Band Structure Of Semiconductors Research Articles

Local electrical properties of thermally grown oxide films formed on duplex stainless steel surfaces

The local electrical properties of thermally grown oxide films formed on ferrite and austenite surfaces of duplex stainless steel at different temperatures were investigated by Current sensing atomic force microscopy, X-ray Photoelectron Spectroscopy (XPS) and Auger Electron Spectroscopy (AES). The current maps and XPS/AES analyses show that the oxide films covering austenite and ferrite surfaces formed at different temperatures exhibit different local electrical characteristics, thickness and composition. The dependence of electrical conductivity of oxide films covering austenite and ferrite surface on the formation temperature is attributed to the film thickness and semiconducting structures, which is intrinsically related to thermodynamics and kinetics process of film grown at different temperature. This is well elucidated by corresponding semiconductor band structures of oxide films formed on austenite and ferrite phases at different temperature.

Tight-binding electronic band structure and surface states of Cu-chalcopyrite semiconductors

We report theoretical calculations of the electronic band structure and surface states of Cu-chalcopyrite semiconductors. The systems under consideration are CuGaS2, CuAlSe2, and CuGaS2 doped with Cr. The calculations are carried out using semi-empirical Tight-Binding formalism. By reproducing the band gap of these systems obtained by ab-initio calculations we report the Tight-Binding parameters required for the band structure calculations. We present bulk band structure calculations of the above mentioned semiconductors, and we analyze the (001) and (110) surface states of the corresponding semi-infinite semiconductors.

Intrinsic transmission magnetic circular dichroism spectra of GaMnAs

Transmission magnetic circular dichroism (MCD) spectroscopy has been widely used to reveal the spin-dependent band structure of ferromagnetic semiconductors. In these previous studies, some band pictures have been proposed from the spectral shapes observed in transmission MCD; however, extrinsic signals originating from optical interference have not been appropriately considered. In this study, we calculate the MCD spectra taking into account the optical interference of the layered structure of samples and show that the spectral shape of MCD is strongly influenced by optical interference. To correctly understand the transmission MCD, we also calculate the intrinsic MCD spectra of GaMnAs that are not influenced by the optical interference. The spectral shape of the intrinsic MCD can be explained by the characteristic band structure of GaMnAs, that is, the spin-polarized valence band and the impurity band existing above the valence band top.

Band structure and thermoelectric properties of half-Heusler semiconductors from many-body perturbation theory

Band structure and thermoelectric properties of half-Heusler semiconductors from many-body perturbation theory

Коррелированное изменение электрических характеристик тонкопленочного полевого транзистора при модификации физических свойств его оксидного полупроводникового канала (InZnO : N)

AbstractWe 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 ( C _G– V _G) and transmission ( I _D– V _G) curves of the TFT under illumination exhibit correlated similar variations. The obtained results justify joint use of the C _G– V _G and I _D– V _G curves in the analysis of peculiarities of the energy band structure of oxide semiconductors.

Band Structure and Photoelectric Characterization of GeSe Monolayers

AbstractGermanium selenide monolayer is promising in photoelectric applications for its natural p‐type semiconductor and complicated band structures. Basic experimental investigations of few‐to‐monolayer germanium selenide are still absent; major scientific challenge is to develop of techniques for controllably thinned monolayers. In this study laser thinned monolayer germanium selenide on SiO2/Si substrates is demonstrated. A broad photoluminescence spectrum with eight continues peaks is observed from visible to infrared wavebands centered at ≈589, 655, 737, 830, 1034, 1178, 1314, and 1456 nm, respectively. First‐principle calculations based on density functional theory illuminate the band structures of few‐to‐monolayer germanium selenide. Photoluminescence investigation combined with first‐principle calculations indicates that the indirect to direct bandgap transition happens at few layers of N = 3. Current–voltage and photoresponse characteristics of monolayer based devices show 3.3 times the photosensitivity and much faster falling edges compared with those of the pristine nanosheet based devices. The present results provide useful insight into deep understanding of thickness dependent performances of germanium selenide monocrystalline.

Magneto-optical spectroscopy of diluted magnetic semiconductors GaMnAs prepared by ion implantation and further impulse laser annealing

Ga1−xMnxAs layers prepared by ion implantation and subsequent pulsed laser annealing with the planned Mn concentrations of x = 0.01–0.08 have been studied using the magneto-optical transversal Kerr effect (TKE) and spectral ellipsometry. The spectral dependences of the diagonal and nondiagonal components of the permittivity tensor (PT), as well as the spectrum of magnetic circular dichroism (MCD) have been calculated for the layers. The obtained spectra of the diagonal PT components show that the layers under study maintain the zinc-blende crystal structure of the parent GaAs semiconductor. All studied samples reveal a strong TKE response at low temperatures with a dependence of an effective Curie temperature (at which TKE appears) on the Mn concentration. A number of extrema in the low-temperature TKE and MCD spectra are close to the energies of transitions in the Γ and L critical points of the parent semiconductor band structure that confirms the intrinsic ferromagnetism of the Ga1−xMnxAs layers. The MCD spectra shape and its change with Mn concentration increasing are discussed on a base of the valence-band model of ferromagnetism in DMS.

Hole-phonon coupling effect on the band dispersion of organic molecular semiconductors

The dynamic interaction between the traveling charges and the molecular vibrations is critical for the charge transport in organic semiconductors. However, a direct evidence of the expected impact of the charge-phonon coupling on the band dispersion of organic semiconductors is yet to be provided. Here, we report on the electronic properties of rubrene single crystal as investigated by angle resolved ultraviolet photoelectron spectroscopy. A gap opening and kink-like features in the rubrene electronic band dispersion are observed. In particular, the latter results in a large enhancement of the hole effective mass (> 1.4), well above the limit of the theoretical estimations. The results are consistent with the expected modifications of the band structures in organic semiconductors as introduced by hole-phonon coupling effects and represent an important experimental step toward the understanding of the charge localization phenomena in organic materials.

Electronic and magnetic properties of antiferromagnetic TbAs via modified Becke Johnson potential plus an on-site Coulomb U

In this work, theoretical study of structural, elastic, electronic properties and magnetic ordering of Terbium arsenide (TbAs) in NaCl-B1 and CsCl-B2 phases are investigated using first-principles calculations based on the full-potential augmented plane wave plus local orbitals (FP-L/APW ‏+ lo) method within the framework of density functional theory (DFT) in the generalized gradient approximation (GGA) with the on-site Hubbard Ueff parameter (GGA+U). At ambient conditions Terbium monopnictides stabilize in NaCl (B1) structure. The calculations indicate that there exists a phase transition from the NaCl-type (B1) structure to CsCl-type (B2) structure at the transition pressure of 16GPa. The calculated structural parameters, such as the lattice constant, bulk modulus and second-order elastic constants, are presented. Elastic constants and their related parameters such as Poisson's ratio, Young modulus and shear modulus were calculated. The anti-ferromagnetic of type II state is found to be the most stable than the other possible phases, which is in agreement with experiment. By varying the Hubbard U parameter from 0 to 10eV, a detailed study of magnetism of this compound via the density of states DOS, is presented. We find that semiconducting band structures can be obtained using the modified Becke-Johnson potential plus an on-site Coulomb U (the mBJ+U approach).

Gauge factors for piezotronic stress sensor in polycrystalline ZnO

The piezotronic effect gained considerable interest during the last decade. It utilizes the interaction of stress-induced piezoelectric charges and the electronic band structure in piezoelectric semiconductors. This could lead to new applications like strain-triggered transistors or transparent strain/stress sensors. Apart from single Schottky barriers, double Schottky barriers in varistor boundaries in ZnO can be modified extensively by the application of stress. The gauge factors obtained by this method far exceed values for commercial strain sensors. The determination of the underlying physical mechanisms is therefore of utmost importance for applications in strain sensing. In this work, the experimental results of the influence of mechanical stress on the current–voltage characteristics of ZnO-based varistor ceramics are contrasted to simulations. It is verified that a recently introduced simplified physical model based on parallel conductive regions is capable of explaining the stress sensitivity of varistor ceramics and the evolution of the gauge factor as a function of mechanical stress. At electric fields below the breakdown voltage, small percolating pathways through the microstructure determine the current response of the ceramic. This current localization effect becomes even more pronounced when the material is mechanically stressed with an externally applied uniaxial compressive load. Considering application in stress sensors, current localization in the material could possibly lead to local heating effects and degradation of the material.

P25@CoAl layered double hydroxide heterojunction nanocomposites for CO2 photocatalytic reduction

Artificial photosynthesis driven by inorganic photocatalysts offers a promising route to renewable solar fuels, however efficient CO2 photoreduction remains a challenge. A family of hierarchical nanocomposites, comprising P25 nanoparticles encapsulated within microporous CoAl-layered double hydroxides (CoAl-LDHs) were prepared via a one-pot hydrothermal synthesis. Heterojunction formation between the visible light absorbing CoAl-LDH and UV light absorbing P25 semiconductors extends utilisation of the solar spectrum, while the solid basicity of the CoAl-LDH increases CO2 availability at photocatalytic surfaces. Matching of the semiconductor band structures and strong donor–acceptor coupling improves photoinduced charge carrier separation and transfer via the heterojunction. Hierarchical P25@CoAl-LDH nanocomposites exhibit good activity and selectivity (>90%) for aqueous CO2 photoreduction to CO, without a sacrificial hole acceptor. This represents a facile and cost-effective strategy for the design and development of LDH-based nanomaterials for efficient photocatalysis for renewable solar fuel production from particularly CO2 and water.

Long‐Term Structural Evolution of an Intercalated Layered Semiconductor

Intercalated molecules can dramatically modify the electronic band structure of layered semiconductors, significantly altering the optical properties of the material. In the layered monochalcogenide Gallium Telluride (GaTe), exposure to air induces a nearly 1 eV reduction of its band gap due to the interlayer diffusion and chemisorption of oxygen. The effect of oxygen chemisorption at the Te‐terminated surfaces on the structure of GaTe, however, is much less known. To better understand the structure–property relationship of intercalated GaTe, a systematic, long‐term, X‐ray diffraction study has been performed on GaTe exfoliated crystals exposed to ambient conditions. It is observed that the structural changes are not limited to a previously observed short‐term increase in lattice expansion. Over the course of months and even years after exfoliation, the oxygen adsorbates continue to modify the structure of GaTe, inducing significant disorder and grain reorientation. It is estimated that approximately one out of every two grains is slightly displaced by the intercalating oxygen, demonstrating a significant increase in grain mosaicity, while still maintaining the original {−2 0 1} out‐of‐plane texture. Correlating these structural transformations to observed changes in electrical and optical properties will enable capitalization of the use of adsorbates to engineer novel properties in these layered materials.

Magnetoexcitons Break Antiunitary Symmetries

We show analytically and numerically that the application of an external magnetic field to highly excited Rydberg excitons breaks all antiunitary symmetries in the system. Only by considering the complete valence band structure of a direct-band-gap cubic semiconductor, the Hamiltonian of excitons leads to the statistics of a Gaussian unitary ensemble without the need for interactions with other quasiparticles like phonons. Hence, we give theoretical evidence for a spatially homogeneous system breaking all antiunitary symmetries.

Observation of spontaneous spin-splitting in the band structure of an n-type zinc-blende ferromagnetic semiconductor.

Large spin-splitting in the conduction band and valence band of ferromagnetic semiconductors, predicted by the influential mean-field Zener model and assumed in many spintronic device proposals, has never been observed in the mainstream p-type Mn-doped ferromagnetic semiconductors. Here, using tunnelling spectroscopy in Esaki-diode structures, we report the observation of such a large spontaneous spin-splitting energy (31.7–50 meV) in the conduction band bottom of n-type ferromagnetic semiconductor (In,Fe)As, which is surprising considering the very weak s-d exchange interaction reported in several zinc-blende type semiconductors. The mean-field Zener model also fails to explain consistently the ferromagnetism and the spin-splitting energy of (In,Fe)As, because we found that the Curie temperature values calculated using the observed spin-splitting energies are much lower than the experimental ones by a factor of 400. These results urge the need for a more sophisticated theory of ferromagnetic semiconductors.

Theoretical study of two-dimensional boron silicide from first-principles

Using first-principles calculations and evolutionary algorithm, we predict two honeycomb (P1- and C2-type) structures of BSi with two-atom thickness. Phonon spectrum calculations show that both structures are thermodynamically stable. A weak covalent bonding interaction between B atoms along [001] direction plays a very important role in structure stability of P1- and C2-type BSi. P1-type BSi is semi-metallic, while C2-type exhibits a semiconductor band structure and has a good absorption of photons in the range of 1.5–2.5eV that is very promising for photoelectric applications. An e22 piezoelectric coefficient of 1.65×10−10Cm−1 is predicted for C2-type BSi, which makes it a potential candidate for piezoelectric applications at nano-scale. Moreover, three-phonon interactions areadoptedtoevaluate phonon lifetimes, phonon linewidths and lattice thermal conductivity. Although both P1- and C2-type BSi have a similar honeycomb structure and same elements, the maximum lattice thermal conductivity of P1-type BSi is around 6 times higher than that of C2-type BSi. Additionally, P1-type BSi exhibits remarkably higher phonon lifetimes than C2-type BSi.

Perovskite-based nanocubes with simultaneously improved visible-light absorption and charge separation enabling efficient photocatalytic CO2 reduction

Finding an ideal model to disclose the role upon tuning band structure and charge separation of wide-bandgap perovskite semiconductors by introducing suitable heteroatoms remains a huge challenge in photocatalysis. Herein, we propose an efficient pathway to increase the light absorption and charge separation for nitrogen and oxygen-vacancy confined in sodium tantalate nanocubes (Vo-NaTaON) and nitrogen-doped graphene quantum dots (N-GQDs) grafted Vo-NaTaON nanocubes by solution-etching-induced phase-transition and in-situ reduction strategies. First-principles calculations demonstrate that the simultaneous incorporation of nitrogen and oxygen-vacancy in sodium tantalate can effectively regulate the electronic structure of sodium tantalate. The analysis of UV–vis spectra and electron paramagnetic resonance reveal that the synergistic contribution of nitrogen and oxygen-vacancy endows the wide-bandgap perovskites tuning the band absorption region from UV (315nm) to visible regime beyond 600nm. As expected, an optimized Vo-NaTaON catalyst was developed, exhibiting superior broad spectrum photochemical reduction of CO2 to fuels. Moreover, N-GQDs/Vo-NaTaON heterojunctions further improve the broad spectrum CO2 photoreduction due to the synergetic catalytic effect of simultaneously improved light-absorption and charge separation. This work may open up more opportunities in the design of efficient photocatalysts for applications in solar photochemical conversion.

Stability and accuracy control of k · p parameters

The k · p method is a successful approach to obtain band structure, optical and transport properties of semiconductors and it depends on external parameters that are obtained either from experiments, tight binding or ab initio calculations. Despite the widespread use of the k · p method, a systematic analysis of the stability and the accuracy of its parameters is not usual in the literature. In this work, we report a theoretical framework to determine the k · p parameters from state-of-the-art hybrid density functional theory including spin–orbit coupling, providing a calculation where the gap and spin–orbit energy splitting are in agreement with the experimental values. The accuracy of the set of parameters is enhanced by fitting over several directions at once, minimizing the overall deviation from the original data. This strategy allows us to systematically evaluate the stability, preserving the accuracy of the parameters, providing a tool to determine optimal parameters for specific ranges around the Γ-point. To prove our concept, we investigate the zinc blende GaAs that shows results in excellent agreement with the most reliable data in the literature.

Magnetic properties of transition metal Mn, Fe and Co dimers on monolayer phosphorene

We studied the geometries, electronic structure and magnetic properties of substitutional doping and adsorption of transition metal (Mn, Fe and Co) dimers on phosphorene monolayer in the framework of the generalized gradient approximation (GGA) and GGA + U. Electronic band structures and magnetic properties were dependent on the doping type and dopant materials. For Mn and Fe substitutional and adsorption dimers, we obtained semiconducting band structures with spin polarization. However, we found a half-metallic feature in Co substitutional dimer while the Co adsorption dimer showed a semiconducting behavior without any spin polarization. With GGA + U, all the systems showed spin polarized semiconducting band structures except Co adsorption dimer which remained unaffected. The hybridization between transition metal (TM) and phosphorene sheet contributed to suppressing the magnetic moment of TM dimers. For instance, the total magnetic moments of −2.0, 4.24 and 1.28 μB/cell for Mn, Fe and Co substitutional dimers were obtained while the Mn and Fe adsorption dimers showed magnetic moments of −1.69 and 0.46 μB/cell. These magnetic moments were enhanced with GGA + U. The same magnetic ground states were obtained both from GGA and GGA + U approaches except for the Mn dimers. We observed that the Mn and Fe substitutional dimers showed an out-of-plane magnetization while an in-plane magnetization was observed in Co substitutional dimer. The Mn adsorption dimer still displayed a perpendicular magnetization whereas the Fe adsorption dimer had an in-plane magnetization. We found that the both GGA and GGA + U showed the same magnetization direction in all the systems.

Review of functional titanium oxides. I: TiO2 and its modifications

An extensive and wide-ranging literature about the polymorphs of titanium dioxide (TiO2) has accumulated during the last few decades, providing a very large resource of data on its properties, functionality and many present and potential industrial uses. This review focuses on the structural, kinetic, thermodynamic and electrical properties of TiO2 from the viewpoint of the relationship between the crystal structure and its present or potential useful functionality, via the electronic structure. The reason for this focus is the fundamental relationship between the electronic band structure of this wide band-gap semiconductor and its interaction with light and chemical species. Intense interest in the photoactivity of TiO2 followed the demonstration by Fujishima and Honda in 1972 of its ability to dissociate water using sunlight. Approaches to band gap engineering via chemical modifications are surveyed and correlated with band-structure calculations using Density Functional Theory and Hartree Fock methods. In the last section, progress in TiO2 applications and prospects for new applications of this material are summarised.

Crystal face regulating MoS2/TiO2(001) heterostructure for high photocatalytic activity

Semiconductor heterostructure is usually constructed to separate carriers, and the energy band structures of component semiconductors are mainly considered to achieve the separation. However, the interface of heterostructure also has a great influence on the transmission of carriers which is always neglected. Here, we employed {001} and {101} facets of anatase TiO2 as substrate to composite with two-dimension MoS2 and compared the photocatalytic activity. The results show that MoS2/TiO2(001) composite with optimal ratio demonstrates a large enhancement of photocatalytic activity as much as 3.4 times than that of pristine TiO2 nanosheets with exposed {001} facets. As compared, P25 with main exposed surface of anatase {101} facets was composited with MoS2, and the composite only exhibits 31% boost of photocatalytic activity than pristine P25. This contrast confirmed that {001} facets of anatase TiO2 are the key to construct carrier exchange heterostructure between MoS2 and TiO2 to enhance the photocatalytic activity. The HRTEM images show that the two-dimension MoS2 grown on the {001} facets of TiO2 nanosheets to show a face-to-face contact. This contact is an ideal transmission channel for photo-induce carriers with less scattering. Moreover, due to the higher valence band compared with {101} facets, the photo-induced holes will transfer to {001} facets and finally be extracted by MoS2 for photocatalysis degradation. The strategy that employs different crystal faces to regulate heterostructure interface can also be applied in other fields such as solar cell and electron device.