Band Structure Of Semiconductors Research Articles

Electronic structures of CsPb(XxY1–x)3 perovskites

We investigated electronic structures of CsPb(XxY1–x)3 perovskites, where X, Y = Cl, Br, or I, and x = 0, 1/3, or 2/3, based on the density functional theory (DFT) with generalized gradient approximation (GGA). The results show that these materials exhibit direct and inverted gaps, in the sense that the character of electronic states near the valence band maximum (VBM) is derived from atomic s-like orbital of Pb and p-like orbital of halide atom, whereas the character of states near the conduction band minimum (CBM) is derived from atomic p-like orbital of Pb in contrast with the electronic band structures of other common semiconductors. Their density of states also indicates that the optical transition in both absorption and luminescence are stronger.

Influence of the free-electron concentration on the optical properties of zincblende GaN up to 1×1020cm−3

We analyze the optical properties of zincblende gallium-nitride in the infrared and ultraviolet spectral range ($\ensuremath{\approx}27\phantom{\rule{0.16em}{0ex}}\mathrm{meV}--6.5\phantom{\rule{0.16em}{0ex}}\mathrm{eV}$) experimentally by spectroscopic ellipsometry and provide a quantitative description of these results by $\mathbf{k}\ifmmode\cdot\else\textperiodcentered\fi{}\mathbf{p}$ perturbation theory. Free-electron concentrations above ${10}^{20}\phantom{\rule{0.16em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$ are achieved by introducing germanium as a donor. We determine the dielectric function as well as band filling effects like the Burstein-Moss shift and band gap renormalization. The Kane model for the band structure of semiconductors near the $\mathrm{\ensuremath{\Gamma}}$-point allows to calculate the effective electron mass and to determine the nonparabolicity of the conduction band. At the same time, these results can be used to derive the free-electron concentration all-optically. The combination of Kane's model, Burstein-Moss shift, and band-gap renormalization can be used to expertly describe the measured transition energies up to $\ensuremath{\approx}3.7\phantom{\rule{0.16em}{0ex}}\mathrm{eV}$ dependent on the carrier concentration, yielding an averaged hole mass of $\ensuremath{\approx}0.61\phantom{\rule{0.16em}{0ex}}{\mathrm{m}}_{\mathrm{e}}$ for the contributing valence bands.

Rational design of visible-light-driven Pd-loaded α/β-Bi2O3 nanorods with exceptional cationic and anionic dye degradation properties

Here, we report a simple sono-chemical method to fabricate Pd-loaded α/β-Bi2O3 nanorods with exceptional methylene blue (MB), brilliant green (BG) and acid red 1 (AR) dye degradation properties. The unique nanorods morphology was accomplished by treating of bismuth complexes with sodium hydroxide at room temperature. Palladium was introduced by UV-photo-deposition method from palladium (II) chloride source. The successful substitution of Bi3+ by Pd2+ in bulk of Pd-loaded α/β-Bi2O3 was confirmed by X-ray diffraction (XRD) and Fourier-transform infrared spectroscopy (FTIR). The surface presence of α-, β-Bi2O3, PdO and metallic Pd0 was confirmed by X-ray photoelectron spectroscopy (XPS). The rational design and fabrication approach is based on matching of (1) valence band (VB) and conduction band (CB) of as-prepared catalyst with redox potentials of dyes, (2) VB and CB with reactive oxygen species (ROS), and (3) redox potentials of dyes with ROS. By using the approach of rational design the material is able to degrade cationic MB (90.8%), BG (83.3%) and anionic AR (83.2%) dyes within 20 min under LED light irradiation (100 mW/cm2), as well as to degrade mixtures of cationic/anionic MB-AR and BG-AR dyes. Further optimization of dye degradation parameters, such as initial concentration of dye, amount of catalyst, and solution pH was carried out. A mechanism of dye degradation was proposed based on ROS scavenging experiments and mapping out of semiconductor band structures with redox potentials of dyes and ROS. Pd-loaded α/β-Bi2O3 catalyst has potential applications in wastewater treatment, water splitting and healthcare industries.

Tuning topological surface magnetism by bulk alloying

Deploying an analytical atomistic model of the bulk band structure of the IV-VI semiconductors we connect the spin structure of the topological surface state to the crystal field and spin orbit coupling parameters of the bulk material. While the Dirac-Weyl (or equivalently, Rashba) type topological surface state is often assumed universal, we show that the physics of the surface state is strikingly non-universal. To see this explicitly we calculate the RKKY interaction, which may be viewed as a probe of this surface state spin structure, finding its \emph{qualitative form} depends on the values the bulk spin-orbit and crystal field parameters take. This opens the way to tune the spin interaction on the surface of a IV-VI topological insulator by, for instance varying the composition of the IV-VI ternary compounds, as well as highlighting the importance of the connection between bulk and surface physics in topological insulators.

Second highest occupied molecular orbital effects on the valence band structure of organic semiconductors

We investigate the electronic states of organic semiconductors with herringbone structures using the multi-orbital tight-binding model for highest and second highest occupied molecular orbitals (HOMOs and SHOMOs) to elucidate the effect of SHOMOs on the valence-band state. We diagonalize the Hamiltonian analytically and derive the valence band width considering hybridization of HOMO and SHOMO. It is shown that the SHOMO effect is enhanced in the condition where the sum of the intermolecular transfer integrals of HOMO-SHOMO in the sublattice and the sum between the sublattices is respectively so large that their product is not negligible compared to the energy difference between HOMO and SHOMO. We also investigate acene, phenacene and thienoacene using density functional theory. It is shown that [1]benzothieno[3,2-b][1]benzothiophene(BTBT) based materials satisfy this condition, and SHOMO contributes significantly. It implies that the origins of the high-mobility of BTBT based materials can be ascribed to the light effective mass due to the contribution of SHOMO.

Density Functional Theory Calculations Revealing Metal-like Band Structures and Work Function Variation for Ultrathin Gallium Arsenide (111) Surface Layers.

Density functional theory (DFT) calculations have been performed on tunable numbers of gallium arsenide (100), (110), and (111) planes for their electron density of states (DOS) plots and the corresponding band diagrams. The GaAs (100) and (110) planes show the same semiconducting band structure with tunable plane layers and a band gap of 1.35 eV around the Fermi level. In contrast, metal-like band structures are obtained with a continuous band structure around the Fermi level for 1, 2, 4, 5, 7, and 8 layers of GaAs (111) planes. For 3, 6, and 9 GaAs (111) planes, the same semiconducting band structure as seen in the (100) and (110) planes returns. The results suggest the GaAs {111} face should be more electrically conductive than its {100} and {110} faces, due to the merged conduction band and valence band. GaAs (100) and (110) planes give a fixed work function, but the (111) planes have variable work function values that are smaller than that obtained for the (100) and (110) planes. Furthermore, bond length, bond geometry, and frontier orbital electron number and energy distribution show notable differences between the metal-like and semiconducting plane cases, so the emergence of plane-dependent electronic properties have quantum mechanical origin at the orbital level. GaAs should possess similar facet-dependent electronic properties to those of Si and Ge.

Experimental realization of a semiconducting quasicrystalline approximant in Al-Si-Ru system by band engineering

We found that an Al-Si-Ru cubic quasicrystalline approximant has a semiconducting band structure by performing an orbital analysis based on density functional theory. These semiconducting transport properties have been confirmed in an experimentally synthesized sample. The temperature dependences of the electrical conductivity and the Seebeck coefficient were consistent with the trends of an intrinsic semiconductor with a band gap of $0.15\phantom{\rule{0.28em}{0ex}}\mathrm{eV}$ above $350\phantom{\rule{0.28em}{0ex}}\mathrm{K}$. The lattice thermal conductivity had a low value of approximately $1.0\phantom{\rule{0.28em}{0ex}}\mathrm{W}\phantom{\rule{0.16em}{0ex}}{\mathrm{m}}^{\ensuremath{-}1}\phantom{\rule{0.16em}{0ex}}{\mathrm{K}}^{\ensuremath{-}1}$ above $400\phantom{\rule{0.28em}{0ex}}\mathrm{K}$, which is close to the theoretical minimum.

Photocatalytic Conversion of CO2 into Oxygenate Fuels/Chemicals Using Efficient, Eco-Friendly, Titania/Hematite-Based Nanostructured Solar-Energy Materials

Photocatalytic hydrogenation of carbon dioxide (CO₂), using water feedstock (H source), sunlight and appropriate semiconducting low-cost/eco-friendly/solar-energy materials, is a promising route for sequestration of this greenhouse gas and its conversion into value-added oxygenate fuels (compounds). Herein, by employing three hematite-based nanostructured photocatalyst/solar-energy materials, the CO₂ photoreduction (hydrogenation) process was carried out inside a water photosplitting reactor, and various oxygenate (C/H/O) products [including ethanol, methanol and formaldehyde as well as oxalic, acetic and formic acids] were synthesized in the reaction medium. Concerning the complexity of system and the diversity of products-being photocatalytically synthesized during the photoconversion process, two straightforward problem-solving strategies were proposed [one, focusing on a single/particular product and pursuing its quantity (concentration) at different illumination periods, and the other, simultaneous investigation of various products being produced at a fixed reaction period, in the aqueous or gas-phase medium]. Finally, the matter of CO₂ photoconversion into oxygenate fuels/chemicals was explained in detail from photoelectrochemical [semiconductor band structure and redox potential] as well as mechanistic perspectives.

Molten salts synthesis and visible light photocatalytic activity of crystalline poly(triazine imide) with different morphologies

Graphite-phase carbon nitride (g-C3N4) has a unique semiconductor band structure (band gap width 2.7 eV), excellent chemical stability and non-toxicity, which has potential application in the field of environmental pollution treatment and energy conversion. However, low specific surface area and high recombination photo-generated electrons–holes, which lead to low quantum efficiency of photocatalytic process and seriously restrict its photocatalytic activity. Herein, crystalline poly(triazine imide) (PTI) with different morphologies was synthesized by molten salts method. The results showed that the layer structure, hollow tubes and nanosheets of crystalline PTI were obtained according to different mass ratio of raw materials and molten salt. The degradation of rhodamine B under visible light irradiation was used to evaluate the catalytic activity of the photocatalyst. The PTI nanosheets exhibited highest visible light catalytic activity due to the increase in specific surface area and improvement of separation efficiency of electron hole pairs. A possible mechanism was also raised according to the results of trapping experimental and spectroscopic properties.

Design of p -type transparent conductors from inverted band structure: The case of inorganic metal halide perovskites

Transparent conductors (TCs), which bring two seemingly contraindicated properties, conductivity and transparency, together into one material, enable many critical technologies. Significant successes have been achieved for $n$-type TCs, such as Sn-doped $\mathrm{I}{\mathrm{n}}_{2}{\mathrm{O}}_{3}$, but developing their $p$-type counterparts has still encountered a big challenge, mainly due to the intrinsic band structure of conventional semiconductors. Here, we propose that a class of wide-gap inorganic metal halide perovskites have great potential as ideal $p$-type TCs, because of their inverted band structure compared to $n$-type TCs, i.e., they have $s$-like wave-function character at the top of the valence band, and $p$-like wave-function character at the lower valence bands and the bottom of the conduction band, which results in significantly improved $p$-type dopability, high hole mobility, and good optical transparency. This insight of designing $p$-type TCs from an inverted band structure opens an avenue for the future design of critical optoelectronic materials.

Organic Semiconducting Alloys with Tunable Energy Levels.

Continuous band structure tuning, e.g., doping with different atoms, is one of the most important features of inorganic semiconductors. However, this can hardly be realized in organic semicondutors. Here, we report the first example of fine-tuning organic semiconductor band structures by alloying structurally similar derivatives into one single phase. By incorporating halogen atoms on different positions of the backbone, BDOPV derivatives with complementary intramolecular or intermolecular charge distributions were obtained. To maximize the Coloumbic attractive interactions and minimize repulsive interactions, they form antiparallel cofacial stacking in monocomponent or in alloy single crystals, resulting in efficient π orbital overlap. Benefiting from self-assembly induced solid state "olefin metathesis" reaction, it was observed, for the first time, that three BDOPV derivatives cocrystallized in one single crystal. Molecules with different energy levels serve like the dopants in inorganic semiconductors. Consequently, as the total number of halogen atoms increased, highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels of the alloy single crystals decreased monotonously in the range from -5.94 to -6.96 eV and -4.19 to -4.48 eV, respectively.

One-Pot Ionothermal Synthesized Carbon Nitride Heterojunction Nanorods for Simultaneous Photocatalytic Reduction and Oxidation Reactions: Synergistic Effect and Mechanism Insight

Band structure and crystallinity engineering of polymeric semiconductors provides effective strategies for photocatalytic activity enhancement. Here, a facile one-step ionothermal method was developed to achieve the in situ growth of carbon nitride (CN) isotype heterojunctions (ms-CN-x) using KCl/LiCl eutectic salts as the high-temperature solvent and urea as the starting material, which are demonstrated to be efficient photocatalysts for removal of toxic heavy metal and organic pollutant in a single or coexisting system. The as-obtained ms-CN-x composite material appears as well-ordered nanorod structure with high crystallinity and consists of phases of both heptazine-based CN and triazine-based CN. With structural and morphological merits, the ms-CN-0.5 photocatalyst exhibits remarkable adsorption–photodegradation performance for high-level methylene blue (MB). Moreover, simultaneously enhanced photocatalytic reaction rate of Cr(VI) reduction and MB oxidation over ms-CN-0.5 are achieved and systematical...

Magneto-optical spectra and the presence of an impurity band in p-type ferromagnetic semiconductor (Ga,Fe)Sb with high Curie temperature

By using magnetic circular dichroism (MCD) spectroscopy with photon energy in both visible (1.5–5 eV) and infrared light regions (0.6–1.7 eV), we systematically investigate the band structure of p-type ferromagnetic semiconductor (Ga1−x,Fex)Sb with various Fe concentrations x = 2%–20% grown by low-temperature molecular beam epitaxy. We observed two peaks in the infrared MCD spectra that can be explained by the optical transitions related to the Fermi level (EF) located in an Fe-related impurity band (IB) in the bandgap. As x increases, the energy shifts of the two peaks suggest that the Fe-related IB extends into the bandgap and EF rises correspondingly. Furthermore, the mobility of hole carriers in these (Ga,Fe)Sb thin films estimated by Hall measurements is very low (0.2–2 cm2/Vs), which is consistent with our conclusion that the hole carriers and EF reside in the IB rather than in the valence band. Our results provide insights into the band structure of p-type ferromagnetic semiconductors (Ga,Fe)Sb with high Curie temperature, which is promising for the realization of spintronic devices operating at room temperature.

Calculation of the band structure of a non-chiral semiconductor and metallic carbon nanotubes

A new form of the linear augmented cylindrical wave method is proposed. For the construction of basis functions, the electron potential is taken to be spherically symmetric in atomic regions, constant in the intermediate region and cylindrically symmetric in the vacuum regions. The basis functions of the method, obtained from the solution of the Schrodinger equation in the corresponding domains, are sewn on the boundaries of the MT-spheres and the cylindrical surfaces of the tube, forming everywhere continuous and differentiable functions. In order to approve a method, the band structure of the non-chiral semiconductor and metallic single-wall carbon nanotubes was calculated.

Measuring the Effective Masses of the Electrical Conductivity and Density of States by Contactless Microwave Means

Knowledge of the effective mass of quasiparticles is needed for studying the band structure of semiconductors, simulating processes of conductivity, and developing actual semiconductor devices. The problem of determining the effective mass of carriers in a particular sample is therefore of great interest. Using the example of silicon, it is shown in this work that it is possible to determine the effective masses of conductivity and the density of states by treating a measured microwave reflection spectrum as a set of information parameters. The inverse problem is solved of finding the conditions for the minimum of the difference of squares of values corresponding to known theoretical and experimentally measured spectral dependences. Calculations and experiments are performed for the temperature range of 130–190 K, in which the highest accuracy of measurements is ensured. For Ga-doped p-silicon and Sb-doped n-silicon, values of the desired effective masses are obtained that coincide with ones given in the literature. The proposed contactless technique allows simultaneous determination of the effective masses of conductivity and density of states of charge carriers using conventional equipment. The approach can be used to measure the parameters of other types of semiconductors, including ones that are little studied.

Spin-Polarized Semiconducting Band Structure of Monolayer Graphene on Ni(111)

The dream of graphene modified to have a spin-polarized semiconducting band structure has remained elusive for over a decade. This study uses spin-polarized scanning tunneling microscopy to reveal that the strong interaction between graphene and Ni(111) substrate generates a large gap in graphene, and simultaneously leads to a relative shift of majority- and minority-spin bands. This band structure persists even at room temperature, which is promising for graphene-based spintronic information technologies.

Correlated Variation of Electrical Characteristics of a Thin-Film Field-Effect Transistor during Modification of the Physical Properties of an InZnO:N Oxide Semiconductor Channel

We have studied changes in the transmission and current–voltage characteristics of a thin-film field-effect transistor (TFT) during modification of the physical properties of its oxide semiconductor channel based on InZnO:N layer. Modification of the electrical parameters of the device was based on the phenomenon of photoinduced charge accumulation in the semiconductor. It is established that the slopes and intercepts of the capacitance–voltage (CG–VG) and transmission (ID–VG) curves of the TFT under illumination exhibit correlated similar variations. The obtained results justify joint use of the CG–VG and ID–VG curves in the analysis of peculiarities of the energy band structure of oxide semiconductors.

Computational study of Fermi kinetics transport applied to large-signal RF device simulations

A detailed description and analysis of the Fermi kinetics transport (FKT) equations for simulating charge transport in semiconductor devices is presented. The fully coupled nonlinear discrete FKT equations are elaborated, as well as solution methods and work-flow for the simulation of RF electronic devices under large-signal conditions. The importance of full-wave electromagnetics is discussed in the context of high-speed device simulation, and the meshing requirements to integrate the full-wave solver with the transport equations are given in detail. The method includes full semiconductor band structure effects to capture the scattering details for the Boltzmann transport equation. The method is applied to high-speed gallium nitride devices. Finally, numerical convergence and stability examples provide insight into the mesh convergence behavior of the deterministic solver.

Series of Lanthanide–Mercury Compounds with Three-Dimensional Structures: Rational Preparation, Structures and Properties

Three novel Ln-Hg complexes [Ln(H2O)2(μ-IA)3Hg3Br6] n (Ln = Pr (1), Nd (2), and Er (3); HIA is isonicotinic acid) are synthesized and characterized. They feature three-dimensional (3-D) motifs. Solid-state UV/vis diffuse reflectance spectroscopy found that their band gaps are 4.91, 4.59, and 2.68 eV. It is found that lanthanide ions could adjust the band structures of semiconductors. Their photoluminescence comes from their characteristic emissions of 1D2 → 3H4 of Pr3+, 7F7/2 → 4S3/2 and 4F3/2 → 4I9/2 of Nd3+, and 4I15/2 → 4F7/2 and 4I15/2 → 4S3/2 of Er3+. The CIE chromaticity coordinate is ( x = 0.5726, y = 0.4206), ( x = 0.7268, y = 0.2732), and ( x = 0.2923, y = 0.4317). Their magnetization susceptibility totally obeys the Curie-Weiss equation with antiferromagnetic performances.

Improved LDA-1/2 method for band structure calculations in covalent semiconductors

The LDA-1/2 method for self-energy correction is a powerful tool for calculating accurate band structures of semiconductors, while keeping the computational load as low as standard LDA. Nevertheless, controversies remain regarding the choice between (1/2)e and (1/4)e charge stripping from the atoms in group IV semiconductors, the incorrect direct band gap predicted for Ge, and inaccurate band structures for III–V semiconductors. Here we propose an improved method named shell-LDA-1/2 (shLDA-1/2 for short), which is based on a shell-like trimming function for the self-energy potential. With the new approach, we obtained improved band structures for group IV, and for III–V and II–VI compound semiconductors. In particular, with shLDA-1/2 we reproduced the complete band structures of Ge in good agreement with experimental data. Moreover, we have defined clear rules for choosing the amount of charge that ought to be stripped in covalent semiconductors, i.e., (1/2)e or (1/4)e.