Valence Band Of Semiconductor Research Articles

To the theory of dimensional quantization in crystals in the Kane approximation

The microscopic mechanism of size quantization of the conduction band and valence band of semiconductors of cubic and tetrahedral symmetry has been theoretically studied in the Kane approximation. It is shown that the matrix Schrödinger equation obtained on the basis of the Kane model cannot be solved analytically for a potential well of an arbitrary profile. Therefore, expressions for the energy spectrum are obtained depending on the two-dimensional wave vector directed along the interface of the heterostructure for various cases that differ in the range of current carrier energy values.

Electrical Conductivities and Conduction Mechanism of Lithium-Doped High-Entropy Oxides at Different Temperature and Pressure Conditions.

Li-doped high-entropy oxides (Li-HEO) are promising electrode materials for Li-ion batteries. However, their electrical conduction in a wide range of temperatures and/or at high pressure is unknown, hindering their applications under extreme conditions. Especially, a clear understanding of the conduction mechanism is needed. In this work, we determined the carrier type of several Li-doped (MgCoNiCuZn)O semiconductor compounds and measured their electrical conduction at temperatures 79-773 K and/or at pressures up to 50 GPa. Three optical band gaps were uncovered from the UV-vis-NIR absorption measurements, unveiling the existence of defect energy levels near the valence band of p-type semiconductors. The Arrhenius-like plot of the electrical conductivity data revealed the electronic conduction in three temperature regions, i.e., the ionization region from 79 to 170 K, the extrinsic region from ∼170 to 300 K, and the intrinsic region at ≥300 K. The closeness of the determined electronic band gap and the second optical band gap suggests that the conduction electrons in the intrinsic region originate from a thermal excitation from the defect energy levels to the conduction band, which determines the electronic conductivity. It was also found that at or above room temperature, ionic conduction coexists with electronic conduction with a comparable magnitude at ambient pressure and that the intrinsic conduction mechanism also operates at high pressures. These findings provide us a fundamental understanding of the band structure and conduction mechanism of Li-HEO, which would be indispensable to their applications in new technical areas.

Bifunctional phosphate-modulated Cu2O/CeO2 redox heterojunction: A promising approach for proficient CO2 reduction

The construction of extremely dispersed redox-heterojunctions is a critical step toward improving of photocatalytic activity. Nonetheless, electron transfer faces momentous hurdles at the interfaces of these redox-heterojunction, mainly as a result of the considerable gap and potential-barrier between them. To address this obstacle, a bifunctional phosphate was deliberately introduced as an electron-bridge into the Cu2O/CeO2 redox-heterojunction system, aiming to necessitate a potential-barrier, establish a photoelectron transport route to improve the surface catalysis, and serve as a catalyst stabilizer, leading to improved dispersion of Cu2O on the CeO2 surface. The resultant intricately dispersed and optimized redox-heterojunction showcased an unparalleled advancement in apparent quantum efficiency (AQE), attaining an approximate value of 1.41 % at 420 nm during the photocatalytic CO2 reduction. The experimental findings established a conspicuous association between the substantial increase in AQE and the diminished interfacial migration resistance, heightened dispersion, and abundance of more active-sites, all accredited to phosphate-modulation in the redox-heterojunction. The work emphasizes the significant importance of phosphate in bridging the conduction and valence bands of semiconductors, leading to enhanced photocarrier separation efficiency and improved molecular dispersion.

Impact of CVD chemistry on band alignment at the MoS2/SiO2 interface

Reliable knowledge of energy alignment of electronic bands at interfaces of few layered Molybdenum Disulfide (MoS2) is crucial for designing MoS2 based electronic devices. In this work, we have applied internal photoemission spectroscopy (IPE) to characterize this band alignment. MoS2 films grown on SiO2(50 nm)/Siusing two different methods have been analyzed by IPE to determine the energy position of the semiconductor valence band (VB) relative to the reference level of the insulator conduction band (CB). We found that changing the MoS2 growth method from Vapor Phase Reaction (VPR) of MoO3 and sulfur, to Solid Precursor Film (SPF) of sulfurized AHM-NaOH spin coated MoS2 film, results in significant (≈600 meV) enhancement in the electron barrier. Such effects are ascribed to the interaction of hydroxy groups from NaOH and AHM promoters with the SiO2 surface leading to interface dipole modification.

Dice Selection in Hybrid Microcircuits For Aerospace Applications

Hybrid Microcircuits which bring about miniaturization with better performance and reliability are ideally suited for application specific low volume production requirement of space missions. A large number of HMCs are used in every spacecraft. HMCs generally use passive components and interconnections in thick film form, and active devices (MOSFETs, Transistor and Diode dice) and monolithic ICs (as bare chips) bonded to a substrate. Environmental radiation threat to ICs and other active circuitry is one of the major causes of failures in HMCs. The ionizing radiation present in the Van Allen Belt penetrates the satellite resulting in the excitation of electrons in the semiconductor valence band to conduction band in the active circuit and can be trapped. Secondary photon radiation, which are generated due to slowing down of high energy particles also strike interior of the spacecraft. The total interaction of various charged particles and photons to the internal circuitry of HMC depends on mass, charge state, kinetic energy of the striking photon as well as on the mass, charge and density of the die structure internal to HMC. The MOS die is prone to failure due to total ionization dose as well as single event effects which in turn may cause HMC failure. HMCs for space applications are mostly characterized with die passivation and hermetically sealed packages for its inherent advantages. In spite of the advantages offered by hermetic sealing like reliability, radiation shielding, out gassing etc, the radiation hardened dice are preferred for realization of HMCs for space systems. A feasibility study is carried out for using MIL and Commercial grade dice in HMCs without compromising on reliability aspects. The layout design, die attachment process, testing and extensive qualification become important in HMC apart from hermetic sealing so as to avoid catastrophic failure. This article details the radiation phenomena and its effects on dice used in HMCs. The advantages of hermetically sealed packages and use of MIL/Commercial grade dice in HMCs along with various mitigation techniques to have reliable circuit meeting the desired performance in space environment are also presented.

Band alignment study at the SrTaO2N / H2O interface varying lattice constants and surface termination from first-principles calculations

In the search for new renewable energy to replace fossil fuels, Hydrogen is one of the most promising candidates for clean energy production. But cheap Hydrogen separation and storage is still a big challenge. Photoelectrochemical devices look promising for the decomposition of the water molecule into 2H2 + O2. Every day new materials and combinations are discovered or invented to improve the efficiency of the complex total process. A necessary condition for the photoelectrochemical process to work without a bias voltage is that the minimum of the semiconductor conduction band must be more positive than the reduction potential H+ to H2, whereas the maximum of the semiconductor valence band must be more negative than the oxidation potential of H2O to O2. Thus, band alignment studies in interfaces of semiconductors with water become of vital importance.In this work, first-principles calculations based on density–functional theory (DFT) in the all-electron and the pseudo-potential approaches have been performed for the analysis of the band alignment in SrTaO2N/H2O interfaces. Different surface orientations were analyzed, together with the dependence of the gap and band alignment with lattice constants for systems grown on mismatched substrates. Water structures were built from classical molecular dynamics and its electronic structure calculated using DFT. The calculations show that the SrTaO2N (001) is suitable for photoelectrochemical applications on a wide range of lattice constant a, except for a compression/elongation of −2%, −1% and 3%, while SrTaO2N (110) results suitable for photoelectrochemical devices over a wider range of lattice constants from −1% to 3%.

Prediction of bipolar VSi2As4 and VGe2As4 monolayers with high Curie temperature and strong magnetocrystalline anisotropy

Recent studies have demonstrated that two-dimensional intrinsic magnetic materials with high Curie temperature $({T}_{\text{c}})$ and large magnetic anisotropy energy (MAE) are highly desirable for future spintronics. After careful investigations through density functional theory, bipolar ${\mathrm{VSi}}_{2}{\mathrm{As}}_{4}$ and ${\mathrm{VGe}}_{2}{\mathrm{As}}_{4}$ monolayers, with semiconductor valence and conduction band edges fully spin polarized in different spin directions, are demonstrated to be highly stable and have in-plane ferromagnetism (FM) with large MAE of $\ensuremath{\sim}5.5$ meV. The FM interaction is found to be dominated by the superexchange between d orbitals of V atoms through p orbitals of anions. More interestingly, ${T}_{\text{c}}$ is estimated to be $\ensuremath{\sim}900$ K through Monte Carlo simulation, which is significantly higher than room temperature. In addition, both MAE and ${T}_{\text{c}}$ can be substantially regulated and increased under biaxial strain and the transition from FM semiconductors to metals will occur. Our calculations and analyses indicate that ${\mathrm{VSi}}_{2}{\mathrm{As}}_{4}$ and ${\mathrm{VGe}}_{2}{\mathrm{As}}_{4}$ monolayers are ideal systems for the fundamental understanding of magnetic physics as well as building blocks for magnetoelastic applications, high-temperature, or/and gate-tunable spintronic nanodevices.

P-Type Ohmic Contact to Monolayer WSe2 Field-Effect Transistors Using High-Electron Affinity Amorphous MoO3

Monolayer tungsten diselenide (1L-WSe2) has been widely used for studying emergent physics due to the unique properties of its valence bands. However, electrical transport studies have been impeded by the lack of a reliable method to realize Ohmic hole-conducting contacts to 1L-WSe2 especially at low carrier densities and low temperatures. Here, we report low-temperature p-type Ohmic contact to 1L-WSe2 field-effect transistors at carrier densities (n) below n = 1 × 1012 cm–2 with negligible temperature dependence down to the lowest measured temperature (10 K). The non-rectifying barrier is achieved between 1L-WSe2 and molybdenum trioxide (MoO3), where 1L-WSe2 underneath MoO3 is heavily hole-doped through surface transfer doping. Electrical transport measurements reveal linear current–voltage relations at a temperature of 10 K and carrier densities from n = 7.7 × 1011 cm–2 to below the threshold. The finding is also supported by nearly temperature-independent output curves up to room temperature and a negligible contact barrier down to the subthreshold regime. Furthermore, the contact resistivity of MoO3-contacted 1L-WSe2 FET is 30.2–64.8 kΩ μm at n = 1.5 × 1012 cm–2, which is the lowest reported for 1L-WSe2 FETs at such low carrier density. Realizing robust p-type Ohmic contact to a 2D transition metal dichalcogenide semiconductor will enable direct electronic measurements of quantum transport in correlated phases in the valence bands of monolayer semiconductors.

Synthesis and applications of TiO2/ZnO hybrid nanostructures by ZnO deposition on TiO2 nanotubes using electrochemical processes

Abstract In recent years, TiO2/ZnO hybrid nanostructures have been attracting the interest of the scientific community due to their excellent photoelectrochemical properties. The main advantage of TiO2/ZnO hybrid nanostructures over other photocatalysts based on semiconductor materials lies in their ability to form heterojunctions in which the valence and conduction bands of both semiconductors are intercalated. This factor produces a decrease in the band gap and the recombination rate and an increase in the light absorption range. The aim of this review is to perform a revision of the main methods to synthesise TiO2/ZnO hybrid nanostructures by ZnO deposition on TiO2 nanotubes using electrochemical processes. Electrochemical synthesis methods provide an easy, fast, and highly efficient route to carry out the synthesis of nanostructures such as nanowires, nanorods, nanotubes, etc. They allow us to control the stoichiometry, thickness and structure mainly by controlling the voltage, time, temperature, composition of the electrolyte, and concentration of monomers. In addition, a study of the most promising applications for TiO2/ZnO hybrid nanostructures has been carried out. In this review, the applications of dye-sensitised solar cell, photoelectrocatalytic degradation of organic compounds, photoelectrochemical water splitting, gas sensors, and lithium-ion batteries have been highlighted.

Enhanced interfacial electron transfer between semiconductor and non-photosynthetic microorganism under visible light

Under visible light, non-photosynthetic microorganism/semiconductor has recently shown promising applications in biofuel production, bioenergy generation and pollutant removal. However, the understanding of electron transfer mechanism at the biotic-abiotic interface is limited. Herein, mixed culture/carbon nitride and pure culture (Stenotrophomonas maltophilia HP14)/carbon nitride were constructed to reveal the energy conversion pathway under visible light. Enhanced electron transfer was observed on the biotic-abiotic interface. The photocurrent of non-photosynthetic S. maltophilia HP14/g-C3N4/ITO with riboflavin addition was 10.2-fold higher than that of g-C3N4/ITO control. As the electron donor, S. maltophilia HP14 transfer bio-electrons to self-secreted riboflavin and valence band of semiconductor. Riboflavin (RF) accept bio-electrons and photogenerated electrons to form 1,5-dihydroflavin (RFH2). While RFH2 is able to transfer electrons to the valence band of carbon nitride and transform into RF again. However, riboflavin is not stable under visible light irradiation, which may lead to a decreased photocurrent. This study demonstrated the role of redox active compounds at the microorganism/semiconductor interface and described a possibility for non-photosynthetic microorganisms to capture and utilize visible light.

Studying VOC in lead free inorganic perovskite photovoltaics by tuning energy bandgap and defect density

In hybrid perovskite photovoltaics (hPPV) highest reported efficiency is ∼25.2%. However, stability and toxicity are major concern towards commercialization. To improve stability, various approaches were made by research communities. Utilizing inorganic perovskite photoactive materials is one such approach to improve stability. Toxic element ‘Pb’ can be substituted with suitable non-toxic elements such as ‘Sn’ without sacrificing efficiency and stability. In CsPbI3, replacing Sn in place of Pb results in the PCE of ∼13%. However, efficiency of this PV is lagging 12% as compared with hybrid perovskite PV. This difference should be overcome to compete with hPPV. To improve the efficiency of inorganic PPV various experimental strategies were adopted. In this study, SCAPS simulation was carried out on FTO/TiO2/CsSnI3/spiro-OMeTAD/Au by altering Eg, energetic position of conduction and valence band, defect density, and thickness of semiconductor. Obtained results were analyzed and reported in this work.

Boosting Hydrogen Evolution at Visible Light Wavelengths by Using a Photocathode with Modal Strong Coupling between Plasmons and a Fabry-Pérot Nanocavity.

Hot‐hole injection from plasmonic metal nanoparticles to the valence band of p‐type semiconductors and reduction by hot electrons should be improved for efficient and tuneable reduction to obtain beneficial chemical compounds. We employed the concept of modal strong coupling between plasmons and a Fabry‐Pérot (FP) nanocavity to enhance the hot‐hole injection efficiency. We fabricated a photocathode composed of gold nanoparticles (Au−NPs), p‐type nickel oxide (NiO), and a platinum film (Pt film) (ANP). The ANP structure absorbs visible light over a broad wavelength range from 500 nm to 850 nm via hybrid modes based on the modal strong coupling between the plasmons of Au−NPs and the FP nanocavity of NiO on a Pt film. All wavelength regions of the hybrid modes of the modal strong coupling system promoted hot‐hole injection from the Au−NPs to NiO and proton/water reduction by hot electrons. The incident photon‐to‐current efficiency based on H2 evolution through water/proton reduction by hot electrons reached 0.2 % at 650 nm and 0.04 % at 800 nm.

Structure and Energetics of Dye-Sensitized NiO Interfaces in Water from Ab Initio MD and Large-Scale GW Calculations

The energy-level alignment across solvated molecule/semiconductor interfaces is a crucial property for the correct functioning of dye-sensitized photoelectrodes, where, following the absorption of solar light, a cascade of interfacial hole/electron transfer processes has to efficiently take place. In light of the difficulty of performing X-ray photoelectron spectroscopy measurements at the molecule/solvent/metal-oxide interface, being able to accurately predict the level alignment by first-principles calculations on realistic structural models would represent an important step toward the optimization of the device. In this respect, dye/NiO surfaces, employed in p-type dye-sensitized solar cells, are undoubtedly challenging for ab initio methods and, also for this reason, much less investigated than the n-type dye/TiO2 counterpart. Here, we consider the C343-sensitized NiO surface in water and combine ab initio molecular dynamics (AIMD) simulations with GW (G0W0) calculations, performed along the MD trajectory to reliably describe the structure and energetics of the interface when explicit solvation and finite temperature effects are accounted for. We show that the differential perturbative correction on the NiO and molecule states obtained at the GW level is mandatory to recover the correct (physical) interfacial energetics, allowing hole transfer from the semiconductor valence band to the highest occupied molecular orbital (HOMO) of the dye. Moreover, the calculated average driving force quantitatively agrees with the experimental estimate.

К ТЕОРИИ ДВУХ - И ТРЕХ ФОТОННОГО ЛИНЕЙНО - ЦИРКУЛЯРНОГО ДИХРОИЗМА В ПОЛУПРОВОДНИКАХ КУБИЧЕСКОЙ СИММЕТРИИ

The linear - circular dichroism of two and three photon absorption of light in semiconductors of cubic symmetry of hole conductivity is theoretically investigated. The matrix elements of two and three - photon optical transitions occurring between the subbands of the semiconductor valence band are calculated. In this case, transitions associated with both non - simultaneous absorption of individual photons and simultaneous absorption of two photons are taken into account, and the spectral and temperature dependences of the coefficient of two and three - photon absorption of polarized radiation are determined

Two- and Three-Photon Linear Circular Dichroism in Semiconductors of Cubic Symmetry

The linear-circular dichroism of two- and three-photon absorption of light in semiconductors of hole conductivity with cubic symmetry is theoretically investigated. The matrix elements of two- and three-photon optical transitions occurring between the subbands of the semiconductor valence band are calculated. Transitions associated with both non-simultaneous absorption of individual photons and simultaneous absorption of two photons are taken into account, and the spectral and temperature dependences of the coefficients of two- and three-photon absorption of polarized radiation are determined.

Schottky barrier modification of GaSSe/graphene heterojunctions based on density functional theory

Constructing van der Waals heterojunctions is an effective way to solve the defects of two-dimensional (2D) material properties. The crystal structure and electronic properties of 2D monolayer GaSSe have been studied using first-principles calculation. Based on density functional theory calculations, we designed two different GaSSe/graphene heterojunctions and computed their electrical and interfacial properties. The calculated results confirm that graphene and GaSSe can maintain the original band structure after compounding, and the interaction between the two layers shows a weak van der Waals effect. The energy band gap of graphene can be opened to 25 meV when the GaSSe/graphene heterojunctions are formed. Graphene can be used as an electrode, and electrons will flow from the graphene layer to the GaSSe channel. Moreover, the interlayer distance and the applied electric field can be adjusted to modulate the Schottky barrier height and the Schottky contact type (n-type and p-type) of GaSSe/graphene heterojunctions. The two different heterojunctions can achieve Schottky contact-type conversion at a layer spacing of 3.00 and 2.70 Å, respectively. The position of the Dirac point of GaSSe/graphene heterojunctions will move with the change of the applied electric field. The Dirac point gradually moves towards the semiconductor valence band as the forward electric field of the heterojunctions increases. In addition, the n-type contact formed at the interface of the GaSSe/graphene heterojunctions will be converted into an Ohmic contact when the reverse electric field is about 0.5 eV Å−1. All the theoretical results clarify the fundamental properties of GaSSe/graphene heterojunctions and predict that the GaSSe/graphene heterojunctions can be used to design high-performance field effect transistor devices.

Two- and Three-Photon Linear-Circular Dichroism in Cubic-Symmetry Semiconductors

The linear-circular dichroism of two- and three-photon absorption in p-type cubic-symmetry semiconductors is theoretically studied. The matrix elements of two- and three-photon optical transitions between subbands of the semiconductor valence band are calculated. In this case, transitions associated with both the non-simultaneous absorption of individual photons and the simultaneous absorption of two photons are taken into account. The spectral and temperature dependences of the coefficient of the two- and three-photon absorption of polarized radiation are determined.

Conductivity and Photoconductivity of a p-Type Organic Semiconductor under Ultrastrong Coupling.

During the past decade, it has been shown that light-matter strong coupling of materials can lead to modified and often improved properties which has stimulated considerable interest. While charge transport can be enhanced in n-type organic semiconductors by coupling the electronic transition and thereby splitting the conduction band into polaritonic states, it is not clear whether the same process can also influence carrier transport in the valence band of p-type semiconductors. Here we demonstrate that it is indeed possible to enhance both the conductivity and photoconductivity of a p-type semiconductor rr-P3HT that is ultrastrongly coupled to plasmonic modes. It is due to the hybrid light-matter character of the virtual polaritonic excitations affecting the linear response of the material. Furthermore, in addition to being enhanced, the photoconductivity of rr-P3HT shows a modified spectral response due to the formation of the hybrid polaritonic states. This illustrates the potential of engineering the vacuum electromagnetic environment to improve the optoelectronic properties of organic materials.

Pentacene and tetracene molecules and films on H/Si(111): level alignment from hybrid density functional theory

The electronic properties of hybrid organic–inorganic semiconductor interfaces depend strongly on the alignment of the electronic carrier levels in the organic/inorganic components. In the present work, we address this energy level alignment from first principles theory for two paradigmatic organic–inorganic semiconductor interfaces, the singlet fission materials tetracene and pentacene on H/Si(111), using all-electron density functional theory calculations with a hybrid exchange–correlation functional. For isolated tetracene on H/Si(111), a type I-like heterojunction (lowest-energy electron and hole states on Si) is found. For isolated pentacene, the molecular and semiconductor valence band edges are degenerate. For monolayer films, we show how to construct supercell geometries with up to 1192 atoms, which minimize the strain between the inorganic surface and an organic monolayer film. Based on these models, we predict the formation of type II heterojunctions (electron states on Si, hole-like states on the organic species) for both acenes, indicating that charge separation at the interface between the organic and inorganic components is favored. The paper discusses the steps needed to find appropriate low-energy interface geometries for weakly bonded organic molecules and films on inorganic substrates from first principles, a necessary prerequisite for any computational level alignment prediction.

A novel type-II Bi2W2O9/g-C3N4 heterojunction with enhanced photocatalytic performance under simulated solar irradiation

In this work, the Bi2W2O9/g-C3N4 type-II heterojunction system has been prepared for the first time by an incipient impregnation method. From the results, we have stated that the addition of low amounts of Bi2W2O9 (≤5 wt%) does not provide changes in the structural and textural properties of g-C3N4, but it improves substantially the photocatalytic performance under simulated solar irradiation. The best photoactive behavior was attained in the sample with 2 wt% of Bi2W2O9, providing a removal degree of the tetracycline antibiotic about 2.6 times higher than the pristine g-C3N4. This result can be explained due to the difference in the edge positions of the valence and conduction bands of both semiconductors, which provides an efficient separation of the photogenerated charge carriers. From the study of the photocatalytic activity under several conditions, it can be inferred that the Bi2W2O9 works as a sink for the photoexcited electrons of the g-C3N4 and provides the formation of hydroxyl radicals through sequential reactions involving the superoxide ions. Furthermore, a possible photocatalytic mechanism of tetracycline removal is discussed.