Nonequilibrium distribution of two-dimensional electrons in the presence of transverse high-frequency pumping

A mechanism is described which leads to the formation of a nonequilibrium distribution of two-dimensional electrons on account of the effect of the electric field of radiation on the interaction of the electrons with a phonon thermostat in the absence of absorption. An analysis is made of the case of a nonresonant, transversely polarized pump, where the intersubband transitions and Drude absorption of the radiation are very weak but the probabilities of transitions involving the participation of phonons do not satisfy the principle of detailed balance. The character of the nonequilibrium distribution of highly degenerate electrons is discussed and their effective temperature is found. Pis’ma Zh. éksp. Teor. Fiz. 68, No. 1, 44–49 (10 July 1998)     

The kinetics of low-temperature electron-phonon relaxation in a metallic film following instantaneous heating of the electrons

The theoretical analysis of experiments on pulsed laser irradiation of metallic films sputtered on insulating supports is usually based on semiphenomenological dynamical equations for the electron and phonon temperatures, an approach that ignores the nonuniformity and the nonthermal nature of the phonon distribution function. In this paper we discuss a microscopic model that describes the dynamics of the electron-phonon system in terms of kinetic equations for the electron and phonon distribution functions. Such a model provides a microscopic picture of the nonlinear energy relaxation of the electron-phonon system of a rapidly heated film. We find that in a relatively thick film the energy relaxation of electrons consists of three stages: the emission of nonequilibrium phonons by “hot” electrons, the thermalization of electrons and phonons due to phonon reabsorption, and finally the cooling of the thermalized electron-phonon system as a result of phonon exchange between film and substrate. In thin films, where there is no reabsorption of nonequilibrium phonons, the energy relaxation consists of only one stage, the first. The relaxation dynamics of an experimentally observable quantity, the phonon contribution to the electrical conductivity of the cooling film, is directly related to the dynamics of the electron temperature, which makes it possible to use the data of experiments on the relaxation of voltage across films to establish the electron-phonon and phonon-electron collision times and the average time of phonon escape from film to substrate. Zh. éksp. Teor. Fiz. 111, 2106–2133 (June 1997)     

Pair scattering of electrons in edge channels of opposite chiralities in the presence of a disorder potential

The nonequilibrium transfer of the energy between electrons of counter-propagating quasi-one-dimensional systems has been perturbatively calculated for edge channels in a two-dimensional system in the integer quantum Hall effect. The processes involving two electrons that are allowed only in the system with disorder have been taken into account. Expressions for the cases of Coulomb scattering and transfer of nonequilibrium phonons have been obtained. The energy transferred per unit time has a quasi-threshold dependence on the degree of nonequilibrium of the hot channel. According to numerical estimates for electrons in GaAs, Coulomb scattering processes dominate in the energy transfer and the expected effect can be experimentally observed.     

Terahertz emission from square wells in a longitudinal electric field

Terahertz emission accompanying heating of two-dimensional electrons by a strong electric field applied along size-quantized GaAs/AlGaAs layers is observed and investigated. The emission is due to indirect optical transitions of hot electrons in the bottom size-quantization band. The experimentally obtained emission spectra are compared with the spectra calculated taking into account scattering of electrons by optical phonons, impurities, and interfacial roughness and electron-electron scattering. Satisfactory agreement is obtained. The temperature of the hot electrons is determined from a comparison of the spectra. Pis’ma Zh. éksp. Teor. Fiz. 67, No. 7, 507–511 (10 April 1998)     

A hybrid-phonon resonance in a quasi-two-dimensional nanostructure

The coefficient of absorption of electromagnetic radiation by a quasi-two-dimensional electron gas placed in an oblique magnetic field is found. The scattering of electrons by optical phonons is shown to lead to resonant absorption. The shape of the resonance peaks on the absorption curve is studied, and their doublet nature is demonstrated. Finally, the dependence of the resonance peaks on the angle between the magnetic field vector and the confinement plane is investigated. Zh. éksp. Teor. Fiz. 111, 1092–1106 (March 1997)     

Heating of two-dimensional excitons by nonequilibrium acoustic phonons

The energy distribution functions of two-dimensional excitons in the presence of nonequilibrium acoustic phonons have been calculated for the geometry used in heat-pulse experiments. The results were obtained by solving numerically the kinetic equation for the case where the exciton gas equilibrates with phonons during its lifetime. The cases of the low and high exciton-gas density limits are considered. It is shown that at low exciton-gas densities the distribution does not follow the Boltzmann function and depends on the quantum-well width. A comparison with earlier experimental data is made. Fiz. Tverd. Tela (St. Petersburg) 41, 1707–1711 (September 1999)     

The Peierls system in a light field

This paper studies the behavior of the low-temperature phase of the Peierls system in a quasimonochromatic time-independent random light field whose frequency is much lower than the frequency of the lower edge of band-to-band transitions. The density matrix method in the dipole approximation is used to derive equations for the band gap. A dependence between the band gap and the light-field intensity is established in an approximation in which the concentration of the nonequilibrium electrons in the conduction band is low. The possibility of, and the conditions for, the existence of a light-induced semiconductor-semiconductor phase transition and cavityless optical bistability with increasing absorption are established. Zh. éksp. Teor. Fiz. 111, 1398–1409 (April 1997)     

Kinetic theory of semiconductor cascade laser based on quantum wells and wires

The paper presents a numerical solution of a system of nonlinear equations for the electron distribution functions in the upper and lower subbands between which lasing transitions occur and the number of nonequilibrium optical phonons in semiconducting cascade lasers based on quantum wells and wires. For the case of quantum wells, we propose an analytical solution of this system of equations, which is a generalization of the previously found solution [V. F. Elesin and Yu. V. Kopaev, Zh. éksp. Teor. Fiz. 108, 2186 (1995) [JETP 81, 1192 (1995)]; V. F. Elesin and Yu. V. Kopaev, Sol. St. Commun. 96, 897 (1995)] in a wider range of injection rates. The threshold injection rate can be significantly reduced owing to reabsorption and accumulation of nonequilibrium optical phonons, nonparabolicity of the subbands and different effective masses of electrons in different subbands. In the case of quantum wires, the threshold injection rate is considerably lower, and its decrease is even larger than in quantum wells. It is remarkable that, owing to the lower electron-electron relaxation rate in the one-dimensional case, the decrease in the threshold injection rate may be two or three orders of magnitude. The relation between the density of states and threshold current has also been studied. Zh. éksp. Teor. Fiz. 111, 681–695 (February 1997)     

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)     

Two types of fundamental luminescence of ionization-passive electrons and holes in optical dielectrics—Intraband-electron and interband-hole luminescence (theoretical calculation and comparison with experiment)

A short high-power pulse of ionizing radiation creates a high concentration of nonequilibrium electrons and holes in a dielectric. They quickly lose their energy, generating a multiplicity of secondary quasiparticles: electron—hole pairs, excitons, plasmons, phonons of all types, and others. When the kinetic energy of an electron becomes less that some value E_Δ≈(1.3-2)E_g it loses the ability to perform collisional ionization and electron excitations of the dielectric medium. Such an electron is said to be ionization-passive. It relaxes to the bottom of the lower conduction band by emitting phonons. Similarly a hole becomes ionization-passive when it “floats up” above some level E_H and loses the ability for Auger ionization of the dielectric medium. It continues to float upward to the ceiling of the upper valance band only by emitting phonons. The concentrations of ionization-passive electrons and holes are larger by several orders of magnitude than those of the active electrons and holes and consequently make of a far larger contribution to many kinetic processes such as luminescence. Intraband and interband quantum transitions make the greatest contribution to the fundamental (independent of impurities and intrinsic defects) electromagnetic radiation of ionization-passive electrons and holes. Consequently the brightest types of purely fundamental luminescence of strongly nonequilibrium electrons and holes are intraband and interband luminescence. These forms of luminescence, discovered relatively recently, carry valuable information on the high-energy states of the electrons in the conduction band and of the holes in the valence band of a dielectric. Experimental investigations of these types of luminescence were made, mainly on alkali halide crystals which were excited by nanoseconal pulses of high-current-density electrons and by two-photon absorption of the ultraviolet harmonics of pulsed laser radiation beams of nanosecond and picosecond duration. The present article gives the results of theoretical calculations of the spectra and other characteristics of intraband electron and interband hole luminescence which are compared with the experimental data. Institute of High-Current Electronics, Sibrian Branch of the Russian Academy of Sciences, Polytechnic University, Tomsk. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 11, pp. 13–41, November, 1997.     

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)     

Ion-implanted buried layer in diamond as a source of ballistic phonons at liquid-helium temperatures

It is shown that an approximately 150 nm thick ion-implanted buried layer in diamond and excited by a pulsed laser at wavelength λ=337 nm is a source of nonequilibrium acoustic phonons propagating ballistically through the diamond sample at temperatures ∼2 K. Pis’ma Zh. éksp. Teor. Fiz. 64, No. 4, 270–272 (25 August 1996)     

Effect of electron-phonon interaction on the thermoelectric properties of superlattices

The electron relaxation time on acoustical phonons, the electrical conductivity, and the phonon-drag thermopower of a semiconductor superlattice with quasi-two-dimensional quantum wells are calculated. The inelasticity of the scattering of charge carriers is taken into account. It is shown that the phonon-drag thermopower of a superlattice can be an order of magnitude greater than the corresponding thermopower of a bulk semiconductor. Pis’ma Zh. éksp. Teor. Fiz. 69, No. 4, 290–295 (25 February 1999)     

Direct measurements of energy relaxation times on an AlGaAs/GaAs heterointerface in the range 4.2–50 K

The temperature dependence of the energy relaxation time τ_e (T) of a two-dimensional electron gas at an AlGaAs/GaAs heterointerface is measured under quasiequilibrium conditions in the region of the transition from scattering by acoustic phonons to scattering with the participation of optical phonons. The temperature interval of constant τ_e, where scattering by the deformation potential predominates, is determined. In the preceding, low-temperature region, where piezoacoustic and deformation-potential-induced scattering processes coexist, τ _e decreases slowly with increasing temperature. Optical phonons start to participate in the scattering processes at T∼25 K (the characteristic phonon lifetime was equal to τ_LOτ4.5 ps). The energy losses calculated from the τ_e data in a model with an effective nonequilibrium electron temperature agree with the published data obtained under strong heating conditions. Pis’ma Zh. éksp. Teor. Fiz. 64, No. 5, 371–375 (10 September 1996)     

Excitonic effects in the recombination radiation spectra of completely filled Landau levels of two-dimensional electrons

The recombination radiation spectra of two-dimensional electrons in an asymmetrically doped GaAs/AlGaAs quantum well are investigated at different temperatures and laser-excitation energies. At low temperatures and in high magnetic fields the recombination lines of the electrons from completely filled Landau levels are split into narrow sublevels. It is shown that this fine structure of the Landau levels is due to the presence of excitonic effects in the initial and final states of the photoexcited system. It is demonstrated that the recombination process is accompanied by the excitation of intersubband and cyclotron magnetoplasma modes. Pis’ma Zh. éksp. Teor. Fiz. 65, No. 1, 38–43 (10 January 1997)     

Electron and phonon effects in superconducting tunnel detectors of x-radiation

Nb/Al/AlO_x/Nb superconducting tunnel junctions were investigated in the role of x-ray detectors. Amplitude spectra of pulses arising upon irradiation of tunnel junctions of different sizes by ⁵⁵Mn x-radiation were recorded at a temperature T=1.4 K. We also analyzed the temporal shape of the pulses. We considered the influence of diffuse motion of nonequilibrium quasiparticles, the inverse tunneling effect, and exchange of 2Δ phonons between electrodes, on the characteristics of the tunnel detectors. It is shown that phonon processes can bring about changes in the amplitude, duration, and polarity of the signal. Fiz. Tverd. Tela (St. Petersburg) 41, 1168–1175 (July 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)     

Kinetic properties of a system of spatially separated excitons and electrons in the presence of a Bose condensate of excitons

A system of interacting, spatially separated excitons and electrons is examined in the presence of a Bose condensate of excitons. The kinetic properties of the system that are governed by the interaction of excitations in the exciton subsystem with electrons are investigated. It is shown that a nonequilibrium distribution of excitations in the exciton subsystem gives rise to an induced electron current. Experimental observation of the kinetic phenomena described can provide new information on the exciton phase state. Zh. éksp. Teor. Fiz. 116, 1440–1449 (October 1999)     

Spatial correlation of an electron and hole in quasi-two-dimensional electronic system in a strong magnetic field and its relationship to the light-scattering tensor

The spatial correlation of light-generated electrons and holes in a quasi-two-dimensional electron gas in a strong magnetic field is investigated in an approximation linear in the intensity of the exciting light. The correlation is due to the interaction of the electrons and holes with longitudinal optical (LO) phonons. The theory permits calculating, on the basis of a special diagrammatic technique, two distribution functions of an electron-hole pair with respect to the distance between the electron and the hole after the emission of N phonons: first, the function determining the total number of pairs which have emitted N phonons and, second, the function related to the rank-4 light-scattering tensor in interband resonance Raman scattering of light. A special feature of the system is that the electron and hole energy levels are discrete. The calculation is performed for a square quantum well with infinitely high barriers. The distribution function and the total number of electron-hole pairs before the emission of phonons as well as the distribution function corresponding to two-phonon resonance Raman scattering are calculated. The theory predicts the appearance of several close-lying peaks in the excitation spectrum under resonance conditions. The number of peaks is related to the number of the Landau level participating in the optical transition. The distance between peaks is determined by the electron-phonon coupling constant. Far from resonance there is one peak, which is much weaker than the peaks obtained under resonance conditions. Zh. éksp. Teor. Fiz. 111, 2194–2214 (June 1997)     

Contribution of nonequilibrium optical phonons to the Peltier and Seebeck effects in polar semiconductors

Additive contributions to the Seebeck and Peltier coefficients made by nonequilibrium longitudinal optical phonons have been calculated. The results obtained are valid for any temperature and applicable to polar nondegenerate semiconductors with low carrier concentrations. The calculated components of the thermoelectric coefficients are exponentially small in the low-temperature domain and reach a maximum at k _BT∼ħω ₀. In materials with a large carrier mass and strong electron-phonon coupling the contribution of optical phonons to the Seebeck coefficient can exceed 1 mV/K. Fiz. Tverd. Tela (St. Petersburg) 40, 1209–1215 (July 1998)