HYDROSTATIC-PRESSURE EFFECTS ON THE EXCITONS AND EXCITON-PHONON INTERACTION IN BII3 CRYSTALS

Optical absorption and photoluminescence due to excitons in BiI3 crystals have been measured under hydrostatic pressures up to 1.5 GPa at 77 K and 300 K. The direct exciton absorption band shows a red shift under pressure with a coefficient of -1.0 X 10(2) meV/GPa. The low energy tail of this band obeys the Urbach rule under all pressures. The steepness coefficient sigma in the Urbach rule increases from 1.03 to 1.22 as pressure increases from atmospheric pressure to 1 GPa. The intensity of a photoluminescence from a self-trapped exciton state diminishes with increasing pressure, which is complemented by strengthening of the band edge photoluminescence. The behavior of the photoluminescence intensities and the steepness coefficient under pressure is discussed in terms of the instability of the self-trapped exciton state due to the change in the exciton-phonon interaction.