Optical detection of an electric field in a GaAs/AlGaAs <Emphasis Type="Italic">n</Emphasis>-<Emphasis Type="Italic">i</Emphasis>-<Emphasis Type="Italic">n</Emphasis> heterostructure with double quantum wells

Processes occurring when a static transverse electric field is applied to a GaAs/AlGaAs n-i-n heterostructure with single quantum wells and asymmetric tunnel-coupled double quantum wells have been investigated by optical methods. The difference between the energies of exciton transitions for quantum wells of different widths makes it possible to attribute the observed photoluminescence peaks to particular pairs of wells or particular single quantum wells. The local electric field for each quantum well has been determined in terms of the Stark shift and splitting of exciton lines in a wide range of external voltage. A qualitative model has been proposed to explain the nonmonotonic distribution of the electric field over the depth of the heterostructure.     

Magnetoluminescence of Ge/Ge1−x Six heterostructures

This paper reports on the first investigation made of luminescence of Ge/Ge_1−x Si_x heterostructures at liquid-helium temperatures in a magnetic field of up to 14 T. The luminescence lines observed in the spectra are due to both free and impurity bound excitons in Ge layers. The diamagnetic shift of the quasi-two-dimensional exciton has been measured. From the experimental data the size of the exciton has been estimated to be 75–90 Å. Zh. éksp. Teor. Fiz. 114, 619–627 (August 1998)     

Effect of free electron-hole pairs on the saturation of excitonic absorption in GaAs/AlGaAs quantum wells

The pump-probe experimental method is used to investigate the effect of photoexcited carriers on the dynamics of the exciton absorption spectra of GaAs / Al_xGa_1–x As-multilayer quantum wells. Use of the method of moment analysis for processing the results makes it possible to identify the simultaneous contribution of changes in oscillator strength and width of the exciton lines in the saturation of exciton absorption. It was found that the oscillator strength recovers its initial value in the course of the first 100–130 ps, whereas broadening and energy-shift of the exciton lines is observed for 700–800 ps. These are the first experimental measurements of the excitation densities at which the oscillator strength of the excitonic state saturates when the latter is perturbed only by free-electron-hole pairs, and when it is perturbed only by other excitons. Fiz. Tverd. Tela (St. Petersburg) 40, 1130–1133 (June 1998)     

Extrinsic two dimensional excitons and their high density effects in BiI3 single crystals

Sharp absorption lines near the fundamental edge caused by planar stacking faults in BiI₃ crystals have been investigated on absorption and luminescence spectra. Under high excitation-photon densities these lines show peak-shift and line broadening. The high density effects on the exciton bound at the defects shows a two-dimensional feature of the exciton motion.     

Cancellation of fine-structure splitting in quantum dots by a magnetic field

We demonstrate control of the fine-structure splitting of the exciton emission lines in single InAs quantum dots by the application of an in-plane magnetic field. The composition of the barrier material and the size and symmetry of the quantum dot are found to determine decrease or increase in the linear polarization splitting of the dominant exciton emission lines with increasing magnetic field. This enables the selection of dots for which the splitting can to be tuned to zero, within the resolution of our experiments. General differences in the g-factors and exchange splittings are found for different types of dot.     

Luminescence of CuInS2: II. Exciton and near edge emission

The near-edge (exciton) emission of CuInS₂ is investigated for various material-compositions as a function of temperature. From these investigations the exciton ionization energy (20 meV) and the temperature dependence of the energy gap were determined. For the first time, recombination of the free exciton belonging to the deeper lying Γ₇ valence bands has been observed. Moreover, six different bound exciton emission lines and a donor to valence band transition were detected. These emissions could be assigned in terms of the defect-chemical model presented in Part I.     

Fine structure of the exciton states in InAs quantum dots

The fine structure of the exciton states in InAs quantum dots grown by the Stranski-Krastanov method with short growth interruptions has been studied by microphotoluminescence at cryogenic temperatures. It has been demonstrated that, with increasing quantum-dot size, the splitting of the exciton states increases steadily to ～10² μeV. It has been shown that, in the exciton energy range of 1.3–1.4 eV, the magnitude of this splitting is comparable to the natural width of the exciton lines. This result is important for the development of entangled photon pair emitters based on InAs quantum dots.     

Low-temperature photoluminescence of MBE-grown GaAs subject to an electric field

Low-temperature photoluminescence of GaAs has been investigated in MBE-grown Al _x Ga_1–x As-GaAs single heterojunctions subject to an electric field. No peak energy shift is observed in the emission lines due to free excitons and excitons bound to isolated centers when the electric field is applied. In contrast, the excitonic lines arising from the previously described defect-induced bound exciton (DIBX) transitions exhibit a prominent low-energy shift when the electric field is increased. We attribute these lines to excitons bound to acceptor pairs. The excitons bound to distant pairs have smaller binding energies than those bound to closer pairs. They are, therefore, easily dissociated in a weak electric field. The electrons and holes thus dissociated may again be trapped by closer pairs, which results in a low-energy shift of the overall spectrum. The photocurrent measured as a function of the electric field supports Dingle's rule for the valence bandedge discontinuity.     

Excitons in monoclinic zinc diphosphide. Longitudinal exciton and the mixed mode

Exciton absorption spectra in high-quality β-ZnP₂ single crystals have been investigated at T=1.7 K for various directions of the wave vector and various polarization states of the radiation. It has been unambiguously established that the additional high-energy A series, which in some works has been called a D series and ascribed to ZnP₂ crystals, of so-called “rhombic” symmetry,^1,8,10,11 is an intrinsic exciton of the β-ZnP₂ series. A mixed mode has been detected for the first time, and the energy of the longitudinal exciton has been determined. The selection rules for the exciton transitions have been analyzed by a group-theoretical approach, and the symmetry of the nS states of the single exciton has been established on the basis of the experimental data — Γ ₂ ⁻ (z). Fiz. Tverd. Tela (St. Petersburg) 41, 193–202 (February 1999)     

有机-无机杂化钙钛矿自组装量子阱结构的能带调控和光电性能的研究

有机-无机杂化钙钛矿材料具有分子尺度上调节能带结构的特点,在光、电、磁等领域均表现出了优异的性能.通过简单的旋涂方法,成功的制备了具有不同无机层层数的杂化钙钛矿材料(C₆H₁₃NH₃)₂(CH₃NH₃)_n-1Pb_nI_3n+1 (简写为C₆Pb关键词： 杂化钙钛矿 量子阱 带隙 光电性能     

Exciton states in strongly coupled asymmetric semimagnetic double quantum dots

Exciton states in a pair of strongly coupled artificial asymmetric quantum dots (QDs) have been studied in magnetic fields up to B = 8T by means of photoluminescence spectroscopy. The QD molecules have been fabricated using a selective interdiffusion technique applied to asymmetric CdTe/(Cd,Mg,Mn)Te double quantum wells. The lateral confinement potential within the plane induced by the diffusion gives rise to effective zero-dimensional exciton localization. Incorporation of the Mn ions in only one dot results in a pair of QDs with a markedly different spin splitting. In contrast to a positive value of the exciton Lande g factor in nonmagnetic (Cd,Mg)Te-based single QDs, the ground exciton transition in the nonmagnetic QD demonstrates nearly zero g factor, thus, indicating a strong electron coupling between the dots. A new low-energy band with a strong red shift appears at high B signifying formation of the indirect exciton in accordance with our calculations. The text was submitted by the authors in English.     

Effets de défauts cristallins sur le spectre d’absorption excitonique de Cu2O

A theory of the effects of lattice defects on the yellow series of the exciton absorption spectrum of Cu₂O is presented. In particular, the effects of lattice defects due to fast neutron irradiation is treated. It is shown that the modification of exciton absorption spectrum, the appearing of the exciton fine structure lines in particular, is due to the deformation potential associated with the large defect clusters which are produced by the fast neutron bombardment. An analogy is established between this effect and the Stark exciton effect observed under externally applied electric field. It is shown that the Stark effect of exciton may be considered as a particular example of the problem that we treat. Several models of deformation potentials are proposed to calculate the variation of the oscillator strength of the exciton absorption lines in Cu₂O as a function of flux of fast neutrons. The effects of point defects are also studied. It is shown that the point defects are not at the origin of the observed fine structure of exciton, but they may shift and broaden exciton obsorption lines. These theoretical considerations are compared qualitatively with experimental results.     

在正向电压激发下ZnSe MIS二极管的自由激子发光

研究了最低温度为20~30K时,在正向电压激发下ZnSe MIS二极管的激子发光光谱,在这一温度下,二极管有可能通过足以产生发光的电流.对于利用通常气相技术生长的高纯晶体所制备的二极管,其电致发光几乎完全由Γ8→Γ6自由激子发光的1LO和2LO声子伴线所组成.根据Gross等人的半经典理论,讨论了两个谱带的形状.结果是谱带的宽度和不对称性归结为激子服从Maxwell-Boltzmann分布,其有效激子温度接近于晶格温度.     

High-energy frenkel cation exciton and specific features of its self-trapping in the CdI<Subscript>2</Subscript>-PbI<Subscript>2</Subscript> crystal system

It has been shown using atomic-force microscopy that the PbI₂ impurity is embedded in the CdI₂ crystal lattice in the form of nanocrystalline inclusions. The model of a high-energy cation exciton related to the ³ P ₂ state of a free Pb²⁺ ion has been considered for the impurity absorption (excitation) band at 3.23 eV. The resonance narrow photoluminescence bands with the split absorption band at 3.12 and 3.20 eV have been compared with the emission of a free Frenkel exciton. It has been demonstrated that, in the temperature range 25–45 K, there arises a self-trapped exciton state, and the main role in its formation is played by the bending vibrations of the CdI₂ crystal lattice. The potential barrier separating the self-trapped state from the free exciton is 23 meV. The photoluminescence band at 2.4 eV is assigned to the emission of the self-trapped high-energy cation exciton of PbI₂ in the CdI₂ crystal lattice.     

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.     

Exciton dynamics in ZnCdSe/ZnSe quantum-well structures

Exciton dynamics in ZnCdSe/ZnSe quantum-well structures have been studied from luminescence spectra obtained at T=2 K. The energy and phase relaxation times of localized exciton states have been determined from a study of the destruction of exciton optical alignment by an external magnetic field and direct measurements of the polarized-radiation decay kinetics in the picosecond range. The exciton polarization lifetimes measured by two independent techniques are found to be in a good agreement. Fiz. Tverd. Tela (St. Petersburg) 40, 809–810 (May 1998)     

Exciton-electron interaction in quantum wells with a two dimensional electron gas of low density

II–VI quantum-well structures containing a 2DEG of low density have been investigated by means of polarized photoluminescence, photoluminescence excitation and reflectivity in external magnetic fields up to 20 T. The spin splittings of the exciton X and the negatively charged exciton X ⁻ are measured as a function of the magnetic field strength. The behavior of the magnetic-field-induced polarization degree of the luminescence line related to X ⁻ demonstrates the formation process of negatively charged excitons from excitons and free carriers polarized by the external magnetic field. We have determined the binding energies of the trion formed either with the heavy-hole or the light-hole exciton. The optically detected magnetic resonance (ODMR) technique was applied for the first time to study the optical transition processes in a nanosecond timescale. The electron ODMR was observed with the detection on either the direct exciton or the negatively charged exciton X. Further evidence for the interaction of excitons with the electrons of the two-dimensional gas are demonstrated by a combined exciton-cyclotron resonance line observed in reflectivity and luminescence excitation, shake-up processes observed in photoluminescence, as well as inelastic and spin-dependent scattering processes. Fiz. Tverd. Tela (St. Petersburg) 41, 831–836 (May 1999) Published in English in the original Russian journal. Reproduced here with stylistic changes by the Translation Editor.     

Optical spectroscopy of a single InAs/GaAs quantum dot in high magnetic fields

We report on the measurements of the photoluminescence from the s-shell of a single InAs/GaAs quantum dot in magnetic fields up to 23 T. The observed multiline emission is attributed to different charge states of a single dot. Characteristic anticrossing of emission lines is explained in terms of hybridization of final states of a triply charged exciton (X⁻³).     

The effect of the initial exciton numbers on <Superscript>54,56</Superscript>Fe(<Emphasis Type="Italic">p,xp</Emphasis>) Pre-Equilibrium Reactions

In pre-equilibrium nuclear reactions, the geometry-dependent hybrid model is applied with the use of the neutron and proton densities to investigate the effect of initial exciton numbers on the nucleon emission spectra. The initial exciton numbers calculated with the theoretical neutron and proton densities have been obtained within the Skryme-Hartree-Fock method with SKM* and SLy4 forces on target nuclei in the ^54,56Fe(p, xp) reaction at 61.5-MeV incident proton energy by using a new calculationmethod of Tel et al. Also, the differences between the initial exciton numbers for protons and neutrons as a function of nuclear radius, focusing on systematic discrepancies correlated to differences in the proton and neutron densities have been investigated.     

Magnetic-field-induced polariton effects in light reflection spectra of structures with wide exciton quantum wells

A theoretical model has been developed to describe the behavior of exciton polaritons in a wide quantum well for structures with the zinc blende symmetry in a transverse magnetic field (the Voigt geometry). The model takes into account the mixing of the 1s-1s and 1s-2p states of heavy excitons by the magnetic field and makes it possible to explain and quantitatively describe the activation of optically inactive states in the reflection spectra and the magnetic-field-induced increase in the translational mass of the exciton. The quantitative calculations of the spectra have been preformed using typical parameters for CdTe/ZnCdTe quantum-well structures.