1. Balslev, I.: Proc. Int. Conf. Phys. Semicond., Kyoto 1966, J. Phys. Soc. Jpn. Suppl. 21 (1966) 101.
2. Dean, P.J., Faulkner, R.A., Schönherr, E.G.: Proc. 10th Int. Conf. Phys. Semicond., Cambridge 1970, Keller, S.P., Hensel, J.C., Stern, F. (eds.), Oak Ridge: USAEC, 1970, p. 286.
3. Faulkner, R.A., Dean, P.J.: J. Lumin. 1/2 (1970) 552.
4. Onton, A., Morgan, T.N.: Phys. Rev. B 1 (1970) 2592.
5. Merz, J.L., Baldereschi, A., Sergent, A.M.: Phys. Rev. B 6 (1972) 3082.