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.

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.

The emission spectra of GaSe single crystals of c-modification doped with Co of 0.001 to 0.5 wtyh concentration are investigated a t different temperatures (T = 4.2 to 35 K ) using polarized light.It is shown that the wide emission band with energy 2.031 eV is due to structural defects while the other wide bands of 2.039 and 2.008 eV m e due to indirect exciton transitions with radiation of TO-phonons. The narrow emission lines correspond to the decay of direct excitons bound on different defects and to their phonon sattelites involving phonons of (14.5 & 0.5) meV. The temperature dependence of a doublet structure of the free exciton emission line, which is thought to be due to the peculiarities of the GaSe crystal band structure, namely the splitting of the valence band by inter-layer interaction is investigated.

GaSe crystals belonging to A%" semiconductor compounds have layer structure /l/. Such crystals are characterized by a strong covalent bond in the layers and a weak van der Waals bond between them. Their optical axis is directed perpendicular to the layers.According to recent calculations of the band structure of GaSe /2/, both direct and indirect transitions with formation of exciton states can occur in such crystals. The difference between their energetic positions is less than 0.1 eV. In accordance with the selection rules, the direct and indirect optical transitions are allowed for thepolarization if I 8. Taking into account the spin-orbit interaction these transitions become allowed also for the polarization 8 1 b.However, the absorption intensity in this case is by far weaker than for the other polarization which practically excludes indirect transitions at the polarization 2 I E .The optical properties of gallium selenide, largely its luminescence spectra have been investigated in many papers. The absorption of GaSe due to indirect transitions was studied elsewhere /3/. The light was directed at an angle as large as possible (up to 45' ) to the z-axis. The experimental results show that for the polarization 8 tl 8 the absorption edge is somewhat shifted towards the red. This fact was attributed to absorption due to indirect transitions.Thus, up to now no data in the literature give coiivincing evidence of the presence of indirect exciton transitions in GaSe crystals. In this paper the absorption edge is measured for the first time with polarized light propagating along the crystal layers. Such an experiment provides a direct and unambiguous information on the nature of optical transitions in the absorption edge region.shown in Fig. 1. Curves 1 and 1 correspond to the polarizations 3 1 if and 3 II 8 , respectively; its comparison shows that in the latter case the absorption edge is red-shifted. The shape of the curve becomes markedly protracted. AsThe experimental results for a GaSe crystal at different temperatures are I 1) Prospekt Nauki 115, 252650 Kiev-28, USSR.