Infrared Absorption by Excitons and Their Associates in Silicon

The absorption of light, Λ = 1.4 μm, has been investigated in undoped silicon at T < 30 °K under powerful laser light with hω = 1.21 eV. Under these conditions the absorption modulation pulse shows two distinct decay times with τ_c 0.4 μs and τ_ex. ≈ 5 to 7 μs, the proportion of either of the two components being strongly dependent on temperature and excitation intensity. The results are discussed in terms of the formation of a condensed exciton phase having a short lifetime of τ_c 0.4 μs. In specimens immersed in liquid He absorption oscillations induced by laser light have been observed.