Distribution Of Electronic States Research Articles

Mechanism of the Generation of Donor–Acceptor Pairs in Heavily Doped n-ZrNiSn with the Ga Acceptor Impurity

The nature of the mechanism of the simultaneous generation of donor–acceptor pairs under heavy doping of n-ZrNiSn intermetallic semiconductor with the Ga acceptor impurity is established. Such spatial arrangement in the crystal lattice of ZrNiSn1–xGa x is found when the rate of movement of the Fermi level eF found from calculations of the density distribution of electron states coincides with that experimentally established from dependences lnρ(1/T). It is shown that when the Ga impurity atom (4s24p1) occupies the 4b sites of Sn atoms (5s25p2), structural defects of both acceptor nature and donor nature in the form of vacancies in the 4b site are simultaneously generated. The results are discussed in the scope of the Shklovskii–Efros model of a heavily doped and compensated semiconductor.

Density functional theory of electron transfer beyond the Born-Oppenheimer approximation: Case study of LiF.

We perform model calculations for a stretched LiF molecule, demonstrating that nonadiabatic charge transfer effects can be accurately and seamlessly described within a density functional framework. In alkali halides like LiF, there is an abrupt change in the ground state electronic distribution due to an electron transfer at a critical bond length R = Rc, where an avoided crossing of the lowest adiabatic potential energy surfaces calls the validity of the Born-Oppenheimer approximation into doubt. Modeling the R-dependent electronic structure of LiF within a two-site Hubbard model, we find that nonadiabatic electron-nuclear coupling produces a sizable elongation of the critical Rc by 0.5 bohr. This effect is very accurately captured by a simple and rigorously derived correction, with an M-1 prefactor, to the exchange-correlation potential in density functional theory, M = reduced nuclear mass. Since this nonadiabatic term depends on gradients of the nuclear wave function and conditional electronic density, ∇Rχ(R) and ∇Rn(r, R), it couples the Kohn-Sham equations at neighboring R points. Motivated by an observed localization of nonadiabatic effects in nuclear configuration space, we propose a local conditional density approximation-an approximation that reduces the search for nonadiabatic density functionals to the search for a single function y(n).

Superconductivity described with Electron-Phonon Synchronic Coupling

Superconductivity physical description is proposed in terms of synchronic phonon electron coupling which is expected to facilitate definition of superconducting solid state configuration. It is based on electron channeling with dual classical/wave representation which consider locality of involved phonon and conduction electron, contrary to usual quantum mechanical formalism and which does not achieve satisfactory physical description up to now. It is considered a) 3D spacial configuration of electronic bands, b) transverse phonon vibration amplitude c) temperature and energy (frequency) dependent phonon density and additional conditions about material structure, and distribution of electronic and phonon states. Higher Tc materials with specific structures and properties can thus be expected. For instance with stacks of new doped hard materials which can be associated with specific atomic transition layers to new generation 2D materials large which have large planar electron rich conduction channels and offering possible new investigation routes.

Механизм генерирования донорно-акцепторных пар при сильном легировании n-ZrNiSn акцепторной примесью Ga

AbstractThe nature of the mechanism of the simultaneous generation of donor–acceptor pairs under heavy doping of n -ZrNiSn intermetallic semiconductor with the Ga acceptor impurity is established. Such spatial arrangement in the crystal lattice of ZrNiSn_1– x Ga_ x is found when the rate of movement of the Fermi level εF found from calculations of the density distribution of electron states coincides with that experimentally established from dependences lnρ(1/ T ). It is shown that when the Ga impurity atom (4 s ^24 p ^1) occupies the 4 b sites of Sn atoms (5 s ^25 p ^2), structural defects of both acceptor nature and donor nature in the form of vacancies in the 4 b site are simultaneously generated. The results are discussed in the scope of the Shklovskii–Efros model of a heavily doped and compensated semiconductor.

Electron localization in niobium doped CaMnO3 due to the energy difference of electronic states of Mn and Nb.

The electron localization in Nb-doped CaMnO3 is analyzed in terms of the space and energy distribution of electronic states employing first-principles calculations. The energy difference of Mn 3d states and Nb 4d states makes NbO6 octahedra impede electrical conduction, so the random distribution of Nb in lattices leads to the localization of electrons near the bottom of the conduction bands. Therefore, although more carriers are introduced when Nb-doping content increases, both the electrical conductivity and absolute thermopower decrease in Nb heavy doped CaMnO3. The calculated transport properties agree well with the experimental data, supporting the analysis of localization.

Spectroelectrochemical analysis of TiO2 electronic states – Implications for the photocatalytic activity of anatase and rutile

The information on the electronic structure of nano- and microstructured semiconductors is crucial for their applications in photovoltaics, optoelectronics and photocatalysis. The distribution of intra-bandgap electronic states is one of the most relevant parameters which may be tuned in order to provide desired properties of a photoactive material. We propose the use of a modified spectroelectrochemical method for the characterisation of vacant electronic states distributed close to the edge of the titanium dioxide conduction band. These additional levels localized within the bandgap were semi-quantitatively characterized for several samples made of different polymorphs (or their blends) of TiO2. The applicability of the method for the determination of deep and shallow electron traps was confirmed. A quantitative analysis of the latter was also conducted for the selected samples. The presented approach provides also the information on the stability of electronic states, which is crucial for numerous practical applications.

Controlling Magnetic Properties at BiFe1−xMnxO3/La2/3Ca1/3MnO3 Interfaces by Tuning the Spatial Distribution of Interfacial Electronic States

AbstractOne major challenge in engineering the magnetism of oxide heterostructures is controlling orbital reconstruction by tuning the charge transfer effect. This paper investigates the forward and backward charge transfer effect and the induced magnetic properties of BiFe1−xMnxO3/La2/3Ca1/3MnO3 heterostructures. Interfacial ferromagnetism is found to be qualitatively tunable by tuning the spatial distribution of interfacial electronic states. The subtle balance of magnetic coupling between FeOMn and MnOMn changed as the electronic structures are modified by charge transfer. The ions' valence is strongly correlated with the interfacial magnetic properties, extending the concept of the charge degrees of freedom affecting the magnetic coupling properties.

Shock Radiation Tests for Saturn and Uranus Entry Probes

This paper describes a test series in the Electric Arc Shock Tube at NASA Ames Research Center with the objective of quantifying shock-layer radiative heating magnitudes for future probe entries into Saturn and Uranus atmospheres. Normal shock waves are measured in hydrogen–helium mixtures ( by volume) at freestream pressures between 13 and 66 Pa (0.1 and 0.5 torr) and velocities from . No shock-layer radiation is detected within measurement limits below , a finding consistent with predictions for Uranus entries. Between 25 and , radiance is quantified from the vacuum ultraviolet through near infrared, with focus on the Lyman- and Balmer series lines of hydrogen. Shock profiles are analyzed for electron number density and electronic state distribution. The shocks do not equilibrate over several centimeters and in many cases the state distributions are non-Boltzmann. Radiation data are compared to simulations of Decadal Survey entries for Saturn and shown to be as much as eight times lower than predicted with the Boltzmann radiation model. Radiance is observed in front of the shock layer, the characteristics of which match the expected diffusion length.

Theoretical prediction of phonon-mediated superconductivity with Tc ≈ 25 K in Li-intercalated hexagonal boron nitride bilayer

A superconducting transition temperature (Tc) up to 25 K in the Li-intercalated bilayer of hexagonal boron nitride (h-BN) is predicted according to ab-initio calculations. A Tc higher than that of metal-intercalated graphene (MIG) is ascribed to the characteristic spatial distribution of electronic states near the Fermi level, which is distinctly different from that in MIG. In the Li-intercalated bilayer h-BN, the breaking of the symmetrical restriction and the increase in the overlap between the charge density and the Li in-plane motion enhance the electron–phonon coupling. Our results provide a new design guideline for two-dimensional superconductors based on intercalated layered materials.

Correlation between Electronic Defect States Distribution and Device Performance of Perovskite Solar Cells.

In the present study, random current fluctuations measured at different temperatures and for different illumination levels are used to understand the charge carrier kinetics in methylammonium lead iodide CH3NH3PbI3‐based perovskite solar cells. A model, combining trapping/detrapping, recombination mechanisms, and electron–phonon scattering, is formulated evidencing how the presence of shallow and deeper band tail states influences the solar cell recombination losses. At low temperatures, the observed cascade capture process indicates that the trapping of the charge carriers by shallow defects is phonon assisted directly followed by their recombination. By increasing the temperature, a phase modification of the CH3NH3PbI3 absorber layer occurs and for temperatures above the phase transition at about 160 K the capture of the charge carrier takes place in two steps. The electron is first captured by a shallow defect and then it can be either emitted or thermalize down to a deeper band tail state and recombines subsequently. This result reveals that in perovskite solar cells the recombination kinetics is strongly influenced by the electron–phonon interactions. A clear correlation between the morphological structure of the perovskite grains, the energy disorder of the defect states, and the device performance is demonstrated.

Time evolution of the population distribution in charge exchange collisions

SynopsisWe have calculated the population distributions of many electronic excite states as a function of time in collisions of bare, hydrogen-like and helium-like C, N and O ions with neutral targets, H, He, and H2. Using the population distribution, we have calculated theoretical emission spectra in the collision system. Comparing the theoretical spectra with the observation, we have estimated the detection efficiency of the spectrometer and discuss the singlet-triplet ratio in charge exchange collisions of hydrogen-like ions.

Structural and electronic inhomogeneity of graphene revealed by Nano-ARPES

Electronic structure describes the distribution of electronic states in reciprocal space, being one of the most fundamental concepts in condensed matter physics, since it determines the electrical, optical and magnetic behaviours of materials. Due to its two-dimensional honeycomb lattice with covalent bonding, pristine graphene exhibits unsurpassed in-plane stiffness and stable structural properties. Here by employing angle-resolved photoemission spectroscopy with spatial resolution ∼ 100 nm (Nano-ARPES), we discuss in detail the structural and electronic properties of graphene grown on cooper by chemical vapour deposition (CVD). Our results reveal the spatial inhomogeneity of graphene film, demonstrating the power of Nano-ARPES to detect the microscopic inhomogeneity of electronic structure for different materials.

(Invited) Characterization of Thin Passive Film-Electrolyte Junctions. The Amorphous Semiconductor (a-SC) Schottky Barrier Approach

A detailed study of the electronic properties of thin (< 20 nm) anodic TiO2 potentiostatically grown on titanium in two different solutions is presented. The results show that the nature of the anodizing solution affects the electronic properties of the anodic film and in particular the density of electronic state (DOS) distribution. Different DOS were derived from the experimental data analyzed according to the theory of amorphous semiconductor (a-SC) Schottky barrier. It is shown that the usual non-linear and frequency dependent Mott-Schottky plots are in agreement with expected theoretical behaviour of a-SC Schottky barrier. It is shown the importance of the DOS distribution in determining the frequency response of the junction which is also related to the electrical behaviour of anodic films as “thin” or “thick” in terms of the ratio between the space-charge region and oxide film thickness.

The Amorphous Semiconductor Schottky Barrier Approach to Study the Electronic Properties of Anodic Films on Ti

A detailed study of the electronic properties of thin (<20 nm) anodic TiO2 potentiostatically grown on titanium in two different solutions is presented. The results show that the nature of the anodizing solution affects the electronic properties of the anodic film and, more specifically, the density of electronic states (DOS) distribution. Different DOS were derived from the experimental data analyzed according to the theory of amorphous semiconductor (a-SC) Schottky barrier. It is shown that the usual non-linear and frequency dependent Mott-Schottky plots are in agreement with expected theoretical behavior of a-SC Schottky barrier.

On the nature of high field charge transport in reinforced silicone dielectrics: Experiment and simulation

The high field charge injection and transport properties in reinforced silicone dielectrics were investigated by measuring the time-dependent space charge distribution and the current under dc conditions up to the breakdown field and were compared with the properties of other dielectric polymers. It is argued that the energy and spatial distribution of localized electronic states are crucial in determining these properties for polymer dielectrics. Tunneling to localized states likely dominates the charge injection process. A transient transport regime arises due to the relaxation of charge carriers into deep traps at the energy band tails and is successfully verified by a Monte Carlo simulation using the multiple-hopping model. The charge carrier mobility is found to be highly heterogeneous due to the non-uniform trapping. The slow moving electron packet exhibits a negative field dependent drift velocity possibly due to the spatial disorder of traps.

The physical mechanism on the threshold voltage temperature stability improvement for GaN HEMTs with pre-fluorination argon treatment

In this paper, a normally-off AlGaN/GaN MIS-HEMT with improved threshold voltage (VTH) thermal stability is reported with investigations on its physical mechanism. The normally-off operation of the device is achieved from novel short argon plasma treatment (APT) prior to the fluorine plasma treatment (FPT) on Al2O3 gate dielectrics. For the MIS-HEMT with FPT only, its VTH drops from 4.2 V at room temperature to 0.5 V at 200 °C. Alternatively, for the device with APT-then-FPT process, its VTH can retain at 2.5 V at 200 °C due to the increased amount of deep-level traps that do not emit electrons at 200 °C. This thermally stable VTH makes this device suitable for high power applications. The depth profile of the F atoms in Al2O3, measured by the secondary ion mass spectroscopy, reveals a significant increase in the F concentration when APT is conducted prior to FPT. The X-ray photoelectron spectroscopy (XPS) analysis on the plasma-treated Al2O3 surfaces observes higher composition of Al-F bonds if APT was applied before FPT. The enhanced breaking of Al-O bonds due to Ar bombardment assisted in the increased incorporation of F radicals at the surface during the subsequent FPT process. The Schrödinger equation of Al2OxFy cells, with the same Al-F compositions as obtained from XPS, was solved by Gaussian 09 molecular simulations to extract electron state distribution as a function of energy. The simulation results show creation of the deeper trap states in the Al2O3 bandgap when APT is used before FPT. Finally, the trap distribution extracted from the simulations is verified by the gate-stress experimental characterization to confirm the physical mechanism described.

Molecular rectifier composed of DNA with high rectification ratio enabled by intercalation.

The predictability, diversity and programmability of DNA make it a leading candidate for the design of functional electronic devices that use single molecules, yet its electron transport properties have not been fully elucidated. This is primarily because of a poor understanding of how the structure of DNA determines its electron transport. Here, we demonstrate a DNA-based molecular rectifier constructed by site-specific intercalation of small molecules (coralyne) into a custom-designed 11-base-pair DNA duplex. Measured current-voltage curves of the DNA-coralyne molecular junction show unexpectedly large rectification with a rectification ratio of about 15 at 1.1 V, a counter-intuitive finding considering the seemingly symmetrical molecular structure of the junction. A non-equilibrium Green's function-based model-parameterized by density functional theory calculations-revealed that the coralyne-induced spatial asymmetry in the electron state distribution caused the observed rectification. This inherent asymmetry leads to changes in the coupling of the molecular HOMO-1 level to the electrodes when an external voltage is applied, resulting in an asymmetric change in transmission.

Excited state electron distribution and role of the terminal amine in acidic and basic tryptophan dipeptide fluorescence

The results of quantum yield (QY) study of tryptophanyl glutamate (Trp–Glu), tryptophanyl lysine (Trp–Lys) and lysinyl tryptophan (Lys–Trp) dipeptides over the pH range, 1.5–13, show that the charge state of the N-terminal amine, and not the nominal molecular charge determines the QY. When the terminal amine is protonated, QY is low (10−2) for all three dipeptides. As the terminal amine cation is found proximal to the indole ring in Trp–Glu and Trp–Lys conformers but not in those for Lys–Trp, its effect may lie only in the partitioning of energy between nonradiative processes, not on QY reduction. QY is also low when both the N-terminal amine and indole amine are deprotonated. These two low QY states can be distinguished by fluorescence lifetime measurement. Molecular dynamics simulation shows that the Chi 1 conformers persist for tens of nanoseconds such that 100–101 ns lifetimes may be associated with individual Chi 1 conformers. The ground state electron density or isosurface of high QY (0.30) 3-methyindole has a uniform electron density over the indole ring as do the higher QY Trp dipeptide conformers. This validates the association of ground state isosurfaces with QY. Excited state orbitals from calculated high intensity, low energy absorption transitions are typically centered over the indole ring for higher QY dipeptide species and off the ring in lower QY species. Thus excited state orbitals substantiate the earlier finding that the ground state isosurface charge density pattern on the indole ring can be predictive of QY.

Effect of step-edge on spectral properties and planar stability of metallic bigraphene

Phonon and electron spectra of metallic bigraphene are analyzed in the presence of step-edge crystal imperfection. Different geometries of step-edge are considered. The dynamic planar stability of the considered structure is proved for temperatures above the ambient. The number of phonon states is shown to grow near the K-point of the first Brillouin zone, compared to pristine graphene. It is found, that this type of defects causes substantially nonuniform distribution of electron states and the pronounced increase in the number of states with energies close to Fermi energy can be expected in electron spectrum of the graphene-based compounds. The performed calculations are in good agreement with inelastic neutron, x-ray and Raman measurements.

Valence Band Profile in Two-Dimensional Silicon-Oxygen Superlattices Probed by Internal Photoemission

Energy distribution of electron states in two-dimensional superlattices (SLs) of crystalline silicon layers separated by monolayers (MLs) of oxygen atoms is studied using internal photoemission (IPE) spectroscopy. SLs made of 1, 2 or 5 periods of one ML of O in between 25, 15, 7, 3 or ∼1 nm thick Si layers were studied by using IPE from these SLs into 20-nm thick Al2O3 dielectric deposited on top of the SL. We observed the reduction of direct optical transition intensity in Si-O SLs when decreasing the Si thickness to 1 nm, indicative of the Si electronic structure distortion inside the SLs. The field dependence of IPE spectral threshold indicates the absence of both the change in Si-O SLs VB energy and any additional field-induced band bending across the SLs. Further, electron spin resonance indicates the absence of additional Si dangling bonds in the Si-O SLs within the sensitivity limit of ≈1011 cm−2. These findings suggest that Si-O SLs enhance the carrier mobility by breaking the crystal symmetry and tuning the location of the channel to the region where electrons or holes travel without additional scattering.