Petro M. Bukivskij

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.

Study of the photoluminescence and photoelectric properties of Pb1−XCdXI2 alloys

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.

Optical, photoelectric, and photorefractive properties of Ti-doped CdTe crystals

The photorefractive characteristics of Ti-doped CdTe semi-insulating crystals were measured. Additional optical and photoelectric measurement confirmed that titanium has advantages over other dopants and that this material has better characteristics for potential applications. The high optical holographic gain coefficient, Γ≈0.60 cm−1, low background absorption, kb≈0.2 cm−1, high optical quality and homogeneity, and almost monopolar (electronic) photoconductivity show that these materials can be effectively used for both optical and photoelectric applications in the near infrared region. Studies of the optical absorption and photodiffusion current made it possible to determine the nature and energy structure of impurity and intrinsic defects as well as to establish their role in the photorefractive effect. It is shown that the excited impurity 4T1(F) state is in resonance with the conduction band. As a result, autoionization of electrons to the conduction band under laser excitation takes place. An energy...

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.

Elaboration of new uncooled detector materials highly sensitive in the near-IR region

The studies of the optical and photoelectric properties of Cd1-xHgxTe:V (x≤0.02, Nv = 1019 cm-3) were carried out. The investigated semiinsulating (ρ = 108 - 109 Ωxcm) crystals were grwn by the vertical Bridgman technique. All obtained samples had n-type conductivity. The measurements of absorption, photoluminescence and photodiffusion spectra allowed us to obtain the information about the impurity centers 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 the impurity centers are in the two- and three-ionized states. In the case of V2+ ions excited 4T1(F)- and 4A2(F)-states for Cd1-xHgxTe:V (x =0.018) crystal is in resonance with the conduction band. It was found that for these crystals the photogeneration of electrons from impurity levels are determined both by direct photoionization and autoionization of electrons from excited states to the conduction band. It was found that the photosensitivity region for Cd1-xHgxTe:V crystals is protracted up to 1800 nm. The dynamic of electronic processes with the participation of impurity and intrinsic defects were investigated using a time-resolved photoelectric spectroscopy. It was shown that the electric processes, which determine the photosensitivity region of this crystal is high speed and corresponds to the subnanosecond region.

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.

Carrier transport and trapping process in photorefractive CdTe:V

Time-resolved photoelectric spectroscopy measurements of photorefractive CdT:V crystals were carried out by using a short light pulse with 9 ns duration from a nitrogen laser 337.1 nm. The light pulse was focused through the semitransparency Au-electrode. The stationary monochromatic illumination of crystals allowed to measure the time-resolved photocurrent, which is caused by the detrapping of electrons photogenerated by the pulse laser excitation. The dependence of intensity of pulse photocurrent at the delay time (formula available in paper), which corresponds to its maximum value, on the energy of additional monochromatic illumination was investigated. In the case, the spectral dependence of pulse photocurrent caused by the detrapping process of electrons in CdTe:V crystals has been measured under the different intensity of the electric field. It was shown that the additional illumination at (formula available in paper)leads to the increasing of photocurrent intensity that is caused by the detrapping processes of electrons from impurity centers and intrinsic defects. Obtained results indicate that CdTe:V crystals are high-sensitive ultrafast photorefractive materials which may be also used for the elaboration of fast photodetectors in the near IR-region.