Impurity Cu+ centers in LiF single crystals

LiF single crystals with copper impurity (0.0004–0.002%) have been grown by the Czochralski method and investigated. The luminescence, excitation, and optical absorption spectra have been recorded. The luminescence spectrum contains a band at 450 nm upon 250-nm excitation. This band is attributed to Cu⁺ centers in the samples grown. The mechanisms of capture and recombination during thermoluminescence are considered.     

Pulsed photo- and cathodoluminescence of LiF crystals doped with tungsten oxide

We have studied the spectral and kinetic characteristics of activated photo- and cathodoluminescence of LiF-O and LiF-WO₃ crystals in the spectral range of 3.6–1.6 eV using methods of pulsed spectrometry with nanosecond time resolution in the temperature range of 15–300 K and in the range of ionizing radiation dose absorbed by crystals of 10²−2 × 10³ Gy.     

Pulsed cathodoluminescence of irradiated LiF-O and LiF(U)-O crystals

The spectral kinetic characteristics of the luminescence excited in previously irradiated (≤10⁶ Gy) LiF-O and LiF(U)-O crystals have been investigated in the spectral range 4–1.8 eV by pulsed spectrometry with nanosecond time resolution. The luminescence of the crystals was excited by nanosecond nitrogen-laser or electron-beam pulses at 300 K. The inertial character of uranium luminescence enhancement and the dependence of the number of enhancement stages on the excitation technique are revealed. A difference in the character of the dependences of the intensities of the pulse photo-and cathodoluminescence of uranium on the irradiation dose is found.     

Intraband radioluminescence of LiF crystals

Spectra of high-energy electrons are calculated in the electron-hole ionization-passive region of lithium fluoride crystals for the conditions of intense irradiation by short pulses of accelerated electrons. The yield of intraband radioluminescence of these crystals is estimated in comparison with the yield of this kind of luminescence of more extensively studied NaCl crystals. The calculations demonstrate that the yield of radioluminescence determined by electron transitions in the conduction band of LiF crystals is two orders of magnitude weaker than the yield of analogous luminescence of NaCl crystals. This is explained, first, by special features of the energy band structure and, second, by the form of the energy dependence of the density of states in the conduction band of LiF crystals. The yield of hole-type intraband radioluminescence is estimated for various assumed relations between the widths of the valence and the forbidden bands.     

Low-voltage cathodoluminescence of ZnS single crystals

Bright blue and green cathodoluminescence from low resistivity ZnS crystals has been observed under the excitation of low-energy electron beams of several tens of volts; i.e., 40 fL at 50 V. Properties of the surface of the crystals are studied by the dependence of current and brightness on applied voltage and by the spectra of cathodoluminescence and photoluminescence.     

Low-voltage red cathodoluminescence of ZnSe single crystals

Red cathodoluminescence from low-resistivity ZnSe:Cu, Al crystals has been observed under the excitation of low-energy electron beams below 50 V. Dependence of current and brightness on applied voltage as well as the emission spectrum are presented. The red emission of ZnSe tends to be subjected to degradation, while the blue and the green of ZnS previously observed do not.     

Dual nature of RN-centers in LiF crystals

An alternative model of the N_c-center as a complex defect consisting of the F ₂ ⁺ -center perturbed by the F_L defect (where F_L is Li⁰ in the F-center) is confirmed by the methods of optical spectroscopy and X-ray diffraction analysis of LiF crystals. Upon selective optical excitation, these centers are decomposed into F₂ and F_L defects.     

Short-lived primary radiation defects in LiF crystals

The spectral and kinetic parameters of electron-pulse-initiated transient absorption and emission of LiF crystals were studied using pulsed spectrometry with a nanosecond time resolution. The measurements were performed in the spectral region of 6 eV, the temperature range of 11–150 K, and within 10^?8–10 s after the termination of an electron pulse. It is shown that the electron-pulse irradiation not only gives rise to F, V _k , and H centers in the LiF crystal but also to certain short-lived defects of two types that differ in the spectral positions of the absorptive and radiative transitions, the lifetime, and the temperature dependence of the production efficiency. Defects of type I feature absorptive transitions at 5.5 and 5.1 eV and a radiative transition at 5.8 eV, whereas the absorptive transitions at 5.3 and 4.75 eV and a radiative transition at 4.4 eV are characteristic of type-II defects. It is found that a variation in the ratio between the concentrations of the different types of short-lived centers in the range of 11–150 K does not affect the quantum efficiency of the F centers. It is assumed that the observed centers are self-trapped excitons of various types.     

Bound-exciton cathodoluminescence in ZnSe crystals and two-phonon resonance

A study has been made of the cathodoluminescence of ZnSe crystals annealed in vacuum [ZnSe(Vac)], in vacuum and, subsequently, in antimony melt [(ZnSe(Vac)(Sb)], or in a zinc melt with subsequent annealing in antimony [ZnSe(Zn)(Sb)]. The emission of all samples contained the I ₁ ^s,d -nLO series. The LO-phonon replicas of the emission line I ₁ ^s observed in ZnSe(Vac) samples are accompanied by single-plasmon satellites. The plasmon energy determining the replica separation is ?ω_p?10 meV. The emission lines of ZnSe(Zn)(Sb) samples have the smallest half-width. We report the first observation of anomalous broadening of the zero-phonon line I ₁ ^s in ZnSe(Vac) samples caused by a high zinc vacancy content. A theory on the shape of the emission spectrum under two-phonon resonance is developed including bound-exciton interaction with mixed plasmon-phonon vibrational modes. It is shown that the splitting of the I ₁ ^d line at T?2 K may originate from resonance exciton-phonon interaction between exciton-impurity complexes.     

Nonmonotonic creep in LiF crystals containing Mg

Precision measurements of the creep rate using interferometric recording of the process in LiF crystals containing 0.002 to 0.03 wt % Mg using interferometric recording of the process show that nonmonotonic episodes in the form of alternating segments with relatively high and low creep rates periodically appear against a background of overall creep attenuation. The accumulation of creep strain takes place mostly on the segments with increased rates, which causes kinks to appear in the creep curve that are noticeable when shifts are resolved at a level of fractional microns. Measurements were made of the height of steps L based on “rate-strain increment” curves for stresses smaller and closer to the yield point. Increasing the stresses causes the value of L to fall off, while increasing the Mg content causes it to increase. It is proposed that the nonmonotonic behavior of creep is connected with structural nonuniformity of the material, which is determined by the impurity content and the nonuniformity of the strain process itself. For small strains in the neighborhood of the yielding area, where strain comes about as a result of a broadening of a pre-existing slip band, the impurity atoms enhance the nonuniformity of the strain (L increases). As the strain (stress) increases, L decreases, which corresponds to a smoothing out of the nonuniformities. It is emphasized that the step-like character of the strain accumulation is a general property. Fiz. Tverd. Tela (St. Petersburg) 40, 690–693 (April 1998)     

Impurity micropotentials in commercial silicon crystals

The use of very cold neutrons, of laser mass spectrometry, and of secondary-ion mass spectrometry has revealed the presence of microprecipitates in commercial Si crystals grown by the Czochralski method and by float zoning.Neutron-Physics Laboratory, Lebedev Physics Institute, Academy of Sciences of the USSR. Translated from Preprint No. 149 of the Lebedev Physics Institute, Academy of Sciences of the USSR, Moscow, 1988.     

Impurity band conduction in CdHgTe crystals

Electrical conduction at 77 K in Cd_xHg_1−xTe, with the composition x ⩽ 0.2, is by electrons in the conduction band, by holes in the valence band and by holes in the impurity band. In samples with zero energy gap, x < 0.14, electrical conduction by holes in the valence band is comparable to electrical conduction by holes in the impurity band. In the open energy gap CdHgTe, electrical conduction by holes in the valence band is negligible in comparison to electrical conduction by holes in the impurity band. In CdHgTe samples, electrical conduction in the impurity band is described by the “Fermi Glass” model.     

Deformation of LiF crystals in DC magnetic field

Active deformation of LiF crystals in a dc magnetic field B is shown to produce strong plasticization, which is the more pronounced, the higher is the magnetic induction B and the lower the strain rate $\dot \varepsilon $ . The measured dependences of the yield point σ _y on B and $\dot \varepsilon $ find a reasonable explanation within a simple kinetic model based on the competition between the thermally activated and magnetically stimulated unpinning of dislocations from impurity centers.     

Metastable F2 center states in LiF crystals

Using time-resolved pulsed absorption and luminescence spectroscopy we have studied the creation ofF ₂ centers during electron pulses of nanosecond duration in the temperature range 80 to 300 K. We show that the formation ofF ₂ centers in long-lived metastable states is thermally activated and competes withF ₂ center relaxation to the singlet state. It is shown that the S-T crossing in the set of states which describe theFC₂ center lies above the¹ _u level. We propose that when the crystal is excited with the lowest energy quanta which produce a photocurrent, the formation of tripletF ₂ centers takes place by localization of a conduction electron on anF ₂ ⁺ center.State Academy of Architecture and Construction, Tomsk. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 11, pp. 17–22, November, 1994.     

Energy transfer among color centers in LiF crystals

In colored LiF crystals there are many absorption and emission bands which cover a wide region of the spectrum from 200 up to 1 300 nm without breaking the continuity. In the frame of these favourable conditions we have performed some experiments of energy transfer among various color centers. The results indicate the existence of efficient exchanges of radiative energy among several bands by using only one exciting wavelength. In particular, the emissions of the F₃ ^- and F₂ ^- color centers centered at 900 and 1 100 nm, respectively, have been observed by pumping at 672 nm completely outside their absorption bands. Received 25 June 2001 and Received in final form 16 July 2001     

Magnetoplastic effect in irradiated NaCl and LiF crystals

The effect of low x-ray irradiation doses (≈10² rad) on the magnetoplastic effect — the detachment of dislocations from paramagnetic centers under the action of an external magnetic field B — in alkali-halide crystals has been investigated. The measurements were performed on LiF crystals and three types of NaCl crystals, differing in impurity content. The dependence of the mean free path l of the dislocations on the rotational frequency ν of a sample in a magnetic field was especially sensitive to low irradiation doses. In unirradiated crystals this dependence is a single-step dependence and is characterized by a critical frequency ν _c ∝B ² above which the magnetoplastic effect is not observed. The frequency ν _c depends only on the type of paramagnetic centers, and not on their density. Even the lowest irradiation dose employed (<100 rad) leads to a sharp restructuring of the dependence l(ν), converting it into a two-step dependence (for edge dislocations) with an additional critical frequency ν _c2, that is insensitive to the irradiation dose, and that corresponds to the appearance of magnetically sensitive stoppers of a new type under irradiation. The initial critical frequency ν _c1, as a rule, also varies with the dose, reflecting the change in state of the impurity complexes (Ca in NaCl and Mg in LiF). Specifically, it is shown for NaCl(Ca) crystals that as the irradiation dose increases, the frequency ν _c1 increases, gradually approaching the value ν _c2, so that by the time the dose is ≈300 rad, the dependence l(ν) once again becomes a single-step dependence, dropping sharply only for ν⩾ν _c2. It is shown that the addition of a small number of Ni atoms to a NaCl crystal makes the Ca complexes radiation resistant, and the critical frequency ν _c1 corresponding to them initially equals ν _c2 for crystals with no Ni. The recombination kinetics of radiation defects in the case in which the samples are irradiated under a tungsten lamp was investigated. A possible physical model of the observed dependences is discussed. Zh. éksp. Teor. Fiz. 111, 615–626 (February 1997)