1. F. A. Kröger, Physica,7, 1 (1940).
2. C. C. Klick, Phys. Rev.,89, 174 (1953).
3. L. R. Furlong, Phys. Rev.,95, 1086 (1954).
4. K. V. Shalimova and V. N. Khirin, Abstracts of Reports to the IInd All-Union Conference on Photoelectric and Optical Phenomena in Semiconductors [in Russian], Izd. AN USSR, Kiev (1961), p. 152.
5. K. V. Shalimova, A. F. Andrushko, V. N. Khirin, and N. K. Morozova, Izv. VUZ. Fiz., No. 5, 119 (1964).