1. G. Pensl and R. Helbig, in Festkoerperprobleme, Vol. 30 of Advances in Solid State Physics, Ed. by U. Roessler (Vieweg, Braunschweig, 1990), p.133.
2. G. A. Lomakin, Sov. Phys. Solid State 7, 600 (1965).
3. M. M. Anikin, A. A. Lebedev, A. L. Syrkin, and A. V. Suvorov, Sov. Phys. Semicond. 19, 69 (1985).
4. H. Kuwabara and S. Yamada, Phys. Status Solidi A 30, 739 (1975).
5. M. Ikeda, H. Matsunami, and T. Tanaka, Phys. Rev. B 22, 2842 (1980).