Crystal structure analysis of spherical silicon using X-ray pole figure

Transcription

1 Solid State Phenomena Vol. 9 (00) pp 9-56 (00) Trans Tech Publications, Switzerland doi:0.08/ Tel.No.: FaxNo.: ro00986@se.ritsumei.ac.jp Crystal structure analysis of spherical silicon using X-ray pole figure S. Omae, C. Okamoto, H. Takakura, Y. Hamakawa and Mikio Murozono Faculty of Science and Engineering, Ritsumeikan University, -- Nojihigashi, Kusatsu, Shiga, Japan, Clean Venture, -8- Tudayamate, Hirakata, Osaka, Japan Keywords: pole figure, spherical silicon, crystal grain, crystallinity, rocking curve, Abstract. Spherical silicon can be produced directly from melted silicon allowing them to solidify into spherical shape by surface tension; several organizations have carried out trial investigation for fabricate solar cells. Spherical silicon produced by this method are generally polycrystalline and solar cells fabricated from these product are strongly affected by the crystallinity. In this report, an X-ray pole plot analysis of crystal structure of spherical silicon is described. We use pole figure measurement in X-ray diffraction, because distribution and number of the small crystals are directly observable. From () pole figure of single crystal as well as polycrystal silicon wafers, we found four poles for the (00) single crystal test sample wafer. In case of the polycrystal, the number of poles is proportional to the number of crystal grains. We have also successfully analyzed the crystallinity of spherical silicon by the pole figure measurement. The () pole figure has only four poles in the case of the single crystal spherical silicon. The limitation of sampling position is also discussed. Introduction In the 0 th century, various kinds of productions have appeared by the industrialization and they yielded the environmental issues at the same time owing to mass consumptions of fossil fuels. In that circumstance, solar cells have been paid great attention as a clean energy resource because solar cells are able to generate electricity directly from the sun light without exhausting any poisonous gases. In 95, almost a century after the discovery of photovoltaic effects by A.H.Becquerel, a silicon-based solar cell with a diffused p-n junction having a conversion efficiency of 6% was developed []. At present a high efficiency single-crystal-silicon-based solar cell shows an efficiency of.7 % using extremely pure semiconductor-grade silicon []. For the preparation of semiconductor-grade silicon, however, a high temperature process of single crystalline silicon growth at 0 C is required. Moreover silicon-based solar cells require a relatively thick active layer of about 0.mm because of the optical and mechanical properties of silicon. Furthermore, bulk silicon solar cells are made from silicon ingots which are cut into wafers for the fabrication of flat solar cells. About 50% of the ingots is lost by the cutting and polishing process. Nevertheless, the cost reduction of crystalline silicon solar cells is extremely difficult nowadays. Spherical silicon makes it possible to reduce the cost because spherical silicon can be produced directly from melted silicon and has the potential for realizing high efficiency, high reliability, and All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications, (ID: , Pennsylvania State University, University Park, USA-09/0/5,0:0:0)

2 50 Polycrystalline Semiconductors VII low cost solar cells [] []. Silicon spheres produced by this method are generally polycrystalline and solar cells fabricated from these product are strongly affected by the crystallinity. Therefore it is very important to clarify the relation between the crystallinity and solar cell performance. Analysis of the crystal structure of the spherical silicon are performed by XRD (X-ray diffraction). Pole figure measurements Figure (a) shows a schematic illustration of the XRD diffractometor [5]. X-ray pole figures were measured by X pert MRD (PHILIPS) operated at 5kV and 0 ma using Cu K ( =.505Å) radiation. In X-ray analysis, important is penetration depth of sample. Penetration depth for silicon is calculated from the absorption coefficient for Cu K in silicon. Figure shows the relation between the diffracted X- ray intensity ratio and the thickness of silicon when X-ray impinge vertical to the silicon wafer. Crystalline analysis range obtained from this study extends to about 00 m below the surface because diffraction the intensity ratio at 00 m depth under the surface of silicon wafer is around. X-ray pole figure measurement was done rotating the sample holder along two axes,, keeping and for a particular Bragg diffraction condition corresponding to the crystallographic plane being investigated (see Fig.(b)). is varied from 0 to 90 and from 0 to 60 for each value of. An X-ray pole figure is drawn as a contour map of the X-ray intensity as a function of (radial axis) and (circumferential axis). When a sample is flat, the measurement surface is the same for all and [6]. However, when the sample is spherical, the sampling area is changed by the rotation of the sample holder. Figure shows the area of the X-ray irradiation seen from a detector at =. for () pole measurement of spherical silicon. In this figure, parts colored in black area represent X-ray irradiation, whereas white color parts are A S H C B F E (a) A, B; Soller slits C; sample E; detector F; slit H; sample holder S; X-ray source Incident X-ray Sample holder (b) Fig. : (a) Schematic of X-ray diffractometor (b)rotation of sample

3 Solid State Phenomena Vol. 9 5 shaded area from X-ray irradiation. Moreover, the arrows show the rotation of. A small part of spherical silicon is irradiated and the area of X-ray irradiation is changed by rotation. The sampling area is only a part of the sample in spherical silicon pole figure measurements. Furthermore, the sampling position is changed by and rotation. X-ray diffraction ratio Si wafer thickness (µm) Fig. : Ratio of Diffraction X-ray intensity to Si (x thickness) when =90 Spherical silicon Sample holder surface Psi = 0 Psi = 0 Psi = 60 Fig. : Area of X-ray irradiation Discussion Firstly, to analyze the X-ray pole figure results, we measured three types of crystalline silicon wafer Fig. : Single crystal silicon wafer

4 5 Polycrystalline Semiconductors VII Fig. 5: Silicon wafer consisting of two crystals Fig. 6: Poly crystal silicon wafer Figure shows a pole figure of a single crystal (00) silicon wafer by setting and values corresponding to () Bragg diffraction. A single crystal is generally characterized by the observation of four poles. The angle between adjacent poles is about 7. Figure 5 is a pole figure showing () diffractions for a test piece consisting of two silicon crystals. In this measurement eight poles appeared consistently, indicating that the structure of the test piece comprises two single crystals. Figure 6 shows pole figure of polycrystal silicon wafer consisting of many crystal grains. As a conclusion, the number of poles in the pole figure is proportional to the number of crystal grains. Figure 7 shows a () pole figure of single crystal spherical silicon made from CZ silicon by polishing, while Fig. 8 shows the () pole figure of single crystal spherical silicon (produced by Ball Semiconductor) which is made by non-contact casting Fig. 7: CZ single crystal spherical silicon

5 Solid State Phenomena Vol Fig. 8: Single crystal from Ball Semiconductor As can be seen in Figs. 7 and 8, four poles appear in the pole figure. These poles satisfy the angular condition for single crystal silicon. This measurement shows that single crystal spherical silicon that is made by non-contact casting shows the same results as a single crystal silicon wafer. The pole figure does not depend on the shape of the sample when the sample is single crystal. X-ray rocking curve reflects the crystallinity of the sample. If dislocations and stress exist in the crystal, diffracted X-ray intensity becomes weak and FWHM (full width half maximum) is broadened. intensity [a.u.] FWHM: 50.(s) intensity [a.u.] FWHM: 50.(s) θ θ Fig. 9: Si wafer rocking curve Fig. 0: Si ball rocking curve Figure 9 shows the rocking curve of the single crystal silicon control wafer. The shape of the rocking curve is sharp, the half band width is 50. second. Figure 0 shows the rocking curve measured for one of the () pole (No.. in Fig. 8) of single crystal spherical silicon that is made by non-contact casting. The result shows high crystallinity because the FWHM is the same for the single crystal silicon control wafer. We have also investigated the development of production of spherical silicon by free fall of melting silicon from a nozzle at the bottom of a crucible.

6 5 Polycrystalline Semiconductors VII m (a) (b) Fig. : () pole figure (a) and SEM image (b) of our spherical silicon Figure shows a () pole figure of our spherical silicon. As can be seen in this pole figure, four poles (No.~) of high intensity are seen showing the same orientations as the single crystal control wafer in Fig.. This implies that our spherical silicon mainly consists of one large. In addition, three poles (No.5~7) where low intensity are also observed have relationship of angle. These results indicate the similar crystallinity of Fig. 5. Furthermore, rotating 60 along () axis, these three poles coincide. As a conclusion, our spherical silicon consists of two kinds of grains. Probably stacking fault exists at the boundary. Summary We have successfully analyzed the crystallinity of spherical silicon by pole figure measurements. It became clear that () pole figure has only four pole in the case of single crystal spherical silicon. But, a range of pole figure analysis for silicon by Cu K line is a range of at most 0.mm in depth from the surface and is not whole crystal analysis. In addition, we have to consider the X-ray irradiation range which varies by Psi and Phi rotation. Acknowledgment This work is partly supported by NEDO. References [] D. M. Chapin, C. S. Fuller and G. L. Pearson, J. Appl. Phys. 5, 676 (95). [] J. Zhao, A. Wang, M. Green, F. Ferrazza, Appl. Phys. Letters 7, 99 (998). [] J. D. Levine, G. B. Hotchkiss and M. D. Hammerbacher, in Proc. th IEEE Photovol. Spec. Conf. (Las Vegas, 99), 05z [] R. R. Schmit, W. J. Van Cak and E. S. Graf, in Proc. th Europ. Photovol. Solar Energy Conf, ed. By. R. Hill, W. Palz, P. Helm (H. S. Stephens & Associates, Bedford, UK, 99), p. 77. [5] B. D. Cullity, Elements of X-Ray Diffraction (Addison-Wesley Publishing, Massachusetts, 959), p. 79. [6] M. Schwartz, J. Appl. Phys. 6, 507 (955).

7 Polycrystalline Semiconductors VII 0.08/ Crystal Structure Analysis of Spherical Silicon Using X-Ray Pole Figures 0.08/ DOI References [] D. M. Chapin, C. S. Fuller and G. L. Pearson, J. Appl. Phys. 5, 676 (95). doi:0.06/.77