It is necessary to achieve an efficiency of more than 20 % using low cost Si wafers. To meet this demand, growth method of single crystal Si by directional solidification, so called mono cast Si, has been proposed by BP solar [1,2]. Although a lot of efforts have been made to realize this technique, the mono-cast Si ingots of high quality have not yet appeared in the market due to several problems.

The first problem is to set a seed before growth. If we grow larger ingot, we need either to place the multiple seeds at the bottom of a cast crucible or conduct the crystal growth from a small seed. We have selected the latter choice and have partly succeeded in the growth of ingots of 50 cm square. The crystal growth condition was determined according to the result of numerical calculation [3]. The mushroom shaped interface is realized during the growth.

The second problem is to avoid light elements such as C, N, and O from the ingots. The reduction of C was realized by optimizing the gas flow condition.

The third problem is to reduce the residual strain and dislocations. By controlling the crystal growth and the following cooling history, the residual strain is reduced to the level without breakage. The dislocations/lineage and twins are also reduced by optimizing the growth procedure.

Now, the device performance of the mono-Si is going to be characterized to prove the advantage of this method.

Acknowledgement

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

REFERENCES

[1] B. Wu, N. Stoddard, R. Ma, R. Clark, J. Cryst. Growth, 310 (2008) 2178.

[2] N. Stoddard, B. Wu, I. Witting, M. Wagner, Y. Park, G. Rozgonyi, R. Clark, Solid State Phenom, 131-133 (2008) 1.

[3] B. Gao, S. Nakano, H. Harada, Y. Miyamura, T. Sekiguchi, K. Kakimoto, J. Cryst. Growth, 352 (2012) 47.

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