This article appeared in a journal published by Elsevier. The attached copy is furnished to the author for internal non-commercial research and

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2 Journal of Alloys and Compounds 491 (2010) Contents lists available at ScienceDirect Journal of Alloys and Compounds journal homepage: Synthesis of amorphous Si Ge alloys using microwave energy J. Cheng a,, D. Agrawal a, Y. Zhang a,r.roy a, A.K. Santra b a Materials Research Institute, Pennsylvania State University, 129A MRL, University Park, PA 16802, United States b Halliburton Duncan Technology Center, Duncan, OK 73536, United States article info abstract Article history: Received 23 September 2008 Accepted 31 October 2009 Available online 10 November 2009 Keywords: Amorphous materials Microwave processing Thermoelectric Amorphous silicon germanium alloys with 70% Si and 30% Ge have been formed by rapid heating in a pure E-field in a single mode 2.45 GHz microwave cavity. The synthesis of amorphous Si Ge alloy by all conventional processing is difficult. This polarized microwave process provides another striking example of its unique abilities: producing bulk amorphous Si Ge alloys. The microwave fabricated amorphous Si 0.7 Ge 0.3 alloy composition exhibited relatively high Seebeck coefficient with high potential of its application in high efficient thermoelectric materials. Published by Elsevier B.V. 1. Introduction 2. Experimental The demand for alternative energy technologies to reduce our dependence on fossil fuels is becoming an ever more important regime of research, including that of high temperature energy harvesting via the direct recovery of waste heat and its conversion into useful electrical energy. Thus, there is a renewed interest in the field of thermoelectrics. Semiconductor silicon germanium (Si Ge) alloy elements are typical thermoelectric power generator for deep-space missions [1,2]. In the past, Si Ge alloys were prepared mostly in single crystal and polycrystalline forms. Recently there has been an emphasis on amorphous phases, including silicon germanium films that are attracting great attention for high performance applications in microelectronics and multi-junction solar cells [3,4]. Meanwhile, efforts have been made to produce bulk amorphous Si Ge alloys using the melt spinning method, which exhibited about ten times higher figure of merit (ZT) values than that of the conventional crystalline Si Ge bulk materials [5]. The present work is a follow up of our earlier work of the synthesis of polycrystalline Si Ge alloys in the 2.45 GHz microwave [6]. Herein, we report the use of this method for the formation of bulk amorphous Si Ge alloys in a pure microwave electric field at 2.45 GHz. The initial results have demonstrated that the microwave synthesized amorphous Si 0.7 Ge 0.3 alloy samples exhibited good thermoelectric properties. Corresponding author. Tel.: ; fax: address: jxc44@psu.edu (J. Cheng). High purity silicon powder (Alfa Aesar, % pure, particle size: 1 m) and germanium powder (Alfa Aesar, %) were used as the starting materials. The powders were mixed in a ratio of 70 mol% Si and 30 mol% Ge using agate mortar with 2 wt% of binder in ethanol. After drying, the powder mixture was uniaxially pressed into mm (3/8 in.) diameter and 5 mm high pellets at 200 MPa pressure, followed by binder removal in a tube furnace at 500 C under flowing Ar gas. The microwave heating experiments were carried out using a lab-made TE 103 single mode cavity powered by a 2.45 GHz, 2 kw microwave generator (Sairum, France) as shown in Fig. 1. The microwave power used in our experiments was kept at 1 kw. A quartz tube is inserted through the microwave cavity to hold the sample crucible (hot pressed BN) and control the operating atmosphere. The sample temperatures were measured using an infrared pyrometer (Raytek, Marathon series, temperature range C). During the experiments, at ambient pressure gas mixture of Ar/H 2 (90%/10%) was passed through the quartz tube to avoid oxidation of the sample at high temperature. X-ray diffraction (XRD) and scanning electron microscope (SEM) were used to determine the phase compositions and microstructures of the microwave processed samples. A lab-made setup was used for measurement of the Seebeck coefficient of the sample (Fig. 2). The Seebeck measurement was conducted in a temperature range from 30 to 180 C, since the sample holder was made of Teflon material that limited the operation temperature. 3. Results and discussion Classically there are various mechanisms identified in the microwave-materials interaction. Some of them are due to dipole reorientation, space and ionic charge, which are primarily found in insulators or dielectric materials. Other losses, depending upon the material under interaction, include through electric conduction and/or magnetic losses in conductive materials. Using a single mode (TE 10n ) chamber, we are able to physically separate the microwave electric field (E-field) and magnetic field (H-field) in different location. By changing the position of the sample inside the microwave /$ see front matter. Published by Elsevier B.V. doi: /j.jallcom

3 518 J. Cheng et al. / Journal of Alloys and Compounds 491 (2010) Fig. 1. The microwave single mode setup for E H heating experiments. single mode cavity, the sample can be exposed to either pure E-field or pure H-field to investigate the microwave-materials interaction with different microwave field excitation (Fig. 3). Our earlier research has demonstrated that various materials exhibited remarkably different heating behavior under different microwave fields [7]. More surprisingly, some crystalline solids such as magnetite and the most common hard magnetic phases such as barium hexaferrite can be rendered noncrystalline in a few seconds when heated in the microwave H-field [8,9]. The phase composition of the starting sample (a physical mixture of Si and Ge with a nominal compositional ratio of 70 mol% Si and 30 mol% Ge) is shown in Fig. 4 (upper green line). There was trace amount of germanium oxide (GeO 2 ) in the starting Ge powder, which formed possibly due to the exposure to atmosphere. After microwave heated in the pure H-field for 3 min at 1 kw power, the GeO 2 phase completely disappeared or leached out, and the germanium dissolved into silicon lattice forming a single-phase Si Ge alloy, all the peaks exhibited in the XRD pattern were corresponding to Si (middle blue line in Fig. 4). This observation is same as our previous work [6]. It was found that in the microwave H-field, the sample s temperature remained constant at around 900 C even when we increased the microwave power much Fig. 2. Schematic of the Seebeck coefficient measurement setup.

4 J. Cheng et al. / Journal of Alloys and Compounds 491 (2010) Fig. 3. Microwave field distribution within a TE 10n single mode cavity. The sample can be positioned in (a) the maximum electric (E) field where the magnetic field is minimum; and/or (b) the maximum magnetic (H) field where the electric field is minimum. higher than 1 kw, indicating the absorption of microwaves by the sample at this temperature got saturated or the material started behaving as bulk metal and therefore instead of absorption only microwave reflection was occurring. Further increase of microwave energy caused more reflected power and more microwave cavity wall loss (the cavity wall became much warmer at higher power). However, when we re-located the same sample from the H-field to the E-field and applied the same microwave power (1 kw), the sample could be heated up to much higher temperature ( C), and the sample s shape changed from the original cylindrical pellet to ball-like, revealing some melt during the heating in the microwave E-field. The sample became amorphous as confirmed by XRD examination (Fig. 4, bottom red line). In the next step we heated another sample (a physical mixture of Si and Ge with a nominal compositional ratio of 70 mol% Si and 30 mol% Ge) in the microwave E-field first and then in H-field to see if the amorphous alloy converts into crystalline phase based on our earlier work in which H-field had produced polycrystalline Si Ge alloy formation. After heating in the microwave E-field for 3 min at 1 kw microwave power, the sample completely transformed into an amorphous phase (XRD pattern shown in Fig. 5, upper red pattern). The SEM morphology of microwave E-field heated sample is shown in Fig. 6. Highly dense with rather flat surface was observed. There are lots of small light colored islands formed on the surface, mainly consisting of GeO 2, which was melted and floated onto the sample surface at high temperature (the melting point of GeO 2 is 1116 C). It is interesting that when we put the microwave E-field processed amorphous Si 0.7 Ge 0.3 sample in the microwave H-field, it got converted into crystalline phase of Si Ge alloy at the same microwave power (1 kw). Meanwhile, the sample exhibited orientated recrystallization as indicated by its XRD pattern showing only a strong Si (4 0 0) peak (Fig. 5, bottom blue line). Fig. 7 shows the Raman spectra of the Si Ge samples. The bottom two lines belong to the samples heated in microwave E-field. A big drop of Si first order peak at 520 cm 1 was found, indicating disorder of the Si lattice after microwave E-field heating. This confirms the XRD results that the material obtained in E-field is in fact amorphous. Germanium silicon alloys are commonly used for thermoelectric power generators at elevated temperatures. In comparison to the crystalline Si Ge alloys, amorphous Si Ge alloys show the extremely large non-dimensional figure of merit (Z) values and that is mainly caused by the amorphous phase not having a superlattice structure [5]. The figure of merit depends on interdependent material parameters by the equation, Z = 2/ (1) where is the Seebeck coefficient, is electrical conductivity and is thermal conductivity. It is challenging to optimize the figure of merit by making suitable compromise among these three properties. The use of amorphous structures had been shown to provide a promising strategy for designing materials with a large value of the figure of merit Z. The microwave synthesized amorphous Si 0.7 Ge 0.3 alloy sample was cut to a 2 mm 2mm 4 mm rectangle piece for Seebeck

5 520 J. Cheng et al. / Journal of Alloys and Compounds 491 (2010) Fig. 5. The XRD patterns of microwave E H heated Si 0.7Ge 0.3 alloy samples. Fig. 4. The XRD patterns of the Si 0.7Ge 0.3 starting sample and microwave H E heated samples. coefficient measurements (Fig. 8 ), and the results are shown in Fig. 9.The sample exhibited p-type conductivity, the measured Seebeck coefficient is higher than 200 at temperatures above 100 C. Currently, we are modifying the setup for high temperature measurement up to 700 C. Because of the present size limit, we are unable to measure the thermal conductivity of the microwave fabricated amorphous Si 0.7 Ge 0.3 alloy sample at this time. The future work will include fabrication of larch size samples to get the Z value for further development of bulk amorphous Si Ge alloys using microwave single mode heating technology. Fig. 7. Raman spectra of Si Ge samples. Fig. 6. The SEM images of Si 0.7Ge 0.3 alloy sample. (a) The starting powder mixture. (b) The sample heated in microwave E-field for 180 s.

6 J. Cheng et al. / Journal of Alloys and Compounds 491 (2010) Conclusion Bulk amorphous Si 0.7 Ge 0.3 alloy samples were successfully fabricated using a microwave heating method in a pure E-field at 2.45 GHz. The pure H-field microwave heated samples produce only crystalline single-phase alloys. The amorphous Si 0.7 Ge 0.3 alloy samples exhibited relatively high Seebeck coefficients indicating their high potential in applications as high efficient thermoelectric materials. Acknowledgements Fig. 8. Bulk amorphous Si 0.7Ge 0.3 alloy sample for the Seebeck coefficient measurement. The authors thank to Dr. Manju Rao for the Raman spectra measurement. The financial support for this research was provided by Halliburton Enegey Services, USA which is gratefully acknowledged. References [1] J.P. Dismukes, L. Ekstrom, E.F. Steigeier, J. Appl. Phys. 35 (1964) [2] B. Cronon, A. Vining, J. Appl. Phys. 69 (1991) 331. [3] E. Kasper, K. Lyertovich, Properties of Silicon germanium and Silver: Carbon, INSPEC, IEE, London, [4] K. Tanaka, M. Matsuda, Thin. Solid Films 163 (1998) 123. [5] S.M. Lee, Y. Okamoto, T. Kawahara, J. Morimoto, The fabrication and thermoelectric properties of amorphous Si Ge Au bulk samples, Mater. Res. Soc. Symp. Proc. 691 (2002) G [6] D. Dube, M. Fu, D. Agrawal, R. Roy, Rapid alloying of silicon with germanium in microwave field using single mode cavity, Mater. Res. Innov. 12 (2008) 119. [7] J. Cheng, R. Roy, D. Agrawal, Radically different effects on materials by separated microwave electric and magnetic fields, Mater. Res. Innov. 5 (2002) 170. [8] R. Roy, Y. Fang, J. Cheng, D. Agrawal, Decrystallizing solid crystalline titania, without melting, using microwave magnetic fields, J. Am. Ceram. Soc. 88 (6) (2005) [9] R. Roy, R. Peelamedu, L. Hurtt, J. Cheng, D. Agrawal, Definitive experimental evidence for microwave effects: radically new effects of separated E and H field, such as decrystallization of oxides in seconds, Mater. Res. Innov. 6 (2002) 128. Fig. 9. Seebeck coefficient of the amorphous bulk Si 0.7Ge 0.3 alloy sample.