Asymmetric Double Quantum Wells Research Articles

Spatially Indirect Excitons in GaAs/Al0.35Ga0.65As Double Quantum Wells in the Presence of In-Plane Magnetic Fields

We studied the optical properties of GaAs/Al0.35Ga0.65As asymmetric double quantum wells at T = 4.2 K and in the presence of in-plane magnetic fields up to 20 T. In an electric field F ≳ 45 kV/cm, electrons and holes are respectively confined in the wide and narrow well and form spatially indirect excitons with a binding energy of∼3.5 meV. The photoluminescence (PL) peak shifts diamagnetically by 0.02 meV/T2 and is quenched by fields of ∼5 T, the suppression being stronger at higher biases. The experimental results are well explained by numerical calculations which show that the magnetic field makes the e–h recombination indirect also in k-space. The suppression of the PL cannot be described in the picture of free e–h pairs, but rather scales as the ratio between the magnetic length and the e–h distance; we therefore believe that it is a peculiar manifestation of the exciton binding.

Mechanism of THz emission from asymmetric double quantum wells.

Impulsive interband optical excitation of the lowest two conduction subbands of a suitably engineered GaAs/AlGaAs double quantum well can lead to coherent THz emission. We demonstrate that, contrary to previous expectations, the dominant emission mechanism can involve the beating of either continuum or exciton states, depending on excitation conditions. The coherence of the continuum beats persists for several picoseconds even for excitation an optical phonon energy above the band edge. We attribute this to the small energy difference between the component eigenstates, which substantially reduces the number of relevant scattering events.

Quantum control of charge carriers in quantum wells

We present theoretical studies of the quantum control of charge carriers in a direct-current-biased (DC-biased) asymmetric double quantum well (ADQW). The objective in this work is to find the laser pulse that drives an electronic wave packet to maximum overlap with a target distribution in a chosen well at a chosen time. We use a genetic algorithm to determine the optimal parameters of the excitation pulses. The robustness of the control is analyzed with respect to the magnitude of the DC field, and two types of defects in the ADQW potential profile, a point defect and a notch. In all cases studied, the genetic algorithm is able to reoptimize the laser field to achieve the desired objective. Finally, we consider the effects of coupling the ADQW to a harmonic bath. We find that a coupling large enough to distort the effective potential significantly does not eliminate the control. Our results indicate that control in the ADQW is quite robust, which should make it a favorable system for further experimental study. © 2000 John Wiley & Sons, Inc. Int J Quant Chem 77: 393–402, 2000

Quantum control of charge carriers in quantum wells

We present theoretical studies of the quantum control of charge carriers in a direct-current-biased (DC-biased) asymmetric double quantum well (ADQW). The objective in this work is to find the laser pulse that drives an electronic wave packet to maximum overlap with a target distribution in a chosen well at a chosen time. We use a genetic algorithm to determine the optimal parameters of the excitation pulses. The robustness of the control is analyzed with respect to the magnitude of the DC field, and two types of defects in the ADQW potential profile, a point defect and a notch. In all cases studied, the genetic algorithm is able to reoptimize the laser field to achieve the desired objective. Finally, we consider the effects of coupling the ADQW to a harmonic bath. We find that a coupling large enough to distort the effective potential significantly does not eliminate the control. Our results indicate that control in the ADQW is quite robust, which should make it a favorable system for further experimental study. © 2000 John Wiley & Sons, Inc. Int J Quant Chem 77: 393–402, 2000

Two-color picosecond experiments on anti-Stokes photoluminescence in GaAs/AlGaAs asymmetric double quantum wells

Two-color photoluminescence experiments are performed on the anti-Stokes photoluminescence in GaAs/AlGaAs asymmetric double quantum wells. Direct evidence for forbidden absorption is shown, and its many intriguing aspects, particularly the role of long-lived defects, are revealed. Our experiments shed light on the ongoing controversies between many different models.

Phonon enhanced inverse population in asymmetric double quantum wells

Interwell optical-phonon-assisted transitions are studied in an asymmetric double-quantum-well heterostructure comprising one narrow and one wide coupled quantum wells (QWs). We show that the depopulation rate of the lower subband states in the narrow QW can be significantly enhanced thus facilitating the intersubband inverse population, if the depopulated subband is aligned with the second subband of the wider QW, while the energy separation from the first subband is tuned to the highest energy optical-phonon mode.

Linear electroabsorption in semi-insulating GaAs/AlGaAs asymmetric double quantum wells

Electroabsorption has been investigated in semi-insulating asymmetric GaAs/AlGaAs double quantum wells presenting high linear Stark responses, adequate for photorefractive applications. We have used the envelope function approximation to calculate the linear Stark shifts of the energy levels and select a suitable structure for the experimental study. The experimental data indicate that the response to the applied field critically depends on a complicated interplay of effects that compete or cooperate to suppress or enhance the electroabsorption. For positive field polarity, the competing contributions of the overlapping e1–hh1 and e1–hh2 transitions partially cancel the electroabsorption despite large linear Stark shifts. On the other hand, small negative fields induce large electroabsorption because the Stark shifts of the two transitions have opposite signs.

MBE growth of AlGaAs/GaAs heterostructure and silicon doping on GaAs( n 1 1)A ( n=1–4) substrates

Undoped AlGaAs/GaAs asymmetric double quantum wells (ADQWs) and Si-doped single epitaxial GaAs layers were prepared on non-(1 0 0) oriented GaAs( n 1 1)A ( n=1–4) substrates in molecular beam epitaxy (MBE) at different As pressures. The grown thickness was the same for (1 0 0) and ( n 1 1)A substrates as revealed by a transmission electron microscope observation. The surfaces of ( n 1 1)A samples change from rough to smooth and featureless with increasing As pressure, while that of (1 0 0) sample shows an inverse change according to the atomic force microscopy results. Photoluminescence spectra of ADQWs have shown that, the luminescence peak energies shift towards lower energy direction with increasing As pressures. The conduction type of ( n 1 1)A was p-type for lower As pressure and n-type for higher As pressure.

Second Harmonic Electroreflectance Study of AlGaAs–GaAs Asymmetric Triangular and Coupled Double Quantum Wells

Electroreflectance (ER) spectroscopy is used to study AlGaAs–GaAs asymmetric triangular and coupled double quantum wells. The ER spectra are taken at both the fundamental (1f ) and second harmonic (2f ) modulation frequencies. A detailed ER lineshape analysis appropriate for a Lorentzian dielectric function is presented. The experimental 1f and 2f spectra are, respectively, shown to be related to the first and second order field induced changes in the dielectric function. A detail comparison of the line shapes of the 1f, 2f and energy derivative of the 1f spectra and their respective spectral forms enable a qualitative deduction of the dominant mechanism of the experimental ER spectra. In particular, for the asymmetric triangular well structure, the dominance of excitonic modulation mechanism can be inferred. Useful additional information on the sample signature such as the features associated with the red and blue shifts are obtained through such a comparative study. In addition, several weak features are observed to be significantly enhanced in the 2f spectra and the characteristics of these features are deduced.

Optical transitions of Al0.35Ga0.65As/GaAs asymmetric double quantum wells grown on GaAs(n11)A (n≤4) substrates

The optical transitions in Al0.35Ga0.65As/GaAs asymmetric double quantum wells (ADQWs) grown on GaAs(n11)A (n≤4) substrates were studied by photoluminescence at low temperatures. The redshift of two PL peaks for substrate orientation far from the (100) plane is attributed to the large anisotropy of the heavy-hole band in the different GaAs orientations. The energy difference of the two transitions also shows a similar shift. By comparing the PL measurements with a simple effective mass calculation, the effective mass of the heavy hole on GaAs(n11)A (n=1, 2, 3, 4) planes is found to be about 0.95, 0.73, 0.54 and 0.41m0, respectively. The effect of As pressures on PL peak energies in ADQWs is also studied. The As pressure-dependence of PL peak energies is attibuted to a decrease of the Ga desporption rate with increasing As pressures on GaAs(n11)A substrates.

Arsenic vapor pressure dependence of surface morphology and silicon doping in molecular beam epitaxial grown GaAs (n11)A (n=1–4) substrates

Surface morphology of undoped AlGaAs/GaAs asymmetric double quantum wells (ADQWs) and Si-doped single epitaxial GaAs layers were studied by atomic force microscopy. Samples were prepared on GaAs(n11)A (n=1–4) substrates in molecular beam epitaxy at different As pressures and the same substrate temperature and Ga flux. The surfaces of (n11)A ADQWs change from rough to smooth and featureless with increasing As pressures, while that of (100) sample shows an inverse change according to the atomic force microscopy results. Triangular pyramidal facets were observed at lower As pressure for (111)A and (211)A orientations. The interface roughness of ADQWs was improved at higher As pressure showing by a transmission electron microscope observation. The conduction type of (n11)A (n<=4) was p-type for lower As pressure and n-type for higher As pressure.

Exciton–phonon interaction in single and double quantum wells

A review is given on studies of exciton–phonon interaction in single quantum wells by means of a nonequilibrium phonon technique. It is shown that in the presence of nonequilibrium phonons the exciton energy distribution is non-Boltzmann. It is sensitive to the phonon energy and momentum distribution and also depends on the quantum well width. The results clearly show the role of 2D exciton confinement in the exciton–phonon interaction. In slightly asymmetric double quantum wells a rapid phonon-assisted relaxation of coupled excitons is observed.

Manifestation of collective properties of spatially indirect excitons in GaAs/AlGaAs asymmetric double quantum wells

Low-temperature (T=1.8, 4.2 K) luminescence of GaAs/Al0.3Ga0.7As double-coupled asymmetric quantum wells (DQW) is observed under variation of an electric field Vdc applied along the normal to the DQW plane. The FWHM of the indirect exciton (IX) emission line was found to undergo, within a certain interval of Vdc, a strong (up to a factor 3.5) narrowing, accompanied by anomalously large IX intensity fluctuations in time within the Vdc region where a strong decrease (or increase) of the FWHM is observed to occur. The dependence of the FWHM on the pumping level Ip also exhibits a substantial decrease of the FWHM within a certain Ip interval. The results obtained are discussed in the frame of an assumption which relates the observed phenomena to the onset of a condensed state in the interacting ensemble of spatially indirect excitons in DQWs.

Sign of the Piezoelectric Field in Asymmetric GaInN/AlGaN/GaN Single and Double Quantum Wells on SiC

AbstractWe study both GaInN/GaN/AlGaN quantum wells with an asymmetric barrier structure grown on SiC substrate and GaN/AlGaN asymmetric double quantum well (ADQW) structures. In the first case, a time-resolved study reveals an enhanced oscillator strength when the AlGaN barrier is on top of the GaInN quantum well. In comparison to our previous study of the same structure grown on sapphire, we find that the sign of the field is the same in both cases: the field points towards the substrate. In the case of ADQW, we observed not only intrawell transitions of both a 4 nm and a 2 nm QW separated by a 2.5 nm AlGaN barrier but also an interwell transition between the two QWs in the photoluminescence. The lifetimes and emission energies of the transitions can be well explained by the existence of the piezoelectric field built in the QWs.

23aB3 Dependence of Ga sticking coefficient on Arsenic vapor pressure and crystal orientation from GaAs substrate grown by MBE using PL of asymmetric double Quantum wells

23aB3 Dependence of Ga sticking coefficient on Arsenic vapor pressure and crystal orientation from GaAs substrate grown by MBE using PL of asymmetric double Quantum wells

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

Photoluminescence and electroreflectance measurements in Si δ-doped GaAs/Al0.35Ga0.65-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.

Selectively observed spatially indirect Stark-ladder transitions in asymmetric double-well GaAs/AlAs superlattices

The Wannier-Stark localization of asymmetric double well superlattices (ADW-SLs) has been investigated by low-temperature photocurrent spectroscopy. The ADW-SLs consist of arrays of asymmetric double quantum wells formed by coupling of two different size wells. Reflecting the difference in the eigenenergies of the two wells, the occurrence of the Stark-ladder transitions is, in principle, characterized by the superposition of two fan diagrams. Hence, four ±1st-order spatially indirect transitions are expected to exist. However, they are experimentally observed only partially. This selectivity is interpreted by considering the coupling pattern between two wells in the ADW-SLs. The control of the coupling pattern is a new concept to design SL structures and its Wannier-Stark localization.

The excition tunneling in ZnCdSe/ZnSe asymmetric double quantum well

Photoluminescence spectra of asymmetric double-quantum-well structure are studied in this paper. We show the excitation power dependence of exciton tunneling. Due to the different tunneling time of electrons and holes, space-charge effect is observed.

Luminescence of GaAs/AlGaAs asymmetric double-quantum-wells excited by a mid-IR free electron laser

Luminescence has been observed from a GaAs/AlGaAs asymmetric double-quantum-wells (ADQWs) structure when excited by intense mid-infrared (IR) radiation from a free-electron laser (FEL). The intensity of the luminescence is found to depend strongly on the mid-IR FEL average power, while the dependence on temperature is quite weak compared to that of photoluminescence (PL) excited by a He–Ne laser. The experimental results also show that the peak energy of the luminescence is almost independent of the external applied voltage. However, the detailed mechanism for the luminescence induced by the intense mid-IR radiation only remains unidentified in the present study.

Anisotropic Zeeman splitting in semimagnetic quantum-well structures

The Zeeman splitting pattern in semimagnetic superlattices and asymmetric double quantum wells composed of different sequences of semimagnetic and nonmagnetic well and barrier materials have been investigated experimentally and theoretically, in particular, in dependence on the orientation of an external magnetic field and the lattice mismatch induced biaxial strain. The magneto-optical anisotropy, which appears by varying the tilting angle of the magnetic field, can be explained by coupling of heavy- and light-hole states of the ${\ensuremath{\Gamma}}_{8}$ valence band. Theoretical results have been obtained by transfer matrix method and are compared with experimental photoluminescence and photoluminescence excitation spectra obtained on superlattices and asymmetric double quantum wells.