Empirical Tight-binding Method Research Articles

AlN/GaN Superlattice band structure calculation Using Empirical Tight Binding Model

AlN/GaN superlattices are studied using the empirical tight-binding method (ETBM) with sp3 approximation. The model includes first-nearest-neighbor orbital interactions and accounts for the strain effect by modifying the ETBM parameters based on variations in bond lengths and angles. According to the authors knowledge, this study marks the first application of ETBM to determine the full band structure of AlN/GaN superlattices. On average, the ETBM band gaps resulting from this work are 7.48 % closer to the experimental data in comparison with Kronig-Penney outputs. In addition to the high accuracy of the model, the presented model is easier to implement compared to the density functional theory model.

Empirical tight-binding method for large-supercell simulations of disordered semiconductor alloys

We analyze and present applications of a recently proposed empirical tight-binding scheme for investigating the effects of alloy disorder on various electronic and optical properties of semiconductor alloys, such as the band gap variation, the localization of charge carriers, and the optical transitions. The results for a typical antimony-containing III-V alloy, GaAsSb, show that the new scheme greatly improves the accuracy in reproducing the experimental alloy band gaps compared to other widely used schemes. The atomistic nature of the empirical tight-binding approach paired with a reliable parameterization enables more detailed physical insights into the effects of disorder in alloyed materials.

Machine learned environment-dependent corrections for a spds∗ empirical tight-binding basis

Abstract Empirical tight-binding (ETB) methods have become a common choice to simulate electronic and transport properties for systems composed of thousands of atoms. However, their performance is profoundly dependent on the way the empirical parameters were fitted, and the found parametrizations often exhibit poor transferability. In order to mitigate some of the the criticalities of this method, we introduce a novel Δ-learning scheme, called MLΔTB. After being trained on a custom data set composed of ab-initio band structures, the framework is able to correlate the local atomistic environment to a correction on the on-site ETB parameters, for each atom in the system. The converged algorithm is applied to simulate the electronic properties of random GaAsSb alloys, and displays remarkable agreement both with experimental and ab-initio test data. Some noteworthy characteristics of MLΔTB include the ability to be trained on few instances, to be applied on 3D supercells of arbitrary size, to be rotationally invariant, and to predict physical properties that are not exhibited by the training set.

Perspective: Theory and simulation of highly mismatched semiconductor alloys using the tight-binding method

The electronic structure of highly mismatched semiconductor alloys is characterized by carrier localization and strongly influenced by the local alloy microstructure. First-principles calculations can deliver valuable quantitative insight, but their associated computational expense limits alloy supercell size and imposes artificial long-range ordering, which can produce misleading results. The empirical tight-binding method (ETBM) provides a transparent approach to investigate large-scale supercells on an atomistic level, to quantitatively predict the electronic structure of semiconductor alloys. Here, we overview key aspects and considerations for establishing ETBMs. Additionally, we discuss and highlight, given that the ETBM matrix elements are described in the language of overlaps between localized atomic orbitals, that ETBMs have proven highly successful in analyzing the impact of localized and resonant impurity states, as well as disorder, on the optoelectronic properties of highly mismatched alloys. The ETBM continues to provide valuable insight for emerging material systems, including two-dimensional materials, perovskites and their heterostructures, and provides a framework to address technologically relevant questions including the importance of short-range disorder for loss mechanisms such as non-radiative Auger–Meitner recombination. Moreover, the ETBM furnishes a quantitative basis for continuum models such as k⋅p or localization landscape theories, allowing to explicitly incorporate disorder effects in nanostructures to underpin predictive device-level analysis.

Statistical model of electronic structure in InAs, InP, GaSb, and Si quantum dots with surface roughness

Simulations based on the sp 3 d 5 s * empirical tight-binding method were performed to provide a statistical understanding of the electronic structures and bandgap distributions of III–V (InAs, InP, GaSb) and IV (Si) semiconductor quantum dots (QDs) with surface roughness. The electronic states and wavefunctions of QDs with surface roughness of different sizes, shapes, and materials were computed. The effects of surface roughness on the electronic structures and the bandgap distributions of QDs were investigated. The results show that the bandgaps of QDs of considered materials/sizes/shapes increase on average when introducing surface roughness. It is shown that the simulated bandgap distributions of QDs with surface roughness can be reproduced by a simple model formula, which can be applied to different materials, sizes, and shapes. The model formula was derived by assuming that removing and adding of one atom procedures are independent random processes.

Full-band Monte Carlo analysis of strain effects on carrier transport in GaN

The effects of strain on the carrier transport in gallium nitride (GaN) are investigated using a full-band Monte Carlo method combined with an empirical tight-binding method. The impacts on the carrier mobility, carrier drift velocity, and breakdown characteristics are discussed. Compressive uniaxial or tensile biaxial strain is beneficial for achieving higher hole mobility in vertical GaN devices due to the light-hole band being lifted above the heavy-hole band. Analysis of the breakdown phenomena indicates that strain does not degrade the breakdown characteristics in terms of its effect on the band structure.

Tight-binding analysis of the effect of strain on the band structure of GaN

The effects of strain on the band structure of GaN are investigated by using an empirical tight-binding method. The impacts on its bandgap, carrier effective mass, and group velocity are discussed. By analyzing the orbital components at the top of the valence band, the cause of the variation of the band structure including effective-mass exchange is discussed. Analysis of the average group velocity indicates that tensile uniaxial or compressive biaxial strain may be beneficial for achieving higher breakdown voltage in vertical GaN devices due to the smaller group velocity of the valence band. For the same reason, we also predict higher breakdown voltages due to tensile biaxial strain for horizontal devices.

Modeling of temperature effects on band structure in type-II superlattices using an empirical tight-binding method

Modeling of temperature effects on band structure in type-II superlattices using an empirical tight-binding method

Simulation of the Band Structure of InAs/GaSb Type II Superlattices Utilizing Multiple Energy Band Theories

Antimonide type II superlattices is expected to overtake HgCdTe as the preferred materials for infrared detection due to their excellent photoelectric properties and flexible and adjustable band structures. Among these compounds, InAs/GaSb type II superlattices represent the most commonly studied materials. However, the sophisticated physics associated with the antimonide-based bandgap engineering concept started at the beginning of the 1990s gave a new impact and interest in the development of infrared detector structures within academic and national laboratories. InAs/GaSb superlattices are a type II disconnected band structure with electrons and holes confined in the InAs and GaSb layers, respectively. The electron miniband and hole miniband can be regulated separately by adjusting the thickness of InAs and GaSb layers, which facilitates the design of superlattice structures and optimizes the value of band offset. In recent years, both domestic and foreign researchers have made many attempts to quickly and accurately predict the bandgaps of superlattice materials before superlattice materials grow. These works constituted a theoretical basis for the effective utilization of the InAs/GaSb system in material optimization and designing new SL structures; they also provided an opportunity for the preparation and rapid development of InAs/GaSb T2SLs. In this paper, we systematically review several widely used methods for simulating superlattice band structures, including the k·p perturbation method, envelope function approximation, empirical pseudopotential method, empirical tight-binding method, and first-principles calculations. With the limitations of different theoretical methods proposed, the simulation methods have been modified and developed to obtain reliable InAs/GaSb SL energy band calculation results. The objective of this work is to provide a reference for designing InAs/GaSb type II superlattice band structures.

Atomic segregation effects on (AlxGa1-xAs)m/(GaAs)n superlattice structure

In this paper, we used the Empirical Tight-Binding Method to model (AlxGa1-xAs)m/(GaAs)n superlattice structure (SLS) grown on GaAs substrate. To increase the accuracy of our modeling method, we also considered the atomic segregation by Kinetic Model, and analyzed the behavior of energy gap variation. As the Al composition increases, the energy gap of the SLS has an upward trend and ranges from 1.42 to 1.8eV. Meanwhile, for Al composition more than x = 0.45, a bump appears in the energy bandgap diagram verses m and n that determines where the bandgap changes from indirect to direct. When we consider the segregation in the energy gap calculation, we observe that the segregation causes the bump to shift 2 ML backward and reduces the bandgap energy. Furthermore, we realized that the segregation could cause a redshift phenomenon and increase the Auger recombination rate in the SLS.

Snapshots of the Fragmentation for C70@Single-Walled Carbon Nanotube: Tight-Binding Molecular Dynamics Simulations.

In previously reported experimental studies, a yield of double-walled carbon nanotubes (DWCNTs) at C70@Single-walled carbon nanotubes (SWCNTs) is higher than C60@SWCNTs due to the higher sensitivity to photolysis of the former. From the perspective of pyrolysis dynamics, we would like to understand whether C70@SWCNT is more sensitive to thermal decomposition than C60@SWCNT, and the starting point of DWCNT formation, which can be obtained through the decomposition fragmentation of the nanopeapods, which appears in the early stages. We have studied the fragmentation of C70@SWCNT nanopeapods, using molecular dynamics simulations together with the empirical tight-binding total energy calculation method. We got the snapshots of the fragmentation structure of carbon nano-peapods (CNPs) composed of SWCNT and C70 fullerene molecules and the geometric spatial positioning structure of C70 within the SWCNT as a function of dynamics time (for 2 picoseconds) at the temperatures of 4000 K, 5000 K, and 6000 K. In conclusion, the scenario in which C70@SWCNT transforms to a DWCNT would be followed by the fragmentation of C70, after C70, and the SWCNT have been chemically bonding in the early stages. The relative stability of fullerenes in CNPs could be reversed, compared to the ranking of the relative stability of the encapsulated molecules themselves.

Band structure calculation in type-II InAs/GaSb superlattices detector with thermal strain using an empirical tight-binding method

When the operating temperature rapidly reduces from room temperature to 77 K, the band structure of InAs/GaSb superlattices device will be affected by thermal strain. Two strain models of the InAs/GaSb superlattices device with and without substrate are designed and fabricated. The effective bandgap and cutoff wavelength of the superlattices devices were calculated using the empirical tight-binding method and compared with the experimental measurement results. The modeling calculations show that the thickness of silicon ROIC is the main factor affecting band structure. The horizontal strain component and bending strain component are selected to assess the effects of the thinning thickness of silicon ROIC, respectively. The model containing Al2O3 substrate and silicon ROIC thickness below 500 μm has less bandgap deviation and, therefore, is recommended. The measured PL spectrum and spectral response confirm the simulation results. By appropriate choice of the layer thicknesses, cutoff wavelength closed to the design value is achievable.

Modeling of (GaP)m/(AlP)n superlattices using empirical tight-binding method

In this paper, we investigated behavior of energy gap and refractive index of (GaP) m /(AlP) n superlattices structure (SLS) grown on GaP (100) substrate in terms of GaP and AlP thicknesses. For this purpose, we used the empirical tight-binding method (ETBM) with sp 3 s* approximation and considering first nearest neighbor and spin-orbit interactions. Since in this method, predicting 15 parameters in individual balk Hamiltonian matrix is very important, we used genetic algorithm to obtain a new set of parameters that can accurately predict the superlattice structures energy gaps with about 1.2% deviation from the experimental data points. We used these parameters and also considered stress, strain and energy band lineups for this SLS to simulate the energy gap and refractive index for a range of GaP and AlP thicknesses. • Introduction of new orbitals interaction parameters for GaP/AlP by using genetic algorithm, which help to have better description in bulk and superlattices. • We compared the result of simulation by experimental values and results with other method and other orbitals interaction parameters. • Prediction of GaP/AlP superlattice bandgap energy and refractive index by varying the thicknesses of both AlP and GaP and extracting a range of bandgap variations.

Electronic band structure of silver low-index surfaces: a tight-binding study

We studied the electronic band structure and corresponding local density of states of low-index fcc Ag surfaces (100), (110), and (111) by using the empirical tight-binding method in the framework of the Surface Green’s Function Matching formalism. The energy values for different surface and resonance states are reported and a comparison with the available experimental and theoretical data is also done.

The modeling of a SWIR type-II InAs/AlSb superlattice using an ETBM and interface engineering

A short-wavelength infrared InAs/AlSb type-II superlattice photodetector has been modeled with 15 Mono Layer (ML) of InAs and 4 Mono Layer (ML) of AlSb. In this work, the Empirical Tight-Binding Method (ETBM) was used to calculate the band structure and predict the material of the interfaces. In addition, the effective mass of the heavy hole, light hole, and electron was calculated for each type of the interfaces by tight binding s p 3 s ∗ modeling. Based on the results, the material of the interfaces plays a critical role in lattice mismatch and cutoff wavelength. The detector cutoff wavelength can be changed by atomic engineering of the superlattice interfaces and changing the thicknesses of InAs and AlSb. The simulation results are in good agreement with the experimental values and prove that the [ ( I n A s ) 15 − I n S b − ( A l S b ) 4 − A l A s ] is the most suitable structure. In order to match the simulation results with experimental results, the effect of the segregation on the band structure was evaluated. In this case, the cutoff wavelength of the SWIR detector is 3.27 μ m and the lattice mismatch between the superlattice and the GaSb (100) substrate is 0.63%. • A SWIR InAs/AlSb T2SL photodetector has been modeled with 15 ML of InAs and 4 ML of AlSb. • (ETBM) is used to calculate and predict the material of the interfaces. • The effective mass of the heavy hole, light hole, and electron was calculated for each type of the interfaces by tight binding s p 3 s ∗ modeling. • The flowchart of extracting the band structure by ETBM is presented. • The migration of atoms in interfaces is investigated.

Atomistic tight-binding theory of structural and optical properties in PbX (X = S, Se, and Te) nanocrystals

Abstract

Importance of second-order deformation potentials in modeling of InAs/GaAs nanostructures

Accurate modeling of electronic properties of nanostructures is a challenging theoretical problem. Methods making use of continuous media approximation, such as k.p, sometimes struggle to reproduce results obtained with more accurate atomistic approaches. On the contrary, atomistic schemes generally come with a substantially larger cost of computation. Here, we bridge between these two approaches by taking 8-band k.p method augmented with non-linear strain terms fit to reproduce sp3d5s* tight-binding results. We illustrate this method on the example of electron and hole states confined in quantum wells and quantum dots of photonics applications relevant InAs/GaAs material system, and demonstrate a good agreement of a non-linear k.p scheme with empirical tight-binding method. We discuss limits of our procedure as well as provide non-linear 8-band k.p parameter sets for InAs and GaAs. Finally, we propose a parameterization for effective term used to improve the accuracy of the standard effective mass method.

Band structure of Ge1−xSnx alloy: a full-zone 30-band k · p model

A full-zone 30-band k · p model is developed as an efficient and reliable tool to compute electronic band structure in Ge1−xSnx alloy. The model was first used to reproduce the electronic band structures in Ge and α-Sn obtained with empirical tight binding and ab initio methods. Input parameters for the 30-band k · p model are carefully calibrated against prior empirical predications and experimental data. Important material properties such as effective mass for electrons and holes, Luttinger parameters, and density of states are obtained for Ge1−xSnx alloy with the composition range 0 < x < 0.3. The 30-band k · p model that requires far less computing resources is a necessary capability for optimization of sophisticated devices made from Ge1−xSnx alloy with a large parameter space to explore.

Simulation of Electron and Hole States in Si Nanocrystals in a SiO2 Matrix: Choice of Parameters of the Empirical Tight-Binding Method

The problem of the optimal choice of parameters of the empirical tight-binding method to simulate the quantum-confined levels of Si nanocrystals embedded into an amorphous SiO2 matrix is studied. To account for tunneling from nanocrystals to SiO2, the amorphous matrix is considered as a virtual crystal with a band structure similar to that of SiO2 β-cristobalite and with a lattice constant matched to the lattice constant of bulk Si. The electron density distributions in k space for electrons and holes quantum-confined in a Si nanocrystal in SiO2 are calculated in a wide energy region, which provides a means to see clearly the possibility of the existence of efficient direct optical transitions for hot electrons at the upper quantum-confined levels.

Моделирование уровней размерного квантования Si-нанокристаллов в матрице SiO-=SUB=-2-=/SUB=-: подбор параметров эмпирического метода сильной связи

AbstractThe problem of the optimal choice of parameters of the empirical tight-binding method to simulate the quantum-confined levels of Si nanocrystals embedded into an amorphous SiO_2 matrix is studied. To account for tunneling from nanocrystals to SiO_2, the amorphous matrix is considered as a virtual crystal with a band structure similar to that of SiO_2 β-cristobalite and with a lattice constant matched to the lattice constant of bulk Si. The electron density distributions in k space for electrons and holes quantum-confined in a Si nanocrystal in SiO_2 are calculated in a wide energy region, which provides a means to see clearly the possibility of the existence of efficient direct optical transitions for hot electrons at the upper quantum-confined levels.