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At least 1,081 records · Page 60
All sourcesNASA NTRSDOE OSTI
DOE OSTI2014

Induced magnetic ordering in alloyed compounds based on Pauli paramagnet YCo{sub 2}

Intermetallic YCo{sub 2} compound is a Pauli exchange-enhanced paramagnet. Structural and magnetic properties melt-spun YCo{sub 2} pure and alloyed with Nb or Ti are presented. The samples crystallize in MgCu{sub 2}-type phase with lattice constant a changing from 7.223 Å for YCo{sub 2}, through 7.213 Å for Y{sub 0.9}Nb{sub 0.1}Co{sub 2} to 7.192 Å for Y{sub 0.9}Ti{sub 0.1}Co{sub 2}, where Y atoms are replaced by Nb or Ti atoms. Nanocrystalline phases can be produced by appropriate cooling rates for the solidification process. By the synthesis process free volumes, vacancies, and alloyed atoms are introduced into the YCo{sub 2} intermetallic. Ab-initio calculations have been performed to investigate the effects of substitution on the spin-split electronic band structure in the ordered YCo{sub 2}. A ferrimagnetic ground state is found in the alloyed systems with substitution on the Y-site which is energetically favorable compared to point defects on Co-sites. However, the experimentally found increased magnetic ordering in alloyed YCo{sub 2} appears to be based on microstructure effects.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Quantitative comparison between Z{sub 1∕2} center and carbon vacancy in 4H-SiC

In this study, to reveal the origin of the Z{sub 1∕2} center, a lifetime killer in n-type 4H-SiC, the concentrations of the Z{sub 1∕2} center and point defects are compared in the same samples, using deep level transient spectroscopy (DLTS) and electron paramagnetic resonance (EPR). The Z{sub 1∕2} concentration in the samples is varied by irradiation with 250 keV electrons with various fluences. The concentration of a single carbon vacancy (V{sub C}) measured by EPR under light illumination can well be explained with the Z{sub 1∕2} concentration derived from C-V and DLTS irrespective of the doping concentration and the electron fluence, indicating that the Z{sub 1∕2} center originates from a single V{sub C}.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Irradiation-induced nano-voids in strained tin precipitates in silicon

We report on self-assembling of spherically shaped voids in nanometer size strained Sn precipitates after irradiation with He{sup +} ions in different conditions. It is found that high-temperature irradiation induces vacancies which are collected by compressively strained Sn precipitates enhancing of out-diffusion of Sn atoms from the precipitates. Nano-voids formation takes place simultaneously with a β- to α-phase transformation in the Sn precipitates. Post-irradiation thermal treatment leads to the removal of voids and a backward transformation of the Sn phase to β-phase. Strain-enhanced separation of point defects along with vacancy assisted Sn out-diffusion and precipitate dissolution are discussed.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Diffusion and impurity segregation in hydrogen-implanted silicon carbide

Diffusion and segregation behavior of hydrogen and oxygen in silicon carbide subjected to H implantation and subsequent annealing were studied with a number of analytical techniques including Secondary Ion Mass Spectrometry (SIMS), Rutherford backscattering spectrometry in channeling geometry, field emission scanning electron microscopy, optical microscopy, cross-sectional transmission electron microscopy, and atomic force microscopy. H{sup +} implantation was carried out with energies of 200 keV, 500 keV, or 1 MeV to doses of 1 × 10{sup 16}, 1 × 10{sup 17}, or 2 × 10{sup 17} ion/cm{sup 2}, and thermal treatment was conducted in flowing argon for 1 to 2 h at temperatures of 740, 780, 1000, or 1100 °C. The process of migration and eventual loss of hydrogen in a point defect regime is postulated to proceed to a large extent through ionized vacancies. This conclusion was derived from the observed substantial difference in H mobilities in n- vs. p-type SiC as the population of ionized vacancies is governed by the Fermi-Dirac statistics, i.e., the position of the Fermi level. For higher doses, a well defined buried planar zone forms in SiC at the maximum of deposited energy, comprising numerous microvoids and platelets that are trapping sites for hydrogen atoms. At a certain temperature, a more or less complete exfoliation of the implanted layer is observed. For a 1 MeV implant heated to 1100 °C in nominally pure argon, SIMS profiling reveals a considerable oxygen peak of 10{sup 16} O atoms/cm{sup 2} situated at a depth close to that of the peak of the implanted H{sup +}. Similarly, 1100 °C annealing of a 200 keV implant induces the formation of a thin oxide (4 nm), located at the interface between the implanted layer and the substrate as evidenced by both SIMS and HRTEM. The measurements were taken on the part of the sample that remained un-exfoliated. In view of a lack of convincing evidence that a hexagonal SiC might contain substantial amounts of oxygen, further investigation is under way to elucidate its presence in the irradiation-damaged films.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Macro- and microscopic properties of strontium doped indium oxide

Solid state synthesis and physical mechanisms of electrical conductivity variation in polycrystalline, strontium doped indium oxide In{sub 2}O{sub 3}:(SrO){sub x} were investigated for materials with different doping levels at different temperatures (T = 20–300 °C) and ambient atmosphere content including humidity and low pressure. Gas sensing ability of these compounds as well as the sample resistance appeared to increase by 4 and 8 orders of the magnitude, respectively, with the doping level increase from zero up to x = 10%. The conductance variation due to doping is explained by two mechanisms: acceptor-like electrical activity of Sr as a point defect and appearance of an additional phase of SrIn{sub 2}O{sub 4}. An unusual property of high level (x = 10%) doped samples is a possibility of extraordinarily large and fast oxygen exchange with ambient atmosphere at not very high temperatures (100–200 °C). This peculiarity is explained by friable structure of crystallite surface. Friable structure provides relatively fast transition of samples from high to low resistive state at the expense of high conductance of the near surface layer of the grains. Microscopic study of the electro-diffusion process at the surface of oxygen deficient samples allowed estimation of the diffusion coefficient of oxygen vacancies in the friable surface layer at room temperature as 3 × 10{sup −13} cm{sup 2}/s, which is by one order of the magnitude smaller than that known for amorphous indium oxide films.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Synchrotron-based analysis of chromium distributions in multicrystalline silicon for solar cells

Chromium (Cr) can degrade silicon wafer-based solar cell efficiencies at concentrations as low as 10 10 cm –3 . In this contribution, we employ synchrotron-based X-ray fluorescence microscopy to study chromium distributions in multicrystalline silicon in as-grown material and after phosphorous diffusion. We complement quantified precipitate size and spatial distribution with interstitial Cr concentration and minority carrier lifetime measurements to provide insight into chromium gettering kinetics and offer suggestions for minimizing the device impacts of chromium. We observe that Cr-rich precipitates in as-grown material are generally smaller than iron-rich precipitates and that Cr i point defects account for only one-half of the total Cr in the as-grown material. This observation is consistent with previous hypotheses that Cr transport and CrSi 2 growth are more strongly diffusion-limited during ingot cooling. Here, we apply two phosphorous diffusion gettering profiles that both increase minority carrier lifetime by two orders of magnitude and reduce [Cr i ] by three orders of magnitude to ≈10 10 cm –3 . Some Cr-rich precipitates persist after both processes, and locally high [Cr i ] after the high-temperature process indicates that further optimization of the chromium gettering profile is possible.

14 SOLAR ENERGY↗
DOE OSTI2015

A-site stoichiometry and piezoelectric response in thin film PbZr 1-x Ti x O 3

Lead zirconate titanate (PZT) films with Zr/Ti ratios of 52/48 and 30/70 annealed at varying partial pressures of PbO within the perovskite phase field exhibited permittivities of 1150 and 600, respectively, with loss tangents of 0.02. Many of the functional properties, including the permittivity, piezoelectricity as indicated via the Rayleigh coefficients, and the aging rates were found to be weakly dependent of the lead content in the single phase field. Minor polarization electric field hysteresis loops and piezoelectric coefficient e 31,f values after a hot poling process suggest that the point defect helps stabilize the aligned domain states. Measurements of the local nonlinear response show an increased low response cluster size with decreasing PbO content, indicating that PbO deficiency acts to reduce domain wall motion where it is already low

36 MATERIALS SCIENCE↗
DOE OSTI2015

Elimination of carbon vacancies in 4H-SiC epi-layers by near-surface ion implantation: Influence of the ion species

The carbon vacancy (V{sub C}) is a prevailing point defect in high-purity 4H-SiC epitaxial layers, and it plays a decisive role in controlling the charge carrier lifetime. One concept of reducing the V{sub C}-concentration is based on carbon self-ion implantation in a near surface layer followed by thermal annealing. This leads to injection of carbon interstitials (C{sub i}'s) and annihilation of V{sub C}'s in the epi-layer “bulk”. Here, we show that the excess of C atoms introduced by the self-ion implantation plays a negligible role in the V{sub C} annihilation. Actually, employing normalized implantation conditions with respect to displaced C atoms, other heavier ions like Al and Si are found to be more efficient in annihilating V{sub C}'s. Concentrations of V{sub C} below ∼2 × 10{sup 11} cm{sup −3} can be reached already after annealing at 1400 °C, as monitored by deep-level transient spectroscopy. This corresponds to a reduction in the V{sub C}-concentration by about a factor of 40 relative to the as-grown state of the epi-layers studied. The negligible role of the implanted species itself can be understood from simulation results showing that the concentration of displaced C atoms exceeds the concentration of implanted species by two to three orders of magnitude. The higher efficiency for Al and Si ions is attributed to the generation of collision cascades with a sufficiently high energy density to promote C{sub i}-clustering and reduce dynamic defect annealing. These C{sub i}-related clusters will subsequently dissolve during the post-implant annealing giving rise to enhanced C{sub i} injection. However, at annealing temperatures above 1500 °C, thermodynamic equilibrium conditions start to apply for the V{sub C}-concentration, which limit the net effect of the C{sub i} injection, and a competition between the two processes occurs.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Elimination of carbon vacancies in 4H-SiC employing thermodynamic equilibrium conditions at moderate temperatures

The carbon vacancy (V{sub C}) is a major point defect in high-purity 4H-SiC epitaxial layers limiting the minority charge carrier lifetime. In layers grown by chemical vapor deposition techniques, the V{sub C} concentration is typically in the range of 10{sup 12 }cm{sup −3}, and after device processing at temperatures approaching 2000 °C, it can be enhanced by several orders of magnitude. In the present study, both as-grown layers and a high-temperature processed one have been annealed at 1500 °C and the V{sub C} concentration is demonstrated to be strongly reduced, exhibiting a value of only a few times 10{sup 11 }cm{sup −3} as determined by deep-level transient spectroscopy measurements. The value is reached already after annealing times on the order of 1 h and is evidenced to reflect thermodynamic equilibrium under C-rich ambient conditions. The physical processes controlling the kinetics for establishment of the V{sub C} equilibrium are estimated to have an activation energy below ∼3 eV and both in-diffusion of carbon interstitials and out-diffusion of V{sub C}'s are discussed as candidates. This concept of V{sub C} elimination is flexible and readily integrated in a materials and device processing sequence.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Improved thermoelectric properties in heavily doped FeGa 3

FeGa 3 , a hybridization gap semiconductor, has been substituted with an n-type dopant Ge to form a series of compositions FeGa 3-x Ge x . Electrical and thermal transport properties of these compositions have been studied. Change in carrier density (n) is evident from the Hall measurements. The carrier density (n) can be as high as ~10 21 cm -3 in these compositions. In order to study the role of heavy doping on the thermoelectric properties of FeGa 3 , an alloy series Fe 1-y Co y Ga 3-x Ge x has also been synthesized with higher concentrations of Ge (x = 0.1–0.35) and Co (y = 0.1–0.5). From resistivity and Seebeck coefficient measurements, it appears that heavy doping is accomplished by the simultaneous substitutions of Ge and Co. The systematic change in both resistivity (ρ) and Seebeck coefficient (α) is possibly due to change in the carrier density (n). The power factor (PF) α 2 /ρ improves steadily with increasing carrier density and the best PF ~1.1 mW/m K 2 is observed for the heavily doped compositions at 875 K. In the alloy series Fe 1-y Co y Ga 3-x Ge x , thermal conductivity is also reduced substantially due to point defect scattering. Finally, due to higher power factors, the figure of merit ZT improves to 0.25 at 875 K for the heavily doped compositions.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Interplay between mass-impurity and vacancy phonon scattering effects on the thermal conductivity of doped cadmium oxide

Understanding the impact and complex interaction of thermal carrier scattering centers in functional oxide systems is critical to their progress and application. In this work, we study the interplay among electron and phonon thermal transport, mass-impurity scattering, and phonon-vacancy interactions on the thermal conductivity of cadmium oxide. We use time domain thermoreflectance to measure the thermal conductivity of a set of CdO thin films doped with Dy up to the saturation limit. Using measurements at room temperature and 80 K, our results suggest that the enhancement in thermal conductivity at low Dy concentrations is dominated by an increase in the electron mobility due to a decrease in oxygen vacancy concentration. Furthermore, we find that at intermediate doping concentrations, the subsequent decrease in thermal conductivity can be ascribed to a large reduction in phononic thermal transport due to both point defect and cation-vacancy scattering. With these results, we gain insight into the complex dynamics driving phonon scattering and resulting thermal transport in functional oxides.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Elemental intermixing within an ultrathin SrRuO{sub 3} electrode layer in epitaxial heterostructure BaTiO{sub 3}/SrRuO{sub 3}/SrTiO{sub 3}

Aberration corrected scanning transmission electron microscopy is used to directly observe atom columns in an epitaxial BaTiO{sub 3} thin film deposited on a 3.6 nm-thick SrRuO{sub 3} electrode layer above an SrTiO{sub 3} (001) substrate. Compositional gradients across the heterointerfaces were examined using electron energy-loss spectroscopy techniques. It was found that a small amount of Ba and Ti had diffused into the SrRuO{sub 3} layer, and that this layer contained a non-negligible concentration of oxygen vacancies. Such point defects are expected to degrade the electrode’s electronic conductivity drastically, resulting in a much longer screening length. This may explain the discrepancy between experimental measurements and theoretical estimates of the ferroelectric critical thickness of a BaTiO{sub 3} ferroelectric barrier sandwiched between metallic SrRuO{sub 3} electrodes, since theoretical calculations generally assume ideal (stoichiometric) perovskite SrRuO{sub 3}.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Features of primary damage by high energy displacement cascades in concentrated Ni-based alloys

Alloying of Ni with Fe or Co reduces primary damage production under ion irradiation. Similar results have been obtained from classical molecular dynamics simulations of 1, 10, 20, and 40 keV collision cascades in Ni, NiFe, and NiCo. In all cases, a mix of imperfect stacking fault tetrahedra, faulted loops with a 1/3 {111} Burgers vector, and glissile interstitial loops with a 1/2 {110} Burgers vector were formed, along with small sessile point defect complexes and clusters. Primary damage reduction occurs by three mechanisms. First, Ni-Co, Ni-Fe, Co-Co, and Fe-Fe short-distance repulsive interactions are stiffer than Ni-Ni interactions, which lead to a decrease in damage formation during the transition from the supersonic ballistic regime to the sonic regime. This largely controls final defect production. Second, alloying decreases thermal conductivity, leading to a longer thermal spike lifetime. The associated annealing reduces final damage production. These two mechanisms are especially important at cascades energies less than 40 keV. Third, at the higher energies, the production of large defect clusters by subcascades is inhibited in the alloys. A number of challenges and limitations pertaining to predictive atomistic modeling of alloys under high-energy particle irradiation are discussed.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2016

Optical studies of the uniaxial stress-induced orbital alignment of the Cr{sup 2+} centers in KZnF{sub 3} single crystal

Observation of an intense optical linear dichroism arising in cubic KZnF{sub 3}:Cr crystal at low temperatures under uniaxial stress applied along the four-fold axis is reported. Dichroism occurs in the range of the wide vibronic absorption band corresponding to {sup 5}E{sub g} → {sup 5}T{sub 2g} transition of the Cr{sup 2+} ions. Strain dependences of the dichroism value were studied at the temperatures of 2.0 K, 4.2 K, and 77 K. We associate our observations with the Jahn-Teller effect in the {sup 5}E{sub g} ground state of the Cr{sup 2+} ion. The model is proposed based on a redistribution of the centers between the minima of the E⨂e problem adiabatic potential that become inequivalent under uniaxial stress applied along the four-fold axis of the crystal. It is shown that random strains in the sample have to be taken into account to achieve the quantitative agreement of the model predictions with experimental data. It is found that random strains in the studied sample originate predominantly from point defects. Obtained parameter values are inversion splitting δ = 9.2 ± 1.6 cm{sup −1}, electron-strain coupling constant qV{sub ES} = 16 500 ± 600 cm{sup −1}, width of the random strain distribution w = (6.9 ± 0.5) ⋅ 10{sup −5}. It is shown also that the minima of the [CrF{sub 6}]{sup 4−} cluster adiabatic potential correspond to the elongated along the four-fold axes configurations.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2017

Spatial luminescence imaging of dopant incorporation in CdTe Films

State-of-the-art cathodoluminescence (CL) spectrum imaging with spectrum-per-pixel CL emission mapping is applied to spatially profile how dopant elements are incorporated into Cadmium telluride (CdTe). Emission spectra and intensity monitor the spatial distribution of additional charge carriers through characteristic variations in the CL emission based on computational modeling. Our results show that grain boundaries play a role in incorporating dopants in CdTe exposed to copper, phosphorus, and intrinsic point defects in CdTe. Furthermore, the image analysis provides critical, unique feedback to understand dopant incorporation and activation in the inhomogeneous CdTe material, which has struggled to reach high levels of hole density.

14 SOLAR ENERGY↗
DOE OSTI2018

Sb-induced strain fluctuations in a strained layer superlattice of InAs/InAsSb

Here, we show that Sb substitution for As in a MBE grown InAs/InAsSb strained layer superlattice (SLS) is accompanied by significant strain fluctuations. The SLS was observed using scanning transmission electron microscopy along the [100] zone axis where the cation and anion atomic columns are separately resolved. Strain analysis based on atomic column positions reveals asymmetrical transitions in the strain profile across the SLS interfaces. The averaged strain profile is quantitatively fitted to the segregation model, which yields a distribution of Sb in agreement with our scanning tunneling microscopy result. The subtraction of the calculated strain reveals an increase in strain fluctuations with the Sb concentration, as well as isolated regions with large strain deviations extending spatially over ~1 nm, which suggest the presence of point defects.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2017

Doping properties of cadmium-rich arsenic-doped CdTe single crystals: Evidence of metastable AX behavior

Cd-rich composition and group-V element doping are of interest for simultaneously maximizing the hole concentration and minority carrier lifetime in CdTe, but the critical details concerning point defects are not yet fully established. Herein, we report on the properties of arsenic doped CdTe single crystals grown from Cd solvent by the travelling heater method. The photoluminescence spectra and activation energy of 74 +/- 2 meV derived from the temperature-dependent Hall effect are consistent with AsTe as the dominant acceptor. Doping in the 10^16 to 10^17/cm^3 range is achieved for measured As concentrations between 10^16 and 10^20/cm^3 with the highest doping efficiency of 40% occurring near 10^17 As/cm^3. We observe persistent photoconductivity, a hallmark of light-induced metastable configuration changes consistent with AX behavior. Additionally, quenching experiments reveal at least two mechanisms of increased p-type doping in the dark, one decaying over 2-3 weeks and the other persisting for at least 2 months. These results provide essential insights for the application of As-doped CdTe in thin film solar cells.

14 SOLAR ENERGY↗
DOE OSTI2018

Initial stages of ion beam-induced phase transformations in Gd 2 O 3 and Lu 2 O 3

The atomic-scale evolution of lanthanide sesquioxides Gd 2 O 3 and Lu 2 O 3 irradiated with 1 MeV Kr ions at room temperature and 120 K, up to fluences of 1x 10 16 ions/cm 2 (~20 dpa), has been characterized by in situ transmission electron microscopy. At room temperature, both oxides exhibited high radiation tolerance. Irradiation did not cause any observable structural change in either material, likely due to the mobility of irradiation-induced point defects, causing efficient defect annihilation. For Gd 2 O 3 , having the larger cation ionic radius of the two materials, an irradiation-induced stacking fault structure appeared at low fluences in the low temperature irradiation. As compared with the cubic-to-monoclinic phase transformations known to result from higher energy (~GeV) ion irradiation, Kr ions of lower energies (~MeV) yield much lower rates of damage accumulation and thus less extensive structural modification. Finally, at a fluence of 2.5 x 10 15 ions/cm 2 , only the initial stages of the cubic-to-monoclinic (C to B) phase transformation process, consisting of the formation and aggregation of defects, have been observed.

36 MATERIALS SCIENCE↗