1. D.G. Thomas and J.J. Hopfield, J. Appl. Phys. 33, 3243 (1962)
2. N.G. Basov and O.V. Bogdankevich, In Proc. 7th Int. Conf. Phys. Semicond., Paris, 1964, Dunod, Paris, p. 225 (1964)
3. J. Bille, Festkörperprobleme/Adv. Solid State Phys. 13, 111 (1973)
4. C. Klingshirn, phys. stat. sol. (b) 71, 547 (1975)
5. W. Wünstel and C. Klingshirn, Opt. Commun. 32, 269 (1980)