1. 2) S. Nakamura, M. Senoh, S. Nagahama, N. Iwasa, T. Yamada, T. Matsushita, Y. Sugimoto, and H. Kiyoku: Jpn. J. Appl. Phys. 36 (1997) L1059-L1061.
2. 3) S. Goto, T. Tojyo, Y. Yabuki, T. Hino, S. Ansai, H. Yamanaka, Y. Moriya, Y. Ito, Y. Hamaguchi, S. Uchida, and M. Ikeda: 28th International Symposium on Compound Semiconductors (ISCS2001) Komaba Oct. 1-4 (2001) TuA-4.
3. 4) H. Hirayama, Y. Enomoto, A. Kinoshita, A. Hirata, and Y. Aoyagi: The Tenth International Conference on Metalorganic Vapor Phase Epitaxy (ICOMVPE-X'2000) Sapporo, June 5-9 (2000) Fr-A8.
4. 5) H. Hirayama, A. Kinoshita, M. Ainoya, T. Yamabi, T. Yamanaka, A. Hirata, and Y. Aoyagi: to be published in phys. stat. sol. (a) (2001).
5. 7) H. Hirayama, A. Kinoshita, A. Hirata, and Y. Aoyagi: GaN and Related Alloys, Mater. Res. Soc. 639 (2001) G2.8.