Double Quantum Well Structure Research Articles

Antiparallel Spin Polarization and Spin Current Induced by Thermal Current and Locally Broken Inversion Symmetry in a Double‐Quantum Well Structure

Generating a nonequilibrium spin polarization with a driving force is first realized by the electric current in a system with broken inversion symmetry and extended to that induced by the thermal current and that appearing in an inversion‐symmetric system with locally broken inversion symmetry. This article theoretically explores the spin polarization generated by the thermal current and the locally broken inversion symmetry in a symmetric double‐quantum well structure (DQWS). This thermally induced spin polarization (TISP) appears in the antiparallel configuration with the TISP of two wells in opposite directions. The calculation using the Boltzmann equation in the relaxation‐time approximation under the condition of zero charge current shows that the local TISP exhibits the maximum at a finite Rashba spin–orbit interaction when the electron density is fixed. This is because the local TISP in the DQWS is enhanced at the chemical potential near the bottom of the first‐excited sub‐band. This enhancement also occurs in a single quantum well with globally broken inversion symmetry. Another finding is that the maximum of the local TISP appears at a nonzero interwell coupling. The spin current by the diffusion of the local TISP into an adjacent electrode is also calculated.

Enhancement of electron transport mobility in GaAs/InGaAs asymmetrically doped narrow quantum well pHEMT structure

We study the effect of asymmetric doping concentrations on the electron mobility μ in GaAs/InGaAs-based single quantum well (SQW) as well as double quantum well (DQW) pseudomorphic high electron mobility transistor (pHEMT) structures. Unequal doping in the substrate and surface barriers (n d1 and n d2 ) causes asymmetric distributions of subband wave functions, ψ 0 and ψ 1, which influence the subband scattering rate matrix elements (SSRME), thereby affecting the subband mobility μ n . For narrow well widths (w w ), in SQW structures, mostly a single subband is occupied. We show that an increase in n d2 , keeping n d1 fixed, enhances μ nonlinearly. The interface roughness (ir-) scattering mostly dominates μ in thin wells (w w < 70 Å), while generally, μ is determined by ionized impurity (ii-) scattering and to some extent by alloy disorder (ad-) scattering. The influence of ir-scattering enhances, while ad-scattering diminishes, by reducing n d2 . For DQW, a double subband is occupied. In a symmetric DQW structure at resonance, n d1 = n d2 , ψ 0 and ψ 1 equally extend into both the wells. For a minor variation, say n d1 > n d2 , ψ 0 mostly lies in one well while ψ 1 is in the other well. In the case of n d1 < n d2 , the distribution reverts. The substantial changes in ψ 0 and ψ 1 influence the intra- and inter-SSRME differently through intersubband effects, leading to nonlinear μ n as a function of n d2 . Taking n d1 + n d2 = 3 × 1018 cm−3, we show that for w w1 = w w2 = 80 Å, a shallow dip in μ occurs at n d1 = n d2 = 1.5 × 1018 cm−3. Whereas, for w w1 = 60 Å and w w2 = 100 Å, the dip in μ occurs near the corresponding resonance, n d1 = 2.3 × 1018 cm−3 and n d2 = 0.7 × 1018 cm−3. Our results of nonlinear μ can be utilized for performance analysis of pHEMT.

Spin-current generation from local spin polarization induced by current through local inversion asymmetry: Double quantum well structure

The current-induced spin polarization (CISP) in a system without the inversion symmetry is known to efficiently generate the spin current. In this paper we propose another approach, generating the spin current from the CISP locally arising in a system with the inversion symmetry in the form of the antiparallel CISP in sublattice structure. In this approach the local CISP is extracted from one of sublattices by a selective contact between the sublattice and an electrode. As the simplest system with such antiparallel CISP, we consider a symmetric double-quantum-well structure (DQWS) and calculate the antiparallel CISP and the spin current to a metallic layer in parallel contact with one well of the DQWS using the Boltzmann equation in the relaxation time approximation. When the Fermi energy is large enough that the first-excited sub-band is occupied, the magnitude of the spin current, which is proportional to that of the antiparallel CISP, increases with the interwell tunnel coupling and reaches twice the value of the decoupled quantum well with the broken inversion symmetry. Such an estimate suggests that inversion-symmetric systems can be useful in generating the spin current if the inversion symmetry is locally broken.

Efficient Nondoped Pure Red/Near-Infrared TADF OLEDs by Designing and Adjusting Double Quantum Wells Structure

The selection of host materials has a great impact on the performance of doping devices, especially near-infrared (NIR) emitters. In this investigation, four diverse host materials serve as a potential barrier layer (PBL) to confine and balance holes and electrons within the potential well layer (PWL), TPA-DCPP, for fabricating nondoped NIR thermally activated delayed fluorescence (TADF) organic light-emitting diodes (OLEDs) (NNT-OLEDs) with double quantum wells’ (DQWs) structure. The hole-type host (mCP) forms an optimum interface energy barrier (IEB) and disperses carriers and excitons in each well, which helps to widen the recombination interval of carriers and restrain the quenching of excitons. Finally, OLEDs with pure red emission and a maximum external quantum efficiency (EQEmax) of nearly 15% (with 0.5 nm well width), deep-red emission with an EQEmax of 12% (with 1.0 nm well width), and NIR emission with an EQEmax of 3.3% (with 5.5 nm well width) are achieved with tunable emission peaks within the range of 641–700 nm by adjusting well width, which are significantly superior to those of doped devices based on the same emitter. This investigation demonstrates a simple, feasible, and effective design strategy for achieving efficient fluorescent OLEDs with controllable wavelength, which solves the blue shift problem in traditional NIR devices of host–guest structure.

Resonant tunneling driven metal-insulator transition in double quantum-well structures of strongly correlated oxide

The metal-insulator transition (MIT), a fascinating phenomenon occurring in some strongly correlated materials, is of central interest in modern condensed-matter physics. Controlling the MIT by external stimuli is a key technological goal for applications in future electronic devices. However, the standard control by means of the field effect, which works extremely well for semiconductor transistors, faces severe difficulties when applied to the MIT. Hence, a radically different approach is needed. Here, we report an MIT induced by resonant tunneling (RT) in double quantum well (QW) structures of strongly correlated oxides. In our structures, two layers of the strongly correlated conductive oxide SrVO3 (SVO) sandwich a barrier layer of the band insulator SrTiO3. The top QW is a marginal Mott-insulating SVO layer, while the bottom QW is a metallic SVO layer. Angle-resolved photoemission spectroscopy experiments reveal that the top QW layer becomes metallized when the thickness of the tunneling barrier layer is reduced. An analysis based on band structure calculations indicates that RT between the quantized states of the double QW induces the MIT. Our work opens avenues for realizing the Mott-transistor based on the wave-function engineering of strongly correlated electrons.

Compensation mechanism of carriers within weakly coupled quantum wells

The study of coupling effect (CE) within low-dimensional quantum structures is essential not only for the comprehensive understanding of quantum transport but also for improving the performances of photoelectric devices. Recently, electronically weakly coupled quantum well (QW) structures pave the way for low threshold lasing, but the related coupling mechanism has not been clarified yet. Here, we report the direct experimental evidence of compensation mechanism in the carrier migration process within weakly coupled double QWs (DQWs). It is proved that the leakage rate of charges could be modulated within the two InGaSb QWs of the DQW structure owing to CE. Compared to the single QW configuration, the dissipation of carriers from the QWs within the DQW structure is alleviated under an external field. Further, radiative recombination efficiency for the DQW structure could be enhanced to obtain a lower threshold power. Thus, it may be deduced that the carrier compensation effect between QWs profoundly impacts the multiple-QW laser performance.

Optical absorption in the terahertz region in HgTe/HgCd(Mn)Te double quantum well structures with a topological phase and structural inversion asymmetry

On the basis of the eight-band kp model, the theoretical engineering of HgTe/HgCd(Mn)Te double quantum well (DQW) structures is presented in this study. An external magnetic field and structural inversion asymmetry are taken into account. The advantages of HgTe two-dimensional topological insulator (2D TI) layers with x-Cd or x-Mn over pure 2D TI HgTe films are theoretically and experimentally proven. All advantages, such as the larger Fermi velocity, very high mobility of the surfaces states, the larger g factor for such mixed layers together with the sharpens on the edge layers in the 2D TI structures and the structural inversion asymmetry (which can be easy realized in the 2D DQW case), mean that the presented structures are encouraging as the basis of, for example, high resolution terahertz detectors. Considering this possible application, the optical transition for the cyclotron resonance and the excitation transfer in the terahertz region is analyzed. The results of the presented investigation are very promising for the application of HgTe/HgCd(Mn)Te DQW structures in terahertz detector.

Suppressing effective magnetic field and spin-relaxation rate by tuning barrier compositions in a (111) quantum well

Adjusting compositions of two barriers forming a (111)-oriented InGaAs/AlGaAsSb quantum well (QW) is proposed as a method to eliminate the effective magnetic field due to the Rashba and linear-in-momentum Dresselhaus spin–orbit interactions in the zeroth and first orders in the gate electric field. The resulting suppressed Dyakonov–Perel spin relaxation is used to achieve a high on/off ratio of the spin-relaxation rate at room temperature in a double-QW structure.

Biaxial strain effects on the band structure and absorption coefficient of GaAs1-x-yNxBiy/GaAs MQWs calculated using k.p method

In this work, we derive the 16 band strain-dependent k·p Hamiltonian using finite difference method for dilute bismide-nitride compound GaNAsBi-based strained quantum wells (QWs). The effect of the biaxial strain (-0.57≤ε∥≤0.50%) on electronic band structure of GaNAsBi/GaAs double QWs (DQWs) operating at the near-infrared spectrum and telecommunications window (1.3 and 1.55 μm) are examined. We use the band anti-crossing model, envelop function formalism and Bir-Pikus theory in conjugation with k·p perturbation method to describe the optoelectronics properties (band offsets, absorption coefficient) behavior of the DQWs structures. Results show that the tensile strained GaNAsBi/GaAs DQWs can be used for the detection of 1, 0.95, and 0.80 eV energy range. This device seem be differentiated by higher carrier confinement and optical absorption compared to compressive strained one. The enhancement of spectral response is analyzed by increasing the QWs number and the coupling between them.

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.

Switching of exciton character in double InGaN/GaN quantum wells

The dependence on exciton density of the interwell coupling scheme in a series of InGaN/GaN symmetrical double quantum wells (DQWs) with varying central barrier width was observed. Continuous-wave photoluminescence (cw-PL) and time-resolved photoluminescence (TRPL), measured at low temperature (6--13 K), allowed us to examine the competition between three optical recombination channels, namely, the recombination of (i) intrawell excitons (DXs), (ii) interwell indirect excitons (IXs), and (iii) presumably coupled well excitons (CWXs), built of electron and hole wave functions that are widely spread over the entire DQW structure. We demonstrate a rather abrupt switching effect that relies on the actual exciton density in the system. In cw-PL experiments as a function of the pumping laser power, this switching is characterized by a threshold laser power density above which we observe (i) a significant change of slope of both the power-dependent blueshift and intensity of the ground-state exciton and (ii) the appearance of higher-energy optical emissions. In TRPL, as the PL intensity decays with time, both these effects are visible but in opposite direction, including the PL intensity transfer from the higher-energy state to the ground state. The observed switching is assigned to a change of the dominant excitonic recombination regime: at low pumping densities the dominant emission arises from the extremely long-lived IX, whereas above threshold the dominant emission corresponds to DXs or CWXs, depending on the barrier width. The threshold power density (or threshold time for TRPL) presents a clearly exponential dependence upon the width of the central barrier, which demonstrates the role of carrier tunneling in the overall switching process. The comparison of IXs in nitride QWs with bias electric-field induced IXs in GaAs/AlGaAs DQWs shows that the spectral blueshifts are a few times larger in the former structures. This enhancement originates from the built-in electric field in group-III nitrides, which has truly microscopic character. Moreover, it is argued that the presence of the potential fluctuations due to alloy disorder and the fluctuations of the coupling barrier width have only a secondary effect on the observed switching.

Study of direct bandgap type-I GeSn/GeSn double quantum well with improved carrier confinement

The GeSn-based quantum wells (QWs) have been investigated recently for the development of efficient GeSn emitters. Although our previous study indicated that the direct bandgap well with type-I band alignment was achieved, the demonstrated QW still has insufficient carrier confinement. In this work, we report the systematic study of light emission from the Ge0.91Sn0.09/Ge0.85Sn0.15/Ge0.91Sn0.09 double QW structure. Two double QW samples, with the thicknesses of Ge0.85Sn0.15 well of 6 and 19 nm, were investigated. Band structure calculations revealed that both samples feature type-I band alignment. Compared with our previous study, by increasing the Sn composition in GeSn barrier and well, the QW layer featured increased energy separation between the indirect and direct bandgaps towards a better direct gap semiconductor. Moreover, the thicker well sample exhibited improved carrier confinement compared to the thinner well sample due to lowered first quantized energy level in the Γ valley. To identify the optical transition characteristics, photoluminescence (PL) study using three pump lasers with different penetration depths and photon energies was performed. The PL spectra confirmed the direct bandgap well feature and the improved carrier confinement, as significantly enhanced QW emission from the thicker well sample was observed.

Lattice-matched double dip-shaped BAlGaN/AlN quantum well structures for ultraviolet light emission devices

Ultraviolet light emission characteristics of lattice-matched BxAlyGa1−x−y N/AlN quantum well (QW) structures with double AlGaN delta layers were investigated theoretically. In contrast to conventional single dip-shaped QW structure where the reduction effect of the spatial separation between electron and hole wave functions is negligible, proposed double dip-shaped QW shows significant enhancement of the ultraviolet light emission intensity from a BAlGaN/AlN QW structure due to the reduced spatial separation between electron and hole wave functions. The emission peak of the double dip-shaped QW structure is expected to be about three times larger than that of the conventional rectangular AlGaN/AlN QW structure.

The effect of barrier width on the electronic properties of double GaAlAs/GaAs and GaInAs/GaAs quantum wells

In this study, the electronic properties of symmetric and asymmetric double Ga1-xAlxAs/GaAs and Ga1-xInxAs/GaAs quantum wells have been investigated depending on the barrier width. Using the effective mass approach, the energy levels and wave functions of double quantum wells (DQW) have been calculated by solving the Schrödinger equation. Our results show that the potential height and the energy levels of Ga1-xInxAs/GaAs DQW are always higher than of Ga1-xAlxAs/GaAs DQW structure. Also, for Ga1-xInxAs/GaAs DQW the variation of the energy difference between these levels as dependent on the barrier width is more than for Ga1-xAlxAs/GaAs DQW. We say that the barrier width have a great effect on the electronic characteristics of different symmetric and asymmetric DQW structures.

Nanoscale engineering of photoelectron processes in quantum well and dot structures for sensing and energy conversion

Advanced selective doping provides effective tool for nanoscale engineering of potential barriers and photoelectron processes in quantum well (QW) and quantum dot (QD) optoelectronic nanomaterials for IR sensing and wide band photovoltaic conversion. Photoelectron kinetics and device characteristics are investigated theoretically and experimentally. Asymmetrical doping of QWs is employed in a double QW structure for tuning electron transitions in QWs by voltage bias. These QW devices demonstrate bias-tunable multicolor detection and capability of remote temperature sensing. The QD structures with bipolar doping are proposed to independently control photocarrier lifetime (photocurrent) and dark current. The bipolar doping allows us to increase the height of nanoscale potential barriers around QDs without changing the electron population in QDs, which determines dark current. The QD devices with bipolar doping demonstrate significant enhancement of photocurrent, while dark current is close to that in corresponding reference devices with unipolar doping.

Fano resonance via bonding and antibonding states in nonadiabatically-pumped double-quantum-well systems

In this work, transport properties of the nonadiabatically pumped double-quantum-well (DQW) structure are studied. Different from a single quantum well, band mixing in the DQW generates bonding and antibonding states, whose wave functions have different spatial symmetry. By applying a time-dependent electric potential to the two well regions simultaneously, Floquet sidebands are formed, which constitutes additional quantum tunneling paths. When one of the Floquet sidebands coincides with the bonding or antibonding quasibound states within the DQW structure, sharp Fano resonances are found in the transmission coefficients as well as in the differential shot noise spectra. While such Fano resonances originate from quantum interference, their shapes are strikingly different for transport via the bonding state and via the antibonding state. The Fano resonance via the even-parity bonding state shows a perfect transmission followed by a total reflection and the Fano resonance via the odd-parity antibonding state has a reversed symmetry and shows a total reflection before a perfect transmission.

Generation of optical picosecond pulses with monolithic colliding‐pulse mode‐locked lasers containing a chirped double‐quantum‐well active region

In this study, the authors present experimental results on 6 mm long monolithic passively colliding-pulse mode-locked (CPM) lasers. The AlGaAs-based devices with In x Ga1−x As y P1−y double-quantum-well (DQW) active regions emit between 860 and 880 nm. To optimise the laser performance and to shorten the pulses the optical gain is spectrally broadened to increase the number of longitudinal lasing modes by using a chirped DQW structure where the In-content of the QWs differs by Δx = 0.08. Lasers having the chirped DQW with lasers having a conventional, unchirped DQW active region are compared. The mode locking operation is investigated in dependence on gain current, absorber voltage and absorber length. According to the experimental results, the lasers with the chirped DQW exhibit an overall better performance and generate shorter pulses with higher average optical output powers over a wider range of parameters. For an optimum set of parameters, the generated pulses have a full width at half maximum of the autocorrelation function of 1.9 ps at a repetition frequency of 12.9 GHz.

Tunneling-induced optical bistability in an asymmetric double quantum well.

Optical bistable behaviors induced by tunneling coupling are demonstrated in an asymmetric double quantum-well structure including a unidirectional ring cavity. We analytically find that optical bistability can be obtained with the existence of resonant tunneling. Furthermore, the threshold of the optical bistability can be modulated by the intensity or the detuning of the driving fields.

Coherent transport and manipulation of spins in indirect-exciton nanostructures

We report on the coherent control and transport of indirect exciton (IX) spins in GaAs double quantum well (DQW) nanostructures. The spin dynamics was investigated by optically generating spins using a focused, circularly polarized light spot and by probing their spatial distribution using spatially and polarization resolved photoluminescence spectroscopy. Optically injected exciton spins precess while diffusing over distances exceeding 20 {\mu}m from the excitation spot with a spatial precession frequency that depends on the spin transport direction as well as on the bias applied across the DQW structure. This behavior is attributed to the spin precession in the effective magnetic field induced by the spin-orbit interaction. From the dependence of the spin dynamics on the transport direction, bias and external magnetic fields we directly determined the Dresselhaus and Rashba spin splitting coefficients for the structure. The precession dynamics is essentially independent on the IX density, thus indicating that the long spin lifetimes are not associated with IX collective effects. The latter, together with the negligible contribution of holes to the spin dynamics, are rather attributed to spatial separation of the electron and hole wave functions by the electric field, which reduces the electron-hole exchange interaction. Coherent spin precession over long transport distances as well as the control of the spin vector using electric and magnetic fields open the way for the application of IX spins in the quantum information processing.

Development of Charge Sensitive Infrared Phototransistors for the Far-Infrared Wavelength

Ultra-highly-sensitive far-infrared detectors are developed for potential application to astronomy. The detectors exploit a novel mechanism called Charge Sensitive Infrared Phototransistors (CSIPs), in which an upper quantum well (QW) in GaAs/AlGaAs double QW structures is positively charged up by photo-excitation via inter-subband transition. This causes the conductance of the lower QW channel to increase. The device is effectively a phototransistor, in which the upper QW serves as a photo-sensitive gate to the source-drain channel provided by the lower QW. Resultant extraordinary high photoconductive gain makes CSIPs so sensitive as to detect single photons. CSIPs are well established in the mid-infrared (\(\lambda \) = 12–20 \(\upmu \)m), achieving noise equivalent power around 1.9 \(\times \) 10\(^{-19}\) W/Hz\(^{1/2}\) with a quantum efficiency of 7 %. CSIPs have been demonstrated to work in longer wavelengths up to 45 \(\upmu \)m, but the sensitivity was not as high as in the shorter wavelengths, probably due to lower quantum efficiency. Reported here is a remarkable improvement in the performance of longer wavelength CSIPs (45 \(\upmu \)m), achieved primarily by optimizing the doping concentration in the upper QW. This work indicates that longer wavelength CSIPs are promising detectors for the astronomical application.