Subband Energy Research Articles

Control of Subband Energies via Interlayer Twisting in an Artificially Stacked WSe2 Bilayer.

Tuning the electronic structure of artificially stacked bilayer crystals using their twist angle has attracted a significant amount of interest. In this study, resonant tunneling spectroscopy was performed on trilayer WSe2/h-BN/twisted bilayer (tBL) WSe2 devices with a wide range of twist angles (θBL) of tBL WSe2, from 0° to 34°. We observed two resonant tunneling peaks, identified as the first and second lowest hole subbands at the valence band Γ point of tBL WSe2. The subband separation, which directly measured the interlayer coupling strength, was tuned by ∼0.1 eV as θBL increased toward 6° and remained nearly constant for larger θBL values. The θBL dependence was attributed to the emergence of a stable W/Se (Se/W) stacking domain in the small θBL region, owing to the atomic reconstruction of the moiré lattice in tBL WSe2. Our findings demonstrate that the twist-controlled subband energies in tBL WSe2 are predominantly determined by local atomic reconstruction.

Parametric Analysis of Indium Gallium Arsenide Wafer-based Thin Body (5 nm) Double-gate MOSFETs for Hybrid RF Applications.

The electrical behavior of a high-performance Indium Gallium Arsenide (In- GaAs) wafer-based n-type Double-Gate (DG) MOSFET with a gate length (LG1= LG2) of 2 nm was analyzed. The relationship of channel length, gate length, top and bottom gate oxide layer thickness, a gate oxide material, and the rectangular wafer with upgraded structural characteristics and the parameters, such as switch current ratio (ION/IOFF) and transconductance (Gm) was analyzed for hybrid RF applications. This work was carried out at 300 K utilizing a Non-Equilibrium Green Function (NEGF) mechanism for the proposed DG MOSFET architecture with La2O3 (EOT=1 nm) as gate dielectric oxide and source-drain device length (LSD) of 45 nm. It resulted in a maximum drain current (IDmax) of 4.52 mA, where the drain-source voltage (VDS) varied between 0 V and 0.5 V at the fixed gate to source voltage (VGS) = 0.5 V. The ON current(ION), leakage current (IOFF), and (ION/IOFF) switching current ratios of 1.56 mA, 8.49Í10-6 μA, and 18.3Í107 μA were obtained when the gate to source voltage (VGS) varied between 0 and 0.5 V at fixed drain-source voltage (VDS)=0.5V. The simulated result showed the values of maximum current density (Jmax), one and twodimensional electron density (N1D and N2D), electron mobility (μn), transconductance (Gm), and Subthreshold Slope (SS) are 52.4 μA/m2, 3.6Í107 cm-1, 11.36Í1012 cm-2, 1417 cm2V-1S-1, 3140 μS/μm, and 178 mV/dec, respectively. The Fermi-Dirac statistics were employed to limit the charge distribution of holes and electrons at a semiconductor-insulator interface. The flat-band voltage (VFB) of - 0.45 V for the fixed threshold voltage greatly impacted the breakdown voltage. The results were obtained by applying carriers to the channels with the (001) axis perpendicular to the gate oxide. The sub-band energy profile and electron density were well implemented and derived using the Non-Equilibrium Green's Function (NEGF) formalism. Further, a few advantages of the proposed heterostructure-based DG MOSFET structure over the other structures were observed. This proposed patent design, with a reduction in the leakage current characteristics, is mainly suitable for advanced Silicon-based solid-state CMOS devices, Microelectronics, Nanotechnologies, and future-generation device applications.

A 30-nm-tunable photoluminescence caused by remote Γ-X-mixings in a GaAs/AlAs asymmetric seven-folded quantum-well superlattice separating with very thin barriers

We investigated the photoluminescence (PL) properties of a GaAs/AlAs asymmetric seven-folded quantum well (ASFQW) superlattices (SLs) with thin separating barriers within the ASFQW period. The linearly blue-shifted PL was proportional to the applied bias voltage. By performing a numerical analysis of the electric-field dependence of subband energies in the ASFQW, we concluded that this PL originated from consecutive Γ-X mixings between spatially long-range-separated Γ-subband states and an X-state in this ASFQW. These results indicate that various phenomena appear in ASFQW-SLs due to the increased degree of freedom of carrier transport paths, but a precise analysis of the electric field dependence of subband resonances is required. Therefore, the integrated study of AMQW-SLs definitely reveals new physical properties and leads to the development of new devices.

Theoretical investigation of optoelectronic properties in PbS/CdS core/shell spherical quantum dots under the effect of the electric field intensity, hydrogenic impurity and geometric parameters

In this paper, we theoretically investigated the combined effects of the external electric field (EF) strength and the geometric parameters on the linear, nonlinear and total dielectric functions as well as the effective dielectric function (DF) coefficients of PbS/CdS spherical core/shell quantum dots (CSQD) in the presence of the hydrogenic impurity located at the center. The subband energy eigenvalues and their corresponding wave functions are obtained by solving the time-independent Schrödinger equation and using the variational method (VM) in the framework of the effective mass approximation (EMA). The linear, nonlinear and total DF as well as the effective DF were discussed and evaluated under the influence of EF intensity, geometric parameters, the change of the number of quantum dots (QDs) per unit volume and optical intensity I based on the compact density matrix (CDM) approach. The obtained results show that the height peaks of the linear, nonlinear and Total DF as well as the effective DF coefficients increase and their resonant peak moves towards lower energies as the EF and geometric factors increase in both cases with and without considering the hydrogenic impurity effect. Furthermore, our findings show that the impact of optical intensity and the number of QDs per unit volume does not change the resonance peaks of imaginary parts of DF and effective DF but decreases their magnitudes. As a result, we believe that numerical results will present important developments and provide great contributions in designing new optoelectronic devices related to CSQD hetero-nanostructure.

Quantum confinement of carriers in the type-I quantum wells structure

Abstract Quantum confinement is recognized to be an inherent property in low-dimensional structures. Traditionally, it is believed that the carriers trapped within the well cannot escape due to the discrete energy levels. However, our previous research has revealed efficient carrier escape in low-dimensional structures, contradicting this conventional understanding. In this study, we review the energy band structure of quantum wells along the growth direction considering it as a superposition of the bulk material dispersion and quantization energy dispersion resulting from the quantum confinement across the whole Brillouin zone. By accounting for all wave vectors, we obtain a certain distribution of carrier energy at each quantized energy level, giving rise to the energy subbands. These results enable carriers to escape from the well under the influence of an electric field. Additionally, we have compiled a comprehensive summary of various energy band scenarios in quantum well structures relevant to carrier transport. Such a new interpretation holds significant value in deepening our comprehension of low-dimensional energy bands, discovering new physical phenomena, and designing novel devices with superior performance.

Modulation of nonlinear optical rectification, second, and third harmonic generation coefficients in n-type quadruple δ-doped GaAs quantum wells under external fields

The paper theoretically explores the variations in nonlinear optical rectification (NOR), second (SHG), and third harmonic generation (THG) coefficients of an n-type quadruple δ-doped GaAs quantum well in the presence of electric, magnetic, and THz laser fields. The energies of subbands and associated wave functions are derived through the solution of the Schrödinger equation, utilizing effective mass and parabolic band approximations. The findings indicate that the applied external electric and magnetic fields significantly influence both the magnitudes and the positions of the peaks in NOR, SHG, and THG concerning the incident photon energies. In addition to the influences of electric and magnetic fields, subjecting the system to an intense laser field results in noticeable changes in the THG, SHG, and NOR coefficients. These numerical findings regarding the optical properties of quadruple δ-doped quantum wells in the presence of external fields could offer valuable guidance for experimental studies.

Effects of external fields and structural parameters on third harmonic generation of GaAs/AlGaAs Manning-like double quantum well structure

In this theoretical work, we investigate how external fields like as electric, magnetic, and intense laser fields, as well as structural factors, affect the third harmonic generation (THG) coefficient of an AlGaAs/GaAs Manning-like double quantum well heterostructure. To achieve our goals, we numerically solve the time-independent Schrödinger equation using the diagonalization approach with the effective mass approximation and then derive the subband energy levels and corresponding wave functions of the structure. After that, we derive the mathematical expression of the THG coefficient by using the compact density matrix method. According to our results, the resonance peaks of the THG coefficient show shifting to the high-energy region with an increase in the field’s magnitude in cases where external fields (electric, magnetic, and intense laser) are applied separately. At the same time, increasing the depth (width) of the quantum well structure causes the THG peaks to shift to the high (low) energy region. We believe that the findings from this search will have a substantial impact on existing experimental device designs and applications.

Dynamics of the nonlinear optical second- and third-harmonic generation in quantum well due to hydrostatic pressure and temperature

We studied the effects of hydrostatic pressure and temperature on the second- and third-harmonic generation (SHG and THG) in semi-parabolic GaAs/Al0.3Ga0.7As quantum wells. By solving the Schrödinger equation numerically, we can determine the system's sub-band energy levels and wave functions, and then calculate its optical gain. The numerical simulation results show that both temperature and hydrostatic pressure can effectively improve the nonlinear optical performance of the system. Notably, we explore the mechanisms underlying these interactions. These findings suggest a viable approach to adjust the nonlinear optical performance of GaAs/Al0.3Ga0.7As quantum system through the subtle interaction between hydrostatic pressure, temperature and structural parameters.

Near-unity quantum yield and long-term emission stability in halide perovskite nanocrystal glass composite

Metal halide perovskites are promising optoelectronic materials due to their outstanding luminescent properties. However, the instability of perovskites has long been the bottleneck to their practical applications. Here Cs4PbBr6 nanocrystals based glass composite (Cs4PbBr6 NCs@glass) are successfully prepared, which displays green emission color (520 nm), narrow bandwidth (23 nm) and a near-unity photoluminescence quantum yield (PLQY). The H2O molecules permeating in the lattice of Cs4PbBr6 were found to be a crucial role in the subband energy emission. The Cs4PbBr6 NCs@glass has excellent emission stability; maintains 93 % of initial PL intensity after ultraviolet light irradiation for over 5000 h. In addition, by adjusting the halogen content, we have achieved tunable emission color from blue (450 nm) to green (520 nm) and red (670 nm) on Cs4PbX6 NCs@glass (X = Cl, Br, I), which covers up to 127 % of the National Television Systems Board (NTSC) standard system. Our finding indicates the commercial applications of perovskite materials in lighting and display.

Extracting brain behavior change in patients with migraine by quantitative analysis of electroencephalogram signal of patients compared to healthy people

Background. Migraine disease is the second most common cause of headaches. Despite the high prevalence, the exact etiology of migraine is yet unknown. In this study, to evaluate the behavior change of electroencephalography (EEG) signals in migraine patients, various features of the EEG signals of migraine patients and healthy controls (HCs) were extracted and compared. Methods. This cross-sectional analytical study was conducted on 21 HCs and 18 migraine patients. Various features, such as fractal dimension (FD), approximate entropy (ApEn), and largest Lyapunov exponent (LLE), were calculated from the EEG signals of migraine patients and HCs. Then different frequency sub-bands of delta, theta, alpha, beta, and gamma were extracted using the wavelet transform, and the energy of these sub-bands was computed. By calculating the mean and variance of the features and applying statistical tests, the feature changes were compared between two groups, and channels with significant differences were identified. Results. The mean of ApEn, FD and energy of all frequency sub-bands in most of the analyzed channels was higher in migraine patients than in HCs. The mean LLE was mostly lower in migraine patients than in healthy controls. According to the statistical tests, the energy of theta and delta frequency sub-bands with 36 and 35 channels was the feature with the highest number of channels, with a significant difference. In this study, P values less than 0.05 were considered statistically significant. Conclusion. Migraine patients may have a less sophisticated brain dynamic system due to an increase in irregularity and randomness, as indicated by an increase in ApEn and a decrease in FD in their EEG signals compared to HCs. Anxiety, tension, and other intense sentiments and emotions, as well as the creation of new neural circuits in the brain, can all contribute to an overall increase in energy across all frequency sub-bands in migraine patients. Practical Implications. Considering the EEG signal behavior as the response of a dynamic system, we can say that the brain function of migraine patients, even in the inter-ictal phase, leaves the definite chaotic state, which is a healthy brain behavior, and enters the random state.

Analytical Modeling for Photocurrent and Detectivity of TiO2/ZnS Core‐Shell Quantum Dot Photodetectors

AbstractIn this paper, a theoretical model has been presented to calculate the photocurrent and detectivity in a TiO2/ZnS core‐shell quantum dot (CSQD) photodetector. Analytical modeling permits to calculate the available photocurrent and corresponding detectivity from the CSQD structures. In this theoretical model, the subband energy levels of the CSQD are estimated using the 3D symmetric quantum box structures with a finite potential barrier. The finite band offset determines the number of available subband energy levels in the CSQD structures depending on the size of the core. Theoretical results show the variation of photocurrent and corresponding detectivity with different core sizes of the CSQD for a constant shell thickness. The calculations indicate that for electronic intersubband transitions detectivity is higher compared with the hole transitions.

Wannier–Stark localization of electronic states in 4H-SiC MOS inversion layer

The electronic states in 4H-SiC MOS inversion layers are theoretically analyzed using the empirical pseudopotential method (EPM). The analysis shows that the Wannier–Stark localization occurs, which is absent in an effective mass approximation (EMA). The Wannier–Stark localization modifies the electronic states in the MOS inversion layers. A model is proposed to describe the in-plane dispersion of subbands affected by the Wannier–Stark localization. The differences between the EPM and EMA results for the subband energy levels and the in-plane effective masses are discussed.

Combined effect of rashba spin-orbit interaction, hydrostatic pressure and temperature on energy dispersion based ballistic conductance of InAs tunnel-coupled (double) quantum wire under exterior magnetic and electric field

In this work the change in energy dispersion curve and ballistic conductance for InAs tunnel-coupled double quantum wire (DQWR) under the influence of the external magnetic field, electric field, hydrostatic pressure, temperature, and Rashba spin-orbit interaction (SOI) are studied theoretically in detail. A constant magnetic field transverse to the fundamental plane is applied to the DQWR. The Landuer-Büttiker formalism is used to determine the transport characteristics, and the energy eigenvectors and eigenvalues are derived within the parameters of the diagonalization approach. The numerical results indicate that the energy spectra of the system are influenced by external factors such as magnetic field, electric field, temperature, hydrostatic pressure, and Rashba SOI, resulting in variations in lateral/vertical and downward/upward shifts. These factors also cause oscillatory patterns in ballistic conductance due to the oddity of energy subbands. These results have significant implications for the advancement of double-quantum wire-based devices.

A Closed-Form Model for 2-DEG Charge and Surface Potential in N-Polar Gallium Nitride Heterostructures

A closed-form model to calculate the 2-D electron gas (2-DEG) charge density and surface potential in an N-polar GaN/AlGaN heterostructure is presented. The proposed model is developed from the solution of Schrödinger’s equation for a finite triangular potential well. Asymptotic expressions of Airy functions are used to compute an initial value of subband energy. A new, unified expression to update the surface potential due to Poisson’s equation is presented. The modified Fang–Howard (M.F.H.) function is then employed to develop a closed-form correction to the subband energy using first-order perturbation theory. No fitting parameters are used. The results from our model are validated with numerical data for a wide range of bias conditions, channel layer thickness, and spacer layer mole fraction. Finally, the gate capacitance of the heterostructure is evaluated using automatic differentiation and validated against experimental data.

Giant quantum oscillations of acoustoelectric current in narrow graphene nanoribbons

A theory is presented for the acoustoelectric (AE) effect in narrow graphene nanoribbons (GNRs) deposited on a piezoelectric substrate when a surface acoustic wave (SAW) is launched onto the surface of the piezoelectric. It is assumed that the electron density in such GNRs can be controlled by applying an external gate voltage, and the acoustic wavelength is much smaller than the mean free path of electrons, so that the quantum mode of interaction of those electrons with the SAW in realized. Using the kinetic theory approach, we calculate the AE current flowing through the GNR sample and arising as a result of momentum transfer from coherent SAW phonons to conduction electrons. It is shown that size quantization of the electron energy spectrum in narrow GNRs leads to giant periodic oscillations of the AE current with a change in the gate voltage. AE current surges occur whenever the Fermi level, rising with increasing gate voltage, crosses in turn the bottom of each of the successive size-quantized electron energy subbands. The oscillations predicted are giant in the sense that the maximum values of the current exceed its minimum values by at least an order of magnitude, and they can be observed in narrow GNRs ∼10 nm wide even at room temperature.

Stochastic simulation of electron transport in a strong electrical field in low-dimensional heterostructures

Abstract In this paper we develop a stochastic simulation algorithm for electron transport in a DA-pHEMT heterostructure. Mathematical formulation of the problem of electron gas transport in the heterostructure in the form of a coupled system of Poisson, Schrödinger and kinetic Boltzmann equations is given. A Monte Carlo model of electron transport in DA-pHEMT heterostructures which accounts for multivalley parabolic band structure, as well as relevant formulas for calculating electron scattering rates and scattering phase functions on polar optical, intervalley phonons and on impurities are developed. The results of a computational experiment involving the solution of the system of Poisson–Schrödinger–Boltzmann equations for the AlGaAs/GaAs/InGaAs/GaAs/AlGaAs heterostructure are presented. The distribution of electrons by energy subband in the main and satellite valleys and the field dependences of the electron drift velocity in each valley are calculated. It was discovered that there is no spatial transfer of electrons into wide-gap AlGaAs layers due to high barriers created by modulated-doped impurities. A comparative analysis of the electron drift velocities in the studied DA-pHEMT heterostructures and in the unstrained layer of the InGaAs is given.

An Efficient Voice Authentication System using Enhanced Inceptionv3 Algorithm

Automatic voice authentication based on deep learning is a promising technology that has received much attention from academia and industry. It has proven to be effective in a variety of applications, including biometric access control systems. Using biometric data in such systems is difficult, particularly in a centralized setting. It introduces numerous risks, such as information disclosure, unreliability, security, privacy, etc. Voice authentication systems are becoming increasingly important in solving these issues. This is especially true if the device relies on voice commands from the user. This work investigates the development of a text-independent voice authentication system. The spatial features of the voiceprint (corresponding to the speech spectrum) are present in the speech signal as a result of the spectrogram, and the weighted wavelet packet cepstral coefficients (W-WPCC) are effective for spatial feature extraction (corresponding to the speech spectrum). W- WPCC characteristics are calculated by combining sub-band energies with sub-band spectral centroids using a weighting scheme to generate noise-resistant acoustic characteristics. In addition, this work proposes an enhanced inception v3 model for voice authentication. The proposed InceptionV3 system extracts feature from input data from the convolutional and pooling layers. By employing fewer parameters, this architecture reduces the complexity of the convolution process while increasing learning speed. Following model training, the enhanced Inception v3 model classifies audio samples as authenticated or not based on extracted features. Experiments were carried out on the speech of five English speakers whose voices were collected from YouTube. The results reveal that the suggested improved method, based on enhanced Inception v3 and trained on speech spectrogram pictures, outperforms the existing methods. The approach generates tests with an average categorization accuracy of 99%. Compared to the performance of these network models on the given dataset, the proposed enhanced Inception v3 network model achieves the best results regarding model training time, recognition accuracy, and stability.

Laser field effect on the normalized self-polarization, self-polarization and binding energy in square quantum wells made of different materials

In this study, we examine the effect of the laser field on normalized self-polarization, self-polarization, and binding energy in square quantum wells made of four different materials under effective mass approximation. The effects of well width, laser field, and impurity position on normalized self-polarization, self-polarization, and binding energies are shown in detail. The subband energies are obtained by the finite difference method, and the impurity energies are calculated by the variational method. The laser field significantly affects binding energy, self-polarization, and normalized self-polarization. The term normalized self-polarization is defined for the first time in this study. This allows a more detailed examination of the self-polarization change depending on the impurity position. Examining the laser field effect, especially in square quantum wells made of different materials, will provide researchers with helpful information about the importance of material selection in calculating binding energy, self-polarization, and normalized self-polarization.

Automated diagnosis of EEG abnormalities with different classification techniques.

Automatic seizure detection and prediction using clinical Electroencephalograms (EEGs) are challenging tasks due to factors such as low Signal-to-Noise Ratios (SNRs), high variance in epileptic seizures among patients, and limited clinical data constraints. To overcome these challenges, this paper presents two approaches for EEG signal classification. One of these approaches depends on Machine Learning (ML) tools. The used features are different types of entropy, higher-order statistics, and sub-band energies in the Hilbert Marginal Spectrum (HMS) domain. The classification is performed using Support Vector Machine (SVM), Logistic Regression (LR), and K-Nearest Neighbor (KNN) classifiers. Both seizure detection and prediction scenarios are considered. The second approach depends on spectrograms of EEG signal segments and a Convolutional Neural Network (CNN)-based residual learning model. We use 10000 spectrogram images for each class. In this approach, it is possible to perform both seizure detection and prediction in addition to a 3-state classification scenario. Both approaches are evaluated on the Children's Hospital Boston and the Massachusetts Institute of Technology (CHB-MIT) dataset, which contains 24 EEG recordings for 6 males and 18 females. The results obtained for the HMS-based model showed an accuracy of 100%. The CNN-based model achieved accuracies of 97.66%, 95.59%, and 94.51% for Seizure (S) versus Pre-Seizure (PS), Non-Seizure (NS) versus S, and NS versus S versus PS classes, respectively. These results demonstrate that the proposed approaches can be effectively used for seizure detection and prediction. They outperform the state-of-the-art techniques for automatic seizure detection and prediction. Block diagram of proposed epileptic seizure detection method using HMS with different classifiers.

Multi‐Subband Polar Optical Phonon Scattering in InAlN/AlN/GaN Heterostructures

The high density of two‐dimensional electron gas (2DEG) in InAlN/AlN/GaN heterostructures results in significant occupation of higher subbands, it is thus imperative to consider multi‐subband occupation and inter‐subband scattering in the calculations of polar optical phonon (POP) scattering. The physical parameters, such as subband energies, wave functions of the four lowest subbands, and Fermi level of InAlN/AlN/GaN heterostructures, are calculated by solving the Schrödinger–Poisson equations and applied to the multi‐subband POP scattering model. The dependence of mobility, which is limited by multi‐subband POP scattering, on temperature and 2DEG density is studied. The results show that multi‐subband occupation and inter‐subband scattering play important roles in electron transport. The higher subbands contribute 26% to room‐temperature electron mobility. Two simplified POP scattering models, which either ignore multi‐subband occupation or simplify the in‐scattering processes are evaluated. The results show that both simplified models introduce significant deviations in the mobilities that are limited by POP scattering. The relative magnitudes of mobility in each subband depend on the energy dependence of momentum relaxation time and the energy distribution of each subband.