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At least 1,567 records · Page 87
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DOE OSTI2011

Formation of a semi-insulating layer in n-type 4H-SiC by electron irradiation

Electron irradiation has been applied to the formation of a semi-insulating 4H-SiC(0001) layer. The resistivity of the semi-insulating layer, which was irradiated with a fluence of 1.9 x 10{sup 18} cm{sup -2} at 400 keV, exceeded 10{sup 10{Omega}} cm at room temperature. From capacitance-voltage characteristics of Schottky structure, the depth of the semi-insulating layer was estimated to be 10 {mu}m, indicating that the whole region of lightly-doped n-type epilayer was converted to the semi-insulating layer by electron irradiation. The semi-insulating property can be ascribed to electron trapping at the Z{sub 1/2} and EH{sub 6/7} centers generated by electron irradiation. The threshold energy for the generation of Z{sub 1/2} center was about 100 keV.

36 MATERIALS SCIENCE↗
DOE OSTI2012

Anharmonic vibrations of the dicarbon antisite defect in 4H-SiC

Dicarbon antisite defects were created by either electron irradiation or ion implantation into 4H-SiC. The no-phonon lines from the dicarbon antisite defect center were observed with their phonon replicas. The stretch frequencies of the defect were observed up to the fifth harmonic. The Morse potential model accounts for the anharmonicity quite well and gives a very good prediction of the vibration energies up to the fifth harmonic with an error of less than 1%. First principles calculations show that the model of a dicarbon antisite defect along with its four nearest neighboring carbon atoms can explain the observed anharmonicity.

36 MATERIALS SCIENCE↗
DOE OSTI2011

A study of radiation damage effects on the magnetic structure of bulk Iron

Defects, defect interactions, and defect dynamics in solids created by fast neutrons are known to have significant impact on the performance and lifetime of structural materials. A fundamental understanding of the radiation damage effects in solids is therefore of great importance in assisting the development of improved materials - materials with ultrahigh strength, toughness, and radiation resistance. In this presentation, we show our recent theoretical investigation on the magnetic structure evolution of bulk iron in the region of the radiation defects. We applied a linear scaling ab-initio method based on density functional theory with local spin density approximation, namely the locally self-consistent multiple scattering method (LSMS), to the study of magnetic moment distributions in a cascade at the damage peak and for a series of time steps as the interstitials and vacancies recombined. Atomic positions correspond to those in a low energy cascade in a 10|000 atom sample, in which the primary damage state and the evolution of all defects produced were simulated using molecular dynamics with empirical, embedded-atom inter-atomic potentials. We will discuss how a region of affected moments expands and then recedes in response to a cascade evolution.

36 MATERIALS SCIENCE↗
DOE OSTI2011

Effects of irradiation and annealing on deep levels in rhodium-doped p-GaAs grown by metal-organic chemical-vapor deposition

This paper reports a detailed study of the effects of irradiation and thermal annealing on deep levels in Rh-doped p-type GaAs grown by low-pressure metal-organic chemical-vapor deposition, using deep level transient spectroscopy (DLTS) technique. It is found upon irradiation with alpha particles that, in addition to the radiation-induced defect peaks, all the Rh-related peaks observed in majority, as well as minority-carrier emission DLTS scans show an increase in their respective concentrations. The usually observed {alpha}-induced defects H{alpha}1, H{alpha}2, and H{alpha}3 are found to have lower introduction rates in Rh-doped samples, as compared to reference samples (not doped with Rh). Alpha-irradiation has been found to decompose the two minority carrier emitting bands (one at low temperature {approx}150 K and the other at {approx}380 K) observed prior to irradiation into distinct peaks corresponding to deep levels Rh1 and Rh2 and EL2 and Rh3, respectively. A similar effect is also observed for the majority-carrier emitting band composed of hole emission from deep levels RhA and RhB, which separate out well upon irradiation. Further, from the double-correlation DLTS measurements, the emission rates of carriers from the radiation-enhanced peaks corresponding to deep levels Rh1, Rh2, Rh3, and RhC were found to be dependent on junction electric field. For RhC, the field dependence data have been analyzed in terms of the Poole-Frenkel model employing a 3-dimensional Coulomb potential with q = 2e (electronic charge). Temperature dependence of the hole capture cross-sections of the levels RhA and RhC was also studied quantitatively. The observed dependence of the hole capture cross-section of RhC on temperature can be interpreted in terms of multiphonon capture model, yielding a capture barrier of 0.2 eV and {sigma}({infinity}) = 2.3 x 10{sup -14} cm{sup 2}. The results of irradiation and isochronal thermal annealing study, in combination with the theoretical analysis of the field dependence of hole emission data, lead us to interpret the levels RhA and RhB as charged centers and probably complexes of arsenic anti-site defect (As{sub Ga}) with Rh-impurity, (As{sub Ga}-Rh). The levels Rh1 and Rh2 are also proposed to be complexes, albeit of a different type, of arsenic anti-site defects (As{sub Ga}) and Rh-impurity (As{sub Ga}-Rh), while the level RhC is likely to be a doubly-charged complex center composed of arsenic interstitial (I{sub As}) and Rh-impurity (I{sub As}-Rh).

36 MATERIALS SCIENCE↗
DOE OSTI2011

Iron distribution in silicon after solar cell processing: Synchrotron analysis and predictive modeling

The evolution during silicon solar cell processing of performance-limiting iron impurities is investigated with synchrotron-based x-ray fluorescence microscopy. We find that during industrial phosphorus diffusion, bulk precipitate dissolution is incomplete in wafers with high metal content, specifically ingot border material. Postdiffusion low-temperature annealing is not found to alter appreciably the size or spatial distribution of FeSi{sub 2} precipitates, although cell efficiency improves due to a decrease in iron interstitial concentration. Gettering simulations successfully model experiment results and suggest the efficacy of high- and low-temperature processing to reduce both precipitated and interstitial iron concentrations, respectively.

36 MATERIALS SCIENCE↗
DOE OSTI2010

Effect of irradiation parameters on defect aggregation during thermal annealing of LiF irradiated with swift ions and electrons

Absorption spectroscopy were performed to study the effects of thermal annealing on the aggregation of color centers in LiF crystals irradiated with different ions between carbon and uranium of megaelectron volt-gigaelectron volt energy. The beam parameters such as energy, energy loss, and fluence have a pronounced influence on the initial defect composition and concentration as well as their evolution upon thermal annealing. A distinct phenomenon was observed, viz., the enhancement of F{sub n} centers for annealing temperatures between 500 and 700 K, followed by Li colloid formation above 700 K. The phenomenon requires specific irradiation conditions whereas the formation of Mg colloids from Mg impurities occurs in all irradiated crystals. The mechanisms of annealing and colloid formation are discussed.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2010

Structural investigation of the Cu{sub 2}Se-In{sub 2}Se{sub 3}-Ga{sub 2}Se{sub 3} phase diagram, X-ray photoemission and optical properties of the Cu{sub 1-z}(In{sub 0.5}Ga{sub 0.5}){sub 1+z/3}Se{sub 2} compounds

Structures of compounds in the Cu{sub 2}Se-In{sub 2}Se{sub 3}-Ga{sub 2}Se{sub 3} system have been investigated through X-ray diffraction. Single crystal structure studies for the so-called stoichiometric compounds Cu(In,Ga)Se{sub 2} (CIGSe) confirm that the chalcopyrite structure (space group I4-bar 2d) is very flexible and can adapt itself to the substitution of Ga for In. On the other hand a structure modification is evidenced in the Cu{sub 1-z}(In{sub 0.5}Ga{sub 0.5}){sub 1+z/3}Se{sub 2} series when the copper vacancy ratio (z) increases; the chalcopyrite structure turns to a modified-stannite structure (I4-bar 2m) when z{>=}0.26. There is a continuous evolution of the structure from Cu{sub 0.74}(In{sub 0.5}Ga{sub 0.5}){sub 1.09}Se{sub 2} to Cu{sub 0.25}(In{sub 0.5}Ga{sub 0.5}){sub 1.25}Se{sub 2} ((i.e. Cu(In{sub 0.5}Ga{sub 0.5}){sub 5}Se{sub 8}), including Cu{sub 0.4}(In{sub 0.5}Ga{sub 0.5}){sub 1.2}Se{sub 2} (i.e. Cu(In{sub 0.5}Ga{sub 0.5}){sub 3}Se{sub 5}). From this single crystal structural investigation, it is definitively clear that no ordered vacancy compound exists in that series. X-ray photoemission spectroscopy study shows for the first time that the surface of powdered Cu{sub 1-z}(In{sub 0.5}Ga{sub 0.5}){sub 1+z/3}Se{sub 2} compounds (z{ne}0) is more copper-poor than the bulk. The same result has often been observed on CIGSe thin films material for photovoltaic applications. In addition, optical band gaps of these non-stoichiometric compounds increase from 1.2 to 1.4 eV when z varies from 0 to 0.75. - Abstract: Pseudo-ternary diagram in the Cu{sub 2}Se-In{sub 2}Se{sub 3}-Ga{sub 2}Se{sub 3} system showing the composition of all the synthesized compounds. The crystal structure of the compounds corresponding to red circles are presented in this study.

36 MATERIALS SCIENCE↗
DOE OSTI2011

Vacancy ordering and oxygen dynamics in oxide ion conducting La{sub 1-x}Sr{sub x}Ga{sub 1-x}Mg{sub x}O{sub 3-x} ceramics: {sup 71}Ga, {sup 25}Mg and {sup 17}O NMR

The oxygen vacancies distribution in the rigid lattice and the thermally activated motion of oxygen atoms are studied in La{sub 1-x}Sr{sub x}Ga{sub 1-x}Mg{sub x}O{sub 3-x} (x=0.00; 0.05; 0.10; 0.15 and 0.20) compounds. For that {sup 71}Ga, {sup 25}Mg and {sup 17}O NMR was performed from 100 K up to 670 K, and ion conductivity measurements were carried out up to 1273 K. The comparison of the electric field gradients at the Ga- and Mg-sites evidences that oxygen vacancies appear exclusively near gallium cations as a species trapped below room temperature in local clusters, GaO{sub 5/2}-{open_square}-GaO{sub 5/2}. These clusters decay at higher temperature into mobile constituents of the structural octahedra Ga(O{sub 5/6{open_square}1/6}){sub 6/2}. At the same time, the nearest octahedral oxygen environment of magnesium cations persists at different doping levels. The case of two adjacent vacant anion sites is found highly unlikely within the studied doping range. The thermally activated oxygen motion starts to develop above room temperature as is observed from both the motional narrowing of {sup 17}O NMR spectra and the {sup 17}O nuclear spin-lattice relaxation rate. The obtained results show that two types of motion exist, a slow motion and a fast one. The former is a long-range diffusion whereas the latter is a local back and forth oxygen jumps between two adjacent anion sites. These sites are strongly differentiated by the probability of the vacancy formation, like the vacant apical site and the occupied equatorial site in the orthorhombic compositions x <0.15. -- Graphical abstract: Up to 700 K the vacant oxygen sites are near Ga but not near Mg. Display Omitted

36 MATERIALS SCIENCE↗
DOE OSTI2011

Crystal-chemistry of mullite-type aluminoborates Al{sub 18}B{sub 4}O{sub 33} and Al{sub 5}BO{sub 9}: A stoichiometry puzzle

Orthorhombic Al{sub 2}O{sub 3}-rich aluminoborate is an important ceramic material for which two slightly different compositions have been assumed: Al{sub 5}BO{sub 9} (5Al{sub 2}O{sub 3}:B{sub 2}O{sub 3}) and Al{sub 18}B{sub 4}O{sub 33} (9Al{sub 2}O{sub 3}:2B{sub 2}O{sub 3}). The formula Al{sub 18}B{sub 4}O{sub 33} (=Al{sub 4.91}B{sub 1.09}O{sub 9}) was derived from results of chemical analyses when crystal structure data were not yet available. Subsequent structural investigations indicated Al{sub 5}BO{sub 9} composition. Nevertheless, Al{sub 18}B{sub 4}O{sub 33} was still accepted as the correct stoichiometry assuming that additional B replaces 9% Al. Powder samples of both compositions and ones with excess boron were prepared by solid state reactions between {alpha}-Al{sub 2}O{sub 3} and B{sub 2}O{sub 3}/H{sub 3}BO{sub 3} at temperatures above 1100 {sup o}C and single-crystals were grown from flux at 1100 and 1550 {sup o}C. Products were investigated by single-crystal and powder XRD, {sup 11}B and {sup 27}Al solid-state MAS-NMR, Raman and FTIR spectroscopy as well as Laser-ablation ICP-MS. No indication of the predicted 9% B{yields}Al substitution was found. LA ICP-MS indicated 12.36(27) wt% B{sub 2}O{sub 3} corresponding to Al{sub 4.97}B{sub 1.03}O{sub 9}. Hence, the suggested Al{sub 18}B{sub 4}O{sub 33} stoichiometry can be excluded for all synthesized samples. A very low amount of Al vacancies at a five-fold coordinated site are likely, charge balanced by an additional nearby three-fold coordinated B site. All evidences indicate that the title compound should be reported as Al{sub 5-x}B{sub 1+x}O{sub 9} with x<0.038(6), which is close to Al{sub 5}BO{sub 9}. -- Graphical abstract: A chemical composition of Al{sub 18}B{sub 4}O{sub 33}=Al{sub 4.91}B{sub 1.09}O{sub 9}=9Al{sub 2}O{sub 3}:2B{sub 2}O{sub 3} has been assumed for mullite-type aluminoborate with Al{sub 5}BO{sub 9} structure. However, samples prepared by different routes showed compositions close to 5Al{sub 2}O{sub 3}:B{sub 2}O{sub 3}. Display Omitted

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2011

Direct identification of interstitial Mn in heavily p-type doped GaAs and evidence of its high thermal stability

We report on the lattice location of Mn in heavily p-type doped GaAs by means of {beta}{sup -} emission channeling from the decay of {sup 56}Mn. The majority of the Mn atoms substitute for Ga and up to 31% occupy the tetrahedral interstitial site with As nearest neighbors. Contrary to the general belief, we find that interstitial Mn is immobile up to 400 deg. C, with an activation energy for diffusion of 1.7-2.3 eV. Such high thermal stability of interstitial Mn has significant implications on the strategies and prospects for achieving room temperature ferromagnetism in Ga{sub 1-x}Mn{sub x}As.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2011

Enhanced photoelectrochemical performance of rutile TiO{sub 2} by Sb-N donor-acceptor coincorporation from first principles calculations

An effective non-metal (N) and non-transition metal (Sb) passivated co-doping approach is proposed to improve the photoelectochemical performance of rutile TiO{sub 2} for water-splitting by using first-principles calculations. It is found that the band edges of N + Sb co-doped TiO{sub 2} match with the redox potentials of water, and a narrow band gap (2.0 eV) is achieved for enhanced visible light absorption. The compensated donor (Sb) and acceptor (N) pairs could prevent the recombination of photo-generated electron-hole pairs. In addition, the N + Sb defect pairs tend to bind with each other, which could enhance the stability and N concentration of the system.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Effects of neutron irradiation of ultra-thin HfO{sub 2} films

Neutron irradiation at low fluence decreases the Pb-type and E′ defect levels in ultra-thin hafnium dioxide films because electrons can fill existing states. These electrons come from electron-hole pairs generated by neutron interactions with silicon and oxygen. Thus, a low fluence of neutrons “anneals” the sample. However, when neutron fluence increases, more neutrons collide with oxygen atoms and cause them to leave the lattice or to transmute into different atoms. This causes the E′ states to increase. As defect-state concentrations increase, leakage currents increase, but since the E′ is much lower than the Pb concentration, this is not a dominant factor.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2010

The anti-perovskite type hydride InPd{sub 3}H{sub 0.89}

Hydrogenation of tetragonal InPd{sub 3} in the ZrAl{sub 3} type structure (four-fold ccp superstructure) yields a hydride with a cubic AuCu{sub 3} type structure (one-fold ccp superstructure). Deuterium can be located by neutron powder diffraction in octahedral voids surrounded exclusively by palladium, [Pd{sub 6}], which are 88.5(6)% occupied in a statistical manner. The resulting deuteride InPd{sub 3}D{sub 0.89} thus crystallizes in a cubic anti-perovskite type structure (space group Pm3-bar m (no. 221), a=402.25(1) pm at 299(2) K). The Pd-D distance of 201.13(1) pm is typical for interstitial hydrides with palladium. Inelastic neutron scattering on the hydride InPd{sub 3}H{sub 0.89}, which shows a spectrum similar to that of binary palladium hydride, confirms the cubic site symmetry of hydrogen in [Pd{sub 6}] interstices. This is also confirmed by the absence of any quadrupole splitting in the {sup 2}D-NMR signal of the deuteride. {sup 1}H NMR spectra of InPd{sub 3}H{sub 0.89} do not show any motional narrowing. Values found for the H jump rate {tau}{sup -1} in InPd{sub 3}H{sub 0.89} remain below 10{sup 6} s{sup -1} in the studied temperature range 28-360 K, indicating a small hydrogen mobility in InPd{sub 3}H{sub 0.8} as compared with binary palladium hydride, PdH{sub {<=}1}. This can be attributed to the large spatial separation of the [Pd{sub 6}] sites. - Graphical abstract: Hydrogen induces a rearrangement in InPd{sub 3} from a ZrAl{sub 3} type structure to a cubic AuCu{sub 3} type structure, thus forming an anti-perovskite type hydride InPd{sub 3}H{sub 0.89}.

36 MATERIALS SCIENCE↗
DOE OSTI2010

Light controlled magnetoresonant softening of {gamma}-irradiated KCl:Fe crystals

Magnetoresonant softening of {gamma}-irradiated KCl:Fe crystals controlled by visual light illumination was observed. Resonant magnetoplasticity detected by microhardness before illumination of the crystals is provided by two types of paramagnetic defects: (1) Fe{sup 2+}-v{sub c} ion-vacancy pair with 7.0 and 3.5 g-factors, (2) Fe{sup +} ions with g-factors equal to 2.2 and 4.1. Illumination of the {gamma}-KCl:Fe crystals by {lambda}=500-600 nm light causes disappearance of the electron paramagnetic resonance lines with 7.0 and 3.5 g-factors corresponding to Fe{sup 2+}-v{sub c} resonances. The spectrum of the illuminated sample changes due to recombination of F-electrons with Fe{sup 2+}-v{sub c} centers and their transformation to the Fe{sup +} centers. Two lines with 2.2 and 4.1 g-factors corresponding to the Fe{sup +} resonances were observed before and after light illumination.

36 MATERIALS SCIENCE↗
DOE OSTI2010

Acceptor Type Vacancy Complexes In As-Grown ZnO

One of the many technological areas that ZnO is interesting for is the construction of opto-electronic devices working in the blue-UV range as its large band gap ({approx}3.4 eV at 10 K) makes them suitable for that purpose. As-grown ZnO shows generally n-type conductivity partially due to the large concentration of unintentional shallow donors, like H, but impurities can also form complexes with acceptor type defects (Zn vacancy) leading to the creation of compensating defects. Recently, Li{sub Zn} and Na{sub Zn} acceptors have been measured and H could form similar type of defects. Doppler Broadening Positron Annihilation spectroscopy experimental results on the observation of Zn related vacancy complexes in ZnO thin films, as-grown, O implanted and Al doped will be presented. Results show that as-grown ZnO film show small Zn vacancy related complexed that could be related to presence of H as a unintentional doping element.

36 MATERIALS SCIENCE↗
DOE OSTI2011

Evaluation of Carbon Interstitial in C-ion Implanted ZnO Bulk Single Crystals by a Nuclear Reaction Analysis Study: An Origin of Low Resistivity

Nuclear reaction analysis (NRA) of carbon-implanted ZnO bulk single crystals (carbon concentration: 1.5x10{sup 20} cm{sup -3}), in conjunction with the channeling technique, using the {sup 12}C(d,p){sup 13}C and {sup 16}O(d,p){sup 17}O reactions shows the presence of the interstitial carbon (C{sub i}) and the occupancy of substitute sites of oxygen atoms. These results suggest that the variation in resistivity from the order of 10{sup 4} {Omega}cm(for un-implanted samples) to that of 10 {Omega}cm (for as-implanted ones) is attributed to the C{sub i} and/or its complex defects, which would act as a shallow donor in ZnO.

36 MATERIALS SCIENCE↗
DOE OSTI2011

O K-energy loss near-edge structure change induced by tantalum impurity in monoclinic hafnium oxide

The present paper reports the energy loss near-edge structure (ELNES) study of monoclinic HfO{sub 2} (m-HfO{sub 2}) and tantalum doped m-HfO{sub 2} (Ta{sub 0.1}Hf{sub 0.9}O{sub 2}) thin films prepared by radio frequency magnetron co-sputtering method. A change in the O K-ELNES spectra was observed as the amount of dopant increases. In order to precise the common features and the differences as a function of Ta defect nature (substitutional or interstitial) in HfO{sub 2}, the O K-ELNES were commented with respect to density functional theory calculations implemented in Vienna ab initio simulation package code. The calculated Ta doped HfO{sub 2} band structure showed that substitutional tantalum is the dominant defect and the spectral differences between doped and non-doped HfO{sub 2} are mainly originated from the change in the local cation distribution around the oxygen atoms.

36 MATERIALS SCIENCE↗
DOE OSTI2011

Effect of nitrogen-doping temperature on the structure and photocatalytic activity of the B,N-doped TiO{sub 2}

B,N-TiO{sub 2} photocatalysts were synthesized by boron doping firstly and subsequently nitrogen doping in NH{sub 3} at variable temperatures. The effects of the nitrogen doping temperature on the structure and photocatalytic activity of the B,N-codoped TiO{sub 2} were investigated. The as-prepared samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), UV-vis diffuse reflectance spectrum (DRS), electron paramagnetic resonance (EPR) and X-ray photoelectron spectroscopy (XPS). The photocatalytic activity was evaluated with photocatalytic degradation of methyl orange dye (MO) under visible light and UV-visible light irradiation. The results suggested that the boron and nitrogen can be incorporated into the TiO{sub 2} lattice either interstitially or substitutionally or both, while the Ti-O-B-N structure plays a vital role in photocatalytic activity in visible light region. The optimal nitrogen doping temperature is 550 {sup o}C. Higher temperature may form many oxygen vacancies and Ti{sup 3+} species, resulting in the decrease of photocatalytic activity in visible light. -- Graphical abstract: The changes of photocatalytic activity of B,N-TiO{sub 2} with variable nitrogen doping temperatures are attributed to the transformation of surface structure and oxygen vacancies, and the Ti-O-B-N structure plays a vital role in photocatalytic activity under visible light irradiation. Display Omitted Research highlights: > B,N-TiO{sub 2} photocatalysts are synthesized by boron doping firstly and subsequently nitrogen doping in NH{sub 3} at variable temperatures. > The boron and nitrogen can be incorporated into the TiO{sub 2} lattice either interstitially or substitutionally or both. > The temperature of nitrogen doping affects the transformation of surface structure and oxygen vacancies. > The Ti-O-B-N structure plays an important role in photocatalytic activity under visible light irradiation.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗