Calculated Band Offsets Research Articles

Rationally designed C3N4/ TiO2 (anatase/brookite) heterojunction for enhanced photocatalytic hydrogen generation under visible light

In this study, C3N4 was prepared using the polycondensation method, and TiO2 (ansate/Brookite) was synthesized using the solvothermal method. These materials were subsequently utilized to prepare C3N4/TiO2 heterojunction materials via the photoanchoring method, with the aim of improving photocatalytic hydrogen production. The physical, chemical, and optical properties of the composites were investigated to verify the formation of heterojunctions, as well as to assess the impact of varying levels of C3N4 content (1%, 5%, and 10%) in the C3N4/TiO2 composite on hydrogen production. Notably, the composite with 5% C3N4 demonstrated superior photocatalytic hydrogen production (approximately 692 μmol h-1 g-1), and the underlying reasons were elucidated using photoelectrochemical characterization. To establish the band alignment of C3N4 and TiO2 before and after contact, a comprehensive array of techniques was employed, encompassing Kelvin force microscopy to acquire work functions, UV–Vis spectral analysis to ascertain band gaps, XPS valence spectra to identify the Valence Band Maxima, and the Kraut method to calculate Band Offsets. These analyses revealed that the formation of the heterojunction is staggered in nature. Finally, utilization of ESR analysis has conclusively verified that the charge transfer mechanism inherent in the C3N4/TiO2 heterojunction adheres to the Z-scheme.

Slab surface passivation and its application to band offset calculations for polar heterointerfaces of zinc-blende semiconductors

Slab surface passivation and its application to band offset calculations for polar heterointerfaces of zinc-blende semiconductors

A comparative analysis of ZnCdTe and ZnCdTeO semiconductor alloys as competent materials for optoelectronic applications

In this paper, we report the comparative study of some parameters of II–VI ternary alloy ZnCdTe and II–VI–O dilute oxide ZnCdTeO. The purpose of this comparative study is to establish both the ternary and quaternary alloys as superior materials for optoelectronic and solar cell applications in which the quaternary materials show more superiority than the ternary material. In this purpose, we take the data from the experiments previously done and published in renowned journals and books. The parameters of these alloys are mainly being calculated using Vegard’s law and interpolation method of those collected data. It was certainly demonstrated that the incorporation of O atoms produces a high bandgap ([Formula: see text]) reduction in host ZnCdTe (Zn[Formula: see text]CdxTe) in comparison to the bandgap reduction in host ZnTe material with Cd incorporation. The bandgap of ZnCdTeO (Zn[Formula: see text]CdxTe[Formula: see text]Oy) was found to be reduced to 1.1357 at [Formula: see text] and the spin–orbit splitting energy ([Formula: see text]) value of ZnCdTeO was calculated to be 1.175[Formula: see text]eV for Cd concentration of 0.5[Formula: see text]mole and O concentration of 0.1[Formula: see text]mole both of which showed excellent results with the prospect of optoelectronic and solar cell applications. The constant rise in the spin–orbit curve signifies a very less internal carrier recombination which decreases the leakage current and augments the efficiency of solar cell. The lattice constants and strain calculation values give very good results and confirm the stability of the materials. Besides, the calculated band offsets values show that for ZnCdTeO, there is higher bandgap reduction than that of ZnCdTe. Moreover, ZnCdTeO covers a wide range of wavelength in the visible region starting from violet region at 393[Formula: see text]nm upto red region at 601[Formula: see text]nm. Both ZnCdTe and ZnCdTeO are found to have excellent applications in optoelectronic and solar cell devices though quaternary ZnCdTeO proves much supremacy over ternary ZnCdTe in all aspects of the properties.

Optimized Sample Adaptive Offset Filter in HEVC

The high-efficiency video coding (HEVC) standard incorporates sample adaptive offset (SAO) as an in-loop filtering technique to reduce distortion. The fundamental tenet of SAO is to classify every reconstructed pixel and then simply add the offset to each group of pixels. The previous existing technique employs 32 bands in band offset (BO) calculation to determine the proper offset value to minimize the distortion in the SAO estimate algorithm for statistics collection and parametric determination procedure. The suggested method will reduce the necessary band count by 16 bands. It does not alter how edge offset is determined in any way. It offers enough and efficient room for creating superior band group predictions and selections, as well as an ideal decrease in the implemented area. Thus, reducing the number of bands to 16 also decreases the amount of storage needed to hold each pixel’s information in binary files by up to 0.65%.

First-principles study of oxygen vacancy defects in orthorhombic Hf0.5Zr0.5O2/SiO2/Si gate stack

The gate defect of the ferroelectric HfO2-based Si field-effect transistor (Si FeFET) plays a dominant role in its reliability issue. The first-principles calculations are an effective method for the atomic-scale understanding of gate defects. However, the first-principles study on the defects of FeFET gate stacks, i.e., the metal/orthorhombic-Hf0.5Zr0.5O2/SiOx/Si structure, has not been reported so far. The key challenge is the construction of metal/orthorhombic-Hf0.5Zr0.5O2/SiOx/Si gate stack models. Here, the atomic structure and defect property of orthorhombic-Hf0.5Zr0.5O2/SiO2/Si gate stack are systematically studied by first-principles calculations. We use the Hf0.5Zr0.5O2(130) high-index crystal face as the orthorhombic ferroelectric layer and construct a robust atomic structure of the orthorhombic-Hf0.5Zr0.5O2/SiO2/Si gate stack without any gap states based on the electron counting rule. The calculated band offsets show that this gate structure is of type-I band alignment. Furthermore, the formation energies and charge transition levels (CTLs) of defects reveal that the oxygen vacancy defects are more favorable to form compared with other defects such as oxygen interstitial and Hf/Zr vacancy, and their CTLs are mainly localized near the Si conduction band minimum and valence band maximum, in agreement with the reported experimental results. The oxygen vacancy defects are responsible for charge trapping/de-trapping behavior in Si FeFET. This work provides an insight into gate defects and paves the way to carry out the first-principles study of ferroelectric HfO2-based Si FeFET.

Overcoming the poor performance of n-CdS/p-SnS solar cells by plasmonic effect of gold and silver nanoparticles

It is known that one of the biggest problems in CdS/SnS solar cells is the efficiency deterioration, which hinders their wider use in application fields. Consequently, in this research, we manipulated the poor photovoltaic performance of CdS/SnS solar cells by the plasmonic effect of silver and gold nanoparticles of particle size of about 7.1 ± 1.4 nm, and 10.01 ± 2.2 nm, respectively. The structural and morphological properties of each layer of the CdS/SnS heterojunction are inspected individually and in a bilayer form with and without nanoparticles (NPs) decoration. The EDAX analysis and surface roughness analysis are evaluated in detail. Furthermore, the measured optical absorption and reflection profile witnessed significant enhancements where the absorption is increased by 235.9% and 835.6% at 550 nm by Au-NPs and Ag-NPs decoration, respectively. The electrical characteristics of the engineered CdS/SnS heterojunction are tested under dark and different irradiances (20−100) mW/cm2. The decoration by the plasmonic nanoparticles induced marvelous improvements in the Voc and Jsc values. Excitingly, the Au-NPs improved the poor efficiency of the virgin CdS/SnS solar cell from (0.022–0.066) % by one order of magnitude to be in the range (0.436–0.51) %, while Ag-NPs decorated cell achieved efficiency in the range (2.507–4.363) %. This new record in CdS/SnS efficiencies is explained in terms of the heterojunction type transformation using the calculated band offsets depending on the resulted values of valance band maximum of CdS and SnS and the energy difference of core levels in the heterojunction form using the photoelectron spectroscopy results.

Direct calculations on the band offsets of large-lattice-mismatched and heterovalent Si and III–V semiconductors

Band offset in semiconductors is a fundamental physical quantity that determines the performance of optoelectronic devices. However, the current method of calculating band offset is difficult to apply directly to the large-lattice-mismatched and heterovalent semiconductors because of the existing electric field and large strain at the interfaces. Here, we proposed a modified method to calculate band offsets for such systems, in which the core energy level shifts caused by heterovalent effects and lattice mismatch are estimated by interface reconstruction and the insertion of unidirectional strain structures as transitions, respectively. Taking the Si and III–V systems as examples, the results have the same accuracy as what is a widely used method for small-lattice-mismatched systems, and are much closer to the experimental values for the large-lattice-mismatched and heterovalent systems. Furthermore, by systematically studying the heterojunctions of Si and III–V semiconductors along different directions, it is found that the band offsets of Si/InAs and Si/InSb systems in [100], [110] and [111] directions belong to the type I, and could be beneficial for silicon-based luminescence performance. Our study offers a more reliable and direct method for calculating band offsets of large-lattice-mismatched and heterovalent semiconductors, and could provide theoretical support for the design of the high-performance silicon-based light sources.

In2O3−Ga2O3 Alloys as Potential Buffer Layers in CdTe Thin-Film Solar Cells

The efficiency of state-of-the-art $\mathrm{Cd}\mathrm{Te}$ solar cells remains limited by the relatively low open-circuit voltage (${V}_{OC}$). Improving the front interface is key towards realizing a higher ${V}_{OC}$ after achieving the necessary bulk carrier density and lifetime. Recent efforts in identifying buffer layers beyond $\mathrm{Cd}\mathrm{S}$ have focused on ${\mathrm{Mg}}_{x}{\mathrm{Zn}}_{1\ensuremath{-}x}\mathrm{O}$, which offers tunability of the band offsets, but often suffers from high interfacial defect densities. ${\mathrm{Ga}}_{2}{\mathrm{O}}_{3}$-based buffer layers demonstrate tremendous improvements in interfacial defect passivation in crystalline silicon and dye-sensitized solar cells, leading to record high ${V}_{OC}$, yet remain largely unexplored in $\mathrm{Cd}\mathrm{Te}$-based devices. Here, we perform hybrid density-functional-theory calculations to investigate pure ${\mathrm{Ga}}_{2}{\mathrm{O}}_{3}$ and ${\mathrm{In}\mathrm{Ga}\mathrm{O}}_{3}$ alloys as a window layer in $\mathrm{Cd}\mathrm{Te}$ photovoltaics. We report calculated band offsets for several pairs of solid-solid interfaces comprising transparent conducting oxide (TCO) and $\mathrm{Cd}\mathrm{Te}$ heterojunctions. The results support a large conduction band offset spike of 0.67 eV for the $\mathrm{Cd}\mathrm{Te}/{\mathrm{Ga}}_{2}{\mathrm{O}}_{3}$ (100) interface, while the offset is reduced to 0.18 eV for the ${\mathrm{In}\mathrm{Ga}\mathrm{O}}_{3}$ alloy and matches closely with the preferred optimum value of 0.2 eV. Device-level modeling tests of $\mathrm{Cd}\mathrm{Te}$ solar cells integrating our results indicate that the highest efficiency is achieved with ${\mathrm{In}\mathrm{Ga}\mathrm{O}}_{3}$ acting both as a buffer layer and TCO. Our results suggest that alloys of ${\mathrm{In}}_{2}{\mathrm{O}}_{3}$ and ${\mathrm{Ga}}_{2}{\mathrm{O}}_{3}$ may be attractive alternatives to ${\mathrm{Mg}}_{x}{\mathrm{Zn}}_{1\ensuremath{-}x}\mathrm{O}$ for tailoring optimal conduction-band offsets of the buffer and TCO layers in high-efficiency $\mathrm{Cd}\mathrm{Te}$ thin-film solar cells.

Exploring the mechanism of Ta3N5/KTaO3 photocatalyst for overall water splitting by first-principles calculations

Abstract The rational fabrication of heterostructures is one of efficient strategies for improving photocatalytic performance of semiconductor photocatalysts. Very recently, Domen and co-workers found that Ta3N5 single crystals grown on the surface of KTaO3 can accomplish photocatalytic overall water splitting for the first time. In order to comprehend the underlying mechanism of this photocatalytic system, we have performed a systematic study based on density functional theory first-principles calculations. Ta3N5(010)/KTaO3(110) slab models have been built according to experimental observations by considering two common terminations of KTaO3(110) surface, named as Ta3N5/O2 and Ta3N5/KTaO. The formations of interfacial bonds are thermodynamically stable, showing a covalent interaction between two components of a heterostructure. Ta3N5/O2 has a higher mobility of photogenerated charge carriers and lower recombination rate of charge carriers than Ta3N5/KTaO. The light absorption of heterostructures displays the feature of KTaO3 in the short wavelength region and the characteristic of Ta3N5 in the long wavelength region. The calculated band offsets show that Ta3N5/O2 and Ta3N5/KTaO have distinct Type-II band alignments, with Ta3N5 as the accumulator of photoinduced electrons in the former and the collector of photogenerated holes in the latter, respectively. The difference in charge density and electrostatic potential between two components acts as a driving force to promote the transfer of electrons and holes to different domains of the interface, which is beneficial to extend the lifetime of photoinduced carriers. Our results demonstrate that the function of Ta3N5 in Ta3N5/KTaO3 photocatalytic system is determined by the termination property of KTaO3(110) surface, which provides a likely reason of the observed photocatalytic activity of overall water splitting achieved by Ta3N5 synthesized by using KTaO3 as a precursor for the nitridation reaction.

A density functional theory study on the interface stability between CsPbBr3 and CuI

This paper assesses the interface stability of the perovskite CsPbBr3 and transport layer CuI using density functional theory and band offset calculations. As a low-cost, more stable alternative to current hole transport materials, CuI may be used to template the epitaxial growth of perovskites such as CsPbBr3 owing to a 1% lattice constant mismatch and larger bulk modulus. We compare all eight atomic terminations of the interfaces between the (100) low-energy facet for both CsPbBr3 and CuI, increasing material thickness to consider charge density redistribution and bonding characteristics between surface and bulk-like regions. A low energy atomic termination is found to exist between these materials where alternating charge accumulation and depletion regions stabilize bonds at the interface. Band offset calculations reveal a type I straddling gap offset in the bulk shifting to a type II staggered gap offset as the thickness of the materials is increased, where the built-in potential changes as layer thickness increases, indicating the tunability of charge separation at the interface. CuI may, thus, be used as an alternative hole transport layer material in CsPbBr3 optoelectronic devices.

Band alignment at the CaF2/Si(111) interface through advanced electronic structure calculations

We determine the band alignment at the ${\mathrm{CaF}}_{2}/\mathrm{Si}(111)$ interface through various advanced electronic structure methods. This interface is experimentally well studied and serves as an ideal test case to examine the accuracy of theoretical schemes. We use both global and range-separated hybrid functionals as well as $GW$ calculations including self-consistency and vertex corrections. Our calculation procedure accounts for residual strain resulting from the small mismatch in the lateral lattice constants at the interface to minimize the systematic error in the comparison with experiment. Both the hybrid functional and the $GW$ schemes give band alignments in overall good agreement with the experimental characterization. However, the considered methods yield sizable variations in the calculated band offsets, which do not originate from incorrect evaluations of the band gaps but rather from different inherent relative positions of the band edges. The comparison with experiment reveals that the global hybrid functional and the quasiparticle self-consistent $GW$ with vertex corrections give the most accurate description of the band alignment. We then determine the variation of the band offsets as a function of the amount of excess fluorine at the interface and attribute the experimental spread in the measured offsets to uncontrolled fluorine contamination.

Hybrid band offset calculation for heterojunction interfaces between disparate semiconductors

Accurate band offset calculations are challenging for heterojunction interfaces that consist of two very different host materials. For this, the key requirement is to have the correct bandgap of each material at the same time. A hybrid calculation scheme (HSE/-U scheme) is proposed to model the band offsets of such interfaces. Our HSE/-U method applies the hybrid functional for the whole interface supercell, but with an additional “reverse GGA+U” on the narrow gap semiconductor side, guaranteeing the correct bandgaps on both sides. Several supercell calculations of dielectric films including HfO2, ZrO2, Al2O3, TiO2, and GaN on an insulating phase VO2 are tested to verify it. All the studied oxides show the type-I band alignment with VO2, and the band edge line-up agrees well with the available experimental reports, supporting the reliability of the proposed hybrid calculation scheme.

First Principles Calculation of Band Offsets and Defect Energy Levels in Al₂O₃/β-Ga₂O₃ Interface Structures with Point Defects

The β -Ga₂O₃ (beta-gallium oxide) is one of the most promising candidate materials for the future power and RF (radio frequency) devices. The Al₂O₃ (aluminum oxide) is widely adopted as the gate dielectric layer, which is mandatory for developing the Metal-Oxide-Semiconductor Field Effect Transistor (MOSFET) based on Ga₂O₃. Therefore, the theoretical investigation on the properties of the Al₂O₃/ β -Ga O interface is required. We have generated several atomistic models for the Al₂O₃ / β -Ga₂O₃ interface, whose structural parameters are consistent with the experimental findings. From the electronic band structure based on the density-functional theory (DFT) calculation, it is confirmed that the generated interface structures are physically stable. The band offsets between the Ga₂O₃ channel and the Al₂O₃ dielectric layer are calculated for various interface structures. For both the conduction band and the valence band, the Al₂O₃ / β -Ga₂O₃ interface provides sufficiently large band offsets, which makes it attractive for the MOSFET application. The point defects in the interface structure play a critical role in determining the device characteristics. The formation energies and transition levels of various point defects are calculated by using the DFT. It is found that the Ga-, O-, and Al- vacancies have the energy levels close to the conduction band minimum. It is expected that those vacancy defects act as the electron traps at the interface.

Ab-initio calculation of band alignments for opto-electronic simulations

A modified core-to-valence band maximum approach is applied to calculate band offsets of strained III/V semiconductor hetero junctions. The method is used for the analysis of (In,Ga)As/GaAs/Ga(As,Sb) multi-quantum well structures. The obtained offsets and the resulting bandstructure are used as input for the microscopic calculation of photoluminescence spectra yielding very good agreement with recent experimental results.

Band Offset Models of Three-Dimensionally Bonded Semiconductors and Insulators

The band offsets of heterojunctions of three-dimensionally (3D) bonded semiconductors lie between two limits, the electron affinity rule (unpinned limit) and the matching of the charge neutrality levels (CNLs), equivalent to the pinned limit. It is shown that it has been difficult to compare the validity of these cases because most experimental and theoretical tests require a lattice matching across the heterojunction, and most semiconductors with the same lattice constant have similar average band energies referred to the vacuum level. A second point is that the CNL when referenced to the vacuum level varies surprisingly weakly with the midgap energy with respect to the vacuum level. A calculation of band offsets for heterojunctions with larger lattice mismatch provides a stronger test, and this result is found to favor the CNL matching model. This result is important for many practical device heterojunctions, where unmatched interfaces are common.

Layer-resolved band bending at the n−SrTiO3(001)/p−Ge(001) interface

The electronic properties of epitaxial heterojunctions consisting of the prototypical perovskite oxide semiconductor, $n\ensuremath{-}{\mathrm{SrTiO}}_{3}$, and the high-mobility Group IV semiconductor $p$-Ge have been investigated. Hard x-ray photoelectron spectroscopy with a new method of analysis has been used to determine band alignment while at the same time quantifying a large built-in potential found to be present within the Ge. Accordingly, the built-in potential within the Ge has been mapped in a layer-resolved fashion. Electron transfer from donors in the $n\ensuremath{-}\mathrm{SrTi}{\mathrm{O}}_{3}$ to the $p$-Ge creates a space-charge region in the Ge resulting in downward band bending, which spans most of the Ge gap. This strong downward band bending facilitates visible light, photogenerated electron transfer from Ge to STO, favorable to drive the hydrogen evolution reaction associated with water splitting. Ti 2p and $\mathrm{Sr}\phantom{\rule{0.28em}{0ex}}3d$ core-level line shapes reveal that the STO bands are flat despite the space-charge layer therein. Inclusion of the effect of Ge band bending on band alignment is significant, amounting to a $\ensuremath{\sim}0.4\phantom{\rule{0.16em}{0ex}}\mathrm{eV}$ reduction in valence band offset compared to the value resulting from using spectra averaged over all layers. Density functional theory allows candidate interface structural models deduced from scanning transmission electron microscopy images to be simulated and structurally optimized. These structures are used to generate multislice simulations that reproduce the experimental images quite well. The calculated band offsets for these structures are in good agreement with experiment.

Accurate and efficient band-offset calculations from density functional theory

We perform a systematic study of the factors that determine the accuracy of heterostructure band alignments in density functional calculations. Band alignments are calculated for a number of representative test structures using either the generalized gradient approximation (GGA) or the more accurate hybrid functional of Heyd, Scuseria, and Ernzerhof (HSE). The alignment of the bulk band structures is achieved through a potential-alignment term from an explicit superlattice calculation, with full inclusion of atomic relaxations. The potential alignments calculated within the GGA are in agreement with those calculated using HSE to within 50 meV, despite the GGA calculations being 10–100 times less computationally expensive. The GGA potential alignments are remarkably accurate, even for heterostructures between materials with a large lattice mismatch, and even when the GGA lattice parameters are markedly different from HSE. We also find that the errors in the potential alignment are small even when the optimum HSE mixing parameter (α) (to bring the band gap in agreement with experiment) is markedly different for the two materials at the interface; the error associated with using a fixed α for a superlattice calculation is small. To obtain band offsets, the potential-alignment values are combined with bulk band structure calculations. GGA exhibits large errors in the positioning of the eigenvalues at the band extrema with respect to the average electrostatic potential, and the resulting all-GGA band alignments are unreliable. HSE produces much more accurate band structures than GGA. Given the accuracy of potential alignments calculated within the GGA, they can be combined with HSE calculations of the bulk electronic band structure to achieve accurate band alignments with a high computational efficiency.

The band offset barrier and optical properties calculation of Co2VGa/GaAs(001) interfaces: A DFT study

The structural, electronic, optical properties and band offsets of Co2VGa/GaAs(001) interfaces are discussed within the framework of density functional theory (DFT) using the FP-LAPW method, and the exchange-correlation potential is approximated by GGA. All interface structures are stable in the energy point of view, however the V–Ga/As case is found to be more stable than the others. A remarkable potential difference ([Formula: see text]V) appeared in all the interfaces, so the Co2VGa/GaAs(001) interfaces are good candidates for electron injection. In all the cases, there is no full spin polarization at the Fermi level, but high CBO and [Formula: see text] coefficients make them promising candidates for spin injection in the transport devices. Optical studies confirm the high metallic treatment of these interfaces as the main electron transitions had occurred in the infrared and visible regions. The real parts of the dielectric function in the x-direction indicate the different behaviors of “Co–Co/As and V–Ga/Ga” and “Co–Co/Ga and V–Ga/As” in the infrared area. In addition, the plasmon frequencies had occurred at high UV energies.

Alignment of Redox Levels at Semiconductor–Water Interfaces

We determine the band alignment between various semiconductors and liquid water by combining molecular dynamics (MD) simulations of atomistic interface models, electronic-structure calculations at the hybrid-functional and GW level, and a computational standard hydrogen electrode. Our study comprises GaAs, GaP, GaN, CdS, ZnO, SnO2, rutile TiO2, and anatase TiO2. For each semiconductor, we generate atomistic interface models with liquid water at the pH corresponding to the point of zero charge. The molecular dynamics are started from two kinds of initial configurations, in which the water molecules are either molecularly (m) or dissociatively (d) adsorbed on the semiconductor surface. The calculated band offsets are found to be strongly influenced by the adsorption mode at the semiconductor–water interface, leading to differences larger than 1 eV between m and d models of the same semiconductor. We first assess the accuracy of various ab initio electronic-structure schemes. The use of a standard hybrid fun...

Effects of the c-Si/a-SiO2 interfacial atomic structure on its band alignment: an ab initio study.

The crystalline-Si/amorphous-SiO2 (c-Si/a-SiO2) interface is an important system used in many applications, ranging from transistors to solar cells. The transition region of the c-Si/a-SiO2 interface plays a critical role in determining the band alignment between the two regions. However, the question of how this interface band offset is affected by the transition region thickness and its local atomic arrangement is yet to be fully investigated. Here, by controlling the parameters of the classical Monte Carlo bond switching algorithm, we have generated the atomic structures of the interfaces with various thicknesses, as well as containing Si at different oxidation states. A hybrid functional method, as shown by our calculations to reproduce the GW and experimental results for bulk Si and SiO2, was used to calculate the electronic structure of the heterojunction. This allowed us to study the correlation between the interface band characterization and its atomic structures. We found that although the systems with different thicknesses showed quite different atomic structures near the transition region, the calculated band offset tended to be the same, unaffected by the details of the interfacial structure. Our band offset calculation agrees well with the experimental measurements. This robustness of the interfacial electronic structure to its interfacial atomic details could be another reason for the success of the c-Si/a-SiO2 interface in Si-based electronic applications. Nevertheless, when a reactive force field is used to generate the a-SiO2 and c-Si/a-SiO2 interfaces, the band offset significantly deviates from the experimental values by about 1 eV.