Crystal Structure Analysis of Spherical Silicon Using X-Ray Pole Figures

Spherical silicon can be produced directly from melted silicon all owing them to solidify into spherical shape by surface tension; several organizations have c arried out trial investigation for fabricate solar cells. Spherical silicon produced by this method are gen rally polycrystalline and solar cells fabricated from these product are strongly affecte d by the crystallinity. In this report, an X-ray pole plot analysis of crystal structure of spherical sil icon is described. We use pole figure measurement in X-ray diffraction, because distribution and number of the small crystals are directly observable. From (111) pole figure of single crystal as well as pol ycrystal silicon wafers, we found four poles for the (100) single crystal test sample wafer. In ca se of the polycrystal, the number of poles is proportional to the number of crystal grains. We have also suc ces fully analyzed the crystallinity of spherical silicon by the pole figure measureme nt. The (111) pole figure has only four poles in the case of the single crystal spherical silicon. The l imitation of sampling position is also discussed. Introduction In the 20 century, various kinds of productions have appeared by the industrialization a nd they yielded the environmental issues at the same time owing to m ass consumptions of fossil fuels. In that circumstance, solar cells have been paid great attention a s a clean energy resource because solar cells are able to generate electricity directly fr om the sun light without exhausting any poisonous gases. In 1954, almost a century after the discovery of photovoltaic effects by A.H. Becquerel, a silicon-based solar cell with a diffuse d p-n junction having a conversion efficiency of 6% was developed [1]. At present a high efficiency single-cryst al-silicon-based solar cell shows an efficiency of 24.7 % using extremely pure semiconductor-grade sil icon [2]. For the preparation of semiconductor-grade silicon, however, a high temperature process of s ingle crystalline silicon growth at 1420 ̊C is required. Moreover silicon-based solar cells requi re a relatively thick active layer of about 0.2mm because of the optical and mechanical properties of s licon. 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 spher ical silicon can be produced directly from melted silicon and has the potential for realizing high efficiency, high reliability, and Tel.No.:+81-77-561-3938 FaxNo. :+81-77-561-3938 e-mail: ro002986@se.ritsumei. ac.jp Solid State Phenomena Online: 2003-06-10 ISSN: 1662-9779, Vol. 93, pp 249-256 doi:10.4028/www.scientific. net/SSP.93. 249 © 2003 Trans Tech Publications Ltd, Switzerland 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 Ltd, www.scientific. net. (Semanticscholar.org-18/03/20,13:45:44) low cost solar cells [3] [4]. Silicon spheres produced by this method a re generally polycrystalline and solar cells fabricated from these product are strongly affec ted by the crystallinity. Therefore it is very important to clarify the relation between the crystallinit y and solar cell performance. Analysis of the crystal structure of the spherical silicon are performed by XRD (X-ray diffraction). Pole figure measurements Figure 1 (a) shows a schematic illustration of the XRD diffractometor [5]. X-ray pole figures were measured by X’pert MRD (PHILIPS) operated at 45kV and 40 mA using Cu K ( =1.5405Å) 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 2 shows the relation between the diffracted Xray 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 100 m below the surface because diffraction the intensity ratio at 100 m depth under the surface of silicon wafer is around 1. X-ray pole figure measurement was done rotating the sample holder along two axes, , keeping and 2 for a particular Bragg diffraction condition corresponding to the crystallographic plane being investigated (see Fig.1(b)). is varied from 0 to 90 ̊ and from 0 to 360 ̊ 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 3 shows the area of the X-ray irradiation seen from a detector at =14.2 for (111) pole measurement of spherical silicon. In this figure, parts colored in black area represent X-ray irradiation, whereas white color parts are H A S