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.     

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)     

Optically induced phase transition of excitons in coupled quantum dots

The weak classical light excitations in many semiconductor quantum dots have been chosen as important solid- state quantum systems for processing quantum information and implementing quantum computing. For strong classical light we predict theoretically a novel phase transition as a function of magnitude of this classical light from the deformed to the normal phases in resonance ease, and the essential features of criticality such as the scaling behaviour, critical exponent and universality are also present in this paper.     

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.     

Diamagnetic shift of exciton energy levels in GaAs-Ga1−xAlxAs quantum wells

The results of direct measurements of the diamagnetic shift of axciton levels in narrow quantum wells of a thickness varying between 25 and 150 Å are reported. A perturbation type approach is used to calculate the diamagnetic shift of 1s exciton levels in quantum well structures of Ga_1−xAl_xAs-GaAs-Ga_1−xAl_xAs. The calculations are applicable in the weak field range for which the Coulomb energy dominates over the magnetic one. The experimental results are in satisfactory agreement with the theory throughout the entire well thicknesses range.     

Magnetoreflectance and luminescense study of beryllium acceptors in selectively doped GaAs/AlxGa1−xAs multiple quantum wells

We present a magneto-optical study of p-type GaAs/Al_xGa_1−xAs quantum wells doped with Be acceptors over the central one-third of the GaAs layers. Using magneto-reflectance and magneto-luminescence spectroscopies, we have investigated (a) interband Landau transitions and (b) transitions from the conduction band Landau levels to the Be acceptors. Binding energies of the acceptors were determined and the dependence of the impurity ground state energy on magnetic field has been studied.     

Fractional-dimensional approach for biexcitons in GaAs/AlxGa1−xAs quantum wells

The energy of a biexciton in a GaAs/Al_xGa_1?xAs quantum well structure with finite barriers is investigated by using the geometrical model of two-dimensional biexcitons proposed by Singh et al. [J. Singh, D. Birkedal, V.G. Layssenko, J.M. Hvam, Phys. Rev. B 53 (1996) 15909; I.-K. Oh, J. Singh, Phys. Rev. B 60 (1999) 2528]. A fractional-dimensional approach is used to obtain the binding energy of the biexciton in both square quantum wells and parabolic quantum wells. Theoretical results show that the binding energy of a biexciton in a finite quantum well exhibits a maximum with increasing well width. The ratio of the binding energy of a biexciton to that of an exciton in a quantum well structure is found to be sensitive to the electron-to-hole mass ratio and larger than that in the three-dimensional system. The results agree fairly well with previous experimental results. The results of our approach are also compared with those of earlier theories.     

Photoluminescence and dark current of p-doped InGaAs/AlxGa1−xAs strained multiple quantum wells

We report the temperature dependence of the photoluminescence spectra and current-voltage (I_d-V) characteristics of p-doped In_0.15Ga_0.85As/AlGaAs quantum well infrared photodetectors (QWIPs) with different barrier heights grown by molecular beam epitaxy (MBE). The dark current at low temperatures is found to be about three orders of magnitude lower than that reported for the n- and p-doped QWIPs made of other material systems. The PL spectra show two emission peaks which correspond to an intersubband absorption and are tunable by changing the mole fraction of Al. The low energy emission peak of the In_0.15Ga_0.85As/Al_0.45Ga_0.55As QWIP is found to be much lower in intensity than that of the high one, due possibly to excess Al which may result in defects or imperfection at/or near well-barrier interfaces.     

Formation of excitons from free electron–hole pairs due to acoustic phonon interactions in quantum wells

A study of the process of exciton formation due to acoustic phonon interaction in quantum wells (QWs) is presented. Considering that excitons are formed from photoexcited free electron–hole pairs, we have derived the rate of such formation as a function of density and temperature of charge carriers and wavevectorK_|| of the center-of-mass motion of exciton, and finally applied our theory to GaAs/AlGaAs QWs. We have found that the formation of an exciton due to acoustic phonon emission is more efficient at relatively large values ofK_|| (hot excitons) whereas that due to longitudinal optical (LO) phonon emission is more efficient at relatively small values of K_||.     

Index of refraction of GaAsAlxGa1−xAs superlattices and multiple quantum wells

Recent research of superlattices and multiple quantum wells has generated considerable interest in the optical waveguiding properties of these structures for optoelectronic applications. As a result we present a theoretical study of the index of refraction of superlattices and determine its variation as a function of frequency and the superlattice parameters, i.e., layer width and AlAs composition. Γ-region exciton and valence-band mixing effects are included in the model. It is found that these two effects have an important influence on the value of the index of refraction and that superstructure effects rapidly decrease for energies greater than the superlattice potential barriers. Because of the quasi-two-dimensional character of the Γ-region excitons, our results indicate that the superlattice index of refraction can vary by ∼ 2% at the quantized, bound-exciton, transition energies. Overall, the theoretical results are in good agreement with the experimental data.     

Quantum entanglement transition in vertically coupledtwo single-electron quantum dots with charged impurity

Effects of a charged impurity on the ground state of two vertically coupled identical single-electron quantum dots with and without applied magnetic field are investigated. In the absence of the magnetic field, the investigations of the charged impurity effect on the quantum entanglement (QE) in some low-lying states are carried out. It is found that, both the positive charged impurity (PCI) and the negative charged impurity (NCI)reduce the QE in the low-lying states under oonsideration except that the QE in the ground state is enhanced by the NCI. Additionally, in the domain of B from 0 Tesla to 15 Tesla, the ground state energy E, the ground state angular momentum L and the ground state QE entropy S are worked out. As far as the ground state are concerned, the PCI (NCI) blocks (induces) the angular momentum phase transition and the QE phase transition besides the known fact (i. e., the PCI/NCI decreases/increases the energy) in the magnetic field.     

Type III - Type I transition and strain-effect in Hg1−xCdxTeCdTe and Hg1−xZnxTeCdTe superlattices

In this paper, the results of Hg_1−xZn_xTeCdTe strained layer superlattices grown by MBE are reported, and compared to Hg_1−xCd_xTeCdTe superlattices. Both Type III and Type I Hg_1−xZn_xTeCdTe superlattices with different strain have been grown on CdTe(111)B/GaAs(100) and CdTe(100)/GaAs(100) substrates and characterized by electron, X-ray diffraction, infrared transmission and Hall measurements. The values of hole mobility between 5×10³ up to 2×10⁴cm²v⁻¹s⁻¹ at T = 23K along (111)B growth orientation and up to 4.9×10⁴cm²v⁻¹s⁻¹ at T = 5K along (100) growth orientation are obtained for Type III superlattices whereas in Type I superlattices, the hole mobility is between 200–300cm²v⁻¹s⁻¹. This drastic change in the hole mobility between Type III and Type I superlattices along with the role of the strain are discussed in this paper.     

Alloy-disorder scattering of the interacting electron gas in quantum wells and heterostructures of Al x Ga1 − x As

The question whether alloy disorder is screened or unscreened is of fundamental importance. Therefore, we calculate the mobility of the interacting two-dimensional electron gas as realized in Al _x Ga_{1 ? x} As quantum wells and heterostructures in the presence of alloy-disorder scattering. For the screening we use the randomphase approximation and we include many-body effects due to exchange and correlation. We propose to determine the alloy disorder potential V _AD from mobility measurements. If we use V _AD = 1.04 eV we can explain recent experimental results obtained for quantum wells and heterostructures with ultrahigh mobility. From the anomalous linear temperature dependence of the mobility measured in heterostructures, we conclude that the alloy disorder is screened. More experiments are needed to confirm the screening of the alloy disorder and we propose some measurements.     

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.     

Effect of external uniaxial stress on the electronic properties and symmetry of p-GaAs/AlxGa1−xAs quantum wells

Abstract

The results of experimental and theoretical investigations of the energy spectrum and electronic properties of symmetric p-[001] Al_0.5Ga_0.5As/GaAs/Al_0.5Ga_0.5As heterostruc-tures under uniaxial [110] compression are presented. The stress-induced piezoelectric field breaks the confining potential symmetry in the quantum well and lifts the degeneracy of the hole subbands. A redistribution of holes in the spin subbands of the ground state takes place, which is revealed in the different shifts of the Shubnikov-de Haas oscillation maxima corresponding to the different spin subbands. The [110] uniaxial compression significantly modifies the band structure, which leads to a strong aniso-tropy of the Fermi surface. The electrical resistance becomes strongly anisotropic under applied compression, decreasing in the direction parallel to the compression and increasing in the perpendicular direction.