Ground state of excitons in quantum-dot quantum-well nanoparticles： stochastic variational method

Within the framework of effective mass approximation, the ground state of excitons confined in spherical core-shell quantum-dot quantum-well (QDQW) nanoparticles is solved by using the stochastic variational method, in which the finite band offset and the heavy (light) hole exciton states are considered. The calculated 1s_e－1s_h transition energies for the chosen CdS/HgS/CdS QDQW samples are in good agreement with the experimental measurements. Moreover, some previous theoretical results are improved.     

On the binding energies of excitons in polar quantum well structures in a weak electric field

The binding energies of excitons in quantum well structures subjected to an applied uniform electric field by taking into account the exciton longitudinal optical phonon interaction is calculated. The binding energies and corresponding Stark shifts for Ⅲ-Ⅴ and Ⅱ-Ⅵ compound semiconductor quantum well structures have been numerically computed. The results for GaAs/A1GaAs and ZnCdSe/ZnSe quantum wells are given and discussed. Theoretical results show that the exciton-phonon coupling reduces both the exciton binding energies and the Stark shifts by screening the Coulomb interaction. This effect is observable experimentally and cannot be neglected.     

In—Plane Propagation of Cavity Polaritons in a Quantum Semiconductor Microcavity

Considering that the coupling among the heavy-hole exciton,light-hole exciton and the cavity photon can form bipolaritons in a quantum semiconductor microcavity,we calculate the group velocities of the cavity polaritons at different incident angles using the coupling model of three harmonic oscillators.The result indicates that the group velocities of the low and middle branches of the cavity polaritons have extrema,but the group velocities of the high branch increase with the increasing incident angle.     

Electronic Properties of p-Type δ-Doped GaAs Structure under Electric Field

We theoretically investigate the electronic properties of p-type δ-doped GaAs inserted into a quantum well under the electric field, at T = 0 K. We will investigate the influence of the electric field on the δ-doping concentration for a uniform distribution. The depth of confining potential, the density profile, the Fermi level, the subband energies and the subband populations calculate by solving the Schrodinger and Poisson equations self consistently. It is found that the changes of the electronic properties are quite sensitive to the applied electric field and the doping concentration. As different from single n-type δ-doped structure, we see a replace between the ground light-hole （lh1 ） subband and the first excited heavy-hole （hh2） subband whenever the external electric field reaches a critical value. We find the abrupt changing of the subband energies and the subband populations whenever the applied electric field reaches a certain value. Also, it is found that the heavy-hole subbands contain many more energy states than the light-hole ones, the population of the heavy-hole levels represent approximately 91% of all the carriers.     

Effect of Magnetic Field on a p-Type δ-Doped GaAs Layer

The energy levels of holes in a p-type δ-doped GaAs structure under a magnetic field are theoretically calculated within the framework of the effective mass approximation for a uniform aceeptor distribution. The electronic structure is calculated by solving the Schrodinger and Poisson equations self-consistently. The effect of the magnetic field on the potential profile changes the degree of the confinement and localization, and thus this behavior can be used to study these systems in regions of interest, without the need to grow many different samples. It is found that the heavy-hole subbands contain many more energy states than the light-hole ones; the population of the heavy-hole levels represents approximately 91 % of all the carriers without magnetic field. With increasing magnetic field the total population of the heavy-holes increases and the number of filled states changes.     

Bound polaron in quantum dot quantum well structures

The problem of bound polarons in quantum dot quantum well (QDQW) structures is studied theoretically. The eigenfrequencies of bulk longitudinal optical (LO) and surface optical (SO) modes are derived in the framework of the dielectric continuum approximation. The electron--phonon interaction Hamiltonian for QDQW structures is obtained and the exchange interaction between impurity and LO-phonons is discussed. The binding energy and the trapping energy of the bound polaron in CdS/HgS QDQW structures are calculated. The numerical results reveal that there exist three branches of eigenfrequencies of surface optical vibration in the CdS/HgS QDQW structure. It is also shown that the binding energy and the trapping energy increase as the inner radius of the QDQW structure decreases, with the outer radius fixed, and the trapping energy takes a major part of the binding energy when the inner radius is very small.     

Optical anisotropy and the direction of polarization of exciton emissions in a semiconductor quantum dot:Effect of heavy-and light-hole mixing

The dependence of the directions of polarization of exciton emissions, fine structure splittings(FSS), and polarization anisotropy on the light-and heavy-hole(LH–HH) mixing in semiconductor quantum dots(QDs) is investigated using a mesoscopic model. In general, all QDs have a four-fold exciton ground state. Two exciton states have directions of polarization in the growth-plane, while the other two are along the growth direction of the QD. The LH–HH mixing does affect the FSS and polarization anisotropy of bright exciton states in the growth-plane in the low symmetry QDs(e.g., C_(2V),C_S, C_1), while it has no effect on the FSS and polarization anisotropy in high symmetry QDs(e.g., C_(3V), D_(2d)). When the hole ground state is pure HH or LH, the bright exciton states in the growth-plane are normal to each other. The LH–HH mixing affects the relative intensities and directions of bright exciton states in the growth-plane of the QD. The polarization anisotropy of exciton emissions in the growth-plane of the QD is independent of the phase angle of LH–HH mixing but strongly depends on the magnitude of LH–HH mixing in low symmetry QDs.     

Paths for the Non-radiative Recombination Occurring in CdS：CdO/Si Multi-Interface Nanoheterostructure Array

A CdS：CdO/Si multi-interface nanoheterostructure array （CdS：CdO/Si-NPA） is prepared by a chemical bath deposition method, and three emission bands are observed in the as-grown CdS：CdO film. By measuring its temperature-dependent photoluminescence （PL） spectrum, the variation trends of the peak energies and intensities with temperature for the three bands are obtained. Based on the theoretical analyses and fitting results, the non-radiative recombination processes corresponding to the PL quenching for the three emission bands are attributed to the thermally activated transition between heavy-hole and light-hole levels （at low temperature） and the thermal escape due to the scattering from longitudinal optical phonons （at high temperature）, the transition from acceptor levels to surface states, and the transition related to surface defect states, respectively. The clarification of the non-radiative recombination processes in CdS：CdO/Si-NPA might provide useful information for promoting the performance of optoelectronic devices based on CdS/Si nanoheterostructures.     

Temperature-Dependent Photoluminescence in Coupling Structures of CdSe Quantum Dots and a ZnCdSe Quantum Well

A coupling structure of CdSe quantum dots (QDs) and a ZnCdSe quantum well (QW) is fabricated by using the molecular-beam epitaxy technique. The effect o~ temperature on the photoluminescence (PL) of the structure is studied. The results reveal that the activation energy of exciton dissociation in the coupling QDs/QW structure is much higher than that of simple CdSe QDs, which is attributed to the exciton tunnelling from the QW to QDs through a thin ZnSe barrier layer. The results also reveal that the position and width of the emission band of the QDs vary discontinuously at certain temperatures. This phenomenon is explained by the QD ionization and exciton tunnelling from the QW to the QDs. It is demonstrated that the coupling structure significantly improves the PL intensity of CdSe QDs.     

Surface Effects on Wannier Excitons in Superlattices:Monte Carlo Simulation

Using a Monte Carlo quadrature,we calculate by a variational method the binding energy Eb of Wannier excitons in N-period m-CdMnTe/n-CdTe superlattices,including or not including the step discontinuity in the potential barrier at the interface,and with varying N as well as well width.The calculation is performed in the framework of the effective mass approximation for both heavy-hole and light-hole excitons.The degree of exciton localization dependence on these parameters is accurately illustrated.     

Exciton/plasmon mixing in metal–semiconductor heterostructures

We report on the strong coupling between surface plasmons and inorganic quantum well excitons. The sample is formed by a corrugated silver film deposited on the top of a heterostructure consisting of five GaAs/GaAlAs quantum wells grown by molecular beam epitaxy. Reflectometry experiments at low temperature (77 K) evidence the formation of plasmon/heavy-hole exciton/light-hole exciton mixed states. The interaction energies, deduced by fitting the experimental data with a coupled oscillator model, amount to 22 meV for the plasmon/light-hole exciton and 21 meV for the plasmon/heavy-hole exciton. Some particularities of the plasmon–exciton coupling are also discussed and qualitatively related to the plasmon polarization.     

Dominance of Fermi-surface holes in p-type tunneling

In-plane uniaxial stress is used to tune continuously the mixing between the heavy-hole (HH) and light-hole (LH) states in a p-type double-barrier structure. The LH1 and HH2 resonant tunneling peaks shift at almost the same rate with stress, in contrast to the corresponding exciton peaks observed by photoreflectance, which exhibit a strong Fano-related anticrossing. Comparison between the observed shifts and a four-band k x p calculation of the state energies in the well provides the first experimental proof that the flow of holes through off-zone center states dominates the resonant tunneling current in p-type structures.     

Oscillator strength of type-II light-hole exciton in InxGa1−xAs/GaAs strained single quantum wells

We have investigated the excitonic properties of In_0.15Ga_0.85As/GaAs strained single quantum wells by using photoreflectance spectroscopy and a variational calculation method. We clearly detected the photoreflectance signal of the type-II light-hole exciton, which consists of an electron confined in the InGaAs layer and a light hole located in the thick GaAs layer, in addition to the type-I heavy-hole exciton confined in the InGaAs layer. The calculated results of the overlap integral of the envelope function in the type-II light-hole exciton predict that the oscillator strength is remarkably enhanced with decreasing the InGaAs-layer thickness. This is demonstrated by the layer-thickness dependence of the photoreflectance intensity of the type-II light-hole exciton.     

Excitonic transitions in Be-doped GaAs/AlAs multiple quantum well

A series of GaAs/AlAs multiple-quantum wells doped with Be is grown by molecular beam epitaxy. The photoluminescence spectra are measured at 4, 20, 40, 80, 120, and 200 K, respectively. The recombination transition emission of heavy-hole and light-hole free excitons is clearly observed and the transition energies are measured with different quantum well widths. In addition, a theoretical model of excitonic states in the quantum wells is used, in which the symmetry of the component of the exciton wave function representing the relative motion is allowed to vary between the two- and threedimensional limits. Then, within the effective mass and envelope function approximation, the recombination transition energies of the heavy- and light-hole excitons in GaAs/AlAs multiple-quantum wells are calculated each as a function of quantum well width by the shooting method and variational principle with two variational parameters. The results show that the excitons are neither 2D nor 3D like, but are in between in character and that the theoretical calculation is in good agreement with the experimental results.     

Fractional-dimensional approach for excitons in GaAsfilms on AlxGa1-xAs substrates

Binding energies of excitons in GaAs films on Al_xGa_1-xAs substrates are studied theoretically with the fractional-dimensional approach. In this approach, the real anisotropic “exciton+film” semiconductor system is mapped into an effective fractional-dimensional isotropic space. For different aluminum concentrations and substrate thicknesses, the exciton binding energies are obtained as a function of the film thickness. The numerical results show that, for different aluminum concentrations and substrate thicknesses, the exciton binding energies in GaAs films on Al_xGa_1-xAs substrates all exhibit their maxima with increasing film thickness. It is also shown that the binding energies of heavy-hole and light-hole excitons both have their maxima with increasing film thickness.     

Effect of a Coulomb well in (In,Ga)As/GaAs quantum wells

Magnetooptical investigation of exciton transitions in high-quality quantum wells of an (In, Ga)As/GaAs heterosystem has been carried out. Investigation of transmission of free-hanging samples detached from the substrate in the magnetic fields of up to 12 T revealed a rich fine structure associated with various heavy-hole and light-hole exciton transitions. In particular, transitions from the excited states of light holes localized in a Coulomb potential produced by an electron along the heterojunction axis (a Coulomb well) have been detected. Taking into account consistently stresses, formation of Landau levels, the binding energies of excitons (diamagnetic excitons), and the effect of a Coulomb well, we have succeeded to describe the experimental results with the use of a self-consistent variational procedure. As a result, new features in the structure of optical transitions have been explained and the effective masses of electrons and holes of excitons formed by both heavy and light holes have been determined with a high accuracy.     

Coupled ultrathin InAs layers in GaAs as a tool for the determination of band offsets

We have determined the band offsets at the highly strained InAs/GaAs heterointerface by photoluminescence excitation (PLE) measurements of the symmetric and antisymmetric states in two coupled ultrathin InAs layers embedded in a GaAs matrix. The conduction band offset ΔE_ccould be separated from the valence band offsets, since in a 32 monolayer (ML) barrier sample, the splitting between the heavy-hole exciton transitions is solely determined by ΔE_c. Knowing ΔE_c, the heavy-hole (hh) and light-hole (lh) band offsets ΔE_hhand ΔE_lhcould subsequently be determined from the coupling-induced shift and splitting in samples with a 16, 8 and 4 ML barrier. We find a conduction band offset of 535 meV, a conduction band offset ratio ofQ_c= 0.58 and a strain induced splitting between the hh and lh subbands of 160 meV.     

Modification of surface states in ultrathin films via hybridization with the substrate: a study of Ag on Ge

The Shockley surface state of Ag(111) develops unusual band dispersion relations for Ag films of decreasing thicknesses on Ge(111), as observed by angle-resolved photoemission. Its parabolic dispersion in the thick-film limit shifts toward higher binding energies and splits into multiple bands with dispersions that reflect the valence band structure of Ge including the heavy-hole, light-hole, and split-off bands. The results are explained in terms of a hybridization interaction between the Ag surface state and the Ge substrate states.     

Magnetoexciton in semiconductor concentric double rings

We investigate theoretically the magnetoexciton states in semiconductor concentric quantum double rings using the multi-band effective mass theory. We find that a perpendicular magnetic field can lead to oscillations in the exciton energy which appear as kinks in the magneto-photoluminescence (PL) spectra as the magnetic field increases. The spatial distribution of the exciton over the rings depends sensitively on the thicknesses of the inner and outer rings. The tunneling coupling between the inner and outer rings and the heavy-hole and light-hole mixing results in different anticrossing behaviors. Exciton can be converted into a spatially separated type-II exciton by tuning the thickness, the inner and/or outer ring radius and the magnetic field. We show that this type I–type II transition is reflected in the oscillator strength of the PL spectrum which will be the experimental signature that will provide us with information about the spatial distribution of the exciton.