Behavior of carbon in electron beam melted mc-silicon

Vacuum 121 (2015) 207e211

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Behavior of carbon in electron beam melted mc-silicon Shiqiang Qin a, b, Dachuan Jiang a, b, Yi Tan a, b, *, Pengting Li a, b, Peng Wang a, b, Shuang Shi a, b a b

School of Materials Science and Engineering, Dalian University of Technology, Dalian 116023, China Key Laboratory for Solar Energy Photovoltaic System of Liaoning Province, Dalian 116023, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 24 July 2015 Accepted 24 August 2015 Available online 29 August 2015

The behavior of carbon in multicrystalline silicon scraps by electron beam melting was investigated in this study. It was found that the process favors nucleation of SiC on Si3N4. Furthermore, carbon tends to gather to top surface of the ingots with increasing melting time, and the reaction between oxygen and carbon favors carbon migration. The melt convection and temperature gradient caused by electron beam are employed to be the dominate reason the phenomenon occurs. The results can provide guidance in silicon recycling. © 2015 Elsevier Ltd. All rights reserved.

Keywords: Silicon Carbides Melting Solar cells Electron beam

Multi-crystalline silicon (mc-silicon) is the most commonly used materials for solar cells. As an effective way, metallurgical process is utilized to produce mc-silicon for solar cells due to its advantages such as low cost and environmentally friendly. The purpose of metallurgy method is to purify silicon from 98% to 99.9999%. Efforts have been made by researchers to reduce impurities like Fe, Al, Ca [1e3] and P, B [4,5] effectively from metallurgical-grade silicon (mg-silicon). However, carbon behavior in silicon haven't been studied enough to satisfy the development of metallurgy industry in silicon production. When carbon exceeds its solubility limit in silicon, silicon carbide will generate and precipitate. The presence of SiC will cause severe ohmic shunts in solar cells and then result in nucleation of new grains in silicon ingots [6,7]. Both carbon and SiC precipitates can cause significant deterioration of the conversion efficiency of solar cells. In metallurgy method process, carbon will distribute in certain area of silicon ingots after directional solidification, and the carbon-rich parts have to be cut off to obtain inclusion-free silicon feedstock. Tons of these kinds of silicon scraps are wasted since its high impurity content, mainly SiC and Si3N4. Carbon contamination is inevitable because of the graphite heaters used in the industry process [8]. Therefore, to understand

* Corresponding author. School of Materials Science and Engineering, Dalian University of Technology, No 2 Linggong Road, Ganjingzi District, Dalian City, Liaoning Province 116023, China. E-mail address: [email protected] (Y. Tan). http://dx.doi.org/10.1016/j.vacuum.2015.08.022 0042-207X/© 2015 Elsevier Ltd. All rights reserved.

the behavior of carbon and its precipitates in silicon ingots has important meanings to obtain high quality silicon feedstock for solar cells, even to recycle the silicon scraps which are wasted. Some studies on the characteristics of carbon segregation and SiC precipitation in metallurgical process have been conducted [9e12]. However, there has been no research about the behavior of SiC by electron beam melting (EBM) process. Since electron beam can heighten melt convection and have high temperature gradient, it will be beneficial to impurity redistribution in the melt [13]. The purpose of this study was to understand the redistribution behavior of carbon and SiC during EBM process. And then it can provide guidance in silicon scraps recycling. The samples studied in this work were silicon scraps which were cut from directional solidification silicon ingots that have high content of SiC and Si3N4 precipitates. The raw material was cleaned ultrasonically in deionized water for 30 min and then dried in an oven. The EBM procedure was carried out using a 30-keV electron beam at 300 mA in an electron beam melting furnace with a vacuum <5
102 Pa for 5 min, 10 min and 20 min, respectively. Then parts of samples were immerged in HF:HNO3 ¼ 3:1 solution to extract SiC and Si3N4. The structure of SiC and Si3N4 was characterized by X-ray diffraction (XRD). Field-emission scanning electron microscopy (FE-SEM) was used to investigate the morphology of the samples. The distribution of the precipitation was observed by EPMA. The backscatter images of the ingot edge after EBM are shown in Fig. 1. And the corresponding EPMA mapping is carried out to

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Fig. 1. Backscatter images of the ingot surface after EBM: (a1) Ingot bottom after EBM for 5 min; (a2) Ingot surface after EBM for 5 min; (b1) Top surface of the ingot after EBM for 10 min; (b2) Ingot bottom after EBM for 10 min; (c1) Top surface of the ingot after EBM for 20 min; (c2) Ingot bottom after EBM for 20 min; (d1) Top surface of the separated part of the ingot after EBM for 20 min; (d2) Bottom of the separated part of the ingot after EBM for 20 min. Macroscopic images of the ingot: (a3) 5 min's EBM ingot; (b3) 10 min's EBM ingot; (c3) 20 min's EBM ingot. (d3) The separated part of the 20 min's melting ingot.

further investigate the element distribution of the edge area of the ingot. After 5 min's melting, impurities precipitate to bottom and no impurities are found near the top surface. Then after 10 min's melting, the tadpoles-like impurities migrate towards top surface as shown in Fig. 1 (b1). After 20 min's melting, impurities tend to pin on the surface (Fig. 1 (c1)) and the ingot separates into two parts. The surface of the separated part bestrews lots of protuberance as shown in Fig. 1 (d3). After pickling in HF and HNO3 solution, undissolved substance is found. Impurities tend to gathering to surface and the separated part has grass-like morphology (Fig. 1(d1)), which indicates they gather to the top surface more and more as melting time increases. The ingot bottom is impurity free after 10 and 20 min's melting. By comparing the EPMA analysis results of the elements distribution with different melting time as shown in Fig. 2, it can be clearly seen that the main impurities which gather to the surface are carbon. The outline of carbon distribution corresponds to the backscatter morphology in Fig. 1. It can be obviously identified that carbon goes through a process of precipitation-transfer-pinninggrowing as melting time goes longer. Furthermore, oxygen gathering also observed by EPMA mapping, the oxygen-rich area is also carbon-rich area, it indicates that carbon gathering must relate with oxygen. During 5 min's melting, the temperature initially reaches the melting point of silicon. Temperature will increase during the process and then result in melt convection. But in this stage, the convection is not strong enough to take SiC to flow in the melt, since the higher density of SiC than Si, SiC particles tend to precipitate to the bottom, furthermore, the temperature of the bottom is lower than melt due to the contact of water-cooling crucible, then it can be assumed that the fluid flow extent is weaker than the other part of Si melt. When SiC particles precipitate to bottom, there is not enough impetus to transfer them and then result in carbon gathering as shown in Fig. 2 (a1). As meting time increases, the melt convection caused by electron beam will enhance due to the increasing temperature. It is reported that SiC particles are continuously distributed in the Si-melt and are mainly transported by melt convection [7]. During 10 min's melting, the increased

temperature heightens the solubility of carbon in silicon, the following chemical reaction occurs:

SiC/Si þ C

(1)

SiC particles are thus reduced and the substitutional carbon is increased in the Si-melt. It is obvious that substitutional carbon can be easier transferred than SiC particles. Research has been suggested that the rate of SiC formation is controlled by the rate of SiO formation [14]. Carbon behavior in silicon has a close relationship with oxygen. As is reported, oxygen can be removed by EBM. It contains a process of oxygen migration from Si melt upward to surface, and then evaporate to vacuum environment [15]. In present work, the gathering of carbon on the top surface can be contributed by oxygen. A schematic of the reaction between carbon and oxygen during EBM is demonstrated in Fig. 2 (e). The presence of oxygen which can be seen from Fig. 2 will migrate upward during EBM. Meanwhile, carbon will react with oxygen as the following reaction:

Si þ 2½C þ ½O ¼ SiC þ CO[

(2)

The occurrence of this reaction can be identified by the distribution consistency of carbon and oxygen from Fig. 2. Thus, the generation of CO makes carbon transfer from silicon melt to surface. Oxygen takes carbon to the surface and then evaporates to vacuum environment. The evaporated oxygen will be forced back to surface due to the electron beam bombardment which can be confirmed by oxygen gathering on surface shown in Fig. 2 (b3), (c3) and (d3). The Si-rich area under the surface is consistent with the backscatter morphology, as displayed in Fig. 2 (a2), (b2), (c2) and (d2). Thus, it can be identified that Si is left and gathering with carbon under the surface, while CO constantly transfers to surface then finally evaporates to the vacuum environment. The EPMA mapping of the ingot interior displays in Fig. 3 reveals that there has been carbon residual in the ingot. And the quantity increases as melting time is longer. This can be attributed to the presence of nitrogen in raw materials as shown in Fig. 4. The 3C-SiC and rod-shape b-Si3N4 inclusions in silicon scrap before EBM were

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Fig. 2. EPMA mapping of elements distribution of the ingot bottom and surface: (a) Ingot bottom after EBM for 5 min. (b) Ingot top surface after EBM for 10 min. (c) Ingot top surface after EBM for 20 min. (d) Ingot top surface of the separated part after EBM for 20 min. (e) The schematic of the reaction between carbon and oxygen during EBM.

Fig. 3. EPMA mapping of C and N distribution in the ingot: (a) Ingot interior after EBM for 5 min. (b) Ingot interior after EBM for 10 min. (c) Ingot interior after EBM for 20 min. (d) Ingot interior of the separated part after EBM for 20 min.

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Fig. 4. (a) SEM image of pore and Si3N4 rods in raw material. (b) SEM image of SiC particles and Si3N4 rods. (c) SEM image of SiC particles growing on Si3N4 rods. (d) SEM image of SiC particles after 5 min's melting. (e) SEM image of SiC particles after 10 min's melting. (f) SEM image of SiC cluster after 20 min's melting. (g) SEM image of SiC cluster in the separated part of the ingot after 20 min's melting. (h) XRD patterns of SiC and Si3N4 before and after EBM.

embedded in silicon, as shown in Fig. 4 (a) and (b). Pores were also observed, it indicates the presence of oxygen in raw material. No presence of other polytypes of SiC is generated after EBM process, as shown in Fig. 4 (h). Lotnyk [16] investigated the SiC particles and SiC filament-type precipitates in mc-Si, they proposed that donors like nitrogen favor the formation of cubic SiC, and the Gibbs energy of b-SiC is lower than a-SiC, but since the distinction of their Gibbs energy is small, the former reason can explain the presence of cubic SiC. Thus, it can explain the nucleation of SiC on Si3N4 in present work. Before EBM, very little SiC generates on Si3N4 (Fig. 4c). Small particles nucleate on Si3N4 after 5 min's melting (Fig. 4d). Then SiC particles aggregate to bulk after 10 min's melting (Fig. 4e). As melting time is set longer, massive SiC nucleates on Si3N4 (Fig. 4f and g). The result is consistent with EPMA mapping of carbon and nitrogen in Fig. 3. The presence of Si3N4 provides nucleation sites for SiC. It can be seen from SEM images that massive SiC can nucleate and grow on Si3N4 rods as melting time increases. Two reasons can be employed to explain SiC nucleation: After melting, the electron beam is dropped to 0 mA within 2s to obtain rapid solidification and result in higher degree of supercooling. The degree of supercooling has a relationship with the threshold radius as follows:

r0 ¼

2sLS Tm Lm DT

here, r0 is the threshold radius of nucleation, sLS is the tension of solideliquid interface, Tm is the equilibrium temperature of crystallization, Lm is the latent heat of crystallization, DT is degree of supercooling. When the radius of crystal embryo r > r0, reduction of

the free energy in system can occur, then embryo can nucleate to stable nucleus. Thus, the higher degree of supercooling can improve SiC nucleation on Si3N4. Furthermore, the temperature gradient from surface to bottom can get higher as melting time goes longer, and then crystal growth can be improved. EBM was conducted to investigate the SiC behavior in silicon. Carbon distribution presents a precipitation-transfer-pinninggrowing process with the increasing melting time. The convection of melt during EBM is proposed to be the dominated impetus of carbon transition from bottom to top surface. And the reaction between oxygen and carbon is employed to be another reason that carbon gathers to top surface, indicating by the consistency of oxygen and carbon distribution in surface area. It is found that SiC continues to nucleate and grow on Si3N4 rods as melting time goes longer, high degree of supercooling and the melt convection caused by EBM can be the main reason that the phenomenon occurs. The results can be a possible guidance to separate SiC from silicon scrap. Acknowledgments The authors gratefully acknowledge financial support from the Specialized Research Fund for the Doctoral Program of Higher Education (Grant No. 20130041110004), the Natural Science Foundation of China (Grant No. 51304033), and the Fundamental Research Funds for the Central Universities (Grant No. DUT15LAB06). References [1] J. Wu, Y. Li, W. Ma, K. Liu, K. Wei, K. Xie, B. Yang, Y. Dai, Impurities removal from metallurgical grade silicon using gas blowing refining techniques, Silicon

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