Band Anticrossing Research Articles

Lattice constant, bandgap energy, absorption coefficient and dielectric function of the antimony-rich InBixSb1-x alloy using first-principles calculations

So far, little is known about the electronic and optical properties of InBixSb1-x. In this work, the first-principles calculations are performed to research the lattice constant, the bandgap energy, the absorption coefficient and the dielectric function of the antimony-rich InBixSb1-x. The results show that the bowing coefficient for the lattice constant is merely −0.012 Å. According to the band structures, it is found that the Sb-rich InBixSb1-x possesses a direct bandgap at G point. Its bandgap reduction is due to the ascending of the valence band maximum (VBM) and the descending of the conduction band minimum (CBM). For the sake of providing a good description for the bandgap energy in the Sb-rich range, the modified valence band anticrossing (MVBAC) model plus a linear equation is utilized. The result shows that the predicted positive to negative bandgap transition occurs at x = 0.13. Besides, the valence band offset between InSb and InBi is identified to be 0.27eV. For the optical properties, it is found that the Bi component is effective in raising the static dielectric constant of InBixSb1-x. However, it has a minor effect on the transition ability of the electrons. In the Sb-rich component range, the critical point energies E0+Δ0E1, E1+Δ1E2 and E1′ are found to shift toward the low energy direction. The result of the absorption spectra supports this opinion as increasing Bi component leads to a redshift of the absorption spectra. The adjustable bandgap energy and the optical properties of InBixSb1-x demonstrate that it is a strong candidate for fabricating the photodetectors in the 8–12 μm spectral region.

Group Theory on Quasisymmetry and Protected Near Degeneracy.

In solid state systems, group representation theory is powerful in characterizing the behavior of quasiparticles, notably the energy degeneracy. While conventional group theory is effective in answering yes-or-no questions related to symmetry breaking, its application to determining the magnitude of energy splitting resulting from symmetry lowering is limited. Here, we propose a theory on quasisymmetry and near degeneracy, thereby expanding the applicability of group theory to address questions regarding large-or-small energy splitting. Defined within the degenerate subspace of an unperturbed Hamiltonian, quasisymmetries form an enlarged symmetry group eliminating the first-order splitting. This framework ensures that the magnitude of splitting arises as a second-order effect of symmetry-lowering perturbations, such as external fields and spin-orbit coupling. We systematically tabulate the quasisymmetry groups within 32 crystallographic point groups and find all the possible unitary quasisymmetry group structures regarding double degeneracy. Applying our theory to the realistic material AgLa, we predict a "quasi-Dirac semimetal" phase characterized by two tiny-gap band anticrossings.

Characterization of induced quasi-two-dimensional transport in n-type InxGa1−xAs1 − yBiy bulk layer

The temperature-dependent transport properties of n-type InGaAsBi epitaxial alloys with various doping densities are investigated by conducting magnetoresistance (MR) and Hall Effect (HE) measurements. The electronic band structure of the alloys and free electron distribution were calculated using Finite Element Method (FEM). Analysis of the oscillations in the transverse (Hall) resistivity shows that quasi-two-dimensional electron gas (Q-2D) in the bulk InGaAsBi epitaxial layer (three-dimensional, 3D) forms at the sample surface under magnetic field even though there is no formation of the spacial two-dimensional electron gas (2DEG) at the interface between InGaAs and InP:Fe interlayer. The formation of Q-2D in the 3D epitaxial layer was verified by temperature and magnetic field dependence of the resistivity and carrier concentration. Analysis of Shubnikov-de Haas (SdH) oscillations in longitudinal (sample) resistivity reveals that the electron effective mass in InGaAsBi alloys are not affect by Bi incorporation into host InGaAs alloys, which verifies the validity of the Valence Band Anti-Crossing (VBAC) model. The Hall mobility of the nondegenerate samples shows the conventional 3D characteristics while that of the samples is independence of temperature for degenerated samples. The scattering mechanism of the electrons at low temperature is in long-range interaction regime. In addition, the effects of electron density on the transport parameters such as the effective mass, and Fermi level are elucidated considering bandgap nonparabolicity and VBAC interaction in InGaAsBi alloys.

Undoing band anticrossing in highly mismatched alloys by atom arrangement

The electronic structures of three highly mismatched alloys (HMAs)—GeC(Sn), Ga(In)NAs, and BGa(In)As—were studied using density functional theory with HSE06 hybrid functionals, with an emphasis on the local environment near the mismatched, highly electronegative atom (B, C, and N). These alloys are known for their counterintuitive reduction in the bandgap when adding the smaller atom, due to a band anticrossing (BAC) or splitting of the conduction band. Surprisingly, the existence of band splitting was found to be completely unrelated to the local displacement of the lattice ions near the mismatched atom. Furthermore, in BGaAs, the reduction in the bandgap due to BAC was weaker than the increase due to the lattice constant, which has not been observed among other HMAs but may explain differences among experimental reports. While local distortion in GeC and GaNAs was not the cause for BAC, it was found to enhance the bandgap reduction due to BAC. This work also found that mere contrast in electronegativity between neighboring atoms does not induce BAC. In fact, surrounding the electronegative atom with elements of even smaller electronegativity than the host (e.g., Sn or In) consistently decreased or even eliminated BAC. For a fixed composition, moving Sn toward C and In toward either N or B was always energetically favorable and increased the bandgap, consistent with experimental annealing results. Such rearrangement also delocalized the conduction band wavefunctions near the mismatched atom to resemble the original host states in unperturbed Ge or GaAs, causing the BAC to progressively weaken. These collective results were consistent whether the mismatched atom was a cation (N), anion (B), or fully covalent (C), varying only with the magnitude of its electronegativity, with B having the least effect. The effects can be explained by charge screening of the mismatched atom's deep electrostatic potential. Together, these results help explain differences in the bandgap and other properties reported for HMAs from different groups and provide insight into the creation of materials with designer properties.

Band dispersion, scattering rate, and carrier mobility using the poles of Green’s function for dilute nitride alloys

The band-anticrossing (BAC) model provides the basis for the self-consistent Green’s function method that we have previously developed to calculate the density of states of GaNxAs1−x dilute nitride alloys. In this paper, we extend this Green’s function method to include the complex energy states and to find the poles of the Green’s function, thereby allowing one to calculate the dispersion relation, group velocity, and the carrier decay rate in disordered dilute nitride alloys. Two different models of the N states have been studied to investigate the band structure of these materials: (1) the conventional two-band BAC model, which assumes that all N states are located at the same energy, and (2) a model which includes N states distributed over a range of energies, as expected in actual dilute nitride samples. Our results for the second model show a much shorter carrier mean-free path, and lower carrier mobility for GaNxAs1−x, with the magnitude of the calculated mobility in good agreement with the experimental data.

The influence of N and Bi on the band structure and band offsets of InAsNBi alloys

We present the electronic properties of InAsNBi with N and Bi concentrations based on 16×16 band Hamiltonian model. This model is extended from Band Anti-crossing (BAC) and the dimension of basis states used in the Hamiltonian matrix is 16. Under the condition that the quaternary compound matches the lattice configuration of the InAs substrate, the variations of band gap (Eg), spin-orbit coupling energy (ΔSO) has been examined as a dependency on the mole fractions of N and Bi. Our findings indicate the integration of N and Bi atoms in InAs gives rise to a substantial band gap energy minimization and the growth of spin-orbit coupling energy. Subsequently a crossover (ΔSO = Eg) is observed at a Bi concentration of 0.37%, which contributes to enhancing the effectiveness of photonic devices by suppress Auger recombination. We further reveal the band offset with different N and Bi concentrations and considered the influence of the substrate on the material deviation. The conduction band energy mislignment (ΔEc) of InAsNBi exceeds energy misalignment in the valence bands (ΔEv), which is required to improve thermal-insensitive characteristics of semiconductor devices. Moreover, the significant mastery over the edges of the conduction and valence bands by adjusting the concentration of N and Bi will increase the flexibility to design InAsNBi/InAs structures.

First principles phase diagram and electronic structure estimation of ZnO1-xSex photoanodes

Terminal solid solutions in the ZnO1−xSex system (0≤x≤0.15,0.95≤x≤1) exhibit extreme bandgap reduction attributable to band anti-crossing (BAC). In this work, we perform a theoretical investigation of alloying in this system (0≤x≤1). The temperature-composition phase diagram of ZnO1−xSex is obtained via first principles and cluster expansion-based Monte–Carlo simulations. For 0≤x≤0.05, a solid solution in the wurtzite structure and for 0.5≤x≤1, a solid solution in the sphalerite structure is obtained. The alloy system exhibits a miscibility gap in the range of 0.05≤x≤0.5. Only the solid solutions are seen to obey bandgap reduction predicted by BAC. The bandgap of the alloys, calculated using the Δ-sol method, shows a bowing behavior as predicted by the BAC model. Difference in the electronegativities of O and Se atoms in the lattice leads to hybridization of O-2p and Se-4p electronic states. Interaction between these electronic states also leads to a split in the valence band edge at the O-rich end and a split in the conduction band edge at the Se-rich end. The effective mass, estimated from the density of states, of holes at the O-rich end and that of electrons at the Se-rich end, increases with alloying. These fundamental insights should help in choosing suitable alloy compositions for optimal photocurrent density when these materials are used as photoanodes.

Stable, self-biased Cs2AgBiBr6 thin-film based photodetector by three-step vapor-deposition

A lead-free, pure phase double perovskite Cs2AgBiBr6 thin film (TF) is fabricated on an ITO glass plate through a three-step sequential vacuum thermal evaporation method followed by annealing. X-ray diffraction (XRD) analysis of the synthesized material revealed a cubic phase with a lattice constant of α = 11.26 Å. The TF exhibits UV and visible absorption with a bandgap estimated at 2.31 eV (303 K) and a red shift of 0.04 eV (363 K). The optical absorption spectrum is simulated using the modified band anti-crossing (BAC) model, determining the Cs2AgBiBr6 TF's theoretical relative refractive index (r.i.) as 2.10 (303 K) and 1.90 (363 K). Furthermore, a photodetector based on Ag/Cs2AgBiBr6/ITO/glass architecture was fabricated, showcasing excellent stability and self-powered photodetection properties under continuous illumination across a wide temperature range of 303–373 K. The device demonstrates a fast temporal response (13/9 ms), a responsivity of ∼1.0 × 10−4 A/W, and a detectivity of ∼2.5 × 107 Jones. Light sensitivity measurements reveal that the device is twice as sensitive at 373 K compared to room temperature, with negligible (1%) degradation in light sensitivity after 60 days of storage under normal ambient conditions.

Theoretical calculations to investigate thermodynamic properties of 2D electron gas in InP/InPBiN quantum structure

The electronic band structure of InP/InNPBi has been theoretically investigated within the framework of the band anticrossing (BAC) model. The matched InP/InNPBi heterostructure has been proposed as an interface that confines electrons in a 2D sheet. The physical parameters of this structure are determined considering the BAC model and based on the self-consistent calculation of coupled Schrodinger-Poisson equations. Thermodynamic properties of the quantum gas of noninteracting electrons are investigated by calculating the chemical potential (μ), the magnetization (M), and the specific heat capacity (Cv) in the presence of a quantizing magnetic field and a finite temperature. The effect of characteristic parameters of the structure (broadening parameter, doping level, and effective mass) on the thermodynamic properties of gas is discussed. More interest is focused on the study of the evolution of Cv as a function of temperature for different estimated effective masses of InNPBi material.

Structural and electronic properties of the Te-rich ZnOxTe1-x

The structural and electronic properties of the Te-rich ZnOxTe1-x are calculated via first-principles calculations. It is found that the lattice constant exhibits a decreasing trend and it has a negative bowing coefficient. The bandgap narrowing in the Te-rich range is by reason of the introduction of the O level in the bandgap, the downward movement of the conduction band minimum (CBM) and the upward movement of the valence band maximum (VBM). The band crossing is due to that the bottom of the Zn-4 s states in the conduction band is lowered after more O atoms are bonded with Zn atoms. In order to solve the problem of the description for the CBM due to the band crossing in the Te-rich range, the energy which the initial lowered Zn-4 s states correspond to in the modified band anticrossing (BAC) model choose the same value with that of the initial O level which the O-2 s states introduce. Additionally, the combination of the modified BAC model and the standard bowing equation is proposed to describe the bandgap energy in the Te-rich range. It is found that the model in this work can give a good description.

Predicting the bandgap energy of distorted GaSbxAs1-x and InSbxAs1-x using design of experiment (DoE) and artificial intelligence (AI): A comparative study

The band anticrossing (BAC) theory is widely used to model the bandgap energy of GaSbxAs1-x and InSbxAs1-x materials and is based on two effects: impurity-host interaction in the range rich in antimony and arsenic content, and intraband coupling within the conduction and valence bands in the moderate range. In this study, the response surface method (RSM) was applied to the data and an attempt was made to develop an artificial neural network (ANN) based on the Levenberg-Maquardt backpropagation algorithm to predict the bandgap energy in a more precise manner. The predicted results were verified using experimental data. Using R2 and RMSE, we found that the neural network model provided more accurate losses, with correlation coefficients of 99.28% and 97.42% for GaSbAs and InSbAs, respectively. Subsequently, we exploited the predicted values of the bandgap energy to study the effect of the lattice mismatch on the optoelectronic properties of the materials. We have shown that the non-degenerate bandgap energies of the deformed layer associated with heavy holes (hh) and light holes (lh) calculated from the values predicted by the ANN undergo a highly nonlinear variation compared to their BAC and RSM counterparts, and the energy range narrows with increasing Sb(x) content. These materials are of scientific interest for infrared photodetectors, which are used in a wide variety of applications including medical diagnostics, thermal imaging for industrial process control, and military applications.

The structure and electronic properties of InBixAs1-x alloy (0 ≤ x < 0.1) predicted by first-principles calculations

The generalized gradient approximation (GGA) plus Hubbard U parameter method is adopted to made a research on the structure and electronic properties of the As-rich InBixAs1-x. It is found that the standard bowing equation is more suitable for giving a description for the lattice constant than the linear equation. The bowing coefficient for the lattice constant of InBixAs1-x is −0.022 Å. Based on the experimental lattice constant of InAs and the difference of the lattice constant between InAs and InBi acquired by the GGA method, the acquired lattice constant of InBi is 6.657 Å. It is also found that incorporating bismuth atoms in InAs can lower the bandgap energy quickly. The bandgap narrowing is on account of the ascending of the valence band maximum (VBM) and the descending of the conduction band minimum (CBM). For the sake of depicting the bandgap energy, the combination of the modified valence band anticrossing (MVBAC) model and a linear equation is employed. It is observed that the combination can give an accurate estimation. Besides, when the bismuth concentration reaches up to 0.105, the bandgap energy of InBixAs1-x can be lowered to 0 eV.

Progress in Spinconversion and its Connection with Band Crossing (Ann. Phys. 4/2022)

Spintronics Recent progress in spinconversion research, with particular focus on studies involving band crossing, is reviewed by Jorge Puebla, Yoshichika Otani, and co-workers in article number 2100398. If the coupling strength of quasiparticles is enhanced, hybrid quasiparticles are formed and band anticrossings appear, with repercussions in other research fields beyond spinconversion, such as quantum information.

Impact of Band Anticrossing on Band-to-Band Tunneling in Highly Mismatched Semiconductor Alloys

We theoretically analyze band-to-band tunneling (BTBT) in highly mismatched, narrow-gap dilute nitride and bismide alloys, and quantify the impact of the $\mathrm{N}$- or $\mathrm{Bi}$-induced perturbation of the band structure---due to band anticrossing (BAC) with localized impurity states---on the electric-field-dependent BTBT generation rate. For this class of semiconductors, the assumptions underpinning the widely employed Kane model of BTBT break down, due to strong band-edge nonparabolicity resulting from BAC interactions. Via numerical calculations based on the Wentzel-Kramers-Brillouin approximation we demonstrate that BAC leads, at fixed band gap, to reduced (increased) BTBT current at low (high) applied electric field compared to that in a conventional ${\mathrm{In}\mathrm{As}}_{1\ensuremath{-}x}{\mathrm{Sb}}_{x}$ alloy. Our analysis reveals that BTBT in ${\mathrm{In}\mathrm{N}}_{x}{\mathrm{As}}_{1\ensuremath{-}x}$ and ${\mathrm{In}\mathrm{As}}_{1\ensuremath{-}x}{\mathrm{Bi}}_{x}$ is governed by a field-dependent competition between the impact of $\mathrm{N}$ ($\mathrm{Bi}$) incorporation on (i) the dispersion of the evanescent Bloch band linking the valence and conduction band edges, which dominates at low field strengths, and (ii) the conduction- (valence-) band-edge density of states, which dominates at high field strengths. The implications of our results for applications in long-wavelength avalanche photodiodes (APDs) and tunneling field-effect transistors (TFETs) are discussed. For APDs, we describe that leakage currents in ideal dilute nitride and bismide devices are likely comparable to those in devices based on conventional III-V materials, but that the expected reduction of the hole impact ionization rate in ${\mathrm{In}\mathrm{As}}_{1\ensuremath{-}x}{\mathrm{Bi}}_{x}$ is promising from the perspective of obtaining low excess noise factor. For TFETs, we describe that $\mathrm{Bi}$ incorporation provides the potential to obtain both reduced subthreshold swing and increased on-state current, making ${\mathrm{In}\mathrm{As}}_{1\ensuremath{-}x}{\mathrm{Bi}}_{x}$ of interest for the development of low-power, high-speed devices.

Giant nonlinear Hall effect in twisted bilayer WTe2

In a system with broken inversion symmetry, a second-order nonlinear Hall effect can survive even in the presence of time-reversal symmetry. In this work, we show that a giant nonlinear Hall effect can exist in twisted bilayer WTe2 system. The Berry curvature dipole of twisted bilayer WTe2 (θ = 29.4°) can reach up to ~1400 Å, which is much larger than that in previously reported nonlinear Hall systems. In twisted bilayer WTe2 system, there exist abundant band anticrossings and band inversions around the Fermi level, which brings a complicated distribution of Berry curvature, and leads to the nonlinear Hall signals that exhibit dramatically oscillating behavior in this system. Its large amplitude and high tunability indicate that the twisted bilayer WTe2 can be an excellent platform for studying the nonlinear Hall effect.

Magneto-optical spectroscopy on Weyl nodes for anomalous and topological Hall effects in chiral MnGe

Physics of Weyl electrons has been attracting considerable interests and further accelerated by recent discoveries of giant anomalous Hall effect (AHE) and topological Hall effect (THE) in several magnetic systems including non-coplanar magnets with spin chirality or small-size skyrmions. These AHEs/THEs are often attributed to the intense Berry curvature generated around the Weyl nodes accompanied by band anti-crossings, yet the direct experimental evidence still remains elusive. Here, we demonstrate an essential role of the band anti-crossing for the giant AHE and THE in MnGe thin film by using the terahertz magneto-optical spectroscopy. The low-energy resonance structures around ~ 1.2 meV in the optical Hall conductivity show the enhanced AHE and THE, indicating the emergence of at least two distinct anti-crossings near the Fermi level. The theoretical analysis demonstrates that the competition of these resonances with opposite signs is a cause of the strong temperature and magnetic-field dependences of observed DC Hall conductivity. These results lead to the comprehensive understanding of the interplay among the transport phenomena, optical responses and electronic/spin structures.

Quasiparticle twist dynamics in non-symmorphic materials

Quasiparticle physics underlies our understanding of the microscopic dynamical behaviors of materials that govern a vast array of properties, including structural stability, excited states and interactions, dynamical structure factors, and electron and phonon conductivities. Thus, understanding band structures and quasiparticle interactions is foundational to the study of condensed matter. Here we advance a ‘twist’ dynamical description of quasiparticles (including phonons and Bloch electrons) in non-symmorphic chiral and achiral materials. Such materials often have structural complexity, strong thermal resistance, and efficient thermoelectric performance for waste heat capture and clean refrigeration technologies. The twist dynamics presented here provides a novel perspective of quasiparticle behaviors in such complex materials, in particular highlighting how non-symmorphic symmetries determine band crossings and anti-crossings, topological behaviors, quasiparticle interactions that govern transport, and observables in scattering experiments. We provide specific context via neutron scattering measurements and first-principles calculations of phonons and electrons in chiral tellurium dioxide. Building twist symmetries into the quasiparticle dynamics of non-symmorphic materials offers intuition into quasiparticle behaviors, materials properties, and guides improved experimental designs to probe them. More specifically, insights into the phonon and electron quasiparticle physics presented here will enable materials design strategies to control interactions and transport for enhanced thermoelectric and thermal management applications.

Eg[formula omitted]t2g Sub band splitting via crystal field and band anticrossing interaction in NixCd1-xO thin films

In the present work, full composition range of NixCd1-xO thin films, from Cadmium oxide to Nickel oxide are prepared with sol-gel chemical synthesis route on corning glass substrates. The Fourier transform infrared spectra provide a direct indication of compositional dependence for different vibrational modes with increasing Ni doping. The microscopy images reflect the reduced crystallinity and closely packed morphology with increasing Ni doping which endorses the changes reflected in electronic structure. The O K edge, soft X-ray absorption spectra (XAS) for thin films indicate a clear evidence of eg and t2g sub band splitting and has been explained via crystal field splitting and three level band anti-crossing (BAC) interaction phenomena. At higher doping fraction, the inhomogeneous electric field of neighboring ions surrounding the unfilled Ni 3d level results in rupturing of L−S coupling and further crystal field splitting. The BAC interaction brings about partially unoccupied localized Ni 3d and empty d acceptor levels above conduction band minima (CBM). As saturated Fermi level goes below CBM with increasing Ni doping, transitions to antibonding orbital [eg(π*) and t2g(σ*)] with finite energy difference, is reflected as separated eg-t2g peaks in XAS spectra. On the other hand, extended X-ray absorption fine structure for Ni K edge fitting provides a complete information for nearest (Ni-O) and next nearest (Ni-Ni) neighbor bond length.

Vapour transport grown photosensitive GeO2 thin film

GeO2 thin film (TF) was fabricated by vapour deposition technique in a two zone horizontal quartz tube. Field emission-scanning electron microscopy showed the formation of 200 nm TF on the Si (100) substrate. The X-ray diffraction depicted the presence of the polycrystalline GeO2 TF. The UV–vis absorption spectrum showed humps at 3.1 eV, 4.0 eV and 5.4 eV. The band anticrossing (BAC) model is modified using the Urbach equation to understand the different transitions in the optical absorption spectrum. We have theoretically calculated the relative refractive index (r.i.) (nr) of the GeO2 as 1.8. The temperature dependent current (I)–voltage (V) characteristics and photocurrent of the Au/GeO2/p-Si Schottky device predicted the tunnelling of the carriers, presence of oxygen defects at the junction. The device’s temporal response indicated good photosensitivity at all temperatures and a significant increase in photocurrent was obtained as the temperature increased from 303 K to 403 K.

Calculation of the Bandgap of Dilute Nitride GaAsSbN Alloys

Dilute nitrides are semiconductor alloys, obtained from the conventional III-V compounds by incorporating a small amount of nitrogen. In this work, we focus on GaAsSbN considered as a perspective material for incorporation in multijunction solar cells. Nitrogen creates a localized level inside the conduction band continuum. The interaction of this level with the conduction band is usually described by the single band anti-crossing (BAC) model. The double BAC model of GaAsSbN considers both the N and the Sb localized levels in the conduction and the valence band, respectively. We calculate the bandgap energy of GaAsSbN employing the double BAC model for different concentrations of Sb and N. Parameters of the BAC model taken from different literature sources are used in the calculations and their influence on the final result is explored. Finally, the calculated bandgap energies are compared to experimental data of GaAsSbN layers grown on n-GaAs substrates by low-temperature liquid phase epitaxy. These data include the optical absorption edge of the material determined by surface photovoltage spectroscopy and the energy position of the photoluminescence peak at room temperature.