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At least 1,639 records · Page 91
All sourcesNASA NTRSDOE OSTI
DOE OSTI2014

Band to band tunneling in III-V semiconductors: Implications of complex band structure, strain, orientation, and off-zone center contribution

In this paper, we use a tight binding Hamiltonian with spin orbit coupling to study the real and complex band structures of relaxed and strained GaAs. A simple d orbital on-site energy shift coupled with appropriate scaling of the off-diagonal terms is found to correctly reproduce the band-edge shifts with strain. Four different 〈100〉 strain combinations, namely, uniaxial compressive, uniaxial tensile, biaxial compressive, and biaxial tensile strain are studied, revealing rich valence band structure and strong relative orientation dependent tunneling. It is found that complex bands are unable to provide unambiguous tunneling paths away from the Brillouin zone center. Tunneling current density distribution over the Brillouin zone is computed using non-equilibrium Green's function approach elucidating a physical picture of band to band tunneling.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

On the bonding nature of electron states for the Fe-Mo double perovskite

The electronic transport as well as the effect of an external magnetic field has been investigated on manganese-based materials, spinels and perovskites. Potential applications of double perovskites go from magnetic sensors to electrodes in solid-oxide fuel cells; besides the practical interests, it is known that small changes in composition modify radically the physical properties of double perovskites. We have studied the Sr{sub 2}FeMoO{sub 6} double perovskite compound (SFMO) using first-principles density functional theory. The calculations were done within the generalized gradient approximation (GGA) scheme with the Perdew-Burke-Ernzerhof (PBE) functional. We have made a detailed analysis of each electronic state and the charge density maps around the Fermi level. For the electronic properties of SFMO it was used a primitive cell, for which we found the characteristic half-metallic behavior density of states composed by e{sub g} and t{sub 2g} electrons from Fe and Mo atoms. Those peaks were tagged as bonding or antibonding around the Fermi level at both, valence and conduction bands.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Power-law photoluminescence decay in quantum dots

Some quantum dot samples show a long-time (power-law) behavior of their luminescence intensity decay. This effect has been recently explained as being due to a cooperation of many tunneling channels transferring electrons from small quantum dots with triplet exciton to quantum dots at which the electrons can recombine with the holes in the valence band states. In this work we show that the long-time character of the sample luminescence decay can also be caused by an intrinsic property of a single dot, namely, by a non-adiabatic effect of the electron occupation up-conversion caused by the electron-phonon multiple scattering mechanism.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2014

Electronic states of lithium passivated germanium nanowires: An ab-initio study

A study of the electronic and structural properties of germanium nanowires (GeNWs) was performed using the ab-initio Density Functional Theory within the generalized gradient approximation where electron-ion interactions are described by ultrasoft pseudopotentials. To study the effects of the lithium in the surface of the GeNWs we compare the electronic band structures of Hydrogen passivated GeNWs with those of partial and totally Li passivated GeNWs. The nanowires were constructed in the [001], [111] and [110] directions, using the supercell model to create different wire diameters. The results show that in the case of partial Li passivation there are localized orbitals near the valence band maximum, which would create a p-doped-kind of state. The total Li passivation created metallic states for all the wires.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

The vibrational spectrum of FeO{sub 2}{sup +} isomers—Theoretical benchmark and experiment

Infrared photodissociation is used to record the vibrational spectrum of FeO{sub 2}{sup +}(He){sub 2–4} which shows three bands at 1035, 980, and 506 cm{sup −1}. Quantum chemical multi-reference configuration interaction calculations (MRCISD) of structures and harmonic frequencies show that these bands are due to two different isomers, an inserted dioxo complex with Fe in the +V oxidation state and a side-on superoxo complex with Fe in the +II oxidation state. These two are separated by a substantial barrier, 53 kJ/mol, whereas the third isomer, an end-on complex between Fe{sup +} and an O{sub 2} molecule, is easily converted into the side-on complex. For all three isomers, states of different spin multiplicity have been considered. Our best energies are computed at the MRCISD+Q level, including corrections for complete active space and basis set extension, core-valence correlation, relativistic effects, and zero-point vibrational energy. The average coupled pair functional (ACPF) yields very similar energies. Density functional theory (DFT) differs significantly from our best estimates for this system, with the TPSS functional yielding the best results. The other functionals tested are BP86, PBE, B3LYP, TPSSh, and B2PLYP. Complete active space second order perturbation theory (CASPT2) performs better than DFT, but less good than ACPF.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2014

Modeling of microstructural effects on electromigration failure

Current electromigration models used for simulation and analysis of interconnect reliability lack the appropriate description of metal microstructure and consequently have a very limited predictive capability. Therefore, the main objective of our work was obtaining more sophisticated electromigration tools. The problem is addressed through a combination of different levels of atomistic modeling and already available, continuum level macroscopic models. A novel method for an ab initio calculation of the effective valence for electromigration is presented and its application on the analysis of EM behavior is demonstrated. Additionally, a simple analytical model for the early electromigration lifetime is obtained. We have shown that its application provides a reasonable estimate for the early electromigration failures including the effect of microstructure. A simulation study is also applied on electromigration failure in tin solder bumps, where it contributed the understanding of the role of tin crystal anisotropy in the degradation mechanism of solder bumps.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Bi-O covalency in PbTiO{sub 3}-BiInO{sub 3} with enhanced ferroelectric properties: Synchrotron radiation diffraction and first-principles study

High-T{sub C} ferroelectric (1 − x)PbTiO{sub 3}-xBiInO{sub 3} shows a sustainable spontaneous polarization in tetragonal phase by amount of BiInO{sub 3} augmented. It is rare and interesting that weak ferroelectric In{sup 3+} substitution does not reduce ferroelectricity as most of BiMeO{sub 3} materials. To understand this abnormality behavior, the structure and the cation displacements are studied by means of synchrotron radiation diffraction. The cation displacements of A-site atoms and B-site atoms exhibit a coupling property. Their sustainedly increasing trend is quantitatively associated with spontaneous polarization. Moreover, the structure, the valence electron density distributions, electron localization function, and Bader analysis have been researched on the chemical bond in (1 − x)PbTiO{sub 3}-xBiInO{sub 3} through first-principles calculations here. Although In atoms substitution reduces the covalency degree, this loss is offset by enhanced covalency between Bi-O bonds. The crucial role of Bi atom substitution not only contributes to enhance the covalency but also promotes the polarization.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Charge transfer from an adsorbed ruthenium-based photosensitizer through an ultra-thin aluminium oxide layer and into a metallic substrate

The interaction of the dye molecule N3 (cis-bis(isothiocyanato)bis(2,2-bipyridyl-4,4′-dicarbo-xylato) -ruthenium(II)) with the ultra-thin oxide layer on a AlNi(110) substrate, has been studied using synchrotron radiation based photoelectron spectroscopy, resonant photoemission spectroscopy, and near edge X-ray absorption fine structure spectroscopy. Calibrated X-ray absorption and valence band spectra of the monolayer and multilayer coverages reveal that charge transfer is possible from the molecule to the AlNi(110) substrate via tunnelling through the ultra-thin oxide layer and into the conduction band edge of the substrate. This charge transfer mechanism is possible from the LUMO+2 and 3 in the excited state but not from the LUMO, therefore enabling core-hole clock analysis, which gives an upper limit of 6.0 ± 2.5 fs for the transfer time. This indicates that ultra-thin oxide layers are a viable material for use in dye-sensitized solar cells, which may lead to reduced recombination effects and improved efficiencies of future devices.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2014

The semi-empirical tight-binding model for carbon allotropes “between diamond and graphite”

The new carbon allotropes “between diamond and graphite” have come under intensive examination during the last decade due to their numerous technical applications. The modification of energy gap in thin films of these allotropes was studied experimentally using optical methods. The proposed simple model of carbon clusters with variable lengths of chemical bonds allows us to imitate the transfer from diamond and diamond-like to graphite-like structures, as well as the corresponding modification of hybridization sp{sup 3}/sp{sup 2} for diamond-like and sp{sub z} for graphite-like phases. This enables us to estimate various allotropes parameters, like the gap E{sub g}, energies of valence E{sub v}, and conduction E{sub c} band edges, and the value of electronic affinity, i.e., optical work function X, which are all of practical importance. The obtained estimations correspond to the experimental data.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Buffer transport mechanisms in intentionally carbon doped GaN heterojunction field effect transistors

Temperature dependent pulsed and ramped substrate bias measurements are used to develop a detailed understanding of the vertical carrier transport in the buffer layers in a carbon doped GaN power heterojunction field effect transistor. Carbon doped GaN and multiple layers of AlGaN alloy are used in these devices to deliver an insulating and strain relieved buffer with high breakdown voltage capability. However, understanding of the detailed physical mechanism for its operation is still lacking. At the lowest electric fields (<10 MV/m), charge redistribution within the C doped layer is shown to occur by hole conduction in the valence band with activation energy 0.86 eV. At higher fields, leakage between the two-dimensional electron gas and the buffer dominates occurring by a Poole-Frenkel mechanism with activation energy ∼0.65 eV, presumably along threading dislocations. At higher fields still, the strain relief buffer starts to conduct by a field dependent process. Balancing the onset of these leakage mechanisms is essential to allow the build-up of positive rather than negative space charge, and thus minimize bulk-related current-collapse in these devices.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Hydrogenation of the Cu{sub PL} center in silicon

The Cu{sub PL} center, a complex of four copper atoms in silicon with the zero-phonon photoluminescence line at 1014 meV and the donor level at 0.1 eV above the top of the valence band, is studied in the process of hydrogenation at 380 K. Complexes of a substitutional copper atom (Cu{sub s}) with one and two hydrogen atoms are observed to form in the hydrogenated region at the expense of Cu{sub PL}, while no isolated Cu{sub s} atoms are detected. Our results indicate that the addition of a single hydrogen atom induces the dissociation of all interstitial Cu atoms which decorate the Cu{sub s} core of the Cu{sub PL} center.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Band alignment between GaN and ZrO{sub 2} formed by atomic layer deposition

The band alignment between Ga-face GaN and atomic-layer-deposited ZrO{sub 2} was investigated using X-ray photoelectron spectroscopy (XPS). The dependence of Ga 3d and Zr 3d core-level positions on the take-off angles indicated upward band bending at GaN surface and potential gradient in ZrO{sub 2} layer. Based on angle-resolved XPS measurements combined with numerical calculations, valence band discontinuity ΔE{sub V} of 1 ± 0.2 eV and conduction band discontinuity ΔE{sub C} of 1.2 ± 0.2 eV at ZrO{sub 2}/GaN interface were determined by taking GaN surface band bending and potential gradient in ZrO{sub 2} layer into account.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Structural and electronic properties of cubic boron nitride doped with zinc

Structural and electronic properties of Zn-doped cubic boron nitride (cBN) were investigated via first principle calculation based on density functional theory. Our simulation suggests that Zn can substitute for both B (Zn{sub B}) and N (Zn{sub N}) atom; Zn{sub B} is energetically favorable, and Zn{sub N} can only be prepared under B-rich conditions. Zn{sub B} induced a shallow acceptor level; however, the large difference in electronegativity between Zn and N makes the acceptor level strongly localized, which reduces effective carrier density. In the case of Zn{sub N}, both deep acceptor levels within band gap and shallow acceptor levels at the top of valence band were induced, which produced more free carriers than Zn{sub B}. The calculated results account for experimental results of enhanced electric conductivity of Zn-doped cBN films prepared under B-rich conditions.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Impact of composition and crystallization behavior of atomic layer deposited strontium titanate films on the resistive switching of Pt/STO/TiN devices

The resistive switching (RS) properties of strontium titanate (Sr{sub 1+x}Ti{sub 1+y}O{sub 3+(x+2y)}, STO) based metal-oxide-metal structures prepared from industrial compatible processes have been investigated focusing on the effects of composition, microstructure, and device size. Metastable perovskite STO films were prepared on Pt-coated Si substrates utilizing plasma-assisted atomic layer deposition (ALD) from cyclopentadienyl-based metal precursors and oxygen plasma at 350 °C, and a subsequent annealing at 600 °C in nitrogen. Films of 15 nm and 12 nm thickness with three different compositions [Sr]/([Sr] + [Ti]) of 0.57 (Sr-rich STO), 0.50 (stoichiometric STO), and 0.46 (Ti-rich STO) were integrated into Pt/STO/TiN crossbar structures with sizes ranging from 100 μm{sup 2} to 0.01 μm{sup 2}. Nano-structural characterizations revealed a clear effect of the composition of the as-deposited STO films on their crystallization behavior and thus on the final microstructures. Local current maps obtained by local-conductivity atomic force microscopy were in good agreement with local changes of the films' microstructures. Correspondingly, also the initial leakage currents of the Pt/STO/TiN devices were affected by the STO compositions and by the films' microstructures. An electroforming process set the Pt/STO/TiN devices into the ON-state, while the forming voltage decreased with increasing initial leakage current. After a RESET process under opposite voltage has been performed, the Pt/STO/TiN devices showed a stable bipolar RS behavior with non-linear current-voltage characteristics for the high (HRS) and the low (LRS) resistance states. The obtained switching polarity and nearly area independent LRS values agree with a filamentary character of the RS behavior according to the valence change mechanism. The devices of 0.01 μm{sup 2} size with a 12 nm polycrystalline stoichiometric STO film were switched at a current compliance of 50 μA with voltages of about ±1.0 V between resistance states of about 40 kΩ (LRS) and 1 MΩ (HRS). After identification of the influences of the films' microstructures, i.e., grain boundaries and small cracks, the remaining RS properties could be ascribed to the effect of the [Sr]/([Sr] + [Ti]) composition of the ALD STO thin films.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Chemical potential shift and gap-state formation in SrTiO{sub 3−δ} revealed by photoemission spectroscopy

In this study, we report on investigations of the electronic structure of SrTiO{sub 3} annealed at temperature ranging between 550 and 840 °C in an ultrahigh vacuum. Annealing induced oxygen vacancies (O{sub vac}) impart considerable changes in the electronic structure of SrTiO{sub 3}. Using core-level photoemission spectroscopy, we have studied the chemical potential shift (Δμ) as a function of annealing temperature. The result shows that the chemical potential monotonously increases with electron doping in SrTiO{sub 3−δ}. The monotonous increase of the chemical potential rules out the existence of electronic phase separation in the sample. Using valence band photoemission, we have demonstrated the formation of a low density of states at the near Fermi level electronic spectrum of SrTiO{sub 3−δ}. The gap-states were observed by spectral weight transfer over a large energy scale of the stoichiometric band gap of SrTiO{sub 3} system leading finally to an insulator-metal transition. We have interpreted our results from the point of structural distortions induced by oxygen vacancies.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2014

Graphene defect formation by extreme ultraviolet generated photoelectrons

We have studied the effect of photoelectrons on defect formation in graphene during extreme ultraviolet (EUV) irradiation. Assuming the major role of these low energy electrons, we have mimicked the process by using low energy primary electrons. Graphene is irradiated by an electron beam with energy lower than 80 eV. After e-beam irradiation, it is found that the D peak, I(D), appears in the Raman spectrum, indicating defect formation in graphene. The evolution of I(D)/I(G) follows the amorphization trajectory with increasing irradiation dose, indicating that graphene goes through a transformation from microcrystalline to nanocrystalline and then further to amorphous carbon. Further, irradiation of graphene with increased water partial pressure does not significantly change the Raman spectra, which suggests that, in the extremely low energy range, e-beam induced chemical reactions between residual water and graphene are not the dominant mechanism driving defect formation in graphene. Single layer graphene, partially suspended over holes was irradiated with EUV radiation. By comparing with the Raman results from e-beam irradiation, it is concluded that the photoelectrons, especially those from the valence band, contribute to defect formation in graphene during irradiation.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Tunable electronic properties and low thermal conductivity in synthetic colusites Cu{sub 26−x}Zn{sub x}V{sub 2}M{sub 6}S{sub 32} (x ≤ 4, M = Ge, Sn)

We have first synthesized Cu{sub 26−x}Zn{sub x}V{sub 2}M{sub 6}S{sub 32} (x ≤ 4, M = Ge, Sn) with the cubic colusite structure and measured the thermoelectric properties. For both M = Ge and Sn, the samples with x = 0 show moderately large thermopower of +27 μV/K at 300 K. The metallic conduction of p-type carriers and Pauli-paramagnetic behavior are consistent with the electron-deficient character expected from the formal charge Cu{sub 26}{sup 1+}V{sub 2}{sup 5+}M{sub 6}{sup 4+}S{sub 32}{sup 2−}. The substitution of Zn for Cu results in significant increases in both the electrical resistivity and thermopower. The resistivity of the samples with x = 4 displays a three-dimensional variable-range hopping behavior at low temperatures. These facts indicate that the doped electrons fill the unoccupied states in the valence band and thereby the Fermi level moves to the localized electronic states at the top of the band. The lattice thermal conductivity is as low as ∼1 W/Km at 300 K for all samples. The structural and thermoelectric properties of the colusites are discussed in comparison with those of doped tetrahedrite Cu{sub 12−x}Zn{sub x}Sb{sub 4}S{sub 13}.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2014

Substitution effect on the magnetic and transport properties of CeNi{sub 0.8−x}Mn{sub x}Bi{sub 2}

We report the results of Mn substitution for Ni in CeNi{sub 0.8}Bi{sub 2} (i.e., CeNi{sub 0.8−x}Mn{sub x}Bi{sub 2}). All the samples have an antiferromagnetic ordered state below T{sub N} = 5.0 K due to localized 4f-magnetic moment on the Ce ions. Besides this antiferromagnetic ordering caused by Ce, the magnetic and transport properties are abruptly changed with increasing Mn contents at the boundary composition of x = 0.4. The magnetic state is changed into a ferromagnetic state around 200 K for x > 0.4, where the electrical resistivity is strongly suppressed to become simple metallic. These results of ferromagnetism and metallicity can be explained by the double exchange mechanism, rather than the simple picture of Doniach phase diagram. The mixed valence states of Ni and Mn ions are confirmed by X-ray photoelectron spectroscopy. For x ≤ 0.4, the initial Ni{sup 3+} state gradually changes to the Ni{sup 2+} state with increasing x up to 0.4. On further increase of x > 0.4, the Ni{sup 2+} state is replaced by the Mn{sup 2+} state, which gradually changes to the final Mn{sup 3+} state. We also present an inelastic neutron scattering (INS) measurements on CeNi{sub 0.8}Bi{sub 2} (i.e., x = 0) between 1.2 and 12 K. The high energy INS study reveals the presence of two well-defined crystal electric field (CEF) excitations near 9 meV and 19 meV at 1.2 K and 6 K, while the low energy INS study reveals the presence of quasielastic scattering above 4 K. We will discuss our INS results of CeNi{sub 0.8}Bi{sub 2} based on the CEF model.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗