Photoluminescence spectroscopy on excitonic states of narrow GaAs–AlGaAs single quantum wells in high magnetic field

Magneto-photoluminescence (PL) experiments of very thin GaAs/Al_0.3Ga_0.7As quantum wells were performed in a magnetic field (B) of up to 20 T. It has been observed that the diamagnetic shift changes abruptly from βB²to αBaround 5 T asBincreases, and both α and β become larger as the well-width increases. ThisB-dependent transition is indicative of the change of electron–hole (e–h) coupling as the cyclotron radius becomes comparable with that of the exciton. The change of PL linewidth byBalso supports the concept that thee–hrecombination is sensitive to the dimensionality.     

Excitonic state in quantum wells formed from “above-barrier” electronic states

Lines corresponding to localized excitonic states formed from “above-barrier” electron and/or hole states (specifically, excitation lines of excitons formed by an electron localized in a QW and a free heavy hole) have been observed in the photoluminescence excitation spectra of GaAs/Al_0.05Ga_0.95As structures with quantum wells (QWs), each containing one single-particle size-quantization level for charge carriers of each type. A computational method is proposed that permits finding the binding energy and wave functions of excitons in QWs taking the Coulomb potential into account self-consistently. The computed values of the excitonic transition energies agree quite well with the experimental results. Pis’ma Zh. éksp. Teor. Fiz. 70, No. 9, 613–619 (10 November 1999)     

Self-consistent inhomogeneity of quantum wells in II–VI semiconductors

An X-ray diffraction method that uses a slightly diverging (3′) beam and maximally attainable diffraction angles ? _B (as large as 77°) was developed to study quantum wells (QWs) with widths of 5–8 nm separated by wide (100–220 nm) barrier layers. The advantage of this method compared to the use of a parallel beam is an increase by two orders of magnitude in the intensity of the beam incident on the sample and an increase in the probability of diffraction for all QWs as a unified single crystal. It is found that the growth on GaAs substrates misoriented by 10° from the (001) plane in the [111]_II direction brings about monoclinization of crystal lattices of the QW layers and barrier layers in opposite directions. Inhomogeneity of composition over the thickness of each well is observed. In the case of growth of a ZnSe/ZnMgSSe structure in which the layers have a crystal-lattice period close to the lattice period of the GaAs substrate, the QWs are inhomogeneously doped with elements from the composition of the barrier layers. The inhomogeneity of QW composition observed in the growth of mismatched layers in ZnCdSe/ZnSSe and ZnCdS/ZnSSe structures is caused by the fact that mismatch between the lattice parameters of QWs and barriers stimulates the growth of self-consistent compositions; this occurs due to a decrease in the Cd concentration in the Zn_1?x Cd _x Se QW in the initial stages of growth compared to the Cd concentration in the flow of gases and an increase in the Zn concentration in the Cd_1?x Zn _x S QW at small values of x up to the concentration matching GaAs (x = 0.4). The mismatch stresses are partially relaxed via dislocations with the (111)_II glide planes, as a result of which is observed the combination of rotation of the crystal planes of the layers and QW around the [1\(\overline 1 \)0] axis and almost cylindrical bending of the entire sample around the perpendicular [110] axis. Mismatch between lattice parameters of the ZnMgSSe barrier layers and the substrate brings about decomposition of these layers into two phases; this decomposition is caused by thermodynamic instability of the alloy.     

Temperature dependence of excitonic transitions in AlxGa1 − xAs/GaAs quantum wells

The temperature dependence of excitonic transitions in double quantum well heterostructures in the temperature range of 2–300 K were investigated. A crossing between excitonic transition experimental curves as a function of temperature in quantum wells of the same thickness and different barrier height is observed. The influence of the barrier height on the temperature dependence of excitonic states in the quantum wells is analyzed.     

Subband and excitonic binding of graded GaAs/Ga1–xAlxAs quantum wells under an electric field

The effects of an applied electric field on subband energies and excitonic binding for a graded GaAlAs quantum well are calculated variationally within the effective mass approximation. The ver sensitive dependence of subband energies on the applied field is calculated using a model potential profile and exact electron and hole wavefunctions. Our calculations have revealed the dependence of the energy shifts of subbands, and excitonic binding on the field direction in the graded quantum well. This pemits control over tunnelling which could be desirable for some applications.     

The symmetry of donor–bound electron wavefunctions in quantum wells

In order to investigate the symmetry (i.e. sphericity) of donor–bound electron wavefunctions in quantum wells, we have invoked a two-parameter trial wavefunction. One parameter is the Bohr radius λ, whilst the other is the eccentricity parameter ζ. The latter incorporates the effect of the quantum well (QW) on the carrier motion in the growth (i.e. the z) direction. Working within the envelope function approximation it is shown that the donor wavefunction has the form of a prolate spheroid. However, calculations of the ratio λ/ζ shows that it is the value of λ which determines the essential symmetry of the wavefunction.     

Photoluminescence properties of ZnS/CdS/ZnS quantum dot–quantum wells doped with Ag<Superscript>+</Superscript> ions

Enhanced photoluminescence and postirradiation luminescence is reported from Ag⁺-doping ZnS/CdS/ZnS quantum dot–quantum wells (QDQWs) prepared via a reverse micelle process. Controlling the final mole ratio of water-to-surfactant in H₂O/Heptane system, the size of a QDQW was estimated to be ~6 nm. Compared to undoped QDQWs, the doped QDQWs exhibited a much stronger orange emission, with a peak blue shift from 615 to 590 nm; the quantum yield was increased from 2.63 to 9.31%, and the remaining luminescence intensity after 2 h ultraviolet irradiation was increased from 71.2 to 94.7%. This improved quantum yield and postirradiation luminescence intensity for doped QDQWs was ascribed to the introduction of Ag⁺ ions to CdS wells.     

Investigation of the radiative lifetime in core–shell CdSe/ZnS and CdSe/ZnSe quantum dots

Using the one band effective mass approximation model we computed the optical properties of the spherical shaped CdSe/ZnS and Cdse/ZnSe core–shell quantum dot (CSQD). For each structure we calculated the charge carrier energies and corresponding wave functions. We investigated the dependence of the carrier energies on various parameters of the CSQD, including its size. Then we calculated the radiative recombination lifetime for the two types of CSQDs nanocrystals. We found that as the size of the dot is increased the optical gap of CSQD is reduced, resulting in a reduction in electron energies and an increase in hole energies. We have shown that the radiative recombination lifetime in the CdSe/ZnS and CdSe/ZnSe CSQDs decreased by increasing the shell thickness around the core of the QD. We also showed that the radiative lifetime in the CdSe/ZnS is less than that in the CdSe/ZnSe CSQDs and is sensitive to the size and nature of shell of the semiconductor's material.     

Orthorhombic symmetry of valence-band states in CdTe/Cd1−x MnxTe quantum wells

A study is reported of the anisotropy in magnetic-field-induced linear polarization in (001) CdTe/Cd_1−x Mn_xTe quantum wells. The observed limiting anisotropy is shown to be due to the low C _2v symmetry of the quantum well. The relations obtained for the C _2v point group are in a good agreement with experiment. Considered on the microscopic scale, the effect is associated with the heavy-hole g-factor anisotropy in the well plane. Fiz. Tverd. Tela (St. Petersburg) 41, 903–906 (May 1999)     

Excited states of the negatively charged exciton X−in wide CdTe/CdZnTe quantum wells

A magneto-optical study of ≈350 Å wide CdTe/CdZnTe quantum wells containing an electron gas is presented. For undoped structures the absorption spectra show lines associated with centre-of-mass quantization of the exciton in the wide CdTe well. Modulation-doped structures show absorption lines corresponding to the creation of the negatively charged exciton X⁻(two electrons, one hole). We observe not just the ground state of X⁻butalsoa series of lines which are attributed to higher states of X⁻where an electron is attached to the centre of mass quantized states of the exciton.     

Controlled switching between quantum states in the exciton–polariton condensate

Optically controlled switching between modes of a polariton laser having different symmetries has been demonstrated experimentally. The microscopic shift of the optical excitation spot dramatically changes the shape of the polariton condensate formed in a cylindrical micropillar on the basis of the planar semiconductor microcavity. Switching between the ring and lobed condensate is achieved owing to the violation of the cylindrical symmetry of the effective potential formed by the lateral surface of the pillar and by the cloud of incoherent excitons created by optical pumping.     

Impurity magneto-optical absorption with the participation of resonance states of D 2 − centers in quantum wells

The dependence of the average binding energy of the resonance g-state of a D ₂ ^? center on the induction of an external magnetic field in a quantum well with a parabolic confining potential is studied using the zero-range potential method. It has been shown that with an increasing exchange interaction, the character of the dependence of the average binding energy of the resonance g-state of the D ₂ ^? center on the induction of the external magnetic field changes. It has been assumed that in GaAs/AlGaAs quantum wells alloyed with small Si donors, resonance D ₂ ^? states can exist under conditions of exchange interaction. It has been found that in spectra of impurity magneto-optical absorption in multiwall quantum structures, exchange interaction manifests itself as oscillations of interference origin.     

Temperature dependent dynamic of excitons in Zn1 − xCdxSe/ZnSe multiple quantum wells

The exciton dynamics in Zn_1 − xCd_xSe/ZnSe multiple quantum wells have been investigated by temperature-dependent time-resolved photoluminescence experiments. A polariton effect (leading to a linearT-dependence of the decay time τ_PL) and thermal escape of carriers from the quantum well at relatively high temperature (resulting in a decrease of τ_PLwith the temperature) are observed in samples of different stoichiometry and well.     

Scattering states of Woods–Saxon interaction in minimal length quantum mechanics

We propose a very simple approach to deal with the problems of the modified Schrödinger equation due to minimal length and thereby solve the minimal length Schrödinger equation in the presence of a non-minimal Woods–Saxon interaction. The transmission and reflection coefficients are reported as well.