Introduction:

  Generally, Floating Zone (FZ)-grown silicon (Si) is used in high power devices that require long carrier lifetime.  FZ Si has longer lifetime than magnetic-field-induced Czochralski (MCZ) Si, because FZ Si has little oxygen precipitate nuclei that act as the recombination center of the carrier.  However, the application of FZ Si for the mass production might not be possible, because FZ Si is difficult to make enlargement in diameter.  Therefore, in order to support the rapidly increasing volume of high power devices, we have started the development of high quality MCZ Si with a longer lifetime by reducing carbon impurities that act as heterogeneous nucleation sites for oxygen precipitates.  In general, MCZ Si has more carbon incorporation than FZ Si, because many graphite parts and a silica crucible are used in a CZ furnace.  The pathway for carbon incorporation into a crystal is schematically shown in Fig. 1¹⁾.  The main factor of carbon incorporation is the back-diffusion of carbon mono-oxide (CO) from the hot graphite parts, where CO is formed by the reaction of silicon mono-oxide (SiO) with graphite, to the melt.  Therefore, in order to grow MCZ Si with lower carbon concentration than FZ Si, it is necessary to prevent CO back-diffusion.  Furthermore, in order to achieve ultra low carbon concentration such as less than 10¹⁴ atoms/cm³, carbon impurities that originate from the poly-Si starting material must be reduced.  For achieving this, one applicable method is to evaporate carbon impurities from the Si melt.  Endo et al.²⁾ suggested that carbon impurities in the Si melt evaporate as CO in accordance with following reaction (1).

    C (diss) + O (diss) ↔ CO (gas)                              (1)

  In this study, we demonstrate how to prevent CO back-diffusion and to promote CO evaporation in order to grow MCZ Si with ultra low carbon concentration.

Fig. 1. Pathway for carbon incorporation into a crystal.

Experimental and results:

  In this study, CZ puller for 3 inches Si crystal in diameter was used.  Carbon concentration was evaluated by Photoluminescence (PL) spectroscopy with a detection limit of 10¹³ atoms/cm³³⁾.  First, the flow path of argon (Ar) gas in a CZ furnace and growth parameters (Ar gas flow rate, heater power, etc.) were adjusted to reduce CO back-diffusion, and CZ Si with lower carbon concentration than FZ Si (<10¹⁵ atoms/cm³) was grown.  Next, the effect of CO evaporation on the carbon concentration in CZ Si was investigated by increasing the Ar gas flow rate.  The results confirmed that the carbon concentration was markedly reduced to below 10¹⁴ atoms/cm³.  This indicates that the reaction (1) proceeded to the right side with a reduction in the CO concentration just above the melt surface.  On the basis of the results, it is possible to grow CZ Si with lower carbon concentration (<10¹⁴ atoms/cm³) than FZ Si by the reduction of CO back-diffusion and the promotion of CO evaporation from the melt.

References:

1) D. E. Bornside, R. A. Brown, J. Electrochem. Soc. 142 (1995) 2790

2) Y. Endo, Y. Yatsurugi, Y. Terai, T. Nozaki, J. Electrochem. Soc. 126 (1979) 1442

3) S. Nakagawa, K. Kashima, M. Tajima, Proceedings of the Forum on the Science and Technology of Silicon Materials 2010 (2010) 326

Acknowledgments:

  This work was partly supported by the New Energy and Industrial Technology Development Organization (NEDO) under the Ministry of Economy, Trade and Industry (METI) of Japan.

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