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Growth of Spherical Si Crystals on Porous Si3N4 Substrate that Repels Si Melt
|Hironori Itoh , Hideyuki Okamura , Chihiro Nakamura , Takashi Abe , Masaharu Nakayama , Ryuichi Komatsu|
Yamaguchi University, 2-16-1 Tokiwadai, Ube 755-8611, Japan
Almost 90% of solar cells currently on the PV market use crystalline Si as the main material . Cutting loss of grown Si ingot to wafers during the conventional manufacturing process of crystalline Si solar cells is up to 60% of the initial weight of grown Si crystals , which is considered to be one of the main obstacles of the cost reduction of solar cells. Therefore, direct growth technique of shaped Si crystals without cutting process has been remarkably investigated to fabricate low-cost Si solar cells [3-5]. We have also been investigated to develop a novel growth technique of shaped Si crystals using the Si3N4 porous ceramic substrate . The solar cells consist of spherical Si crystals are on the market recently. These spherical Si crystals have been fabricated by the drop tower method . In this method, droplets of Si melt are crystallized during free-fall within a few seconds and most of the fabricated spherical Si becomes fine multicrystalline because of the high cooling rate of Si melt. These multicrystalline spherical Si need further re-melting and re-crystallizing process at slow cooling rate to improve crystalline quality for use as solar cells . In this study, we report successful results of growth of high quality spherical Si crystals with slow cooling on the developed porous substrate.
Powders of Si3N4, SiO2 and PMMA particles (5μm in diameter) were mixed and pressed into pellet. The porous ceramic film with same composition was also prepared on AlN board by spin coating method. These substrates were degreased under air at 873-1373 K and then fired under N2 atmosphere at 1873 K. Porous structure was obtained by the thermal decomposition of mixed PMMA particles (Fig. 1). Spherical Si crystals were grown on the porous substrate in the horizontal tube furnace. Multiple pieces of semiconductor grade Si material (10 mg) was melted on the substrate at a temperature of 1753 K for 6 minutes with the heating rate set to 200 K/h and the cooling rate to 150 K/h.
Spherical Si crystals with diameters of up to 2 mm were successfully grown on the porous substrate with good reproducibility (Fig. 2a). The degree of undercooling of Si melt at crystallization of spherical Si was estimated to be less than 70 K, based on the difference between the heater temperature at melting of Si and that at crystallization of Si. Grown spherical Si crystals show metallic luster and most of them consist of single grain or only 2-3 grains with twinning planes observed from their etched cross sections (Fig. 2b). This confirmed that spherical Si crystals close to single crystals in their properties can be obtained stably by crystallizing Si on the porous substrate with a sufficiently low cooling rate without additional re-melting and re-crystallizing process of grown Si. The concentration of carbon and oxygen atoms in the grown Si crystals (C: 6 ppm, O: 5 ppm) analyzed by SIMS satisfies the specification of solar grade Si (C: 10 ppm, O: 20 ppm) . Neither sticking nor infiltration of Si against the porous substrate was observed and the contact angle of Si against porous substrate was not changed through growth of spherical Si crystals on the same substrate with multiple times. Thus, the developed porous substrate is continuously reusable for the growth of high quality spherical Si crystals.
This study is supported by Regional Innovation Cluster Program (Global Type) by Ministry of Education, Culture, Sports, Science and Technology; Yamaguchi Green Materials Cluster (2009-2013).
Figure 1. SEM micrograph of surface of the fabricated Si3N4 porous ceramic substrate. Holes formed by thermal decomposition of mixed PMMA particles were connected to each other.
Figure 2. (a) As-grown spherical Si crystals and (b) their cross sections after polishing and NaOHaq etching. They are composed of single grain or only 2-3 grains with twinning planes.
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Presentation: Poster at 17th International Conference on Crystal Growth and Epitaxy - ICCGE-17, Topical Session 5, by Hironori Itoh
See On-line Journal of 17th International Conference on Crystal Growth and Epitaxy - ICCGE-17
Submitted: 2013-03-26 12:39 Revised: 2013-07-18 13:03