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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 1,675 records · Page 93
All sourcesNASA NTRSDOE OSTI
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↗
DOE OSTI2017

Surface texture of single-crystal silicon oxidized under a thin V{sub 2}O{sub 5} layer

The process of surface texturing of single-crystal silicon oxidized under a V{sub 2}O{sub 5} layer is studied. Intense silicon oxidation at the Si–V{sub 2}O{sub 5} interface begins at a temperature of 903 K which is 200 K below than upon silicon thermal oxidation in an oxygen atmosphere. A silicon dioxide layer 30–50 nm thick with SiO{sub 2} inclusions in silicon depth up to 400 nm is formed at the V{sub 2}O{sub 5}–Si interface. The diffusion coefficient of atomic oxygen through the silicon-dioxide layer at 903 K is determined (D ≥ 2 × 10{sup –15} cm{sup 2} s{sup –1}). A model of low-temperature silicon oxidation, based on atomic oxygen diffusion from V{sub 2}O{sub 5} through the SiO{sub 2} layer to silicon, and SiO{sub x} precipitate formation in silicon is proposed. After removing the V{sub 2}O{sub 5} and silicon-dioxide layers, texture is formed on the silicon surface, which intensely scatters light in the wavelength range of 300–550 nm and is important in the texturing of the front and rear surfaces of solar cells.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Influence of the doping type and level on the morphology of porous Si formed by galvanic etching

The formation of porous silicon (por-Si) layers by the galvanic etching of single-crystal Si samples (doped with boron or phosphorus) in an HF/C{sub 2}H{sub 5}OH/H{sub 2}O{sub 2} solution is investigated. The por-Si layers are analyzed by the capillary condensation of nitrogen and scanning electron microscopy (SEM). The dependences of the morphological characteristics of por-Si (pore diameter, specific surface area, pore volume, and thickness of the pore walls), which determine the por-Si combustion kinetics, on the dopant type and initial wafer resistivity are established.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Single crystals of (FeIn{sub 2}S{sub 4}){sub x} · (CuIn{sub 5}S{sub 8}){sub 1–x} alloys: Crystal structure, nuclear gamma resonance spectra, and thermal expansion

FeIn{sub 2}S{sub 4} and CuIn{sub 5}S{sub 8} compounds and (FeIn{sub 2}S{sub 4}){sub x} · (CuIn{sub 5}S{sub 8}){sub 1–x} alloy single crystals are grown by planar crystallization. It is shown that both of the initial FeIn{sub 2}S{sub 4} and CuIn{sub 5}S{sub 8} compounds and alloys on their basis crystallize with the formation of the cubic spinel structure. It is established that the unit-cell parameter a linearly varies with the composition parameter x. By means of nuclear gamma resonance spectroscopy in the transmission mode of measurements, the local states of iron ions in the alloys are studied. For the single crystals grown in the study, thermal expansion is explored using the dilatometry technique, the thermal-expansion coefficients are determined, and the Debye temperature and rms (root-mean-square) dynamic displacements are calculated.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Crystal defects in solar cells produced by the method of thermomigration

The results of studying the crystal structure of regions in silicon, recrystallized during the course of thermomigration of the liquid Si–Al zone in the volume of the silicon substrate, are reported (similar regions doped with an acceptor impurity are used to obtain high-voltage solar cells). X-ray methods (including measurements of both diffraction-reflection curves and topograms) and also high-resolution electron microscopy indicate that single-crystal regions in the form of a series of thin strips or rectangular grids are formed as a result of the thermomigration of liquid zones. Dislocation half-loops are detected in the surface layers of the front and back surfaces of the substrate. (311)-type defects are observed in the recrystallized regions.

14 SOLAR ENERGY↗
DOE OSTI2017

Silicon nanowire array architecture for heterojunction electronics

Photosensitive nanostructured heterojunctions n-TiN/p-Si were fabricated by means of titanium nitride thin films deposition (n-type conductivity) by the DC reactive magnetron sputtering onto nano structured single crystal substrates of p-type Si (100). The temperature dependencies of the height of the potential barrier and series resistance of the n-TiN/p-Si heterojunctions were investigated. The dominant current transport mechanisms through the heterojunctions under investigation were determined at forward and reverse bias. The heterojunctions under investigation generate open-circuit voltage V{sub oc} = 0.8 V, short-circuit current I{sub sc} = 3.72 mA/cm{sup 2} and fill factor FF = 0.5 under illumination of 100 mW/cm{sup 2}.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Effect of doping with rare-earth elements (Eu, Tb, Dy) on the conductivity of Bi{sub 2}Te{sub 3} layered single crystals

The temperature dependences of the resistivity in the directions parallel and perpendicular to the layer plane in the range of temperatures T = 5–300 K and the Hall and transverse magnetoresistance effects (magnetic fields <80 kOe, T = 5 K) are studied for doped and undoped Bi{sub 2}Te{sub 3} layered single crystals. It is shown that, upon the doping of Bi{sub 2}Te{sub 3} crystals with atoms of rare-earth elements (Eu, Tb, Dy), the resistivity in the directions parallel and perpendicular to the layer plane in Bi{sub 2}Te{sub 3} increases. The increase in the resistivity is caused mainly by a decrease in the charge-carrier mobility because of an increased contribution of charge-carrier scattering at defects to scattering processes. The charge-carrier concentrations and mobilities as well as the Hall factor defined by the anisotropy of the effective masses and by the orientation of ellipsoids with respect to the crystallographic axes are estimated.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Structure of bismuth films obtained using an array of identically oriented single-crystal bismuth islands preliminarily grown on a substrate

An array of identically oriented single-crystal bismuth islands formed on mica plates is used as a substrate to improve the structure of bismuth thin films grown by vacuum thermal evaporation. The array of islands is formed by the chemical etching of a single-crystal bismuth film 1 μm thick grown by floating-zone recrystallization under a coating. The structure of the obtained films is studied by X-ray diffraction, atomicforce microscopy, and electron backscatter diffraction scanning electron microscopy.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Mn{sub 0.1}Ag{sub 0.9}In{sub 4.7}S{sub 7.6} single crystals: Crystal structure, band gap, and thermal expansion

Mn{sub 0.1}Ag{sub 0.9}In{sub 4.7}S{sub 7.6} single crystals are for the first time grown by the Bridgman method (vertical variant). The single crystals crystallize in the cubic spinel structure. The band gap of the single crystals is determined from the transmittance spectra in the region of the fundamental absorption edge at temperatures of T = 295 and 80 K. Thermal expansion is studied by the dilatometric method in the temperature range 80–500 K. The coefficients of thermal expansion, the Debye temperatures, and the rms (root mean square) dynamic displacements of atoms are calculated.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Optical thyristor based on GaAs/InGaP materials

The possibility of creating thyristor structures with external optical control by laser radiation with a wavelength of ~800 nm, based on single-crystal wafers of semi-insulating GaAs and layers of InGaP with the lattice parameter compatible with GaAs, is shown.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Investigation of the Far-IR Reflection Spectra of SmS Single Crystals and Polycrystals in the Homogeneity Range

The far- and mid-IR reflection spectra of Sm{sub 1+} {sub x}S (x = 0–0.17) samples are recorded and analyzed, as well as their electrical and structural parameters at a temperature of T = 300 K. The bond ionicity in SmS is shown to fall with a decrease in the area of the X-ray coherent scattering region and an increase in the concentration of donor impurities and, consequently, conduction electron concentration. The electrical conductivity of stoichiometric SmS single crystals and polycrystals can be determined with an error of 10% from the IR reflection spectra. Due to the low structural quality of the samples, the electrical conductivity cannot be determined in the case of deviation from stoichiometry.

36 MATERIALS SCIENCE↗
DOE OSTI2018

On the Growth of FeIn{sub 2}S{sub 2}Se{sub 2} Single Crystals and the Study of their Properties

FeIn{sub 2}S{sub 2}Se{sub 2} single crystals are grown for the first time by the Bridgman method, and their composition and properties are determined. It is found that the single crystals are hexagonal in structure. The arrangement of Fe ions in the structure of the FeIn{sub 2}S{sub 2}Se{sub 2} crystals is studied by nuclear γ-resonance spectroscopy. From the transmittance spectra in the region of the fundamental absorption edge, the band gap of the single crystals grown in the study is determined.

36 MATERIALS SCIENCE↗