1. P. A. Ivanov and V. E. Chelnokov, Fiz. Tekh. Poluprovodn. (St. Petersburg) 29, 1921 (1995) [Semiconductors 29, 1003 (1995)].
2. F. Engelbrecht, J. Zeman, G. Wellenhofer, et al., Phys. Rev. B 56, 7348 (1997).
3. V. V. Sobolev and V. V. Nemoshkalenko, Electronic Structure of Solids in the Region of Fundamental Absorption (Naukova Dumka, Kiev, 1992), Chap. 2, p. 403.
4. Ch. Keffer, T. M. Hayes, and A. Bienenstock, Phys. Rev. Lett. 21, 1676 (1968).
5. Y. W. Tsang and M. L. Cohen, Phys. Rev. B 3, 1254 (1971).