Subband Separation 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.

Tunability of electronic and thermoelectric properties of hexagonal boron nitride with carbon impurities under magnetic field: Tight binding investigation

Utilizing the Kubo-Greenwood formula, Tight Binding calculations were employed to examine the electronic and thermoelectric properties of hexagonal boron nitride (h-BN) with carbon impurity instead of boron, nitrogen and pairs boron-nitrogen. The electronic properties of the pristine monolayer BN are markedly impacted by the introduction of carbon dopants and its band gap reduction is directly correlated with the concentration of carbon impurities. The electronic properties of doped h-BN are influenced by the presence of a magnetic field, leading to subband separation and band gap narrowing, independent of the impurity types. The thermal conductivity and magnetic susceptibility of the CBN-doped monolayer BN structure are higher than those of the BC and NC doped h-BN structures and for all structures, their properties have a strong dependence on the magnetic field. The Lorenz Number for all structures has peak at the TM temperature which shifts to a lower temperature as the impurity concentration decreases.

Investigation the three dimensional bound states in quantum dot nanowire systems

Electronic confined states of a cylindrical InP quantum dot embedded in infinite GaP nanowire is studied theoretically. To this end the Schrodinger equation is solved numerically in the effective mass approximation. The subband separation between the GaP nanowire and InP/GaP core–shell nanowire is utilized to predict the possible existence of the three dimensionally confined states of the system. Using this, the nanowire critical radius is calculated for each channel of angular momentum, the frontier of type I-Type II transition in the InP/GaP quantum dot nanowire system. It is shown that the lower limit of the continuum energy depends on the angular momentum and increases by it, causing discrete bound states of higher angular momentum to lie in the continuum of the lower ones. Surviving the energy states in the presence of external magnetic field reveals stronger confinement for electrons in states with negative angular momentum quantum numbers.

Electron–electron scattering rate in CdTe/CdMnTe single quantum well

CdMnTe is demonstrated to be a good candidate in the X-ray and [Formula: see text]-ray detector application, however, there are few reports on theoretical analysis of electron scattering rate in CdMnTe quantum well. Within the framework of effective mass approximation and envelope function approximation, the influence of the Mn alloy composition ([Formula: see text], the well width ([Formula: see text], the electron temperature ([Formula: see text] and the electron density ([Formula: see text] on the electron–electron scattering rate (1/[Formula: see text] in the CdTe/Cd[Formula: see text]Mn[Formula: see text]Te single quantum well (SQW), are simulated by shooting method and Fermi’s Golden Rule. The results show that 1/[Formula: see text] is significant inverse proportional to [Formula: see text], but positively proportional to [Formula: see text] and [Formula: see text]. Except for a small peak at 20 K, 1/[Formula: see text] is not sensitive to [Formula: see text]. The above differential dependency of 1/[Formula: see text] on [Formula: see text] and [Formula: see text] can be interpreted by sub-band separation ([Formula: see text], which is proportional to [Formula: see text] but inversely proportional to [Formula: see text]. When [Formula: see text] decreases gradually, the electron transition becomes easier, which leads to 1/[Formula: see text] increases. The dependency of 1/[Formula: see text] on [Formula: see text] can be interpreted by kinetic energy of electrons. The larger the electron kinetic energy is, the more difficult the electron transition from first excited state to ground state is, which leads to 1/[Formula: see text] decreasing. The dependency of 1/[Formula: see text] on [Formula: see text] can be interpreted by the Coulomb interaction between electrons, i.e., the increase of electron collision probability caused by the increase of [Formula: see text].

High resolution sub-band decomposition underdetermined blind signal separation using virtual sensor based ICA method for low latency applications

Underdetermined blind source separation (UBSS) objective is to recover the source signals from a number of mixtures without any information about the mixing system. Technologies such as teleconferencing, hearing aids and hands-free telephony require real time processing, as long delays are considered intolerable in interactive two-way communication. This is a major challenge in BSS, as algorithms generally require significant amounts of data (several seconds of data or more) to generate sufficient statistics for separation. In this paper, we propose a hybrid algorithm which combines sub-band decomposition and well-known Independent Component Analysis (ICA) based algorithm, Extended-Infomax. We first employ sub-band decomposition algorithm in sparse time domain to compensate low data efficiency of short time block lengths and estimate the mixing matrix. Subsequently, the proposed virtual sensor based underdetermined Extended-Infomax source model is used to estimate the source signals. As the separation evaluation results are so sensitive to the mixing matrix estimation outcome, achieving nearly optimum results in the first phase of the proposed algorithm significantly improves the results of source separation performance. A weighted version of k-plane clustering algorithm is derived to obtain the mixing coefficients. Experimental evaluations reveal the effectiveness of our proposed method over the state-of-the-art techniques.

Gate-All-Around FET Design Rule for Suppression of Excess Non-Linearity

Gate-All-Around Field Effect Transistors (GAAFETs) for the future technology nodes will have highly confined channel cross-sections. Effects like subband separation and geometry dependent density of states result in kinks, peaks and valleys appearing in terminal characteristics like capacitance and transconductance. This has significant effect on the circuit linearity performance, in analog and RF domains. In this paper, we have proposed a design rule for the maximum allowed overdrive voltages or minimum silicon body width for a given body thickness so that the effects of subband separation may be avoided. Effect of corner radius has also been included.

Multi-band CNN architecture using adaptive frequency filter for acoustic event classification

Although Convolutional Neural Networks (CNNs) architecture based learning systems have shown impressive results in the performance of numerous classification tasks, their effectiveness has been limited in certain cases of acoustic based classification. This vulnerability is particularly evident in the acoustic event classification tasks using spectral features. For example, spectral based features may suffer from a typical normalization process when it is fed to a neural network for training since the magnitudes in high-frequency band are inadvertently attenuated even though they may yet contain useful discriminant features. Although some research efforts try to mitigate this problem by introducing a multi-band approach for attaining salient and stable features, it requires empirically preset frequency bands to separate the spectral features. Being heuristic, however, this process is difficult to ensure the consistency required for high correlation between manually separated features and good classification performance. In this paper, we propose a novel filter parameter modeling framework performing optimized frequency sub-band separation via CNN based end-to-end training for achieving high acoustic event classification performance. In particular, the filter response characteristics, namely, cut-off frequencies and damping ratio for roll off are considered as added learning parameters to the CNN architecture for the proposed end-to-end learning framework so that the filter’s frequency response is optimized for producing salient features. The proposed training process is shown to not only automatically select the filter parameters for multi-band frequency separation but also guarantee high correlation between the resulting sub-band features and accurate classification performance.

An Intelligent Sleep Apnea Classification System Based on EEG Signals.

Sleep Apnea is a sleep disorder which causes stop in breathing for a short duration of time that happens to human beings and animals during sleep. Electroencephalogram (EEG) plays a vital role in detecting the sleep apnea by sensing and recording the brain's activities. The EEG signal dataset is subjected to filtering by using Infinite Impulse Response Butterworth Band Pass Filter and Hilbert Huang Transform. After pre-processing, the filtered EEG signal is manipulated for sub-band separation and it is fissioned into five frequency bands such as Gamma, Beta, Alpha, Theta, and Delta. This work employs features such as energy, entropy, and variance which are computed for each frequency band obtained from the decomposed EEG signals. The selected features are imported for the classification process by using machine learning classifiers including Support Vector Machine (SVM) with Kernel Functions, K-Nearest Neighbors (KNN), and Artificial Neural Network (ANN). The performance measures such as accuracy, sensitivity, and specificity are computed and analyzed for each classifier and it is inferred that the Support Vector Machine based classification of sleep apnea produces promising results.

The electron-longitudinal optical phonon scattering rate in GaInAsP/InP stepped quantum well

Within the framework of effective mass approximation, the scattering rate via longitudinal optical (LO) phonon emission for an electron and the mean scattering rate via LO phonons emission for electrons initially in the first excited sub-band and finally in the ground sub-band in [Formula: see text] stepped quantum well (QW) is calculated adopting the shooting method and Fermi’s golden rule. The results show that the scattering rate and the mean scattering rate are highly dependent on alloy compositions, well width, initial electron energy, electron temperature and sub-band separation [Formula: see text] between the ground sub-band and the first excited sub-band. When [Formula: see text] is larger than the LO phonon energy, the scattering rate and the mean scattering rate increases with increasing Ga composition, decreasing As composition and increasing well width. However, when [Formula: see text] is smaller than the LO phonon energy, its change tendency is contrary. The scattering rate increases with decreasing initial electron energy if the separation between the initial electron energy and the ground state energy [Formula: see text] is not smaller than the LO phonon energy. The scattering rate and the mean scattering rate increases with rising electron temperature. The mean scattering rate reaches the maximum value when [Formula: see text] is equal to the LO phonon energy. The interruption in the scattering rate happens when the separation between the initial electron energy and [Formula: see text] is smaller than the LO phonon energy. The rapid decrease of the mean scattering rate happens when [Formula: see text] is smaller than the LO phonon energy if [Formula: see text] continues decreasing. In addition, both the scattering rate and the mean scattering rate show little change with different stepped layer widths.

Effects of Electrode Layer Band Structure on the Performance of Multilayer Graphene-hBN-Graphene Interlayer Tunnel Field Effect Transistors.

Interlayer tunnel field-effect transistors based on graphene and hexagonal boron nitride (hBN) have recently attracted much interest for their potential as beyond-CMOS devices. Using a recently developed method for fabricating rotationally aligned two-dimensional heterostructures, we show experimental results for devices with varying thicknesses and stacking order of the graphene electrode layers and also model the current-voltage behavior. We show that an increase in the graphene layer thickness results in narrower resonance. However, due to a simultaneous increase in the number of sub-bands and decrease of sub-band separation with an increase in thickness, the negative differential resistance peaks becomes less prominent and do not appear for certain conditions at room temperature. Also, we show that due to the unique band structure of odd number of layer Bernal-stacked graphene, the number of closely spaced resonance conditions increase, causing interference between neighboring resonance peaks. Although this can be avoided with even number of layer graphene, we find that in this case the bandgap opening present at high biases tend to broaden the resonance peaks.

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.

Phase diagrams for the stability of theν=12fractional quantum Hall effect in electron systems confined to symmetric, wide GaAs quantum wells

We report an experimental investigation of fractional quantum Hall effect (FQHE) at the even-denominator Landau level filling factor $\nu$ = 1/2 in very high quality wide GaAs quantum wells, and at very high magnetic fields up to 45 T. The quasi-two-dimensional electron systems we study are confined to GaAs quantum wells with widths $W$ ranging from 41 to 96 nm and have variable densities in the range of $\simeq 4 \times 10^{11}$ to $\simeq 4 \times 10^{10}$ cm$^{-2}$. We present several experimental phase diagrams for the stability of the $\nu=1/2$ FQHE in these quantum wells. In general, for a given $W$, the 1/2 FQHE is stable in a limited range of intermediate densities where it has a bilayer-like charge distribution; it makes a transition to a compressible phase at low densities and to an insulating phase at high densities. The densities at which the $\nu=1/2$ FQHE is stable are larger for narrower quantum wells. Moreover, even a slight charge distribution asymmetry destabilizes the $\nu=1/2$ FQHE and turns the electron system into a compressible state. We also present a plot of the symmetric-to-antisymmetric subband separation ($\Delta_{SAS}$), which characterizes the inter-layer tunneling, vs density for various $W$. This plot reveals that $\Delta_{SAS}$ at the boundary between the compressible and FQHE phases increases \textit{linearly} with density for all the samples. Finally, we summarize the experimental data in a diagram that takes into account the relative strengths of the inter-layer and intra-layer Coulomb interactions and $\Delta_{SAS}$. We conclude that, consistent with the conclusions of some of the previous studies, the $\nu=1/2$ FQHE observed in wide GaAs quantum wells with symmetric charge distribution is stabilized by a delicate balance between the inter-layer and intra-layer interactions, and is very likely described by a two-component ($\Psi_{311}$) state.

WDM-Coherent OCDMA over one single device based on short chip Super structured fiber Bragg gratings

We theoretically propose and demonstrate experimentally a Coherent Direct Sequence OCDMA en/decoder for multi-channel WDM operation based on a single device. It presents a broadband spectral envelope and a periodic spectral pattern that can be employed for en/decoding multiple sub-bands simultaneously. Multi-channel operation is verified experimentally by means of Multi-Band Super Structured Fiber Bragg Gratings with binary phase encoded chips fabricated with 1mm inter-chip separation that provides 4x100 GHz ITU sub-band separation at 1.25 Gbps. The WDM-OCDMA system verification was carried out employing simultaneous encoding of four adjacent sub-bands and two different OCDMA codes.

Effect of doping on the mid-infrared intersubband absorption in GaN/AlGaN superlattices grown on Si(111) templates

We report on the effect of Si doping on the mid-infrared intersubband absorption in GaN/AlGaN superlattices. For increasing doping levels, interband luminescence displays a blueshift and a broadening of the band edge caused by the screening of the internal electric field and band-filling effects. The intersubband absorption energy is mainly governed by many-body effects like exchange interaction and depolarization shift, which increase the e1–e2 subband separation. The ISB blueshift induced by many-body effects can be more than 50% of the e1–e2 transition energy.

Evidence for Developing Fractional Quantum Hall States at Even Denominator1/2and1/4Fillings in Asymmetric Wide Quantum Wells

We report the observation of developing fractional quantum Hall states at Landau level filling factors nu = 1/2 and 1/4 in electron systems confined to wide GaAs quantum wells with significantly asymmetric charge distributions. The very large electric subband separation and the highly asymmetric charge distribution at which we observe these quantum Hall states, together with the fact that they disappear when the charge distribution is made symmetric, suggest that these are one-component states, possibly described by the Moore-Read Pfaffian wave function.

Magneto-intersubband oscillations in triple quantum wells

We present magnetotransport studies of high-density triple quantum well samples with different barrier widths. Because of electron transitions between three occupied 2D subbands, the magnetoresistance shows magneto-intersubband oscillations whose periodicity is determined by the subband separation energies. Temperature-dependent measurements allow us to extract quantum lifetime of electrons. A theoretical consideration of the observed phenomenon is also presented.

Optimum quantum well width for III-nitride nonpolar and semipolar laser diodes

The advantage of using wider quantum wells in III-nitride lasers offered by nonpolar/semipolar technology is limited by narrower valence subband separation, thermal hole redistribution, and resulting optical gain degradation in wider wells. We show that corresponding increase in radiative carrier lifetime in wider quantum wells can lower the laser threshold, thus inferring the existence of an optimum quantum well width for laser design.

Optical characteristics of III-nitride quantum wells with different crystallographic orientations

This article presents a direct comparison of calculated optical characteristics of polar, nonpolar, and semipolar III-nitride quantum wells. We show that the advantage of using wider quantum wells offered by nonpolar/semipolar technology is severely limited by narrower valence subband separation, thermal hole redistribution, and resulting optical gain degradation in wider wells. However, we emphasize the importance of using wider quantum wells to prevent electron leakage. We also show that gain characteristics of laser structures grown in nonpolar/semipolar orientations are less vulnerable to detrimental effect of nonradiative recombination.

Narrow inhomogeneous broadening of V-groove quantum wires grown on vicinal substrates

Significant reduction in inhomogeneous broadening of GaAs/AlGaAs V-groove quantum wires (QWRs) is achieved by growing them on vicinal (001) GaAs substrates misaligned by several degrees with respect to the [11¯0] groove direction. Low temperature photoluminescence spectra exhibit QWR linewidths as low as 3.7 meV for subband separation of 41 meV and 3 meV for subband separation of 27 meV. Atomic force microscopy evidences a change in the growth dynamics as compared with QWRs made on exact (001) GaAs substrates. The impact of the different growth dynamics on the wire interface structure is discussed.

Magnetic oscillations of resistivity and absorption of radiation in quantum wells with two populated subbands

The dynamic (ac) conductivity tensor of quantum wells with two populated subbands in the presence of a magnetic field perpendicular to the well layer is calculated theoretically. The microscopic theory is based on the Kubo formalism assuming a detailed consideration of elastic scattering of electrons by the random disorder potential with arbitrary correlation length. The results describe the influence of magnetic field on the linear absorption of low-frequency electromagnetic radiation, and demonstrate the existence of magnetic oscillations that survive at high temperatures and whose maxima correspond to absorption of electromagnetic radiation at combined frequencies, determined by both the magnetic field and the subband separation. Different polarizations of the radiation field with respect to the quantum-well layer are considered. Analytical expressions are derived for the case of sufficiently weak magnetic field when the Landau levels are overlapping. Application of the theory to the static (dc) limit provides a consistent description of the magneto-intersubband oscillations of the resistivity in the systems with two populated subbands.