Intersubband Interactions Research Articles

π\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\pi$$\\end{document} quantization in the Berry phase by the multiple degenerate points

Semiconductors have a valence band consisting of multi-subbands. Inter-subband interaction causes multiple degenerate points near the Brillouin zone center Γ\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\Gamma$$\\end{document} even when the system exhibits space-reversal asymmetry. In the present study, we analyzed the effect of the existence of multiple degenerate points on topological features using Berry parameter investigation. For reasonable generalization, we propose a model Hamiltonian that represents multiple degeneracies in a two-dimensional quantum-well system. We also explored “topological” modifications through numerical calculations of the effective magnetic field and Berry’s connection and curvature. We found that the energy dependence of the Berry phase is affected by the interaction between degenerate points.Graphical abstract

Effect of parabolic structure potentials on electron mobility in AlxGa1-xAs quantum well based field effect transistor structure

Here, we theoretically calculate and analyze electron mobility μ in AlxGa1-xAs double parabolic and semi-parabolic quantum well (DPQW and DSPQW) based field effect transistor structures. We compute μ as a function of applied electric field F by considering ionized impurity (II) and alloy disorder (AL) screened scattering potentials (VscrII/AL) and show that μ(DPQW) > μ (DSPQW). The symmetric property of VscrII/AL makes μ to vary symmetrically around F = 0 at which two subbands are occupied. The shape of the DSPQW structure potential, compared to DPQW, causes the subband wave functions ψ0 and ψ1 to lie nearer and also penetrate more into the central barrier. Accordingly, the II-scattering is reduced due to increased distance of the electrons from the ionized donors (lying in the side barriers), while AL-scattering is enhanced through increased scattering in the central barrier. As a result, we obtain for F = 0, μII (DSPQW) > μII (DPQW) and μAL (DSPQW) < μAL (DPQW). Further, we show that μ enhances in DPQW by increasing design parameters such as well width, spacer width, and central barrier width. However, μ reduces with the increase in alloy concentrations inside the well through intersubband interaction. The obtained results reflect the suitability of parabolic potential based quantum well structures in emerging optoelectronic devices to enhance the transport and optical properties.

Improvement of electron mobility mediated by interface roughness scattering in pseudomorphic GaAs/In0.15Ga0.85As asymmetric double quantum well structure

We analyse low temperature multisubband electron mobility mediated by interface roughness (ir-) scattering i.e., µirs as a function of electric field F in a pseudomorphic modulation δ-doped GaAs/In0.15Ga0.85As asymmetry-double-quantum-well (ADQW) structure. The asymmetry is obtained by considering different (I) well widths (w1 = 100 Å and w2 = 150 Å) and (II) doping concentrations (Nd1 = 1.0 × 1018 cm−3 and Nd2 = 1.5 × 1018 cm−3). We adopt random phase approximation and calculated µirs by considering screened ir-scattering potential. The cusp like behaviour of Fermi Energy levels at resonance field Fr makes µirs non-monotonic through intersubband interaction. We show that there is an enhancement of µirs and shifting of the resonance position with increasing central barrier width b in both cases. However, in case I, the increase in Nd enhances µirs but Fr doesn’t change. Whereas, in case II, as w increases, not only µirs but also Fr changes. The effect of the structure parameters on µirs can be utilized to improve the surface roughness effect in new emerging opto-electronic devices.

Effect of structure parameters on electron mobility in asymmetric V-shaped double quantum well structure

Here, we analyse the effect of structure parameters like well width (w), central barrier width (b), and alloy concentration (x) on multisubband electron mobility μ in a GaAs-AlxGa1-xAs based modulation doped asymmetric V-shaped-double-quantum-well (VDQW) structure. The asymmetry in the structure potential is generated through the difference in the doping concentrations (N d ) in the side barriers i.e., N d1 (0 to 4 × 1018 cm−3) and N d2 (2 × 1018 cm−3). The mobility μ is calculated by considering ionised impurity (imp) and alloy disorder (ad) scattering mechanisms. The continuous variation of x inside the well makes μ ad < μ imp resulting in the dominance of ad-scattering on μ as a function of N d1. As a result, at the interface an increase in x from 0.1 to 0.3 reduces μ around 40%. However, an increase in w symmetrically (w 1 = w 2) enhances μ. Further, the introduction of non-symmetric well profile (w 1 ≠ w 2) not only causes asymmetric redistribution of subband wave functions ψ 0 and ψ 1 in the wells, but also changes the position and hence occupation of subband energy levels, thereby influencing the subband mobility. As the difference in w 1 and w 2 increases, the system becomes more and more single subband occupied as a function of N d1 and hence the mobility enhances due to the absence of intersubband scattering. Our results also reflected that an increase in b from 20 Å to 80 Å has a marginal effect on μ during single subband occupancy but improves μ during double subband occupancy through intersubband interaction.

Spin Textures and Berry Phases for Holes Confined in SiGe Mixed‐Alloy 2D Quantum Well System: Quantization of Berry Phase via Intersubband Interaction

In their last article [Phys. Lett. A 2021, 389, 127091], the authors have extended the approach by considering crossings with the spin–orbit interaction (SOI) up to the second order and studied the spin texture and Berry phase of the heavy‐mass holes (HHs) confined in the SiGe 2D quantum well system. HHs cause quasi‐degeneracy via the intersub‐band interaction (ISI) working as Dirac‐like singularity, leading to the energy‐dependent Berry phase “quantized” by π. This π quantization is understandable by counting the number of quasi‐degenerate points in the Brillouin zone alongside recognizing the sign of the Berry curvature there. Herein, the remaining holes of the light‐mass holes (LHs) and separate holes (SHs) are studied. The spin textures and the Berry curvatures and phases are explored comprehensively, focusing on the competition between the Rashba and Dresselhaus SOIs via ISI. Comparisons with HHs elucidate that LHs and SHs cause similar quasi‐degeneracy through ISI, functioning as the Dirac‐like singularity. As such, the Berry phase is quantized by π, confirming the validity of the counting model irrespective of hole type. In some peculiar cases, HHs and LHs cause a possible sign inversion in the Berry phase, leading to zero Berry phase against temperature.

Electron mobility in asymmetric coupled AlxGa1-xAs parabolic quantum well structure – Impact of external electric field

We study the impact of external electric field FEXT on electron mobility μ in asymmetric AlxGa1-xAs coupled parabolic quantum well structure. The asymmetry in the structure potential is obtained by varying the doping concentration NDSUB (0–1.5 × 1018 cm−3) towards the substrate and keeping NDSUR (1.0 × 1018 cm−3) fixed along the surface. We apply FEXT = 0, 5, and 10 kV/cm from the substrate side of the structure and calculate μ as function of NDSUB by considering ionized impurity and alloy disorder scattering mechanisms. We show that as FEXT increases μ enhances through intersubband interaction. The occupation of subband states changes from single to double, double, and double to single leading to enhancement, oscillation, and depreciation of μ for FEXT = 0, 5, and 10 kV/cm respectively. We also show that in the absence of FEXT, increasing the well width or the central barrier width enhance μ as function of NDSUB.

Spin texture and Berry phase for heavy-mass holes confined in SiGe mixed-alloy two-dimensional system: Intersubband interaction via the coexistence of Rashba and Dresselhaus spin-orbit interactions

By extending the k⋅p approach, we study the spin texture and Berry phase of heavy-mass holes (HHs) confined in the SiGe two-dimensional (2D) quantum well system, focusing on the intersubband interaction via the coexistence of the Rashba and Dresselhaus spin-orbit interactions (SOIs). The coexistence of both SOIs generates spin-stabilized(+)/destabilized(-) HHs. The strong intersubband interaction causes quasi-degenerate states resembling the 2D massive Dirac fermion. Consequently, the Berry phases of HHs have unique energy dependence understood by counting the quasi-degenerate points with the signs of the Berry phases. Thermal averaging of the Berry phase demonstrates that HH+/− has a peculiar plateau of +π/−π at less than 30 K and then changes its sign at approximately 200 K.

Effect of external electric field on multisubband electron mobility in n-V-shaped double quantum well HEMT structure

In this work, we present theoretically the effect of external electric field Fe on low temperature multisubband electron mobility μ in V-shaped double quantum well (V-DQW) HEMT structure. We consider the impact of ionised impurity and alloy disorder scatterings for the calculation of μ. We show that, in the proposed structure, when Fe, is absent, there are two subbands occupied below the Fermi levels. However, as Fe increases, there is an alteration of the potential profile, which changes the energy levels and wave function distributions leading to variation of occupation of subband states, i.e. from double to single. During double subband occupancy, initially, μ enhances with Fe, attains a peak value and then decreases. Whereas, for Fe where the transition from double to single subband occupancy occurs, there is a sudden rise in μ due to the cease of inter-subband interaction. It is interesting to note that different structure parameters, e.g. well widths Ww, central barrier width BC, doping concentrations ND, alloy concentrations xv at the well edges of the V-DQW have a fascinating impact on μ. We show that increasing Ww, and BC and decreasing ND, and xv enhances μ.

D'yakonov-Perel' spin relaxation in a bilayer with local structural inversion asymmetry

The spin relaxation in the D'yakonov-Perel' mechanism is theoretically studied in a symmetric double-quantum-well structure (DQWS) with an intersubband spin-orbit interaction (SOI) due to the local structural inversion asymmetry and the linear-in-wave-number Dresselhaus SOI. It is found that the spin relaxation rate induced by the intersubband SOI exhibits a suppression with $\ensuremath{\omega}{\ensuremath{\tau}}_{p}$ in the Lorentzian form of ${\left(1+{\ensuremath{\omega}}^{2}{\ensuremath{\tau}}_{p}^{2}\right)}^{\ensuremath{-}1}$, where $\ensuremath{\hbar}\ensuremath{\omega}$ is the intersubband energy separation and ${\ensuremath{\tau}}_{p}$ is the momentum relaxation time. The present Lorentzian suppression leads to a crossover with increasing $\ensuremath{\omega}{\ensuremath{\tau}}_{p}$ from the D'yakonov-Perel'-type relaxation (the spin relaxation time ${\ensuremath{\tau}}_{s}\ensuremath{\propto}{\ensuremath{\tau}}_{p}^{\ensuremath{-}1}$) to the Elliott--Yafet-type relaxation (${\ensuremath{\tau}}_{s}\ensuremath{\propto}{\ensuremath{\tau}}_{p}$). It is also shown that the spin relaxation rate in the presence of both the linear Dresselhaus SOI and the intersubband SOI is isotropic with respect to the in-plane spin direction in the present DQWS in contrast to a single quantum well exhibiting an in-plane anisotropy.

Effect of illumination on the interplay between Dresselhaus and Rashba spin-orbit couplings in InAs quantum wells

We studied the dependence on carrier density of the spin–orbit coupling (SOC) and of the semiconducting properties of a thin n-type InAs quantum well in the limit of high carrier density, when the second subband starts to populate. Measurements of sheet and Hall resistances were performed in a variable magnetic field, temperature, and under illumination with wavelengths of 400 nm up to 1300 nm. Beats in the Shubnikov de Haas oscillations indicated the presence of strong spin–orbit coupling, and the fast Fourier transforms of the oscillations point toward the presence of both Rashba and Dresselhaus spin–orbit interactions. We used simulations to extract the carrier density dependence of the SOCs, based on the Hamiltonian of a single subband 2D electron system with both types of spin–orbit interactions and subject to a perpendicular magnetic field. The results indicate that the Rashba coefficient decreases with increasing the carrier concentration, with an accelerated decrease close to the transition to double occupancy. The Dresselhaus coefficient is not only smaller but also drops in the proximity of the transition, together with the carrier mobility and the Landau level broadening energy. This behavior is discussed in the context of phenomena associated with the onset of double occupancy and based on intersubband interactions.

Kinetic processes in heavily doped semiconductor heterojunctions

We report results of study of electron - electron relaxation processes in the 2D electrons system with the fine structure of the energy spectrum and the spatial distribution of the electron density. For heavily doped heterojunction, when two subbands are filled, expressions for the time of intra- and inter-subband electron-electron interactions, matrix elements of a complete screening potential and the dielectric function were obtained beyond the long-wavelength limit. It is shown that the oscillations of temperature and concentration dependence of the electron - electron interactions are associated with the excitation of plasma oscillations components in 2D electron system.

Effects of inter-subband couplings on bound and extended states of terahertz cascade structures

We theoretically analyze the effect of inter-subband couplings on the eigenstates of a terahertz heterostructure in the presence of coulombic donors. We find that the inter-subband electron-donor interaction causes a drastic reorganization of the eigenenergies and a strong admixture between the subbands that decisively influences the optical absorption spectrum of the structure. These results illustrate the importance of the inter-subband couplings in the formulation of the eigenstates of imperfect terahertz heterostructures.

Intraband and intersubband many-body effects in the nonlinear optical response of single-wall carbon nanotubes

We report on the nonlinear optical response of a monochiral sample of (6,5) single-wall carbon nanotubes by means of broadband two-color pump-probe spectroscopy with selective excitation of the ${S}_{11}$ excitons. By using a moment analysis of the transient spectra, we show that all the nonlinear features can be accurately accounted for by elementary deformations of the linear absorption spectrum. The photogeneration of ${S}_{11}$ excitons induces a broadening and a blueshift of both the ${S}_{11}$ and ${S}_{22}$ excitonic transitions. In contrast, only the ${S}_{11}$ transition shows a reduction of oscillator strength, ruling out population up-conversion. These nonlinear signatures result from many-body effects, including phase-space filling, wave-function renormalization, and exciton collisions. This framework is sufficient to interpret the magnitude of the observed nonlinearities and stress the importance of intersubband exciton interactions. Remarkably, we show that these intersubband interactions have the same magnitude as the intraband ones and bring the major contribution to the photobleaching of the ${S}_{22}$ excitonic transition upon ${S}_{11}$ excitation through energy shift and broadening.

Artificial Gauge Field and Topological Phase in a Conventional Two-dimensional Electron Gas with Antidot Lattices

Based on the Born-Oppemheimer approximation, we divide the total electron Hamiltonian in a spin-orbit coupled system into the slow orbital motion and the fast interband transition processes. We find that the fast motion induces a gauge field on the slow orbital motion, perpendicular to the electron momentum, inducing a topological phase. From this general designing principle, we present a theory for generating artificial gauge field and topological phase in a conventional two-dimensional electron gas embedded in parabolically graded GaAs/InxGa1−xAs/GaAs quantum wells with antidot lattices. By tuning the etching depth and period of the antidot lattices, the band folding caused by the antidot potential leads to the formation of minibands and band inversions between neighboring subbands. The intersubband spin-orbit interaction opens considerably large nontrivial minigaps and leads to many pairs of helical edge states in these gaps.

Enhancement of multisubband electron mobility in asymmetrically doped coupled double quantum well structure

We study the effect of coupling of subband wave functions on the multisubband electron mobility in a barrier delta doped GaAs/AlxGa1−xAs asymmetric double quantum well structure. We use selfconsistent solution of the coupled Schrödinger equation and Poisson's equation to calculate the subband wave functions and energy levels. The low temperature mobility is considered by using scatterings due to ionized impurities, interface roughness and alloy disorder. We show that variation of the width of the central barrier considerably affect the interplay of different scattering mechanisms on electron mobility through intersubband effects. Under single subband occupancy, the mobility increases with decrease in the barrier width as functions of doping concentration as well as function of well width. However, in case of double subband occupancy, effect of intersubband interaction yields opposite trend, i.e., increase in mobility with increase in barrier width. It is gratifying to show that in case of asymmetric variation of well widths the mobility shows nonmonotonic behavior which varies with change in the width of the central barrier under double subband occupancy.

Enhancement of low temperature electron mobility due to an electric field in an InGaAs/InAlAs double quantum well structure

The effect of external electric field F on multisubband electron mobility μ in an In0.53Ga0.47As/In0.52Al48As double quantum well structure is analyzed. We consider scatterings due to ionized impurities, interface roughness and alloy disorder to analyze μ. The variation of scattering mechanisms as a function of F for different structure parameters shows interesting results through intersubband interactions. For small well widths, the mobility is governed by interface roughness scattering. When two subbands are occupied, the effect of impurity scattering gets enhanced through intersubband interactions. Our results of enhancement in mobility as a function of F, can be utilized for low temperature devices.

Conductance of an interacting quasi-one-dimensional electron gas with a scatterer

We calculate the conductance of a quantum wire with two occupied subbands in a presence of a barrier taking into account the interaction between electrons. We extend the renormalization-group equation for the scattering matrix of the barrier to the case of intersubband interactions, find its fixed points, and investigate their stability. Depending on the interaction parameters, the conductance may be equal to 0, $e^2/h$, or $2e^2/h$ per spin projection. In some parameter ranges, two stable fixed points may coexist, so the ultimate conductance depends on the properties of the bare barrier. For spinful electrons, the conductance of the wire may nonmonotonically depend on the Fermi level and temperature.

Enhancement of electron mobility in asymmetric coupled quantum well structures

We study the low temperature multisubband electron mobility in a structurally asymmetric GaAs/AlxGa1-xAs delta doped double quantum well. We calculate the subband energy levels and wave functions through selfconsistent solution of the coupled Schrodinger equation and Poisson's equation. We consider ionized impurity scattering, interface roughness scattering, and alloy disorder scattering to calculate the electron mobility. The screening of the scattering potentials is obtained by using static dielectric response function formalism within the random phase approximation. We analyze, for the first time, the effect of asymmetric structure parameters on the enhancement of multisubband electron mobility through intersubband interactions. We show that the asymmetric variation of well width, doping concentration, and spacer width considerably influences the interplay of scattering mechanisms on mobility. Our results of asymmetry induced enhancement of electron mobility can be utilized for low temperature device applications.

Multisubband electron mobility in asymmetric GaAs/AlGaAs quantum well structures

The effect of structural asymmetry on multisubband electron mobility is studied by considering a barrier delta doped GaAs/AlxGa1−x As quantum well structure. The subband wave functions and energy levels are obtained by adopting selfconsistent solution of the coupled Schrodinger and Poisson’s equations. We consider scatterings due to ionized impurities, interface roughness and alloy disorder. The screening of the scattering potentials is obtained by adopting static dielectric response function formalism. We show that the interplay of different scattering mechanisms on low temperature electron mobility yields interesting results through intersubband interaction. Further, we show that asymmetry in structure parameters, such as, doping concentrations and spacer widths, influence in enhancing the mobility considerably.

Tunable coupling of two-dimensional superconductors in bilayer SrTiO3heterostructures

Interlayer coupling effects between high mobility two-dimensional superconductors are studied in bilayer $\ensuremath{\delta}$-doped SrTiO${}_{3}$ heterostructures. By tuning the undoped SrTiO${}_{3}$ spacer layer between the dopant planes, clear tunable coupling is demonstrated in the variation of the sheet carrier density, Hall mobility, superconducting transition temperature, and the temperature and angle dependencies of the superconducting upper critical field. Systematic variation is found between one effective (merged) two-dimensional superconductor to two decoupled two-dimensional superconductors. In the intermediate coupled regime, a crossover is observed due to coupling arising from intersubband interactions.