Low-temperature studies of the absorption, photoluminescence, photodiffusion and photoconductivity spectra of Sn2P2S6 crystals were carried out in the wide spectral range 0.8–3.5 eV. The position of defect energy levels relative to the crystal energy bands has been determined. It was shown that the photoionization transitions from the valence band to the level with the energy Ev+1.35 eV are caused by the presence of the hole metastable state. In the optical and photoelectric spectra several bands were revealed with energy greater than the band gap of the crystal (Eg = 2.5 eV). It was established that these bands are caused by the optical transitions between the valence band and upper conduction bands. It was shown that the electron–hole recombination, caused by the band-to-band transitions with the participation of the upper conduction subbands, is fast and corresponds to the nanosecond region. The combined scheme of the defect energy level and the band-to-band electronic phototransitions in Sn2P2S6 crystals was constructed. A mechanism for the photorefractive effect in these crystals is proposed.

No abstract

No abstract

No abstract

The low-temperature photoluminescence (PL), photodiffusion current, photoconductivity (PC), and nuclear quadrupolar resonance spectra of the layered PbI2 single crystals and Pb1−XCdXI2 alloys were investigated. It was shown that the alloys are formed for any composition ratio and they have monopolar (hole) PC. The exciton PL band energy depends linearly on the composition ratio. It was found that the PbI2 clusters with various sizes are formed in Pb1−XCdXI2 alloys. For X≤0.50 the clusters are large [more than the Bohr radius of exciton in PbI2 (RB)] while for X=0.50 they are both large and small (smaller than RB). It was shown that these clusters are formed within the crystal layer. For Pb1−XCdXI2 alloys with X=0.70 the broad PL band at 2.1 eV is observed. This band is caused by the formation of the self-trapping excitons in PbI2 clusters. It was found that this PL band determines the emission of the alloys at T=300 K.