Lowest Subband Research Articles

Effect of strain on multisubband electron transport in GaAs/InxGa1-xAs coupled quantum well structures

We analyze the strain induced changes in the low temperature multisubband electron mobility mediated through the intersubband interactions in a pseudomorphic GaAs/InxGa1−xAs coupled double quantum well structure. We consider the non-phonon scattering mechanisms and study the effect of strain on them. We show that strain reduces the mobility due to ionized impurity (imp-) scattering μimp but enhances the mobility due to interface roughness (IR-) scattering μIR. For alloy disorder (AL-) scattering as long as the lowest subband is occupied, the effect of strain enhances the mobility μAL. However, once the second subband is occupied, there is almost no change, rather decrease in μAL for larger well widths. It is gratifying to note that for single subband occupancy, the effect of strain enhances the total mobility μ. On the other hand, for double subband occupancy, initially there is almost no change, but with increase in well width the total mobility reduces. We vary the In composition x from 0.15 to 0.2 and 0.25 and the barrier width between the two wells to analyze their effects on the mobility which shows interesting results. Our study of multisubband mobility can be utilized for the low temperature device applications.

Effects of an electric field on the electronic and optical properties of zigzag boron nitride nanotubes

We have investigated the electro-optical properties of zigzag BNNTs, under an external electric field, using the tight binding approximation. It is found that an electric field modifies the band structure and splits the band degeneracy. Also the large electric strength leads to coupling the neighbor subbands which these effects reflect in the DOS and JDOS spectrum. It has been shown that, unlike CNTs, the band gap of BNNTs can be reduced linearly by applying a transverse external electric field. Also we show that the larger diameter tubes are more sensitive than small ones. The semiconducting metallic transition can be achieved through increasing the applied fields. The number and position of peaks in the JDOS spectrum are dependent on electric field strength. It is found that at a high electric field, the two lowest subbands are oscillatory with multiple nodes at the Fermi level.

Correlation effects for a quasi‐one‐dimensional polaron gas

AbstractIn this work, we investigate the plasmon–LO phonon interaction effects on the intrasubband structure factor, electron–electron effective potential, and plasmon energy associated with the lowest subband in a GaAs‐AlGaAs rectangular quantum‐well wire (QWW) as a function of the electronic density. Our calculations are performed using the self‐consistent field approximation, which includes the local‐field correction (LFC) within the Singwi, Tosi, Land, and Sjolander (STLS) theory, at zero temperature and assuming a three‐subband model, where only the first subband is occupied by electrons. We report for the first time dips in the structure factor spectra as a function of the quasi‐one‐dimensional (Q1D) plasmon–LO phonon wavevector that are directly related with the resonant split of the collective excitation energy into two branches due to the polaronic effects.

Temperature dependence of dark current in a p-i-n photodiode incorporating a resonant tunneling structure

Analyses of the slope variations, as a function of temperature, of the dark current–voltage curves obtained in the case of p–i–n diodes incorporating in their intrinsic region a double barrier single quantum well (QW) structure indicate that below the flat band condition a conduction mechanism by tunneling assisted by recombination centers prevails over most of the temperature range considered (20–300 K). Above the flat band condition and for temperatures below 180 K, negative differential resistance (NDR) regions displaying Z-shaped bistability were observed. The appearance of these NDRs is attributed to resonant tunneling of holes via the lowest light-hole subband in the QW and electrons via the second quasi-bound level in the conduction band.

Error-Resilient 3-D Wavelet Video Coding with Duplicated Lowest Sub-Band Coefficients and Two-Step Error Concealment Method

In this paper, we propose a two-step error concealment algorithm based on an error resilient three-dimensional discrete wavelet transform (3-D DWT) video coding scheme. The proposed scheme consists of an error-resilient encoder duplicating the lowest sub-band bit-streams for dispersive grouped frames and an error concealment decoder. The error concealment method of this decoder is decomposed of two steps, the first step is replacement of erroneous coefficients in the lowest sub-band by the duplicated coefficients, and the second step is interpolation of the missing wavelet coefficients by minimum mean square error (MMSE) estimation. The proposed scheme can achieve robust transmission over unreliable channels. Experimental results provide performance comparisons in terms of peak signal-to-noise ratio (PSNR) and demonstrate increased performances compared to state-of-the-art error concealment schemes.

Spin Hall effect due to intersubband-induced spin-orbit interaction in symmetric quantum wells

We investigate the intrinsic spin Hall effect in two-dimensional electron gases in quantum wells with two subbands, where a new intersubband-induced spin-orbit coupling is operative. The bulk spin Hall conductivity sigma(z)(xy) is calculated in the ballistic limit within the standard Kubo formalism in the presence of a magnetic field B and is found to remain finite in the B=0 limit, as long as only the lowest subband is occupied. Our calculated sigma(z)(xy) exhibits a nonmonotonic behavior and can change its sign as the Fermi energy (the carrier areal density n(2D)) is varied between the subband edges. We determine the magnitude of sigma(z)(xy) for realistic InSb quantum wells by performing a self-consistent calculation of the intersubband-induced spin-orbit coupling.

Low-Temperature Behaviors of Phonon-Limited Electron Mobility of Sub-10-nm-Thick Silicon-on-Insulator Metal–Oxide–Semiconductor Field-Effect Transistor with (001) and (111) Si Surface Channels

In this study we simulate the impact of silicon layer thickness and temperature on the phonon-limited electron mobility of the inversion layer for ultrathin-body (111) and (001) surface silicon-on-insulator (SOI) layers in single-gate (SG) and double-gate (DG) SOI metal–oxide–semiconductor field-effect transistors (MOSFETs); one-dimensional self-consistent calculations and relaxation time approximations are performed. For a sub-10-nm-thick SOI layer, it is demonstrated that intra-subband phonon scattering at the lowest subband in the inversion layer on the (111) Si surface of the DG SOI MOSFET is strongly suppressed in a medium-to-high effective field (Eeff) at a low temperature. On the other hand, it is revealed that the contribution of inter-subband phonon scattering in the inversion layer on the (001) Si surface of the DG SOI MOSFET is very large in a medium-to-high Eeff regardless of temperature. Simulation results suggest that the phonon-limited electron mobility on the (111) Si surface of the DG MOSFET will be highly advantageous for future high-performance low-temperature device applications including space applications.

Fractional quantum Hall state atν=14in a wide quantum well

We investigate, with the help of Monte Carlo and exact-diagonalization calculations in the spherical geometry, several compressible and incompressible candidate wave functions for the recently observed quantum Hall state at the filling factor $\ensuremath{\nu}=1/4$ in a wide quantum well. The quantum well is modeled as a two-component system by retaining its two lowest subbands. We make a direct connection with the phenomenological effective-bilayer model, which is commonly used in the description of a wide quantum well and we compare our findings with the established results at $\ensuremath{\nu}=1/2$ in the lowest Landau level. At $\ensuremath{\nu}=1/4$, the overlap calculations for the Halperin (5,5,3) and (7,7,1) states, the generalized Haldane-Rezayi state and the Moore-Read Pfaffian, suggest that the incompressible state is likely to be realized in the interplay between the Halperin (5,5,3) state and the Moore-Read Pfaffian. Our numerics show the latter to be very susceptible to changes in the interaction coefficients, thus indicating that the observed state is of multicomponent nature.

Intersubband exchange interaction induced by optically excited electron spins in GaAs/AlGaAs quantum wells

Spin-dependent intersubband excitonic interactions have been investigated in GaAs/AlGaAs quantum wells by two-color pump and probe spectroscopy. We generated spin-polarized electrons in the lowest subband by resonant excitation of the heavy-hole exciton (E1-HH1) and observed polarization-dependent broadening of the second-subband exciton resonance (E2-HH2 and E2-LH1). The exchange interaction between the first and the second-subband excitons is found to play a crucial role in polarization-dependent spectral modulation as well as spin-independent Coulomb screening.

An analytic model for threshold voltage shift due to quantum confinement in surrounding gate MOSFETs with anisotropic effective mass

Threshold voltage shift due to quantum confinement in surrounding gate MOSFETs with anisotropic effective mass is calculated from the solution of 2D Schrödinger equation in elliptic coordinates. The solutions are of the Mathieu function type. It is shown that for some intermediate range of radius in silicon, several subbands need be taken into account to obtain accurate threshold voltage shifts. However, for small radius, only the lowest subband need be considered, and even anisotropic effective mass can be replaced by reduced isotropic effective mass, for which Bessel function is sufficient.

Quantum wires coupled by tunneling

The electron tunneling between two quantum wires at a point contact is calculated. Several subbands and a finite tunneling amplitude are considered, while the Coulomb interaction between electrons is neglected. The salient feature is the hybridization-split lower band edge states, which have a large spectral weight because of the diverging one-dimensional Van Hove singularities. For the lowest subband, the bonding state lies below the continuum of states arising from the motion of carriers along the conductors, while the antibonding state is embedded into the continuum and hence has a broadened structure. A slight disorder could lead to localized states close to the band edges, which would reduce this broadening and thus render the peak observable.

Gate control of the spin-splitting energy in a quantum dot: Application in single qubit rotation

We numerically calculate modulation of the spin-splitting energy in an asymmetric quantum dot due to an external electrostatic potential (applied via gates) that induces the Rashba spin–orbit interaction. The quantum dot has finite potential barriers and contains a built-in electric field (due to doping or compositional variation) that makes the potential profile asymmetric in space. It is placed in a magnetic field which lifts the spin degeneracy of every subband due to the Zeeman effect and we calculate by how much the Rashba interaction can change the total spin-splitting energy in the lowest subband. In the past, a perturbative solution that ignored “spin texturing” effects indicated that (1) the Rashba interaction always increases the total spin-splitting energy and that (2) the increase is a negligible fraction ( < 0.1 % ) of the total spin-splitting energy for reasonable parameters. The more accurate non-perturbative solution presented here accounts for spin texturing and shows that (1) the total spin-splitting energy may actually decrease (instead of increasing) owing to the Rashba effect and that (2) the Rashba interaction can decrease the spin-splitting energy by > 50 % for reasonable parameters. A modulation this large can be used for single qubit rotation in a quantum gate.

Many body theory of THz intervalence gain in quantum wells

This paper presents calculations of THz gain in III–V quantum wells based on transitions between the valence bands. The system is out of equilibrium and the optical susceptibility leading to absorption and gain is calculated after a Keldysh nonequilibrium Greens’ functions approach. The relevance of many body and nonresonant contribution to the spectrum is discussed as a function of local population inversion in k-space for a system where the injected holes are distributed within the two lowest subbands without global population inversion.

Spin-orbit coupling in an In0.52Ga0.48As quantum well with two populated subbands

Structural inversion asymmetry controls the magnitude of Rashba spin-orbit coupling in the electron energy spectrum of a narrow band gap semiconductor. We investigate this effect for a series of two-dimensional electron gases in In0.52Ga0.48As quantum wells, surrounded by In0.52Al0.48As barriers, where either one or two electric subbands are populated. Structural inversion asymmetry does not exist at low carrier density while at higher carrier densities (above (4–5)×1011cm−2), a finite spin splitting is observed. The spin orbit coupling coefficients (α) are determined from the power spectrum of the oscillatory magnetoresistance, although this is complicated by magnetointersubband scattering in the region where two subbands are occupied. In the lowest subband, α1≈(0.6–1.0)×10−11eVm over a range of (4–9)×1011cm−2 total carrier density. In the second subband, α2≈2.2×10−11eVm, and this reduces with increasing carrier density. The spin-orbit coupling effect disappears at high carrier density although a strong structural inversion asymmetry still exists. We discuss these results with the consequences for spintronic devices operating on the principle of charge density dependent or electric field dependent spin manipulation in narrow band gap semiconductors.

Type of the second hole subband on the Si(110) surface: Spin splitting anisotropy

Magnetoresistance measurements have been performed for the cases of one and two occupied size-quantization hole subbands in silicon field-effect transistors prepared on the Si (110) surface. In both cases, Shubnikov-de Haas oscillations exhibit very weak sensitivity to the in-plane component of the magnetic field. This indicates that both lowest subbands are formed by heavy holes. This conclusion disagrees with the wide-spread opinion that the second subband is the ground subband of light holes in the system under consideration.

One-dimensional semirelativity for electrons in carbon nanotubes

It is shown that the band structure of single-wall semiconducting carbon nanotubes (CNT) is analogous to relativistic description of electrons in vacuum, with the maximum velocity $u={10}^{8}\phantom{\rule{0.3em}{0ex}}\mathrm{cm}∕\mathrm{s}$ replacing the light velocity. One-dimensional semirelativistic kinematics and dynamics of electrons in CNT is formulated. Two-band $\mathbf{k}∙\mathbf{p}$ Hamiltonian is employed to demonstrate that electrons in CNT experience a zitterbewegung (trembling motion) in absence of external fields. This zitterbewegung should be observable much more easily in CNT than its analogue for free relativistic electrons in vacuum. It is argued that in the lowest subband of metallic CNT, where the rest effective mass of electrons vanishes, the zitterbewegung should not occur.

Carrier transfer in coupled asymmetric GaAs/AlGaAs double quantum wells after ultrafast intersubband excitation

Intersubband scattering processes are investigated in selectively doped asymmetric tunnel-coupled GaAs/AlGaAs quantum wells by infrared pump probe spectroscopy. Various subbands are excited by resonant absorption of a mid-infrared pump pulse and the carrier relaxation is observed via time- and frequency-resolved absorption changes monitored by a subsequent infrared probe pulse. The experimental data show that most of the carriers remain in the quantum well in which the excitation process mainly takes place. Only a minor fraction of the carriers is transferred between excited non-resonant subbands of the neighbouring quantum wells. The situation is different for electrons in the two lowest subbands which are separated only by 10 meV. Here a rapid and efficient transfer is observed. The scattering time is in the order of a picosecond at room temperature; it increases to values between 4 ps and 20 ps at 80 K. The data are interpreted with detailed model calculations taking into account phonon and impurity scattering. Both experimental and calculated data demonstrate the possibility of population inversion between excited states of quantum wells under optical or current electron injection.

Resonant Raman scattering of electrons in quantum wells: Identification of elementary excitations

Inelastic scattering of light by electrons in a quantum well was studied. We considered the resonant Raman scattering associated with intersubband electronic excitations. Evidence is provided that when the two lowest subbands are considered four types of excitations are present. They give rise to two scattering lines associated with single-particle excitations and another two associated with collective charge-density excitations, while spin-density excitations are not considered. The shape of the Raman spectrum strongly depends on several characteristics in the experiment, namely, the quantum well width, the concentration of the carriers, many-body effects, the temperature, and, mainly, the momentum-transfer dependent on the laser-photon frequency and the experimental geometry. Particular experimental results are satisfactorily described.

Photoluminescence Investigation of Two-Dimensional ElectronGas in an Undoped AlxGa1−xN/GaN Heterostructure

Low-temperature photoluminescence measurement is performed on anundoped AlxGa1−xN/GaN heterostructure.Temperature-dependent Hall mobility confirms the formation oftwo-dimensional electron gas (2DEG) near the heterointerface. A weakphotoluminescence (PL) peak with the energy of ~79 meV lower thanthe free exciton (FE) emission of bulk GaN is related to the radiativerecombination between electrons confined in the triangular well and theholes near the flat-band region of GaN. Its identification is supportedby the solution of coupled one-dimensional Poisson and Schrödingerequations. When the temperature increases, the red shift of the 2DEGrelated emission peak is slower than that of the FE peak. The enhancedscreening effect coming from the increasing 2DEG concentration and thevarying electron distribution at two lowest subbands as a function oftemperature account for such behaviour.

Multisubband effect in spin dephasing in semiconductor quantum wells

Multi-subband effect on spin precession and spin dephasing in $n$-type GaAs quantum wells is studied with electron-electron and electron-phonon scattering explicitly included. The effects of temperature, well width and applied electric field (in hot-electron regime) on the spin kinetics are thoroughly investigated. It is shown that due to the strong inter-subband scattering, the spin procession and the spin dephasing rate of electrons in different subbands are almost identical despite the large difference in the D'yakonov-Perel' (DP) terms of different subbands. It is also shown that for quantum wells with small well width at temperatures where only the lowest subband is occupied, the spin dephasing time increases with the temperature as well as the applied in-plane electric field until the contribution from the second subband is no longer negligible. For wide quantum wells the spin dephasing time tends to decrease with the temperature and the electric field.