Effect of the size of x-ray coherent-scattering regions on the electrical parameters of semiconducting SmS

The relation of the electrical properties of semiconducting samarium sulfide (SmS) to the size of coherent scattering regions for x-rays is considered. The dependence of the carrier concentration on the size of coherent scattering regions in single-crystal and polycrystalline SmS samples, as well as in thin samarium sulfide films, is established experimentally. A satisfactory agreement of the calculated curves with the experiment suggests that the size of coherent scattering regions has a dominant effect on the concentrations of charge carriers and defect samarium ions in SmS. The effect of thermal shocks on the size of coherent scattering regions in SmS single crystals and polycrystals, as well as in polycrystalline Sm_0.4Eu_0.6S samples, is investigated.     

Defect samarium ions and electromotive-force generation in SmS

A model explaining electromotive-force generation in SmS under heating in the absence of external temperature gradients is considered. An analysis of data on the density of SmS single crystals with compositional deviations from stoichiometry in the homogeneity range suggests that excess samarium ions occupy vacancies on the sulfur sublattice. Possible concentrations of defect samarium ions are determined (up to 2.8×10²¹ cm^?3). The temperature interval within which electromotive force appears in various SmS samples (440–640 K) and the critical conduction-electron concentrations at which the generation sets in [(6.0–8.5)×10¹⁹ cm^?3] are calculated. An expression permitting estimation of the magnitude of the observed effect is proosed.     

Emf induced by a change in the samarium ion valence as a result of a phase transition in SmS single crystals

The mechanism of the emf appearing in a semiconductor under heating in the absence of external temperature gradients, an effect revealed by the present authors, is considered. The experiments were performed on samarium sulfide (SmS) single crystals. It is shown that the emf is generated by an abrupt change in the samarium ion valence, which results from the ion screening by the electrons activated into the conduction band. We succeeded in obtaining emf pulses 1.3 s long with an amplitude of up to 2.5 V at T ～ 460 K, as well as CW emf generation within the 375-to 405-K temperature interval with a maximum value of ～50 mV.     

Study of the electrical properties of thin SmS films at high pressures

The pressure-induced shift of impurity levels under hydrostatic compression (?1.9 × 10^?2 meV/MPa) at T = 300 K has been derived from measurements of the behavior with temperature of the electrical resistance of thin polycrystalline SmS films on glass substrates at different pressures. The difference between the pressure-induced shifts of impurity levels in thin films and single crystals has been attributed to the effect of elastic properties of the substrate material. It has been shown that the semiconductor-metal phase transition in SmS films does not occur at pressures of up to 1000 MPa, because the impurity levels triggering the mechanism of phase transition at such pressures are not in the conduction band.     

Chemical analysis of semiconducting and metallic SmS thin films by X-ray photoelectron spectroscopy

We studied the chemical state of semiconducting and metallic SmS thin films by X-ray photoelectron spectroscopy (XPS), which were fabricated using dual-target magnetron sputtering by controlling the power applied to both metal and chalcogenide targets. On the basis of the valence band spectra obtained, it was suggested that semiconducting SmS has the final state corresponding to the Sm²⁺(4f⁶) configuration below the Fermi level, and metallic SmS has mainly the Sm³⁺(4f⁵) final state and a virtual band state in the Sm 5d band, contributing to the delocalization of 4f electrons and the emergence of metallic conductivity (4f⁶d⁰-4f⁵d¹). Thus, the spectra of our fabricated SmS thin films correspond to the band structure obtained from the dielectric property. This is the first work performed on the intrinsically prepared metallic SmS while the former works done for the sample transformed from semiconductor to metal phase by hard polishing.     

SmS phase transition in thin films prepared by reactive evaporation

Preparation and analysis of thin films containing samarium sulfide (SmS) is presented along with a review of relevant SmS properties. Films were deposited onto unheated substrates by the reactive evaporation of samarium in a backpressure of hydrogen sulfide. This technique yields films that contain significant quantities of impurities; however high-quality SmS crystals are also formed. A phase transition in the SmS crystals was observed both by spectrophotometry and x-ray diffraction. The film optical properties can be modeled with an effective medium calculation. The predicted spectra successfully reproduce the observed qualitative features over a wide range of wavelengths.     

Defect structure in SmS

The experimental dependence of the concentration of conduction electrons on the size of x-ray coherent scattering regions is obtained. The dependence is analyzed within the previously developed concentration model of the energy spectrum of samarium sulfide SmS. It is shown that the impurity donor levels in the SmS compound correspond to the samarium defect ions located at the boundaries of coherent scattering regions.     

Influence of the mechanical effect and impurity doping on the endothermic effect in SmS

The effect of heat absorption has been found for powder samples of samarium sulfide (SmS) in a temperature range of 235–250 K. The comparison with a similar effect in single-crystal SmS has been performed and thermodynamic parameters of the process have been determined. It has been shown that the collective injection of electrons into the conduction band from impurity donor levels is responsible for the appearance of this effect in both cases. The shift of the endothermic peak upon powdering toward lower temperatures and an increase in the enthalpy are associated with a decrease in the characteristic size of the X-ray coherent scattering regions and, as a consequence, with an increase in the concentration of impurity donor levels.     

Destruction of SmS single crystals during the semiconductor-metal phase transition under hydrostatic compression

The behavior of internal microstresses, the size of the X-ray coherent scattering region, and the residual amount of the metal phase during the cyclic loading of SmS single crystals at hydrostatic pressure above the critical pressure of the semiconductor-metal phase transition has been investigated. It has been shown that the samples are destroyed as the microstresses reach the values corresponding to the ultimate stress of SmS single crystals. As the number of loading cycles increases, the coherent scattering region gradually decreases, which is accompanied by a decrease in the amount of the metal phase in the samples.     

The influence of hydrostatic pressure on the electronic transport properties of semiconducting SmS

The Hall coefficient, resistivity and transverse magnetoresistance of nominally stoichiometric SmS single crystals were measured as a function of pressure up to 6–8 kbar, at temperatures from 40 to 300 K. Hall coefficient variations versus pressure led to the conclusion that the energy difference between 4f⁶5d⁰ and 4f⁵5d¹ configurations in Sms was at least 0.1 eV at 300 K at first order transition pressure.     

Influence of the defect state of samples on the luminescence spectra of CdS single crystals irradiated by electrons

The photoluminescence spectra of CdS single crystals irradiated by electrons (E = 1.2 MeV, Φ = 2×10¹⁷ cm^?2) are investigated in the visible and near-infrared regions of electromagnetic radiation. Some samples of the CdS single crystals are preliminarily irradiated by neutrons (E = 2 MeV, Φ = 2 × 10¹⁸ cm^?2) with the aim of increasing the concentration of initial structural defects. From analyzing the peak intensities of photoluminescence in the irradiated single crystals at the wavelengths λ_m = 0.720, 1.030, and 0.605 μm, it is concluded that the CdS samples with a low concentration of structural defects in the initial state possess the highest resistance to electron radiation. It is assumed that the observed transformation of the photoluminescence spectra of the imperfect CdS single crystals subjected to electron irradiation is determined by either the mechanisms of subthreshold defect formation or the transformation of the defect complexes in elastic and electric fields near the large structural damages of the crystal lattice.     

Pecularity of lithium electrodiffusion into fulleride single crystals

Diffusion of lithium cations in C₆₀ single crystals driven by an electric field has been studied. It was found that the stoichiometry of C₆₀ single crystals can be changed with respect to lithium by injection of Li⁺ ions through the heterojunction Li₇SiPO₈|C₆₀ and electrons through the heterojunction C₆₀| Graphite. The double charge injection changes the stoichiometry of lithium in a C₆₀ single crystal and increases both the lithium ionic and electronic conductivity. The electronic conductivity in Li_xC₆₀ crystals is non-metallic in nature. The temperature dependence of the electronic conductivity lithium doped C₆₀ single crystals was investigated. It was found that electrons occupied the lowest singlet exited states (LUMO) and that this leads to the appearance of an intensive EPR signal. The temperature dependence of the concentration of paramagnetic centers was investigated. Paper presented at the 6th Euroconference on Solid State Ionics, Cetraro, Calabria, Italy, Sept. 12–19, 1999.     

Formation of radiation defects in single crystals of NaCl and kcl irradiated with channeled electrons with energies 10‐50 keV

We present the results of investigations of the process of formation of F-centers in single crystals of NaCl and kCl upon exposure to 10–50 keV electrons with the incident electron beam directed at various angles to the crystallographic axes < 100> of single crystals. We showed that when the electrons are channeled, the rate of formation of F-centers increases, and the concentration of F-centers near the surface of the single crystals increases. We established a connection between the observed orientation phenomena in the formation of F-centers and the radiation dose and initial energy of the electrons. We determined characteristic angles for channeling the electrons.     

Additive coloring rate and intensity for pure and doped fluorite crystals

The mechanism of the additive coloring of calcium fluoride crystals, both pure and doped with I- and III-group elements, has been investigated. The coloring rate and intensity and the dependences of these parameters on the experimental conditions, vapor mixture pressure, and temperature have been experimentally determined. It is shown that the concentration of anion vacancies/electrons that can be introduced into crystals is in the range of 10¹⁷–10¹⁸ cm⁻³; in crystals with a high concentration of rare earth ions, the limiting concentration can be an order of magnitude higher.     

Space-charge limited currents in n-GaSe crystals

Measurements performed on n-GaSe crystals by means of the space-charge limited current method indicate the presence of two deep electron traps, at 0.62 and 0.68 eV below the conduction band, with an energy width of 0.13 eV and a concentration of about 10¹³ cm^?3. Various tests confirm that single carrier (electrons) injection in a space-charge regime is the dominant mechanism in measured I–V curves. The connection between traps and defective structure of n-GaSe is discussed.     

Electrical conductivity of SmS polycrystals

The electrical conductivity of SmS polycrystals has been studied in the temperature range 300?C870 K. It has been shown that, at 300 K ?? T ?? 700 K, the concentration of conduction electrons is determined by electron transfer from impurity donor levels, and at T > 700 K, by that from the samarium 4f levels.     

Investigation of the temperature dependences of the electrical resistance of SmS single crystals at different pressures

The pressure-induced shift of impurity levels under hydrostatic pressure at T = 300 K (−9.6 meV/MPa) has been obtained from measurements of the temperature dependences of the electrical resistance of SmS single crystals at different pressures. The obtained value confirms the validity of the existing model of the semiconductor-metal phase transition in samarium monosulfide.     

On the correlation between the magnetic structure and the electrical properties of EuB6

Single crystals of EuB₆ were prepared by the floating-zone method. Magnetic and electric measurements on these crystals in the temperature range 1.6–300 K revealed that the pure EuB₆ is an antiferromagnet with a Néel temperature of 5–6 K and has a carrier concentration of 4.2 × 10²⁰ cm^-3. It was found that in polycrystalline sample, defects of Eu ions reduce the carrier concentration and make the sample ferromagnetic. It is suggested that the conduction electrons in pure EuB₆ originate from the overlap of the 4f band tail and the conduction band.     

Temperature dependence of the SmS lattice parameter

The behavior of the lattice parameter of single-crystal SmS with temperature was studied by x-ray diffractometry in the range 100–700 K. The observed features are assigned to a temperature-induced variation in the filling of the Sm²⁺f-shell ground-state multiplet levels. The temperature dependence of the thermal expansion coefficient of SmS was measured. It is shown that the lattice constant behavior in samples that exhibit a pronounced emf generation effect under heating is influenced by the transition of defect samarium ions from the divalent to trivalent state and that the effect itself derives from phase transitions in SmS.