Valence Band Density Of States Research Articles

Electronic structure of single-crystal solid solutions Pb1-xBaxTiO3 (0 ≤ x ≤ 1) from X-ray photoelectron spectroscopy and real-space multiple electron scattering calculations

Valence band and core level X-ray photoelectron spectra (XPS) are measured in single-crystal solid solutions Pb1-xBaxTiO3 (x = 0, 0.021, 0.146, 0.24, 0.541, 0.784, 0.906, 1). Valence band densities of states (DOS) and theoretical valence band XPS are calculated for Pb1-xBaxTiO3 (x = 0, 0.12, 0.25, 0.5, 0.875, 1) via the method of multiple electron scattering in real space. Experimental and theoretical valence band XPS are in good agreement. The increase of ionicity of the TiO bonds in Pb1-xBaxTiO3 upon the growth of relative Ba content x is predicted theoretically and shown experimentally using the energy position of the charge transfer satellites in the Ti2p XPS. The increase of Ti2p-spectrum line widths against relative Ba content x is observed.

Band Ordering and Dynamics of Cu2–xTe and Cu1.98Ag0.2Te

63Cu, 65Cu, and 125Te NMR measurements are reported for Cu2–xTe and Cu1.98Ag0.2Te. The results demonstrate an onset of Cu-ion hopping below room temperature, including an activation behavior consistent with high-temperature transport measurements but with a significant enhancement of the hopping barriers with Ag substitution. We also separated the Korringa behavior by combining NMR line shape and relaxation measurements, thereby identifying large negative chemical shifts for both nuclei, as well as large Cu and Te s-state contributions in the valence band. Further comparison was obtained through heat capacity measurements and chemical shifts computed by density functional methods. The large diamagnetic chemical shifts coincide with behavior previously identified for materials with topologically nontrivial band inversion, and in addition, the large metallic shifts point to analogous features in the valence band density of states, suggesting that Cu2Te may have similar inverted features.

Unraveling The Origin of Enhanced Field Emission from Irradiated FeCo-SiO2 Nanocomposites: A Combined Experimental and First-Principles Based Study.

This work is driven by the vision of engineering planar field emitters with ferromagnetic metal-insulator nanocomposite thin films, using swift heavy ion (SHI) irradiation method. FeCo nanoparticles inside SiO2 matrix, when subjected to SHI get elongated. Using this, we demonstrate here a planar field emitter with maximum current density of 550 μA/cm(2) at an applied field of 15 V/μm. The film, irradiated with 5 × 10(13) ions/cm(2) fluence (5e13) of 120 MeV Au(9+) ions, shows very high electron emitting quantum efficiency in comparison to its unirradiated counterpart. Surface enhanced Raman spectroscopy analysis of unirradiated and 5e13 films further confirms that the field emission (FE) enhancement is not only due to surface protrusions but also depends on the properties of entire matrix. We find experimental evidence of enhanced valence band density of states (VB DOS) for 5e13 film from XPS, which is verified in the electronic structure of a model FeCo cluster from first-principles based calculations combining density functional theory (DFT) and molecular dynamics (MD) simulations. The MD temperature is selected from the lattice temperature profile inside nanoparticles as deduced from thermal spike model. Increasing the irradiation fluence beyond 5e13, results in reduced VB DOS and melting of surface protrusions, thus causing reduction of FE current density. We finally conclude from theoretical analysis that change in fluence alters the co-ordination chemistry followed by the charge distribution and spin alignment, which influence the VB DOS and concurrent FE as evident from our experiment.

DFT studies of hydrogen retention in beryllium nitride

The valence band density of states of bulk beryllium nitride (alpha-Be3N2) and the solvation energy of hydrogen atoms in this nitride is herein studied using the generalized gradient approximation density functional theory. The DOS is compared to photoelectron spectroscopy investigations of nitrogen-implanted beryllium. The presence of defects (atomic vacancies) was taken into consideration in the calculations on the trapping of deuterium. The barriers of activation for hydrogen atom diffusion in the bulk and from the defect were calculated and compared to their values in pure beryllium metal and in beryllium oxide. Some consequences regarding the effects of nitrogenation of beryllium as a plasma-facing material in nuclear fusion devices are eventually proposed.

Junctionless Silicon and In0.53Ga0.47As Transistors—Part II: Device Variability From Random Dopant Fluctuation

Random dopant fluctuation (RDF) variability in nanoscale junctionless FETs (JLFETs) utilizing either Si or In0.53Ga0.47As channels has been studied using technology computer-aided design (TCAD) simulations. The 15nm node Si and InGaAs JLFETs are equivalently designed and calibrated using nonequilibrium Green’s function simulations for statistical TCAD analysis. We find that n-InGaAs JLFETs exhibit reduced RDF variability compared with n-Si JLFETs in terms of threshold voltage, subthreshold swing, and drain-induced barrier lowering as a result of high degeneracy effects. By contrast, the variability of p-InGaAs JLFETs is comparable with that of n- and p-Si JLFETs because of the larger valence band density of states (DOS). The normalized variations in ON-state drive current are roughly equal ( $\sim 16$ %) for all device types, because the major effects of degenerate screening on current transport effectively cancel one another. From these results, we find that high carrier degeneracy in small DOS materials can have a significant effect on the electrostatic integrity of JLFETs in the presence of RDF (especially in subthreshold), and that degenerately doped n-InGaAs devices are slightly (and inherently) more immune to RDF compared with Si JLFETs.

Spectral weight shift in the valence band density of states and concurrent increase in field emission by hydrogenation of FeCo–SiO2 nanocomposites

Roughened surface and enhanced valence electronic states of hydrogen plasma treated FeCo–SiO2 nanogranular films has made it an efficient field emitter in comparison to its as-grown and furnace annealed counterparts.

Oxygen 2p Partial Density of States in a Typical Oxide Glass B2O3

The valence- and conduction-band 2p partial densities of states (DOSs) of oxygen in a typical oxide glass, B2O3, were investigated by soft X-ray emission spectroscopy (SXES) and soft X-ray absorption spectroscopy, respectively. An ab initio molecular dynamics (MD) simulation was carried out, and the calculated valence-band DOS is in good agreement with the SXES experimental result. The bond angle dependence of the O 2p partial DOS was also examined by the ab initio MD simulation.

Substrate-controlled band positions in CH₃NH₃PbI₃ perovskite films.

Using X-ray and ultraviolet photoelectron spectroscopy, the surface band positions of solution-processed CH3NH3PbI3 perovskite thin films deposited on an insulating substrate (Al2O3), various n-type (TiO2, ZrO2, ZnO, and F:SnO2 (FTO)) substrates, and various p-type (PEDOT:PSS, NiO, and Cu2O) substrates are studied. Many-body GW calculations of the valence band density of states, with spin-orbit interactions included, show a clear correspondence with our experimental spectra and are used to confirm our assignment of the valence band maximum. These surface-sensitive photoelectron spectroscopy measurements result in shifting of the CH3NH3PbI3 valence band position relative to the Fermi energy as a function of substrate type, where the valence band to Fermi energy difference reflects the substrate type (insulating-, n-, or p-type). Specifically, the insulating- and n-type substrates increase the CH3NH3PbI3 valence band to Fermi energy difference to the extent of pinning the conduction band to the Fermi level; whereas, the p-type substrates decrease the valence band to Fermi energy difference. This observation implies that the substrate's properties enable control over the band alignment of CH3NH3PbI3 perovskite thin-film devices, potentially allowing for new device architectures as well as more efficient devices.

Auger recombination of biexcitons and negative and positive trions in individual quantum dots.

Charged exciton states commonly occur both in spectroscopic studies of quantum dots (QDs) and during operation of QD-based devices. The extra charge added to the neutral exciton modifies its radiative decay rate and also opens an additional nonradiative pathway associated with an Auger process whereby the recombination energy of an exciton is transferred to the excess charge. Here we conduct single-dot spectroscopic studies of Auger recombination in thick-shell ("giant") CdSe/CdS QDs with and without an interfacial alloy layer using time-tagged, time-correlated single-photon counting. In photoluminescence (PL) intensity trajectories of some of the dots, we resolve three distinct states of different emissivities ("bright", "gray", and "dark") attributed, respectively, to the neutral exciton and negative and positive trions. Simultaneously acquired PL lifetime trajectories indicate that the positive trion is much shorter lived than the negative trion, which can be explained by a high density of valence band states and a small hole localization radius (defined by the QD core size), factors that favor an Auger process involving intraband excitation of a hole. A comparison of trion and biexciton lifetimes suggests that the biexciton Auger decay can be treated in terms of a superposition of two independent channels associated with positive- and negative-trion pathways. The resulting interdependence between Auger time constants might simplify the studies of multicarrier recombination by allowing one, for example, to infer Auger lifetimes of trions of one sign based on the measurements of biexciton decay and dynamics of the trions of the opposite sign or, alternatively, estimate the biexciton lifetime based on studies of trion dynamics.

Fermi level stabilization and band edge energies in CdxZn1−xO alloys

We have measured the band edge energies of CdxZn1−xO thin films as a function of composition by three independent techniques: we determine the Fermi level stabilization energy by pinning the Fermi level with ion irradiation, measure the binding energy of valence band states and core levels by X-ray photoelectron spectroscopy, and probe shifts in the conduction band and valence band density of states using soft X-ray absorption and emission spectroscopy, respectively. The three techniques find consensus in explaining the origin of compositional trends in the optical-bandgap narrowing upon Cd incorporation in wurtzite ZnO and widening upon Zn incorporation in rocksalt CdO. The conduction band minimum is found to be stationary for both wurtzite and rocksalt alloys, and a significant upward rise of the valence band maximum accounts for the majority of these observed bandgap changes. Given these band alignments, alloy disorder scattering is found to play a negligible role in decreasing the electron mobility for all alloys. These band alignment details, combined with the unique optical and electrical properties of the two phase regimes, make CdZnO alloys attractive candidates for photoelectrochemical water splitting applications.

Elucidating the 3d Electronic Configuration in Manganese Phthalocyanine

To shed light on the metal 3d electronic structure of manganese phthalocyanine, so far controversial, we performed photoelectron measurements both in the gas phase and as thin film. With the purpose of explaining the experimental results,three different electronic configurations close in energy to one another were studied by means of density functional theory. The comparison between the calculated valence band density of states and the measured spectra revealed that in the gas phase the molecules exhibit a mixed electronic configuration, while in the thin film, manganese phthalocyanine finds itself in the theoretically computed ground state, namely, the b1(2g)e3(g)a1(1g)b0(1g) electronic configuration.

Ruthenium core-activated platinum monolayer shell high redox activity cathodic electrocatalysts for dye-sensitized solar cells

Here, we have combined the understanding of density functional theory in relation with lattice strain-to-activity correlation to design a Rucore–Ptshell structured electrocatalyst in a dye-sensitized solar cell (DSC) cathode. This electrocatalyst possesses a compressive lattice strain in the shell region which translocates the Pt valence charge to neighboring atoms to decrease the Pt valence band density of states and thus lower the Pt d-band levels. Consequently, compared with DSCs using Pt nanoparticles (NPs) cathode, the Rucore–Ptshell NPs improves the triiodide to iodide redox reduction activity by 2.11-fold and thus boosts the photovoltaic efficiency of DSCs by ∼30.4%. These results provide mechanistic information for the development of DSCs with reduced Pt utilization and programmable electrochemical performance.

Size- and Orientation-Dependent Strain Effects on Ballistic Si p-Type Nanowire Field-Effect Transistors

The size- and orientation-dependent uniaxial strain effects on ballistic hole transport in nanowire field-effect transistors are investigated using an sp <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> d <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</sup> s <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">*</sup> -based tight-binding formalism coupled with a compact electrostatics model and a semiclassical transport model. It is found that the strain-induced reduction of the valence band density of states leads to an increased ballistic hole current. This is explained by the product of a small reduction in hole density and a significant increase in the average ballistic hole velocity under uniaxial compression. While uniaxial compressive strain is beneficial for both 〈110〉 and 〈100〉 devices, the strain response of 〈110〉 nanowires is much larger than their 〈100〉 counterparts. Ultrascaled 〈110〉 nanowires have the highest hole drive current under both strained and unstrained conditions, despite the reduction of strain-induced ballistic hole current enhancement for narrower devices.

Energy and angle dependent photoemission study on Si/Ge multilayers using synchrotron radiation

The present report adds to the ongoing research on Si/Ge nanostructures by discussing the results obtained utilizing unique features of Synchrotron radiation (SR). Based on the structural and electronic properties of this system investigated in our earlier work, we have synthesized [Si(5 nm)/Ge(5 nm)]×10 multilayers (MLS) and exposed it to a range of incident photon energies (utilizing synchrotron radiation) and at different incident angles for the photoemission spectroscopic measurements. It is shown how the interface features are enhanced simultaneously suppressing the signals from topmost atomic layers. Also the effect of incident angle variation is discussed, which gives a clear picture of interface electronic properties of Si/Ge system. Significant changes in Si 3p, Ge 4p and s band features are observed in the valence band density of states. The overall results present an interesting observation of thoroughly investigating the basic building bilayer stack of the multilayer structure by combining energy and/or angle variation with low energy sputter depth profiling.

Modeling of low-voltage oxide-based electric-double-layer thin-film transistors fabricated at room temperature

The room-temperature-made low-voltage electric-double-layer (EDL) thin-film transistors (TFTs) are reported previously with good performance including a huge EDL gate capacitance above 1 μF/cm2. We report a two-dimensional simulation of the carrier transport and subgap density of states (DOS) in low-voltage indium tin oxide EDL TFTs. The simple model with a constant mobility and two-step subgap DOS reproduces well the characteristics of EDL TFTs. A nice fitting to the experimental data was obtained with a changeable effective conduction band DOS and valence band DOS model, which is reasonable to EDL electrostatic modulation mechanism. The EDL TFTs show much lower DOS than the InGaZnO4 TFTs.

The interatomic Auger transition and Auger-photoelectron coincidence spectroscopy (APECS)

Abstract The Ta bulk N 7-level of TaC(1 1 1) decays either by the on-site Auger decay or by the interatomic off-site one. By the interatomic off-site Auger decay the subsurface core-hole is filled by a subsurface valence electron, and the excess energy is dissipated by the excitation of the neighboring surface N 7 electron to an unoccupied state, just above the Fermi level. The interatomic Auger decay is followed by the surface on-site Auger decay [R.A. Bartynski, S. Yang, S.L. Hulbert, C.-C. Kao, M. Weinert, D.M. Zehner, Phys. Rev. Lett. 68 (1992) 2247]. We showed by a many-body theory that the Ta N 7-VV Auger-electron spectroscopy (AES) spectrum of TaC(1 1 1) produced by the surface Auger decay via the interatomic Auger decay, measured in coincidence with the bulk core-level photoemission coincides in both line shape and peak kinetic energy (KE) with the singles (noncoincidence) Ta N 7-VV AES one produced by the surface Auger decay. This explains why Bartynski et al. [Phys. Rev. Lett. 68 (1992) 2247] could describe the coincidence Ta N 7-VV AES spectrum of TaC(1 1 1) produced by the surface Auger decay via the interatomic Auger decay by a self-convolution of the valence band density of states (SCDOS) from the first atomic layer.

Theoretical Investigation of Anisotropic Gain Mechanisms in InGaAsP-Based 1.5-$\mu$m Quantum Dash Lasers

We examine the electronic structure and optical properties of 1.5-μm InAs/InGaAsP/InP quantum dash-in-a-well (DWELL) and dash-in-a-barrier (DBAR) lasers. Using 1-D and 3-D k.p calculations, we show that the electron states are not confined to the dash layer in the DWELL structures and are poorly confined in the DBAR case, due to the small conduction band offset in InGaAsP systems. The built-in strain induces a large HH-LH splitting, resulting in a significant reduction in the calculated valence band density of states (DOS). Coupled together, these properties can be engineered to give a nearly symmetric conduction and valence band DOS within 0.1 eV of the band edge. The calculated gain due to light polarized along the TE(110) and TE(1-10) directions is anisotropic, with the degree of anisotropy dependent on the dash height and the area density of the dashes. We conclude that the dashes can provide a high modal gain with reduced transparency and threshold carrier density but similar threshold current density compared to equivalent quantum well devices.

Atomic Contribution to Valence Band Density of States in Gallium Oxide and Silicon Oxide Nano Layered Films

High resolution X-ray photoelectron spectroscopy (XPS) is used to investigate the spectra of nanolayered films. Amorphous gallium oxide (Ga2O3)-silicon dioxide (SiO2) nanolayered thin films are grown using ultrahigh vacuum radio frequency (rf) magnetron sputtering on sapphire substrates at room temperature. Films are layered with 15-angstrom Ga2O3 oxide and 75-angstrom SiO2 for a total of 10 layers. Referring to atomic core levels, atomic contribution to valence band density of states is experimentally nominated. This analytical technique has particular applicability to the evaluation of the density of states with atomic contributions.

Kondo scaling of the pseudogap inCeOs4Sb12 andCeFe4P12

CeOs4Sb12 and CeFe4P12 are classified as Kondo semiconductors, which show coupled changesin electrical transport, thermodynamic and magnetic properties with alow-temperature semiconductor-like electrical resistivity. We have carried outcore level and valence band photoemission spectroscopy on single crystalCeOs4Sb12 andCeFe4P12 tostudy their electronic structure and the evolution of states at the Fermi level as a function of temperature(∼10–300 K). The Ce 3d core level spectra show the presence off0,f1 andf2 final states with very different relative intensities in the two compounds.Single-impurity Anderson model calculations provide f electron counts ofnf = 0.97 and 0.86 per Ce atom, suggestive of a low- and high-TK (= single ion Kondotemperature) for CeOs4Sb12 and CeFe4P12, respectively. The high-resolution temperature-dependent near-Fermi level spectra show pseudogaps ofenergy ∼ 50 meV and ∼ 110 meV in the valence band density of states (DOS) ofCeOs4Sb12 andCeFe4P12, respectively. The temperature dependence of the DOS at the Fermi levelfollows the change in effective magnetic moment estimated from magneticsusceptibility for both materials, confirming the Kondo nature of the pseudogap inCeOs4Sb12 andCeFe4P12. Acompilation of measured pseudogaps using photoemission and optical spectroscopy identifies the chargegaps ΔC for Ce-based Kondo semiconductors and provides a direct relation withTK givenby ΔC ∼ 2kBTK. In conjunction with the known behaviour of the spin gapsΔS ∼ kBTK, the results establish the coupled energy scaling of the spin and charge gaps in Kondosemiconductors.

Band structure and optical gain of tensile-strained germanium based on a 30 band k⋅p formalism

We have investigated the band structure of tensile-strained germanium using a 30 band k⋅p formalism. This multiband formalism allows to simultaneously describe the valence and conduction bands, including the L, Δ, and Γ valleys. We calculate the energy band variation as a function of strain and obtain that the crossover from indirect to direct band gap occurs for a tensile in-plane strain of 1.9%. The effective masses of density of states are deduced from the calculated conduction and valence band density of states. Significant deviations are observed as compared to the effective masses of density of states values of unstrained bulk germanium. We finally calculate the optical gain that can be achieved with tensile-strained bulk germanium. An optical gain larger than 3000 cm−1 is predicted for a carrier density of 1×1018 cm−3 and a 3% in-plane biaxial strain. This optical gain is larger than the one of GaAs calculated with the same formalism and is much larger than the experimental free-carrier absorption losses. This gain should be sufficient to achieve lasing in these structures.