(Invited) Growth of Czochralski Silicon Crystals with Ultralow Carbon Concentrations for High-Power Insulated-Gate Bipolar Transistor Devices

Introduction

   To achieve lower ON-resistances in insulated-gate bipolar transistor (IGBT) devices, silicon crystals should exhibit long carrier lifetimes. Generally, floating-zone (FZ) silicon exhibits longer carrier lifetimes than magnetic-field-induced Czochralski (MCZ) silicon, and therefore, it is used in high-power IGBT devices. However, since FZ silicon has lower productivity than MCZ silicon, development of high-quality MCZ silicon with long carrier lifetimes is crucial for supporting the growing demand for IGBT devices. To this end, we focused on reducing carbon impurities in MCZ silicon, because these impurities act as heterogeneous nucleation sites for oxygen precipitates and consequently shorten carrier lifetime.

   Carbon with concentrations lower than 10¹⁵ atoms/cm³, which is the detection limit of FT-IR, was evaluated by photoluminescence (PL) spectroscopy. Carbon can be detected in silicon crystals at lower concentrations of the order of 10¹³ atoms/cm³ by using the quantitative relationship between the carbon concentration and the PL intensity of carbon complexes generated by electron irradiation [1].

Growth of CZ silicon crystals with ultralow carbon concentrations

   During CZ growth, carbon contamination is mainly influenced by the back-diffusion of carbon monoxide (CO) into the melt; here, CO is generated by the reaction between silicon monoxide (SiO) and the graphite parts. Moreover, the starting polycrystalline silicon contains carbon impurities in concentrations of ~5×10¹⁴ atoms/cm³. Therefore, to reduce carbon concentration in CZ silicon below that in FZ silicon, CO back-diffusion and the carbon impurities originating from the starting polycrystalline silicon both need to be reduced. Fig. 1 shows the dependence of carbon concentrations in 6-inch CZ silicon on the solidified fraction [2]. During growth in the conventional hot zone, the carbon concentration in CZ silicon was higher than that in FZ silicon because of CO back-diffusion (Fig. 1a). In this study, CO back-diffusion was prevented by using a hot zone that can reduce the reaction between SiO and the graphite parts such as the heater. Because the carbon distribution in the crystal growth axis direction was similar to that predicted by the effective segregation law, it was confirmed that CO incorporation into the melt was almost prevented during the growth processes (Fig. 1b). Furthermore, upon increasing the argon (Ar) flow rate, carbon concentrations decreased gradually for solidified fractions lower than 30% (Fig. 1c). This indicates that CO evaporation from the melt dominated over CO incorporation into the melt. By promoting CO evaporation, we managed to grow 6-inch CZ silicon crystals with a carbon concentration lower than 1.0×10¹⁴ atoms/cm³.

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.

References

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

[2] Y. Nagai, S. Nakagawa, K. Kashima, J. Cryst. Growth, in press (2014).

Figure 1. Dependence of the carbon concentrations in 6-inch CZ silicon on the solidified fraction.