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Anisotropy of the Seebeck coefficient in Czochralski grown p-type SiGe single crystal
Materials Science and Engineering B 119 (2005) 182–184
Anisotropy of the Seebeck coefficient in Czochralski grown p-type SiGe single crystal Zhongwei Jiang a , Weilian Zhang a,∗ , Liqin Yan a , Xinhuan Niu b a b
Semiconductor Material Institute, Hebei University of Technology, Tianjin 300130, China School of Information Engineering, Hebei University of Technology, Tianjin 300130, China
Received 16 November 2004; received in revised form 11 January 2005; accepted 13 February 2005
Abstract To discuss the possibility of improvement in thermoelectric properties of SiGe alloys, we examined anisotropy of the Seebeck coefficient of the p-type SiGe single crystals with orientation of 1 1 1 and 1 0 0 grown by Czochralski method. For measurement of the Seebeck coefficient, we developed an apparatus that is capable of measuring the Seebeck coefficient in the temperature range of 300–900 K. The Seebeck coefficient of the sample with 1 1 1 orientation was around 325–400 V/K, while that of the sample with 1 0 0 orientation was around 450–530 V/K. The difference was larger than experimental error, and appeared to attribute to the difference in crystallographic direction. © 2005 Elsevier B.V. All rights reserved. Keywords: SiGe single crystal; p-Type; Seebeck coefficient; Anisotropy
1. Introduction Silicon–germanium (Si1−x Gex or germanium–silicon Gex Si1−x where x indicates the mole fraction of germanium) alloy is a complete solid solution semiconductor with a cubic diamond-type structure. According to composition, the lattice parameters of the solid solution fellows Vegard’s Law closely from 0.566 nm (pure Ge) to 0.543 nm (pure Si), while the band gap change from 0.66 eV (Ge) to 1.12 eV (Si). Thus SiGe alloys are important for both microelectronic and optoelectronic devices and various functional materials in view of the potential for band gap and lattice parameter engineering they offer. Bulk SiGe crystals have also the application to photo-detectors, X-ray and neutron monochromators, etc. The most well-known application of SiGe alloys is as a materials for thermoelectric power generators at elevated temperature [1]. In fact, SiGe thermoelectric devices have been successfully used as a power generator with radioisotope heat source in deep space probes Voyager, Galileo, etc. Now, SiGe
∗
Corresponding author. Tel.: +86 22 26564175; fax: +86 22 26546709. E-mail address: jiangzhong [email protected] (W. Zhang).
0921-5107/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2005.02.057
has attracted increasing keen interest as a material with environmental compatibility. Usually SiGe alloys for such works were prepared by the zone leveling method or by hot pressing or sintering of powder materials. The maximum possible conversion efficiency of SiGe alloys has been discussed theoretically by some groups [1,2]. Slack and Hussain [2] have suggested that a high quality single crystal of SiGe alloys might be the most useful material for no boundary scattering effect. For such a theoretical expectation, it is necessary to use bulk single crystals of SiGe alloys with high purity and quality. Additionally, a single crystal has many advantages like as the mechanical stability, the uniformity, etc. The obstacle to conducting such a project has been the difficulty in growing bulk single crystals of SiGe alloys of a suitable size due to a wide separation of solidus and liquidus and the differences in the densities, lattice parameters and melting temperatures of the constituent elements. Recently, one of the present authors has succeeded in growing bulk crystals of SiGe alloys by Czochralski technique [3,4]. Several thermoelectric properties were measured on SiGe alloys have been reported by Abeles [5], Dismukes et al. [6] and Yonenaga et al. [7,8]. However, none of them stud-
Z. Jiang et al. / Materials Science and Engineering B 119 (2005) 182–184
183
Table 1 Crystal fundamental properties Sample number
Ge content (wt.%)
Diameter (mm)
Crystal length (mm)
Crystal orientation
Conductive type
Electrical resistivity ( cm)
1 2
5 5
60 60
170 170
1 1 1 1 0 0
p p
8.26 0.58
ied the potential anisotropies of the Seebeck coefficient in SiGe single crystal with different orientation. In this paper the Seebeck coefficient of p-type CZ-SiGe (Ge 5 wt.%) single crystals with 1 1 1 and 1 0 0 orientation are investigated at 300–900 K, and found obviously difference between them. The difference appeared to attribute to the difference in crystallographic direction.
2. Experiment procedure 2.1. Single crystal preparation p-Type SiGe single crystals with 5 wt.% Ge were grown by the Czochralski technique with 1 1 1 and 1 0 0 seed crystal. High purity materials of Ge and Si single crystals were charged together into a fused-silica crucible. Born (B) was added as a p-type dopant. The crystals were grown with a very low pulling rate during the crystal growth from head to tail is 0.8–0.1 mm/min in a flowing argon gas atmosphere. The silicon seed was rotated clockwise at constant rate of 15 rpm, and the crucible was rotated at a constant rate counter-clockwise between 3 and 5 rpm. Table 1 shows the SiGe crystal growth parameters. The samples used in the experiments were prepared from the homogeneous parts of the grown crystals. The composition and homogeneity were determined by PHILIPS XL30W/TMP SEM-energy dispersive X-ray (EDX) spectroscopy and secondary ion mass spectroscopy (SIMS). The
electrical resistivity and conductivity type were determined by the four-point probe method of ASTM-F84-84a. 2.2. Measurement Seebeck coefficient is measured in the temperature range of 300–900 K. The specimens for measurements are cut to the disk shape with 40 mm diameter and the surface were ground to 5 mm in thickness. The normal axis of the disks in single crystals was parallel along the growth direction. The Seebeck coefficient is measured by the apparatus which we designed and made. Fig. 1 shows a schematic of the apparatus. The samples were held between two copper block in a flowing argon gas atmosphere. It has two probe made of a fine type K (chromel/alumel) thermocouple to detect temperature on the copper surface. The electromotive force induced by the temperature difference of those blocks is detected by the potential difference meter. The Seebeck coefficient was estimated from the relationship between electromotive force and temperature difference (about 10 K). The measured Seebeck coefficient is not corrected by the Seebeck coefficient of the copper block, so it is relative Seebeck coefficient.
3. Results and discussion Fig. 2 shows the Seebeck coefficient of SiGe single crystals as a function of temperature. As seen in this figure, the
Fig. 1. Schematic diagram of apparatus used for Seebeck coefficient measurement.
184
Z. Jiang et al. / Materials Science and Engineering B 119 (2005) 182–184
Fig. 2. The Seebeck coefficient of SiGe single crystals with different direct plotted against the temperature.
Seebeck coefficient of sample 2 (1 0 0 orientation) against temperature decreased more sharply than sample 1 (1 1 1 orientation). It is obviously that sample 2 has higher Seebeck coefficient than sample 1. The Seebeck coefficient of sample 1 was around 325–400 V/K, while that of sample 2 was around 450–530 V/K. The difference was larger than experimental error, and appeared to attribute to the difference in crystallographic direction. Both the Si and Ge are cubic diamond-type structure. When current carriers transport along 1 1 1 orientation, they are scattering strongly by the lattice vibration of lots of atoms. The same lattice vibration effect which occur in SiGe single crystal with 1 0 0 orientation is weaker.
4. Conclusion The anisotropy of the Seebeck coefficient of the p-type SiGe single crystals with orientation of 1 1 1 and 1 0 0 grown by Czochralski method was examined in this paper. For measurement of the Seebeck coefficient, we developed an apparatus that is capable of measuring the Seebeck coefficient in the temperature range of 300–900 K. The Seebeck coefficient of the sample with 1 1 1 orientation was around 325–400 V/K, while that of the sample with 1 0 0 orientation was around 450–530 V/K. The difference was larger than experimental error, and appeared to attribute to the difference in crystallographic direction.
Acknowledgement
Fig. 3. Band structure of Si or dilute Si1−x Gex alloys.
Seebeck coefficient, positive for p-type, with the magnitude of 300–500 V/K in the whole temperature range investigated. The transportation of current carriers and phonons, accompany with the shift of energy and charges, in crystal lattice is the intrinsical reason of Seebeck effect. Gerballe and Hull measured the absolute Seebeck coefficients, S, for doped silicon [9] and germanium [10]. They discovered that there is a phonon-drag contribution as well as a pure electronic contribution to S. In the SiGe single crystals considered here the phonon mean free paths are so short that the phonon-drag contribution disappear, and only the electronic part remains. The Seebeck coefficient is observed to decrease at high temperature, which may originate in the effect of thermal excitation of carriers across the gap from the conduction band, as derived theoretically by Slack and Hussain [2]. The band structure of Si or dilute Si1−x Gex alloys [11] as shown in Fig. 3, the band minima lie in 1 0 0 orientation. So the
The authors would like to thank the Hebei Natural Science Foundation of China (No. E2004000061) for the financial support of this study.
References [1] M.N. Tripathi, C.M. Bhandari, J. Phys.: Condens. Matter 15 (2003) 5359. [2] G.A. Slack, M.S. Hussian, J. Appl. Phys. 69 (1991) 331. [3] W. Zhang, H. Zhao, H. Chen, J. Sun, E. Zhang, Res. Progr. Solid State Electr. 24 (2) (2004) 253. [4] N. Xinhuan, Z. Weilian, L. Guoqi, J. Zhongwei, J. Crystal Growth 267 (2004) 424. [5] B. Abeles, Phys. Rev. 131 (1963) 1906. [6] J.P. Dismukes, L. Ekstrom, E.F. Steigmeier, I. Kudman, D.S. Beers, J. Appl. Phys. 35 (1964) 2899. [7] I. Yonenaga, T. Goto, T.F. Tang, S. Yamaguchi, Proceedings of 18th International Conference on Thermoelectrics (ICT’99), 2000, p. 436. [8] I. Yonenaga, T. Akashi, T. Goto, J. Phys. Chem. Solid 62 (2001) 1313. [9] T.H. Geballe, G.W. Hull, Phys. Rev. 98 (1955) 940. [10] T.H. Geballe, G.W. Hull, Phys. Rev. 94 (1954) 1131. [11] J.C. Chen, Proceedings of the IEEE, vol. 80, 1992, p. 571.
Anisotropy of the Seebeck coefficient in Czochralski grown p-type SiGe single crystal Zhongwei Jiang a , Weilian Zhang a,∗ , Liqin Yan a , Xinhuan Niu b a b
Semiconductor Material Institute, Hebei University of Technology, Tianjin 300130, China School of Information Engineering, Hebei University of Technology, Tianjin 300130, China
Received 16 November 2004; received in revised form 11 January 2005; accepted 13 February 2005
Abstract To discuss the possibility of improvement in thermoelectric properties of SiGe alloys, we examined anisotropy of the Seebeck coefficient of the p-type SiGe single crystals with orientation of 1 1 1 and 1 0 0 grown by Czochralski method. For measurement of the Seebeck coefficient, we developed an apparatus that is capable of measuring the Seebeck coefficient in the temperature range of 300–900 K. The Seebeck coefficient of the sample with 1 1 1 orientation was around 325–400 V/K, while that of the sample with 1 0 0 orientation was around 450–530 V/K. The difference was larger than experimental error, and appeared to attribute to the difference in crystallographic direction. © 2005 Elsevier B.V. All rights reserved. Keywords: SiGe single crystal; p-Type; Seebeck coefficient; Anisotropy
1. Introduction Silicon–germanium (Si1−x Gex or germanium–silicon Gex Si1−x where x indicates the mole fraction of germanium) alloy is a complete solid solution semiconductor with a cubic diamond-type structure. According to composition, the lattice parameters of the solid solution fellows Vegard’s Law closely from 0.566 nm (pure Ge) to 0.543 nm (pure Si), while the band gap change from 0.66 eV (Ge) to 1.12 eV (Si). Thus SiGe alloys are important for both microelectronic and optoelectronic devices and various functional materials in view of the potential for band gap and lattice parameter engineering they offer. Bulk SiGe crystals have also the application to photo-detectors, X-ray and neutron monochromators, etc. The most well-known application of SiGe alloys is as a materials for thermoelectric power generators at elevated temperature [1]. In fact, SiGe thermoelectric devices have been successfully used as a power generator with radioisotope heat source in deep space probes Voyager, Galileo, etc. Now, SiGe
∗
Corresponding author. Tel.: +86 22 26564175; fax: +86 22 26546709. E-mail address: jiangzhong [email protected] (W. Zhang).
0921-5107/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2005.02.057
has attracted increasing keen interest as a material with environmental compatibility. Usually SiGe alloys for such works were prepared by the zone leveling method or by hot pressing or sintering of powder materials. The maximum possible conversion efficiency of SiGe alloys has been discussed theoretically by some groups [1,2]. Slack and Hussain [2] have suggested that a high quality single crystal of SiGe alloys might be the most useful material for no boundary scattering effect. For such a theoretical expectation, it is necessary to use bulk single crystals of SiGe alloys with high purity and quality. Additionally, a single crystal has many advantages like as the mechanical stability, the uniformity, etc. The obstacle to conducting such a project has been the difficulty in growing bulk single crystals of SiGe alloys of a suitable size due to a wide separation of solidus and liquidus and the differences in the densities, lattice parameters and melting temperatures of the constituent elements. Recently, one of the present authors has succeeded in growing bulk crystals of SiGe alloys by Czochralski technique [3,4]. Several thermoelectric properties were measured on SiGe alloys have been reported by Abeles [5], Dismukes et al. [6] and Yonenaga et al. [7,8]. However, none of them stud-
Z. Jiang et al. / Materials Science and Engineering B 119 (2005) 182–184
183
Table 1 Crystal fundamental properties Sample number
Ge content (wt.%)
Diameter (mm)
Crystal length (mm)
Crystal orientation
Conductive type
Electrical resistivity ( cm)
1 2
5 5
60 60
170 170
1 1 1 1 0 0
p p
8.26 0.58
ied the potential anisotropies of the Seebeck coefficient in SiGe single crystal with different orientation. In this paper the Seebeck coefficient of p-type CZ-SiGe (Ge 5 wt.%) single crystals with 1 1 1 and 1 0 0 orientation are investigated at 300–900 K, and found obviously difference between them. The difference appeared to attribute to the difference in crystallographic direction.
2. Experiment procedure 2.1. Single crystal preparation p-Type SiGe single crystals with 5 wt.% Ge were grown by the Czochralski technique with 1 1 1 and 1 0 0 seed crystal. High purity materials of Ge and Si single crystals were charged together into a fused-silica crucible. Born (B) was added as a p-type dopant. The crystals were grown with a very low pulling rate during the crystal growth from head to tail is 0.8–0.1 mm/min in a flowing argon gas atmosphere. The silicon seed was rotated clockwise at constant rate of 15 rpm, and the crucible was rotated at a constant rate counter-clockwise between 3 and 5 rpm. Table 1 shows the SiGe crystal growth parameters. The samples used in the experiments were prepared from the homogeneous parts of the grown crystals. The composition and homogeneity were determined by PHILIPS XL30W/TMP SEM-energy dispersive X-ray (EDX) spectroscopy and secondary ion mass spectroscopy (SIMS). The
electrical resistivity and conductivity type were determined by the four-point probe method of ASTM-F84-84a. 2.2. Measurement Seebeck coefficient is measured in the temperature range of 300–900 K. The specimens for measurements are cut to the disk shape with 40 mm diameter and the surface were ground to 5 mm in thickness. The normal axis of the disks in single crystals was parallel along the growth direction. The Seebeck coefficient is measured by the apparatus which we designed and made. Fig. 1 shows a schematic of the apparatus. The samples were held between two copper block in a flowing argon gas atmosphere. It has two probe made of a fine type K (chromel/alumel) thermocouple to detect temperature on the copper surface. The electromotive force induced by the temperature difference of those blocks is detected by the potential difference meter. The Seebeck coefficient was estimated from the relationship between electromotive force and temperature difference (about 10 K). The measured Seebeck coefficient is not corrected by the Seebeck coefficient of the copper block, so it is relative Seebeck coefficient.
3. Results and discussion Fig. 2 shows the Seebeck coefficient of SiGe single crystals as a function of temperature. As seen in this figure, the
Fig. 1. Schematic diagram of apparatus used for Seebeck coefficient measurement.
184
Z. Jiang et al. / Materials Science and Engineering B 119 (2005) 182–184
Fig. 2. The Seebeck coefficient of SiGe single crystals with different direct plotted against the temperature.
Seebeck coefficient of sample 2 (1 0 0 orientation) against temperature decreased more sharply than sample 1 (1 1 1 orientation). It is obviously that sample 2 has higher Seebeck coefficient than sample 1. The Seebeck coefficient of sample 1 was around 325–400 V/K, while that of sample 2 was around 450–530 V/K. The difference was larger than experimental error, and appeared to attribute to the difference in crystallographic direction. Both the Si and Ge are cubic diamond-type structure. When current carriers transport along 1 1 1 orientation, they are scattering strongly by the lattice vibration of lots of atoms. The same lattice vibration effect which occur in SiGe single crystal with 1 0 0 orientation is weaker.
4. Conclusion The anisotropy of the Seebeck coefficient of the p-type SiGe single crystals with orientation of 1 1 1 and 1 0 0 grown by Czochralski method was examined in this paper. For measurement of the Seebeck coefficient, we developed an apparatus that is capable of measuring the Seebeck coefficient in the temperature range of 300–900 K. The Seebeck coefficient of the sample with 1 1 1 orientation was around 325–400 V/K, while that of the sample with 1 0 0 orientation was around 450–530 V/K. The difference was larger than experimental error, and appeared to attribute to the difference in crystallographic direction.
Acknowledgement
Fig. 3. Band structure of Si or dilute Si1−x Gex alloys.
Seebeck coefficient, positive for p-type, with the magnitude of 300–500 V/K in the whole temperature range investigated. The transportation of current carriers and phonons, accompany with the shift of energy and charges, in crystal lattice is the intrinsical reason of Seebeck effect. Gerballe and Hull measured the absolute Seebeck coefficients, S, for doped silicon [9] and germanium [10]. They discovered that there is a phonon-drag contribution as well as a pure electronic contribution to S. In the SiGe single crystals considered here the phonon mean free paths are so short that the phonon-drag contribution disappear, and only the electronic part remains. The Seebeck coefficient is observed to decrease at high temperature, which may originate in the effect of thermal excitation of carriers across the gap from the conduction band, as derived theoretically by Slack and Hussain [2]. The band structure of Si or dilute Si1−x Gex alloys [11] as shown in Fig. 3, the band minima lie in 1 0 0 orientation. So the
The authors would like to thank the Hebei Natural Science Foundation of China (No. E2004000061) for the financial support of this study.
References [1] M.N. Tripathi, C.M. Bhandari, J. Phys.: Condens. Matter 15 (2003) 5359. [2] G.A. Slack, M.S. Hussian, J. Appl. Phys. 69 (1991) 331. [3] W. Zhang, H. Zhao, H. Chen, J. Sun, E. Zhang, Res. Progr. Solid State Electr. 24 (2) (2004) 253. [4] N. Xinhuan, Z. Weilian, L. Guoqi, J. Zhongwei, J. Crystal Growth 267 (2004) 424. [5] B. Abeles, Phys. Rev. 131 (1963) 1906. [6] J.P. Dismukes, L. Ekstrom, E.F. Steigmeier, I. Kudman, D.S. Beers, J. Appl. Phys. 35 (1964) 2899. [7] I. Yonenaga, T. Goto, T.F. Tang, S. Yamaguchi, Proceedings of 18th International Conference on Thermoelectrics (ICT’99), 2000, p. 436. [8] I. Yonenaga, T. Akashi, T. Goto, J. Phys. Chem. Solid 62 (2001) 1313. [9] T.H. Geballe, G.W. Hull, Phys. Rev. 98 (1955) 940. [10] T.H. Geballe, G.W. Hull, Phys. Rev. 94 (1954) 1131. [11] J.C. Chen, Proceedings of the IEEE, vol. 80, 1992, p. 571.