Higher Subbands Research Articles

ELECTRON SPIN DYNAMICS THROUGH FERROMAGNETIC QUANTUM POINT CONTACT

We study spin-dependent tunneling through ferromagnetic quantum point contact. It was found that both the current and the conductance show increasing oscillatory fluctuations with the variation of Fermi energy and contact area at different temperatures. These oscillations are due to polarization inversion at the incoming of each new sub-band and increasing transmission probability through the higher sub-bands. The results are found to be in good concordance with those in the literature.

Numerical Study of $C$–$V$ Characteristics of Double-Gate Ultrathin SOI MOSFETs

Capacitance-voltage (C-V) characteristics of double-gate ultrathin silicon-on-insulator (SOI) MOSFETs are numerically investigated in detail. The measured back-gate bias dependence is reproduced by the Poisson-Schrodinger solver including the highly precise physical models for many-body interactions of carrier-carrier and carrier-ion, and for incomplete ionization of doping impurities in whole semiconductor regions of n+poly-Si/oxide/SOI/oxide/p-Si capacitor including the volume inversion. In addition, we study the higher subband effect at higher temperature in detail, in order to deduce the impacts of self-heating and nonstatic transport

Wideband speech coding robust against packet loss

This paper describes a wideband speech coding method that is robust against packet loss, and a packet transmission control method. The coding is based on 4-kHz-band CELP speech coding and consists mainly of multilevel coding using MDCT transform encoding, with a speech quality enhancement codec for the nonspeech portion and a higher subband enhancement codec for coding the higher subband at or above 4 kHz. This method can provide high-quality coding of nonspeech signals such as background music as well as wideband speech. If this coding method and packet transmission control are combined, speech transmission can continue without much deterioration even when the network becomes congested and packet loss occurs. Subjective and objective testing confirmed that, unlike conventional coding methods, high-quality speech transmission was possible even with 10% random packet loss when this method is used for speech transmission on an IP network. © 2006 Wiley Periodicals, Inc. Electron Comm Jpn Pt 3, 89(12): 20–30, 2006; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/ecjc.20291

Intersubband excitations at finite temperatures and their roles in different quasi-one-dimensional systems

We theoretically study many-body excitations in three different quasi-one-dimensional (Q1D) electron systems: (i) those formed on the surface of liquid Helium; (ii) in two coupled semiconductor quantum wires; and (iii) Q1D electrons embedded in polar semiconductor-based quantum wires. Our results show intersubband coupling between higher subbands and the two lowest subbands affecting even the lower energy intersubband plasmons on the liquid Helium surface. Concerning the second system, we show a pronounced extra peak appearing in the intersubband impurity spectral function for temperatures as high as 20 K. We finally show coupled intersubband plasmon-phonon modes surviving for temperatures up to 300 K.

Monte Carlo simulation of in-plane spin-polarized transport in GaAs/GaAlAs quantum well in the three-subband approximation

We develop a Monte Carlo (MC) tool incorporated with the three-subband approximation model to investigate the in-plane spin-polarized transport in GaAs/GaAlAs quantum well. Using the tool, the effects of the electron occupation of higher subbands and the intersubband scattering on the spin dephasing have been studied. Compared with the corresponding results of the simple one-subband approximation model, the spin dephasing length is reduced four times under 0.125 kV/cm of driving electric field at 300K by the MC tool incorporated with the three-subband approximation model, indicating that the three-subband approximation model predicts significantly shorter spin dephasing length with temperature increasing. Our simulation results suggest that the effects of the electron occupation of higher subbands and the intersubband scattering on the spin-dependent transport of GaAs 2-dimensional electron gas need to be considered when the driving electric field exceeds the moderate value and the lattice temperature is above 100K. The simulation by using the MC tool incorporated with the three-subband approximation model also indicates that, under a certain driving electric field and lattice temperature, larger channel widths cause spins to be depolarized faster. Ranges of the three components of the spins are different for three different injected spin polarizations due to the anisotropy of spin–orbit interaction.

Helicity and Electron-Correlation Effects on Transport Properties of Double-Walled Carbon Nanotubes

Starting from selection rules for intershell tunneling in double-walled nanotubes with commensurate (c-DWNTs) and incommensurate (i-DWNTs) shells, we show that for i-DWNTs the coupling is negligible between lowest energy subbands, but it becomes important as the higher subbands become populated. In turn the elastic mean-free path of i-DWNTs is reduced for increasing energy, with additional suppression at subband onsets and crossings. At low energies, a Luttinger liquid theory for DWNTs with metallic shells is derived. Interaction effects are more pronounced in i-DWNTs than in c-DWNTs.

Generalized double-exchange model for magnetic semiconductors with angular momentum j

To facilitate the search for new magnetic semiconductors with high transition temperatures T C , we use dynamical mean-field theory to evaluate T C for a double-exchange system with general angular momentum j = 1 / 2 , 3 / 2 , 5 / 2 , … . For simplicity, we assume that there is one local moment per site and that the Hund's coupling J c between the local moments and the charge carriers (with undoped bandwidth W ) is large. The maximum Curie temperature T C max ( m j , j ) for a given m j and j occurs when the m j sub-band is half-filled. For a fixed j, T C max ( m j , j ) is the largest in the lowest or the highest sub-band with m j = ± j , where the carriers are most optimally coupled to the local moments. When j ⪢ 1 , T C max ( ± j , j ) scales like W / 2 j + 1 , which is the bandwidth of each m j sub-band. For j = 1 / 2 , T C max ( ± 1 / 2 , 1 / 2 ) is suppressed by fluctuations of the carrier spin. Surprisingly, T C max ( ± j , j ) reaches a maximum for j = 3 / 2 , the same angular momentum as the charge carriers in p-band semiconductors like GaAs and Ge.

Conductance quantization in deep mesa-etched gate-controlled ballistic electron waveguides

We present a theoretical study of the ballistic conductance in electron waveguides created by deep mesa etching from quantum-well structures with a two-dimensional electron gas (2DEG) in the well. The widths of the waveguides are controlled by gate bias voltages. We consider three different cases: the etched waveguide is completely covered by a gate [continuous gate (CG)], the gate is deposited on top of the waveguide [top gate (TG)], and when the gates are located on the etched side walls [side gate (SG)]. The number and periodicity of the quantized conductance steps, as well as the energy separation of the one-dimensional subbands near the Fermi level are determined as functions of the parameters of the device. The CG device provides a fairly periodic quantized conductance staircase. The highest subband separation is achieved for the TG device etched well below the 2DEG layer and for the SG device etched slightly below it.

Intersubband cyclotron resonance of holes in strained Ge/GeSi(111) heterostructures with germanium wide quantum wells and cyclotron resonance of 1L electrons in GeSi layers

The submillimeter (ℏω=0.5–5 meV) magnetoabsorption spectra of strained Ge/Ge1−xSix(111) multilayer heterostructures with thick Ge layers (dGe=300–850 A, dGeSi≈200 A, x≈0.1) are investigated at T=4.2 K upon band-gap optical excitation. It is revealed that the absorption spectra contain cyclotron resonance lines of 1L electrons localized in GeSi solid solution layers (unlike the previously studied structures with thin Ge layers as quantum wells for 3L electrons). The absorption spectra of the samples with thick Ge layers (dGe=800–850 A) exhibit cyclotron resonance lines of holes due to transitions from the lower Landau levels in the first quantum-well subband to the Landau levels belonging to the third and fifth higher subbands.

Inter- and intra-subband relaxation of hot electrons in GaAs/AlGaAs quantum wells

In this work, we have studied the inter- and intra-subband scattering of hot electrons in quantum wells using the hot electron–neutral acceptor luminescence technique. We have observed direct evidence of the emission of confined optical phonons by hot electrons excited slightly above the n=2 subband in GaAs/Al 0.37Ga 0.63As quantum wells. Scattering rates of photoexcited electrons via inter- and intra-subband LO phonon emission were calculated based on the dielectric continuum model. We found that, for wide wells with the Al composition of our experiments, both the calculated and experimental results suggest that the scattering of the electrons is dominated by the confined LO phonon mode. In the calculations, scatterings among higher subbands are also dominated by the same type of phonon at well width of 10 nm.

Scheme for subband adaptive beamforming array implementation using quadrature mirror filter banks

A novel scheme for a subband adaptive beamforming array (SABA) implementation is presented. The higher subband output signal is optimised and then multiplied with a factor to generate an output similar to the optimised lower subband. Both outputs are then combined to produce the desired signal. This arrangement reduces circuit complexity as well as the cost of the SABA, while maintaining the same beamforming performance.

Subband adaptive filtering decimation constraints for oversampled nonuniform filterbanks

To avoid significant inband aliasing, the decimation factors for a real filterbank must satisfy strict constraints. These decimation constraints decrease the real filterbank's computational efficiency by requiring higher subband data rates. To combat this, a set of modified pseudo-quadrature mirror filter equations are proposed which easily construct complex near-perfect reconstruction filterbanks that can be utilized to build both uniform and nonuniform filterbanks with no significant inband aliasing. With a properly designed real or complex filterbank, subband adaptive filters illustrate low m.s.e. after convergence. However, the computational complexity and convergence time associated with the complex filterbank is significantly less. Several numerical simulations are included to illustrate the conclusions.

Population inversion between subbands in simple periodical GaAs/AlAs superlattices

Photoluminescence (PL) from type-I GaAs/AlAs superlattices (SLs) have been studied under an electric field. In such SLs the AlAs layer, which is the barrier for the Γ subbands, is the quantum well for the X electron subbands. The X subbands in the AlAs barriers have a large density of subbands, and have a great influence on carrier transport even in type-I GaAs/AlAs SLs. Furthermore, the X subbands can be expected to be useful for carrier injection into a higher Γ subband using X-Γ resonance because PL from transitions between higher Γ subbands and hh1 has been observed. Intersubband population inversion in simple periodical GaAs/AlAs SLs is presented using X-Γ resonance. Interband PL measurement and calculation of the overlap integral between electrons and hole subbands confirm the intersubband population inversion. Carrier injection into higher subbands using X-Γ resonance is expected to be useful for achieving simple quantum cascade laser structures.

Ballistic conductance of a quantum wire at finite temperatures

The temperature-dependent ballistic conductance of a quantum wire is calculated without consideration of carrier scattering. The contribution to the conductance (G) from size-quantization subbands with E j - μ ≫ kT is described by the Landauer-Buttiker formula G=2e 2/h, where e is the elementary charge, h is Planck’s constant, μ(T) is the electrochemical potential, E j is the j-th subband bottom, T is temperature, and k is the Boltzmann constant. The contribution from other subbands decreases as their number increases, being exponentially small for higher subbands. The quantum staircase disappears when kT approaches the energy spacing between the size-quantization levels. Such temperature quenching of the quantum staircase at gate potentials corresponding to a stepwise change in the ballistic conductance is observed in studies of the quantized conductance of a silicon quantum wire.

Electron-optical phonon interaction and in-plane mobility in lead chalcogenide quantum-well structures

The in-plane electron mobility limited by scattering from optical phonons in lead chalcogenide (PbSnTe-type) quantum-well structures is studied theoretically. It is found that the mobility decreases drastically for quantum-well widths smaller than about 200 \AA{}. The scattering from the interface phonons is responsible for that. The nonparabolicity considerably reduces the mobility. Influence of the nonparabolicity on the mobility essentially depends upon the growth axis of the quantum-well structure. It is found that the nonparabolicity in quantum-well systems can be successfully represented by two following effects: (1) dependence of the in-plane electron effective mass upon the in-plane electron wave vector (subband nonparabolicity), and (2) dependence of the in-plane electron effective mass upon the subband number and quantum-well width (in-plane mass quantization). It is shown that, if the quantum-well width exceeds 250 \AA{}, influence of the nonparabolicity on the mobility is actually represented by the subband nonparabolicity alone, otherwise the in-plane mass quantization effect arises. The mobility of a many-subband system is found to be limited mainly by the intrasubband scattering. The mobility of electrons in a higher subband is smaller than that in the first one, which is due to the larger in-plane effective mass and stronger scattering rates in the higher subband. The mobility is essentially affected by electrons in a higher subband, while the gap between this subband and the ground one does not exceed ${k}_{B}T$. The electrophonon resonance effect is also considered.

Electron-phonon relaxation rates in InGaAs–InP and HgCdTe–CdTe quantum wells

We calculate electron-LO-confined and interface-phonon scattering rates in In1−xGaxAs–InP and Hg1−xCdxTe–CdTe quantum wells considering the influence of nonparabolicity on the energy subbands. A simple k⋅p model is used to take into account this nonparabolicity and a reformulated dielectric continuum slab model is employed to describe the confined phonon modes. We find that the subband nonparabolicity increases the scattering rates significantly for all transitions and that this effect is more pronounced as transitions from higher subbands are involved. We show that this behavior can be understood in terms of the phonon wave vector, the density of final states and the electron-phonon overlap.

A novel hardware configuration for an adaptive beamforming array using quadrature mirror filter banks

The performance of a novel multirate subband adaptive beamforming array is evaluated. Instead of the traditional fast Fourier transform (FFT) filter, the quadrature mirror filter (QMF) bank is used here to partition or to subband the incoming signal frequency band. The QMF bank divides the spectrum into the lower and higher subbands. Beamforming is then carried out for both of these subbands. In this paper, the same concept is used, except for the fact that only one of the two subbands (in our case, the lower) is optimized for the desired subband output. This signal is then passed through a multiplier or a transformer, which generates the same output as the other subband (in our case, higher). Both the optimized and transformer outputs are then combined to produce the final signal. This method reduces the circuit complexity as well as the cost of the subband adaptive beamformer. The radiation responses, the mean-square errors, and the eigenspectrums are subjected to investigation. It is observed that the beamforming performance and the mean-square errors (MSE) remain the same for both the previous and novel configurations. ©1999 John Wiley & Sons, Inc. Microwave Opt Technol Lett 21: 140–145, 1999.

Intersubband transitions in InAs/GaSb semimetallic superlattices

We report parallel magnetic-field-activated intersubband absorption in InAs/GaSb semimetallic superlattices. The samples were studied in the Voigt configuration enabling normal-incidence excitation of the intersubband transition. For samples with narrow wells, the small absorption coefficient meant that an alternative waveguide configuration was used, again with magnetic field applied parallel to the superlattice layers. In this latter configuration we see further activation of the intersubband resonance by the parallel field as well as new transitions to higher subbands. The intersubband energy and absorption characteristics are studied over a large range of well widths and compared with results from eight-band $\mathbf{k}\ensuremath{\cdot}\mathbf{p}$ calculations.

Localized excitonic transitions in a ZnSe-Zn0.75Cd0.25Se double-superlattice grown by molecular beam epitaxy

A group of well-defined exciton transitions from the localized states were observed in ZnSe-Zn0.75Cd0.25Se double-superlattice structure. The photoluminescence and photoreflectance have been employed to study the subband transitions at low temperatures. At 1.4 K, except the two ground states and two higher subbands of n=1 light-hole and n=2 heavy-hole excitonic transitions, other four peaks (A, B, C, and D) also were observed in wider-well superlattice. Those peaks were attributed to the excitonic transitions from n=2 heavy-hole subband due to the fluctuation of well-barrier interface. Another localized excitonic transition from narrower-well superlattice appeared as increasing the modulated intensity in photoreflectance spectra.

Confinement Dependence of Bound Polaronic Energies in Anisotropic Quantum Dots under Magnetic Fields

Energy levels of a polaron bound to hydrogenic impurities in anisotropic double-parabolic quantum dots are investigated by the second order perturbation theory. The polaronic energy corrections are expressed as a function of the confinement strength in different directions and the magnetic field applied along the growth direction. With characteristic parameters pertaining to GaAs, it is found that as the effective thickness reduces, the energy correction to higher subbands increases at a much larger rate than that to the lower subband. The polaron binding energy for higher subbands may become larger than that for the lower subband in the case of sufficiently thin quantum dots, in contrast to the normal energy level structure.