1. M. Herman, Semiconductor Superlattices [Russian translation], Mir, Moscow (1989).
2. A. S. Tager, Electron Tech., UHF Electronics, No. 9, 21 (1987).
3. G. Bastard, Phys. Rev. B24, 5993?5997 (1981).
4. M. Altarelli, Phys. Rev. B28, 842?845 (1983).
5. D. Y. K. Ko and J. C. Inkson, Semicond. Sci. Technol.3, 791?796 (1988).