Quantum cyclotron resonance of two-dimensional holes in the Ge layers of Ge-Ge1−x Six heterostructures

Quantum cyclotron resonance of two-dimensional holes in strained germanium layers of the periodic heterostructure Ge-Ge_1−x Si_x has been observed and investigated for the first time. The results are compared with the data of electrophysical measurements in strong magnetic fields. A clear dependence of the magnitude of the longitudinal effective mass of the holes on the absolute magnitude of the elastic deformation of the germanium layers (splitting energy of the hole subbands) is observed. Fiz. Tverd. Tela (St. Petersburg) 39, 2096–2100 (November 1997)     

Remarkable effects of uniaxial stress on stimulated far IR emission from hot holes in germanium in crossed electric and magnetic fields

Stimulated far IR emission due to l-h as well as cyclotron transitions of hot holes in uniaxially stressedp-Ge (P H E) was studied. The results obtained showed the significance of intersub-band hole tunnelling for these mechanisms of generation and may be explained by a change in tunnelling produced by the stress. A considerable expansion of the stimulated light hole cyclotron emission band was observed in a stressed crystal. This expansion allows covering (in one sample) of all generation bands in the light hole cyclotron resonance (CR) masers in unstressedp-Ge, reported so far.     

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)     

Mixing of excitonic states containing light and heavy holes in an isolated GaAs/AlGaAs quantum well in a magnetic field

The Zeeman splitting of the ground states 1s(hh) and 1s(lh) of excitons with heavy and light holes, respectively, in a 15-nm isolated Al_0.3Ga_0.7As/GaAs quantum well in magnetic fields of up to 20 T is investigated according to the photoluminescence excitation spectra in the Faraday geometry (σ⁺− σ⁻ components). The observed anomalous pattern of nonlinear Zeeman splitting and the nonmonotonic behavior of the effective hole g factor are interpreted in terms of the strong mixing of the magnetoexcitonic states containing light and heavy holes. Pis'ma Zh. éksp. Teor. Fiz. 64, No. 1, 52–56 (10 July 1996)     

Measurement of cyclotron masses of spin-orbit-split quasi-two-dimensional holes in GaAs(100) narrow quantum wells

The magnetic field dependence of the cyclotron mass of heavy holes in asymmetric GaAs(100) quantum wells is measured by optical detection of resonant microwave absorption for various concentrations of quasi-two-dimensional holes. The effect of spin-orbit splitting on the cyclotron masses of heavy holes is discovered and investigated. The energy spectrum of holes is calculated on the basis of experimental data. The energy range in which spin-plasmon oscillations are observed in hole systems with various concentrations is predicted.     

Intervalence-band photoinduced absorption in undoped GaAs/AlxGa1−xAs multiple-quantum-wells

We report the results of photoinduced absorption (PA) measurements in the energy range appropriate for transitions between the heavy holes (hh) and the spin orbit split-off valence-band holes (so), in samples of undoped, bulk GaAs and GaAs/AlGaAs multiple-quantum-wells (MQW). The bulk GaAs hh-so absorption band is broad and asymmetric; its line shape is accounted for by a finite k-space width of the hole Bloch states, arising, for instance, from impurity scattering. A good fit to the data was obtained using a reasonable width of . In the MQW structures, the intervalence band hh1-so1 PA is identified. In addition, impurity related so transitions were also observed.     

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)     

Landau quantization and hot hole stimulated FIR emission in crossed electric and magnetic fields

Theoretical and experimental investigations of the resonant amplification of far-infrared (FIR) radiation by hot holes in germanium in theE B field configuration have shown that stimulated emission is possible not only at the fundamental but also at higher harmonics of the light-hole cyclotron resonance frequency. Single line tunable FIR laser oscillations close to the fundamental and the second cyclotron harmonic are demonstrated and explained within the presented model. In the crossed field configuration a repulsion of light and heavy hole levels is predicted as a result of interaction.     

Charged and neutral excitonic complexes in GaAs/AlGaAs quantum wells

The temperature and magnetic-field dependences of the recombination line of multiparticle excitonic complexes in undoped and lightly doped GaAs/AlGaAs quantum wells are investigated. These dependences have previously been attributed to free charged excitons (trions). It is shown that this line corresponds to a bound state of a complex, specifically, to an exciton bound on a neutral donor in a barrier. It is found that as the temperature or pump power is raised, there appear in the recombination spectrum not only a cyclotron replica shifted downward in energy but also a replica which is symmetrically shifted upwards in energy by an amount equal to the cyclotron energy and which is due to emission from an excited state of an impurity complex. The behavior of the cyclotron replicas is studied as a function of the electron density and temperature. Pis’ma Zh. éksp. Teor. Fiz. 66, No. 11, 730–735 (10 December 1997)     

Far-infrared laser oscillation due to cyclotron emission in p-type germanium

Far-infrared laser oscillation due to cyclotron emission in the light hole band of p-type germanium was observed under crossed electric and magnetic fields. The wavelength is inversely proportional to magnetic fields with a cyclotron mass ofm _c =0.048m _c . Numerical calculations based on Luttinger Hamiltonian show that mixing of wavefunctions between the light and heavy hole bands causes population inversion betweenn=0 andn=1 light-hole Landau levels. It is also shown that non-equidistant energy spacing of light-hole Landau levels is essential to yield net amplification.     

Light hole transport in a strained GaAs/In0.18Ga0.82As quantum well at high pressures

Abstract

Here we report what we believe to be the first observation of the pressure dependence of the light hole behavior in a modulation doped In_0.18Ga_0.82As/GaAs single strained quantum well grown by MBE. Transport measurements have been undertaken as a function of temperature (4–300K) and hydrostatic pressure (4–8kbar). Hole mobilities of ～17000 cm²/Vs have been obtained for sheet carrier densities of ～3.3×10¹¹ cm^?2. At low temperatures (<100K) persistent photogenerated holes have been observed. The hole mobility is found to decrease with increasing pressure at a rate intermediate between that typically observed for holes and electrons in bulk III-V semiconductors.     

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 0.26m ₀ (m ₀ 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.     

Transformation of the dimensionality of excitonic states in quantum wells with asymmetric barriers in an electric field

A transformation of the dimensionality of excitonic states from 2D to 3D with increasing external electric field is observed in single GaAs/Al_xGa_1−x As quantum-well structures with asymmetric barriers. The binding energy of a 2D exciton remains constant over a wide range of variation of the field, since the decrease in the binding energy is compensated by increasingly larger penetration of the electronic wave function into the barrier layer, where the exciton binding energy is higher because the effective mass is larger and the dielectric constant of AlGaAs is lower than that of GaAs. When the maximum of the electron wave function is displaced into the barrier as the field increases, the exciton binding energy decreases. As the field increases further, a 2D exciton transforms into a quasi-3D exciton, with a heavy hole in the quantum well and an electron in a resonant above-barrier state. Pis’ma Zh. éksp. Teor. Fiz. 67, No. 3, 207–211 (10 February 1998)     

Effect of hole-hole scattering on the conductivity of the two-component 2D hole gas in GaAs/(AlGa)As heterostructures

The temperature dependences of the zero-magnetic-field resistivity ρ and magnetoresistance of the 2D hole gas in GaAs/(AlGa)As heterostructures are investigated in the temperature interval 0.4–4.2 K. As the temperature T is increased, (i) the resistivity ρ grows with a decreasing derivative dρ/dT, and (ii) the positive magnetoresistance diminishes from about 40% at T=0.4 K to about 1% at T=4.2 K. The results are explained in terms of a temperature-dependent mutual scattering of the holes, accompanied by momentum transfer between two different spin-split subbands. Pis’ma Zh. éksp. Teor. Fiz. 67, No. 2, 101–106 (25 January 1998) Published in English in the original Russian journal. Edited by Steve Torstveit.     

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)     

Infrared radiation from hot holes during spatial transport in selectively doped InGaAs/GaAs heterostructures with quantum wells

The infrared radiation from hot holes in In_xGa_1−x As/GaAs heterostructures with strained quantum wells during lateral transport is investigated experimentally. It is found that the infrared radiation intensities are nonmonotonic functions of the electric field. This behavior is due to the escape of hot holes from quantum wells in the GaAs barrier layers. A new mechanism for producing a population inversion in these structures is proposed. Pis’ma Zh. éksp. Teor. Fiz. 64, No. 7, 478–482 (10 October 1996)     

Semiclassical energy levels of electrons in metals with band degeneracy lines

We show that in calculating the semiclassical energy levels of electrons in metals located in a magnetic field, one must determine whether or not the corresponding electron paths in the space of wave vectors k are attached to a band degeneracy line. Calculations in the two possible cases, i.e., with and without such attachment, differ by |e|ℏ/2m*c, where e is the electron charge and m* is the cyclotron mass of the electron. This shift in the energy levels is of a topological nature, and its existence depends neither on the specific form of the electron dispersion relation ε(k) near the electron path nor on the shape or size of this path. The reason for this shift lies in the fact that the electron orbit is attached to the band degeneracy line, which is the line of singular points of the Bloch wave functions. In many respects this effect is similar to the Aharonov-Bohm effect if the band degeneracy line is considered an infinitely thin “solenoid.” This shift in energy levels should become apparent in studies of oscillation phenomena in metals. We give examples of metals in which the conditions for observing the shift is probably the most favorable. Zh. éksp. Teor. Fiz. 114, 1375–1392 (October 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)     

THz spectroscopy of diluted magnetic semiconductors and semiconductor quantum structures in ultrahigh magnetic fields

Magneto-absorption spectra in ferromagnetic semiconductor In_1−xMn_xAs films and self-organized PbSe/PbEuTe quantum dot superlattices have been studied in the terahertz range at very high magnetic fields up to 500 T. Both heavy hole (HH) and light hole (LH) cyclotron resonance (CR) have been observed in bulk In_1−xMn_xAs thin films with different Mn concentrations. The detailed Landau level calculation in terms of the effective mass approximation well explained the CR peak positions, line shapes and the dependence of the circular polarization of the incident light on the CR spectra. In InMnAs/GaSb heterostructures that have higher ferromagnetic transition temperature (T_c) than the bulk samples, the observed HH and LH cyclotron masses are larger than that in the bulk thin films. We found that the CR peak position and its line shape suddenly change in the vicinity of the ferromagnetic transition temperature, suggesting the change in the electronic structure due to the ferromagnetic transition. Electron CR in PbSe/PbEuTe quantum dots has been observed and it was found that the effective mass of the electrons is considerably modified by the quantum confinement potential and the lattice strain around the dots. A large wavelength dependence of the absorption intensity was observed due to the interference effect of the radiation inside the sample.     

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)