1. P. Beckman, Phys. Rev. 150, 215 (1966).
2. S. W. Duckett, Phys. Rev. 166, 302 (1968).
3. S. Larach (Ed.), Photoelectronic Materials and Devices, Van Nostrand, New York (1965).
4. J. J. Scheer and J. Van Laar, Solid State Commun. 3, 189 (1965).
5. B. E. Parker, unpublished work at S.E.R.L.