Low-temperature photoacoustic spectra ofBiI3single crystals

The photoacoustic (PA) spectra of layer halide ${\mathrm{BiI}}_3$ has been studied by a piezoelectric transducer as a detector in the temperature region from 115 K to room temperature. Four peaks of G (1.97 eV), A (2.03 eV), B (2.17 eV), and C (2.38 eV) are observed in the PA spectra at 115 K. Peaks A and B are explained by the transition of the ground and the first excited states of the direct exciton. The onset of the transitions from the spin-orbit split valence band occurs at peak C. The decrease of the PA signal which begins at peak C is interpreted by the generation of the different pathways for the relaxation of the excited carriers. Since the intensity of the peak G varies from sample to sample and the peak has not been observed in the photoluminescence measurements, the origin of the peak G is attributed to the nonradiative transitions of the extrinsic centers. This capability to observe the nonradiative-decay processes is one of the chief advantages for the PA measurements.