1. For recent summaries see Mat. Sci. Forum 258-63 (1997); Proceedings of the Eighth International Conference on Shallow-Level Centers in Semiconductors Montpellier France 27 to 30 July 1998 [ Phys. Status Solidi (B) in press].
2. H. J. Queisser Mat. Res. Soc. Symp. Proc. in press.
3. Wetzel C., et al., Phys. Rev. Lett. 78, 3923 (1997).
4. Ramdas A. K., Rodriguez S., Rep. Progr. Phys. 44, 1297 (1981).
5. S. T. Pantelides Ed. Deep Centers in Semiconductors: A State-of-the-Art Approach (Gordon & Breach Lausanne Switzerland ed. 2 1992)