Conduction Band States Research Articles

Hard x-ray photoemission study of Yb1−xZrxB12: the effects of electron doping on the Kondo insulator YbB12

We have carried out hard x-ray photoemission spectroscopy (HAXPES) of Yb1−xZrxB12 () to study the effects of electron doping on the Kondo insulator YbB12. The Yb valences of Yb1−xZrxB12 at 300 K estimated from the Yb 3d HAXPES spectra decreased after substituting Yb with Zr from 2.93 for YbB12 to 2.83 for Yb0.125Zr0.875B12. A temperature dependent valence decrease was found upon cooling for all doping concentrations. We found peak shifts of the B 1s and Zr 3d5/2, and Yb3+ 4f spectra toward the deeper binding-energy with increasing Zr concentration, which indicates a shift of the Fermi level to the higher energy and that of the Yb 4f hole level close to the Fermi level, respectively, due to electron doping. These results qualitatively show the enhanced hybridization between the Yb 4f and conduction-band states with Zr substitution, consistent with magnetic susceptibility measurements.

Photo-induced changes of the surface band bending in GaN: Influence of growth technique, doping and polarity

In this work, we use conductance and contact potential difference photo-transient data to study the influence of the growth technique, doping, and crystal polarity on the kinetics of photo-generated charges in GaN. We found that the processes, and corresponding time scales, involved in the decay of charge carriers generated at and close to the GaN surface via photo-excitation are notably independent of the growth technique, doping (n- and p-types), and also crystal polarity. Hence, the transfer of photo-generated charges from band states back to surface states proceeds always by hopping via shallow defect states in the space-charge region (SCR) close to the surface. Concerning the charge carrier photo-generation kinetics, we observe considerable differences between samples grown with different techniques. While for GaN grown by metal-organic chemical vapor deposition, the accumulation of photo-conduction electrons results mainly from a combined trapping-hopping process (slow), where photo-generated electrons hop via shallow defect states to the conduction band (CB), in hydride vapor phase epitaxy and molecular beam epitaxy materials, a faster direct process involving electron transfer via CB states is also present. The time scales of both processes are quite insensitive to the doping level and crystal polarity. However, these processes become irrelevant for very high doping levels (both n- and p-types), where the width of the SCR is much smaller than the photon penetration depth, and therefore, most charge carriers are generated outside the SCR.

First-principle simulations of electronic structure in semicrystalline polyethylene.

In order to increase our fundamental knowledge about high-voltage cable insulation materials, realistic polyethylene (PE) structures, generated with a novel molecular modeling strategy, have been analyzed using first principle electronic structure simulations. The PE structures were constructed by first generating atomistic PE configurations with an off-lattice Monte Carlo method and then equilibrating the structures at the desired temperature and pressure using molecular dynamics simulations. Semicrystalline, fully crystalline and fully amorphous PE, in some cases including crosslinks and short-chain branches, were analyzed. The modeled PE had a structure in agreement with established experimental data. Linear-scaling density functional theory (LS-DFT) was used to examine the electronic structure (e.g., spatial distribution of molecular orbitals, bandgaps and mobility edges) on all the materials, whereas conventional DFT was used to validate the LS-DFT results on small systems. When hybrid functionals were used, the simulated bandgaps were close to the experimental values. The localization of valence and conduction band states was demonstrated. The localized states in the conduction band were primarily found in the free volume (result of gauche conformations) present in the amorphous regions. For branched and crosslinked structures, the localized electronic states closest to the valence band edge were positioned at branches and crosslinks, respectively. At 0 K, the activation energy for transport was lower for holes than for electrons. However, at room temperature, the effective activation energy was very low (∼0.1 eV) for both holes and electrons, which indicates that the mobility will be relatively high even below the mobility edges and suggests that charge carriers can be hot carriers above the mobility edges in the presence of a high electrical field.

Aharonov-Bohm oscillations in phosphorene quantum rings

The Aharonov-Bohm (AB) effect in square phosphorene quantum rings, with armchair and zigzag edges, is investigated using the tight-binding method. The energy spectra and wave functions of such rings, obtained as a function of the magnetic flux $\mathrm{\ensuremath{\Phi}}$ threading the ring, are strongly influenced by the ring width $W$, an in-plane electric field ${E}_{p}$, and a side-gating potential ${V}_{g}$. Compared to a square dot, the ring shows an enhanced confinement due to its inner edges and an interedge coupling along the zigzag direction, both of which strongly affect the energy spectrum and the wave functions. The energy spectrum that is gapped consists of a regular part, of conduction (valence) band states, that shows the usual AB oscillations in the higher- (lower-) energy region, and of edge states, in the gap, that exhibit no AB oscillations. As the width $W$ decreases, the AB oscillations become more distinct and regular and their period is close to ${\mathrm{\ensuremath{\Phi}}}_{0}/2$, where the flux quantum ${\mathrm{\ensuremath{\Phi}}}_{0}=h/e$ is the period of an ideal circular ring ($W\ensuremath{\rightarrow}0$). Both the electric field ${E}_{p}$ and the side-gating potential ${V}_{g}$ reduce the amplitude of the AB oscillations. The amplitude can be effectively tuned by ${E}_{p}$ or ${V}_{g}$ and exhibits an anisotropic behavior for different field directions or side-gating configurations.

Enhanced Luminescence of La3+-Doped Gadolinium Oxysulfide with Tunable Crystalline Size

The concentration of La3+ in (La x Gd1−x )2O2S:Tb phosphors prepared by the sulfide fusion method from coprecipitated oxalate precursors has been tuned, and the prominent effect on the crystalline size, particle size, and luminescence properties investigated. First-principles calculations were used to characterize the charge deformation, energy gap, and crystal field. According to density functional theory, the electron density of states in conduction bands increased with increase in the La3+ concentration. The increased electronic density strengthened the repulsion and thus decreased the diffusion so as to decrease the crystalline size from 106.2 nm to 37.3 nm. The particle size of (La x Gd1−x )2O2S:Tb increased from 0.21 μm to 1.25 μm as the La3+ concentration was increased from 15 mol.% to 60 mol.%. The excitation spectrum shifted towards shorter wavelength, enhancing the luminescence intensity of (La x Gd1−x )2O2S:Tb when excited at 254 nm. Furthermore, shorter lifetime was obtained due to lower symmetry as more Gd3+ was substituted by La3+ ions.

Doping-induced spin-orbit splitting in Bi-doped ZnO nanowires

Our predictions, based on density-functional calculations, reveal that surface doping of ZnO nanowires with Bi leads to a linear-in-$k$ splitting of the conduction-band states, through spin-orbit interaction, due to the lowering of the symmetry in the presence of the dopant. This finding implies that spin polarization of the conduction electrons in Bi-doped ZnO nanowires could be controlled with applied electric (as opposed to magnetic) fields, making them candidate materials for spin-orbitronic applications. Our findings also show that the degree of spin splitting could be tuned by adjusting the dopant concentration. Defect calculations and ab initio molecular dynamics simulations indicate that stable doping configurations exhibiting the foregoing linear-in-$k$ splitting could be realized under reasonable thermodynamic conditions.

A simple technology for CuO/TiO2 3D interface fabrication using nanocrystalline oxide powders

A CuO/TiO2 interface with potential use for solar light conversion is studied. Thin films are synthesized using a mixture of commercially available nanocrystalline TiO2 and CuO powders. Samples with mass concentrations from 5 to 10% of CuO in TiO2 were prepared from a colloidal suspension using a ‘doctor blade’ technique. Heat treatment (500 °C, 1 h) was used to generate crystals necking and improve adhesion to substrates. X-ray diffraction analysis and scanning electron microscopy indicate that the CuO nanocrystals are smaller than those of TiO2. Since TiO2 does not absorb light in the spectral range where the CuO absorption edge is located, it was possible to obtain an effective CuO absorption coefficient spectra and then extract the bandgap energy. The results confirm the potential use of CuO as a TiO2 inorganic sensitizer for solar light. SEM images show a mesoporous structure for all samples that would facilitate penetration of a hole conductor and guarantee a large three-dimensional interface. Photocurrent direction with no bias confirms electron transfer from the CuO to the TiO2 when CuO/TiO2 films are used as photoelectrodes. Therefore, excited electrons in the CuO conduction band occupy levels with energy greater than the empty states in the TiO2 conduction band. Possible technological improvements to increase electron collection are discussed.

Electron Raman Scattering and Raman Gain in Pyramidal Semiconductor Quantum Dots.

The Raman scattering related with conduction band states in semiconductor pyramidal quantum dots is theoretically investigated. The electron Raman differential cross section and Raman gain coefficient are calculated making use of analytically determined quantum states. The energy spectrum is obtained within the effective mass approximation. The features of the Raman differential cross section are discussed in terms of their dependence on the changes of the quantum dot geometry. The variation of the Raman gain coefficient as a function of the quantum dot size and shape is also analyzed.

Low Density of Conduction and Valence Band States Contribute to the High Open-Circuit Voltage in Perovskite Solar Cells

Hybrid perovskites are widely used for high-performance solar cells. Large diffusion lengths and long charge carrier lifetimes are considered two main factors for their high performance. Here, we argue that not only large diffusion lengths and long carrier lifetimes but also the low densities of the conduction and valence band states (Nc, Nv) contribute to high-performance perovskite solar cells. We estimated Nc and Nv of silicon, CdTe, and typical perovskites with two different methods. It was found that Nc and Nv of perovskites and CdTe are much lower than that of silicon. Using numerical models, we found that the solar cell of a material with same characteristics but lower Nc and Nv can realize a higher open-circuit voltage (Voc) and higher power conversion efficiency (PCE). We put forward and proved that the low Nc and Nv in hybrid perovskite is one of the factors for its high performance. This provides a new guideline for finding and developing new photovoltaic candidate materials.

Spin polarized conduction and valence band states in GdN

AbstractOptical transmission spectra are collected in the ferromagnetic state of GdN at various temperatures. The temperature dependent extinction coefficient extracted from the transmission measurements show that the conduction and valence band states are spin polarized. The results confirm the LSDA+U estimates of the energy gap associated with the majority/minority-spins and also the magnitude of the spin splitting.

Electronic Structure and Optical Gain of InNBiAs/InP Pyramidal Quantum Dots

Dilute bismide alloys and dilute nitride alloys have garnered increasing research interest over the past few years, as it promises increased engineering flexibility in the design of advanced compound semiconductor heterostructure devices. Key device parameters such as lattice constant, bandgap and band offsets can be controlled with greater precision, and this leads to better performance for a wide range of electronic and optoelectronic devices. For dilute nitride alloys, band anticrossing effects are produced due to the coupling of their resonant states with the conduction band states. This lowers the conduction band edge energy. Likewise, the coupling of the resonant states of dilute bismide with the valence band states results in the valence band anticrossing and the valence band edge energy rises. Additionally, the effective bandgap may fall below the spin-orbital-splitting energy, thus inhibiting Auger recombination. This makes them excellent candidates for optoelectronic device applications. In this work, the electronic bandstructure and optical gain of InNBiAs/InP pyramidal quantum dots are investigated using the 16-band k·p model with constant strain. The effective bandgap falls as we increase the composition of nitrogen and bismuth. With an appropriate choice of composition of doped atoms, we can tune the emission wavelength in the range of 2–5µm suitable for mid-infrared device applications. Additionally, we observe the size effect by tuning the size of quantum dot.

Dissecting charge relaxation pathways in CdSe/CdS nanocrystals using femtosecond two-dimensional electronic spectroscopy.

Exciton relaxation dynamics of CdSe and quasi-type-II CdSe/CdS core/shell nanocrystals were examined using femtosecond two-dimensional electronic spectroscopy (2DES). The use of 2DES allowed for determination of structure-specific and state-resolved carrier dynamics for CdSe nanocrystals formed with five, or fewer, CdS passivation monolayers (ML). For CdSe and CdSe/CdS nanocrystals formed with one through three MLs of CdS, excitation using broad bandwidth femtosecond visible laser pulses generated electron-hole pairs among the |X1〉 = 2.14 eV and |X2〉 = 2.27 eV exciton states. For both excitations, the electron is promoted to the lowest energy excited (1Se) conduction-band state and the hole is in the 1S3/2 (X1) or 2S3/2 (X2) valence-band state. Therefore, the relaxation dynamics of the hot hole were isolated by monitoring the-time-dependent amplitude of 2DES cross peaks. The time constant for hot hole relaxation within the CdSe valence band was 150 ± 45 fs. Upon passivation by CdS, this hole relaxation time constant increased to 170 ± 30 fs (CdSe/CdS-3ML). This small increase was attributed to the formation of a graded, or alloyed, interfacial region that precedes the growth of a uniform CdS capping layer. The small increase in hole relaxation time reflects the larger nanocrystal volume of the CdSe/CdS system with respect to the CdSe nanocrystal core. In contrast, the dynamics of larger core/shell nanocrystals (≥4ML CdS) exhibited a picosecond buildup in 2DES cross-peak amplitude. This time-dependent response was attributed to interfacial hole transfer from CdS to CdSe valence-band states. Importantly, the 2DES data distinguish CdSe exciton relaxation from interfacial carrier transfer dynamics. In combination, isolation of structurally well-defined nanocrystals and state-resolved 2DES can be used to examine directly the influence of nanoscale structural modifications on electronic carrier dynamics, which are critical for developing nanocluster-based photonic devices.

D-shell of iron atom of the amorphous FeCr15B15alloy effective charge change during the crystallization

An amorphous metal alloy of the FeCrB system was studied during the crystallization by thermal annealing. Such an alloy is a perspective candidate for the role of an intermediate layer in multilayer covering for cutting tools. By the using of the thermoelectric voltage measurement, positron annihilation spectroscopy, and X-ray photoelectron spectroscopy, the conjoint research was performed for the study of the conduction and d-electron band state in the amorphous metallic alloy FeCr15 B15 , which intersects each other by the energy. The results of all the studies agree with each other and indicate the change in the effective charge of the d-shell by 1 electron during crystallization.

Donor Impurity-Related Optical Absorption in GaAs Elliptic-Shaped Quantum Dots

The conduction band and electron-donor impurity states in elliptic-shaped GaAs quantum dots under the effect of an externally applied electric field are calculated within the effective mass and adiabatic approximations using two different numerical approaches: a spectral scheme and the finite element method. The resulting energies and wave functions become the basic information needed to evaluate the interstate optical absorption in the system, which is reported as a function of the geometry, the electric field strength, and the temperature.

High absorption coefficients of the CuSb(Se,Te)2 and CuBi(S,Se)2 alloys enable high-efficient 100 nm thin-film photovoltaics

We demonstrate that the band-gap energies Eg of CuSb(Se,Te)2 and CuBi(S,Se)2 can be optimized for high energy conversion in very thin photovoltaic devices, and that the alloys then exhibit excellent optical properties, especially for tellurium rich CuSb(Se1−xTex)2. This is explained by multi-valley band structure with flat energy dispersions, mainly due to the localized character of the Sb/Bi p-like conduction band states. Still the effective electron mass is reasonable small: mc ≈ 0.25m0 for CuSbTe2. The absorption coefficient α(ω) for CuSb(Se1−xTex)2 is at ħω = Eg + 1 eV as much as 5–7 times larger than α(ω) for traditional thin-film absorber materials. Auger recombination does limit the efficiency if the carrier concentration becomes too high, and this effect needs to be suppressed. However with high absorptivity, the alloys can be utilized for extremely thin inorganic solar cells with the maximum efficiency ηmax ≈ 25% even for film thicknesses d ≈ 50 − 150 nm, and the efficiency increases to ∼30% if the Auger effect is diminished.

Redistribution of valence and conduction band states depending on the method of modification of SiO2 structure.

The effect of introducing porosity and the insertion of methyl groups in SiO4 tetrahedra on the distribution of valence and conduction band states of SiO2 was studied using high-resolution near edge X-ray absorption fine structure spectroscopy (NEXAFS) and soft X-ray photoelectron spectroscopy (XPS). Alignment of NEXAFS spectra and valence band XPS spectra in a common energy scale was performed using binding energies of the initial levels obtained by XPS. It was established that the insertion of methyl groups into SiO4 tetrahedra leads to a significant shift of the top of the valence band EV to smaller binding energies due to the reduction of the electronegativity of the nearest surrounding neighbors of the Si atoms, while introducing porosity changes the position of EV only slightly. The position of the bottom of the conduction band is affected by neither the introduction of porosity nor the insertion of methyl groups. The formation of the π*C[double bond, length as m-dash]C state near the Fermi level, caused by the presence of porogen residues in the structure of organosilicate glass (OSG) and responsible for the leakage currents, was revealed. It was found that high level porosity in OSG films induces a significant variation of Si-O-Si valence angles. A number of Si-O dangling bonds were found in the surface layers of por-SiO2, while methyl groups effectively passivated these dangling bonds in OSG films. The obtained results are important for understanding the regularities of electronic structure formation in SiO2-based low-k dielectrics, which is necessary for the reduction of energy dissipated in semiconductor integrated circuits (ICs).

Electron effective mass enhancement in Ga(AsBi) alloys probed by cyclotron resonance spectroscopy

The effect of Bi incorporation on the conduction band structure of Ga(AsBi) alloys is revealed by a direct estimation of the electron effective mass via cyclotron resonance absorption spectroscopy at THz frequencies in pulsed magnetic fields up to 65 T. A strong enhancement in the electron effective mass with increasing Bi content is reported, with a value of mass $\ensuremath{\sim}40%$ higher than that in GaAs for $\ensuremath{\sim}1.7%$ of Bi. This experimental evidence unambiguously indicates a Bi-induced perturbation of the host conduction band states and calls for a deep revision of the theoretical models describing dilute bismides currently proposed in the literature, the majority of which neglect or exclude that the incorporation of a small percentage of Bi may affect the conduction band states of the host material.

Influence of nitrogen-doping concentration on the electronic structure of CuAlO2 by first-principles studies

Effect of N doping concentration on the electronic structure of N-doped CuAlO2 was investigated by density functional theory based on generalized-gradient approximation plus orbital potential. Lattice parameters a and c both increase with increasing N-doping concentration. Formation energies increase with increasing N doping concentration and all N-doped CuAlO2 were structurally stable. The calculated band gaps for N-doped CuAlO2 narrowed compared to pure CuAlO2, which was attributed to the stronger hybridization between Cu-3d and N-2p states and the downward shift of Cu-3p states in conduction bands. The higher the N-doping concentration is, the narrower the band gap. N-doped CuAlO2 shows a typical p-type semiconductor. The band structure changed from indirect to direct after N doping which will benefit the application of the CuAlO2 materials in optoelectronic and electronic devices.

Negative Electro-conductance in Suspended 2D WS2 Nanoscale Devices.

We study the in situ electro-conductance in nanoscale electronic devices composed of suspended monolayer WS2 with metal electrodes inside an aberration-corrected transmission electron microscope. Monitoring the conductance changes when the device is exposed to the electron beam of 80 keV energy reveals a reversible decrease in conductivity with increasing beam current density. The response time of the electro-conductance when exposed to the electron beam is substantially faster than the recovery time when the beam is turned off. We propose a charge trap model that accounts for excitation of electrons into the conduction band and localized trap states from energy supplied by inelastic scattering of incident 80 keV electrons. These results show how monolayer transition metal dichalcogenide 2D semiconductors can be used as transparent direct electron detectors in ultrathin nanoscale devices.

Semiconducting Properties of Swift Au Ion-Irradiated ZnO Thin Films at Room Temperature

The semiconducting properties of Au ion-irradiated ZnO thin films were investigated as a function of ion irradiation dose at room temperature. The Au ion irradiation was conducted with acceleration energy of 130 MeV in the ion dose range from 1 × 1011 to 5 × 1012 ions/cm2. The physical properties showed no change regardless of the Au ion irradiation dose; however, the electrical properties of Au ion-irradiated ZnO thin films changed, depending on the Au ion irradiation dose. The electronic structure drastically changed with the evolution of hybridized molecular orbital structure for the conduction band and band edge states below the conduction band. These remarkable changes in electronic structure correlate with changes in electrical properties, such as carrier concentration and mobility.