Browsing by Author "Kovalev, L. E."

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    Deep Centers Influence On Photoresponse Characteristics In High-Resistivity ZnSe
    (MRS Online Proceedings Library, Cambridge University Press, 2011) Ковальов, Леонід Євгенійович; Kovalev, L. E.; Kovalyov, L. E.
    The photocurrent peak after preliminary optical excitation with specific wavelength in high resistivity (ρ≍1012 Ω∗cm), unintentionally doped ZnSe monocrystals was observed. To study photocurrent “flash” phenomenon, the photocurrent relaxation spectra, photon-capture crosssection (PCCS) spectra and photo-electron paramagnetic resonance (photo-EPR) were investigated. Two-levels model involving the recombination of an S-center and an acceptor A has been developed to obtain the relation between recombination probability and instantaneous photocurrent value. The S-centers ionization energies (1.06eV and 1.14eV from the bottom of the conduction band) and charge carriers life times were determined. The acceptor's positions in ZnSe band gap are equal to 2.05, 2.11, 2.16 and 2.21eV from the bottom of the conduction band. The photocurrent and PCCS “flash” nature as well as complicated photo-EPR are associated with the relaxation of S-centers attributed to Fe ion in the band gap of semi-insulated ZnSe.
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    Effect Of Thermal Annealing In Bi And Zn Melts On Local Centers In Znse
    (MRS Online Proceedings Library, Cambridge University Press, 2011) Ковальов, Леонід Євгенійович; Kovalev, L. E.; Kovalyov, L. E.
    Time-resolved photoluminescence (TRPL) at T=4,2K, electron paramagnetic resonance (EPR) in the range of 4,2K–77K, and wavelength modulated transmittance (WMT) spectra for both undoped and annealed in Bi and Zn melts ZnSe single crystals have been investigated. It is found that the annealing in Bi melts leads to recovery of crystal structure. This supposition is based on the increase of fine and hype fine interaction of residual Mn2+, ions with the simultaneous decrease of the EPR signals line width and the decrease of the F-center line. Furthermore, the band with λmax=520 nm disappears, while exciton band I1 (λmax=445 nm) and edge band Q0 – DAP (λmax=460,2 nm), involving AlZn – LiZn, pairs, become more revealed. At the same time, band narrowing of the TRPL spectra was observed. The annealing in Zn melt causes the increase of the number of F-center. From WMT spectra the energy position of local centers and the dominant influence on impurities and defects of the crystal structure were investigated.
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    Kinetics of photoconductivity in ZnSe crystals upon photoexcitation of deep centers
    (Pleiades Publishing, Ltd., 1995) Ковальов, Леонід Євгенійович; Kovalev, L. E.; Kovalyov, L. E.
    It has long been known that investigation of the spectral characteristics of photoconductivity (PC) in wide-band-gap semiconductors does not provide full information on the position of photosensitive centers in the forbidden band. The broad spectrum of intrinsic and impurity PC consists of several overlapping bands. In the long-wavelength region, impurity photoeffect is superimposed over the falling-off spectrum of intrinsic PC. In addition, measurements of steady-state PC are incapable of revealing photoinsensitive recombination and trapping centers. Those levels that were not excited prior to illumination of the semiconductor may provide no contribution to PC and may appear either photoinsensitive or indistinguishable from dominant channels of recombination and trapping on other centers. Preliminary excitation may alter the state of these levels to such a degree that they will show up as a spike in the PC relaxation curve. In this paper, we report kinetic studies of the PC and the effective cross section for photon capture (CSPC) in the photosensitivity range of zinc selenide single crystals containing trace impurities.
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    Spectral memory of the photoconductivity of high-resistivity ZnSe
    (Soviet Physics--Semiconductors, 1989) Ковальов, Леонід Євгенійович; Kovalev, L. E.; Kovalyov, L. E.
    Spectral memory of photoconductivity is manifested by a change in profile of relaxation curve after preliminary excitation with light from a specific part of spectrum. Authors develop model accounting for main features of the photorelaxation curve.