1. A. Rogalski, Infrared Phys. 28, 139 (1988).
2. A. Rogalski and J. Rutkowski, Infrared Phys. 29, 887 (1989).
3. P. Brogowski, M. Mucha, and J. Piotrowski, Phys. Status Solidi 114, K37 (1989).
4. K. A. Kosyachenko, I. M. Rarenko, O. O. Bodnaruk, et al., Sci. Bull. Chernovtsy University, Physics 40, 59 (1998).
5. L. A. Kosyachenko, I. M. Rarenko, O. A. Bodnaruk, and Sun Weiguo, Fiz. Tekh. Poluprovodn. (St. Petersburg) 33, 1438 (1999) [Semiconductors 33, 1293 (1999)].