Valence Band States Research Articles

Gradual Carrier Filling Effect in "Green" InGaN/GaN Quantum Dots: Femtosecond Carrier Kinetics with Sequential Two-Photon Absorption.

Quantum dots (QDs) allow for a significant amount of strain relaxation, which is helpful in GaN systems where a large lattice mismatch needs to be accommodated. InGaN QDs with a large indium composition are intensively investigated for light emitters requiring longer wavelengths. These are especially important for developing high-efficiency white light sources. Understanding the carrier dynamics in this large lattice-mismatched system is essential to improving the radiative efficiency while circumventing high defect density. This work investigates femtosecond carrier and photon dynamics in self-organized In0.27Ga0.73N/GaN QDs grown by molecular beam epitaxy using transient differential absorption spectroscopy, which measures the differential absorption coefficient (Δα) with and without an optical pump. Due to 3D quantum confinement and the small effective mass of InGaN, the low density of states in the conduction band is easily filled with electrons. In contrast, the GaN barrier region is replete with a high density of electrons due to a large effective mass. This contrast in carrier density creates a unique phenomenon in the dynamics, showing a change in the differential absorption coefficient (Δα) sign from negative to positive with time. The ultrafast microscopic processes indicate that right after the optical pump and first photon absorption, the valence (conduction) band states are depleted (replete) of electrons. This ground-state bleaching process makes Δα negative, and the probe beam is not absorbed. The electrons are then gradually transferred from the GaN barrier into InGaN QDs, which absorb the second photon from the probe beam (excited-state absorption), making Δα positive. The presence of excited-state carriers with a long lifetime is indicative of the enhanced availability of carriers for radiative recombination. This effect also promotes stimulated emission and amplified spontaneous emission, which can be used to develop lasers and superluminescent LEDs, respectively. Measurements with multiple pump powers and temperatures further confirm that the efficacy of InGaN QDs is enhanced by this effective mass contrast and 3D reservoir of carriers from the GaN barrier. This effect can be used to improve the internal quantum efficiency of GaN-based light emitters.

The bandgap energy of the dilute bismuth GaBi x Sb1−x alloy depending on temperature

Abstract The bandgap energy of the dilute bismuth GaBi x Sb1−x alloy vs. temperature is investigated in this study. Its reduced temperature-sensitiveness is because of the localized character of the valence band states (VBS). In order to describe the reduced temperature-sensitiveness of the bandgap energy, a new term including localized energy is added to Varshni's equation. It is found that the localized energy exhibits an increasing trend as the bismuth fraction increases, which indicates that the localized character of the VBS becomes strong with the increasing bismuth fraction. It is also found that the influence of the bismuth fraction on the temperature dependence of the bandgap energy of GaBi x Sb1−x is smaller than that of GaBi x As1−x . In addition, the element indium is undoubtedly a good candidate to lessen the bismuth fraction to realize that the spin-orbit-splitting (SOP) energy surpasses the bandgap energy in GaBi x Sb1−x .

Phonon-assisted carrier tunneling with hyperfine-induced spin flip in coupled quantum dot systems

We calculate the rates of phonon-assisted hyperfine spin flips during electron and hole tunneling between quantum dots in a self-assembled quantum dot molecule. We show that the hyperfine process dominates over the spin-orbit-induced spin relaxation in magnetic fields up to a few Tesla for electrons, while for holes this cross-over takes place at field magnitudes of a fraction of Tesla, upon the assumption of a large $d$-shell admixture to the valence band state, resulting in a strong transverse hyperfine coupling. The interplay of the two spin-flip mechanisms leads to a minimum of the spin-flip probability, which is in principle experimentally measurable and can be used as a test for the presence of substantial transverse hyperfine couplings in the valence band.

Resonant photoemission spectroscopy of the ferromagnetic Kondo system CeAgSb2

We study the electronic structure of single crystal CeAgSb2, a low T c ferromagnetic Kondo system, using synchrotron radiation photoemission spectroscopy (PES) and x-ray absorption spectroscopy (XAS). We have carried out Ce 5s, Ce 5p, Sb 4d core level PES as well as Ce 4d–4f resonant PES of the valence band states of CeAgSb2. We also report the resonant PES behavior of Ce 5s and 5p core-levels and the simultaneous intensity suppression of Sb 4d core-level and Ag 4d valence band states across the Ce 4d–4f threshold. The Ce 4d–4f resonant valence band spectra show typical f 0 and f 1 features associated with Kondo behavior. The constant initial state spectra of the f 0 and f 1 features show maxima at different incident photon energies, arising from different relative contributions of surface and bulk character Ce 4f partial density of states. XAS across the Ce 3d–4f M-edge also shows the corresponding final state f 1 and f 2 features. The weak temperature dependence of the XAS spectra between T = 25 K and 300 K could be simulated using a full multiplet calculation combined with a simplified single-impurity Anderson model approach. The calculations confirm the Kondo screening and allows quantification of the bulk Ce 4f electron count in CeAgSb2. The Ce 5s states show an exchange splitting which reflects the local magnetic moment of Ce 4f states. The overall results characterize the bulk and surface sensitive Ce 4f states, and indicate the role of Kondo effect in forming the moderately enhanced heavy-fermion carriers in CeAgSb2.

Adsorption of water on the pristine and defective semiconducting 2D CrPX 3 monolayers (X: S, Se)

The effect of vacancy and water adsorption on the electronic structure of semiconducting 2D trichalcogenide material CrPX 3 (X: S, Se) is studied using state-of-the-art density functional theory (DFT) approach. It is found that chalcogen vacancies play a minor role on the electronic structure of CrPX 3 in the vicinity of the Fermi level leading to the slightly reduced band gap for these materials, however, inducing strongly localised defect states which are placed in the energy gap formed by the valence band states. Our DFT calculations show that the interaction of water molecules with CrPX 3, pristine and defective, can be described as physisorption and the adsorption energy for H2O is insensitive to the difference between pristine and chalcogen-defective surface of trichalcogenide material. These results are the first steps for the theoretical description of the ambient molecules interaction with 2D semiconducting CrPX 3 material, that is important for its future experimental studies and possible applications.

Carrier-stabilized hexagonal Ge

Germanium is crystalized in the cubic diamond structure, but its high energy hexagonal Ge (lonsdaleite) phase has many novel properties such as direct band gap. Using first-principles calculations, we show that the hexagonal lonsdaleite phase of Ge can be stabilized by introducing carriers, either electrons or holes, because Ge in the cubic and hexagonal phases form a type-I band alignment with both electrons and holes localized at the hexagonal site. This result is distinct from that in zinc-blende compounds such as ZnSe, because due to the lack of inversion symmetry, the crystal-field splitting, zone folding, and symmetry-controlled level repulsion between valence and conduction band states lead to a type-II band alignment between its cubic and hexagonal phases, so the hexagonal (wurtzite) phase of ZnSe can only be stabilized, in principle, by holes. This distinction reveals that, due to the symmetry differences, the well-investigated understanding of band structure differences between zinc-blende and wurtzite phases should not be simply extended to that of diamond and lonsdaleite phases despite the remarkable structure resemblance between the two cases.

Attosecond intra-valence band dynamics and resonant-photoemission delays in W(110)

Time-resolved photoelectron spectroscopy with attosecond precision provides new insights into the photoelectric effect and gives information about the timing of photoemission from different electronic states within the electronic band structure of solids. Electron transport, scattering phenomena and electron-electron correlation effects can be observed on attosecond time scales by timing photoemission from valence band states against that from core states. However, accessing intraband effects was so far particularly challenging due to the simultaneous requirements on energy, momentum and time resolution. Here we report on an experiment utilizing intracavity generated attosecond pulse trains to meet these demands at high flux and high photon energies to measure intraband delays between sp- and d-band states in the valence band photoemission from tungsten and investigate final-state effects in resonant photoemission.

Hot-carrier dynamics in InAs/AlAsSb multiple-quantum wells

A type-II InAs/AlAs_{0.16}Sb_{0.84} multiple-quantum well sample is investigated for the photoexcited carrier dynamics as a function of excitation photon energy and lattice temperature. Time-resolved measurements are performed using a near-infrared pump pulse, with photon energies near to and above the band gap, probed with a terahertz probe pulse. The transient terahertz absorption is characterized by a multi-rise, multi-decay function that captures long-lived decay times and a metastable state for an excess-photon energy of >100 meV. For sufficient excess-photon energy, excitation of the metastable state is followed by a transition to the long-lived states. Excitation dependence of the long-lived states map onto a nearly-direct band gap (E{_g}) density of states with an Urbach tail below E{_g}. As temperature increases, the long-lived decay times increase <E{_g}, due to the increased phonon interaction of the unintentional defect states, and by phonon stabilization of the hot carriers >E{_g}. Additionally, Auger (and/or trap-assisted Auger) scattering above the onset of the plateau may also contribute to longer hot-carrier lifetimes. Meanwhile, the initial decay component shows strong dependence on excitation energy and temperature, reflecting the complicated initial transfer of energy between valence-band and defect states, indicating methods to further prolong hot carriers for technological applications.

Isolated Pd atoms in a silver matrix: Spectroscopic and chemical properties.

Over the past decade, single-atom alloys (SAAs) have been a lively topic of research due to their potential for achieving novel catalytic properties and circumventing some known limitations of heterogeneous catalysts, such as scaling relationships. In researching SAAs, it is important to recognize experimental evidence of peculiarities in their electronic structure. When an isolated atom is embedded in a matrix of foreign atoms, it exhibits spectroscopic signatures that reflect its surrounding chemical environment. In the present work, using photoemission spectroscopy and computational chemistry, we discuss the experimental evidence from Ag0.98Pd0.02 SAAs that show free-atom-like characteristics in their electronic structure. In particular, the broad Pd4d valence band states of the bulk Pd metal become a narrow band in the alloy. The measured photoemission spectra were compared with the calculated photoemission signal of a free Pd atom in the gas phase with very good agreement, suggesting that the Pd4d states in the alloy exhibit very weak hybridization with their surroundings and are therefore electronically isolated. Since AgPd alloys are known for their superior performance in the industrially relevant semi-hydrogenation of acetylene, we considered whether it is worthwhile to drive the dilution of Pd in the inert Ag host to the single-atom level. We conclude that although site-isolation provides beneficial electronic structure changes to the Pd centers due to the difficulty in activating H2 on Ag, utilizing such SAAs in acetylene semi-hydrogenation would require either a higher Pd concentration to bring isolated sites sufficiently close together or an H2-activating support.

Room temperature CNOT operation in a diluted magnetic semiconductor quantum dot

Population evolution in a magnetic impurity doped semiconductor quantum dot has been studied by applying a sequence of pulses of chosen pulse area. By optical excitation mechanism, the population in (J_z=+3/2), heavy hole state of valence band is carried over to (J_z=-3/2), valance band state, via the (J=+1/2) conduction band states. The injected microwaves entangle conduction band states. This arrangement is successfully employed to ascertain quantum CNOT operation, and the calculation predicts maximum fidelity of 80\% for the CNOT operation.

Orbital Complexity in Intrinsic Magnetic Topological Insulators MnBi_{4}Te_{7} and MnBi_{6}Te_{10}.

Using angle-resolved photoelectron spectroscopy (ARPES), we investigate the surface electronic structure of the magnetic van der Waals compounds MnBi_{4}Te_{7} and MnBi_{6}Te_{10}, the n=1 and 2 members of a modular (Bi_{2}Te_{3})_{n}(MnBi_{2}Te_{4}) series, which have attracted recent interest as intrinsic magnetic topological insulators. Combining circular dichroic, spin-resolved and photon-energy-dependent ARPES measurements with calculations based on density functional theory, we unveil complex momentum-dependent orbital and spin textures in the surface electronic structure and disentangle topological from trivial surface bands. We find that the Dirac-cone dispersion of the topologial surface state is strongly perturbed by hybridization with valence-band states for Bi_{2}Te_{3}-terminated surfaces but remains preserved for MnBi_{2}Te_{4}-terminated surfaces. Our results firmly establish the topologically nontrivial nature of these magnetic van der Waals materials and indicate that the possibility of realizing a quantized anomalous Hall conductivity depends on surface termination.

First-principles investigation of the electronic and thermoelectric properties of SiGe doped with Sn and one percent B

Silicon-Germanium (SiGe) has recently attracted much interest as a room temperature thermoelectric (TE) material for converting heat into electrical output power. With the advantage of silicon being non-toxic, cost-effective, and abundant on earth, a silicon base TE material has a promising future. A first-principle calculation based on the fully self-consistent Korringa-Kohn-Rostoker method with the coherent potential approximation (KKR-CPA) to treat several forms of chemical disorders of SiGe by Sn-doping was carried out. In SiGe1-xSnx, as the Sn content increases the Fermi level shifts to the conduction band edge. Similarly, in Si1-xSnxGe, a high Sn content (x = 0.4 to 0.9) results in the Fermi level shifting to the conduction band edge. On the contrary, a low amount of Sn content (x = 0.1 to 0.3) causes the Fermi level to fluctuate between the conduction band and the valence band states. With the addition of 1% Boron impurity to the alloys Si1-xSnxGe and SiGe1-xSnx, the number of carriers (electron and hole) states was enhanced by 0.05 states/eV. This makes the alloys Si0.3Sn0.69B0.01Ge and SiGe0.4Sn0.59B0.01 promising for application as n-type electrodes in a thermoelectric generator (TEG).

Structural and optoelectronic properties of cubic Zn1-x-yBexMgySe quaternary alloys nearly lattice matched to GaAs substrate: A density functional investigation

Density functional calculations of structural and optoelectronic properties of cubic Zn1-x-yBexMgySe quaternary alloys are carried out considering their nearly lattice matching to GaAs substrate. Calculations ensure that each quaternary alloy is a direct band gap (Γ−Γ) semiconductor. The mBJ-GGA based computed minimum band gap of each alloy is larger that with EV-GGA scheme. Enhancement in beryllium or magnesium composition nonlinearly reduces the lattice constant, but enhances the bulk modulus and minimum band gap of quaternary alloys. Lower effective mass of electrons compared to holes confirms dominant role of electrons in carrier transportation in each specimen. Electronic transitions from occupied Se-4p state of valence band to unoccupied Zn-5s, Mg-3p, Mg-4s, Be-2p and Be-3s states of conduction band collectively contribute intense peaks in ε2(ω) spectra of each quaternary alloy. Quaternary semiconductor with higher band gap possesses lower value of zero-frequency limits in ε1(ω), n(ω) and R(ω) spectra, but requires higher critical point energies in ε2(ω), k(ω), σ(ω) and α(ω) spectra and vice versa. Computed oscillator strength of each quaternary alloy confirms the presence of sufficient number (>200) of electrons in the unoccupied states of conduction band above 27.0 eV of incident photon energy during any optical excitation.

Thickness- and strain-tunable electronic structures of two-dimensional Bi2O2Se

Two-dimensional (2D) Bi2O2Se that exhibits high carrier mobility, moderate band gap, and good stability is a promising candidate of 2D semiconductors for next-generation computing technologies. However, the electronic properties of ultrathin Bi2O2Se films are poorly understood, especially the thickness and strain tunability of their electronic structures. In this work, we performed a systematic investigation of electronic structures of 2D Bi2O2Se using HSE06 method with high accuracy. Our results show that Bi2O2Se multi-layers display a weak quantum size effect. The origin of this unusual phenomenon is elucidated by the crystal orbital Hamilton population (COHP) analysis. Imposed with biaxial strain, band gaps of monolayer and bilayer can be modulated in a large energy range, e.g., direct band gap of monolayer from 3.08 eV to 1.17 eV which strides over the whole visible light region. Besides, the band gap of bilayer closes up at large tensile strain of ~8%, indicating a semiconductor-metal transition. Further analysis of valence band and conduction band edges shows that high tunability of band gap originates from the sensitivity of band edge with strain, especially conduction band minimum. With the strain from compressive to tensile, Bi-Se bond length gradually increase with the stretch of lattice, which reduces the Bi-Se bond strength and thus lowers the energies of valence band and conduction band edge states due to their antibonding character. Highly tunable electronic properties via changing thickness or adding strain impart Bi2O2Se more possibilities for its application in electronic and optoelectronic devices.

Microstructure and luminescence properties of the high pressure high temperature sintered AlN–TiN ceramics

Composite ceramics of titanium nitride grains incorporated in an aluminium nitride matrix have been synthesized by high pressure high temperature treatment of a mechanical mixture of AlN–TiN (3 mol % TiN) powders. The microstructure of the samples analysed by means of electron beam microanalysis and Raman spectroscopy shows that the formation of cubic AlN in the composite begins near titanium nitride grains. The areas of mixed chemical composition, which can be assigned to the formation of the solid solutions Al1−xTixN, have been observed at the phase interfaces. The luminescence properties of AlN–TiN ceramics have been considered focusing on the choice of high pressure and high temperature treatment conditions. Three main components at 2.0 eV, 2.4 eV and 3.1 eV are revealed in the cathodoluminescence spectra analysed quantitatively. The observed emission originates from the radiative transitions with participation of valence band states, oxygen-vacancy centres (VAl–ON), nitrogen vacancies VN, and shallow donors which form a complex system of energy levels in the bandgap of the wurtzite-type AlN.

Insight into the role of copper in the promoted photocatalytic removal of NO using Zn2-xCuxCr-CO3 layered double hydroxide

In this work the ability of Zn2-xCuxCr-CO3 layered double hydroxides (LDHs) as highly efficient DeNOx photocatalysts was studied. LDHs with x = 0, 0.2 and 0.4 were prepared using a coprecipitation method. The samples were characterized by different techniques such as XRD, XPS, FT-IR, ICP-MS, TG, SBET, SEM and Diffuse reflectance (DR). The increased amount of copper ions in the LDH layers gave rise to slight changes in the structure and morphology and an important variation of the optical properties of the LDHs. The prepared ZnCuCr–CO3 photocatalysts exhibited favourable conversion efficiency (51%) and an extraordinary selectivity (97%) for the photochemical NO abatement. The photochemical mechanism was elucidated from DOS, EPR, Femtosecond transient absorption and in-situ DRIFTS studies. The results suggested that the presence of Cu2+ ions in the LDH framework introduced new states in the valence band states, thus favouring the production and mobility of e−/h+ charge carriers and a greater production of ⋅O2− and ⋅OH.

Fabrication and Characterization of Ni-, Co-, and Rb-Incorporated CH3NH3PbI3 Perovskite Solar Cells

Ni- and Co-incorporated CH3NH3PbI3 perovskite solar cells were fabricated and characterized to optimize the photovoltaic and optical properties related to surface morphology, crystal growth and orientation, and electronic structures. Partially replacing Pb with Ni or Co in the perovskite crystals improved the photovoltaic performance and carrier mobility based on the effective mass in the band structure. In particular, the addition of both Ni and Rb compounds to perovskite improved the long-term stability of the photovoltaic cells, which depended on surface modification and coverage, crystal growth, and the high (100) orientation in the perovskite layer. The short-circuit current density of the cells was increased by promoting the generation and mobility of photoinduced carriers, which were inversely proportional to the effective mass ratio. Electron correlation was associated with the promotion of charge transfer owing to the hybridization between the 3d orbitals of Ni and the 5p orbitals of the I atoms near the valence band state.

The carbon state in dilute germanium carbides

Conduction and valence band states for the highly mismatched alloy (HMA) Ge:C are projected onto Ge crystal states, Ge vacancy states, and Ge/C atomic orbitals, revealing that substitutional carbon not only creates a direct bandgap but also the new conduction band is optically active. Overlap integrals of the new Ge:C conduction band state with states from unperturbed Ge show that the new band cannot be attributed to any single Ge band but is a mixture of multiple Ge states. The Ge Γ conduction band valley state plays the largest single role, but L and X valley states collectively contribute a larger share than Γ due to the multiplicity of degenerate states. C sites structurally resemble uncharged vacancies in the Ge lattice, similar to Hjalmarson's model for other HMAs. C also perturbs the entire Ge band structure even at the deepest crystal core energy levels, particularly if staggered supercells are used to mimic a disordered alloy. Projection onto atomic sites shows a relatively weak localization compared with other HMAs, but it does show a strong anisotropy in probability distribution. L-valley conduction band states in Ge contribute to the conduction band minimum in Ge:C, but the optical transition strength in Ge:C remains within a factor of 2 of the direct gap transition in Ge.

Ultrafast intraband Auger process in self-doped colloidal quantum dots

Summary Investigating the separate dynamics of electrons and holes has been challenging, although it is critical for the fundamental understanding of semiconducting nanomaterials. n-Type self-doped colloidal quantum dots (CQDs) with excess electrons occupying the low-lying state in the conduction band (CB) have attracted a great deal of attention because of not only their potential applications to infrared optoelectronics but also their intrinsic system that offers a platform for investigating electron dynamics without elusive contributions from holes in the valence band. Here, we show an unprecedented ultrafast intraband Auger process, electron relaxation between spin-orbit coupling states, and exciton-to-ligand vibrational energy transfer process that all occur exclusively in the CB of the self-doped β-HgS CQDs. The electron dynamics obtained by femtosecond mid-infrared spectroscopy will pave the way for further understanding of the blinking phenomenon, disproportionate charging in light-emitting diodes, and hot electron dynamics in higher quantum states coupled to surface states of CQDs.

Experimental and theoretical study of the effect of bombardment with Ar+ ions on the spectrum of valence electrons of a Si (111) single crystal

The paper studies the effect of disordering of the surface layers on the electronic and optical properties of single-crystal silicon.An analysis of the photoelectron spectra shows that with complete amorphization of the surface density, the condition of Si valence electrons of changes significantly. In particular, the positions of the main maximum of the electrons of the valence band of Si (111) shift by ~ 0.4 eV towards higher binding energies and the band gap Eg increases by 0.1-0.15 eV. The energy of a valence electron in amorphized silicon, which at low impurity concentrations, i.e. at low bombardment doses (D &lt;1015 cm–2), the potential Mkk and, therefore, the shift of the maximum of the density of states vary linearly with respect to concentration. At high impurity concentrations (at doses D&gt; 1015 cm–2), corresponding to the transition to amorphous silicon, the concentration dependence of Mkk is very weak. Therefore, upon amorphization, the peak A of the density of states of the silicon valence band under consideration is shifted to the region of lower electron binding energies. The theoretical substantiation of the obtained experimental results is given.