Electron Subbands Research Articles

Intersubband resonant enhancement of second order-nonlinear susceptibility in asymmetric AlxGa1−xN/GaN double quantum wells

The present work aims at investigating the second-harmonic generation theoretically in asymmetric AlxGa1−xN/GaN double quantum wells based on a compact density-matrix formalism and a self-consistent approach. Calculations have been made using two- and three-level quantum well models. For this purpose, the electronic subbands and their related wave functions as well as the Fermi energy are computed in asymmetric AlxGa1−xN/GaN double quantum well heterostructures within the effective mass and Hartree approximations. From the given results, it was concluded as follows: (i) The susceptibility exhibits multiple peak structures originating from the intersubband and their corresponding virtual transitions. (ii) The second-order susceptibility strongly depends on the aluminum composition and on the planar-doping design in the barriers as well. (iii) The optimization of electron wave functions' overlaps considerably improved the second-harmonic generation. (iv) Double quantum well systems are revealed to be the most appropriate to generate the second harmonic with an efficient intensity at the suitable resonance frequency. Theoretically, the obtained results would make a useful contribution for experimental studies. From an applied viewpoint, they can open up new opportunities for AlxGa1−xN/GaN optoelectronic devices.

Discrete Electronic Subbands due to Bragg Scattering at Molecular Edges.

The discretization of the electronic structure of nanometer-size solid systems due to quantum confinement and the concomitant modification of their physical properties is one of the cornerstones for the development of nanoscience and nanotechnology. In this Letter we demonstrate that the Bragg scattering of Cu(111) surface-state electrons by the periodic arrangement of tetracyanoquinodimethane molecules at the edges of self-assembled molecular islands, along with the dominant contribution of backscattering processes to the electronic density of states, discretizes the possible values of the electron momentum parallel to the island edge. The electronic structure consists thus of a discrete number of subbands which occur in a nonclosed space, and therefore without quantum confinement.

Synthesizing a \u03bd=2/3 fractional quantum Hall effect edge state from counter-propagating \u03bd=1 and \u03bd=1/3 states

Topological edge-reconstruction occurs in hole-conjugate states of the fractional quantum Hall effect. The frequently studied filling factor, ν = 2/3, was originally proposed to harbor two counter-propagating modes: a downstream v = 1 and an upstream v = 1/3. However, charge equilibration between these two modes always led to an observed downstream v = 2/3 charge mode accompanied by an upstream neutral mode. Here, we present an approach to synthetize a v = 2/3 edge mode from its basic counter-propagating charged constituents, allowing a controlled equilibration between the two counter-propagating charge modes. This platform is based on a carefully designed double-quantum-well, which hosts two populated electronic sub-bands (lower and upper), with corresponding filling factors, vl and vu. By separating the 2D plane to two gated intersecting halves, each with different fillings, counter-propagating chiral modes can be formed along the intersection line. Equilibration between these modes can be controlled with the top gates’ voltage and the magnetic field.

Investigation of Subband Electron Temperatures of Quantum Cascade Lasers

The electron temperature of each subband is the key to understanding the characteristics of quantum cascade lasers (QCLs). We show herein a method, based on the particle swarm optimization algorithm, to obtain the electron temperature of the conduction band in the active region of QCLs. Accurate subband electron temperatures at room temperature, low temperature, and under various external electric fields are obtained. Furthermore, we discuss the variation and distribution of subband electron temperature that occurs during the operation of QCLs. The laser gain and I–V curves are obtained from the subband electron temperature. A comparison of these results with experimental results confirms that consideration of the subband electron temperature can improve our understanding of QCLs.

Electric-field-enhanced circular dichroism of helical semiconductor nanoribbons.

In this Letter, we analyze circular dichroism (CD) enhancement of a helical semiconductor nanoribbon exposed to a weak homogenous electric field. By creating a periodic superlattice for the confined electrons, the electric field splits the electronic sub-bands into minibands and gives rise to critical points in the electronic density of states. We show that the modification of the electronic energy spectrum results in the appearance of new optically active transitions in the CD and absorption spectra, and that the CD signal of the nanoribbon is significantly enhanced at the critical points. The ability to dynamically control the chiroptical response of semiconductor nanoribbons by an external electric field makes them promising for the next-generation nanophotonic devices.

Effect of thin heterogeneous functional nanolayers on electron transport in InGaAs-based quantum wells with high electron density (a review)

A prospective improvement of InGaAs-based quantum well heterostructures design is possible towards the quantum treatment of electron subbands states and electron transport processes in the real structures with thin spatially inhomogeneous functional nanolayers. Insertion of nanosized GaAs, AlAs and InAs layers into quantum well/spacer/barrier layers or applying a graded InyGa1-yAs channel can enhance electron transport properties and gives a novel degree of freedom for the quantum structures engineering. The review and original study for PHEMT/GaAs, HFET/GaAs and InP HEMT structures are discussed.

Engineering Carrier Effective Masses in Ultrathin Quantum Wells of IrO_{2}.

The carrier effective mass plays a crucial role in modern electronic, optical, and catalytic devices and is fundamentally related to key properties of solids such as the mobility and density of states. Here we demonstrate a method to deterministically engineer the effective mass using spatial confinement in metallic quantum wells of the transition metal oxide IrO_{2}. Using a combination of insitu angle-resolved photoemission spectroscopy measurements in conjunction with precise synthesis by oxide molecular-beam epitaxy, we show that the low-energy electronic subbands in ultrathin films of rutile IrO_{2} have their effective masses enhanced by up to a factor of 6 with respect to the bulk. The origin of this strikingly large mass enhancement is the confinement-induced quantization of the highly nonparabolic, three-dimensional electronic structure of IrO_{2} in the ultrathin limit. This mechanism lies in contrast to that observed in other transition metal oxides, in which mass enhancement tends to result from complex electron-electron interactions and is difficult to control. Our results demonstrate a general route towards the deterministic enhancement and engineering of carrier effective masses in spatially confined systems, based on an understanding of the three-dimensional bulk electronic structure.

Broadband bias-tunable terahertz photodetector using asymmetric GaAs/AlGaAs step multi-quantum well

In this letter, a broadband bias-tunable terahertz (THz) quantum well infrared photodetector (QWIP) with a GaAs/AlGaAs asymmetric step multi-quantum well using the intersubband transition was proposed, designed, fabricated, and characterized. It is found that the THz QWIP has two photon response peaks at 4.5 THz and 6.5 THz and the bias-tunable photon response in the 4.0–6.5 THz frequency range of the device is attributed to the field-assisted tunneling of electrons in the second excitation subbands to continuum states.

Theoretical investigation of hot electron cooling process in GaAs/AlAs cylindrical quantum wire under the influence of an intense electromagnetic wave

Hot electrons cooling by phonons in GaAs/AlAs cylindrical quantum wire (CQW), under the influence of an intense electromagnetic wave (EMW), is studied theoretically. Analytic expression for the electron cooling power (CP) is derived from the quantum transport equation for phonons, using the Hamiltonian of interacting electron–optical phonon system. Both photon absorption and emission processes are considered. Numerical results show that the CP reaches maximum when the energy difference between electronic subbands equals the energy of an optical phonon plus the photon energy. Under the influence of the EMW, the negative CP is observed showing that electrons gain energy from phonon and photon instead of losing their energy. Also, the CP increases with increasing the EMW amplitude. Our results theoretically clarify the mechanism of the electron cooling process by phonons in the GaAs/AlAs CQW under the EMW, which is of significance for designing and fabricating high-speed nanoelectronic devices based on this material.

A k · p treatment of edge states in narrow 2D topological insulators, with standard boundary conditions for the wave function and its derivative

For 2D topological insulators with strong electron–hole hybridization, such as HgTe/CdTe quantum wells, the widely used 4 × 4 k · p Hamiltonian based on the first electron and heavy hole sub-bands yields an equal number of physical and spurious solutions, for both the bulk states and the edge states. For symmetric bands and zero wave vector parallel to the sample edge, the mid-gap bulk solutions are identical to the edge solutions. In all cases, the physical edge solution is exponentially localized to the boundary and has been shown previously to satisfy standard boundary conditions for the wave function and its derivative, even in the limit of an infinite wall potential. The same treatment is now extended to the case of narrow sample widths, where for each spin direction, a gap appears in the edge state dispersions. For widths greater than 200 nm, this gap is less than half of the value reported for open boundary conditions, which are called into question because they include a spurious wave function component. The gap in the edge state dispersions is also calculated for weakly hybridized quantum wells such as InAs/GaSb/AlSb. In contrast to the strongly hybridized case, the edge states at the zone center only have pure exponential character when the bands are symmetric and when the sample has certain characteristic width values.

Effect of non-square structure potential on the multisubband electron mobility in double quantum well structure

We study the impact of non-square potential well structure on the electron mobility μ of double quantum well (DQW) based field effect transistors carved out of the AlxGa1-xAs alloy. The barriers lying towards the substrate and surface sides of the DQW are delta doped with Si. We consider DQWs having V-shaped (VDQW), parabolic (PDQW), cubic (CDQW) and square (SDQW) potential wells to obtain the low temperature double subband electron mobility μ. We consider ionized impurity (Imp-) and alloy disorder (Al-) scatterings to calculate μ as a function of well width w and surface electron density NS. We show that the changes in the structure potentials influence the interplay of intersubband effects on the scattering mechanisms differently causing μ(VDQW) < μ(PDQW) ≅ μ(CDQW) as a function of w whereas μ(VDQW) < μ(PDQW) < μ(CDQW) as a function of Ns. We show that μ increases not only with increase in the width of the central barrier b but also decrease with the height of the non-square potentials. Our results can be utilized to analyze the effect of non-square quantum well potentials on the channel conductivity of the quantum well modulation doped field effect transistors.

Impurity-assisted terahertz photoluminescence in quantum wells under conditions of interband stimulated emission

Terahertz and near-infrared photoluminescence under conditions of interband stimulated emission are studied in n-GaAs/AlGaAs quantum well laser structure. The observed terahertz emission is related to the optical transitions of nonequilibrium electrons from the first electron subband and excited donor states to donor ground states in quantum wells. The opportunity to increase the intensity of impurity-assisted terahertz emission due to interband stimulated emission with the participation of impurity centres is demonstrated.

THz emission of donor and acceptor doped GaAs/AlGaAs quantum well structures with inserted thin AlAs monolayer

In this paper, we explore THz range optical intersubband transition energies in a donor doped quantum well of a GaAs/AlGaAs system as a function of the insertion position of an AlAs monolayer in the GaAs quantum well. In simulated models, the optical transition energies between electron subband levels 1 and 2 were higher in the doped structure than in the undoped structure. This may be because the envelope wave function of the second electron subband strongly overlapped the envelope wave function of the first electron subband and influenced the optical intersubband transition between the two levels in the THz range. At different levels of bias voltage at the Schottky barrier on the donor doped structure, the electric field in the growth direction of the structure linearly increased the further away the AlAs monolayer was placed from the reference position. We also simulated the optical transition energies between acceptor energy levels of the acceptor doped structure as a function of the insertion position of the AlAs monolayer. The acceptor doped structure induced THz range emission whereas the undoped structure induced mid-IR emission.

Robust integer and fractional helical modes in the quantum Hall effect.

Electronic systems harboring one-dimensional helical modes, where spin and momentum are locked, have lately become an important field of its own. When coupled to a conventional superconductor, such systems are expected to manifest topological superconductivity; a unique phase hosting exotic Majorana zero modes. Even more interesting are fractional helical modes, yet to be observed, which open the route for realizing generalized parafermions. Possessing non-abelian exchange statistics, these quasiparticles may serve as building blocks in topological quantum computing. Here, we present a new approach to form protected one-dimensional helical edge modes in the quantum Hall regime. The novel platform is based on a carefully designed double-quantum-well structure in a GaAs based system hosting two electronic sub-bands; each tuned to the quantum Hall effect regime. By electrostatic gating of different areas of the structure, counter-propagating integer, as well as fractional, edge modes with opposite spins are formed. We demonstrate that due to spin-protection, these helical modes remain ballistic for large distances. In addition to the formation of helical modes, this platform can serve as a rich playground for artificial induction of compounded fractional edge modes, and for construction of edge modes based interferometers.

Hidden edge Dirac point and robust quantum edge transport in InAs/GaSb quantum wells

The robustness of quantum edge transport in InAs/GaSb quantum wells in the presence of magnetic fields raises an issue on the fate of topological phases of matter under time-reversal symmetry breaking. A peculiar band structure evolution in InAs/GaSb quantum wells is revealed: the electron subbands cross the heavy hole subbands but anticross the light hole subbands. The topologically protected band crossing point (Dirac point) of the helical edge states is pulled to be close to and even buried in the bulk valence bands when the system is in a deeply inverted regime, which is attributed to the existence of the light hole subbands. A sizable Zeeman energy gap verified by the effective g-factors of edge states opens at the Dirac point by an in-plane or perpendicular magnetic field, however it can also be hidden in the bulk valance bands. This provides a plausible explanation for the recent observation on the robustness of quantum edge transport in InAs/GaSb quantum wells subjected to strong magnetic fields.

Heat Exchange Between Electrons and Phonons in Nanosystems at Sub-Kelvin Temperatures

Ultra-sensitive nanoscopic detectors for electromagnetic radiation consist of thin metallic films deposited on dielectric membranes. The metallic films, of thickness d of the order of 10 nm, form the thermal sensing element (TSE), which absorbs the incident radiation and measures its power flux or the energies of individual photons. To achieve the sensitivity required for astronomical observations, the TSE works at temperatures of the order of 0.1 K. The dielectric membranes are used as support and for thermal insulation of the TSE and are of thickness L − d of the order of 100 nm (L being the total thickness of the system). In such conditions, the phonon gas in the detector assumes a quasi-two-dimensional distribution, whereas quantization of the electrons wavenumbers in the direction perpendicular to the film surfaces leads to the formation of quasi two-dimensional electronic sub-bands. The heat exchange between electrons and phonons has an important contribution to the performance of the device and is dominated by the interaction between the electrons and the antisymmetric acoustic phonons.

Exploring interacting Floquet states in black phosphorus: Anisotropy and bandgap laser tuning

Non-adiabatic interacting Floquet (or dressed) states arising from the intrinsic coupling between electrons and holes with off-resonant electromagnetic radiation have been investigated for recently synthesized ultrathin gapped and anisotropic black phosphorus. Our analytical calculations were carried out for the low-energy electronic subbands near the Γ point. Optical dressed states for both linear and circular polarizations of the incoming radiation have been obtained and analyzed. We focus our attention on linearly polarized light with arbitrary polarization direction since this case has not been considered for dressing fields imposed on initially anisotropic structures. We have examined and compared various cases for one- and few-layer phosphorus, including massless Dirac fermions with laser tunable in-plane anisotropy. The electronic properties, such as bandgaps, Fermi velocities, and effective masses, are renormalized in a very different way compared to those for the previously studied gapped Dirac structures. More importantly, material anisotropy and angular dependence of energy dispersions could be tuned by incident polarized light in the individual direction. The interaction of electrons with photons brings out a product term for two wave-vector components in energy dispersions, which leads to non-vanishing off-diagonal elements in inverse effective-mass and momentum-relaxation-time tensors and is expected to have a significant effect on the conductivities of electrons and holes. Such unique features in the band structure can be applied to new designs and fabrications of optical transistors and logic devices used in optical computers and communications.

Gate tunable spin-orbit coupling and weak antilocalization effect in an epitaxial La2/3Sr1/3MnO3 thin film

Epitaxial La$_{2/3}$Sr$_{1/3}$MnO$_3$ (LSMO) films have been grown on SrTiO$_3$ (001) substrates via pulsed laser deposition. In a 22-nm thick LSMO film with a low residual resistivity of $\rho_0$ = 59 $\mu \Omega$ cm, we found a zero-field dip in the magnetoresistance (MR) below 10 K, manifesting the weak antilocalization (WAL) effect due to strong spin-orbit coupling (SOC). We have analyzed the MR data by including the D'yakonov-Perel' spin-relaxation mechanism in the WAL theory. We explain that the delocalized spin-down electron subband states play a crucial role for facilitating marked SOC in clean LSMO. Moreover, we find that the SOC strength and gate voltage tunability is similar to that in the 2DEG at LaAlO$_3$/SrTiO$_3$ interface, indicating the presence of an internal electric field near the LSMO/SrTiO$_3$ interface. In a control measurement on a 5-nm thick high resistivity ($\rho_0$ = 280 $\mu \Omega$ cm) LSMO film, we observe only a small zero-field peak in MR from weak localization effect, indicating negligible SOC.

Effect of picosecond magnetic pulse on dynamics of electron's subbands in semiconductor bilayer nanowire

We report on possibility of charge current generation in nanowire made of two tunnel coupled one-dimensional electron waveguides by means of single magnetic pulse lasting up to 20ps. Existence of interlayer tunnel coupling plays a crucial role in the effect described here as it allows for hybridization of the wave functions localized in different layers which can be dynamically modified by applying a time changeable in-plane magnetic field. Results of time-dependent DFT calculations performed for a bilayer nanowire confining many electrons show that the effect of such magnetic hybridization relies on tilting of electrons’ energy subbands, to the left or to the right, depending on a sign of time derivative of oscillating magnetic field due to the Faraday law. Consequently, the tilted subbands become a source of charge flow along the wire. Strength of such magneto-induced current oscillations may achieve even 0.6μA but it depends on duration of magnetic pulse as well as on charge density confined in nanowire which has to be unequally distributed between both transport layers to observe this effect.

Guest Editorial Mini-Special Issue on International Quantum Cascade Lasers School and Workshop (IQCLSW 2016)

The papers in this special section were presented at the International Quantum Cascade Lasers School and Workshop (IQCLSW 2016). Quantum cascade lasers (QCLs) are unipolar optoelectronic devices that exploit optical transitions between engineered electronic subbands created in multiple coupled semiconductor quantum wells. The large number of nano metre thick semiconductor layers and the complexity of the overall structure, makes the QCL the most impressive demonstration of the capabilities offered by band structure engineering and state-of-the-art materials growth technologies.