Asymmetric Quantum Wells Research Articles

Finite-difference method applied for eight-band kp model for Hg1−xCdxTe/HgTe quantum well

In this work the finite-difference method (FDM) is presented for the undoped Hg[Formula: see text]CdxTe/HgTe quantum well (QW) which allows to calculate the band structures based on Kane’s 8 × 8 kp model including all second-order terms representing the remote-band contributions. In particular the common central-difference form is employed in the discretization procedure. The FDM is applied in the envelope function approach (EFA) for the [001]-oriented system for two cases: without a magnetic field ([Formula: see text] = 0) and with a magnetic field perpendicular to the layer ([Formula: see text]). A proposed presented method can solve all the states simultaneously and can be used for a wide range of temperatures and widths of QW for different values of [Formula: see text] for Hg[Formula: see text]CdxTe/HgTe QW as well as for more complex structures, e.g., asymmetric QW, double quantum wells (DQWs), multiple quantum wells (MQWs) and for the so-called 3D topological insulator with a strained HgTe layer. The results obtained by this method are in a complete agreement with the previous ones. Based on that it is shown that the different [Formula: see text]–Cd compounds in the barrier as well as in the QW make the critical width different than 6.4 nm for HgTe QW. What is also very interesting from the application point of view of the strained mixed HgCdTe QW is that for a different width and a different mismatch lattice it is possible to observe the influence of the upper and lower parts of the Dirac cone in, e.g., a magneto-transport experiment.

Charge and energy transfer in double asymmetric quantum wells with quantum dots

The luminescence and luminescence excitation spectra of CdSe/ZnSe quantum dots are studied in a set of double quantum wells with the ZnSe barrier of width 14 nm, the same amount of a deposited CdSe layer forming a deep well and shallow wells with different depths. It is found that for a certain relation between the depths of shallow and deep wells in this set, conditions are realized under which the exciton channel in the luminescence excitation spectrum of a shallow well dominates in the region of kinetic exciton energies exceeding 10 longitudinal optical phonons above the bottom of the exciton band of the ZnSe barrier. A model is developed for the transfer of electrons, holes, and excitons between the electronic states of shallow and deep quantum wells separated by wide enough barriers. It is shown that the most probable process of electronic energy transfer between the states of shallow and deep quantum wells is indirect tunneling with the simultaneous excitation of a longitudinal optical phonon in the lattice. Because the probability of this process for single charge carriers considerably exceeds the exciton tunneling probability, a system of double quantum wells can be prepared in which, in the case of weak enough excitation, the states of quantum dots in shallow quantum wells will be mainly populated by excitons, which explains experimental results obtained.

Comparison of asymmetric double parabolic-inversed parabolic quantum wells for linear optical (1–2) transition

In present study, for asymmetric double parabolic quantum well (ADPQW) and asymmetric double inverse parabolic quantum well (ADIPQW) the linear optical absorption (OA) coefficient, the linear refractive index change (RIC), resonant peaks of the linear OA and the optical rectification (OR) coefficients are examined as dependent on the structural parameters (barrier width (b) and Al concentration at the well center (σ)). The results display that the variation of all OA and OR coefficients and RIC depend on b and σ-parameter. I have also shown that the σ-parameter has a significant effect on the electronic and optical properties of ADPQW and ADIPQW, and that the energy levels and the dipole moment matrix elements vary depending on the shape of the limiting potential. The intersubband absorption spectrum in the ADPQW shows the blue shift by increasing the b and σ values. Whereas the absorption spectrum for ADIPQW shows blue shift for σ=1 with increasing b-values, this spectrum shows red shifts for σ=5. The electro-optic properties of these structures promise many potential applications in high speed spatial light modulators and switches. So, it can be adjusted RIC and the resonant peak size of the linear OA and OR coefficient by changing Al density at the well center in addition to the barrier dimensions.

The effects of the intense laser field on the optical properties of the asymmetric parabolic quantum well

We have calculated the effects of the intense laser field on the total optical absorption coefficient (the linear and third-order nonlinear) for transition between two lower-lying electronic levels in the asymmetric parabolic \({\text{GaAs/ Ga}}_{{ 1 {\text{ - x}}}} {\text{Al}}_{\text{x}} {\text{As}}\) quantum well. Total absorption coefficient (linear and nonlinear absorption coefficient) for the transitions between any two electronic states was calculated by using density matrix formalism and the perturbation expansion method. Our results show that the effects of intense laser field and the well dimensions on the optical transitions are more pronounced. If well center is changed to be \({\text{L}}_{\text{c}} 0)\), effective well width decreases (increases) and thus we can obtain the red or blue shift in the peak position of the absorption coefficient by changing the intensities of the non-resonant intense laser field as well as dimensions of the well.

Influence of band nonparabolicity on intersub-band optical absorption in GaAs asymmetric quantum wells by difference frequency generation of two midinfrared waves

The linear and nonlinear intersub-band absorption in GaAs asymmetric quantum wells (QWs) by difference frequency generation of two midinfrared waves are theoretically calculated in parabolic and nonparabolic QWs. The calculated results show that terahertz (THz) emission peak shifts in wavelength and THz emission coefficient is substantially reduced after considering band nonparabolicity. For the absorption coefficient of one of the two pump beams, it moves to lower energies by increasing its own wavelength and has little changes with the increase of the other wavelength. The leading causes of these phenomena are the changes of energy levels, the corresponding wave functions, and the transition matrix elements, which are caused by band nonparabolicity. Thus, it is necessary to make an accurate comparison of experimental intersub-band absorption with theoretical predictions.

Nonlinear Intersubband Optical Absorption Coefficient and Refractive Index Changes in Asymmetric Parabolic Double Quantum Wells

The change in optical absorption coefficients and refractive index in GaAs-AlGaAs asymmetric parabolic double quantum wells (DQWs) with applied electric field are studied in detail. Analytical expressions for the linear, nonlinear and total intersubband absorption coefficient and refractive index changes are obtained by using compact density matrix approach. It is found that the magnitude of the nonlinear part of the change in refractive index and absorption coefficients are larger than the linear part in the given frequency region. The value of the total change in refractive index is negative and the asymmetric DQWs becomes left handed media. This property is of great importance for metamaterials science and can contribute substantially to the present search for simple and inexpensive left handed media. Moreover the negative value of the total change in absorption coefficient may be used for developing optical device such as maser at different frequency regime.

Performance improvement of AlGaN-based deep-ultraviolet light-emitting diodes via asymmetric step-like AlGaN quantum wells

Characteristics of AlGaN-based deep-ultraviolet light-emitting diodes (DUV-LEDs) with light-emitting wavelength around 265 nm via step-like AlGaN quantum wells (QWs) have been investigated. Simulation approach yields a result that, there is significant enhancement of light output power (LOP) for DUV-LEDs with two-layer step-like AlGaN QWs compared to that with conventional one. The location and thickness of AlGaN layer with higher Al-content in the step-like QWs are confirmed to significantly affect the distributions and overlap of electron and hole wavefunctions. The best material characteristic is obtained when the step-like QW is designed as an asymmetric structure, such as Al0.74Ga0.26N (1.8 nm)/Al0.64Ga0.36N (1.2 nm), where AlGaN with higher Al-content layer is set to be located nearer from n-side and be thick as far as possible. The key factors for the performance improvements for this specific design is the enhanced hole transport and mitigated auger recombination.

Effect of doping symmetry on electron spin relaxation dynamics in (110) GaAs/AlGaAs quantum wells

Considerable interest has been aroused in the study of the spin dynamics in semiconductors due to its potential applications in spintronics and quantum computation. In this paper, time-resolved circularly polarized pump-probe spectroscopy is used to study the carrier density dependences on the electron spin relaxation in approximately symmetrical and completely asymmetrical doping (110) GaAs/AlGaAs quantum wells. With the increase of the carrier density, the spin relaxation time first increases and then decrease obviously in both of the quantum wells, and the measured spin relaxation time of the approximately symmetrical doping quantum wells is always longer than that of the asymmetrical doping one. By analysis, we find that the spin relaxation is not dominated only by the Bir-Aronov-Pikus (BAP) mechanism in (110) GaAs quantum wells, that though the Dresselhaus spin-orbit coupling does not lead to any spin relaxation, the asymmetry of the doping position contributes to the asymmetry of potential energy structure, thus the built-in electric field which can induce the Rashba spin-orbit coupling to appear, and that the effective magnetic field induced by the Rashba spin-orbit coupling normal to the growth direction can lead to spin relaxation along the growth direction. Therefore, the Dyakonov-Perel (DP) mechanism plays an important role in asymmetrical doping (110) GaAs/AlGaAs quantum wells. In the approximately symmetrical and completely asymmetrical doping (110) GaAs/AlGaAs quantum wells, the DP mechanism dominates the spin relaxation at low carrier density, thus the spin relaxation time increases with carrier density increasing due to the strengthening of the electron-electron scattering and the decreasing of the momentum relaxation time. However, at high carrier density, BAP mechanism plays an important role, thus the spin relaxation time decreases obviously with carrier density increrasing, but the decay rates in both of the quantum wells are slower than that in the casethat only BAP mechanism dominates, because both the DP and BAP mechanism play an important role. The strength of the Rashba spin-orbit coupling depends on the symmetry of the quantum well. The DP mechanism in a completely asymmetrical doping quantum well is stronger than that in an approximately symmetrical doping quantum wells, thus the decay rate in a completely asymmetrical doping quantum wells is always slower than that in an approximately symmetrical doping quantum wells, and the spin relaxation time in a completely asymmetrical doping quantum wells is shorter than that in an approximately symmetrical doping quantum wells.

Quantum well of a new type based on gapless graphene with different fermi velocities

The energy spectrum of a new-type quantumwell composed of gapless graphenes with identical work functions and different Fermi velocities is investigated. Symmetric and asymmetric quantum wells are considered. In a symmetric well, there is always at least one bound state. In an asymmetric well, a bound state appears, beginning at a certain finite momentum. A possibility of appearance of boundary states is investigated.

Interband optical absorption in wurtzite MgxZn1−xO/ZnO/MgyZn1−yO asymmetric quantum wells

Based on Fermi golden rule, the optical absorption induced by interband transition of electrons and holes in wurtzite MgxZn1−xO/ZnO/MgyZn1−yO asymmetric quantum wells at room temperature has been discussed. The built-in electric field (BEF) and Poisson potential are considered to calculate the eigenstates and eigenenergies of electrons and holes. The interband optical absorption coefficients (IOACs) influenced by ternary mixed crystal and size effects as functions of incident photon wavelengths are presented. The results indicate that increasing Mg component in left barrier can enhance the BEF to enforce electrons (holes) close to the left (right) interface, so as to reduce the overlapping of their wave functions. Thus the IOAC peak decreases rapidly and presents a blue shift with the increment of Mg component x. Furthermore, the size effect on IOACs is also discussed. The absorption peak is more sensitive to the change of the well width than the left barrier size. The absorption peak reduces sharply and shows a red shift with the increase of the well width. Our results could provide guidance on experiments and device fabrication.

The effects of the electric and intense laser field on the binding energies of donor impurity states (1s and 2p±) and optical absorption between the related states in an asymmetric parabolic quantum well

We have calculated the effects of electric and intense laser fields on the binding energies of the ground and some excited states of conduction electrons coupled to shallow donor impurities as well as the total optical absorption coefficient for transitions between 1s and 2p± electron-impurity states in a asymmetric parabolic GaAs/Ga1−x AlxAs quantum well. The binding energies were obtained using the effective-mass approximation within a variational scheme. Total absorption coefficient (linear and nonlinear absorption coefficient) for the transitions between any two impurity states were calculated from first- and third-order dielectric susceptibilities derived within a perturbation expansion for the density matrix formalism. Our results show that the effects of the electric field, intense laser field, and the impurity location on the binding energy of 1s-impurity state are more pronounced compared with other impurity states. If the well center is changed to be Lc<0 (Lc>0), the effective well width decreases (increases), and thus we can obtain the red or blue shift in the resonant peak position of the absorption coefficient by changing the intensities of the electric and non-resonant intense laser field as well as dimensions of the well and impurity positions.

Optical investigations of ZnO/ZnMgO quantum wells in self-assembled ZnMgO nanocolumns grown on Si (111) by MBE

Epitaxial growth of semiconductor nanocolumns has attracted large interest in recent years as means to fabricate nano-sized devices. Nanostructures grown on Si (111) substrates are very useful for applications in optoelectronics and integrated optoelectronic circuits. We present the catalyst-free ZnMgO nanocolumns growth under high temperature conditions with asymmetric ZnO double quantum wells separated by ZnMgO barriers of different thickness (3nm and 100nm) deposited on Si (111) substrate using a molecular beam epitaxy method. The high quality quantum structures based on ZnMgO nanocolumns were characterized using scanning electron microscopy and optical spectroscopy (photo- and cathodoluminescence). Their luminescent characteristics were investigated using different techniques. Study of excitons in ZnMgO/ZnO/ZnMgO quantum wells is thus important to understand the optical properties of these semiconductor structures.

Mid/far-infrared photo-detectors based on graphene asymmetric quantum wells

We conducted a theoretical study on the electronic properties of a single-layer graphene asymmetric quantum well. Quantification of energy levels is limited by electron–hole conversion at the barrier interfaces and free-electron continuum. Electron–hole conversion at the barrier interfaces can be controlled by introducing an asymmetry between barriers and taking into account the effect of the interactions of the graphene sheet with the substrate. The interaction with the substrate induces an effective mass to carriers, allowing observation of Fabry–Pérot resonances under normal incidence and extinction of Klein tunneling. The asymmetry, between barriers creates a transmission gap between confined states and free-electron continuum, allowing the large graphene asymmetric quantum well to be exploited as a photo-detector operating at mid- and far-infrared frequency regimes.

Excitons in asymmetric quantum wells

Resonance dielectric response of excitons is studied for the high-quality InGaAs/GaAs heterostructures with wide asymmetric quantum wells (QWs). To highlight effects of the QW asymmetry, we have grown and studied several heterostructures with nominally square QWs as well as with triangle-like QWs. Several quantum confined exciton states are experimentally observed as narrow exciton resonances. A standard approach for the phenomenological analysis of the profiles is generalized by introducing different phase shifts for the light waves reflected from the QWs at different exciton resonances. Good agreement of the phenomenological fit to the experimentally observed exciton spectra for high-quality structures allowed us to reliably obtain parameters of the exciton resonances: the exciton transition energies, the radiative broadenings, and the phase shifts. A direct numerical solution of the Schrödinger equation for the heavy-hole excitons in asymmetric QWs is used for microscopic modeling of the exciton resonances. Remarkable agreement with the experiment is achieved when the effect of indium segregation is taken into account. The segregation results in a modification of the potential profile, in particular, in an asymmetry of the nominally square QWs.

Ground state normalized binding energy of impurity in asymmetric quantum wells under hydrostatic pressure

We have studied and computed variationally the impurity energy, impurity energy turning points, and ground state normalized binding energy as functions of the impurity position for shallow impurity in asymmetric quantum wells under hydrostatic pressure. We found that the normalized binding energy significantly depends on the asymmetry of the well, besides depending on the impurity position and hydrostatic pressure. Also, the dependence of the positive normalized binding energy on the pressure can be used to find out the degree of the asymmetry of the well or the impurity position in the well.

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.

Nonlinear intersubband transitions in asymmetric double quantum wells as dependent on intense laser field

In this study, for asymmetric double quantum well (ADQW) the linear and nonlinear intersubband optical absorption coefficients and the refractive index changes are examined as dependent on the intense laser field (ILF). The obtained results display that the location and the size of all absorption coefficients and refractive index change depend on ILF and the asymmetric parameter. Also, I showed that ILF provides an important effect on the electronic and optical properties of ADQW, and the changes of the energy levels and the dipole moment matrix elements are dependent on the shape of the confinement potential. Such effects can supply methods to drive tunable semiconductor lasers, which may be tailored by quantum potential well parameters and ILF values.

Enhanced refractive index without absorption in four-level asymmetrical double semiconductor quantum well

We investigate the absorptive-dispersive properties of a weak probe field in a four-level asymmetrical double semiconductor quantum well. It is found that the enhanced refraction index without absorption can be easily controlled via adjusting properly the corresponding parameters of the system. Our scheme may provide some new possibilities for technological applications in dispersion compensation and solid-state quantum communication for quantum information processing.

The nonlinear optical rectification and second harmonic generation in asymmetrical Gaussian potential quantum well: Effects of hydrostatic pressure, temperature and magnetic field

In the present work, the effects of hydrostatic pressure, temperature, and magnetic field on the nonlinear optical rectification (OR) and second-harmonic generation (SHG) in asymmetrical Gaussian potential quantum well (QW) have been investigated theoretically. Here, the expressions for the optical properties are calculated by the compact-density-matrix approach and iterative method. Simultaneously, the energy eigenvalues and their corresponding eigenfunctions have been obtained by using the finite difference method. The energy eigenvalues and the shape of the confined potential are modulated by the hydrostatic pressure, temperature, and magnetic field. So the results of a number of numerical experiments indicate that the nonlinear OR and SHG strongly depends on the hydrostatic pressure, temperature, and magnetic field. This gives a new degree of freedom in various device applications based on the intersubband transitions of electrons.

不对称双量子阱中电子布居转移的相干控制研究

不对称双量子阱中电子布居转移的相干控制研究