1. Turjanica, I. D., Chepur, D. V.: Ukr. Fiz. Zh. 12 (1967) 500.
2. Czaja, W., Harbeke, G., Krausbauer, L., Meier, E., Curtis, B. J., Brunner, H.: Solid State Commun. 13 (1973) 1445.
3. Kolosyuk, V. N., Vashchenko, V. I.: Opt. Spectrosc. (USSR) 34 (1973) 236.
4. Shekhmametev, R. I.: Phys. Status Solidi (b) 66 (1974) 371.
5. Kolosyuk, V. N., Vashchenko, V. I., Berezdetskii, T. I.: Opt. Spectrosc. (USSR) 39 (1975) 335.