Decay dynamics of free and trapped excitons in tetragonal mercuric iodide

We present the time-resolved photoluminescence spectra of tetragonal mercuric iodide from 4.2 to 100 K. Decay of photoluminescence is dominated by transport of the exciton-polariton to sites for recombination: the surface (band 1 at about 532 nm) and the band 2 trap site (about 560 nm). The decay rate of the exciton-polariton in the bulk of the material decreases precipitously (a factor of nearly 100) over this temperature range, which we ascribe to a manifestation of polariton (radiative) decay. Inelastic scattering of exciton-polaritons from thermally populated phonons reduces the cooling rate of the exciton-polariton to energies below the knee of the exciton-polariton dispersion curve, which slows transport and thus blocks the dominant pathway for radiative recombination and reduces the decay rate. Band 2, which is present to some extent in all mercuric iodide crystals, shows a rapid rise time and two decay channels. The decay rate of the radiative channel is less than the decay rate of band 1 at low temperatures, and changes at higher temperatures to be indistinguishable from that of band 1. At higher temperatures a nonradiative channel begins to dominate; its decay rate increases strongly with temperature, which leads to quenching of all luminescence even at temperatures well below 300 K.