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At least 1,657 records · Page 92
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DOE OSTI2014

Ab-initio modeling of electromechanical coupling at Si surfaces

The electromechanical coupling at the silicon (100) and (111) surfaces was studied via density functional theory by calculating the response of the ionization potential and the electron affinity to different types of strain. We find a branched strain response of those two quantities with different coupling coefficients for negative and positive strain values. This can be attributed to the reduced crystal symmetry due to anisotropic strain, which partially lifts the degeneracy of the valence and conduction bands. Only the Si(111) electron affinity exhibits a monotonously linear strain response, as the conduction band valleys remain degenerate under strain. The strain response of the surface dipole is linear and seems to be dominated by volume changes. Our results may help to understand the mechanisms behind electromechanical coupling at an atomic level in greater detail and for different electronic and atomic structures.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2014

Ferromagnetism and the electronic band structure in (Ga,Mn)(Bi,As) epitaxial layers

Impact of Bi incorporation into (Ga,Mn)As layers on their electronic- and band-structures as well as their magnetic and structural properties has been studied. Homogenous (Ga,Mn)(Bi,As) layers of high structural perfection have been grown by the low-temperature molecular-beam epitaxy technique. Post-growth annealing treatment of the layers results in an improvement of their structural and magnetic properties and an increase in the hole concentration in the layers. The modulation photoreflectance spectroscopy results are consistent with the valence-band model of hole-mediated ferromagnetism in the layers. This material combines the properties of (Ga,Mn)As and Ga(Bi,As) ternary compounds and offers the possibility of tuning its electrical and magnetic properties by controlling the alloy composition.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Theoretical investigation of La monopnictides: Electronic properties and pressure-induced phase transition

The NaCl-type La monopnictides are proper reference materials for the study of strongly correlated rare-earth pnictides. Yet, despite the simple crystal structure of this system, traditional density functional theory (DFT) calculations have dramatic failures in describing their electronic properties: DFT severely underestimates the band gaps and thus predicts incorrect transport characters of them. Here, we perform a corrected DFT calculation to rectify this failure. Our results show that LaN, LaP, and LaAs are semiconductor with band gaps of 0.82, 0.25, and 0.12 eV, respectively, and LaSb is semimetallic with an overlap of conduction and valence bands approximately 0.28 eV, in agreement with the available experiments. Additionally, under high-pressure, we find that LaN displays a new sequence of phase-transition, B1 → anti-B10 → B2, which is different from the previous theoretical predictions but consistent with the recent experiment.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2014

Charge tuning in [111] grown GaAs droplet quantum dots

We demonstrate charge tuning in strain free GaAs/AlGaAs quantum dots (QDs) grown by droplet epitaxy on a GaAs(111)A substrate. Application of a bias voltage allows the controlled charging of the QDs from −3|e| to +2|e|. The resulting changes in QD emission energy and exciton fine-structure are recorded in micro-photoluminescence experiments at T = 4 K. We uncover the existence of excited valence and conduction states, in addition to the s-shell-like ground state. We record a second series of emission lines about 25 meV above the charged exciton emission coming from excited charged excitons. For these excited interband transitions, a negative diamagnetic shift of large amplitude is uncovered in longitudinal magnetic fields.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

The effects of Si-doped prelayers on the optical properties of InGaN/GaN single quantum well structures

In this paper, we report on the effects of including Si-doped (In)GaN prelayers on the low temperature optical properties of a blue-light emitting InGaN/GaN single quantum well. We observed a large blue shift of the photoluminescence peak emission energy and significant increases in the radiative recombination rate for the quantum well structures that incorporated Si-doped prelayers. Simulations of the variation of the conduction and valence band energies show that a strong modification of the band profile occurs for the quantum wells on Si-doped prelayers due to an increase in strength of the surface polarization field. The enhanced surface polarization field opposes the built-in field across the quantum well and thus reduces this built-in electric field. This reduction of the electric field across the quantum well reduces the Quantum Confined Stark Effect and is responsible for the observed blue shift and the change in the recombination dynamics.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Band alignment study of lattice-matched In{sub 0.49}Ga{sub 0.51}P and Ge using x-ray photoelectron spectroscopy

Lattice-matched In{sub 0.49}Ga{sub 0.51}P was grown on a p-type Ge(100) substrate with a 10° off-cut towards the (111) by low temperature molecular beam epitaxy, and the band-alignment of In{sub 0.49}Ga{sub 0.51}P on Ge substrate was obtained by high resolution x-ray photoelectron spectroscopy. The valence band offset for the InGaP/Ge(100) interface was found to be 0.64 ± 0.12 eV, with a corresponding conduction band offset of 0.60 ± 0.12 eV. The InGaP/Ge interface is found to be of the type I band alignment.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Formation kinetics of trivacancy-oxygen pairs in silicon

The formation of donor and acceptor states of the trivacancy-oxygen (V₃O) complex in p- and n-type silicon, respectively, has been studied in detail by means of deep level transient spectroscopy. The samples were irradiated with 1.8 MeV protons, and it was found that acceptor and donor states of V₃ disappear at annealing temperatures below 200 °C, with a transition from the metastable (110) planar configuration to the stable fourfold coordinated configuration. Annealing above 200 °C unveils the formation of two levels with energy positions at E{sub c} –0.34 eV and E{sub c} –0.46 eV in n-type samples and two levels at E{sub v} + 0.24 eV and E{sub v} + 0.11 eV in p-type samples. (E{sub c} and E{sub v} denote the conduction and valence band edge, respectively.) The amplitudes of these levels show an almost one-to-one correlation with those of the V₃-related levels occurring after irradiation. In accordance with recent reports in the literature, the emerging levels are ascribed to the V₃O complex; single and double negative charge states in n-type samples and single and double positive charge states in p-type samples. By undertaking isothermal formation of the V₃O complex in p-type samples which exhibits first-order kinetics, the diffusivity of V₃ in the neutral charge state is determined to be (8.5 ± 3.5) × 10₋₂ exp [–(1.50 ± 0.04) eV/kT] cm²/s.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

First-principles electronic structure and formation energies of group V and VII impurities in the α-Fe{sub 2}O{sub 3} alloys

Based on density functional theory, the electronic structures, formation energy, and transition level of the selected group V and VII impurities in α-Fe{sub 2}O{sub 3} are investigated by means of first-principles methods. Numerical results show that the group V and VII atoms-doped α-Fe{sub 2}O{sub 3} can be energetically favorable under the Fe-rich condition. Group V atom substituting O atom can induce the acceptor impurity level, while the deep donor impurity states are formed inside the band gap when group VII atom substitute O atom in the α-Fe{sub 2}O{sub 3}. Moreover, our results show that halogen atom F substituting O atom should be very easy in the α-Fe{sub 2}O{sub 3}. In addition, our results also show that for both group V and VII atom-doped α-Fe{sub 2}O{sub 3}, the upper sides of valence band are modified obviously, while the conduction band edge does not change.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Charge transfer and mobility enhancement at CdO/SnTe heterointerfaces

We report a study of the effects of charge transfer on electrical properties of CdO/SnTe heterostructures. A series of structures with variable SnTe thicknesses were deposited by RF magnetron sputtering. Because of an extreme type III band offset with the valence band edge of SnTe located at 1.5 eV above the conduction band edge of CdO, a large charge transfer is expected at the interface of the CdO/SnTe heterostructure. The electrical properties of the heterostructures are analyzed using a multilayer charge transport model. The analysis indicates a large 4-fold enhancement of the CdO electron mobility at the interface with SnTe. The mobility enhancement is attributed to reduction of the charge center scattering through neutralization of the donor-like defects responsible for the Fermi level pinning at the CdO/SnTe interface.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Determination of heterojunction band offsets between CdS bulk and PbS quantum dots using photoelectron spectroscopy

Photoelectron spectroscopy was used to measure the energy discontinuity in the valence band (ΔE{sub V}) of a CdS/PbS quantum dot (QD) heterojunction for which the PbS QD layer was deposited using solution based layer-by-layer dip coating method on top of RF magnetron sputtered CdS. A value of ΔE{sub V} = 1.73 eV was obtained using the Cd 3d and Pb 4f energy levels as references. Given the band gap energies of the CdS and PbS-QD layers, the conduction band offset ΔE{sub C} was determined to be 0.71 eV.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Defect density and dielectric constant in perovskite solar cells

We report on measurement of dielectric constant, mid-gap defect density, Urbach energy of tail states in CH{sub 3}NH{sub 3}PbI{sub x}Cl{sub 1−x} perovskite solar cells. Midgap defect densities were estimated by measuring capacitance vs. frequency at different temperatures and show two peaks, one at 0.66 eV below the conduction band and one at 0.24 eV below the conduction band. The attempt to escape frequency is in the range of 2 × 10{sup 11}/s. Quantum efficiency data indicate a bandgap of 1.58 eV. Urbach energies of valence and conduction band are estimated to be ∼16 and ∼18 meV. Measurement of saturation capacitance indicates that the relative dielectric constant is ∼18.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Landau levels of the C-exciton in CuInSe{sub 2} studied by magneto-transmission

The electronic structure of the solar cell absorber CuInSe{sub 2} is studied using magneto-transmission in thin polycrystalline films at magnetic fields up to 29 T. A, B, and C free excitons are resolved in absorption spectra at zero field and a Landau level fan generated by diamagnetic exciton recombination is observed for fields above 7 T. The dependence of the C band exciton binding energy on magnetic fields, calculated using a hydrogenic approximation, is used to determine the C exciton Rydberg at 0 T (8.5 meV), band gap (1.2828 eV), and hole effective mass m{sub so} = (0.31 ± 0.12)m{sub 0} for the C valence sub-band.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

In operando observation system for electrochemical reaction by soft X-ray absorption spectroscopy with potential modulation method

In order to investigate local structures of electrolytes in electrochemical reactions under the same scan rate as a typical value 100 mV/s in cyclic voltammetry (CV), we have developed an in operando observation system for electrochemical reactions by soft X-ray absorption spectroscopy (XAS) with a potential modulation method. XAS spectra of electrolytes are measured by using a transmission-type liquid flow cell with built-in electrodes. The electrode potential is swept with a scan rate of 100 mV/s at a fixed photon energy, and soft X-ray absorption coefficients at different potentials are measured at the same time. By repeating the potential modulation at each fixed photon energy, it is possible to measure XAS of electrochemical reaction at the same scan rate as in CV. We have demonstrated successful measurement of the Fe L-edge XAS spectra of aqueous iron sulfate solutions and of the change in valence of Fe ions at different potentials in the Fe redox reaction. The mechanism of these Fe redox processes is discussed by correlating the XAS results with those at different scan rates.

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

Density functional and theoretical study of the temperature and pressure dependency of the plasmon energy of solids

The temperature and pressure dependency of the volume plasmon energy of solids was investigated by density functional theory calculations. The volume change of crystal is the major factor responsible for the variation of valence electron density and plasmon energy in the free electron model. Hence, to introduce the effect of temperature and pressure for the density functional theory calculations of plasmon energy, the temperature and pressure dependency of lattice parameter was used. Also, by combination of the free electron model and the equation of state based on the pseudo-spinodal approach, the temperature and pressure dependency of the plasmon energy was modeled. The suggested model is in good agreement with the results of density functional theory calculations and available experimental data for elements with the free electron behavior.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Systematic structure of the neutron drip-line {sup 22}C nucleus

In the present work we systematically discuss the nuclear structure of the the heaviest particle-bound carbon isotope, {sup 22}C. The ground state wave function of the carbon isotope is calculated using the {sup 20}C core plus two-valence neutron based on a phenomenological mean-field MF potential. We apply the deduced wave function to provide the nuclear matter density which is necessary in the calculations of the total reaction cross section. Calculations show that there is a reasonable good description of the experimental binding energy BE and root-mean square RMS radius. The exotic structure and configuration of the ground state carbon isotope is explained and a consistent explanation on the two-neutron halo (Borromean) nucleus is given.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2014

Schottky contact by Ag on In{sub 2}O{sub 3} (111) single crystals

The barrier height of a metal-semiconductor contact was studied by means of angle-resolved photoemission spectroscopy, which was implemented through stepwise Ag deposition on the ultra-high vacuum cleaved (111) surface of melt-grown In{sub 2}O{sub 3} single crystals. A small Schottky barrier height of 0.22 ± 0.08 eV was determined by following the band bending of the valence band and core level spectra with Ag thickness and corrected for the photovoltage effect. In addition, the work function of Ag and the electron affinity of In{sub 2}O{sub 3} were measured in situ to be 4.30 ± 0.05 eV and 4.18 ± 0.06 eV, respectively. Agreement was observed when comparing the barrier height from band bending to the calculated one by applying the Schottky-Mott rule, yielding a value of 0.12 ± 0.11 eV. Due to an additionally appearing photovoltage, an explicit reference to the surface electron accumulation layer is not necessary when discussing the Schottky character of the Ag/In{sub 2}O{sub 3} contact.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Hot-carrier solar cells using low-dimensional quantum structures

We propose a high-conversion-efficiency solar cell (SC) utilizing the hot carrier (HC) population in an intermediate-band (IB) of a quantum dot superlattice (QDSL) structure. The bandgap of the host semiconductor in this device plays an important role as an energy-selective barrier for HCs in the QDSLs. According to theoretical calculation using the detailed balance model with an air mass 1.5 spectrum, the optimum IB energy is determined by a trade-off relation between the number of HCs with energy exceeding the conduction-band edge and the number of photons absorbed by the valence band−IB transition. Utilizing experimental data of HC temperature in InAs/GaAs QDSLs, the maximum conversion efficiency under maximum concentration (45 900 suns) has been demonstrated to increase by 12.6% as compared with that for a single-junction GaAs SC.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗