1. Lawson, W.D., Nielson, S., Putley, E.M. and Young, A.S. (1959) J. Phys. Chem. Solids, 19, 325.
2. De Nobel, D. (1959) Philips Res. Rep., 14, 361;430.
3. Whitsett, C.R., Broerman, J.G. and Summers, C.J. (1981) Semicond. Semimet., 16, 53.
4. Galazka, R.R. and Kossut, J. (1980) in Narrow Gap Semiconductors: Physics and Applications, Springer, Berlin.
5. Berding, M.A., Sher, A. and Chen, A.B. (1987) J. Vac. Sci. Technol. A, 5, 3009.