非对称方势阱中的激子及其与声子的相互作用

采用类LLP(Lee-Low-Pines)变换和分数维变分法,在讨论有限深非对称方势阱Ga_1－xAl_xAs/ GaAs/Ga_0.7Al_0.3As的分数维基础上,计算了其中激子的基态能量以及声子对其影响,随着势阱宽度增加,激子能量先减小后增大,出现一个最小值.讨论了一侧势垒高度变化对分数维、激子基态能量的影响,并发现声子作用使得激子能量明显增大.另外,非对称方势阱中的激子结合能随阱宽的减小而增     

An alternative approach to exciton binding energy in a GaAs-AlxGa1-x as quantum well

A variational-perturbative method is used to calculate the binding energy of an exciton in quantum well structure of Al_xGa_1-xAs-GaAs-Al_xGa_1-xAs. The fitness of potential well heights and differences of electron or hole effective mass in barrier region are both taken into considerations. The binding energies as a function of GaAs well sizes and as a function of alloy compositions, and a photon energy emitted in the recombination of an exciton, are presented. Validity of the calculation is discussed.     

Phonon and electron-hole plasma effects on binding energies of excitons in wurtzite GaN/In<Subscript>x</Subscript>Ga<Subscript>1−x</Subscript>N quantum wells

The binding energy of an exciton screened by the electron-hole plasma in a wurtzite GaN/In _x Ga_1−x N quantum well (in the case of 0.1 < x < 1 within which the interface phonon modes play a dominant role) is calculated including the exciton-phonon interaction by a variational method combined with a self-consistent procedure. The coupling between the exciton and various longitudinal-like optical phonon modes is considered to demonstrate the polaronic effect which strongly depends on the exciton wave function. All of the built-in electric field, the exciton-phonon interaction and the electron-hole plasma weaken the Coulomb coupling between an electron and a hole to reduce the binding energy since the former separates the wave functions of the electron and hole in the z direction and the later two enlarge the exciton Bohr radius. The electron-hole plasma not only restrains the built-in electric field, but also reduces the polaronic effect to the binding energy.     

Exciton binding energy in a double quantum well: effect of the barrier shift

We have calculated the exciton binding energy in an Al _xGa_1 − x As  / GaAs double quantum well by a variational envelope function procedure using a simple two-band model. The influence of the shift of the AlAs separating barrier, introducing an asymmetry into the system, on the value of the exciton binding energy has been analysed. It has been observed that this shift induces significant changes of the exciton binding energy—even several meVs in the case of very thin barriers.     

Polaronic effects on two-dimensional excitons

Summary We calculate the binding energy of a two-dimensional exciton for a set of states labelled by the quantum numberm associated to the angular momentum in the direction perpendicular to the surface. The Fr?hlich electron-phonon and hole-phonon interactions are taken into account. The statem=0 is more bound with respect to that obtained by the Wannier exciton theory with the screening given by the static dielectric constant and the reduced mass calculated by the electron and hole polaronic masses. The opposite effect is found for the statem=1,2,... Whenm becomes large, the hydrogenic series is recovered with the screening and reduced mass defined above. Our results are compared with the experimental data concerning exciton luminescence GaAs/Al _x Ga_1−x As, CuCl/CaF₂ and CdTe/Cd_1−x Zn _x Te quantum wells. The authors of this paper have agreed to not receive the proofs for correction.     

Screening of the electron-hole interaction in quantum well structures

Recent optical non-linearities in GaAs/Ga_1?xAl_xAs quantum well structures have been attributed to the screening of the electron-hole interaction in such structures by the free carriers created. We here present the results of a variational calculation of the ground state energy of an electron-hole pair confined to move in two dimensions when screening of the Coulomb interaction between the electron-hole pair by the free carriers present is taken into account by using the screened potential obtained by Stern and Howard for hydrogenic impurities in semiconducting inversion layers. It is found that the binding energy of the 2D exciton decreases less rapidly with the screening parameter than is the case for a 3D exciton and that the 2D exciton remains bound even for large values of the screening parameter. This is in contrast to the case in bulk semiconductors where the electron-hole pair no longer bind into an exciton when the screening length of the free carriers becomes less than the Bohr radius of the exciton.     

Binding energies of wannier excitons in GaAs-Ga1−xAlxAs quantum well structures

Binding energies of Wannier excitons in a quantum well structure consisting of a single slab of GaAs sandwiched between two semi-infinite slabs of Ga_1?xAl_xAs are calculated using a variational approach. Due to reduction in symmetry along the axis of growth of these quantum well structures and the presence of band discontinuities at the interfaces, the degeneracy of the valence band of GaAs is removed leading to two exciton systems, namely, the heavy hole exciton and the light hole exciton. The variations of the binding energies of these two excitons as a function of the size of the GaAs quantum wells for various values of the heights of the potential barrier are calculated and their behavior is discussed.     

Optics of excitons in systems with sharp heteroboundaries. Approximation of a strongly localized exciton wave function

Optical reflection spectra from a Ga_0.7Al_0.3As/GaAs heteroboundary are calculated using the approximation of a strongly localized exciton wave function. The calculation is based on electron Γ₆ and hole Γ₈ kp-Hamiltonians with position-dependent parameters. Fiz. Tverd. Tela (St. Petersburg) 40, 872–874 (May 1998)     

Kinetics of indirect photoluminescence in GaAs/AlxGa1−x As double quantum wells in a random potential with a large amplitude

The kinetics of indirect photoluminescence of GaAs/Al_xGa_1−x As double quantum wells, characterized by a random potential with a large amplitude (the linewidth of the indirect photoluminescence is comparable to the binding energy of an indirect exciton) in magnetic fields B≤12 T at low temperatures T≥1.3 K is investigated. It is found that the indirect-recombination time increases with the magnetic field and decreases with increasing temperature. It is shown that the kinetics of indirect photoluminescence corresponds to single-exciton recombination in the presence of a random potential in the plane of the double quantum wells. The variation of the nonradiative recombination time is discussed in terms of the variation of the transport of indirect excitons to nonradiative recombination centers, and the variation of the radiative recombination time is discussed in terms of the variation of the population of optically active excitonic states and the localization radius of indirect excitons. The photoluminescence kinetics of indirect excitons, which is observed in the studied GaAs/Al_xGa_1−x As double quantum wells for which the random potential has a large amplitude, is qualitatively different from the photoluminescence kinetics of indirect excitons in AlAs/GaAs wells and GaAs/Al_xGa_1−x As double quantum wells with a random potential having a small amplitude. The temporal evolution of the photoluminescence spectra in the direct and indirect regimes is studied. It is shown that the evolution of the photoluminescence spectra corresponds to excitonic recombination in a random potential. Zh. éksp. Teor. Fiz. 115, 1890–1905 (May 1999)     

Donor impurity in vertically-coupled quantum-dots under hydrostatic pressure and applied electric field

In this work we make a predictive study on the binding energy of the ground state for hydrogenic donor impurity in vertically-coupled quantum-dot structure, considering the combined effects of hydrostatic pressure and in growth-direction applied electric field. The approach uses a variational method within the effective mass approximation. The low dimensional structure consists of three cylindrical shaped GaAs quantum-dots, grown in the z-direction and separated by Ga_1-xAl_xAs barriers. In order to include the pressure dependent Γ – X crossover in the barrier material a phenomenological model is followed. The main findings can be summarized as follows: 1) for symmetrical and asymmetrical dimensions of the structures, the binding energy as a function of the impurity position along the growth direction of the heterostructure has a similar behavior to that shown by the non-correlated electron wave function with maxima for the impurity in the well regions and minima for the impurity in the barrier regions, 2) for increasing radius of the system, the binding energy decreases and for R large enough reaches the limit of the binding energy in a coupled quantum well heterostructure, 3) the binding energy increases for higher Aluminum concentration in the barrier regions, 4) depending of the impurity position and of the structural dimensions of the system (well width and barrier thickness) – and because changing the height of the potential barrier makes possible to induce changes in the degree of symmetry of the carrier-wave function –, the electric field and hydrostatic pressure can cause the impurity binding energy increases or decreases, and finally 5) the line-shape of the binding energy curves are mainly given by the line-shape of the Coulomb interaction.     

Influence of Mg content on optical properties of exciton confinement in wurtzite ZnO/MgxZn1−xO coupled quantum dots

Based on the framework of effective-mass approximation and variational approach, optical properties of exciton are investigated theoretically in ZnO/Mg_xZn_1−xO vertically coupled quantum dots (QDs), with considering the three-dimensional confinement of electron and hole pair and the strong built-in electric field effects. The exciton binding energy, the emission wavelength and the oscillator strength as functions of the structural parameters (the dot height, the barrier thickness between the coupled wurtzite ZnO QDs and Mg content x in the barrier layers) is calculated in detail. The results elucidate that Mg content have a significant influence on the exciton state and optical properties of ZnO coupled QDs. When Mg content x increases, the strong built-in electric field increases and leads to the redshift of the effective band gap of the Mg_xZn_1−xO layer. These theoretical results are useful for design and application of some important photoelectronic devices constructed by using ZnO strained QDs.     

Direct and spatially indirect excitons in GaAs/AlGaAs superlattices in strong magnetic fields

Luminescence and luminescence excitation spectra are used to study the energy spectrum and binding energies of direct and spatially indirect excitons in GaAs/AlGaAs superlattices having different electron and hole miniband widths in high magnetic fields perpendicular to the heterolayers. The ground state of the indirect excitons formed by electrons and holes which are spatially distributed among neighboring quantum wells is found to lie between the ground 1s state of the direct excitons and the threshold of the continuum of dissociated exciton states in the minibands. The indirect excitons have a substantial oscillator strength when the binding energy of the exciton exceeds the scale of the width of the resulting miniband. It is shown that a high magnetic field shifts a system of symmetrically bound quantum wells toward weaker bonding. At high exciton concentrations, spatially indirect excitons are converted into direct excitons through exciton-exciton collisions. Fiz. Tverd. Tela (St. Petersburg) 40, 833–836 (May 1998)     

Optical properties of exciton confinement in wurtzite ZnO/MgxZn1−xO coupled quantum dots

Based on the framework of effective-mass approximation and variational approach, optical properties of exciton are investigated theoretically in ZnO/Mg_xZn_1−xO vertically coupled quantum dots (QDs), with considering the three-dimensional confinement of electron and hole pair and the strong built-in electric field effects due to the piezoelectricity and spontaneous polarization. The exciton binding energy, the emission wavelength and the oscillator strength as functions of the different structural parameters (the dot height and the barrier thickness between the coupled wurtzite ZnO QDs) are calculated with the built-in electric field in detail. The results elucidate that structural parameters have a significant influence on the exciton state and optical properties of ZnO coupled QDs. These results show the optical and electronic properties of the quantum dot that can be controlled and also tuned through the nanoparticle size variation.     

Binding energies of excitons in symmetric and asymmetric quantum wells in a magnetic field

The binding energy of the exciton in the symmetric and asymmetric GaAs/Ga_1 − xAl_xAs quantum wells is calculated with the use of a variational approach. Results have been obtained as a function of the potential symmetry, and the size of the quantum well in the presence of an arbitrary magnetic field. The applied magnetic field is taken to be parallel to the axis of growth of the quantum well structure. The role of the asymmetric barriers, magnetic field, and well width in the excitonic binding is discussed as the tunability parameters of the GaAs/Ga_1 − xAl_xAs system.     

Determination of exciton transition energies and oscillator strengths in AlGaAsGaAs multiple quantum well structures in an electric field using photocurrent spectroscopy

Photocurrent spectroscopy of Al_xGa_1−xAsGaAs multiple quantum well structures in an electric field perpendicular to the heterointerface was used to characterize exciton transition energies and oscillator strengths as a function of applied field. Excitons, some of which grow with increasing electric field, were identified for electric fields in the range of 0–4×10⁴V/cm. The electric field dependence of exciton oscillator strengths and transition energies is attributed to a complicated interplay between local variation of zone center electron and hole wave function overlap and strong valence-band mixing. Excellent agreement between experiment and theory, which incorporates valence subband mixing effects, is found.     

Raman spectroscopy of resonance exciton tunneling in GaAs/AlAs superlattices in electric fields

Optical-resonance-Raman scattering by acoustic phonons is used to study the effect of an electric field on the state of excitons in GaAs/AlAs superlattices. When the energy of the exciting photon coincides with the energy of an exciton bound to Wannier-Stark states of a heavy hole and electron with Δn=0,±1, the acoustic Raman scattering is enhanced. Oscillations in the intensity of the Raman spectrum in the electric field are explained by resonance delocalization of the exciton ground state as it interacts with Wannier-Stark states of neighboring quantum wells or with Wannier-Stark states of a higher electron miniband. Fiz. Tverd. Tela (St. Petersburg) 40, 827–829 (May 1998)     

Exciton bound to an ionized donor impurity in GaAs/Ga1−xAlxAs ellipsoidal finite-potential quantum dots under hydrostatic pressure

In the framework of perturbation theory, a variational method is used to study the ground state of a donor bound exciton in a weakly prolate GaAs/Ga_1−xAl_xAs ellipsoidal finite-potential quantum dot under hydrostatic pressure. The analytic expressions for the Hamiltonian of the system have been obtained and the binding energy of the bound exciton is calculated. The results show that the binding energy decreases as the symmetry of the dot shape reduces. The pressure and Al concentration have a considerable influence on the bound exciton. The binding energy increases monotonically as the pressure or Al concentration increases, and the influence of pressure or Al concentration is more pronounced for small quantum dot size.     

Linear coefficients of photoelasticity in multilayer quantum well structures having a sloping bottom in exciton resonance regions

An analytic expression is obtained for the linear coefficients of photoelasticity in multilayer quantum-well structures having a sloping bottom in the region of the fundamental exciton resonance. The coefficients of photoelasticity are calculated for a GaAs/Al_0.28Ga_0.72As superlattice at the long-wavelength edge of the exciton ground-state resonance. It is shown that these coefficients for multilayer quantum-well structures with the sloping bottom which arises in a varizone quantum-well, are larger, and for those with a bottom slope determined by a constant electric field applied to the multilayer quantum-well structure, are lower than the same quantities for a superlattice with a rectangular quantum-well. The magnitude of the linear contribution of the piezoelectric field stimulated by the slope of the bottom of the quantum-well to the photoelasticity coefficient is calculated for piezoelectric superlattices and this is compared with the contribution introduced by the straining potential. Fiz. Tverd. Tela (St. Petersburg) 40, 1740–1744 (September 1998)     

Infrared radiation from hot holes during spatial transport in selectively doped InGaAs/GaAs heterostructures with quantum wells

The infrared radiation from hot holes in In_xGa_1−x As/GaAs heterostructures with strained quantum wells during lateral transport is investigated experimentally. It is found that the infrared radiation intensities are nonmonotonic functions of the electric field. This behavior is due to the escape of hot holes from quantum wells in the GaAs barrier layers. A new mechanism for producing a population inversion in these structures is proposed. Pis’ma Zh. éksp. Teor. Fiz. 64, No. 7, 478–482 (10 October 1996)     

Combined effects of magnetic and electric fields on the interband optical transitions in InAs/InP quantum wire

Combined effects of magnetic and electric fields on the confined exciton in an InAs_1−xP_x/InP (x=0.2) quantum well wire are investigated taking into account the geometrical confinement effect. Variational formulism, within the frame work of effective mass approximation, is applied to obtain the exciton binding energy. The second order harmonic generation and the optical gain are carried out using compact density method. The strain effects are included with the confinement potential in the Hamiltonian. The energy difference of the ground and the first excited state is found in the presence of magnetic and electric fields taking into the consideration of spatial confinement effect. The result shows that the optical properties are more influenced taking into account the effects of geometrical confinement, magnetic field and electric field. It is shown that the telecommunication wavelength can be achieved with the suitable doping barrier material with the wire material and the external perturbations.