Increasing the bulk lifetime of silicon crystals is crucial for improving insulated-gate bipolar transistor (IGBT) properties such as on-resistance. In order to achieve a longer bulk lifetime, carbon and oxygen impurities in silicon crystals should be reduced. This is because these impurities strongly affect the formation behavior of oxygen precipitates, which is a major cause of decrease in the bulk lifetime [1]. Floating-zone (FZ) silicon is mainly used for high-power devices because the oxygen concentration in FZ silicon is significantly lower than that in magnetic-field-induced Czochralski (MCZ) silicon. However, FZ silicon growth process encounters difficulties regarding mass productivity, because it is difficult to grow crystals with a diameter larger than 8 inches. In order to meet the growing demand for high-power IGBTs, high-quality MCZ silicon with large crystal diameter needs to be developed as a replacement of FZ silicon. We focused on the reduction of carbon impurities in MCZ silicon, which act as heterogeneous nucleation sites for oxygen precipitates. In our recent study, we demonstrated the growth of 8-inch-diameter MCZ silicon crystals with a carbon concentration less than 1.0E14 atoms/cm³, which is significantly lower than that in FZ silicon crystals [2]. In this study, the relationship between ultralow carbon concentration (<1.0E14 atoms/cm³) and the bulk lifetime was investigated in order to clarify the impact of carbon impurities on the bulk lifetime of MCZ silicon crystals.

Experimental

The bulk lifetime of MCZ silicon and FZ silicon crystals was measured by the direct-current photoconductive decay (PCD) method designated by ASTM F28-75[3] in order to avoid the influence of surface recombination. The carrier concentrations of these samples were approximately 1.0E12 atoms/cm³. To analyze the effect of carbon concentration on the bulk lifetime, MCZ crystals having a carbon concentration from 1.0E13 to 1.0E15 atoms/cm³, which was measured by photoluminescence (PL) spectroscopy [4], were used. Furthermore, to investigate the dependence of the oxygen concentration on the bulk lifetime, MCZ and FZ crystals with a wide range of oxygen concentration from 1.0E15 to 1.0E18 atoms/cm³were used.

Results and Discussion

The relationship between the carbon concentration and the bulk lifetime of FZ and MCZ silicon crystals is shown in Figure 1. The bulk lifetime of MCZ silicon dramatically increased with a decrease in the carbon concentration. This indicates that the nuclei formation of oxygen precipitates was suppressed during crystal growth with reduced carbon concentration. Furthermore, when the carbon concentration was less than 1.0E14 atoms/cm³, the dependence of the oxygen concentration on the bulk lifetime was negligible. Therefore, we demonstrated that MCZ silicon crystal with a carbon concentration of less than 1.0E14 atoms/cm³ exhibits a longer bulk lifetime than FZ silicon crystal.

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. Kishino, Y. Matsushita, M. Kanamori and I. Iizuka, Jpn. J. Appl. Phys. 21 (1982) 1-12.
2. Y. Nagai, K. Kashima, S. Nakagawa and M. Higasa, Solid State Phenomena 242 (2015) 3-9.
3. ASTM Designation: F28-75(Reapproved 1981) 195.
4. S. Nakagawa, K. Kashima, M. Tajima, Proceedings of the Forum on the Science and Technology of Silicon Materials 2010 (2010) 326.