Symmetric Quantum Wells Research Articles

Effects of magnetic, electric, and non-resonant intense laser fields on exciton binding energy and exciton absorption in a symmetric Gaussian single quantum well

Effects of magnetic, electric, and non-resonant intense laser fields on exciton binding energy and exciton absorption in a symmetric Gaussian single quantum well

Effects of hydrostatic pressure, aluminium composition, and external magnetic field on intersubband and intrasubband transitions characteristics in symmetric potential quantum wells

Aluminium composition (λ), magnetic field (B), and hydrostatic pressure (P) influences on magneto-optical (MO) properties of intra- and inter-subband transitions in GaAs/AlλGa1−λAs symmetric QWs for four models of confined optic phonons are studied and compared. The findings indicate: the MO characteristics of intra- and inter-subband transitions are significantly influenced by B, λ, and P for phonon absorption and emission. The e–p interaction intensity decreases as P increases, whereas it increases as λ increases. The influences of λ and P on phonon absorption are more significant than those on emission. E–p interaction intensity dependence on λ and P for intra-subband transitions is stronger than that for inter-subband ones. The e–p interaction in QWs is well described by Knipp–Reinecke and Huang–Zhu models. To reduce the influence of P on MO properties, we can increase T and λ or decrease B. At T≈600 K, the influence of P is insignificant and can be neglected.

Nonlinear optical rectification in symmetric double-graded quantum wells dressed by non-resonant laser field under electric and magnetic fields

Nonlinear optical rectification in symmetric double-graded quantum wells dressed by non-resonant laser field under electric and magnetic fields

Comparison of electron scattering by acoustic-phonons in two types of quantum wells with GaAs and GaN materials.

In this work, we report a detailed comparison of electron-acoustic-phonon (EAP) interaction strength in symmetric (parabolic) and asymmetric (semi-parabolic) quantum-wells (QWs) for both GaAs and GaN materials. The operator projection method will be utilized to calculate the acoustic-phonon-assisted cyclotron resonance (CR) absorption power. The EAP interaction strength is determined by measuring the full width at half maximum (FWHM) of the acoustic-phonon-assisted CR absorption peak based on the profile of the curve describing the dependence of the acoustic-phonon-assisted CR absorption power on the photon energy. The studied result reveals that the EAP interaction strengths in the symmetric and asymmetric QWs are functions of the electron temperature (ET), external magnetic field (EMF), and confined potential frequency (CPF). Namely, the larger the ET, the EMF, and the CPF, the stronger the EAP interaction strengths in the symmetric and asymmetric QWs are for both GaN and GaAs materials. More importantly, the obtained result demonstrates that under the influence of the structural (CPF) and external (ET and EMF) parameters, the EAP interaction strength in the symmetric QW is always much stronger than that in the asymmetric QW for both GaN and GaAs materials. Simultaneously, the EAP interaction strength in the GaN material is much stronger than that in the GaAs material for both the symmetric and asymmetric QWs.

Optical gain calculation and structure optimization of type-II GaAsSb/InAs/InGaAs quantum well heterostructures

This paper reports an optimization scheme of type-II GaAsSb/InAs/InGaAs quantum wells using the six bands k · p method. By calculating the optical gain, the optimal structures of the quantum wells are obtained. We observed that without an electric field, the thickness values of symmetrical quantum wells designed by the AdaGrad algorithm are 2.97, 2.81, and 6.21 nm, respectively, and the gain intensity is 6489 cm−1 that is about 1600 cm−1 higher than the original empirical design scheme. The thickness of asymmetric quantum wells designed by the Adam algorithm is 5.56, 2.72, 1.88, 3.16, and 2.46 nm under the external electric field of 75 kV/cm, respectively, and the gain intensity is 5729 cm−1 that is about 2300 cm−1 higher than the original empirical design scheme. The optimized scheme effectively suppresses the gain attenuation caused by the electric field and is helpful to improve the performance of interband cascade lasers.

Parabolic-Gaussian Double Quantum Wells under a Nonresonant Intense Laser Field.

In this paper, we investigate the electronic and optical properties of an electron in both symmetric and asymmetric double quantum wells that consist of a harmonic potential with an internal Gaussian barrier under a nonresonant intense laser field. The electronic structure was obtained by using the two-dimensional diagonalization method. To calculate the linear and nonlinear absorption, and refractive index coefficients, a combination of the standard density matrix formalism and the perturbation expansion method was used. The obtained results show that the electronic and thereby optical properties of the considered parabolic-Gaussian double quantum wells could be adjusted to obtain a suitable response to specific aims with parameter alterations such as well and barrier width, well depth, barrier height, and interwell coupling, in addition to the applied nonresonant intense laser field.

Harmonic-Gaussian Symmetric and Asymmetric Double Quantum Wells: Magnetic Field Effects

In this study, we considered the linear and non-linear optical properties of an electron in both symmetrical and asymmetrical double quantum wells, which consist of the sum of an internal Gaussian barrier and a harmonic potential under an applied magnetic field. Calculations are in the effective mass and parabolic band approximations. We have used the diagonalization method to find eigenvalues and eigenfunctions of the electron confined within the symmetric and asymmetric double well formed by the sum of a parabolic and Gaussian potential. A two-level approach is used in the density matrix expansion to calculate the linear and third-order non-linear optical absorption and refractive index coefficients. The potential model proposed in this study is useful for simulating and manipulating the optical and electronic properties of symmetric and asymmetric double quantum heterostructures, such as double quantum wells and double quantum dots, with controllable coupling and subjected to externally applied magnetic fields.

Effect of electron–electron interaction on magnitude of quantum oscillations of dissipative resistance in magnetic fields

Magneto-intersubband resistance oscillations (MISOs) of highly mobile 2D electrons in symmetric GaAs quantum wells with two populated subbands are studied in magnetic fields B=(B⊥,B∥) tilted from the normal to the 2D electron layer at different temperatures T. The in-plane component (B∥) of the field B induces magnetic entanglement between subbands, leading to beating in oscillating density of states (DOS) and to MISO suppression. Model of the MISO suppression is proposed. Within the model, a comparison of MISO amplitude in the entangled and disentangled (B∥=0) 2D systems yields both difference frequency of DOS oscillations, df, and strength of the electron–electron interaction, described by parameter εF∗, in the 2D system. These properties are analyzed using two methods, yielding consistent but not identical results for both df and εF∗. The analysis reveals an additional angular dependent factor of MISO suppression. The factor is related to spin splitting of quantum levels in magnetic fields.

Comparison of the nonlinear optical properties of asymmetrical and symmetrical quantum wells

Comparison of the nonlinear optical properties of asymmetrical and symmetrical quantum wells

Detailed analysis of the intersubband absorption in a GaAs stepped quantum well for quadratic detection and second harmonic generation

We focus on intersubband transitions tunability in GaAs stepped quantum wells through the step's width variation. A double resonance can then occur due to two-photon absorption with possible promising nonlinear optics applications in absorption or emission. In this work, we identified five structures where the equidistant two-photon absorption (ETPA) is possible. We have evaluated the potential of each structure for quadratic detection (QD) and/or second harmonic generation (SHG). This was accomplished quantitatively through the analysis of the calculations results of the main terms of the nonlinear factors involved in both processes. These calculations are based on those of the optical physical parameters linked to the intersubband absorption, namely the individual absorption coefficients and the corresponding dipolar momentum couplings. To highlight the enhancing effect of the asymmetry degree on the total absorption, an investigation of the optical absorption properties of symmetrical quantum wells was done in parallel.

Photoluminescence of Double Quantum Wells: Asymmetry and Excitation Laser Wavelength Effects

Circularly polarized photoluminescence (PL) spectroscopy measured at 19 K on GaAs/AlGaAs symmetric and asymmetric double quantum wells (DQW) is reported. The PL is obtained by exciting the sample with a circularly polarized (left or right) laser in order to create an initial unbalanced distribution of electron spins in the conduction band and, in this way, obtain the electron spin lifetime . The effects of the excitation laser wavelength are estimated by exciting with laser wavelengths of 701.0, 787.0, 801.5, and 806.5 nm. The increase of with the excitation wavelength is attributed to the lower initial quasimomentum k of the excited carriers, which also reduces spin–orbit relaxation processes. is found to be higher in asymmetric DQWs: this is attributed to the wider QWs in these samples, which reduces spin relaxation due to the Dresselhaus mechanism. In addition, a smaller contribution from the Rashba mechanism is also detected by comparing samples with built‐in electric fields of different orientations defined by doped barrier layers.

Non-equilibrium spin polarization in magnetic two-dimensional electron gas with k-linear and k-cubed Dresselhaus spin–orbit interaction

The current-induced spin polarization (CISP) of charge carriers is one of the main mechanisms of spin-to-charge interconversion effects that can be used in new spintronics devices. Here, CISP is studied theoretically in symmetric quantum wells growing in [001] crystallographic direction, where both k -linear and k -cubed Dresselhaus spin–orbit interactions are present. The exchange interaction responsible for perpendicular to plane net magnetization is also taken into account. The main focus is on the influence of cubic Dresselhaus term on CISP and the interplay between spin–orbit interaction (SOI) and the exchange field. The analytical and numerical results are derived within the linear response theory and Matsubara Green’s function formalism. Apart from detailed numerical results, we also provide some analytical expressions that may be useful for interpretation of experimental results and for characterization of quantum wells with Dresselhaus SOI. Analytical expressions for the relevant Berry curvature are also derived, and it is shown that the Berry curvature in magnetic 2DEG with cubic Dresselhaus interaction oscillates in the k -space, while its averaged value is reduced. We also analyze the temperature behavior of CISP and calculate the low-temperature spin polarizability due to heat current. • The effects of the interplay of Dresselhaus spin–orbit interaction (SOI) (with both k -linear and k -cubed terms) and exchange interaction on the nonequilibrium current-induced spin polarization, have been analyzed. • The k -cubed term in Dresselhaus SOI leads to a decrease in nonequilibrium spin density with increasing chemical potential. • The cubic Dresselhaus SOI in magnetic 2DEG creates Berry curvature oscillations in the k -space and reduces the Berry curvature with increasing energy. • The out of plane magnetization leads to an additional component of spin polarization and also to anomalous Hall conductivity that is robust against impurities. • Thermally-induced spin-polarization is analyzed and is shown to be remarkable in a well-defined range of chemical potential.

Temperature damping of magneto-intersubband resistance oscillations in magnetically entangled subbands

Magneto-intersubband resistance oscillations (MISO) of highly mobile 2D electrons in symmetric GaAs quantum wells with two populated subbands are studied in magnetic fields tilted from the normal to the 2D electron layer at different temperatures $T$. Decrease of MISO amplitude with temperature increase is observed. At moderate tilts the temperature decrease of MISO amplitude is consistent with decrease of Dingle factor due to reduction of quantum electron lifetime at high temperatures. At large tilts new regime of strong MISO suppression with the temperature is observed. Proposed model relates this suppression to magnetic entanglement between subbands, leading to beating in oscillating density of states. The model yields corresponding temperature damping factor: $A_{MISO}(T)=X/\sinh(X)$, where $X=2\pi^2kT\delta f$ and $\delta f$ is difference frequency of oscillations of density of states in two subbands. This factor is in agreement with experiment. Fermi liquid enhancement of MISO amplitude is observed.

Donor impurity atom effect on the inter-subband absorption coefficient for symmetric double n-type δ-doped GaAs quantum well

In this work, the electronic structure and the inter-subband absorption coefficient (IAC) are theoretically studied for symmetric double n-type δ-doped GaAs quantum well considering the donor impurity atom effect. The electron states are determined by a diagonalization procedure, working within the effective mass and parabolic band approximations, and the effect of donor center is treated via the variational method. Meanwhile, linear and nonlinear contributions for the inter-subband absorption coefficient were evaluated from expressions usually derived within the perturbative solution of the Von Neumann equation for density matrix. We report the impurity binding energy by considering a donor atom located at the center of the system (at zi = 0). We found that the reported physical properties become more sensitive to the inter-well separation distance Lw than to the δ-doping density, N2d. In the former case the total optical absorption coefficient undergoes an important red-shift as well as a significant decrease in its magnitude. When N2d values increase, the binding energy exhibits a contrary effect, and the total optical absorption coefficient exhibits an small blue-shift with no significant changes in its magnitude. The presence of the donor impurity atom in the system represents one factor that can modify the location of inter-subband absorption coefficient, by inducing a blue-shift of the optical response. The resonant peak energies are within the range of several terahertz, showing potential device applications within this range of the electromagnetic spectrum.

Electron density effect on spin-orbit interaction in [001] GaAs quantum wells

The spin-orbit interaction of two-dimensional (2D) electrons in semiconductor quantum wells gives rise to a variety of interesting transport and optical spin-dependent effects. In the GaAs/AlGaAs type heterosystems, this interaction consists of the isotropic Bychkov-Rashba term, which is absent in symmetric wells, and the anisotropic Dresselhaus term, reflecting the lattice symmetry. It is well-known that the first term can be controlled by electric fields in the growth direction: external or internal, induced by a charge density of 2D electrons. In this work we reveal that the 2D electron charge can substantially affect also the Dresselhaus interaction in symmetric quantum wells. Within the one-band electron Hamiltonian containing, together with the bulk Dresselhaus interaction, the two contributions to the Dresselhaus term from the quantum well interfaces, we show that the internal electric field from the 2D electron charge density can substantially renormalize the anisotropic spin-orbit interaction of 2D electrons. This effect may be important in quantitative studies of spin-dependent phenomena in quantum wells.

Spin drift-diffusion for two-subband quantum wells

Controlling the spin dynamics and spin lifetimes is one of the main challenges in spintronics. To this end, the study of the spin diffusion in two-dimensional electron gases (2DEGs) shows that when the Rashba and Dresselhaus spin-orbit couplings (SOC) are balanced, a persistent spin helix regime arises. There, a striped spin pattern shows a long lifetime, limited only by the cubic Dresselhaus SOC, and its dynamics can be controlled by in-plane drift fields. Here, we derive a spin diffusion equation for non-degenerate two-subbands 2DEGs. We show that the intersubband scattering rate, which is defined by the overlap of the subband densities, enters as a new nob to control the spin dynamics, and can be controlled by electric fields, being maximum for symmetric quantum wells. We find that for large intersubband couplings the dynamics follow an effective diffusion matrix given by approximately half of the subband-averaged matrices. This extra 1/2 factor arises from Matthiessen's rule summing over the intra- and intersubband scattering rates, and leads to a reduced diffusion constant and larger spin lifetimes. We illustrate our findings with numerical solutions of the diffusion equation with parameters extracted from realistic Schr\"odinger-Poisson calculations.

Non-polar ZnCdO/ZnO step-barrier quantum wells designed for THz emission

Symmetric and asymmetric quantum wells of ZnCdO structures grown on ZnO in non-polar orientations are investigated for their potential in THz emission due to the intersubband transitions between the subbands in the conduction band. The Schrödinger and Poisson equations are solved self-consistently for the square and step-barrier quantum wells of the Zn1−xCdxO layers for x up to 0.25. The results are represented as functions of the Cd concentration in the well and step-barrier layers and their dimensions. The role of the many-body effects on the intersubband transitions is discussed. It is found that emission wavelengths between 4.5 and 8.55 THz in the step-barrier quantum wells are possible in the case of an optical pumping at a wavelength in the range of 8.27–15.7 μm. The depolarization and excitonic shifts are found to contribute to the bare intersubband transition energies as much as their 15% at maximum.

High Output Power GaN-Based Green Resonant-Cavity Light-Emitting Diodes With Trapezoidal Quantum Wells

Green resonant-cavity light-emitting diode (RCLED) has great potential in optical communication but suffers low efficiency. In this study, GaN-based flip-chip green RCLED incorporated with nitrogen face-oriented inclination asymmetric trapezoidal quantum wells (NOAT-QWs) is proposed to enhance the light-output power (LOP). Samples with NOAT-QWs and normal symmetric square quantum wells (SS-QWs) were fabricated and characterized. Although their electrical characteristics and emission spectra are similar, the LOP and emission efficiency are significantly improved. At a driving current of 450 mA, the improvement of LOP for RCLED with NOAT-QWs can be as high as 1.44 times in comparison to the sample with SS-QWs. The unprecedented high output power of 115 mW and the narrow full-width-at-half-maximum (FWHM) of 6.4 nm in the emission spectrum give great potential in optical communication. The inherent mechanism was investigated by the finite element analysis, from which the simulation results match well with the experimental measurements. The simulation results reveal that the NOAT-QWs are beneficial in alleviating the quantum confined stark effect and facilitating easier hole carriers’ flow across the barriers, leading to more electron–hole wave function overlaps and higher radiative recombination rate.

Effect of optical phonons scattering on electron mobility in asymmetric AlGaN/GaN quantum wells

Abstract Based on the dielectric continuum model, uniaxial model and force balance equation, the electron mobility in asymmetric wurtzite AlxGa1-xN/GaN/AlyGa1-yN quantum wells (QWs) is investigated theoretically in consideration of optical phonon scattering and varieties of Al compositions x and y. The dispersion relations and potentials of interface, confssined, half space and propagation optical phonon modes are given by the transfer matrix method including the effect of two-mode transverse optical phonons. Contrary to symmetric QWs, one branch of interface optical phonons vanishes and transforms into half space or confined optical one in certain intervals of Al component, whereas the confined optical phonons exist in a wider Al component range. The electron mobility in asymmetric AlGaN/GaN QWs can be modulated to be higher than that in symmetric ones. It is found that the total electron mobility decreases at first, then induces steps and finally decreases as x increases for certain y. The electron mobility also shows strong temperature-dependence. A significant modification on electron mobility appears for a narrower GaN well layer. Our results indicate that the asymmetric AlGaN/GaN QWs can be modulated to exhibit better transport properties than symmetric ones.

Performance Enhancement of GeSn Transistor Laser with Symmetric and Asymmetric Multiple Quantum Well in the Base

The performance of a Multiple Quantum Well (MQW) Heterojunction Bipolar Transistor Laser (HBTL) has been studied using GeSn alloy. Both symmetric and asymmetric quantum wells have been considered. Main analysis is focused on finding the minority carrier concentration in the base, the base threshold current, light output power of the device and the values are compared with GeSn based Single Quantum Well and InGaAs based Multiple Quantum Well Transistor Laser.