1. W.H. Koschel, U. Kaufmann and S.G. Bishop, Solid State Commun.,21, 1069 (1977).
2. S. Fung, R.J. Nicholas and R.A. Stradling, J. Phys. C: Solid State Phys.,12, 5145 (1979).
3. See, for example, the relevant publications inSemi-Insulating III-V Materials, ed. by Rees (Shiva Pub-lications Limited, UK, 1980).
4. J.K. Rhee and P.K. Bhattacharya, J. Appl. Phys., 53 (to be published June 1982).
5. G.W. Iseler,Proceedings of the Seventh Inter-national Symposium on Gallium Arsenide and Related Compounds, St. Louis, ed. by C.M. Wolfe (The Insti-tute of Physics, London, 1979), p. 144.