S. I. Gubarev

Screening of excitonic states by low-density 2D charge carriers in GaAs/AlGaAs quantum wells

Screening of excitonic states by a system of 2D electrons (or holes) in GaAs/AlGaAs single quantum wells is studied. With increasing concentration of 2D charge carriers, a threshold-type disappearance of excitonic states is observed in both luminescence and reflectance spectra. The higher the quality of the 2D system, the lower the corresponding threshold concentration. In the best systems, the collapse of excitonic states occurs at unexpectedly low electron densities ne=5×109 cm−2, which correspond to the mean dimensionless distance between the particles rs=8. This value far exceeds the threshold values observed for 3D systems (rs≈2), as well as the values obtained for quantum wells in previous studies. The problem of measuring the concentration of low-density 2D charge carriers in photoexcitation conditions is solved by applying the method of optical detection of the dimensional magnetoplasma resonance. This method provides reliable measurements of the density of a 2D system to the values about 109 cm−2.

Effect of screening by two-dimensional charge carriers on the binding energy of excitonic states in GaAs/AlGaAs quantum wells

The spectrum of excitonic excited states in GaAs/AlGaAs quantum wells of different width is studied together with its change due to the screening of electron-hole interaction by two-dimensional electrons. The exciton binding energy decreases sharply with an increase in the concentration of two-dimensional electrons. The temperature dependence of screening parameters is studied for the ground and excited excitonic states down to ultralow temperatures T=50 mK.

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.

Dimensional magnetoplasma resonance of 2D holes in (001) GaAs/AlGaAs quantum wells

Dimensional magnetoplasma resonance is observed and studied in a spatially confined, two-dimensional hole system in (001) GaAs/AlGaAs single quantum wells. From the analysis of the field dependence of the magnetoplasma resonance on the diameter of the 2D system, the semiclassical cyclotron hole mass is determined. Its value is found to be equal to 0.26m0 (m0 is the free electron mass), which considerably exceeds the theoretically predicted value. A method is proposed for a direct determination of the concentration and mobility of 2D holes from the analysis of the magnetoplasma resonance.

Properties of exciton states in GaAs/AlGaAs quantum wells in the presence of a quasi-two-dimensional electron gas

The changes in binding energy and oscillator strength of the exciton state due to the screening by a quasi-two-dimensional electron gas are calculated self-consistently in the approximation of noninteracting electrons and in the local field approximation. It is shown that the collapse of the bound state occurs at very low concentrations, Ns≈5×109 cm−2, which is a consequence of the inclusion of the nonlinearity of the response of the system to a Coulomb perturbation. The temperature dependence of the exciton collapse is investigated. The phase diagram of the dissociation of the given bound state is constructed, and the region in which it is possible to observe experimentally the temperature dependence of the exciton collapse is indicated.

Plasma waves in a two-dimensional electron system laterally screened by a metallic gate

The dispersion of magnetoplasma and plasma excitations in two-dimensional electron systems, whose edges are formed by a voltage applied to a metallic gate, has been studied. A substantial decrease in the plasma wave frequency as compared to the plasma frequency measured in the etched mesas with the same geometry, size, and electron density has been observed. The dependence of the observed frequency softening on the structure size has been studied and the laterally screened plasma excitation has been shown to violate the square-root dispersion relation.

Magnetoplasma resonance in a GaAs/AlGaAs quantum well in a strong parallel magnetic field

The dispersion of magnetoplasma excitations in two-dimensional electron systems in a strong parallel magnetic field has been studied. A considerable increase in the electron cyclotron mass with an increase in the parallel component of magnetic field has been detected. It has been found that the cyclotron mass increment is a quadratic function of the magnetic field parallel to the interface. It has been shown that the mass anisotropy of 2D electrons induced by the parallel magnetic field reaches nearly 2.5 in B‖ = 7 T. The energy of space quantization of the electron in the quantum well has been estimated from the magnetic field dependence of the anisotropy.

Screening of exciton states by quasi-two-dimensional electron gas in quantum wells

Changes in the binding energy and oscillator strength of an exciton state due to screening by a quasi-two-dimensional electron gas were calculated self-consistently in a nonlinear approximation. It was shown that the collapse of the bound state proceeds at very small concentrations Ns≃5×109 cm−2, which is a consequence of taking into account the nonlinearity of the system response to the Coulomb perturbation.

Optical detection of magnetoplasma resonances in indirect-gap AlAs/AlGaAs quantum wells

Magnetoplasma excitations in AlAs/AlGaAs quantum wells, characterized by strong anisotropy in the effective mass of two-dimensional electrons, are investigated using the optical detection of resonance microwave absorption. This technique is used for the first time for an indirect-gap semiconductor. It is found that the magnetic dispersion of the cyclotron magnetoplasma mode deviates significantly from the theoretically expected behavior. This may be related to the considerably more pronounced manifestation of retardation effects in two-dimensional systems with an anisotropic energy spectrum.

Occurrence of a gap in the spectrum of magnetoplasma excitations of a two-dimensional electron disk subjected to a strong in-plane magnetic field

The effect of a parallel magnetic field on the dispersion of bulk and edge magnetoplasmons in a disk-shaped two-dimensional electron system is investigated. It is found that the anisotropy of the electron effective mass that appears in a parallel magnetic field lifts the degeneracy of plasma oscillations in the disk. This is accompanied by the occurrence of a gap in the spectrum of magnetoplasmons, and the magnetic dispersion of these excitations changes from a linear to a parabolic one. The width of the gap is determined by the difference between the frequencies of plasma oscillations along the field and transverse to the field and grows quadratically with the in-plane field strength.