Optical Properties of Crystalline Gallium Telluride and Bismuth Triiodide.

Spontaneous photoluminescence (PL) and transmission measurements near the fundamental absorption edge of GaTe have been investigated under low-level laser excitation conditions in iodine transport vapour-grown high-quality GaTe crystals. The ground-state 1S triplet and singlet excitonic peaks are well resolved in optical continuous wave (cw) argon laser excited PL spectra, the triplet state lying ~1.6 meV lower in energy than the singlet state. Transmission spectra also indicate that the absorption corresponding to the n = 1 exciton is made up of two peaks, of which the higher-energy one is observed only for light polarized perpendicular to the b axis. We propose possible symmetries for the band gap edges of GaTe. We have resolved the n = 2 and n = 3 excited excitonic states under low level laser excitation conditions. We assign two weak lines at energies ~1.7757 eV and ~1.7735 eV in the cw excited PL spectra to bound excitons and a broad band at ~1.73 eV with a full width at half maximum (FWHM) of ~30 meV to donor-acceptor pair recombinations. We have measured that the radiative lifetime of free excitons is less than 100 ps at a photo -excited e-h pair density of ~10 ^{15} cm^ {-3} and crystal temperature of 8.7 K. We have also estimated radiative lifetimes for bound excitons. We find that over the range of 5 to 25 K, the temperature variation of the excitonic emission linewidth and peak energy comes mainly from exciton-acoustic phonon interactions via the deformation potential. We have found some indications for exciton condensations in GaTe at T = 6 K under higher excitation conditions. Finally, we have also studied single crystals of BiI_3, another layered material, using optical transmission measurement.