1. C. C. Wu, J. Tsai, and H. N. Spector,Phys. Rev. B 7, 3836 (1973).
2. V. K. Arora and S. C. Miller,Phys. Rev. B 10, 688 (1974).
3. S. P. Andreev,Zh. Eksp. Teor. Fiz. 77, 1046 (1979) [Sov. Phys. JETP 50, 526 (1979)].
4. A. Matulis and A. Chenis,Zh. Eksp. Teor. Fiz. 77, 1134 (1979) [Sov. Phys. JETP 50, 572 (1979)].
5. V. V. V'yurkov and P. V. Domnin,Fiz. Tekh. Poluprov. 13, 1951 (1979) [Sov. Phys. Semicond. 13, 1137 (1979)].