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)     

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/AlaAs superlattices, with different widths of the electron and hole minibands, located in a high magnetic field perpendicular to the heterolayers. It is found that the ground state of the indirect excitons formed by electrons and holes and spatially separated between neighboring quantum wells lies between the ls ground state of the direct excitons and the continuum threshold for dissociated exciton states in the minibands. Indirect excitons in superlattices have a significant oscillator strength when the binding energy of the exciton exceeds the order of the width of the resulting miniband. The behavior of the binding energy of direct and indirect heavy hole excitons during changes in the tunneling coupling between the quantum wells is established. It is shown that a strong magnetic field, which intensifies the Coulomb interaction between the electron and hole in an exciton, weakens the bond in a system of symmetrically bound quantum wells. The spatially indirect excitons studied here are analogous to first order Wannier-Stark localized excitons in superlattices with inclined bands (when an electrical bias is applied), but in the present case the localization is of purely Coulomb origin. Zh. éksp. Teor. Fiz. 112, 1106–1118 (September 1997)     

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)     

Interwell exciton relaxation in semimagnetic asymmetric double quantum wells

The possibility of magnetic field control of the spectral and polarization characteristics of exciton recombination is examined in Cd(Mg, Mn) Te-based asymmetric double quantum wells. At low fields, the exciton transition in a semimagnetic well is higher in energy than that in a nonmagnetic well and the interwell exciton relaxation is fast. In contrast, when the energy order of the exciton transitions reverses at high fields, unexpectedly slow relaxation of σ⁻ polarized excitons from the nonmagnetic well to the σ⁺-polarized ground state in the semimagnetic well is observed. Strong dependence of the total circular polarization degree on the heavy-light hole splitting Δ _hh-lh in the nonmagnetic well is found and attributed to the spin dependent interwell tunneling controlled by exciton spin relaxation. Such a slowing down of the relaxation allows separation of oppositely spin-polarized excitons in adjacent wells. The text was submitted by the authors in English.     

Exciton molecules and exciton liquids in semiconductors

Excitons in many-valley semiconductors form molecules consisting of four and more excitons. The degeneracy factor g of the conduction band in germanium is 8, and in silicon g=12. As in acceptors, the hole ground state in excitons is fourfold degenerate. The same is valid for exciton molecules, because they are quantum objects with spherical symmetry. The exciton binding energy in molecules is close to that in exciton-liquid droplets. Experimental evidence is considered for the existence, besides biexcitons, of stable exciton molecules consisting of three and four, and, possibly, 11 and 12 excitons. Molecules containing from five to ten excitons are apparently unstable. Fiz. Tverd. Tela (St. Petersburg) 40, 929–931 (May 1998)     

Spin relaxation of Mn ions in (CdMn)Te/(CdMg)Te quantum wells under picosecond optical pumping

Spin relaxation of Mn ions in a Cd_0.97Mn_0.03Te/Cd_0.75Mg_0.25Te quantum well with photogenerated quasi-two-dimensional electron-hole plasma at liquid helium temperatures in an external magnetic field has been investigated. Heating of Mn ions by photogenerated carriers due to spin and energy exchange between the hot electron-hole plasma and Mn ions through direct sd-interaction between electron and Mn spins has been detected. This process has a short characteristic time of about 4 ns, which leads to appreciable heating of the Mn spin subsystem in about 0.5 ns. Even under uniform excitation of a dense electron-hole plasma, the Mn heating is spatially nonuniform, and leads to formation of spin domains in the quantum well magnetic subsystem. The relaxation time of spin domains after pulsed excitation is measured to be about 70 ns. Energy relaxation of excitons in the random exchange potential due to spin domains results from exciton diffusion in magnetic field B=14 T with a characteristic time of 1 to 4 ns. The relaxation time decreases with decreasing optical pump power, which indicates smaller dimensions of spin domains. In weak magnetic fields (_B=2 T) a slow down in the exciton diffusion to 15 ns has been detected. This slow down is due to exciton binding to neutral donors (formation of bound excitons) and smaller spin domain amplitudes in low magnetic fields. The optically determined spin-lattice relaxation time of Mn ions in a magnetic field of 14 T is 270±10 and 16±7 ns for Mn concentrations of 3% and 12%, respectively. Zh. éksp. Teor. Fiz. 112, 1440–1463 (October 1997)     

Propagation of quasi-two dimensional exciton polaritons in a quantum well waveguide

The propagation of exciton polaritons in an optical waveguide with a quantum well is studied. Spatial dispersion of the excitons causes the wave vector of the exciton polaritons to split between waveguide and exciton modes at resonance. The magnitude of this splitting is determined by the radiative decay parameter of excitons with corresponding polarization in the quantum well. The group velocity of the waveguide exciton polaritons in the resonance region can be three or four orders of magnitude lower than the speed of light in vacuum. Fiz. Tverd. Tela (St. Petersburg) 40, 362–365 (February 1998)     

Barrier-disorder induced exciton relaxation via LO-phonons in GaAs/Al x Ga1−x As multiple quantum wells

Hot exciton relaxation is observed in GaAs/Al _x Ga_1–x As multiple quantum wells. The photolumnescence excitation spectra of the localized exciton emission at low temperatures and excitation densities are composed of narrow equidistant peaks exactly separated by the GaAs LO-phonon energy (36 meV). The relaxation mechanism via LO-phonons is found to be important for localized excitons in multiple quantum wells with GaAs layer thicknesses of about 50 Å, where pronounced alloy fluctuations in the barriers provide a strong additional lateral potential which suppresses the dissociation of hot excitons.     

Specific features in the temperature dependence of photoluminescence from CdTe/ZnTe size-quantized islands and ultrathin quantum wells

A study has been carried out of the temperature dependences of luminescence spectra on a large number of CdTe/ZnTe structures differing in average thickness, 〈L _z〉=0.25–4 monolayers (ML), and CdTe layer geometry (continuous, island type). The influence of geometric features in the structure of ultrathin layers on linewidth, the extent of lateral localization of excitons, their binding energy, and exciton-phonon coupling is discussed. It is shown that in island structures there is practically no lateral exciton migration. The exciton-phonon coupling constant in a submonolayer structure has been determined, Γ_ph=53 meV, and it is shown that in structures with larger average thicknesses Γ_ph is considerably smaller. Substantial lateral exciton migration was observed to occur in a quantum well with 〈L _z〉=4 ML, and interaction with acoustic phonons was found to play a noticeable part in transport processes. It has been established that the depth of the exciton level in a quantum well and structural features of an ultrathin layer significantly affect the temperature dependences of integrated photoluminescence intensity. Fiz. Tverd. Tela (St. Petersburg) 41, 717–724 (April 1999)     

Exciton-exciton collisions and conversion of interwell excitons in GaAs/AlGaAs superlattices

The ratio of the densities of intra-and interwell excitons in a symmetric system of coupled quantum wells — a superlattice based on a GaAs/AlGaAs heterostructure — is investigated over a wide range of optical excitation power densities. Conversion of interwell excitons into intrawell excitons as a result of exciton-exciton collisions is observed at high exciton densities. Direct evidence for such a conversion mechanism is the square-root dependence of the interwell exciton density on the optical excitation level. The decrease in the lifetime of interwell excitons with increasing excitation density, as measured directly by time-resolved spectroscopy methods, confirms the explanation proposed for the effect. Pis’ma Zh. éksp. Teor. Fiz. 65, No. 8, 623–628 (25 April 1997)     

Magnetic field effect on transitions between direct and indirect excitons in diluted magnetic semiconductor double quantum wells

Transitions between direct and indirect excitons with change of magnetic field in double quantum well heterostructure Cd_1−xMg_xTe/Cd_1−yMg_yTe/Cd_1−xMg_xTe/Cd_1−zMn_zTe/Cd_1−xMg_xTe in external magnetic field are studied. The structure contains diluted magnetic semiconductor (Cd,Mn)Te layer that forms magnetic quantum well with the depth depending on the magnetic field intensity. Above some magnetic field the indirect exciton becomes the lowest excited state of the system. The indirect exciton lifetime exceeds by several orders of magnitude of the direct exciton one. The range of quantum well widths for which the indirect exciton is the exciton lowest state was estimated for the proposed system.     

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)     

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)     

Intraband transitions in magnetoexcitons in coupled double quantum wells

A theory of far-infrared (FIR) magneto-optical intraband s→p ^± transitions of direct and indirect excitons in semiconductor coupled double quantum wells has been developed. The case of symmetric strained In_xGa_1−x As/GaAs quantum wells with nondegenerate valence band in the regime of both narrow and wide barriers has been analyzed. The energies and dipole matrix elements of transitions between the ground s and excited p ^± states in a quantizing magnetic field B>2 T and electric field ℰ perpendicular to the quantum well plane have been studied. The regimes of direct (in a weak electric field) and indirect (in a strong electric field) transitions, and the transition between the direct and indirect regimes, have been investigated. Zh. éksp. Teor. Fiz. 113, 1446–1459 (April 1998)     

Spectrum of indirect magnetoexcitons in coupled quantum wells

The indirect Mott exciton (spatially-separated electron and hole) in coupled quantum wells in crossed electric and magnetic fields is discussed. The exciton spectrum is calculated for the case where the distance between the quantum wells of the electron and hole is larger than the exciton Bohr radius. The magnetoexciton creation probability is calculated and its dependence on the electric field is found. The absorption of electromagnetic radiation between the indirect magnetoexciton levels in coupled quantum wells is discussed. Fiz. Tverd. Tela (St. Petersburg) 39, 2220–2223 (December 1997)     

Exciton magnetotransport in two-dimensional systems: Weak-localization effects

The paper considers the effect of a magnetic field B on the transport of neutral composite particles, namely excitons, in weakly disordered two-dimensional (2D) systems. In the case of classical transport (when the interference of different paths is neglected), the magnetic field suppresses exciton transport, and the static diffusion constant D(B) monotonically drops with B. When quantum-mechanical corrections due to weak localization are taken into account, D(B) becomes a nonmonotonic function of B. In weak magnetic fields, where the magnetic length is much larger than the exciton Bohr radius, ℓ_B=(ℏc/eB)^1/2≫a _B =ε ℏ²/μe ²,a positive magnetodiffusion effect is predicted, i.e., the exciton mobility should increase with B. Zh. éksp. Teor. Fiz. 114, 359–378 (July 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.     

Drag effects in a two-layer system of spatially separated electrons and excitons

We discuss drag effects in a two-layer system of spatially separated electrons and excitons: the entrainment of excitons by moving electrons, and the entrainment of electrons by moving excitons. For the case of excitons entrained by electrons we find the drag velocity υ _drag, and for electrons entrained by excitons we compute the induced electric field E ₂. These drag effects can be sensitive indicators of the phase state of the excitons and of phase transitions in the exciton system (to a liquid phase, superfluid state, etc.) Zh. éksp. Teor. Fiz. 111, 1107–1119 (March 1997)     

Optical exciton Stark effect and quantum beats at exciton quasienergy levels in quantum wells

The optical Stark effect and quantum beats in a GaAs/AlGaAs quantum well are investigated theoretically for the case when the first two electron size-quantization levels are mixed dynamically by a high-intensity CO₂ laser pulse polarized perpendicular to the plane of the quantum well. The quasienergy spectrum of heavy-hole excitons and the ratio between the probabilities of exciton transition with and without a strong electromagnetic field are obtained. The time-dependent intensity of absorption of the sensing light is determined. It exhibits quantum beats at twice the electron Rabi frequency. Fiz. Tverd. Tela (St. Petersburg) 39, 1291–1294 (July 1997)     

Effect of spin relaxation on the polarization of excitonic luminescence in disordered systems

The kinetics of localized excitons in systems with disorder is studied with allowance for the fine structure of the excitonic state and for spin relaxation processes. The exciton distribution function, formed as a result of the competition between radiative and nonradiative recombination, spin relaxation, and intercenter transitions with an exponentially wide scatter in transition times, is calculated. Pis’ma Zh. éksp. Teor. Fiz. 65, No. 8, 612–617 (25 April 1997)