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At least 1,711 records · Page 95
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DOE OSTI2019

Influence of the Charge State of Xenon Ions on the Depth Distribution Profile Upon Implantation into Silicon

Experimental depth distributions of the concentration of implanted xenon ions depending on their charge state and irradiation energy are presented. Xenon ions in charge states q = 1–20 and energies in the range from 50 to 400 keV are incorporated into single-crystal silicon. Irradiation is performed in the direction not coinciding with the crystallographic axes of the crystal to avoid the channeling effect. The ion fluence varies in the range of 5 × (10{sup 14}–10{sup 15}) ion/cm{sup 2}. The irradiation by singly charged ions and investigation of the samples by Rutherford backscattering spectroscopy is performed using an HVEE acceleration complex at Moscow State University. Multiply charged ions are implanted using a FAMA acceleration complex at the Vinća Institute of Nuclear Sciences. The depth distribution profiles of the incorporated ions are found using Rutherford backscattering spectroscopy. Experimental results are correlated with computer calculations. It is shown that the average projective path of multiply charged ions in most cases is shorter when compared with the average projected path of singly charged ions and the results of computer modeling.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

On the Synthesis and Photoluminescence and Cathodoluminescence Properties of CdSe, CdTe, PbS, InSb, and GaAs Colloidal Quantum Dots

Quantum dots of the group of wide-gap and narrow-gap semiconductors are synthesized and investigated under identical conditions, which makes it possible to perform the comparative analysis and modeling of the mechanisms of radiative recombination and luminescence, for which a stable exciton bond between the electron and hole has an important role. Exciton states are unstable for quantum dots without a shell and narrow-gap semiconductors, which leads to a substantial decrease in the probability of radiative recombination and, correspondingly, the quantum yield of luminescence. The experimental values of the spectral position of the luminescence maximum for quantum dots with clear manifestation of the exciton recombination mechanism noticeably shift to the long-wavelength region with respect to the calculated ones. In calculations and analysis, we use the effective electron mass for bulk semiconductors. The observed good correspondence of the calculated values of the maximum and spectral band with the experiment can mean that quantum dots have a long-range order crystalline structure similar to that one observed in single crystals and polycrystals.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Plasma-Chemical Deposition of Diamond-Like Films onto the Surface of Heavily Doped Single-Crystal Diamond

The plasma-chemical deposition of diamond-like carbon (DLC) films onto heavily boron-doped single-crystal p-type diamond (the concentration ~10{sup 20} cm{sup –3}) in CH{sub 4} + Ar plasma is conducted. The deposition rate is 7 nm min{sup –1}. The elemental composition and properties of the films are studied in detail. It is found that the films are enriched with hydrogen, possess a density of 2.4 g cm{sup –3}, and exhibit an ultrasmooth surface (with a roughness of 0.4 ± 0.2 nm).

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Studies of the Cross Section and Photoluminescence of a GaAs Layer Grown on a Si/Al{sub 2}O{sub 3} Substrate

A GaAs/AlAs/GaAs/AlAs/Ge heterostructure grown on a Si/Al{sub 2}O{sub 3}(1 1-bar 02) substrate is formed and studied. The Ge buffer layer is produced by the “hot wire” technique, whereas the III–V layers are grown by metal–organic vapor-phase epitaxy. The optical quality of the III–V layers is determined by photoluminescence spectroscopy. Structural studies are performed by high-resolution transmission electron microscopy. The elemental composition is determined by energy-dispersive X-ray spectroscopy. In the study, the possibility of growing a single-crystal GaAs layer on a Si/Al{sub 2}O{sub 3} substrate through AlAs/GaAs/AlAs/Ge buffer layers is shown.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Temperature Coefficient of Movement of the Resonance Level of Iron in Pb{sub 1 – x} {sub – y}Sn{sub x}Fe{sub y}Te Alloys

The phase and elemental composition and the temperature dependences of the resistivity and Hall coefficient (temperature range 4.2 K ≤ T ≤ 300 K, magnetic fields B ≤ 0.07 T) are studied in Pb{sub 1 – x– y}Sn{sub x}Fe{sub y}Te alloys with varying matrix composition and iron-impurity concentration along single-crystal ingots synthesized by the Bridgman–Stockbarger method. The distributions of tin and iron along ingots are obtained. Anomalous temperature dependences of the Hall coefficient related to the Fermi-level pinning by the resonance level of iron located in the valence band of the alloys are found. The experimental results are analyzed within the model of transformation of the electronic structure, involving iron level movement with respect to the top of the valence band with increasing tin concentration and temperature. The temperature coefficient of the iron level movement with respect to the midgap is determined. Possible diagrams of transformation of the electronic structure with increasing temperature in alloys with the normal spectrum (0.06 ≤ x ≤ 0.35) are proposed.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2017

Beta-induced decrease in the content of the Si-II, Si-XII, Si-III, and α-Si silicon phases formed under an indenter

Volumes of the metastable Si-II, Si-XII, Si-III, and α-Si silicon phases in a locally deformed (Berkovich pyramid) region of unirradiated and beforehand irradiated Si single crystals are quantitatively estimated. Experimental data obtained by Raman spectroscopy and in situ detection of Si-I → Si-II silicon phase transformations under an indenter are used to calculate the volumes. The preliminary irradiation of silicon by β particles from a {sup 90}Sr–{sup 90}Y source (fluence F = 3.24 × 10{sup 10} cm{sup –2}, intensity I = 1.8 × 10{sup 5} cm{sup –2} s{sup –1}) reduces the volume of these phases by a factor of more than 1.5.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2017

Single-phase epitaxial InFeSb layers with a Curie temperature above room temperature

Epitaxial InFeSb/GaAs heterostructures were obtained by laser deposition in vacuum. Investigations by high-resolution transmission electron microscopy and microdiffraction showed that the InFeSb layers are single-crystal and do not contain additional phase inclusions. Study of their magnetotransport properties have revealed that an anomalous Hall effect and a negative magnetoresistance up to room temperature are observed in the structures.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2017

Optical properties of bulk gallium nitride single crystals grown by chloride–hydride vapor-phase epitaxy

A gallium nitride crystal 5 mm in thickness was grown by chloride–hydride vapor-phase epitaxy on a sapphire substrate, from which the crystal separated during cooling. At an early stage, a three-dimensional growth mode was implemented, followed by a switch to a two-dimensional mode. Spectra of exciton reflection, exciton luminescence, and Raman scattering are studied in several regions characteristic of the sample. Analysis of these spectra and comparison with previously obtained data for thin epitaxial GaN layers with a wide range of silicon doping enabled conclusions about the quality of the crystal lattice in these characteristic regions.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2017

NMR study of topological insulator Bi{sub 2}Te{sub 3} in a wide temperature range

NMR studies of {sup 125}Te in the topological insulator bismuth telluride Bi{sub 2}Te{sub 3} in a wide temperature range from room temperature to 12.5 K are performed. The pulsed NMR spectrometer Bruker Avance 400 is applied. The NMR spectra are obtained for the powder from Bi2Te3 single crystal and monocrystalline plates with the orientations c || B and c ⊥ B. At room temperature, the spectra consist of two lines related to two nonequivalent positions of tellurium nuclei Te1 and Te2. The parameters of the NMR frequency shift tensor are found from the powder spectrum. The temperature dependences of the spectra for the powder and plates with the orientation c ⊥ B agree with each other. The line shift with decreasing temperature is explained by the reduction of the Knight shift. The thermal activation energy of charge carriers is estimated. The spectra for the plates with the orientation c || B demonstrate peculiar behavior below 91 K. The spin-lattice relaxation time for the powder and monocrystalline plates with both orientations at room temperature is measured.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2017

Comparative study of the spectral and structural properties of EuAl{sub 3}(BO{sub 3}){sub 4} single crystals with different morphologies

Europium alumoborate EuAl{sub 3}(BO{sub 3}){sub 4} microcrystals have been synthesized by the flux method at a temperature of 1050°C. The obtained crystals have different morphologies: both plane-faced and skeletal microcrystals have been observed. Infrared spectroscopy, cathodeluminescence, and transmission electron microscopy investigations of individual microcrystals showed that the spectral and structural characteristics of these morphological forms coincide. The obtained crystals are characterized by the rhombohedral symmetry (sp. gr. R32) with the inclusions of C2/c monoclinic phase domains.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2017

IR spectra of carbon-vacancy clusters in the topochemical transformation of silicon into silicon carbide

Using infrared (IR) spectroscopy and spectral ellipsometry, we experimentally confirmed the previously predicted mechanochemical effect of the stoichiometric composition disorder leading to the formation of carbon-vacancy structures in silicon carbide (SiC) films grown on silicon substrates by the atom substitution method. It was found that a band at 960 cm{sup –1} in the IR spectra of SiC films on silicon, corresponding to “carbon-vacancy clusters” is always present in SiC films grown under pure carbon monoxide (CO) or in a mixture of CO with silane (SiH{sub 4}) on Si substrates of different orientation and doping level and type. There is no absorption band in the region of 960 cm–1 in the IR spectra of SiC films synthesized at the optimum ratio of the CO and trichlorosilane (SiHCl{sub 3}) gas pressures. The previously predicted mechanism of the chemical reaction of substitution of Si atoms for carbon by the interaction of gases CO and SiHCl{sub 3} on the surface of the silicon substrate, which leads to the formation of epitaxial layers of single-crystal SiC, is experimentally confirmed.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2017

Initial stages of the growth of barium strontium titanate films on a semi-isolating silicon carbide substrate

The initial stages of the growth of ferroelectric barium strontium titanate films on single-crystal silicon carbide substrates have been studied for the first time. The choice of a substrate with high thermal conductivity has been due to the possibility of applying these structures in powerful microwave devices. The temperature ranges separating the mechanism of the surface diffusion of deposited atoms from the diffusion via a gaseous phase during the growth of multicomponent films have been determined. The studies show that the mass transfer by means of surface diffusion leads to the formation of small-height nuclei that cover a large area of the substrate, whereas the mass transfer via a gaseous phase leads to the formation of a “columnar” islandtype structure with small percentage of covering the substrate and larger island heights.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Effect of structural defects on the magnetic properties of the EuBaCo{sub 1.90}O{sub 5.36} single crystal

The effect of structural defects in cobalt and oxygen sublattices with the constant average oxidation level 3+ of all cobalt ions on the magnetic properties of the EuBaCo{sub 1.90}O{sub 5.36} single crystal has been studied. The magnetic properties of the single crystal and the polycrystalline sample of the corresponding composition are compared in the range T = 200–650 K. The results show that the cobalt-deficient EuBaCo{sub 2–x}O{sub 5.5–δ} samples demonstrate a three-dimensional XY ferromagnetic ordering of magnetic sublattices. The values of the effective magnetic moment at T > 480 K indicate the existence of the IS and HS states of Co{sup 3+} ions. The large difference of values of μ{sub eff} of the EuBaCo{sub 1.90}O{sub 5.36} single crystal and polycrystal can be due to that the magnetic ion spins lie in plane ab. The magnetic field directed along plane ab substantially influences the magnetic ordering at T < 300 K.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Low-temperature luminescence and thermally stimulated luminescence of BeO: Mg single crystals

Luminescence and thermally stimulated luminescence (TL) of BeO: Mg crystals are studied at T = 6–380 K. The TL glow curves and the spectra of luminescence (1.2–6.5 eV), luminescence excitation, and reflection (3.7–20 eV) are obtained. It is found that the introduction of an isovalent magnesium impurity into BeO leads to the appearance of three new broad luminescence bands at 6.2–6.3, 4.3–4.4, and 1.9–2.6 eV. The first two are attributed to the radiative annihilation of a relaxed near-impurity (Mg) exciton, the excited state of which is formed as a result of energy transfer by free excitons. The impurity VUV and UV bands are compared with those for the intrinsic luminescence of BeO caused by the radiative annihilation of self-trapped excitons (STE) of two kinds: the band at 6.2–6.3 eV of BeO: Mg is compared with the band at 6.7 eV (STE{sub 1}) of BeO, and the band at 4.3–4.4 eV is compared with the band at 4.9 eV (STE{sup 2}) of BeO. In the visible region, the luminescence spectrum is due to a superposition of intracenter transitions in an impurity complex including a magnesium ion. The manifestation of X-ray-induced luminescence bands at T = 6 K in BeO: Mg indicates their excitation during band-to-band transitions and in recombination processes. The energy characteristics of the impurity states in BeO: Mg are determined; the effect of the isovalent impurity on the fluctuation rearrangement of the BeO: Mg structure in the thermal transformation region of STE{sub 1} → STE{sub 2} is revealed.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Evolution of domain structure and formation of charged domain walls during polarization reversal in lithium niobate single crystals modified by vacuum annealing

The evolution of the domain structure during polarization reversal has been investigated in plates of lithium niobate with spatially inhomogeneous electrical conductivity produced by vacuum annealing. The formation of charged domain walls in the crystal bulk has been studied. Revealed features of the domain growth in the bulk have been attributed to the formation of nanodomains under the pyroelectric field and inhomogeneous spatial distribution of the electric field. Creation of the charged domain walls with controlled parameters is of great interest for domain walls engineering.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Luminescence of impurity Ce{sup 3+} centers in KH{sub 2}PO{sub 4} : Ce crystals

The photoluminescence, X-ray luminescence, and cathodoluminescence spectra of KH{sub 2}PO{sub 4} : Ce single crystals contain a nonelementary band of radiation with an energy of 3.55 eV and decay time of 27–33 ns. It is formed by fast radiative interconfiguration d → f transitions between the excited and ground states of Ce{sup 3+} ions, with the ground state is split by a crystalline field. In the range of concentrations studied (0.5–3 × 10{sup –2} wt %), Ce3+ ions enter the KH{sub 2}PO{sub 4} : Ce crystal lattice as substitution ions. Local charge compensation takes place by means of defects in the crystal structure that cause luminescence with a large Stokes shift in the region of 2.4–2.2 eV. The presence of hydrogen sublattice defects decreases the efficiency of energy transport by free charge carriers to the luminescent centers. The interaction of defects and impurity centers manifests itself as a slow inertial building-up of the stationary X-ray luminescence yield of Ce{sup 3+} centers.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Specific features of the ferroelectric state in two-layer barium strontium titanate-based heterostructures

The structural properties of one- and two-layer heterostructures based on the barium–strontium titanate of various compositions deposited by the Frank–Van der Merve on a magnesium oxide substrate have been studied. The heterostructures have been prepared by the rf sputtering of the stoichiometric ceramic targets in a Plazma 50 SE deposition system. The principal difference of this method of deposition from known analogs is that the growth of single-crystal films occurs from a disperse oxide phase formed in the plasma of a high-current rf discharge during the ceramic target sputtering at the cluster level. The peculiarities of the manifestation of the ferroelectric state in the two-layer heterostructures when changing the sequence order of the films with various compositions of barium–strontium titanate.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Domain epitaxial growth of ferroelectric films of barium strontium titanate on sapphire

A model of the epitaxial growth of crystalline multicomponent films on single-crystal substrates with a domain correspondence is presented using a solid solution of barium strontium titanate on sapphire substrates (r cut). The domain epitaxial growth suggests the matching of the lattice planes of the film and the substrate having similar structures by comparison of domain multiple of an integral number of the interplanar spacings. Variation of the component composition of the solid solution enables changes in the domain size in the range sufficient for epitaxial growth. This method can be used to project the epitaxial growth of films of various solid solutions on single-crystal substrates.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗