1. T. S. Moss, Photoconductivity in the Elements (London, Butterworth, 1952).
2. W. G. Spitzer and W. Kaiser, Phys. Rev. Lett. 1, 230 (1958).
3. W. Neft and K. Seiler, in Boron: Preparation, Properties and Applications, Ed. by G. K. Gaule (Plenum Press, New York, 1965), p. 143.
4. O. A. Golikova, M. Zhubanov, and D. L. Mirlin, Fiz. Tverd. Tela 11, 1655 (1969) [Sov. Phys. Solid State 11, 1341 (1969)].
5. R. G. Yarovaya, K. P. Tsomaya, I. N. Shklyarovsky, and D. L. Gabunia, Fiz. Tverd. Tela 16, 3522 (1974) [Sov. Phys. Solid State 16, 2291 (1974)].