1. For a review, see, e.g.: C. D. Jefferies, L. V. Keldysh (eds.): Electron-Hole Droplets in Semiconductors ( North-Holland, New York 1983 )
2. H. Ehrenreich, F. Seitz, D. Turnbull (eds.): Solid State Physics, Vol. 32 Academic, New York 1977 )
3. M. Ueta, H. Kanzaki, K. Kobayashi, Y. Toyozawa, F. Hanamura: Excitonic Processes in Solids, Springer Ser. Solid-State Sci., Vol. 60 ( Springer, Berlin, Heideberg 1986 )
4. S. Nikitine: In Exeitons at High Density, ed. by H. Haken, S. Nikitine ( Springer, New York 1975 )
5. M. A. Tamor, J. R Wolfe: Phys. Rev. Lett. 44, 1703 (1980)