1. A. I. Grachev, I. V. Pleshakov, A. P. Paugurt, and S. G. Shul’man, Fiz. Tverd. Tela (St. Petersburg) 38, 2922 (1996) [Phys. Solid State 38, 1598 (1996)].
2. A. I. Grachev and I. V. Pleshakov, Solid State Commun. 101, 507 (1996).
3. A. I. Grachev, V. Yu. Davydov, P. M. Karataev, S. F. Karmanenko, A. P. Paugurt, and I. V. Pleshakov, Physica C 288, 268 (1997).
4. H. S. Kwok, J. P. Cheng, and S. Y. Dong, Phys. Rev. B 43, 6270 (1991).
5. T. S. Moss, G. J. Burrell, and B. Ellis, Semiconductor Opto-Electronics (Butterworths, London, 1973).