COLLECTIVE PROPERTIES OF EXCITONS IN SILICON

CONSIDERABLE interest has developed recently in the properties of a system of excitons of high density. At exciton concentrations such that the distance between them becomes of the order of their radius, there must arise, in the opinion of Keldyshu1 , a collective exciton state of the liquid type. In the study of the collective properties of excitons, one of the fundamental problems is the question of the dominant character of the interaction between them. In the case when repulsion forces are dominant we could hope to find Bose condensation in the exciton-gas; for attractive forces, however, the system will condense into the liquid phase. Phenomena of the Bose-condensation typel11 could then begin to develop in this liquid phase. In the range of exciton concentrations for which the collective interactions become important, the product n1 /.la0 ~ 1 (n is the concentration and a0 the Bohr radius of the excitons) and difficulties arise in the theoretical study of an exciton system of such a density. In these conditions experimental investigations take on a special significance, since they enable us to trace the transition from the gas of excitons to their new state. The first experimental investigations of a system of excitons of high density in germanium were carried out in the workl241 , in which the occurrence of metallic properties in a system of excitons was discovered and data were given which clearly indicate the formation of an exciton condensate. In the present paper, results are given of an investigation of recombination radiation in the exciton region of the spectrum of pure and alloyed silicon at low temperatures. The purpose of the work was to study the character of the collective effects in a system of excitons at high exciton density.