1. C. Hilsum and H. D. Rees, Electron. Lett.6, 277 (1970).
2. C. Hilsum, B. Mullin, B. Prew, H. D. Rees and B. Straughan, Electron. Lett.6, 307 (1970).
3. D. Z. Colliver, C. Hilsum, J. R. Morgan, H. D. Rees and B. C. Taylor, Intern. Symp. on Gallium Arsenide and Related Compounds, Aachen 1970, (The Institute of Physics, London and Bristol) p. 140.
4. K. J. Bachmann, E. Buehler and J. L. Shay, This Journal to be published.
5. G. Wolff, P. H. Keck and J. D. Broder, Phys. Rev.94, 753 (1954).