations stemmed from the great difference in atomic mass between Si and Sn [15]. The power factor can be improved as well on account of the increase of the Seebeck coefficient determined by the degeneracy of the conduction band minima characteristic to the Mg2(Si,Sn) solid solutions and the possibility to fine-tune it by controlling the Sn content [8]. After a number of studies reported on these solid solutions with different stoichiometries, it was established that the best TE properties correspond to the materials of composition Mg2SixSn1x with x between 0.35 and 0.6 in the temperature range of 500e850 K [8,16,17]. These solid solutions were doped with Sb, known to improve the TE properties of the ternary materials by increasing the
574
C. Prahoveanu et al. / Journal of Alloys and Compounds 649 (2015) 573e578
carrier concentration inside the solid solution and therefore the power factor, up to 4.2 103 WK2 m1 for bulk materials [8]. Thus, promising results and high values of the figure of merit (ZT ¼ 1.3) [8] for Mg2(Si,Sn) solid solutions have been obtained for bulk materials. However, their transport properties still need to be examined thoroughly at different stoichiometries as Mg2Si1xSnx is not thermodynamically stable in the expected range of operating temperature. Actually, despite some disputes about its limits, literature data agree on the existence of a miscibility gap within the pseudo binary Mg2SieMg2Sn phase diagram [18e21]. Scaling down to thin films may be advantageous because of the control over the microstructure and texture which may have a beneficial effect on transport properties of the material [22]. Furthermore, the ongoing miniaturization of TE modules for various applications [23,24] makes the synthesis of TE thin films and their integration to such modules a technological objective. The moderately elevated temperature domain for which the Mg2(Si,Sn) solid solutions were reported to have high figures of merit, imposes for power generation a geometry of the TE module in which the inplane transport properties of the thin films are exploited. Such geometries have been proposed in the 60s for monolithic miniaturized thermoelectric generators [14]. They are built up of stacks of two different thermoelectric layers (p and n), each layer separated from the next by an electrically insulating layer and interconnected in series at both ends of the device. Such monolithic devices require the thermoelectric elements and the dielectric materials to be chemically and mechanically compatible. Moreover, even if the bearing surface is lower for the interface between the thermoelectric material and the electrode, this interface needs also to be stable in temperature. The reported work allocated to the investigation as thin films of Mg2Si [25e32], Mg2Sn [33] and the ternary solid solutions [34,35] are mainly related to their synthesis. To our knowledge, only two articles report on the investigation of the TE properties of these materials as thin films. Mg2(Si,Sn) solid solutions were previously investigated as thin films for Si contents between 0.4 and 0.6 [34,35]. It was reported that the power factor of these thin films doesn't match that of bulk materials, with values lower than 3 104 WK2 m1, while the possibility for improvement remained open. In this paper, we investigate crystalline thin films of Mg2(Si,Sn) solid solutions deposited by microwave plasma-assisted co-sputtering. The chosen stoichiometry was close to that of Mg2Si0.4Sn0.6, being the most promising Mg2(Si,Sn) solid solution composition based on the TE performances that were previously reported. The films have been grown on different substrates, SiO2/Si and glass which are considered as feasible insulating layers and Ni as a potential electrode for a miniaturized TE module. In order to investigate the reactivity at the film-insulator and film-electrode interfaces in the working temperature range, the structural and chemical stability of the film as well as the electrical conductivity have been examined at intermediate temperatures, while the Seebeck coefficient was investigated at ambient temperature.
of each material with the targets positioned diametrically opposed on the target holder is performed to ensure a homogeneous deposition of each element on the substrate. This control over the sputtering process permits a fine tuning of the composition of the resulting thin films. The microstructure of the film can also be modified by controlling the flux of sputtered atoms and their energy. The former is controlled by the microwave power and the gas pressure, while the latter is determined not only by target voltage, but also by the couple of pressure-distance between the targets and the substrate. Furthermore, the temperature of the substrate can be adjusted during the deposition by use of a temperature-controlled circulating bath. The parameters used during the deposition process include a base pressure of 106 Torr, 1 mTorr of working Ar pressure, 2 kW of microwave power. The substrate, which was left at floating potential with its temperature either at 300 K or 450 K, was at a distance of 12 cm from the targets. The composition of the solid solution was controlled by fine-tuning the bias applied to each target. The chemical composition of the thin films was ascertained by EDX (energy dispersive x-ray spectroscopy) and EPMA (electron probe micro-analyzer), with an error lower than 1 wt%. The crystal structure investigations were performed using XRD (x-ray diffraction in reflection mode, l ¼ 1.54 Å or 1.79 Å, angular resolution of 0.2 ) and the morphology of the cross section was characterized by FE-SEM (field-emission scanning electron microscope, EHT ¼ 3 kV). The dependence of the thin films electrical conductivity with temperature was determined from the resistivity measurements performed using the 4-point probe method in the temperature range 300e700 K with a lab-contrived set-up (the heating and cooling rates set to 10 K/min). The measurements were done in an inert atmosphere (He) to avert the oxidation of the samples at high temperatures. The Seebeck coefficient was measured at room temperature with a non-commercial device with an estimated accuracy of 5% [37]. A temperature gradient was applied along the plane of the film, ensured by a heating cartridge, while the electrical and thermal contacts were done by mechanical pressure applied to the surface of the sample. Additionally, in order to verify the thermal stability of the thin films, prolonged thermal treatments at temperatures up to 850 K were carried out in an oven with heating and cooling rates of 5 K/min in sealed ampoules under Ar atmosphere. 3. Results and discussions 3.1. Synthesis of Mg2(Si,Sn) solid solution thin films Mg2(Si,Sn) thin films with a thickness of ~4 mm have been deposited on SiO2 (500 nm)/Si, glass and Ni substrates at room temperature (RT) and at high temperature (HT ¼ 450 K). The composition of the films was regulated by the deposition parameters that characterize the PVD process used (specifically, adjusting the bias applied to the targets). In Table 1 are shown the
2. Experimental details Mg2(Si,Sn) thin films have been deposited on SiO2/Si, glass and Ni substrates at different temperatures, using the setup described in greater detail in Refs. [33,34]. The plasma is ignited in an Ar atmosphere by 20 dipolar plasma sources circularly arranged and supplied by microwave generators with a power of 2 kW, evenly distributed to the sources through power dividers [36]. The deposition process is performed through the co-sputtering of all three types of targets (Mg, Si and Sn, with 99.99% purity) by applying a negative dc bias, separately, to each of them. The dual co-sputtering
Table 1 EPMA measurements of the composition of Mg2(Si,Sn) thin films deposited for different sets of bias applied to the targets (relative values given e VMg,Si,Sn/VSn). Mg/Si/Sn relative bias values
Composition
Si content (relative to Sn)
1.94/2.22/1.00 1.75/2.00/1.00 1.67/1.90/1.00 1.56/1.96/1.00 1.59/1.91/1.00 1.82/1.90/1.00
Mg2.04Si0.42Sn0.53 Mg2.03Si0.43Sn0.55 Mg2.00Si0.42Sn0.58 Mg1.98Si0.41Sn0.61 Mg1.99Si0.4Sn0.61 Mg2.02Si0.37Sn0.61
0.44 0.44 0.42 0.40 0.40 0.38
C. Prahoveanu et al. / Journal of Alloys and Compounds 649 (2015) 573e578
compositions determined by EPMA measurements of Mg2(Si,Sn) thin films obtained for different sets of values for the bias applied to each target. These results highlight the advantage of this deposition method in contrast with the widely used magnetron sputtering, due to a better fine tuning of the composition as outcome of the independent control over the process parameters. In the following, the samples described were deposited using the sets of bias values from Table 1, in order to grow thin films of composition close to a stoichiometric Mg2Si0.4Sn0.6. In the third column are presented the corresponding Si content values in the material with respect solely to the Sn content and the results of this work will be discussed with reference to this ratio. Based on the chosen experimental conditions (2 kW power applied to the plasma sources and 1 mTorr of gas pressure) a relatively low deposition rate of 30 nm/min was obtained. In Fig. 1 are shown the cross-sectional SEM images of two depositions performed at RT and HT. In the whole range of substrate temperature that we investigated, the as-deposited films showed no cracks, nor delamination. The film deposited at RT (Fig. 1a) has a relatively dense columnar microstructure with submicron in-plane feature sizes. By increasing the deposition temperature (450 K), it results to an even denser columnar morphology (Fig. 1b) and to highly crystalline films, due to the surplus of energy allocated to the atoms at the surface of the substrate. This translates to an enhancement of the surface diffusion and mobility of the adatoms which directly affects the resulting structure of the film. The X-ray diffraction patterns of the films deposited on SiO2/Si substrates at RT and HT are shown in Fig. 2. All the peaks are
575
Fig. 2. X-ray diffraction patterns of Mg2(Si,Sn) thin films deposited on SiO2/Si substrate at RT and HT (450 K).
accounted for, corresponding to the ternary solid solution and to the Si substrate. Within the angular resolution of the diffractometer, the RT as-deposited films appear as corresponding to a single phased Mg2(Si,Sn) solid solution with an antifluorite structure (space group Fm-3m). Depositing the films at higher temperature leads to their growth in either of two crystal structures although the deposition parameters are supposedly the same for all films. The first polymorph corresponds to the regular antifluorite structure of the solid solution. On the other hand, the second structure might be described as an hexagonal symmetry related to the P63/m space group previously reported not for solid solutions, but for Mg2Sn and Mg2Si synthesized in high pressure and high temperature conditions [38e40]. Additionally, the two polymorphs may also coexist in the same film, as it can be noticed in Fig. 2. The formation of metastable polymorph is relatively common in thin films deposited by PAPVD [41]. Considering the films with an antifluorite structure deposited at RT and HT, the two diffraction patterns testify a difference in the crystallinity of the two samples. As opposed to the film grown at RT, the film deposited at HT shows no preferential orientation, meaning that the higher deposition temperature inhibits the texturing of the film. Also, the broadening of the (111) peak with increasing deposition temperature testifies a decrease of the grain size inside the film. 3.2. Thermal behavior of thin films deposited on SiO2/Si, glass and Ni substrates
Fig. 1. SEM cross-sectional images of Mg2(Si,Sn) thin films deposited on SiO2/Si substrate at (a) RT and (b) HT (450 K).
After post-deposition thermal treatment at 700 K for 2 h, the film deposited on SiO2/Si substrate at RT suffers almost total delamination. This behavior at high temperature treatments might be put on account of the difference in the coefficients of thermal expansion (CTE) between the film and the SiO2/Si substrate. The CTE of SiO2 is widely known to be 0.5 106 K1; however the value for Mg2(Si,Sn) solid solutions has not been yet specifically determined. Nevertheless, the CTE are known for Mg2Si (11.5 106 K1 [42]) and Mg2Sn (9.9 106 K1 [42]), as bulk materials, from where it can be deduced that there is an order of magnitude separating the values of SiO2 and Mg2(Si,Sn). This CTE mismatch might generate compressive stress in the film upon annealing. The HT deposition resulted in a great amelioration of the adherence between the film and the SiO2 substrate after the annealing treatment, though the detachment of the film is still visible now at the interface between the oxide layer and Si. This
576
C. Prahoveanu et al. / Journal of Alloys and Compounds 649 (2015) 573e578
detachment does not affect the entire film and thus it was not delaminated from the substrate. The increase of the substrate temperature might have decreased either the intrinsic stresses arose during the growth of the film or the thermal stress in the subsequent thermal treatments by reducing the difference between the annealing temperature and the deposition temperature [43]. Moreover, we investigated the mechanical stability of the film at high temperatures, on substrates with higher CTE, respectively glass (dielectric, CTEglass ¼ 9 106 K1) and Ni (conductive, CTENi ¼ 13 106 K1). The film grown on glass at HT shows no adherence issues with respect to the substrate after thermal treatment TT at 700 K and it no longer fractures either. Same results have been validated for the Ni substrate. These results show that the fracturing of the film at high temperatures is not an intrinsic property, but strictly an effect of the CTE mismatch between the film and the substrate. Furthermore, according to the post-annealing structural characterization of the films, a separation of the solid solution into two cubic phases is visible for samples with both under- and overstoichiometric Mg content and with Si content between 0.40 and 0.43 (deposited either at RT or HT). In the case of the as-deposited hexagonal phase, the separation is preceded by a phase transformation (exemplified for the Ni substrate in Fig. 3). The two resulting phases have lattice parameters of 6.59 Å and, respectively, 6.71 Å, indicating the co-existence of two solid solutions with different Si content. This shows that the first TT acts as annealing, stabilizing the structure of the film. On the other hand, the phase separation confirms the metastability of the deposited films prior to the thermal treatment for the indicated compositions. However, for samples with Si content of 0.38, the film remains as single-phased after TT at 653 K as depicted in Fig. 4 (the noticed shift of the peak position between as-deposited and after TT thin films is due to the stress relaxation that follows the annealing). The results obtained on the stability and metastability of the material come in agreement with the existence of a miscibility gap in the Mg2SieMg2Sn pseudo-binary phase diagram, although the actual edges of the gap are highly disputed [18e21]. Nevertheless, the stability of the cubic phase for a Si content lower than 0.40 and the phase separation observed for higher content suggest that the former composition lies outside the miscibility gap in the phase diagram, while the latter is within. As for the reactivity, XRD analysis performed after a 40 h TT at 700 K of the Mg2(Si,Sn) film has revealed reactivity with Ni
Fig. 3. X-ray diffraction patterns of Mg2(Si,Sn) thin films deposited on Ni substrate at HT (450 K) before and after thermal treatment at 700 K for 40 h.
Fig. 4. X-ray diffraction patterns of Mg2(Si,Sn) thin films deposited on borosilicate glass at HT (450 K) with a Si content of 0.38, before and after thermal treatment at 653 K for 168 h.
substrate concurred by Bragg peaks assigned to MgNi2Sn alloy. Interdiffusion due to interfacial reactivity has also been observed by SEM on films deposited on SiO2/Si substrates heat treated for extended time. Conversely, films deposited on borosilicate glass do not show any signs of reactivity and maintain their thermal stability for extended TT of 40 h and 168 h at 653 K. The diffraction patterns of the films (for any Si content) after the aforementioned TT reveal no changes compared to the annealed sample. The SEM image appended as Fig. 5 presents the interface between the thin film and the borosilicate glass after TT 168 h at 653 K. Unfortunately EDX mapping is not conclusive to perform elemental investigation of the interface due to the spatial resolution which is at the scale of the thickness of the film. However the morphology of the interface concurs for the absence of reactivity. After some inquiries in the matter, it has been established that the highest temperature for which the Mg2(Si,Sn) thin films maintain their thermal stability is 653 K when no reaction occurs with the substrate. For higher temperature, the films suffer total decomposition into Sn, Si and Sn-rich Mg2(Si,Sn) phases, which is in agreement with recent studies on the stability of Mg2(Si,Sn) solid solutions at intermediate temperatures in bulk materials [44,45]. However, for previously mentioned thin films, the decomposition has been determined only for samples with Si content between 0.40 and 0.43. Further investigations need to be made for samples with lower Si content to validate whether they are more stable for
Fig. 5. ESB-SEM cross-sectional image of film-substrate (borosilicate glass) interface after thermal treatment at 653 K for 168 h.
C. Prahoveanu et al. / Journal of Alloys and Compounds 649 (2015) 573e578
higher temperatures due to the composition that is hypothesized to be outside the miscibility gap.
3.3. Transport properties The electrical conductivity measurement shows that after the first thermal cycle, the following cycles are reproducible and the electrical conductivity follows the same dependence with temperature both during heating and cooling (Fig. 6). The temperature dependence obtained for the electrical conductivity is specific to semiconductors and with values (104 U1 m1) comparable to those reported for bulk materials [16]. The difference noticed between the first heating and cooling cycles can be translated by the microstructure changes and stress relaxation that occur during the first annealing. Measurements of the Seebeck coefficient have been performed at ambient temperature on thermally treated Mg2(Si,Sn) thin films (single-phased and two-phased). The values obtained range between 110e170 mV/K, suggesting a p-type behavior of the film. This result is in stark contrast with most of what was reported on Mg2(Si,Sn) solid solutions as bulk materials [7], which is indicative of n-type materials. Moreover, by comparing the absolute values between the two, the Seebeck coefficient of the film seems to be much lower than for bulk materials (400e600 mV/K at RT). On account of the possible effect of Mg content on the transport properties of the material, the measurements were performed for both under-stoichiometric and over-stoichiometric thin films (with values of the Mg content between 66.3 and 67.3 at%). However, the Seebeck coefficient has remained the same in all cases, still suggestive of a p-type material. On the other hand, the results presented here are similar to those previously reported on Mg2(Si,Sn) thin films [35]. Further investigation, including microstructural characterization and theoretical modeling of the transport properties, is needed to establish how does a switch of majority carriers occurs, i.e. the change from an n-type to a p-type material. Overall, there appears to be no difference in electrical conductivity between single-phased and two-phased thin films, but a slight increase of the Seebeck coefficient (from 110 mV/K to 170 mV/ K) was observed for the single-phased film. However, this may also be attributed to the composition closer to a Si content of 0.35, for which the overlapping of the two conduction bands (corresponding to Mg2Si and Mg2Sn) was previously reported [8]. Therefore, the impact of the structure (single-phased or two-phased) on the thermoelectric properties (including thermal conductivity) still needs to be assessed.
577
4. Conclusions Thin films of Mg2(Si,Sn) solid solutions have been deposited on SiO2/Si, glass and Ni substrates at different temperatures by microwave plasma-assisted co-sputtering. The depositions resulted in films with an antifluorite and, possibly, a hexagonal crystal structure. Subsequent thermal treatments showed that the films are sensitive to the composition. If the Si content lies between 0.40 and 0.43, the films go through a structural stabilization process or phase transition, leading to the separation into two cubic phases, suggesting that the as-deposited films are grown in a metastable phase. If however, the Si content is lower, in this case 0.38, the films do not go through any phase separation, marking probably one edge of the miscibility gap in the phase diagram. The use of multiple substrates during the depositions emphasized the significance of the CTE difference between the Mg2(Si,Sn) solid solutions and the substrates over the adherence properties of the films after thermal treatments at intermediate temperatures (700 K). However, the deposition temperature also proved to be effective in the amelioration of the film mechanical stability, relieving the stress inside the films during their growth. Extended heat treatments revealed the reactivity between the Mg2(Si,Sn) solid solutions and the Ni and SiO2/Si substrates, which implies that their use in a miniaturized Mg2(Si,Sn)- based TE device would necessitate an additional diffusion barrier. Conversely, borosilicate glass shows both a high mechanical and chemical inertness up to the temperature where Mg2(Si,Sn) solid solutions show stability. The electrical properties of the films were investigated, showing values of the electrical conductivities similar to that of undoped bulk materials. The Seebeck coefficient of the films has also been determined at room temperature. As opposed to previous reports on bulk materials, a lower value was found and suggestive of a ptype behavior, but is comparable to that obtained in a previous work on thin films. The thermal stability of the presented Mg2(Si,Sn) thin films has been validated upto 653 K. For the films which suffer phase separation, this temperature marks the highest value for which they maintain stability and do not start to decompose in phases of constituent elements. However, it still rests to further inquiry whether the single-phased thin films are more stable at higher temperatures and how their transport properties (Seebeck coefficient and thermal conductivity) evolve with temperature and in comparison with two-phased thin films. Acknowledgments The authors acknowledge the help of Daniel Bourgault and de ric Gay for the Seebeck coefficient and, respectively, the Fre gion electrical conductivity measurements. L.L. acknowledges Re ^ ne-Alpes for its financial support within the framework of Rho CMIRA Explora-Pro grant 14.004457. References
Fig. 6. Electrical conductivity dependence with temperature of a Mg2(Si,Sn) thin film deposited on glass substrate at HT (450 K): first and second heatingecooling cycles.
[1] B.C. Sales, Critical overview of recent approaches to improved thermoelectric materials, Int. J. Appl. Ceram. Technol. 4 (2007) 291e296. [2] G.J. Snyder, E.S. Toberer, Complex thermoelectric materials, Nat. Mater. 7 (2008) 105e114. [3] A.V. Dmitriev, I.P. Zvyagin, Current trends in the physics of thermoelectric materials, Phys. Usp 53 (2010) 789e803. [4] T.M. Tritt, Thermoelectric phenomena, materials, and applications, in: D.R. Clarke, P. Fratzl (Eds.), Annual Review of Materials Research, Annual Reviews, 2011, pp. 433e448. Palo Alto. [5] H. Alam, S. Ramakrishna, A review on the enhancement of figure of merit from bulk to nano-thermoelectric materials, Nano Energy 2 (2013) 190e212. [6] W. Liu, Q. Zhang, X.F. Tang, H. Li, J. Sharp, Thermoelectric properties of Sbdoped Mg2Si0.3Sn0.7, J. Electron. Mater. 40 (2011) 1062e1066. [7] Q. Zhang, J. He, T.J. Zhu, S.N. Zhang, X.B. Zhao, T.M. Tritt, High figures of merit
578
[8]
[9]
[10]
[11]
[12]
[13] [14] [15]
[16]
[17]
[18]
[19] [20] [21]
[22]
[23]
[24]
[25] [26]
[27]
C. Prahoveanu et al. / Journal of Alloys and Compounds 649 (2015) 573e578 and natural nanostructures in Mg2Si0.4Sn0.6 based thermoelectric materials, Appl. Phys. Lett. 93 (2008) 102109. W. Liu, X.J. Tan, K. Yin, H.J. Liu, X.F. Tang, J. Shi, Q.J. Zhang, C. Uher, Convergence of conduction bands as a means of enhancing thermoelectric performance of n-type Mg2Si1xSnx solid solutions, Phys. Rev. Lett. 108 (2012) 166601. Y. Isoda, T. Nagai, H. Fujiu, Y. Imai, Y. Shinohara, Thermoelectric properties of Sb-doped Mg2Si0.5Sn0.5, in: ICT'06: XXV International Conference on Thermoelectrics, Proceedings, IEEE, New York, 2006, pp. 406e410. W.J. Luo, M.J. Yang, F. Chen, Q. Shen, H.Y. Jiang, L.M. Zhang, Preparation and thermoelectric properties of Bi-doped Mg2Si0.8Sn0.2 compound, Mater. Trans. 51 (2010) 288e291. W. Liu, X.F. Tang, J. Sharp, Low-temperature solid state reaction synthesis and thermoelectric properties of high-performance and low-cost Sb-doped Mg2Si0.6Sn0.4, J. Phys. D Appl. Phys. 43 (2010) 085406. J.P. Heremans, V. Jovovic, E.S. Toberer, A. Saramat, K. Kurosaki, A. Charoenphakdee, S. Yamanaka, G.J. Snyder, Enhancement of thermoelectric efficiency in PbTe by distortion of the electronic density of states, Science 321 (2008) 554e557. D.M. Rowe, CRC Thermoelectrics Handbook: Macro to Nano, 2006. W. Thorpe, Manufacture of Thermo-electric Generators, Ferranti Ltd, United States, 1969. V.K. Zaitsev, M.I. Fedorov, E.A. Gurieva, I.S. Eremin, P.P. Konstantinov, A.Y. Samunin, M.V. Vedernikov, Highly effective Mg2Si1xSnx thermoelectrics, Phys. Rev. B 74 (2006) 045207. W. Liu, X.F. Tang, H. Li, K. Yin, J. Sharp, X.Y. Zhou, C. Uher, Enhanced thermoelectric properties of n-type Mg2.16(Si0.4Sn0.6)(1y)Sby due to nano-sized Sn-rich precipitates and an optimized electron concentration, J. Mater. Chem. 22 (2012) 13653e13661. Q. Zhang, H. Yin, X.B. Zhao, J. He, X.H. Ji, T.J. Zhu, T.M. Tritt, Thermoelectric properties of n-type Mg2Si0.6ySbySn0.4 compounds, Phys. Status Solidi A 205 (2008) 1657e1661. E.N. Nikitin, E.N. Tkalenko, V.K. Zaitsev, A.I. Zaslavskii, A.K. Kuznetsov, A study of the phase diagram for the Mg2Si-Mg2Sn system and the properties of certain of its solid solutions, J. Inorg. Mater. 4 (1968) 1656e1659. S. Muntyanu, E.B. Sokolov, E.S. Makarov, Study of the Mg2SneMg2Si system, J. Inorg. Mater. 2 (1966) 870e875. € bner, R. Schmid-Fetzer, Phase formation in MgeSneSi and A. Kozlov, J. Gro MgeSneSieCa alloys, J. Alloy. Compd. 509 (2011) 3326e3337. tres nano et microstructuraux sur P. Bellanger, Etude de l'influence des parame te s thermoe lectriques des siliciures de magne sium Mg2(Si,Sn) de les proprie type-n in, Bordeaux, 2014. J.H. Sui, J. Li, J.Q. He, Y.L. Pei, D. Berardan, H.J. Wu, N. Dragoe, W. Cai, L.D. Zhao, Texturation boosts the thermoelectric performance of BiCuSeO oxyselenides, Energy Environ. Sci. 6 (2013) 2916e2920. I. Chowdhury, R. Prasher, K. Lofgreen, G. Chrysler, S. Narasimhan, R. Mahajan, D. Koester, R. Alley, R. Venkatasubramanian, On-chip cooling by superlatticebased thin-film thermoelectrics, Nat. Nanotechnol. 4 (2009) 235e238. S. van Herwaarden, Sensor applications of thermoelectric thin films, in: Proceedings ICT '97. XVI International Conference on Thermoelectrics, 1997, pp. 47e55. I. Kogut, M.C. Record, Magnesium silicide thin film formation by reactive diffusion, Thin Solid Films 522 (2012) 149e158. A. Vantomme, G. Langouche, J.E. Mahan, J.P. Becker, Growth mechanism and optical properties of semiconducting Mg2Si thin films, Microelectron. Eng. 50 (2000) 237e242. M.T. Hosseinnejad, M. Ghoranneviss, G.R. Etaati, F. Shahgoli, Fabrication of
[28]
[29]
[30]
[31]
[32]
[33]
[34]
[35]
[36]
[37]
[38]
[39] [40]
[41]
[42] [43] [44]
[45]
magnesium silicide thin films by pulsed ion beam ablation in a 1.6 kJ plasma focus device, Vacuum 94 (2013) 57e63. Q.Q. Xiao, Q.A. Xie, K.J. Zhao, Z.Q. Yu, Effect of annealing atmosphere on the Mg2Si film growth deposited by magnetron sputtering, in: X. Yi, L. Mi (Eds.), Materials and Manufacturing Technology, Pts 1 and 2, Trans Tech Publications Ltd, Stafa-Zurich, 2010, pp. 290e294. J.E. Mahan, A. Vantomme, G. Langouche, J.P. Becker, Semiconducting Mg2Si thin films prepared by molecular-beam epitaxy, Phys. Rev. B 54 (1996) 16965e16971. S.W. Song, K. Striebel, E. Cairns, Electrochemical studies of the Mg2Si thin films prepared with pulsed laser deposition, in: G.A. Nazri, E. Takeuchi, R. Koetz, B. Scrosati (Eds.), Batteries and Supercapacitors, Electrochemical Society Inc, Pennington, 2003, pp. 262e267. T. Serikawa, M. Henmi, K. Kondoh, Microstructure and Mg concentration of Mg-Si thin film deposited by ion beam sputtering on glass substrate, J. Vac. Sci. Technol. A 22 (2004) 1971e1974. H. Yu, Q. Xie, Q. Chen, Effects of annealing on the formation of Mg2Si film prepared by resistive thermal evaporation method, J. Mater. Sci. Mater. Electron. 24 (2013) 3768e3775. s, T.T. Vinh, D. Fruchart, N. Skryabina, H. Le-Quoc, A. Lacoste, E.K. Hlil, A. Be Thin films of thermoelectric compound Mg2Sn deposited by co-sputtering assisted by multi-dipolar microwave plasma, J. Alloy. Compd. 509 (2011) 9906e9911. chu, A. Be s, D. Bourgault, D. Fruchart, Deposition of H. Le-Quoc, A. Lacoste, S. Be thin films of Mg2Si1xSnx solid solution by plasma-assisted co-sputtering, J. Alloy. Compd. 538 (2012) 73e78. chu, S. Populoh, A. Weidenkaff, A. Lacoste, Thermoelectric H. Le-Quoc, S. Be properties of thin films of Sb doped Mg2Si1xSnx solid solutions, J. Alloy. Compd. 546 (2013) 138e144. D. Vempaire, S. Miraglia, A. Sulpice, L. Ortega, E.K. Hlil, D. Fruchart, J. Pelletier, Plasma-based ion implantation: a valuable industrial route for the elaboration of innovative materials, Surf. Coat. Technol. 186 (2004) 245e247. D. Bourgault, C.G. Garampon, N. Caillault, L. Carbone, J.A. Aymami, Thermoelectric properties of n-type Bi2Te2.7Se0.3 and p-type Bi0.5Sb1.5Te3 thin films deposited by direct current magnetron sputtering, Thin Solid Films 516 (2008) 8579e8583. G.A. Roberts, E.J. Cairns, J.A. Reimer, An electrochemical and XRD study of lithium insertion into mechanically alloyed magnesium stannide, J. Electrochem. Soc. 150 (2003) A912eA916. G. Urretavizcaya, G.O. Meyer, Metastable hexagonal Mg2Sn obtained by mechanical alloying, J. Alloy. Compd. 339 (2002) 211e215. J. Hao, B. Zou, P.W. Zhu, C.X. Gao, Y.W. Li, D. Liu, K. Wang, W.W. Lei, Q.L. Cui, G.T. Zou, In situ X-ray observation of phase transitions in Mg2Si under high pressure, Solid State Commun. 149 (2009) 689e692. chu, A. Be s, L. Laversenne, MgH2 thin H. Le-Quoc, A. Lacoste, S. Miraglia, S. Be films deposited by one-step reactive plasma sputtering, Int. J. Hydrogen Energy 39 (2014) 17718e17725. L. Berger, CRC Handbook of Chemistry and Physics, 87th ed., 2006. K. Holmberg, A. Matthews, Coatings Tribology: Properties, Mechanisms, Techniques and Applications in Surface Engineering, Elsevier Science, 2009. J. Bourgeois, J. Tobola, B. Wiendlocha, L. Chaput, P. Zwolenski, D. Berthebaud, F. Gascoin, Q. Recour, H. Scherrer, Study of electron, phonon and crystal stability versus thermoelectric properties in Mg2X (X ¼ Si, Sn) compounds and their alloys, Funct. Mater. Lett. 06 (2013) 1340005. M. Sondergaard, M. Christensen, K.A. Borup, H. Yin, B.B. Iversen, Thermal stability and thermoelectric properties of Mg2Si0.4Sn0.6 and Mg2Si0.6Sn0.4, J. Mater. Sci. 48 (2013) 2002e2008.