Undercooling Solidification Behavior of B or C Coated Bulk Si by Electromagnetic Levitating Process

Yusuke SAKUDA 1, Shuji AZUMO2 and Katsuhisa NAGAYAMA3

1Graduate Student, Shibaura Institute of Technology,Tokyo,Japan,m207024@sic.shibaura-it.ac.jp 2Graduate Student, Shibaura Institute of Technology,Tokyo,Japan,m705101@sic.shibaura-it.ac.jp

3Department of Materials Science, Facultly of Engineering, Shibaura Institute of Technology, Tokyo, Japan,nagayama@sic.shibaura-it.ac.jp

Abstract

Si was heated with an electromagnetic levitation process by 15kW high frequency power supply. Samples were coated with C or B by a vacuum deposition device. Conductivity was given to the sample by C or B coated on the surface and heating was possible. We established new levitating process that solidified bulk Si by coating B or C. The crystal growth model of Si could be classified by faceted growth at low undercooling, dendrite growth at intermediate undercooling and continuous growth at high undercooling. Furthermore, we also classified solidification behavior of Si into the three sections at the primary solidification, the recalescence and the secondary solidification by using high-speed camera. 1. Introduction

It is well known that containerless process is able to produce highly pure materials. Homogeneous nucleation can be put into practice because heterogeneous nucleation by impurities mixture from a container can be avoided. Homogeneous nucleation is able to take place high undercooling and many researchers have studied solidification behavior of Si these days (1-8). In those studies it has been reported that a surface growth configuration in Si is generally shifted from facet growth to dendrite growth, consecutive growth with increasing of undercooling (1).

Recently, it is expected spherical single crystal Si can be applied for three-dimensional device materials in the U.S.A. and Japan. Moreover, a joint experiment by Japan and Germany that used electromagnetic levitation process (TEMPUS) in International Space Station is planned. A tip-shaped element is used now as an IC in a semiconductor maker, but it is possible to write on a circuit at increases by using spherical Si instead of a conventional tip-shaped substrate. As a result, spherical Si is focused by an advantage that writes on liberally circuits in Si of small quantity. For example, spherical Si is used for a solar cell. Generation efficiency varies with an incidence angle from light of the sun with a flat board-shaped thing. But high generation efficiency keep with a spherical thing because it is not influenced by a directivity. In this way, spherical Si is introduced as advanced technology in the field of semiconductor, and it is produced using an electromagnetic levitating process and a drop tube method. However Si does not have conductive property, it cannot be levitated and be heated by the electromagnetic levitation process. So we used heating process which is conducted by radiation heat from a carbon ring and studied solidification behavior in Si. But in this case, it was difficult to

observe a levitating fusion image. Therefore, we focused on penetration depth of electromagnetic field on surface of Si the depth is very small compared with metals and showed a new levitating process. In this process, Si sample is coated a very slight amount of C or B. In this case, the sample is near to pure Si because B or C is added in only the Si surface and is not added in inside, especially. Consequently, we were aimed at establishing new levitating process which solidified Si by coating B or C and particularly analyzing relations of undercooling and crystal growth of Si by electromagnetic levitating process. 2. Experimental

Samples were used high purity bulk Si (99.999%) that was made by CZ method. A vacuum deposition device was evacuated to 10-3 Pa by diffusion pump and samples were coated with C or B by vapor deposition process. The prepared sample was placed on quartz holder. Fig. 1 shows schemat ic d iagram of electromagnetic levitation system. The electromagnetic levitation chamber was evacuated to 10-3 Pa by a turbo molecular pump and then filled with argon gas. A sample was heated by an electromagnetic levitation

Fig. 1 Schematic diagram of electromagnetic levitation

system of B or C coated bulk Si.

process with15kW high frequency power supply. Conductivity was given to the sample by C or B coated on the surface and heating was possible. Fig. 1.2 shows B surface morphology on a glass substrate was observed by atomic force microscope (AFM). In Fig. 2, B is scattered on the glass plate in the shape of a particle without forming the film. The Si melt was solidified by helium gas from the under side and solidified. Temperature change of the sample was measured with a two colors radiation pyrometer. The vertical section of the solidified samples was examined with X-ray diffractometer (XRD) using CuKα radiation at room temperature and optical microscope. The surface morphology was analyzed by scanning electron microscope (SEM).

Fig. 2 B surface morphology on a glass substrate was

observed by atomic force microscope (AFM).

3. Results and Discussion 3.1 Undercooling solidification behavior

Fig. 3 shows the cooling curves and appearance photos for B coated Si that was solidified. In all cooling curves, the B coated samples (a), (b) and (c) were heated until they exceeded melting point 1687K and cooled by helium gas jet flow at 158 K/s, 58 K/s and 56 K/s, respectively.

The undercooling ΔT of the samples (a), (b) and (c) showed 21 K, 96 K and 150 K, respectively. Fig. 4 shows the cooling curves for the solidified C coated Si samples. In all cooling curves, the C coated samples (a), (b) and (c) were heated until they exceeded melting point 1687 K and cooled by helium gas jet flow at 63 K/s, 107 K/s and 68 K/s, respectively. The undercooling ΔT of the samples (a), (b) and (c) showed 32 K, 70 K and 170 K, respectively.

The appearance diagram of the sample with low undercooling presents a luster, the surface is smooth and the luster fades a little in intermediate undercooling. On the other hand, the picture of sample with high undercooling presents a dull luster and the surface is rough. In addition, a projection appears in all samples. These results of cooling curves and appearance of B or C coated Si were almost the same as the result in pure Si. This can say the same thing in both the vertical sections and surface morphologies.

Fig. 4 The cooling curves for the solidified C coated bulk Si in a various cooling rate and undercooling.

Fig. 3 The cooling curves for the solidified B coated bulk Si in a various cooling rate and undercooling.

Therefore, it is thought that there is not the bad influence to Si in the vapor deposition source. In the current study, B or C is confirmed as effective vapor deposition thing for the Si. Addition of B has the characteristic of constructing p-type semiconductor and addition of C has the characteristic of improving physical properties of Si. Fig. 5 shows images of high-speed camera taken with a frame rate of 1,000 fps and a resolution 512×128 pixels at undercooling solidification behavior of Si. The sample indicates undercooling of ΔT=18 K, 96 K and 170 K. The images (a), 0.000 s to 2.335 s range was defined as primary solidification, 2.364 s was recalescence and 5.478 s to 10.219 s range was defined as secondary solidification. It was understood that recalescence did not almost happen in the sample at low undercooling. The faceted growth was observed in the image of 5.478 s when secondary solidification recalescence and 2.498 s to 7.858 s range was defined

as secondary solidification. The dendrite growth was observed in the image of 2.498 s when secondary happened. The images (b), 0.000 s to 2.261 s range was defined as primary solidification, 2.268 s was solidification happened. The images (c), 0.000 s to 4.409 s range was defined as primary solidification, 4.412 s was recalescence and 4.567 s to 9.800 s range was defined as secondary solidification. The continuous growth was observed in the image of 4.567 s when secondary solidification happened. It was thought that crystal grains which generated in recalescence were broken and refined in secondary solidification. Thus, there was only liquid phase in primary solidification, there were liquid and solid phase in recalescence and there was almost solid phase in secondary solidification. By the way, it was understood that all peculiar projections generated after recalescence.

Fig. 7 Optical microscope images of vertical section of Si.

(b) ΔT=70 K (a) ΔT=21 K (c) ΔT=150 K

Fig. 6 X-ray diffraction patterns of Si processed by electromagnetic levitation.

Fig. 5 Images of high-speed camera atundercooling solidification behavior of Si.

(a) ΔT=18 K (b) ΔT=96 K (c) ΔT=170 K

3.2 Vertical structure Fig. 6 shows X-ray diffraction patterns of Si processed by electromagnetic levitation. One peak (111) was obtained in Si with low undercooling (a). Whereas, many peaks were observed in Si with intermediate (b) and high undercooling (c). Those result indicate that Si with low undercooling approached a single crystal, Si with intermediate and high undercooling were poly crystal. It is thought that this approaches single crystal so that the generation of the nucleus decreased because the cooling rate was slow. Fig. 7 shows optical microscope images of vertical section of Si. Large crystal grains were observed everywhere in Si with low undercooling (a). Otherwise, refinement of crystal grains were observed at the center of Si with intermediate undercooling (b), the refinement was observed in all of Si with high undercooling (c). As the result, crystal grains gather toward the center with increasing of undercooling. It was thought that Si was solidified from the surface to the center. Furthermore, refinement in the crystal grain was observed with increasing of undercooling. The crystal structure of all projections was almost the same and did not depend on undercooling degree. Also, the projection is generated for the solidification expansion. It is thought that it approaches more single crystal if the generation of this

projection is lost. It will be necessary to improve it to the problem of the projection in the future. 3.3 Surface morphology Surface morphologies of the sample solidified were examined by scanning electron microscopy (SEM). Fig. 8 shows the result of undercooling of 21 K, 96, 150 K and 170 K. The fluent faceted growth is observed at low undercooling of 21 K. The surface of the sample had luster and the edge stood out. The dendrite growth is observed at intermediate undercooling of 96 K. In addition, the growth in this intermediate undercooling range is called the faceted dendrite growth as a middle growth style of faceted growth in the low undercooling area and the continuous growth in the high undercooling area (4). The surface of the sample had also luster and edges, but a lot of dendrites were observed in comparison with the sample of the low undercooling. Otherwise, the continuous growth was observed at high undercooling of 150 K and 170 K. The continuous growth was due to refinement of the dendrite which was made in intermediate undercooling area. As the evidence, the trace of the dendrite growth was observed in the sample at undercooling of 150 K. Refinement in a crystal grain at undercooling of 170 K is frequently seen in comparison with undercooling of in a crystal grain at undercooling

Fig. 8 The surface morphology for the solidified Si samples by scanning electron microscope.

Fig. 9 The surface morphology of coated Si and

pure Si at low undercooling. of 170 K is frequently seen in comparison with undercooling of 150 K. It is thought that continuous growth was developed by dendrites broken. Furthermore, the continuous surface morphology that generated by dendrites broken was much refined with increasing of undercooling. Projections were faceted growth at low undercooling, intermediate undercooling and high undercooling. Because projections generated after recalescence and rise to melting point, they generated at atmosphere of low undercooling and showed faceted growth. Finally, it was understood that the surface morphologies of Si were classified into three divisions by a difference of undercooling degree. In addition, it is thought that neither B nor C was

influenced to Si from the result of these surface morphology. Fig. 9 shows the surface morphology of pure Si and coated Si with the low undercooling. From this figure, it is understood both indicated faceted growth when undercooling degree is the same low undercooling. Therefore, it can be said that neither B nor C influenced Si. 4. Conclusion

We established new levitating process which

solidified bulk Si by coating B or C and obtained the following conclusion. (1) New levitating process which solidified bulk Si by coating B or C is established. (2) The surface morphology for the solidified B or C coated bulk Si shifted from faceted growth to dendrite growth, continuous growth with increasing of undercooling. (3) The vertical section for the solidified B or C coated bulk Si became refinement of a crystal grain with increasing of undercooling and the growth direction changed. (4) The undercooling solidification behavior is classified by primary solidification, recalescence and secondary solidification with high-speed camera. Acknowledgements The authors thank Y. Kasuya for help with the AFM analysis. References 1) R.P.Liu, T.Volkmann and D.M.Herlach., Acta Mater, 49,

440-444, 2000. 2) K. Nagashio, H. Okamoto, H. Ando, K. Kuribayashi and

I. Jimbo., Japanese Journal of Applied Physics, 45, 623-626, 2006.

3) T. Aoyama, K. Kuribayashi., Mater. Sci. Eng, A 304-306, 231-234, 2001

4) K. Kuribayashi, T. Aoyama., Journal of Crystal Growth, 237-239, 1840-1843, 2002

5) C. Panofen, D.M. Herlach., Mater. Sci. Eng, A 449-451, 699-703, 2007

6) K. Nagashio, K. Kuribayashi., Acta Mater, 53, 3021-3029, 2005.

7) K. Nagashio, H. Murata, K. Kuribayashi., Journal of Crystal Growth, 275, 1685-1690, 2005.

8) K. Nagashio, H. Murata, K. Kuribayashi., Acta Mater, 52, 5295-5301, 2004.