SIMS Study of C, O and N Impurity Contamination for Multi-Crystalline Si Solar Cells
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SIMS Study of C, O and N Impurity Contamination for Multi-Crystalline Si Solar Cells Larry Wang and R. S. Hockett Evans Analytical Group, 810 Kifer Road, Sunnyvale, CA 94086, U.S.A. ABSTRACT This paper is a case study of using SIMS to quantitatively measure C, O and N impurity contamination at two sequential commercial process steps: (1) Si feedstock: 7N (modified Siemens) and 5N feedstock (UMG-Si); and (2) multi-crystalline (mc-Si) solar wafers: cut and etched, from directional solidification bricks grown from 7N and 7N/5N (80:20) feedstock. The conclusion of this study is twofold: (a) the primary opportunity to reduce C, O and N contamination in mc-Si solar cells is at the directional solidification process, and (b) the costly specification of highly pure Si feedstock is unnecessary from a C, O and N perspective if a directional solidification process is used. INTRODUCTION Oxygen, carbon and nitrogen are important elements in silicon photovoltaic (PV) technology. Oxygen Oxygen forms a BOx defect in p-type Si wafers which are used in most Si solar cells, and this defect is responsible for the initial light-induced degradation of solar cell efficiency [1]. Controlling the oxygen concentration below some prescribed level is one approach to reduce this efficiency loss. The SEMI PV wafer standard M6-0707 [2] prescribes that the oxygen concentration should be below 8 x 1017/cm3 in multi-crystalline wafers and 1 x 1018/cm3 in single crystal wafers. Oxygen can enter the silicon from a variety of paths, such as, (1) the crucible material used in Cz-pullers and in directional solidification furnaces; (2) the process of making Si feedstock bricks if it some kind of oxide liner is used in the brick formation; or (3) the process for upgrading metallurgical-grade silicon (UMG-Si) as feedstock for Si solar wafers. The measurement of oxygen in silicon at levels of 8 x 1017/cm3 implies the detection limit should be below 8 x 1016/cm3. The chosen analytical method can depend upon the form of the silicon to be tested. For example, FTIR can be used for single crystal silicon wafers according to SEMI MF1188-1107 [3]. However, this method has some special challenges for multicrystalline wafers in that there may be considerable oxygen precipitation which is not quantified by FTIR, and the backside IR reflectance is most likely not matched to the BLANK silicon wafer used in the method, thus giving a bias error the determination of the oxygen content. If the oxygen needs to be measured in feedstock bricks, chunks, granules or flakes, FTIR is not appropriate. SIMS can measure the oxygen concentration in all these forms of silicon. SIMS has been used for over 20 years to measure [O] in heavily-doped single crystal electronic-grade silicon according to SEMI MF1366-1107 [4], and a variant of this method can be used for non-wafer physical forms as well.
Carbon Carbon can form SiC-type defects including filaments in multi-crystalline PV Si. These defects can cause local shunting behavior [5] and may affect minority carrier lifeti
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