Lowest Subband Research Articles

Theoretical investigation of enhanced nonlinear optical properties of silicene and carbon nanotubes: Potential applications in infrared and ultraviolet optoelectronics

This theoretical study investigates the linear and nonlinear optical properties of zigzag carbon nanotubes (CNTs) and silicene nanotubes (SiNTs) with varying radii, focusing on their behavior in the infrared and ultraviolet energy ranges. In the infrared region, absorption spectra exhibit several peaks resulting from allowed transitions between valence and conduction bands. The number of absorption peaks increases with radius for both nanotube types, with SiNTs showing peaks at lower energy ranges and higher intensities compared to CNTs. Conversely, CNTs display markedly higher absorption intensities in the ultraviolet region. The quadratic electronic optic (DC Kerr) effect reveals sharp peaks near the band gap with multiple sign changes, attributed to allowed optical transitions at band edges. The third-order optical susceptibility for both CNT and SiNT structures show several peaks below the band gap energy due to multiphoton resonance absorption. In the infrared region, two highest and lowest subbands near to the Fermi level play a dominant role in the χTHG(3)(3ω) peaks formation. The position and intensity of χTHG(3)(3ω) peaks demonstrate a strong dependence on nanotube radius and type with higher intensity for the SiNTs. The tunable nature of the optical properties of CNTs and SiNTs by their radius and the enhanced nonlinear optical response of SiNTs, characterized by lower energy peaks and higher intensities, show their significant potential for advanced applications in nonlinear optics, optical detection, and high-energy optical systems.

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.

Spin and valley degrees of freedom in a bilayer graphene quantum point contact: Zeeman splitting and interaction effects

We present a study on the lifting of degeneracy of the size-quantized energy levels in an electrostatically defined quantum point contact in bilayer graphene by the application of in-plane magnetic fields. We observe a Zeeman spin splitting of the first three subbands, characterized by effective Land\'e $g$ factors that are enhanced by confinement and interactions. In the gate-voltage dependence of the conductance, a shoulderlike feature below the lowest subband appears, which we identify as a 0.7 anomaly stemming from the interaction-induced lifting of the band degeneracy. We employ a phenomenological model of the 0.7 anomaly to the gate-defined channel in bilayer graphene subject to in-plane magnetic field. Based on the qualitative theoretical predictions for the conductance evolution with increasing magnetic field, we conclude that the assumption of an effective spontaneous spin splitting is capable of describing our observations, while the valley degree of freedom remains degenerate.

Steady-state and transient electronic transport properties of β-(Al x Ga1–x )2O3/Ga2O3 heterostructures: An ensemble Monte Carlo simulation

The steady-state and transient electron transport properties of β-(Al x Ga1−x )2O3/Ga2O3 heterostructures were investigated by Monte Carlo simulation with the classic three-valley model. In particular, the electronic band structures were acquired by first-principles calculations, which could provide precise parameters for calculating the transport properties of the two-dimensional electron gas (2DEG), and the quantization effect was considered in the Γ valley with the five lowest subbands. Wave functions and energy eigenvalues were obtained by iteration of the Schrödinger–Poisson equations to calculate the 2DEG scattering rates with five main scattering mechanisms considered. The simulated low-field electron mobilities agree well with the experimental results, thus confirming the effectiveness of our models. The results show that the room temperature electron mobility of the β-(Al0.188Ga0.812)2O3/Ga2O3 heterostructure at 10 kV⋅cm−1 is approximately 153.669 cm2⋅V−1⋅s−1, and polar optical phonon scattering would have a significant impact on the mobility properties at this time. The region of negative differential mobility, overshoot of the transient electron velocity and negative diffusion coefficients are also observed when the electric field increases to the corresponding threshold value or even exceeds it. This work offers significant parameters for the β-(Al x Ga1−x )2O3/Ga2O3 heterostructure that may benefit the design of high-performance β-(Al x Ga1−x )2O3/Ga2O3 heterostructure-based devices.

On-Chip Voltage-Controlled Oscillator Based on a Center-Tapped Switched Inductor Using GaN-on-SiC HEMT Technology

This study presents a voltage-controlled oscillator (VCO) in a cross-coupled pair configuration using a multi-tapped switched inductor with two switch-loaded transformers in 0.5 µm GaN technology. Two switch-loaded transformers are placed at the inner and outer portions of the multi-tapped inductor. All the switches are turned off to obtain the lowest sub-band. The outer transformer with three pairs of switches is turned on alternately to provide three sub-band modes. A pair of switches at the inner transformer provide a high-frequency band. Two switch-loaded transformers are turned on to provide the highest sub-band. Six modes are selected to provide a wide tuning range. The frequency tuning range (FTR) of the VCO is 27.8% from 3.81 GHz to 8.04 GHz with a varactor voltage from 13 V to 22 V. At a 1 MHz frequency offset from the carrier frequency of 4.27 GHz, the peak phase noise is −119.17 dBc/Hz. At a power supply of 12 V, the output power of the carrier at 4.27 GHz is 20.9 dBm. The figure of merit is −186.93 dB because the VCO exhibits a high output power, low phase noise, and wide FTR. To the best of the author’s knowledge, the FTR in VCOs made of GaN-based high electron mobility transistors is the widest reported thus far.

The theoretical study of the mobility of a two-dimensional electron gas in ALGaN/GaN/ALGaN double heterostructures

We investigate the drift and Hall mobility of a quasi-two-dimensional electron gas (Q2DEG) confined in wurtzite AlGaN/GaN/AlGaN square quantum wells (QWs). We use the variational-subband-wave-function model for carrier confinement and assume that the electrons only occupy the lowest subband. We take into consideration the most important scattering mechanisms such as the edge and charged dislocation, remote and background impurity, interface roughness, acoustic phonon via deformation potential and piezoelectric field, and polar LO phonon. Assuming the scattering by acoustic phonons to be quasi-elastic and using the iterative method for inelastic regime, where the LO phonon scattering becomes important, we study the dependence of the mobility on the temperature T, carrier density $$N_{\mathrm {s}}$$ , and QW width L.

Conductance quantisation in patterned gate In0.75Ga0.25As structures up to 6 × (2e2/h)

We present electrical measurements from In0.75Ga0.25As 1D channel devices with Rashba-type, spin–orbit coupling present in the 2D contact regions. Suppressed backscattering as a result of the time-reversal asymmetry at the 1D channel entrance results in enhanced ballistic transport characteristics with clear quantised conductance plateaus up to 6 × (2e2/h). Applying DC voltages between the source and drain ohmic contacts and an in-plane magnetic field confirms a ballistic transport picture. For asymmetric patterned gate biasing, a lateral spin–orbit coupling effect is weak. However, the Rashba-type spin–orbit coupling leads to a g-factor in the 1D channel that is reduced in magnitude from the 2D value of 9 to ~6.5 in the lowest subband when the effective Rashba field and the applied magnetic field are perpendicular.

Slantlet filter-bank-based satellite image enhancement using gamma-corrected knee transformation

ABSTRACTIn this paper, an efficient and relatively fast approach for satellite image enhancement is proposed. This technique is based on auto-knee transfer function with suitable gamma correction using slantlet transform for two-scale decomposed image. Dark or low contrast, big-data (or large sized) multispectral images can be easily enhanced by proper tuning for the value of gamma-parameter using slantlet transform. Here, sub-band decomposition is achieved by employing single-level slantlet filter-bank, which is just equivalent to second-level sub-band decomposition using discrete wavelet transform) that has been employed initially. For this purpose, main information of the image is concentrated to lowest sub-band, over which gamma correction is applied after computing the knee transfer function adaptively for low quality input image. In addition to this two-scale decomposition-based enhancement, here, gamma-corrected energy redistributed slantlet transform-based textural enhancement framework is also suggested. The experimentation comprised of relative performance evaluation and comparison on the same scale; clearly reflects the outperformance of proposed methodology over various well-known pre-existing state-of-the-art techniques both quantitatively and qualitatively.

Two-band k.p Hamiltonian of phosphorene based on the infinitesimal basis transformations approach

In this work, the two-band Hamiltonian of phosphorene (the two-dimensional black phosphorus) was derived in the framework of k.p theory using infinitesimal basis transformation approach. Our main focus was on the lowest subband in conduction band and the highest subband in valence band belonging to the irreducible representations Г4− and Г2+, respectively. We have extended our modeling up to the second-order corrections in the Hamiltonian elements. Our results show the indirectness of the band gap and the anisotropy of the energy dispersion relations for electrons and holes, which is naturally endowed by the orthorhombic symmetry of the crystal lattice.

Strain controlled valley filtering in multi-terminal graphene structures

Valley-polarized currents can be generated by local straining of multi-terminal graphene devices. The pseudo-magnetic field created by the deformation allows electrons from only one valley to transmit, and a current of electrons from a single valley is generated at the opposite side of the locally strained region. We show that valley filtering is most effective with bumps of a certain height and width. Despite the fact that the highest contribution to the polarized current comes from electrons from the lowest sub-band, contributions of other sub-bands are not negligible and can significantly enhance the output current.

Angular momentum and topology in semiconducting single-wall carbon nanotubes

Semiconducting single-wall carbon nanotubes are classified into two types by means of orbital angular momentum of valley state, which is useful to study their low energy electronic properties in finite-length. The classification is given by an integer $d$, which is the greatest common divisor of two integers $n$ and $m$ specifying the chirality of nanotubes, by analyzing cutting lines. For the case that $d$ is equal to or greater than four, two lowest subbands from two valleys have different angular momenta with respect to the nanotube axis. Reflecting the decoupling of two valleys, discrete energy levels in finite-length nanotubes exhibit nearly fourfold degeneracy and its small lift by the spin-orbit interaction. For the case that $d$ is less than or equal to two, in which two lowest subbands from two valleys have the same angular momentum, discrete levels exhibit lift of fourfold degeneracy reflecting the coupling of two valleys. Especially, two valleys are strongly coupled when the chirality is close to the armchair chirality. An effective one-dimensional lattice model is derived by extracting states with relevant angular momentum, which reveals the valley coupling in the eigenstates. A bulk-edge correspondence, relationship between number of edge states and the winding number calculated in the corresponding bulk system, is analytically shown by using the argument principle, which enables us to estimate the number of edge states from the bulk property. The number of edge states depends not only on the chirality but also on the shape of boundary.

Modeling the spectral shape of InGaAlP-based red light-emitting diodes

We have developed a spectral model for describing the shape of the emission spectrum of InGaAlP-based red light-emitting diodes (LEDs) with quantum-well structure. The model is based on Maxwell-Boltzmann distribution with junction temperature Tj and an experimental two-dimensional joint density of states (DOS). We model the DOS with a sum of two exponentially broadened step functions describing the two lowest sub-bands in semiconductor quantum well. The relative locations ΔE1 = 0 meV and ΔE2 = 112.7 meV above the band gap energy Eg = 1.983 eV and the ratio 2.13 of the step heights were fixed using an experimental DOS extracted from a LED spectrum measured at known Tj and driving current I. The model can then be fitted to other spectra of other LED samples at varied Tj and I by varying the fitting parameters Eg, Tj, and the broadening of the sub-band edges. The model was tested for three LED samples over I = 200–370 mA and Tj = 303–398 K. Junction temperatures obtained by modeling were compared with calibrated Tj obtained by the forward voltage method. The mean absolute difference was about 2.9 K (0.8%) over the whole region studied and the maximum difference was 8.5 K. The thermal coefficient measured for Eg was −0.509 meV K−1. For the first and second sub-band edges, the thermal broadening coefficients were 18 μeV K−1 and 37 μeV K−1, respectively.

Partial spin polarization of a conductance in a bi-layer [formula omitted] heterostructure based nanowire for the rectangular and the smooth lateral confinement potentials

We simulate the electron transport in a vertical bi-layer nanowire in order to study an influence of the lateral confinement's shape on a spin polarization of wire's conductance. The active part of considered quantum wire constitutes a double inverted heterojunction In0.52Al0.48As/In0.53Ga0.47As which nanostructure can be fabricated in molecular beam epitaxy process while the lateral confinement potential can be finally formed by means of cleaved overgrowth or surface oxidization methods giving the desired rectangular and smooth lateral confinement. In calculations we take into account interaction between charge carriers using DFT within local spin density approximation. We show that if the magnetic field is perpendicular to the wire axis, the pseudogaps are opened in energy dispersion relation E(k) what in conjunction with spin Zeeman shift of spin-up and spin-down subbands may enhance the spin polarization of conductance with reference to a single layer wire. For nanowire with rectangular lateral confinement potential we found that the electron density has two maximums localized at wire edges in each layers. This modificates strongly all magnetosubbands giving up to four energy minimums in lowest subband and considerably diminishes widths of pseudogaps what translates into low maximal spin polarization of conductance, not exceeding 40%. This drawback is absent in wire with smooth lateral confinement. However, in order to gain a large spin polarization simultaneous tuning of magnetic field as well as the Fermi energies in both layers of nanowire are required.

Polaronic effects on the collective excitation in a GaAs parabolic quantum wire

In this paper, the plasmon-longitudinal optical (LO) phonon interaction effects on the intrasubband structure factor, on the pair correlation function and on the intrasubband plasmon energy associated with the lowest subband in a GaAs parabolic quantum well wire (QWW) are investigated by varying the subband separation energy and the electronic density. The calculations are performed using the self-consistent field approximation, which includes the local-field correction within the Singwi, Tosi, Land and Sjölander (STLS) theory, at zero temperature and assuming a three-subband model where only the first subband is occupied by electrons. The theoretical results obtained via STLS theory are compared with the random phase approximation (RPA) approach results, in order to emphasize the importance of the local field corrections (LFC) for the calculation of the collective excitations in quasi-one-dimensional systems.

A Compact Model for Generic MIS-HEMTs Based on the Unified 2DEG Density Expression

In this paper, the 2-D electron gas density <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$({n}_{s})$</tex></formula> and Fermi level <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$({E}_{f})$</tex></formula> analytical expressions as an explicit function of the terminal biases that covers the strong- and moderate-inversion and subthreshold regions and scalable with physical parameters are developed. It is validated by the comparison with the (exact) numerical solutions for different device parameters, in which the device operating region may encompass one or two lowest sub-bands ( <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">${E}_{0}$</tex></formula> and <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">${E}_{1}$</tex> </formula> ) in the triangular well. With the unified <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">${E}_{f}$</tex></formula> model, a surface-potential <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$(\phi_{s})$</tex></formula> based drain-current <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$({I}_{\rm ds})$</tex></formula> model for the metal–insulator–semiconductor (MIS) high electron-mobility transistor (HEMT) is developed. Nonlinear source/drain access region resistances ( <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">${R}_{s}$</tex></formula> and <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">${R}_{d}$</tex></formula> ) can also be modeled via a subcircuit, including an empirical <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">${R}_{s}$</tex></formula> model for capturing the current-collapse effect. The compact drain-current model is shown to match the experimental data of MIS HEMTs very well in both subthreshold and strong-inversion regions, with smooth and symmetric behaviors and including the (dc) self-heating effect. It also models the corresponding MIS diode <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$C-V$</tex></formula> using the same set of physical and minimum fitting parameters.

A Rate Control Algorithm in JPEG2000 Based on Code Content Prediction

In order to accelerate the coding process of JPEG2000 and avoid encoding useless codes, a new algorithm is proposed for JPEG2000 rate control. This algorithm is improved by the model prediction algorithm.The code passes transmit the lowest frequency sub-band without distortion after wavelet decomposition. Then the proposed algorithm allocates the rate for the current code block accords to the number of coding passes and the weight of wavelet quantization. At last, it encodes the code block which brings the greatest amount of information preferentially, accords to the code block bit-plane relationship in the sub-band; In addition, the algorithm truncates the code-stream in real time during the encoding procedure. Compared with the PCRD algorithm, the experimental results indicate that the proposed algorithm can greatly reduce the computation operation and encoding time. DOI: http://dx.doi.org/10.11591/telkomnika.v11i11.3720 Full Text: PDF

Carrier dynamics and stimulated radiative terahertz transitions between Landau levels in cascade GaAs/AlGaAs quantum well structures

The carrier distribution over Landau levels was studied in resonant tunneling GaAs/AlGaAs quantum well structures under tunneling pumping of the upper subband. The numerical calculations of the Landau level populations for various values of pumping intensity (tunneling time), magnetic field and structure doping were carried out. The population inversion between zeroth Landau level of the upper subband and the first Landau level of the lowest subband was shown to exist in wide range of the magnetic field strength. The effect of various scattering mechanisms, both two-particle (electron-electron scattering) and single-particle (acoustic phonon and interface roughness scattering) ones, on level population was studied. The way of lifting the selection rule forbidding the inter-Landau level terahertz transitions of interest and achieving considerable values of the dipole matrix element is proposed.

Analysis of strained double-lightly doped MOSCNT using NEGF

The effects of uniaxial and torsional strains on the double-lightly doped MOSCNT (DLD-MOSCNT) performances are investigated, using the non-equilibrium Green function (NEGF) formalism in mode space approach. The Hamiltonian of the device is obtained by a tight-binding approximation assuming that only pz orbitals are contributing in carrier transport. In all simulation processes, one mode with the lowest subband is considered. DLD-MOSCNT has a small band-to-band tunneling and almost eliminates the ambipolar behavior of IDS–VGS characteristics because of the band engineering. We use a modified model to demonstrate the strain effects on such a low OFF-current device. The results show that the strain effects mainly depend on the chiral vector and diameter of CNT. The strain causes band gap and carrier velocity changes, which result in variation of ON- or OFF-current. In addition, the subthreshold swing of this device under uniaxial strain is calculated, which is about 61 mV/Dec for 2 % tensile strain in (16,0) and for −2 % compressive strain in (17,0). Under the uniaxial strain, in the case that the energy band gap increases, the variation of DIBL is very small.

An Efficient Coding Algorithm for Photogrammetric Image

For solving the problems of storage and transmission existing in the high-resolution photogrammetric image, an efficient photogrammetric image encoding method based on Region of Interest (ROI) is proposed according to the application characters of photogrammetric image. The ROIs of the photogrammetric image are automatically detected and extracted using the binarization segmentation and the image dilation algorithm. The description of ROI mask is achieved by combining the coefficient similarity in the same level wavelet sub-bands with specific wavelet filter at an extremely low bit rate. All ROI coefficients are compressed using the lossless method to completely reserve the image information necessary for higher measurement precision. According to the energy concentration features of wavelet transform, only the BG (Back Ground) coefficients belonging to the lowest frequency sub-band are encoded in order to improve total compression ratio. Experimental results show that the ROI mask and the main context information could be described at less than 0.03bit/pixel. Furthermore, the proposed method has improved its compression ratio by more than 6%, and reduced the encoding time above 30% as compared with those of the MaxShift method based on JPGE2000. Thus, the proposed method realizes a high efficiency encoding of photogrammetric image and lays a good foundation for the storage and transmission of the high-resolution photogrammetric image.

Bistability and hysteresis of intersubband absorption in strongly interacting electrons on liquid helium

We study nonlinear inter-subband microwave absorption of electrons bound to the liquid helium surface. Already for a comparatively low radiation intensity, resonant absorption due to transitions between the two lowest subbands is accompanied by electron overheating. The overheating results in a significant population of higher subbands. The Coulomb interaction between electrons causes a shift of the resonant frequency, which depends on the population of the excited states and thus on the electron temperature ${T}_{e}$. The latter is determined experimentally from the electron photoconductivity. The experimentally established relationship between the frequency shift and ${T}_{e}$ is in reasonable agreement with the theory. The dependence of the shift on the radiation intensity introduces nonlinearity into the rate of the inter-subband absorption, resulting in bistability and hysteresis of the resonant response. The hysteresis of the response explains the behavior in the regime of frequency modulation, which we observe for electrons on liquid ${}^{3}$He and which was previously seen for electrons on liquid ${}^{4}$He.