Density Of States In Conduction Band Research Articles

Comprehensive investigation of Li-based novel quaternary Heusler LiTiPdZ (Z = Al, Ga, In) compounds for thermoelectric performance

In the present article, we used Density Functional Theory and the Boltzmann transport equation to study the structural, electronic, and thermoelectric (TE) properties along with phonon dynamics of recently discovered LiTiPdZ (Z = Al, Ga, In) quaternary Heusler (QH) compounds. We applied generalized gradient approximations (GGA) for band structure and density of states (DOS) calculations. Comprehensive investigations of electronic structure reveal that an indirect band gap of 0.77 eV, 0.793 eV and 0.611 eV for LiTiPdAl, LiTiPdGa, and LiTiPdIn, is there reflecting the semiconducting nature of these compounds. We find that Pd-4d states contribute to the conduction band DOS while Ti-3d states contribute significantly to DOS near Fermi energy (EF) in both valence and conduction band. Phonon dispersion spectra show that all the studied compounds are dynamically stable, as obtained frequency is only in positive range. We have calculated thermoelectric properties up to a fairly high temperature (1600 K), to check the compatibility of these compounds for high temperature TE application. A high value of ZT (∼0.60 1600K) proposes these compounds as potential candidates for high temperature TE applications.

ZnSnN2 Schottky barrier solar cells

The electron density of ZnSnN2 fabricated by different deposition methods is usually much higher than its conduction band density of states and this hinders its device application. By increasing the Zn/Sn atomic ratio of the alloy target, ZnSnN2 with electron density of 1016 cm−3 is fabricated. The obtained ZnSnN2 and silver deposited with radio-frequency sputtering can form Schottky contact which shows photovoltaic effects. The voltage- and frequency-dependent capacitance of the Schottky diodes is further studied. The interface between the Ag-ZnSnN2 is abrupt with interface states of about 1013 eV−1·cm−2 as revealed by the capacitance–voltage curves. The midgap density of states of ZnSnN2 is found to be constant.

Insight into conduction band density of states at c-Si/TiO2 interface for efficient heterojunction solar cell

Carrier selective solar cell has become one of the hot spots in the area of Si solar cell. The proposed architecture FTO/TiO2/c-Si/i-a-Si:H/Cu2O/back contact studied through simulation demonstrates a power conversion efficiency of 20.03%. This study is the first to report detailed exploration of effect of the conduction band density of states on the efficiency of Si solar cell. Through optimization, the conduction band density of state (1017 cm-3) drastically increases the power conversion efficiency from 18% (at 1021 cm-3) to 21.25% (at 1017 cm-3) i.e., an improvement of 18% relatively. Along with this, the parameters like absorber layer thickness, absorber’s defect density, thickness of electron transport layer and interface defect density are also optimized. Moreover, the charge transport properties and the impact of the Schottky barrier height at c-Si/TiO2 interface on band alignment is studied. After optimization of various physical parameters such as thickness (100 μm), conduction band density of states (1017 cm-3) and defect concentration (1010 cm−3) of c-Si layer, thickness of TiO2 layer (20 nm) and interface defect density at c-Si/TiO2 junction (1010 cm−2), a short-circuit current of 38.11 mA cm−2, open-circuit voltage of 0.84 V, fill factor of 85.99% is obtained, leading to an enhanced theoretical power conversion efficiency of 27.77%.

A SPICE compatible physics-based intrinsic charge and capacitance model of InAs-OI-Si MOS transistor

In this work, we present a core model of terminal charge and intrinsic capacitances of InAs-on-Si MOS transistors. The eigenenergy and corresponding wavefunctions are deduced by solving time independent Schrödinger wave equation inside the finite potential well. A second-order non-parabolic correction to the sub-band energy, effective mass of electron, and conduction band density of states have been employed and incorporated into the core model. The developed model effectively captures the variation in gate capacitance and reproduce the staircase like capacitance-voltage characteristics. Effect of conduction band nonparabolicity on gate capacitance is studied for different band-nonparabolicity factor. The model is validated with self-consistent Schrödinger-Poisson solver predicted result for different drain bias and channel thicknesses. The model calculated results are in a reasonable agreement with numerical simulation data. The SPICE compatibility of our model is demonstrated through Verilog-AMS implementation of the model and SPICE simulation of two benchmark circuits. • We propose a core model for intrinsic charge and capacitances of InAs-on-insulator MOS Transistor over Silicon substrate. • The model is derived from explicit solution of Schrödinger and Poisson equations without using any empirical parameters. • The model includes conduction band nonparabolicity effect on sub-band energy, terminal charges and capacitances. • Inversion charge present inside buffer region and its effect on capacitance is modeled using a semi analytical approach. • The model is developed using Verilog-AMS language, and circuit simulation is demonstrated using commercial SPICE simulator.

Finite-temperature full random-phase approximation of bandgap narrowing in quasi-neutral regions: Theory including nonparabolicity and application to silicon and InGaAs

The bandgap narrowing (BGN) in quasi-neutral regions of semiconductors is calculated in a finite-temperature full random-phase approximation formalism based on a simple isotropic dispersion model including band nonparabolicity. The total quasi-particle shift (QPS) is determined by the exchange-correlation self-energy of the free carriers and the correlation energy of the interaction between carriers and ionized dopants. At cryogenic temperatures, the latter part results in giant shifts of the minority band edge in n-type semiconductors with a large ratio of valence to conduction band density of states, as often present in III–V materials. However, at room temperature, the BGN does not exceed common values. The reason for this behavior is explained analytically. Whereas the exchange-correlation energy of free carriers is known to be insensitive to band structure details, the nonparabolicity of the conduction band (CB) has a strong effect on the ionic QPS of the minority carriers in n-type III–V materials. It strongly reduces the BGN at cryogenic temperatures compared to the case of a parabolic CB. This is demonstrated numerically and also analytically for n-type InGaAs. The BGN in n-type silicon becomes independent of temperature at high concentrations, but in p-type silicon, a weak temperature dependence re-emerges above the Mott density, which also can be attributed to the ionic QPS of the minority electrons. The calculated BGN for quasi-neutral regions in silicon is in good agreement with earlier photoluminescence and more recent photo-conductance measurements.

Tunable Infrared Plasmonic Properties of Epitaxial Ti1-xMgxN(001) Layers.

Optical transmission and reflection spectra in combination with ellipsometry and transport measurements on epitaxial rocksalt structure Ti1-xMgxN(001) layers with 0.00 ≤ x ≤ 0.49 are employed to explore their potential as refractory infrared plasmonic materials. A red shift in the reflection edge ℏωe from 2.0 to 0.8 eV and the corresponding unscreened plasma energy ℏωpu from 7.6 to 4.7 eV indicate a linear reduction in the free carrier density N with increasing x. However, nitrogen vacancies in Mg-rich samples act as donors, resulting in a minimum N = 1.6 × 1022 cm-3 for x = 0.49. Photoelectron valence band spectra confirm the diminishing conduction band density of states and indicate a 0.9 eV decrease in the Fermi level as x increases from 0 to 0.49. The dielectric function ε = ε1 + iε2 can be divided into a low-energy spectral region where intraband transitions result in large negative and positive ε1 and ε2, respectively, and a higher energy interband transition region with both ε1 and ε2 > 0. The screened plasma energy Eps that separates these two regions red-shifts from 2.6 to 1.3 eV for x = 0-0.39, indicating a tunable plasmonic activity that extends from the visible to the infrared (470-930 nm). Electron transport measurements indicate a metallic temperature coefficient of resistivity (TCR) for TiN-rich alloys with x ≤ 0.26 but weak carrier localization and a negative TCR <60 K for x = 0.39 and <300 K for x = 0.49, attributed to Mg alloying-induced disorder. The plasmonic quality factor Q is approximately an order of magnitude larger than what was previously reported for polycrystalline Ti1-xMgxN, making Ti1-xMgxN(001) layers competitive with Ti1-xScxN(001).

Correlations in the Electronic Structure of van der Waals NiPS3 Crystals: An X-ray Absorption and Resonant Photoelectron Spectroscopy Study.

The electronic structure of high-quality van der Waals NiPS3 crystals was studied using near-edge X-ray absorption spectroscopy (NEXAFS) and resonant photoelectron spectroscopy (ResPES) in combination with density functional theory (DFT) approach. The experimental spectroscopic methods, being element specific, allow one to discriminate between atomic contributions in the valence and conduction band density of states and give direct comparison with the results of DFT calculations. Analysis of the NEXAFS and ResPES data allows one to identify the NiPS3 material as a charge-transfer insulator. Obtained spectroscopic and theoretical data are very important for the consideration of possible correlated-electron phenomena in such transition-metal layered materials, where the interplay between different degrees of freedom for electrons defines their electronic properties, allowing one to understand their optical and transport properties and to propose further possible applications in electronics, spintronics, and catalysis.

Van Hove Singularities and Trap States in Two-Dimensional CdSe Nanoplatelets.

Semiconductor nanoplatelets, which offer a compelling combination of the flatness of two-dimensional semiconductors and the inherent richness brought about by colloidal nanostructure synthesis, form an ideal and general testbed to investigate fundamental physical effects related to the dimensionality of semiconductors. With low temperature scanning tunnelling spectroscopy and tight binding calculations, we investigate the conduction band density of states of individual CdSe nanoplatelets. We find an occurrence of peaks instead of the typical steplike function associated with a quantum well, that rule out a free in-plane electron motion, in agreement with the theoretical density of states. This finding, along with the detection of deep trap states located on the edge facets, which also restrict the electron motion, provides a detailed picture of the actual lateral confinement in quantum wells with finite length and width.

Study of a new layered ternary chalcogenide CuZnTe2 and its potassium intercalation effect

A new layered ternary chalcogenide CuZnTe2 and its effect due to potassium (K) intercalation have been investigated using ab-initio method under the framework of density functional theory (DFT). Here, we report the structural, electronic and elastic properties of both proposed parent compound CuZnTe2 and intercalated KCuZnTe2. The electronic band structures and the density of states (DOS) of both these chalcogenides have also been studied. The parent compound demonstrates p-type conductivity with the energy band gap of 0.7 eV but surprisingly, the increase of energy gap (1.5 eV) is found in the intercalated KCuZnTe2, a direct-transition type semiconductor. The optical absorption result in KCuZnTe2 also shows the identical value of gap energy calculated by Wood-Tauc theory. The density of states (DOS) in the valence band for both compounds is dominated by the partial contribution of Cu/Zn 3d and Te 5p orbitals but the prime contribution of Cu/Zn 4s and Te 5s mainly in the conduction band DOS. The DOS value at around Fermi level in these chalcogenides is indicating the degeneracy behavior of a semiconductor. Both compounds are mechanically stable and also malleable. We also calculated the thermal properties in the intercalated KCuZnTe2 using quasi-harmonic Debye model. The observed values of Debye temperature, specific heat capacities and volume expansion coefficient using this model is almost consistent with the estimated values given in theory.

The theoretical direct-band-gap optical gain of Germanium nanowires

We calculate the electronic structures of Germanium nanowires by taking the effective-mass theory. The electron and hole states at the Γ-valley are studied via the eight-band k.p theory. For the [111] L-valley, we expand the envelope wave function using Bessel functions to calculate the energies of the electron states for the first time. The results show that the energy dispersion curves of electron states at the L-valley are almost parabolic irrespective of the diameters of Germanium nanowires. Based on the electronic structures, the density of states of Germanium nanowires are also obtained, and we find that the conduction band density of states mostly come from the electron states at the L-valley because of the eight equivalent degenerate L points in Germanium. Furthermore, the optical gain spectra of Germanium nanowires are investigated. The calculations show that there are no optical gain along z direction even though the injected carrier density is 4 × 1019 cm−3 when the doping concentration is zero, and a remarkable optical gain can be obtained when the injected carrier density is close to 1 × 1020 cm−3, since a large amount of electrons will prefer to occupy the low-energy L-valley. In this case, the negative optical gain will be encountered considering free-carrier absorption loss as the increase of the diameter. We also investigate the optical gain along z direction as functions of the doping concentration and injected carrier density for the doped Germanium nanowires. When taking into account free-carrier absorption loss, the calculated results show that a positive net peak gain is most likely to occur in the heavily doped nanowires with smaller diameters. Our theoretical studies are valuable in providing a guidance for the applications of Germanium nanowires in the field of microelectronics and optoelectronics.

Assessing model-dielectric-dependent hybrid functionals on the antiferromagnetic transition-metal monoxides MnO, FeO, CoO, and NiO

Recently, two nonempirical hybrid functionals, dielectric-dependent range-separated hybrid functional based on the Coulomb-attenuating method (DD-RSH-CAM) and doubly screened hybrid functional (DSH), have been suggested by Chen et al (2018 Phys. Rev. Mater. 2 073803) and Cui et al (2018 J. Phys. Chem. Lett. 9 2338), respectively. These two hybrid functionals are both based on a common model dielectric function approach, but differ in the way how to non-empirically obtain the range-separation parameter. By retaining the full short-range Fock exchange and a fraction of the long-range Fock exchange that equals the inverse of the dielectric constant, both DD-RSH-CAM and DSH turn out to perform very well in predicting the band gaps for a large variety of semiconductors and insulators. Here, we assess how these two hybrid functionals perform on challenging antiferromagnetic transition-metal monoxides MnO, FeO, CoO, and NiO by comparing them to other conventional hybrid functionals and the GW method. We find that single-shot DD0-RSH-CAM and DSH0 improve the band gaps towards experiments as compared to conventional hybrid functionals. The magnetic moments are slightly increased, but the predicted dielectric constants are decreased. The valence band density of states (DOS) predicted by DD0-RSH-CAM and DSH0 are as satisfactory as HSE03 in comparison to experimental spectra, however, the conduction band DOS are shifted to higher energies by about 2 eV compared to HSE03. Self-consistent DD-RSH-CAM and DSH deteriorate the results with a significant overestimation of band gaps.

Resonance State Method for Electron Injection in Dye Sensitized Solar Cells

Herein, the ab initio method is applied to examine metastable molecular excited states on a solid surface using resonance state theory and Green's function. A formula for the complex energy correction that determines the decay rate is presented; the configuration interaction effect together with major molecule-surface interactions are considered in more detail as compared to previous studies. Furthermore, the lifetimes of the excited states of Ru-terpyridine dyes adsorbed on an anatase surface are calculated, and the effects of the molecular structure and adsorption mode on the electron injection rate are studied. Also, the adsorption structures and relative stabilities of a series of Ru-terpyridine dyes-including the black dye-are reported. An implicit solvation model is necessary to reliably calculate the alignment between the photoabsorption spectrum and the conduction band density of states, governing the injection rate. Finally, some of the factors that limit the injection ability of dyes are discussed.

The seeds of Zn6Y8 L12-type clusters in amorphous Mg85Zn6Y9 alloy investigated by photoemission spectroscopy

Photoemission and inverse-photoemission spectroscopy (PES and IPES) measurements were carried out on amorphous Mg85Zn6Y9 alloy together with the polycrystal of the same concentrations with a synchronized long-period stacking ordered phase to investigate the valence- and conduction-band electronic structures, respectively. The valence- and conduction-band density of states (DOS) seems to be mostly the same as that of the crystalline phase. The core-level PES measurements were also performed at the Mg 2s, 2p, Zn 3p, 3d, and Y 3d levels to study the chemical natures of the constituent elements. The Y 3d core spectrum exhibits mostly three doublets in the amorphous phase, while mostly only one in the crystalline phase. This result reflects a coexistence of three different sites of the Y atoms in the amorphous phase; ones in the Zn6Y8 L12 clusters or their large fragments of about 57%, ones in their small fragments of about 20%, and isolated atoms of about 23%, which are mostly the same as those in the Mg97Zn1Y2 lightly doped LPSO alloy. Thus, it is concluded that well-fledged seeds of the L12 clusters are already provided even in the amorphous phase.

Characterization of the Valence and Conduction Band Levels of n = 1 2D Perovskites: A Combined Experimental and Theoretical Investigation

AbstractThis study presents a combined experimental and theoretical study of the electronic structure of two 2D metal halide perovskite films. Ultraviolet and inverse photoemission spectroscopies are performed on solution‐processed thin films of the n = 1 layered perovskite butylammonium lead iodide and bromide, BA2PbI4 and BA2PbBr4, characterized by optical absorption and X‐ray diffraction, to determine their valence and conduction band densities of states, transport gaps, and exciton binding energies. The electron spectroscopy results are compared with the densities of states determined by density functional theory calculations. The remarkable agreement between experiment and calculation enables a detailed identification and analysis of the organic and inorganic contributions to the valence and conduction bands of these two hybrid perovskites. The electron affinity and ionization energies are found to be 3.1 and 5.8 eV for BA2PbI4, and 3.1 and 6.5 eV for BA2PbBr4. The exciton binding energies are estimated to be 260 and 300 meV for the two materials, respectively. The 2D lead iodide and bromide perovskites exhibit significantly less band dispersion and a larger density of states at the band edges than the 3D analogs. The effects of using various organic ligands are also discussed.

Carrier-Specific Femtosecond XUV Transient Absorption of PbI2 Reveals Ultrafast Nonradiative Recombination

Femtosecond carrier recombination in PbI2 is measured using tabletop high-harmonic extreme ultraviolet (XUV) transient absorption spectroscopy and ultrafast electron diffraction. XUV absorption from 45 eV to 62 eV measures transitions from the iodine 4d core level to the conduction band density of states. Photoexcitation at 400 nm creates separate and distinct transient absorption signals for holes and electrons, separated in energy by the 2.4 eV band gap of the semiconductor. The shape of the conduction band and therefore the XUV absorption spectrum is temperature dependent, and nonradiative recombination converts the initial electronic excitation to thermal excitation within picoseconds. Ultrafast electron diffraction (UED) is used to measure the lattice temperature and confirm the recombination mechanism. The XUV and UED results support a 2nd-order recombination model with a rate constant of 2.5x10-9 cm3/s.

Pulsed laser deposition of two-dimensional ZnO nanocrystals on Au(111): growth, surface structure and electronic properties

Two-dimensional (2D) ZnO structures have been deposited on the Au(111) surface by means of the pulsed laser deposition technique. In situ scanning tunneling microscopy and scanning tunneling spectroscopy measurements have been performed to characterize morphological, structural and electronic properties of 2D ZnO at the nanoscale. Starting from a sub-monolayer coverage, we investigated the growth of ZnO, identifying different atomic layers (up to the fifth). At low coverage, we observed single- and bi-layer nanocrystals, characterized by a surface moiré pattern that is associated to a graphene-like ZnO structure. By increasing the coverage, we revealed a morphological change starting from the fourth layer, which was attributed to a transition toward a bulk-like structure. Investigation of the electronic properties revealed the semiconducting character of 2D ZnO. We observed a dependence of the density of states (DOS) and, in particular, of the conduction band (CB) on the ZnO thickness, with a decreasing of the CB onset energy for increasing thickness. The CB DOS of 2D ZnO shows a step-like behaviour which may be interpreted as due to a 2D quantum confinement effect in ZnO atomic layers.

Valence and Conduction Band Densities of States of Metal Halide Perovskites: A Combined Experimental-Theoretical Study.

We report valence and conduction band densities of states measured via ultraviolet and inverse photoemission spectroscopies on three metal halide perovskites, specifically methylammonium lead iodide and bromide and cesium lead bromide (MAPbI3, MAPbBr3, CsPbBr3), grown at two different institutions on different substrates. These are compared with theoretical densities of states (DOS) calculated via density functional theory. The qualitative agreement achieved between experiment and theory leads to the identification of valence and conduction band spectral features, and allows a precise determination of the position of the band edges, ionization energy and electron affinity of the materials. The comparison reveals an unusually low DOS at the valence band maximum (VBM) of these compounds, which confirms and generalizes previous predictions of strong band dispersion and low DOS at the MAPbI3 VBM. This low DOS calls for special attention when using electron spectroscopy to determine the frontier electronic states of lead halide perovskites.

Joint density of states in low dimensional semiconductors

We present a different approach to evaluate density of states for quasi-bidimensional systems, which bonds density of states in the confinement direction with in-plane 2D density of states. Applying the convolution operation, we propose an accurately mathematical expression that combines directly the valence band and conduction band density of states functions to generate a joint density of states for direct transitions. When considering low dimensional semiconductors, another expression is found which shows that the density of states for electrons (holes) can be calculated by convolution operations between the confinement direction and in-plane electron (hole) density of states. Using both expressions, we have calculated the quantum well and superlattice absorption coefficient, resulting in positive alignment with experimental data. A more complete description of physical absorption is achieved with this new approach.

Enhanced conduction band density of states in intermetallic EuTSi3 (T = Rh, Ir)

We report on the physical properties of single crystalline EuRhSi3 and polycrystalline EuIrSi3, inferred from magnetization, electrical transport, heat capacity and 151Eu Mössbauer spectroscopy. These previously known compounds crystallise in the tetragonal BaNiSn3-type structure. The single crystal magnetization in EuRhSi3 has a strongly anisotropic behaviour at 2 K with a spin-flop field of 13 T, and we present a model of these magnetic properties which allows the exchange constants to be determined. In both compounds, specific heat shows the presence of a cascade of two close transitions near 50 K, and the 151Eu Mössbauer spectra demonstrate that the intermediate phase has an incommensurate amplitude modulated structure. We find anomalously large values, with respect to other members of the series, for the RKKY Néel temperature, for the spin-flop field (13 T), for the spin-wave gap (20–25 K) inferred from both resistivity and specific heat data, for the spin-disorder resistivity in EuIrSi3 (240 cm) and for the saturated hyperfine field (52 T). The enhanced values of the quantities that depend on the electronic density of states at the Fermi level, imply that the latter must be strongly enhanced in these two materials. EuIrSi3 exhibits a giant magnetoresistance ratio, with values exceeding 600% at 2 K in a field of 14 T.

Structural, optical and electrical properties studies of ultrasonically deposited tin oxide (SnO2) thin films with different substrate temperatures

Undoped thin films of tin oxide (SnO2) were deposited onto microscopic glass substrates using an ultrasonic spray pyrolysis technique. The thin films were deposited on glass at large scale of temperature ranged in 400–500°C and stepped by 20°C. The effect of substrate temperature on structural, electrical and optical properties was studied using X-ray diffraction (XRD), UV–visible spectrophotometer and a conventional four point probe technique. XRD showed that all the films were polycrystalline with major reflex along (110) plane, manifested with amelioration of grain size from 6.51 to 29.80nm upon increasing substrate temperature. Lattice constant ‘a’, ‘c’, microstrains and dislocation densities were affected by the substrate temperature. Transmittance of about 75% at more than 800nm for films prepared at 480°C has been observed. With increasing the substrate temperature the direct band gap energy was averaged in 4.03–4.133eV. Plasma frequency (ωp) was estimated to be 4⋅1014Hz and optically estimated free electrons number N leading to predict that, in SnO2 material, the effective density of conduction band states is about 5⋅1019cm−3. Among all the samples the film that deposited at 480°C exhibited lowest resistivity of about 9.19×10−3Ωcm.