Wannier–Stark resonances in Zener tunneling diodes with GaAs/AlAs superlattices

We present DC transport measurements of the valence to conduction band (Zener) tunneling current in ap–i–ndiode with an ultrathin intrinsic layer containing a (GaAs)₅/(AlAs)₂multi-quantum well structure. According to recent theoretical predictions, the DC current should show maxima as a function of the reverse bias voltage that reflect the formation of Wannier–Stark resonances. So far, Wannier–Stark resonances have only been observed optically and never in a regime of strong Zener tunneling. Experimentally, we find the second derivative of the current-voltage characteristics to show a weak oscillatory structure indeed, indicating the existence of Wannier–Stark resonances in Zener tunneling.     

Oscillations of the magnetoresistance of a two-dimensional electron gas in a GaAs quantum well with AlAs/GaAs superlattice barriers in a microwave field

The effect of microwave radiation in the frequency range from 1.2 to 10 GHz on the magnetoresistance of a high-mobility two-dimensional electron gas has been studied in a GaAs quantum well with AlAs/GaAs superlattice barriers. It has been found that the microwave field induces oscillations of this magnetoresistance, which are periodic in the reciprocal magnetic field (1/B). It has been shown that the period of these oscillations in the frequency range under study depends on the microwave radiation power.     

Magnetophonon resonance in a GaAs quantum well with AlAs/GaAs superlattice barriers at high filling factors

The magnetotransport of a high-mobility 2D electron gas in single GaAs quantum wells with AlAs/GaAs superlattice barriers is studied at high filling factors. For the selectively doped structures under study in the temperature range from 10 to 25 K, magnetoresistance oscillations periodic in the inverse magnetic field are observed with their frequency being proportional to the Fermi wave vector of the 2D electron gas. The experimental results are explained by the interaction of the 2D electron gas with leaky interface acoustic phonons.     

Modified excitonic resonances in GaAs/AlAs multiple quantum well heterostructures with ultrathin barriers

Excitonic resonance structures in GaAs/AlAs multiple quantum well heterostructures with varying barrier-layer thicknessesL _B down to 1.3 nm are investigated for two sets of samples with the nominal well widths ofL _Z =9.2 and 6.4 nm, by 2K photoluminescence excitation spectroscopy. The observed resonance energies of then=1 heavyhole (1 hh) and light-hole (1 lh) free excitons imply that quantum confinement effects persist at least down to the decreased barrier-layer thickness ofL _B =1.3 nm. This result is inconsistent with the red shifts expected from the simple well-coupling theory within the one-band Kronig-Penney model at the point. Instead, blue shifts of 6–8 meV (8–17 meV) are observed for the 1 hh (1 lh) excitonic resonance peaks whenL _B is decreased from 10 to 2 nm. A relative decrease of the oscillator strength of the 1 lh transition compared to the 1 hh transition is also observed asL _B is decreased. These results manifest important effects of the indirect-gap barrier material for the actual wavefunction matching across the interface and the breakdown of the envelope function approach to GaAs/AlAs quantum well heterostructures with ultrathin barriers.     

Upconversion of photoluminescence due to subband resonances in a GaAs/AlAs multiple quantum well structure

We have investigated the upconversion of photoluminescence (PL) due to subband resonances in a simple GaAs(15.3 nm)/AlAs(4.5 nm) multiple quantum well embedded in a p–i–n diode structure. The systematic measurements of the PL spectra and the calculated results of the interband transition energies as a function of electric field strength reveal that the PL bands from the electron subbands with n=3 (E3) and n=4 (E4) sharply appear under the first-nearest-neighbor resonance conditions between the E1 and E3 subbands and the E1 and E4 subbands, respectively, owing to the carrier injection to the E3 and E4 subbands from the E1 subband. This result indicates that the resonant tunneling due to the subband resonance is a dominant mechanism for the carrier population in the higher lying subbands. Utilizing these subband resonances, we have demonstrated the upconversion of PL from the E3 and E4 subbands under the excitation condition of the fundamental interband transition between the E1 and the n=1 heavy-hole subbands.     

Coherent Zeeman resonance from electron spin coherence in a mixed-type GaAs/AlAs quantum well

Coherent Zeeman resonance from electron spin coherence is demonstrated in a Lambda-type three-level system, coupling electron spin states via trions. The optical control of electron density that is characteristic of a mixed-type quantum-well facilitates the study of trion formation as well as the effects of many-body interactions on the manifestation of electron spin coherence in the nonlinear optical response.     

Photoluminescence dynamics due to exciton and free carrier transport in GaAs/AlAs superlattices

Photogenerated carrier transfer is investigated in a set of three GaAs/AlAs short-period superlattices (SPSs) with different barrier thicknesses by steady-state and time-resolved photoluminescence (PL) spectroscopy at 15–20 K as a function of excitation power. The tunneling transport of carriers is evaluated by detecting excitonic PL signals from an embedded GaAs single quantum well (SQW) in the middle of the SPS layer. We find that, as the barrier thickness is decreased, the PL intensity ratio of SQW/SPS increases systematically due to enhanced tunneling efficiencies of both electrons and holes. However, the PL intensity ratio significantly increases with decreases in the excitation power by more than two orders of the magnitude. We attribute the enhanced PL intensity of SQW relative to the SPS to the faster transport of electrons that can recombine with residual holes to form excitons in SQW. The PL dynamics of SQW and SPS thus shows unique density-dependent PL intensity and time behaviors due to variations in relative amounts of excitons and free carriers to be transported into the SQW layer.     

Electro-optical trap for dipolar excitons in a GaAs/AlAs Schottky diode with a single quantum well

An electro-optical trap for spatially indirect dipolar excitons has been implemented in a GaAs/AlAs Schottky diode with a 400-Å-wide single quantum well. In the presence of a bias voltage applied to a gate, the trap for excitons appears upon ring illumination of the structure by a continuous-wave or pulsed laser generating hot electron-hole pairs in the quantum well. A barrier for excitons collected inside the illuminated ring appears because of the screening of the applied electric field by nonequilibrium carriers directly in the excitation region. Excitons are collected inside the ring owing to the ambipolar drift of carriers and dipole-dipole exciton repulsion in the optical pump region. For dipolar excitons thus collected in the center of the ring electrooptical trap, a significant narrowing of the luminescence line that accompanies an increase in the density of excitations indicates the collective behavior of dipolar excitons.     

Magnetotransport properties of a ballistic ring interferometer on the basis of a GaAs quantum well with a high concentration of 2D electron gas

Magnetotransport properties of ballistic ring interferometers made on the basis of 2D electron gas in a GaAs quantum well with AlAs/GaAs superlattice barriers are studied. An asymmetry of magnetoresistance and a phase reversal in h/e oscillations are observed when the bias voltage across the ring exceeds kT/e.     

Commensurate oscillations of the magnetoresistance of a two-dimensional electron gas in GaAs quantum wells with corrugated heteroboundaries

Oscillations of the magnetoresistance commensurate with the spatial modulation period of the growth surfaces were observed in selectively doped GaAs quantum wells with AlAs/GaAs superlattice barriers grown by molecular beam epitaxy. The experimental data obtained are explained by the lateral potential modulation of the two-dimensional electron gas in narrow GaAs quantum wells with corrugated heteroboundaries and agree with the two-dimensional distribution of the local capacitance in such structures.     

Giant magnetoresistance oscillations induced by microwave radiation and a zero-resistance state in a 2D electron system with a moderate mobility

The effect of a microwave field in the frequency range from 54 to 140 GHz on the magnetotransport in a GaAs quantum well with AlAs/GaAs superlattice barriers and with an electron mobility no higher than 10⁶ cm²/V s is investigated. In the given two-dimensional system under the effect of microwave radiation, giant resistance oscillations are observed with their positions in the magnetic field being determined by the ratio of the radiation frequency to the cyclotron frequency. Earlier, such oscillations had only been observed in GaAs/AlGaAs heterostructures with much higher mobilities. When the samples under study are irradiated with a 140-GHz microwave field, the resistance corresponding to the main oscillation minimum, which occurs near the cyclotron resonance, appears to be close to zero. The results of the study suggest that a mobility value lower than 10⁶ cm²/V s does not prevent the formation of zero-resistance states in a magnetic field in a two-dimensional system under the effect of microwave radiation.     

Ring interferometer on the basis of 2D electron gas in a double quantum well

Magnetotransport properties of submicron rings fabricated on the basis of 2D electron gas in a GaAs double quantum well are studied. It is shown that, in such interferometers, the Aharonov-Bohm effect is caused by coherent processes in two weakly coupled rings, which have different widths of electron channels. In these interferometers, a phase inversion of h/e oscillations is observed under the action of the parallel component of a tilted magnetic field. This phenomenon is qualitatively explained by a redistribution of charge carriers in the two rings.     

Coexistence of collective and single-particle effects in the photoresponse of a 2D electron gas to microwave radiation

The photoresponse of magnetoresistance of a high-density two-dimensional electron system to microwave electromagnetic radiation is studied. The damping of the Shubnikov-de Haas oscillation by radiation with a non-monotonic dependence of this effect on the magnetic field and the radiation-induced oscillations of magnetoresistance are observed. The damping is most pronounced within isolated narrow magnetic field intervals that closely correspond to the expected positions of magnetoplasma resonances in the sample under study and also near the cyclotron resonance position. A “window” is observed in the photoresponse near the field value predicted on the basis of a single-particle electron spectrum consisting of broadened Landau levels. The radiation-induced oscillations, the window in the photoresponse, and the damping of the Shubnikov-de Haas oscillations near the cyclotron resonance are described in terms of the theory based on the concept of the nonequilibrium filling of single-electron states. Thus, it is demonstrated that the photoresponse pattern observed in the experiment is formed by both single-particle and collective (magnetoplasma) effects.     

One‐phonon resonant electron Raman scattering in a cylindrical GaAs/AlAs quantum dot

We have presented a theoretical calculation of the differential cross section for the electron Raman scattering process associated with the interface optical phonon modes in cylindrical GaAs quantum dots (QDs) with a AlAs matrix. We consider the Fröhlich electron–phonon interaction in the framework of the dielectric continuum approach. The selection rules for the processes are studied. Singularities are found to be sensitively size‐dependent, and, by varying the size of the QDs, it is possible to control the frequency shift in the Raman spectra. A discussion of the phonon behavior for QDs with different size is presented. Copyright © 2013 John Wiley & Sons, Ltd.     

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)     

Microwave radiation induced collective response in Si/SiGe heterostructures with a 2D electron gas

The collective resonant photoresponse in Si/SiGe structures with a 2DEG under microwave radiation is reported for the first time. The application of microwave radiation of various frequencies results in resonant photoconductivity at magnetic field values that are systematically lower than expected for the cyclotron resonance (CR) in an infinitely large two-dimensional electron system. The analysis of the data shows that the observed microwave radiation induced response is dominated by plasmon excitations.