Yu. P. Gnatenko

Photoluminescence of high optical quality CdSe thin films deposited by close-spaced vacuum sublimation

Polycrystalline CdSe thin films (d1⁄40.1–3.0 μm) have been deposited on a glass substrate by means of the close-spaced vacuum sublimation technique. X-ray diffraction measurements have shown that the films obtained at Тs4473 K have only wurtzite phase. The influence of deposition conditions, in particular, the substrate temperature on the photoluminescence (PL) of CdSe films spectra was investigated. This let us study the effect of glass substrate on their optical quality as well as determine the nature and energy structure of the intrinsic defects and residual impurities in the films. The presence in PL spectrum of the most intense sharp donor bound exciton DX-line for CdSe films obtained at Ts1⁄4873 K indicates the n-type conductivity and their high optical quality. Intensive PL bands in the spectral range 1.65–1.74 eV were also observed, which are associated with the recombination of donor–acceptor pairs with the participation of the shallow donor and acceptor centers caused by Na(Li) residual impurities. As a result of the study of the PL spectra of CdSe films the optimal temperature conditions of their growth were determined, namely, the substrate temperature Ts1⁄4 873 K and the evaporator temperature Te1⁄4973 K. & 2013 Elsevier B.V. All rights reserved.

Optical and photoelectric spectroscopy of photorefractive Sn2P2S6 crystals

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.

Optical and photoelectric properties of vanadium-doped Cd1-xHgxTe crystals

Vanadium-doped semi-insulating Cd1−xHgxTe (x < 0.05) n-type crystals were grown by the Bridgman technique for the first time. Studies were carried out of the low-temperature optical and photoelectric properties of Cd1−xHgxTe (x = 0.014), which provided information on the deep anisotropic impurity centres and intrinsic defects. The nature and the position of their energy levels with respect to the crystal energy band were determined. It was shown that for the investigated crystals there are two photogeneration mechanisms of electrons from deep impurity levels: the auto-ionization of electrons from the discrete state, which is in resonance with the conduction band, and their direct photoionization. It was found that the photosensitivity region for Cd0.986Hg0.014Te:V crystals is about 1.3 μ m.

Spectroscopic study of V doped Hg0.018Cd0.981Mn0.001Te bulk crystals as near-infrared materials for optical applications

It was shown that the photosensitivity region of the semi-insulating V doped Hg0.018Cd0.981Mn0.001Te crystals extends up to 1800 nm and is caused by the electron photogeneration resulting in the transitions from the main T41(F) state to the excited A42(F)- and T41(P) states of V2+ ions, which are in resonance with the conduction band. The electronic processes in the crystals are fast and correspond to the nanosecond region. Codoping by the Mn atoms leads to considerable improvement their optical quality and increase the resistivity. The obtained results indicate that such crystals can be considered as promising near-infrared materials for various optical applications.

Nature and energy structure of impurity and intrinsic defects in V-doped Cd1−xHgxTe

The nature and the energy structure of impurity and intrinsic defects, including the formation of anisotropic impurity centres, are determined in V-doped semi-insulating Cd1−xHgxTe (x ≤ 0.037) crystals which were grown by the Bridgman technique for the first time. From detailed analysis of the spectral dependence of the photodiffusion current data obtained for the different directions of light propagation in crystals as well as the impurity absorption spectra, we established that for the investigated crystals anisotropic V2+ and V3+ centres are formed. Two mechanisms of the electron photogeneration from the ground impurity state to the conduction band (direct photoionization and auto-ionization from the excited state V2+ ions) are established. It is shown that the efficacy of the auto-ionization of electrons depends on the position of the excited state relative to the conduction band bottom. The photoinduced impurity centres are formed as a result of the illumination of the crystals with the light with energy about 1.50 eV. The nature and photoionization energy of these photoinduced centres are determined. The scheme of the impurity and intrinsic defects energy levels in the energy gap of the investigated crystals is presented.

Time-resolved photoluminescence spectroscopy of excitons in layered semiconductor PbI2 nanoclusters

We studied the dynamics of excitons excited in layered semiconductor PbI2 nanoclusters (NCLs) using time-resolved photoluminescence (TRPL) spectroscopy. TRPL spectra reveal formation of self-trapped excitons (STEs). It was shown that the formation of the STEs for larger [more than the Bohr radius of exciton in bulk PbI2 (RB = 1.9 nm)] NCLs occurs in sub-nanosecond time scale, while in the case of small NCLs with sizes about RB, it takes place in nanosecond scale. The effective energy transfer from the small to the larger semiconductor NCLs, which arises from dipole-dipole intercluster interactions, takes place in sub-nanosecond scale. We demonstrate that the STEs are stable states and they define effective radioluminescence of the investigated Pb1−XCdXI2 alloys.

Time-resolved photoluminescence spectroscopy of localized exciton magnetic polarons in Cd0.70Mn0.30Te spin glass compound

We have investigated dynamics of different localized exciton magnetic polarons (LEMPs) in Cd0.70Mn0.30Te spin glass (SG) compound below the freezing temperature Tf in the crystal regions, where various microscopic magnetic spin states (MMSSs), namely, “loose” spins, finite, and infinite clusters, are formed. It was shown that there is a broad distribution of the LEMPs lifetimes. The presence of the long-lived LEMPs is caused by the admixture of the optically active bright exciton states to the dark exciton states, i.e., the “brightening” of the dark LEMPs which exist along with the bright LEMPs. The lifetimes of the dark LEMPs correspond to hundreds of nanoseconds. It was found that the time decay of photoluminescence band intensity is approximated by the sum of two functions: a single exponential function and the Kohlrausch–Williams–Watts stretched exponential function. The stretched exponential function describes the recombination processes of the LEMPs formed in the crystal regions of the finite cluste...

Temperature and concentration dependence of the parameters of the 127I NQR spectrum of the (BiI3)1–x(PbI2)x mixed layered semiconductor

The results of investigations of the temperature coefficients of the parameters of the 127I NQR spectrum of the (BiI3)1–x(PbI2)x mixed layered semiconductor in the temperature range 77–150 K are reported. It is shown that, at low PbI2 contents, x ≤ 0.08, the temperature coefficients of the quadrupole coupling constants and asymmetry parameter for 127I nuclei remain unchanged, while at x = 0.20 and T = 77 K, the NQR spectrum undergoes significant changes. Based on the measurements performed, it is concluded that, at PbI2 contents of x ≤ 0.08, the quasi-two-dimensional pattern of the vibrational states persist.

Spectroscopic and photorefractive properties of Ti-doped CdTe crystals

Studies were made of the optical, photoelectric and photorefractive properties of CdTe:Ti crystals which provided information on the nature and energy structure of the impurity centers and intrinsic defects as well as on their photorefractive parameters.