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At least 1,639 records · Page 91
All sourcesNASA NTRSDOE OSTI
DOE OSTI2019

Jahn–Teller Distortions of Metal Phthalocyanine Anions [M{sup IV}OPc]{sup n–} (M = Ti, V)

Based on X-ray diffraction data, a comparative analysis of the molecular geometry of eight phthalocyanine anions [Ti{sup IV}OPc]{sup n–}, [V{sup IV}OPc]{sup n–} (n = 1, 2) and neutral phthalocyanines [Ti{sup IV}OPc] and [V{sup IV}OPc] is carried out to study Jahn–Teller distortions of their molecular structure. It is experimentally shown that the occupation of a doubly degenerate lowest unoccupied molecular orbital of the [M{sup IV}OPc] molecule level by additional electrons leads to core tetragonal symmetry lowering from C{sub 4v} to C{sub 2v} with a significant change in the bond lengths distribution in the molecule. The effect manifests itself to a greater extent in imine bonds of the C–N{sub im}–C fragment, in which a systemic alternation of bonds into short and long is observed. The degree of distortion is proportional to the charge of the metal phthalocyanine macrocycle of the organic part of the molecule, C{sub 32}H{sub 16}N{sub 8}. The Jahn–Teller distortions of the macrocycle noticeably affect the position of frequencies characteristic of [M{sup IV}OPc] in the Raman spectra of single crystals of anionic phthalocyanine complexes in the range 1100–1700 cm{sup –1}. The experimental patterns agree fairly well with the results of DFT calculations.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Short-Lived Electron Excitations in FeTe{sub 1 –x}Se{sub x} as Revealed by Microwave Absorption

The Fe{sub 1 +y}Te{sub 1 –x}Se{sub x} single crystals with the various Se/Te ratios were studied by the microwave absorption and direct current resistivity measurements. The comparison of the microwave absorption data and the resistivity versus temperature made it possible to separate the contributions of two types of spin fluctuations. One of them is due to the anisotropic magnetic (nematic) fluctuations. It is observed over the wide temperature range from ~30 to 150 or 200 K. In FeSe it has the maximum close to the structural transition temperature. Another MWA anomaly is located in the narrow temperature range above the superconducting transition. It is likely due to the antiferromagnetic fluctuations. Annealing of a sample at the temperature around 300°C in the oxygen atmosphere made it possible to exclude the effect of excess iron on the observed anomalies.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Electronic Structure and Electronic Properties of PtSn{sub 4} Single Crystal

A topological semimetal PtSn{sub 4} single crystal is grown and the following properties are studied: its electrical resistivity in the temperature range from 4.2 to 300 K, galvanomagnetic properties at temperatures from 4.2 to 80 K and in magnetic fields of up to 100 kOe, and optical properties at room temperature. Theoretical calculations of the electronic structure are performed. It is shown that the residual resistivity is rather small: ρ{sub 0} = 0.47 μΩ cm, which is characteristic of a “good” metal. The ρ(T) dependence is of the metallic type, increasing monotonically with temperature. Analysis of the temperature dependences of the magnetoresistivity suggests that the Fermi surface of the PtSn{sub 4} compound may contain closed sheets. Studies of the Hall effect and the estimates made in the single-band model allowed us to conclude that the predominant type of current carriers are holes with concentration n = 6.8 × 10{sup 21} cm{sup –3} and mobility μ ≈ 1950 cm{sup 2}/(V s) at T = 4.2 K. It is shown that the optical properties of PtSn{sub 4} have features characteristic of “bad” metals. The calculation of the electronic structure of PtSn{sub 4} showed that, in general, this compound has a structure characteristic of metallic systems with a sufficiently large number of electronic states at the Fermi level, which is consistent with the experimental results on the electronic transport and optical properties of the PtSn{sub 4} single crystal.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Orientation dependence of relativistic-positron annihilation in single crystals

An effect of the orientation dependence of the cross section for the single-photon annihilation of relativistic positrons with atomic electrons in a crystal is predicted. It is shown that the probability for the single-photon annihilation of a channeled positron in a crystal may be either suppressed in a crystal in relation to a homogeneous medium or, on the contrary, enhanced. The reason is that, depending on their incidence angle, the positrons may be either in the vicinity of ion planes of the crystal, where the electron density is higher, or far away from them, where the electron density is lower.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2013

Simulation of the absorption of a femtosecond laser pulse in crystalline silicon

The effect of the nonlinearity of the absorptivity and absorption coefficient on the process of the intense photoexcitation of silicon is studied on the basis of a model of the two-photon excitation of a semiconductor with consideration for external emission. Correlation between the results of simulation of the absorption of a femtosecond laser pulse in single-crystal silicon and experimental data under conditions of the femtosecond excitation of surface plasmon polaritons makes it possible to refine the mechanism of changes in the absorptivity and to make an inference regarding the necessity of considering these changes when assessing the conditions of the laser treatment of semiconductors. Avenues for further improvement of the theoretical model are discussed.

36 MATERIALS SCIENCE↗
DOE OSTI2013

Anisotropy of the solid-state epitaxy of silicon carbide in silicon

A new method for the solid-state synthesis of epitaxial layers is developed, in which a substrate participates in the chemical reaction and the reaction product grows not on the substrate surface, as in traditional epitaxial methods, but inside the substrate. This method offers new opportunities for elastic-energy relaxation due to a mechanism operating only in anisotropic media, specifically, the attraction of point defects formed during the chemical reaction. The attracting point centers of dilatation form relatively stable objects, dilatation dipoles, which significantly reduce the total elastic energy. It is shown that, in crystals with cubic symmetry, the most favorable arrangement of dipoles is the 〈111〉 direction. The theory is tested by growing silicon carbide (SiC) films on Si (111) substrates by chemical reaction with carbon monoxide CO. High-quality single-crystal SiC-4H films with thicknesses of up to 100 nm are grown on Si (111). Ellipsometric analysis showed that the optical constants of the SiC-4H films are significantly anisotropic. This is caused not only by the lattice hexagonality but also by a small amount (about 2–6%) of carbon atoms remaining in the film due to dilatation dipoles. It is shown that the optical constants of the carbon impurity correspond to strongly anisotropic highly oriented pyrolytic graphite.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Optically detected cyclotron resonance in heavily boron-doped silicon nanostructures on n-Si (100)

Electron and hole cyclotron resonance at a frequency of 94 GHz is detected by a change in the intensity of photoluminescence lines whose positions are identical to those of dislocation luminescence lines D1 and D2 in single-crystal silicon and in heavily boron-doped silicon nanostructures on the Si (100) surface. The angular dependence of the spectrum of the optically detected cyclotron resonance corresponds to the tensor of the electron and hole effective mass in single-crystal silicon, and the resonance-line width indicates long carrier free-path times close to 100 ps. The results obtained are discussed within the framework of the interrelation of the electron-vibration coupling to charge and spin correlations in quasi-one-dimensional chains of dangling bonds in silicon.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Optical properties of nanowire structures produced by the metal-assisted chemical etching of lightly doped silicon crystal wafers

Layers of Si nanowires produced by the metal-assisted chemical etching of (100)-oriented single-crystal p-Si wafers with a resistivity of 1–20 Ω · cm are studied by reflectance spectroscopy, Raman spectros-copy, and photoluminescence measurements. The nanowire diameters are 20–200 nm. The wafers are supplied by three manufacturing companies and distinguished by their different lifetimes of photoexcited charge carriers. It is established that the Raman intensity for nanowires longer than 1 μm is 3–5 times higher than that for the substrates. The interband photoluminescence intensity of nanowires at the wavelength 1.12 μm is substantially higher than that of the substrates and reaches a maximum for samples with the longest bulk lifetime, suggesting a low nonradiative recombination rate at the nanowire surfaces.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Epitaxial growth of hexagonal silicon polytypes on sapphire

The formation of a single-crystal silicon polytype is observed in silicon-on-sapphire structures by high-resolution transmission electron microscopy. The appearance of inclusions with a structure different from that of diamond is attributed to the formation of strong-twinning regions and the aggregation of stacking faults, which form their own crystal structure in the crystal lattice of silicon. It is demonstrated that the given modification belongs to the 9R silicon polytype.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Antireflective nanostructured zinc oxide arrays produced by pulsed electrodeposition

Conditions for the pulsed electrochemical deposition of nanostructured zinc oxide arrays with a certain morphology, crystal structure, and optical properties from aqueous electrolytes onto substrates of transparent electrically conducting tin dioxide and on single-crystal silicon wafers with built-in homojunctions are studied in order to develop antireflection coatings for solar cells. It is shown that it is possible to obtain single-layer planar antireflection coatings or arrays of nanorods of this material, both having the form of hexagonal prisms and exhibiting the moth-eye effect.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Low-temperature conductivity in CuGaS{sub 2} single crystals

CuGaS{sub 2} single crystals are grown by the Bridgman-Stockbarger method. X-ray diffraction and Raman studies are performed. It is shown that the conductivity at low temperatures has an activation mechanism: in the range of 100–300 K, impurity conduction with an acceptor activation energy of 12 meV dominates; at temperatures below 100 K, Mott conductivity, i.e., so-called hopping conductivity with a variable hop length prevails. The density of localized states and the average carrier hop length are estimated.

36 MATERIALS SCIENCE↗
DOE OSTI2015

MBE growth of GaP on a Si substrate

It is shown that single-crystal GaP buffer layers can be formed on a Si substrate by molecular-beam epitaxy, with the “migration-enhanced epitaxy” procedure applied in the stage in which the nucleating layer is formed. When a GaP layer is produced on a p-type silicon substrate, a p-n junction is created in a natural way between the p-Si substrate and the surface n-Si layer produced by the diffusion of phosphorus into the substrate during the course of the epitaxial growth of GaP. This p-n junction can be used as the first junction of a silicon-based multijunction photovoltaic converter.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Surface plasmon polaritons in a composite system of porous silicon and gold

A composite system of silicon quantum dots and gold particles with properties periodically changing along the surface (i.e., a system exhibiting the properties of a diffraction grating) is obtained by a one-step metal-assisted chemical etching. The spectral and angular dependences of the photoresponse for the composite system on single-crystal silicon are studied. The photoresponse peaks were observed, which behavior (the dependence on the parameters of the diffraction grating, wavelength and incidence angles of light) is attributed to the excitation of plasmon-polariton modes at the surface of the composite system with the diffraction grating. At the same time, the obtained values of the wave vectors for these modes are smaller than those calculated for plasmon polaritons excited at the interface between air and metal (gold) diffraction grating.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Electron scattering in the Δ{sub 1} model of the conduction band of germanium single crystals

Electron scattering in the possible Δ{sub 1} models of the conduction band in germanium crystals formed by hydrostatic or uniaxial pressure is investigated. On the basis of the theory of anisotropic scattering, the temperature dependences of the anisotropy parameter of the relaxation times and electron mobility for these models under conditions of scattering at impurity ions, as well as at acoustic and intervalley phonons are obtained. Analysis of the temperature dependences indicates that, in the temperature range of 77–300 K, intervalley scattering becomes substantial. Only for the Δ{sub 1} model formed by uniaxial pressure along the crystallographic direction [100], the electron scattering at intervalley phonons, which correspond to the g transitions, is minor with respect to scattering at acoustic phonons (the intravalley scattering) and impurity ions.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Features of the uniaxial elastic deformation of X-ray-irradiated p-Si crystals

Changes in the conductivity of p-Si single-crystals irradiated at room temperature during their mechanical compression and stress relief are studied. It is shown that irradiation is accompanied by the generation of point defects in silicon, which play the role of stoppers for dislocation motion. The effect of “radiation memory” in “electronic” silicon crystals is detected.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Effect of copper on the recombination activity of extended defects in silicon

The effect of copper atoms introduced by high-temperature diffusion on the recombination properties of dislocations and dislocation trails in p-type single-crystal silicon is studied by the electron-beam-induced current technique. It is shown that, in contrast to dislocations, dislocation trails exhibit an increase in recombination activity after the introduction of copper. Bright contrast appearance in the vicinity of dislocation trails is detected after the diffusion of copper and quenching of the samples. The contrast depends on the defect density in these trails.

36 MATERIALS SCIENCE↗
DOE OSTI2015

The Shubnikov-de Haas effect and thermoelectric properties of Tl-doped Sb{sub 2}Te{sub 3} and Bi{sub 2}Se{sub 3}

The influence of doping with Tl on the Shubnikov-de Haas effect at T = 4.2 K in magnetic fields up to 38 T in p-Sb{sub 2−x}Tl{sub x}Te{sub 3} (x = 0, 0.005, 0.015, and 0.05) and n-Bi{sub 2−x}Tl{sub x}Se{sub 3} (x = 0, 0.01, 0.02, 0.04, and 0.06) single crystals is investigated. Extreme cross-sections of the Fermi surface in both materials decrease upon doping with Tl: the hole concentration decreases in Sb{sub 2−x}Tl{sub x}Te{sub 3} due to the donor effect of Tl and the electron concentration in n-Bi{sub 2−x}Tl{sub x}Se{sub 3} decreases due to the acceptor effect of Tl. The temperature dependences of the Seebeck coefficient, electrical conductivity, thermal conductivity, and dimensionless thermoelectric figure of merit in a temperature range of 77–300 K are measured. The thermal conductivity and electrical conductivity decrease upon doping with Tl both in p-Sb{sub 2−x}Tl{sub x}Te{sub 3} and in n-Bi{sub 2−x}Tl{sub x}Se{sub 3}. The Seebeck coefficient increases in all compositions upon an increase in doping over the entire measured temperature range. The thermoelectric figure of merit increases upon doping with Tl.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Effect of microwave treatment on the luminescence properties of CdS and CdTe:Cl Single Crystals

The effect of microwave radiation on the luminescence properties of CdS and CdTe:Cl single crystals is studied. It is established that the exposure of these semiconductors to short-term (≤30 s) microwave radiation substantially modifies their impurity and defect structure. The mechanisms of transformation of the defect subsystem of II–VI single crystals upon microwave treatment are discussed. It is shown that the experimentally observed changes are defined by the nonthermal effects of microwave radiation at a power density of 7.5 W cm{sup –2}; at 90 W cm{sup –2}, nonthermal effects are prevailing.

36 MATERIALS SCIENCE↗