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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 1,765 records · Page 98
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DOE OSTI2016

Mechanisms of the degradation of Schottky-barrier photodiodes based on ZnS single crystals

The effect of ultraviolet (UV) illumination on the electrical and spectral characteristics of Schottky-barrier photodiodes based on ZnS single crystals is studied. It is found that irradiation deteriorates their photosensitivity and changes the current–voltage and capacitance–voltage characteristics and the surface profile of the blocking electrode. It is shown that the main reason for a decrease in the photosensitivity of the diodes is the photoinduced drift of mobile donors in the electric field of the barrier. This drift depends on the crystallographic orientation of the surface being irradiated. Another photoinduced process observed in the diodes is photolysis of the ZnS crystal. This process mainly determines the change in the electrical characteristics of the diodes and in the surface profile of the electrode at an insignificant change in the photosensitivity.

36 MATERIALS SCIENCE↗
DOE OSTI2016

On the complex structure of the optical spectra of a tetragonal calomel single crystal in a wide energy range

The spectral complex of optical functions of the calomel Hg{sub 2}Cl{sub 2} single crystal is determined in the range 0–20 eV at 300 K in unpolarized light. The spectra of the imaginary part of the permittivity ε{sub 2}(E), the bulk–Imε{sup –1} and the surface–Im(1 + ε){sup –1} electron energy losses are decomposed into elementary bands. Their main parameters, including energies and oscillator strengths of the transition bands are determined. Calculations are performed on the basis of the experimental reflectance spectrum of the crystal cleavage. Computer programs based on Kramers–Kronig relations, analytical formulas, and the advanced parameterfree method of combined Argand diagrams are used. The main features of the spectral set of optical functions and the parameters of expansion band components ε{sub 2}(E),–Imε{sup –1}, and–Im(1 + ε){sup –1} are determined.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Growth, structure, and properties of GaAs-based (GaAs){sub 1–x–y}(Ge{sub 2}){sub x}(ZnSe){sub y} epitaxial films

The possibility of growing the (GaAs){sub 1–x–y}(Ge{sub 2}){sub x}(ZnSe){sub y} alloy on GaAs substrates by the method of liquid-phase epitaxy from a tin solution–melt is shown. X-ray diffraction shows that the grown film is single-crystal with the (100) orientation and has the sphalerite structure. The crystal-lattice parameter of the film is a{sub f} = 0.56697 nm. The features of the spectral dependence of the photosensitivity are caused by the formation of various complexes of charged components. It is established that the I–V characteristic of such structures is described by the exponential dependence I = I{sub 0}exp(qV/ckT) at low voltages (no higher than 0.4 V) and by the power dependence J ∼ V{sup α}, where the exponent α varies with increasing voltage at high voltages (V > 0.5 V). The results are treated within the framework of the theory of the drift mechanism of current transfer taking into account the possibility of the exchange of free carriers within the recombination complex.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Electrochemical characteristics of nanostructured silicon anodes for lithium-ion batteries

High-aspect periodic structures with thin vertical walls are studied as regards their applicability as negative electrodes of lithium-ion batteries. The nanostructures are fabricated from single-crystal silicon using photolithography, electrochemical anodization, and subsequent anisotropic shaping. The capacity per unit of the visible surface area of the electrode and the specific internal surface area are compared for structures of varied architecture: 1D (wires), 2D (zigzag walls), and 3D structures (walls forming a grid). Main attention is given to testing the endurance of anodes based on zigzag and grid structures, performed by galvanostatic cycling in half-cells with a lithium counter electrode. The influence exerted by the geometric parameters of the structures and by the testing mode on the degradation rate is determined. It is shown that the limiting factor of the lithiation and delithiation processes is diffusion. The endurance of an electrode dramatically increases when the charging capacity is limited to ∼1000 mA h/g. In this case, nanostructures with 300-nm-thick walls, which underwent cyclic testing at a rate of 0.36C, retain a constant discharge capacity and a Coulomb efficiency close to 100% for more than 1000 cycles.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Features of photoinduced magnetism in some yttrium–iron-garnet single crystals

Photoinduced magnetic phenomena are considered in yttrium–iron single-crystalline garnets grown from a BaO–B{sub 2}O{sub 3} molten solution with the addition of iridium to the initial melt. The features of the composition and defects of the crystal structure of the samples in the surface layer are determined. In view of this, explanations for features of the photoinduced magnetic phenomena in the investigated crystals are proposed. The determining role of oxygen anions for the photoinduced magnetic phenomena at room temperature is highlighted, and the possible role of a variation in the dopant content and composition is considered.

36 MATERIALS SCIENCE↗
DOE OSTI2016

X-ray conductivity of ZnSe single crystals

The experimental I–V and current–illuminance characteristics of the X-ray conductivity and X-ray luminescence of zinc-selenide single crystals feature a nonlinear shape. The performed theoretical analysis of the kinetics of the X-ray conductivity shows that even with the presence of shallow and deep traps for free charge carriers in a semiconductor sample, the integral characteristics of the X-ray conductivity (the current–illuminance and I–V dependences) should be linear. It is possible to assume that the nonlinearity experimentally obtained in the I–V and current–illuminance characteristics can be caused by features of the generation of free charge carriers upon X-ray irradiation, i.e., the generation of hundreds of thousands of free charge carriers of opposite sign in a local region with a diameter of <1 μm and Coulomb interaction between the free charge carriers of opposite signs.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Optical properties of In{sub 2}Se{sub 3} thin films

In{sub 2}Se{sub 3} films are produced by ion-beam evaporation at substrate temperatures of 313 and 623 K. As the target, In{sub 2}Se{sub 3} single crystals grown by the vertical Bridgman method are used. The composition and structure of the crystals and films are determined by the X-ray spectral analysis and X-ray diffraction techniques, respectively. It is established that the crystals and films crystallize with the formation of a hexagonal structure. The band gap and refractive index of the In{sub 2}Se{sub 3} films are determined from the transmittance and reflectance spectra. It is found that, as the substrate temperature is increased, the band gap increases.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Oxidation model of polycrystalline lead-chalcogenide layers in an iodine-containing medium

Model concepts of control over the formation of oxide shells in the course of oxidation are confirmed on the basis of experimental results obtained in a study of the fundamental aspects of how nanostructured layers are formed upon diffusion annealing on faceted single crystals of lead chalcogenides. Auger-spectroscopic data on the distribution of the elemental composition over the thickness of layers based on lead selenide–cadmium selenide solid solutions are presented.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Effect of thallium doping on the mobility of electrons in Bi{sub 2}Se{sub 3} and holes in Sb{sub 2}Te{sub 3}

The Shubnikov–de Haas effect and the Hall effect in n-Bi{sub 2–x}Tl{sub x}Se{sub 3} (x = 0, 0.01, 0.02, 0.04) and p-Sb{sub 2–x}Tl{sub x}Te{sub 3} (x = 0, 0.005, 0.015, 0.05) single crystals are studied. The carrier mobilities and their changes upon Tl doping are calculated by the Fourier spectra of oscillations. It is found shown that Tl doping decreases the electron concentration in n-Bi{sub 2–x}Tl{sub x}Se{sub 3} and increases the electron mobility. In p-Sb{sub 2–x}Tl{sub x}Te{sub 3}, both the hole concentration and mobility decrease upon Tl doping. The change in the crystal defect concentration, which leads to these effects, is discussed.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Solubility of oxygen in CdS single crystals and their physicochemical properties

The specific features of oxygen dissolution in CdS using the example of single crystals grown by the gas-transport method with deviations from stoichiometry at 1100°C are considered. The effect of various types of intrinsic point defects in crystals of different composition on the form in the presence of oxygen is analyzed. It is shown that the most stable composition thermodynamically is that corresponding to nonstoichiometric “self-activated cadmium sulphide” stabilized with oxygen.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Structural and optical properties of GaAs(100) with a thin surface layer doped with chromium

The aim of this study is to explore the structural and optical properties of single-crystal GaAs(100) doped with Cr atoms by burning them into the substrate at high temperatures. The diffusion of chromium into single-crystal GaAs(100) substrates brings about the formation of a thin (~20–40 μm) GaAs:Cr transition layer. In this case, chromium atoms are incorporated into the gallium-arsenide crystal lattice and occupy the regular atomic sites of the metal sublattice. As the chromium diffusion time is increased, such behavior of the dopant impurity yields changes in the energy structure of GaAs, a decrease in the absorption at free charge carriers, and a lowering of the surface recombination rate. As a result, the photoluminescence signal from the sample is significantly enhanced.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Prompt quality monitoring of InSe and GaSe semiconductor crystals by the nuclear quadrupole resonance technique

To determine the quality of layer-structured semiconductor single crystals, the nuclear quadrupole resonance technique with successive scanning of the entire sample volume and assessment of the structural quality on the basis of the observed spectra is used. The proposed method is appropriate in the case of InSe, GaSe and GaAs ingots grown in evacuated cells by the Bridgman technique and can be used repeatedly in subsequent technological procedures with no operator access to the material. To provide scanning within a limited region of the sample and to ensure efficient interaction of the high-frequency field with the crystal, excitation and detection of the nuclear-spin-induction signal is implemented by the saddle-shaped transceiver coil of the nuclear quadrupole resonance spectrometer.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Dielectric properties of layered FeGaInS{sub 4} single crystals in an alternating electric field

The results of investigations of the frequency and temperature dependences of dielectric losses and the imaginary part of the dielectric permittivity in FeGaInS{sub 4} single crystals are presented. Their experimental values are determined. It is established that the loss tangent and the imaginary part of the permittivity of FeGaInS{sub 4} single crystals in a field with frequencies of 10{sup 4}–10{sup 6} Hz decrease inversely proportional to the frequency (tanδ ~ 1/ω), and the conductivity is characterized by the band–hopping mechanism. For FeGaInS{sub 4}, the relaxation time is calculated, and it is established that there is a mechanism of electron polarization caused by thermal motion in this crystal.

36 MATERIALS SCIENCE↗
DOE OSTI2016

UV and IR emission intensity in ZnO films, nanorods, and bulk single crystals doped with Er and additionally introduced impurities

For ZnO films, nanorods, and bulk single crystals doped with Er{sup +} ions, it is shown that the effect of codopants introduced into the cation and ion sublattices and the observation of a high-intensity emission band at the wavelength λ{sub max} = 1535 nm are defined by the local environment of the Er{sup +} ion. Doping of the films and single crystals with Er{sup +} ions by diffusion brings about an infrared (IR) emission band with a low intensity because of an inadequate concentration of impurity ions. The emission intensity of this band can be raised by introducing additional Ag, Au, or N{sup +} impurities into the ZnO films. The UV-emission intensity of the Er-doped films and single crystals at λ{sub max} = 368–372 nm is identical to that of the undoped films. ZnO nanorods doped with Er only or together with Al or Ga codopants exhibit only one IR band (at λ{sub max} = 1535 nm), whose intensity decreases upon the introduction of codopants. Doping of the nanorods with the N{sup +} gaseous impurity during growth (930 < T < 960°C) and then with the Er{sup +} impurity by diffusion does not yield a substantial increase in the IR-emission intensity compared to the that of the corresponding band for nanorods not doped with the N{sup +} impurity. In the Er-doped nanorods, whose photoluminescence spectra exhibit a high-intensity band at λ{sub max} = 1535 nm, the UV emission band at λ{sub max} = 372 nm is practically lacking.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Epitaxial GaN layers formed on langasite substrates by the plasma-assisted MBE method

In this publication, the results of development of the technology of the epitaxial growth of GaN on single-crystal langasite substrates La{sub 3}Ga{sub 5}SiO{sub 14} (0001) by the plasma-assisted molecular-beam epitaxy (PA MBE) method are reported. An investigation of the effect of the growth temperature at the initial stage of deposition on the crystal quality and morphology of the obtained GaN layer is performed. It is demonstrated that the optimal temperature for deposition of the initial GaN layer onto the langasite substrate is about ~520°C. A decrease in the growth temperature to this value allows the suppression of oxygen diffusion from langasite into the growing layer and a decrease in the dislocation density in the main GaN layer upon its subsequent high-temperature deposition (~700°C). Further lowering of the growth temperature of the nucleation layer leads to sharp degradation of the GaN/LGS layer crystal quality. As a result of the performed research, an epitaxial GaN/LGS layer with a dislocation density of ~10{sup 11} cm{sup –2} and low surface roughness (<2 nm) is obtained.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Polarization of the induced THz emission of donors in silicon

The polarization of the terahertz (4.9–6.4 THz) stimulated emission of Group-V (Sb, P, As, Bi) donors in single-crystal silicon under pumping (photoionization) by a CO{sub 2} laser (photon energy 117 meV), depending on the uniaxial compressive deformation of the crystal along the [100] axis, is experimentally investigated. The influence of the field direction of the pump wave on its efficiency is discussed.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Terahertz injection lasers based on PbSnSe alloy with an emission wavelength up to 46.5 μm

Diffusion injection lasers based on Pb{sub 1} {sub –} {sub x}Sn{sub x}Se alloy, emitting in a wide spectral range of 10–46.5 μm depending on the composition and temperatures are fabricated. A technology for growing high-quality single crystals from the vapor phase under conditions of free growth is developed. The dependences of the total emission intensity on the pump current and the emission spectra of injection lasers based on Pb{sub 1–x}Sn{sub x}Se are studied. In these samples, lasing of long-wavelength radiation to a record wavelength of 46.5 μm is achieved.

36 MATERIALS SCIENCE↗