Manifestation of exitonic effects in the magneto-oscillations of the intensity of the recombination radiation of 2D electrons

A study is made of the temperature dependence of the magnetooscillations of the recombination radiation of 2D electrons from the photoexcited size-quantization subband in an isolated GaAs/AlGaAs quantum well. It is shown that at high temperatures (T>10 K) the period of the oscillations is determined by the ratio of the intersubband energy splitting and the sum of the electron and hole cyclotron energies. It is found that as the temperature decreases (T<5 K), a new series of oscillations (with the same period but with a larger phase shift), which are associated with the appearance of excitonic states under the Landau levels, appears. Pis’ma Zh. éksp. Teor. Fiz. 64, No. 10, 719–724 (25 November 1996)     

Two-dimensional electron gas in double quantum wells with tilted bands

When a voltage is applied to double quantum wells based on AlGaAs/GaAs heterostructures with contact regions (n-i-n structures), a two-dimensional (2D) electron gas appears in one of the quantum wells. Under illumination which generates electron-hole pairs, the photoexcited holes become localized in a neighboring quantum well and recombine radiatively with the 2D electrons (tunneling recombination through the barrier). The appearance, ground-state energy, and density of the degenerate 2D electron gas are determined from the structure of the Landau levels in the luminescence and luminescence excitation spectra as well as from the oscillations of the radiative recombination intensity in a magnetic field with detection directly at the Fermi level. The electron density is regulated by the voltage between the contact regions and increases with the slope of the bands. For a fixed slope of the bands the 2D-electron density has an upper limit determined by the resonance tunneling of electrons into a neighboring quantum well and subsequent direct recombination with photoexcited holes. Pis’ma Zh. éksp. Teor. Fiz. 65, No. 11, 840–845 (10 June 1997)     

Magnetoplasmon replica in the recombination radiation spectra of a quasi-two-dimensional electron gas in GaAs/AlGaAs quantum wells

The radiative recombination spectra of two-dimensional electrons with free photoexcited holes are investigated for a wide variety of GaAs/ AlGaAs quantum wells, with different thicknesses and electron densities. It is found that for certain, close to integral, filling factors an intense line corresponding to an Auger process — radiative recombination with the emission of an additional magnetoplasmon — appears in the luminescence spectrum. The new line is shifted to lower energies with respect to the zero Landau level, and the magnetic field dependence of the energy splitting between these lines agrees with the theoretical concepts of the dispersion of magnetoplasmon excitations. This makes it possible to estimate the magnetoplasmon energy at the roton minimum. Pis’ma Zh. éksp. Teor. Fiz. 66, No. 8, 539–544 (25 October 1997)     

Interband resonant tunneling in superconductor heterostructures in a quantizing magnetic field

The resonant tunneling of electrons through quasistationary levels in the valence band of a quantum well in double-barrier structures based on III–V materials with type-II heterojunctions is considered in a quantizing magnetic field directed perpendicularly to the interfaces. The transmission coefficients of the tunnel structure for transitions from states corresponding to different Landau levels are calculated using the Kane model. It is shown that transitions with a unit change in the Landau level index n as a result of mixing of the wave functions of states with opposite spin orientations are possible on the interfaces due to spin-orbit coupling. The probability of such transitions can be comparable to the probability of transitions without a change in the Landau level index for InAs/AlGaSb/GaSb resonant-tunneling structures. Fiz. Tverd. Tela (St. Petersburg) 40, 2121–2126 (November 1998)     

Manifestation of collective effects in polarization-resolved recombination spectra of the completely filled zeroth landau level of two-dimensional electrons

The luminescence spectra due to recombination of two-dimensional electrons with optically excited holes have been studied in a wide range of electron filling factors in the transverse magnetic field. A nonmonotonic filling-factor dependence of the energy splitting between different circular polarizations of photoluminescence from the completely filled zeroth Landau level of electrons has been observed. It has been shown that this dependence is associated with collective (excitonic) effects that appear due to the interaction between electrons from partially occupied upper Landau levels and holes remaining on the zeroth Landau level after recombination.     

Self-consistent calculation of Landau levels of a quasi-two-dimensional hole gas at a GaAs/AlGaAs p-type heterojunction

An efficient method is proposed for the self-consistent calculation of Landau levels of a quasi-two-dimensional hole gas at a GaAs/AlGaAs heterostructure in a perpendicular magnetic field. The method is based on transforming the Schroedinger and Poisson equations to a system of nonlinear differential equations which are then spatially discretized and solved by the method of relaxation. The method proposed is used to model the optical spectra for recombination of the quasi-two-dimensional hole gas with electrons localized at a dlayer of donors in an isolated p-type heterojunction. Particular attention is paid to effects associated with the dependence of the wave functions and shape of the potential well on the magnetic field, which have not been considered before. Fiz. Tverd. Tela (St. Petersburg) 40, 1117–1125 (June 1998)     

Topological Berry phase and semiclassical quantization of cyclotron orbits for two dimensional electrons in coupled band models

The semiclassical quantization of cyclotron orbits for two-dimensional Bloch electrons in a coupled two band model with a particle-hole symmetric spectrum is considered. As concrete examples, we study graphene (both mono and bilayer) and boron nitride. The main focus is on wave effects – such as Berry phase and Maslov index – occurring at order (^h/_2p)\hbar in the semiclassical quantization and producing non-trivial shifts in the resulting Landau levels. Specifically, we show that the index shift appearing in the Landau levels is related to a topological part of the Berry phase – which is basically a winding number of the direction of the pseudo-spin 1/2 associated to the coupled bands – acquired by an electron during a cyclotron orbit and not to the complete Berry phase, as commonly stated. As a consequence, the Landau levels of a coupled band insulator are shifted as compared to a usual band insulator. We also study in detail the Berry curvature in the whole Brillouin zone on a specific example (boron nitride) and show that its computation requires care in defining the “k-dependent Hamiltonian” H(k), where k is the Bloch wavevector.     

Scattering of electrons by a potential step in a magnetic field

The scattering of two-dimensional electrons drifting in crossed electric and magnetic fields by an abrupt step in the external potential is discussed. The problem is reduced to solving a system of ordinary differential equations. It is shown that for moderate electric fields scattering with a change of Landau level numbers becomes appreciable. Zh. éksp. Teor. Fiz. 112, 1756–1762 (November 1997)     

Conductivity of 2D multi-component electron gas partially-quantized by magnetic field

The 2D semimetal consisting of heavy holes and light electrons is studied. The consideration is based on the assumption that electrons are quantized by magnetic field while holes remain classical. We assume also that the interaction between components is weak and the conversion between components is absent. The kinetic equation for holes colliding with quantized electrons is utilized. It has been stated that the inter-component friction and corresponding correction to the dissipative conductivity σ _xx do not vanish at zero temperature due to degeneracy of the Landau levels. This correction arises when the Fermi level crosses the Landau level. The statement will keep in force until the degeneracy remains. The limits of kinetic equation applicability were found. We also study the situation of kinetic memory when particles repeatedly return to their meeting points.     

Exchange interaction effects in inter-Landau level Auger scattering in a two-dimensional electron gas

We consider the influence of spin effects on the inter-Landau level electron-electron scattering rate in a two-dimensional electron gas. Because of exchange spin splitting, the Landau levels are not equidistant. This leads to the suppression of Auger processes and a nonlinear dependence of the lifetime on the concentration of excited electrons even at very low excitation levels. Pis’ma Zh. éksp. Teor. Fiz. 67, No. 8, 574–579 (25 April 1998) Published in English in the original Russian journal. Edited by Steve Torstveit.     

Kinetics of two-dimensional electrons on a curved surface

A number of effects associated with the curvature of the surface on which a two-dimensional (2D) electron gas is placed are studied. The most significant effect in an external magnetic field (which for 2D electrons becomes effectively nonuniform) is the lifting of the degeneracy of the Landau levels. The intensity and shape of the cyclotron resonance line (inhomogeneously broadened) for different polarizations and the corrections to the Hall constant are found for the example of a circular cylinder. A picture of the quantization of the conductance that is qualitatively different from the case of a flat strip is obtained for a quasi-one-dimensional quantum wire in the form of a hollow cylinder. It is shown that in contradistinction to the planar case the spectrum of 2D electrons on the curved surface is sensitive to the sign of the spin-orbit coupling constant (for a fixed sign of the curvature). For hetero-junctions, for example, this opens up new possibilities for extracting information about their “hidden parameters.” Pis’ma Zh. éksp. Teor. Fiz. 64, No. 6, 421–426 (25 September 1996)     

Optical detection of the absorption of acoustical phonons by a two-dimensional electron gas in a magnetic field

The effect of absorption of nonequilibrium acoustical phonons on the intensity of recombination of a two-dimensional electron gas in a magnetic field is investigated. The nonequilibrium acoustical phonons are emitted in the relaxation of electrons in a tunnel junction deposited on the back side of a sample with a two-dimensional electronic channel. It is demonstrated that the optical signal showing the intensity of the recombination of nonequilibrium electrons from a photoexcited size-quantization subband can serve as a sensitive detector of acoustical phonons. Because the general heating of two-dimensional carriers and the intersubband transitions stimulated by the absorption of nonequilibrium acoustical phonons lead to effects of different sign, the useful signal can be discriminated unambiguously. Pis’ma Zh. éksp. Teor. Fiz. 69, No. 1, 30–35 (10 January 1999)     

Resistivity of 2D electrons with ν=1/2 in a zero magnetic field

The dependence of the resistivity of 2D electrons with Landau level filling factor ν=1/2 in a zero magnetic field is studied experimentally as a function of the carrier density. It is found that the ratio of the resistivity for B=0 and ν=1/2 is a linear function of the carrier density, as predicted by a theory based on the scattering of composite fermions by spatial fluctuations of the effective static magnetic field. Pis’ma Zh. éksp. Teor. Fiz. 66, No. 3, 154–158 (10 August 1997)     

Production of coherent states of excitons in semiconductors by means of the recombination of free carriers

It is shown that the modern experimental techniques make it possible to produce a coherent Bose condensate of excitons in semiconductors by the direct recombination of electrons from the conduction band and holes from the valence band. Pis’ma Zh. éksp. Teor. Fiz. 65, No. 5, 436–440 (10 March 1997)     

Observation of a hybridization gap in cyclotron resonance spectra of semimetallic InAs/GaSb quantum wells

Splitting of the cyclotron resonance (CR) line is observed in the electron CR spectra of InAs/GaSb heterostructures containing tunneling-coupled electron and hole layers. This splitting is interpreted to be a manifestation of a hybridization gap arising as a result of anticrossing of the Landau levels of electrons and holes when their wave functions overlap. The energy splitting of the CR lines is correlated with the magnitude of the overlap and agrees with theoretical estimates of the hybridization gap width. Pis’ma Zh. éksp. Teor. Fiz. 69, No. 4, 313–317 (25 February 1999)     

Cathodoluminescence of ZnSe(Bi):Al crystals

The cathodoluminescence (CL) in ZnSe crystals annealed at T=1200 K in a Bi melt containing an aluminum impurity is investigated. The spectra are recorded for different excitation levels, temperatures, and detection delay times t ₀. As t ₀ is increased, the intensity of the orange band at λ _max=630 nm (1.968 eV) in the CL spectrum decreases in comparison to the intensity of the dominant yellow-green band at λ _max=550 nm (2.254 eV), whose half-width increases in the temperature range 6–120 K and then decreases as the temperature increases further. It is shown that such behavior of the yellow-green band is caused by the competition between two processes: recombination of donor-acceptor pairs and of free electrons with holes trapped on acceptors. The former mechanism is dominant at low temperatures, and the latter mechanism is dominant at high temperatures. At T∼120 mK the contributions of the two mechanisms to the luminescence are comparable. The resultant structureless band then achieves its greatest half-width, which is dictated by the interaction of the recombining charge carriers with longitudinal-optical and longitudinal-acoustic phonons and with the free-electron plasma. The mean number of longitudinal-optical phonons emitted per photon is determined mainly by their interaction with holes trapped on deep acceptors in the form of Al atoms replacing Se. The donor in the pair under consideration is an interstitial Al atom. Fiz. Tverd. Tela (St. Petersburg) 39, 1526–1531 (September 1997)     

Charge transfer mechanism in high-purity silicon-basedn +-π-p + structures

Results of experimental investigations into current-voltage characteristics of n⁺-π-p⁺ structures based on high-purity silicon doped with boron are presented. It is shown that the I–V characteristic at high injection levels is described by Stafeev's theory, when the thickness d of the π-region is smaller than or close to three ambipolar diffusion lengths L_a. For a current density J above (3–50) A/cm² at varying temperatures, the I–V characteristic obeys a relation usually occurring in the case where charge-carrier recombination in the n⁺ and p⁺-regions dominates that found in the π-region. The effect of mutual electron-hole scattering on the behavior of the I–V characteristic is evident at J>630 A/cm³. For d/L_a=9, the I–V characteristic at high injection levels is treated by the theory of double carrier injection into a semiconductor with consideration for diffusion corrections. In the temperature range below 200–276 K, the I–V curve for all samples studied exhibits a linear dependence on J followed by a portion corresponding to a maximum occupancy of recombination levels by injected electrons. Here the Fe or Au donor levels presumably act as recombination levels. The electron and hole capture areas (cross sections) by recombination centers are roughly estimated. Siberian Physicotechnical Institute at Tomsk State University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 7, pp. 35–45, July 2000.     

Features of charge-carrier recombination in amorphous molecular semiconductors doped with polymethine dyes

The photogeneration and recombination of charge carriers in poly-N-epoxypropylcarbazole films with additions of a polymethine dye are investigated irradiation of films with blocking contacts by light both within and outside the absorption range of the dye. The kinetics of the accumulation and relaxation of electron-hole pairs, whose lifetimes exceed tens and hundreds of seconds, are studied. It is postulated that an increase in the recombination luminescence intensity occurs in an electric field as a result of an increase in the efficiency of the bimolecular radiative recombination stimulated by trapped electrons from photogenerated excitons. Fiz. Tverd. Tela (St. Petersburg) 40, 629–635 (April 1998)     

Determination of landau level lifetimes in n-GaAs

The lifetime of electrons in the first excited Landau level of n-GaAs is determined from a combination of measurements of far infrared cyclotron resonance induced absorption and conductivity change. Values of T₁ of the order of 10^?8s for densities of excited electrons of 10¹¹ cm^?3 and a temperature dependence of T^?2.7 are found. An upper limit for the N = 0 Landau level to donor recombination time of the order of 10^?9s was derived from pulsed conductivity measurements.     

Giant blue shift of photoluminescence in strongly excited type-II ZnSe/BeTe superlattices

A giant blue shift (≈0.5 eV) and a large decrease in the emission time of a spectral band corresponding to radiative recombination of spatially separated electrons and holes are observed in ZnSe/BeTe superlattices at high laser excitation levels. On the basis of numerical calculations, the observed defects are attributed to band bending arising in type-II structures at high carrier density. Pis’ma Zh. éksp. Teor. Fiz. 66, No. 5, 351–356 (10 September 1997)