Photon-assisted tunneling in coupled double quantum wells

Using different techniques to individually contact two closely spaced electron gases, we study the tunneling characteristics between weakly coupled GaAs quantum wells, with and without resonant far-infrared excitation. We find that for barriers as thick as 300 Å, the alignment between the subbands in the wells can be observed as an increased tunneling conductivity. To study photon-assisted tunneling in our samples, we use the cyclotron resonance as a strong, tunable electronic excitation in the far-infrared. When the Landau-level spacing [hstrok]ω_c corresponds to the laser energy [hstrok]ω_L, the carriers are effectively pumped to higher Landau levels, which leads to a reduced resistance across the tunnel barrier. This photo-conductive signal is “doubly resonant” in that it is at maximum when ω_c coincides with ω_L, and at the same time the subbands of the two wells are aligned.     

Electroabsorption in narrow coupled double quantum wells: Coulombiccoupling effects

The effects of Coulombic coupling between different subband pairs on the electroabsorption spectra of narrow coupled double quantum wells (QWs) have been studied. It is shown via detailed comparison between electroabsorption spectra calculated using a full excitonic Green's function method, a decoupled excitonic Green's function method, a variational method, and experimental data that inclusion of the Coulombic coupling between different subband pairs is required for correct prediction of electroabsorption in the narrow well system. It is also shown that, due to Coulombic coupling, it is necessary to include unbound QW states, above the QW edge, in the simulation of electroabsorption and electrorefraction. These results are of particular significance for the accurate calculation of electrorefraction, by Kramers-Kronig transformation of QW electroabsorption spectra, in coupled QW structures containing narrow QWs     

The effect of resonant sublevel coupling on intersubband transitions in coupled double quantum wells

The effect of resonant sublevel coupling on intersubband transitions in double quantum wells is investigated using far-infrared spectroscopy. We study widely tuneable parabolic double quantum wells in which potential spikes of different energetic height and thickness provide tunnel barriers for the electron systems on either side of the barrier. The use of gate electrodes enables us to tune both the carrier densities as well as the respective sublevel spacings and allow for a manifold degeneracy scheme like in an artificial molecule where the atomic number of both partners can be intentionally changed. Depending on the actual experimental condition, we observe pronounced level anticrossings into a symmetric and antisymmetric state. This single-particle sublevel coupling manifests itself in a rich spectrum of the observed collective intersubband transitions which occur at the depolarization shifted intersubband energy.     

Transport properties of magnetoexcitons in coupled quantum wells

The transport times τ(P) and the mean free paths of magnetoexcitons in a quantum well and of spatially direct and indirect magnetoexcitons in coupled quantum wells in a random potential produced by thickness fluctuations of the quantum wells or a random distribution of impurity centers in quantum wells (P is the magnetic momentum of an exciton) are calculated. The function τ(P) is nonmonotonic, but as the distance D between the quantum wells increases, the maximum τ _max(P) gradually vanishes in the presence of scattering by surface terraces. As the magnetic field (H) increases, τ(0) decreases as for is the magnetic length) and as 1/H ² for D≫l. The behavior of the computed values of τ for large H agrees qualitatively with the experimental data. The mean free path length of a magnetoexciton exhibits at P≠0 a maximum whose magnitude decreases as the parameter D/l increases. Fiz. Tekh. Poluprovodn. 32, 596–601 (May 1998)     

Electrooptic effects in GaAs-AlGaAs narrow coupled quantum wells

The linear and quadratic electrooptic coefficients in narrow single and strongly coupled GaAs-Al_xGa_1-xAs quantum wells have been measured. The quadratic electrooptic effect is enhanced over that of conventional square quantum wells for both TE and TM polarization in all the structures considered, by up to six times in the case of 2-nm-wide GaAs-Al_0.2Ga_0.8As strongly coupled quantum wells. The origin of the enhanced quadratic electrooptic effect was found to correlate with a larger red shift in the absorption edge exciton and strong Coulombic coupling of the bound exciton states with the quasi-continua     

Intersubband absorption saturation in InGaAs-AlAs-AlAsSb coupled quantum wells

This paper gives saturation (switching) energy (I/sub S/) estimates in an all-optic switch material based on absorption saturation in InGaAs-AlAs-AlAsSb coupled-double quantum wells (cDQWs). A model based on density matrix theory for a 4-level system is used to simulate the pulsed excitation conditions of the experiment in the communication wavelength region. The theoretical estimates are compared with experimentally determined values. A comparison of the I/sub S/ in two different cDQWs (one with an indium composition in the well of about 53% and without an AlAs stopping layer at the well-barrier interface and another with 72% indium and an AlAs stopping layer) clearly shows that the samples with high indium content are of a better quality compared to those with lattice-matched indium composition (53%). An order-of-magnitude reduction in the I/sub S/ in an all-optic switch based on cDQWs with high indium content is reported.     

Tunneling effects in tilted magnetic fields in n-InGaAs/GaAs structures with strongly coupled double quantum wells

The effects of tunneling between two parallel two-dimensional electron gases in n-InGaAs/GaAs nanostructures with strongly coupled double quantum wells with a change in the in-plane component of a tilted magnetic field (up to B _‖ = 9.0 T) in the temperature range T = 1.8–70.0 K are investigated. A nonmonotonic temperature dependence of the inverse quantum lifetime τ _q ^? (T) is obtained from analysis of the dependence of the longitudinal resistance on the parallel component of the tilted magnetic field at fixed temperatures, ρ _xx (B _‖, T). The quadratic portion of this dependence is found to be due to the contribution of inelastic electron-electron scattering. The decrease in the inverse quantum lifetime τ _q ^? (T) at T > 0.1T _F cannot be described within known theories; it seems, it is not related to the processes of electron momentum relaxation.     

The cyclotron resonance in Heterostructures with the InSb/AlInSb quantum wells

The absorption of two-dimensional electrons in InSb-based quantum wells in the quantized magnetic fields in the terahertz spectral region are studied. A p-Ge-based cyclotron laser was used as the radiation source. The effective mass of carriers at the Fermi level equal to 0.0219m ₀ (m ₀ is the mass of a free electron) is determined from the cyclotron resonance spectra. It is shown that the electron spectrum is described by the Kane model in a wide range of magnetic fields. An anomalously pronounced splitting of the cyclotron resonance line not associated with the nonparabolicity of the conduction band of InAs is observed in low magnetic fields, which can be attributed to the effect of the spin-orbit interaction.     

耦合量子阱中激子凝聚研究新进展

玻色-爱因斯坦凝聚成为探索量子世界的一种新方法,而且在半导体纳米结构中激子的凝聚研究取得了很大进展.实验上利用耦合量子阱间接激子中电子和空穴在空间上的分离,显著提高了激子的冷却速度和寿命,成功地把激子冷却到1 K以下,观察到了激子的准凝聚状态.着重介绍冷激子系统凝聚现象、发光图案和宏观有序的激子态.理解这些简并激子系统的形成机理,为其在半导体纳米结构中最终实现玻色-爱斯坦凝聚提供新的机会.     

Hot exciton effects and relaxation in coupled quantum wells

By studying photoluminescence and photoluminescence excitation in coupled quantum wells, we have studied the influence of the tunneling time on the relaxation of the photoexcited carriers. The ability to vary the coupling strength by the barrier thickness, allowed us to evidence hot exciton relaxation effects as well as resonant Raman scattering.     

Electron transport in coupled quantum wells with double-Sided doping

The conductivity and Hall mobility have been measured in heterostructures with coupled quantum wells (QW) as functions of temperature and the QW width. If a tunnel-transparent barrier is inserted in the middle of a QW, the mobility increases in narrow wells and decreases in wide wells. The experimental data have been compared with the calculated dependences. It has been shown that the number of filled quantum-well subbands depends on the well width and the presence of a barrier. The magnetoresistance and Hall resistance were measured at a temperature of 4.2 K in the range of magnetic fields of 1–40 T. The filling of subbands was determined from a Fourier analysis of the Shubnikov-de Haas oscillations, and good agreement with the calculated data was obtained.     

Electric-field-induced refractive index changes in InGaAs-InAlAsasymmetric coupled quantum wells

Two kinds of InGaAs-InAlAs asymmetric coupled quantum wells (QW's) were numerically analyzed in an effort to enhance change in the refractive index (Δn) at longer wavelengths where absorption coefficient is small. The analysis reveals the operating voltage, Δn, chirp parameter, Δn/Δk, (Δk: change in the extinction coefficient) and figure of merit, Δn/α, (α: absorption coefficient) can be improved by these two kinds of the QW's. This improvement is caused by an abrupt electric-field-induced increase or decrease in the oscillator strength for the lowest heavy hole exciton     

Intersubband relaxation of hot carriers in coupled quantum wells

We compare experimental results for the photoluminescence decay in coupled quantum wells with a Monte Carlo simulation of electron and hole dynamics in this structure. The results support the assumption that the measured decay is associated with intersubband relaxation of electrons with little effect due to the holes. Quantitative agreement is found between the measured and theoretical time constants for most fields, although the theoretical values are shorter than the corresponding experimental values close to resonance.     

Relaxation of excitons in semimagnetic asymmetric double quantum wells

The steady-state circular-polarized photoluminescence in semimagnetic asymmetric double quantum wells based on Cd(Mn,Mg)Te is studied thoroughly in relation to the polarization of intrawell nonresonance photoexcitation in magnetic fields Bup to 9 T. In low fields B, in which the exciton in the magnetic well is higher in energy than the exciton in the nonmagnetic well, the complete interwell relaxation of excitons is observed. In fields higher than B _c = 3–6 T, at which the exciton level in the magnetic well crosses the field-independent exciton level in the nonmagnetic well, the magnetic-field-induced red shift of the exciton in the magnetic well is accompanied by the establishment of a nonequilibrium distribution of excitons. This suggests that spin relaxation plays an important part in the interwell separation of excitons in the spin-dependent potential of the heterostructure. The efficiency of spin relaxation is controlled by mixing of valence band states in the nonmagnetic well and by splitting of heavy and light holes Δ _hh-lh . Different modes of interwell tunneling are observed in different field regions separated by the field B _c ^* > B _c corresponding to the crossing of the localized excitons in the nonmagnetic well and free excitons in the magnetic well. Possible mechanisms of interwell tunnel relaxation are discussed.     

Nearly chirp-free electroabsorption modulation usingInGaAs-InGaAlAs-InAlAs coupled quantum wells

We present an electroabsorption modulator based on slightly asymmetric InGaAs-InGaAlAs-InAlAs coupled quantum wells operated in the normally-off mode. The device exhibits a large change of the absorption coefficient in the vicinity of the zero-bias exciton peak wavelength with a very small change of the refractive index. The maximum excursion of the chirp parameter over the entire span of the bias voltage is less than ±0.1 for a specific wavelength or ±0.4 over a 12~14 -nm range. This structure is promising for low-chirp, high bit-rate electroabsorption modulators     

Magnetospectroscopy of double HgTe/CdHgTe quantum wells

The magnetoabsorption spectra in double HgTe/CdHgTe quantum wells (QWs) with normal and inverted band structures are investigated. The Landau levels in symmetric QWs with a rectangular potential profile are calculated based on the Kane 8 × 8 model. The presence of a tunnel-transparent barrier is shown to lead to the splitting of states and “doubling” of the main magnetoabsorption lines. At a QW width close to the critical one the presence of band inversion and the emergence of a gapless band structure, similar to bilayer graphene, are shown for a structure with a single QW. The shift of magnetoabsorption lines as the carrier concentration changes due to the persistent photoconductivity effect associated with a change in the potential profile because of trap charge exchange is detected. This opens up the possibility for controlling topological phase transitions in such structures.     

A high-speed intersubband modulator based on quantum interferencein double quantum wells

Calculations on a modulator based on quantum interference in AlGaAs/GaAs asymmetric double quantum wells (QWs) are performed. The modulation of the absorption is based on the anti-crossing behavior of the two lowest states in the coupled wells. At anti-crossing, the oscillator strengths of the transitions from these two lowest states to a higher state are changed in opposite directions. The width of the barrier between the wells should be thick enough to allow a large change in oscillator strength with applied field, yet thin enough so that the absorption peaks of the transitions are resolved. The QWs are designed so that one absorption peak has only a small energy shift for the transition used for modulation while the absorption varies rapidly with the applied voltage. A complete structure including a surface plasmon waveguide is proposed enabling calculations of modal absorption. Parameters important for the performance of the modulator are then determined. An extinction ratio of 10 dB at a wavelength of 8.4 μm is predicted for a device length of 18 μm and a peak-to-peak voltage of 0.9 V. The resistance-capacitance-limited 3-dB bandwidth is 130 GHz. The predicted performance compares very favorably with present interband modulators based on the quantum-confined Stark effect     

Light-scattering determination of electron tunneling gaps in double quantum wells

Resonant inelastic light-scattering techniques have been used to measure directly the single-particle tunneling gap (Δ_SAS) in Al_xGa_1−xAs/GaAs double quantum wells. We have observed a systematic decrease in Δ_SAS with increasing height of the barrier in agreement with the Δ_SAS calculated self-consistently within a Hartree approximation.     

Quasi-analytical study of the energy levels in double quantum wells

In this work we develop an analytical expression to the eigenvalue equation for the double quantum well (DQW), to describe the energy gap between resonant levels. The calculation of the energy gap does not involve the evaluation of the wavefunctions for the DQW nor the transfer or tunneling integrals in contrast to how it is usually done.