Abstract

Layered BiI₃ crystals exhibit allowed indirect exciton absorptions with unusually large transition probabilities assisted by LO-phonons. This makes us possible to observe clearly the secondary emission (SE) resonant to the exciton state.¹⁾ The SE spectra give deep informations on the dynamical relaxation processes of exciton. The spectra under resonance excitation by a tunable dye laser consist of one-phonon Raman lines, two- and multi-phonon resonant Raman lines (RRL) associated with hot-luminescence (HL) bands. The typical spectra observed in three samples are shown in Fig. 1. The R_A+C line is the two-phonon RRL due to the zone edge LO-phonons A and C. As shown in Fig.2, the Raman intensity of R_A+C shows resonant enhancement at the indirect edge $\left({\text{E}}_{\text{gx}}^{\text{i}}+\text{A}(\text{Z})\right)$ and changes sensitively depending on excitation energy ω_ℓ, lattice temperature and samples. The results can be interpreted by three kinds of damping processes; the sample dependent nonradiative decay σ_o, the scattering by LA- and LO-phonons which depends on temperature and ω_ℓ. These three parameters can be estimated quantitatively from the Raman intensity in Fig.2.²⁾ The HL part L_C due to recombination of the exciton assisted by the LO-phonon C has rather broad and structured lineshape reflecting directly the non-thermal equilibrium distribution which is constructed through the above three relaxation processes. The structures shift with ω_ℓ. The strong and relatively structureless L_C band is observed in sample a (small σ_O), in which multiple LA-phonon scattering is dominant (Fig.1(a)). As going from sample a to b and c, since the nonradiative decay is getting to be significant, the HL band becomes weak, and the Raman-like structures coming from lower order LA-phonon scattering appear clearly (Fig.1(b) and (c)). These results give new informations on interpretation of the resonant Raman and hot-luminescence spectra observed in the secondary emission accompanied by phonons.

© 1984 Optical Society of America