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, N _s≈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.     

Excitons and excitonic complexes in GaAs/AlGaAs quantum wells with a low-density quasi-two-dimensional electron and hole channel

The recombination spectra of excitons and excitonic complexes in un-doped GaAs/AlGaAs single quantum wells are investigated. It is shown on the basis of a study of the magnetic-field dependence of the emission spectra and the degree of optical orientation in zero magnetic field and on the basis of electrooptic measurements that not only the density but also the sign of the charge carriers in a well depend strongly on the photoexcitation energy. It is shown on the basis of a comparative analysis of the spin splitting of the recombination lines of free and bound excitons that the recombination line which was attributed earlier to a positively charged exciton corresponds to the recombination of an exciton bound on a neutral acceptor. Pis’ma Zh. éksp. Teor. Fiz. 67, No. 9, 707–713 (10 May 1998)     

Effect of electron screening on the photoluminescence in GaAs/AlGaAs quantum wells

Magneto-photoluminescence of one-side-doped GaAs/AlGaAs single quantum well is measured. Dependence of the spectra on the well width revealed that the long range screening effect of two-dimensional electrons on a free hole plays an important role in the process of recombination.     

High-frequency magneto-oscillations in GaAs/AlGaAs quantum wells

We have observed very high-frequency, highly reproducible magneto-oscillations in modulation doped GaAs/AlGaAs quantum well structures. The oscillations are periodic in an inverse magnetic field (1/B) and their amplitude increases with temperature up to T approximately 700 mK. Being initially most pronounced around the filling factor nu=1/2, they move towards lower nu with increasing T. Front and back-gating data imply that these oscillations require a coupling to a parallel conducting layer. A comparison with existing oscillation models renders no explanation.     

Absorption and emission of terahertz radiation in doped GaAs/AlGaAs quantum wells

Spontaneous emission of terahertz radiation from structures with GaAs/AlGaAs quantum wells in a longitudinal magnetic field has been studied. It is shown that some bands in the emission spectrum can be related to radiative electron transitions between resonant and localized impurity states, as well as to the transitions with participation of subband states. The temperature dependence of the equilibrium intraband absorption of terahertz radiation and its modulation in a longitudinal electric field in GaAs/AlGaAs quantum wells has been investigated.     

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 N _s ?5×10⁹ cm^?2, which is a consequence of taking into account the nonlinearity of the system response to the Coulomb perturbation.     

Photoreflectance measurements in GaAs/AlGaAs asymmetric quantum wells

In this work we investigated the optical control of the bidimensional electron gas density in a single asymmetric quantum well using, for the first time, photoreflectance. We performed our measurements at 80 and 300 K as a function of the power density of the pump beam. Under strong illumination, the bidimensional electron gas density is washed out of the quantum well and under a dark condition, it reaches its maximum value. The variation of the optical transitions observed in our photoreflectance spectra was related to the induced changes of the band profile in between these two limiting cases.     

Effect of ionized impurities on electron tunneling in GaAs/AlGaAs triple quantum wells

Photoluminescence and electroreflectance measurements in Si δ-doped GaAs/Al_0.35Ga_0.65-A triple quantum wells are performed in order to study the effect of ionized impurities on electron tunneling. It is theoretically shown that a thin quantum well with a δ-doping layer inserted between narrow and wide quantum wells of asymmetric double quantum wells enhances impurity scattering rate significantly. Photoluminescence decay time is found to decrease 30% at maximum compared with a sample without a δ-doping layer.     

Dynamics of excitonic states in GaAs/AlGaAs quantum wells

The influence of photoexcited carriers on the dynamics of the absorption spectra of GaAs/Al_xGa_1−2x As multilayer quantum wells is investigated experimentally. It is found that at quasiparticle densities all the way up to 10¹¹ cm⁻² the saturation of the excitonic absorption is due to both a decrease of oscillator strength and broadening of the excitonic lines. It is shown that in the case of femtosecond resonance laser exci-tation the decrease of oscillator strength is due to free electron-hole pairs, while the broadening and energy shift of the excitonic lines are due to the exciton-exciton interaction. The lifetimes of free electron-hole pairs and excitons (≈65 ps and ≈410 ps, respectively) are determined from the exponential decrease of the change in the oscillator strength and in the width and energy position of the excitonic lines. Pis’ma Zh. éksp. Teor. Fiz. 66, No. 3, 139–144 (10 August 1997)     

Time-resolved studies of electron and hole spin dynamics in modulation-doped GaAs/AlGaAs quantum wells

Spin dynamics in semiconductors have gained much interest in the past years due to the emerging field of semiconductor spintronics. This review is focussed on the observation and control of electron and hole spin dynamics in modulation-doped heterostructures based on the GaAs/AlGaAs material system. Modulation doping allows for the creation of two-dimensional electron and hole systems with high carrier mobility. By confining carriers to a two-dimensional sheet, the spin–orbit interaction is modified significantly. In addition to this, it can be further modified by changing the symmetry of the system, for example by externally applied or built-in electric fields along the growth direction. Our recent experimental results on spin dynamics in two dimensions are reviewed and discussed in connection with theoretical considerations. A brief overview of the current research challenges in this field is given.     

Comparison of thermally detected optical absorption and electromodulation spectra of GaAs/AlGaAs quantum wells

We report for the first time a comparative study of GaAs/AlGaAs quantum well (QW) spectra obtained by thermally detected optical absorption (TD-OA) and photo- (PR) and electroreflectance (ER) experiments, respectively. The excitonic transition energies, obtained at low temperature with these methods, agree within 1 meV. It is demonstrated, using samples with different AlGaAs overlayer thicknesses, that the shape of the TD-OA response is strongly influenced by interference effects for QW's grown on absorbing substrates. A complete PR lineshape analysis yields phase angles for all transitions, whose differences can be attributed to the change of the AlGaAs front barrier thicknesses. ER investigations following successive top layer removal confirm this interpretation.     

Novel excitonic transitions in n-type GaAs/AlGaAs quantum wells

We report on a novel peak, the F-line, observed in photoluminescence spectra of GaAs/AlGaAs quantum wells (QWs) with various donor layer positions and concentrations. The F-line is well-defined and red shifted by approximately 1.3 meV (dependent on the experimental conditions) relatively the free exciton (FE) in a 200 Å wide QW. The F-line exhibits a strong magnetic field dependence. The enhanced intensity with increasing field is due to an increasing wave function overlap caused by the enhanced localization of the involved charge carriers. In accordance, the derived thermal activation energy for the F-line is magnetic field dependent. The F-line exhibits a diamagnetic shift as expected for an excitonic transition and splits into four components with increasing magnetic field. Another associated higher energy peak, the E-line, is observed preferably in the presence of a magnetic field, between the heavy hole- and light hole-FE in PL excitation spectra. The E-line also exhibits a striking magnetic field and temperature dependence. The observed properties of the F-line with a striking dependence on the excitation intensity, magnetic field and temperature are consistent with the observation of an exciton bound at the negatively charged D^- donor state or a negatively charged X^- exciton.     

Absorption and emission of far-IR radiation by hot holes in GaAs/AlGaAs quantum wells

It is found that when holes in GaAs/Al_0.5Ga_0.5As quantum wells are heated by a longitudinal electric field, the absorption in the far-IR region of the spectrum changes. The spontaneous emission spectrum in the far-IR range is measured. It is shown that the absorption and emission are due to direct intersubband transitions of holes near the peaks in the reduced density of states. The experimental data are in agreement with a theoretical calculation. Pis’ma Zh. éksp. Teor. Fiz. 63, No. 12, 928–932 (25 June 1996)     

Shake-up intersubband transitions observed in GaAs/AlGaAs quantum wells

Shake-up transitions involving QW hole subbands have been observed as satellites in selective photoluminescence spectra of undoped GaAs/AlGaAs QWs. These shake-up transitions are explained in terms of an interaction between localized exciton and valence-band hole states attached to the QW subbands, in which holes are shaken up from the n=1 heavy hole subband to higher subbands, either the n=1 light hole subband or the n=2 heavy hole subband. The required localization is due to the interface roughness; thus these new transitions are of intrinsic origin. From the observation of the intersubband shake-up processes we derive direct information about the hole inter-subband energies. Furthermore, the satellite intensity is strikingly enhanced in the presence of a magnetic field due to an increasing exciton localization related to the compression of its wave function in the field. The exciton wave function compression continues until its radius in the plane of the well is comparable with the radius of the "flat island" characterized by constant QW width. Accordingly, from the magnetic field dependence of the shake-up satellite intensity we can roughly estimate the size of the "flat islands" and consequently probe the interface roughness.     

Valence bands spin splitting in GaAs/AlGaAs quantum wells

A quantitative comparison between different model calculations of valence band states in GaAs/AlGaAs heterostructures is presented. We demonstrate that a 14-band k.p Hamiltonian using a completely new parameterization based on fits of the tight-binding band structure leads to energy dispersion relations in excellent agreement with experiment, whereas previous parameterizations result in significant deviations. The relevance of the present results to the calculation of spin-related phenomena is discussed.     

Up-conversion luminescence via a below-gap state in GaAs/AlGaAs quantum wells

We observe a band-to-band photoluminescence (PL) of the well layers in GaAs/AlGaAs quantum well structures (hν_P = 1.56 eV) under below-gap excitation (BGE) with a Nd:YAG laser (hν_B = 1.17 eV) at 77 K. The origin of the up-conversion luminescence was inside the epitaxially grown well layers and is different from those reported in GaAs substrates. A detailed study of a two-wavelength excited PL was carried out by changing the density of both the BGE and the above-gap excitation (AGE) by a He–Ne laser (hν_A = 1.96 eV) individually. The up-conversion process corresponds to the increase in the PL intensity due to the BGE in two-wavelength excited PL, which reveals the mechanism of a cascade excitation via a below-gap state in quantum wells for the first time. A rate- equation analysis explained the measured BGE density dependence of the up-conversion luminescence.     

Interwell radiative recombination in the presence of random potential fluctuations in GaAs/AlGaAs biased double quantum wells

The interwell radiative recombination from biased double quantum wells (DQW) in pin GaAs/AlGaAs heterostructures is investigated at different temperatures and external electrical fields. The luminescence line of interwell recombination of spatially separated electron-hole pairs exhibits systematic narrowing with temperature increase from 4.5 to 30 K. A theoretical model is presented which explains the observed narrowing in terms of lateral thermally activated tunneling of spatially separated e-h pairs localized by random potential fluctuations in the quantum wells. Pis’ma Zh. éksp. Teor. Fiz. 67, No. 8, 580–585 (25 April 1998) Published in English in the original Russian journal. Edited by Steve Torstveit.     

Birefringence and absorption of infrared radiation in tunnel-coupled GaAs/AlGaAs quantum wells in a longitudinal electric field

Birefringence and absorption modulation under electron heating in a longitudinal electric field in the tunnel-coupled GaAs/AlGaAs quantum wells have been found and investigated in the spectral region corresponding to intersubband electron transitions. The observed phenomena are explained by electron heating in electric field and electron transfer in real space. The equilibrium absorption spectra at different lattice temperatures are analyzed.