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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 1,657 records · Page 92
All sourcesNASA NTRSDOE OSTI
DOE OSTI2015

Investigations of nanodimensional Al{sup 2}O{sup 3} films deposited by ion-plasma sputtering onto porous silicon

The purpose of this study is the deposition of nanodimensional Al{sup 2}O{sup 3} films on the surface of nanoporous silicon and also fundamental investigations of the structural, optical, and morphological properties of these materials. Analyzing the results obtained here, it is possible to state that ultrathin nanostructured Al{sup 2}O{sup 3} films can be obtained in the form of threads oriented in one direction and located at a distance of 300–500 nm from each other using ion-plasma sputtering on a layer of porous silicon. Such a mechanism of aluminum-oxide growth is conditioned by the crystallographic orientation of the initial single-crystalline silicon wafer used to fabricate the porous layer. The results of optical spectroscopy show that the Al{sup 2}O{sup 3}/por-Si/Si(111) heterophase structure perfectly transmits electromagnetic radiation in the range of 190–900 nm. The maximum in the dispersion of the refractive index obtained for the Al{sup 2}O{sup 3} film grown on por-Si coincides with the optical-absorption edge for aluminum oxide and is located in the region of ∼5.60 eV. This fact is confirmed by the results of calculations of the optical-absorption spectrum of the Al{sup 2}O{sup 3}/por-Si/Si(lll) heterophase structure. The Al{sup 2}O{sup 3} films formed on the heterophase-structure surface in the form of nanodimensional structured threads can serve as channels of optical conduction and can be rather efficiently introduced into conventional technologies, which are of great importance in microelectronics and optoelectronics.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Specific features of the optical and photoelectric properties of nanocrystalline indium oxide

The spectral dependences of the absorption coefficient and steady-state photoconductivity and the relaxation of photoconductivity (after illumination switch off) in nanocrystalline indium oxide (In{sub 2}O{sub 3}) films synthesized by the sol–gel method are investigated. The mean nanocrystal size varies from 7–8 to 39–41 nm, depending on the sample-preparation conditions. The optical band gap of nanocrystalline In{sub 2}O{sub 3} is determined as a result of combined analysis of the optical absorption spectra and the spectral dependence of the photoconductivity. The optical band gap in nanocrystalline In{sub 2}O{sub 3} is found to be much narrower than the optical band gap in In{sub 2}O{sub 3} single crystals. The photoconductivity decay kinetics in nanocrystalline In{sub 2}O{sub 3} in air, vacuum, and argon at room temperature are studied. A model explaining the observed photoconductivity decay is proposed.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Magnetic and electrical properties of Fe{sub 0.9}Ag{sub 0.1}In{sub 2.3}S{sub 4.4} single crystals

The magnetic and electrical properties of the Fe{sub 0.9}Ag{sub 0.1}In{sub 2.3}S{sub 4.4} single crystal are studied in the temperature range 4–300 K and in magnetic fields of 0–14 T. It is established that the sample under study is paramagnetic. In the ground state, short-range-order correlations typical of a spin glass with a freezing temperature of 10 K are detected. The magnetic ordering temperature is 15 K. The sample is a semiconductor with a resistivity of 3.5 kΩ cm at room temperature. For the Fe{sub 0.9}Ag{sub 0.1}In{sub 2.3}S{sub 4.4} single crystal, a mechanism for the formation of magnetic and electrical states is suggested.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Binding energy of excitons formed from spatially separated electrons and holes in insulating quantum dots

It is found that the binding energy of the ground state of an exciton formed from an electron and a hole spatially separated from each other (the hole is moving within a quantum dot, and the electron is localized above the spherical (quantum dot)–(insulating matrix) interface) in a nanosystem containing insulating Al{sub 2}O{sub 3} quantum dots is substantially increased (by nearly two orders of magnitude) compared to the exciton binding energy in an Al{sub 2}O{sub 3} single crystal. It is established that, in the band gap of an Al{sub 2}O{sub 3} nanoparticle, a band of exciton states (formed from spatially separated electrons and holes) appears. It is shown that there exists the possibility of experimentally detecting the ground and excited exciton states in the band gap of Al{sub 2}O{sub 3} nanoparticles at room temperature from the absorption spectrum of the nanosystem.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Photodetectors on the basis of Ge/Si(001) heterostructures grown by the hot-wire CVD technique

The fabrication of photodetectors for the wavelength range of communications λ = 1.3–1.55 µm on the basis of Ge/Si(001) heterostructures with thick (∼0.5 µm) Ge layers grown by the hot-wire technique at reduced growth temperatures (350°C) is reported. The single-crystal Ga layers are distinguished by a low density of threading dislocations (∼10{sup 5} cm{sup –2}). The photodetectors exhibit a rather high quantum efficiency (∼0.05 at λ = 1.5 µm and 300 K) at moderate reverse saturation current densities (∼10{sup –2} A cm{sup –2}). Thus, it is shown that the hot-wire technique offers promise for the formation of integrated photodetectors for the wavelength range of communications, especially in the case of limitations on the conditions of heat treatments.

36 MATERIALS SCIENCE↗
DOE OSTI2016

On the specific electrophysical properties of n-InSe single crystals

The temperature dependences of physical parameters (the conductivity and the Hall constant) are experimentally investigated for pure indium-selenide (n-InSe) crystals and those lightly doped with rareearth elements (gadolinium, holmium, and dysprosium). It is established that the obtained results depend on the origin of the samples under investigation and prove to be contradictory for different samples. The obtained experimental results are treated taking into account the presence of chaotic large-scale defects and drift barriers caused by them in these samples.

36 MATERIALS SCIENCE↗
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↗