Effect of heteroboundary spreading on the properties of exciton states in Zn(Cd)Se/ZnMgSSe quantum wells

Exciton states in Zn(Cd)Se/ZnMgSSe quantum wells with different diffusion spreading of interfaces are studied by optical spectroscopy methods. It is shown that the emission spectrum of quantum wells at low temperatures is determined by free excitons and bound excitons on neutral donors. The nonlinear dependence of the stationary photoluminescence intensity on the excitation power density and the biexponential luminescence decay are explained by the neutralization of charged defects upon photoexcitation of heterostructures. With the stationary illumination on, durable (about 40 min) reversible changes in the reflection coefficient near the exciton resonances of quantum wells are observed. It is shown that, along with the shift of exciton levels, the spreading of heteroboundaries leads to three effects: an increase in the excitonphonon interaction, an increase in the energy shift between the emission lines of free and bound excitons, and a decrease in the decay time of exciton luminescence in a broad temperature range. The main reasons for these effects are discussed.     

Exciton-exciton collisions and conversion of interwell excitons in GaAs/AlGaAs superlattices

The ratio of the densities of intra-and interwell excitons in a symmetric system of coupled quantum wells — a superlattice based on a GaAs/AlGaAs heterostructure — is investigated over a wide range of optical excitation power densities. Conversion of interwell excitons into intrawell excitons as a result of exciton-exciton collisions is observed at high exciton densities. Direct evidence for such a conversion mechanism is the square-root dependence of the interwell exciton density on the optical excitation level. The decrease in the lifetime of interwell excitons with increasing excitation density, as measured directly by time-resolved spectroscopy methods, confirms the explanation proposed for the effect. Pis’ma Zh. éksp. Teor. Fiz. 65, No. 8, 623–628 (25 April 1997)     

Excitonic transitions in Be-doped GaAs/AlAs multiple quantum well

A series of GaAs/AlAs multiple-quantum wells doped with Be is grown by molecular beam epitaxy. The photoluminescence spectra are measured at 4, 20, 40, 80, 120, and 200 K, respectively. The recombination transition emission of heavy-hole and light-hole free excitons is clearly observed and the transition energies are measured with different quantum well widths. In addition, a theoretical model of excitonic states in the quantum wells is used, in which the symmetry of the component of the exciton wave function representing the relative motion is allowed to vary between the two- and threedimensional limits. Then, within the effective mass and envelope function approximation, the recombination transition energies of the heavy- and light-hole excitons in GaAs/AlAs multiple-quantum wells are calculated each as a function of quantum well width by the shooting method and variational principle with two variational parameters. The results show that the excitons are neither 2D nor 3D like, but are in between in character and that the theoretical calculation is in good agreement with the experimental results.     

Exciton/plasmon mixing in metal–semiconductor heterostructures

We report on the strong coupling between surface plasmons and inorganic quantum well excitons. The sample is formed by a corrugated silver film deposited on the top of a heterostructure consisting of five GaAs/GaAlAs quantum wells grown by molecular beam epitaxy. Reflectometry experiments at low temperature (77 K) evidence the formation of plasmon/heavy-hole exciton/light-hole exciton mixed states. The interaction energies, deduced by fitting the experimental data with a coupled oscillator model, amount to 22 meV for the plasmon/light-hole exciton and 21 meV for the plasmon/heavy-hole exciton. Some particularities of the plasmon–exciton coupling are also discussed and qualitatively related to the plasmon polarization.     

Temperature dependent time-resolved exciton luminescence in GaAs/AlGaAs quantum wires and dots

Transient photoluminescence of GaAs/AlGaAs quantum wires and quantum dots formed by strain confinement is studied as a function of temperature. At low temperature, luminescent decay times of the wires and dots correspond to the radiative decay times of localized excitons. The radiative decay time can be either longer or shorter than that of the host quantum well, depending on the size of the wires and dots. For small wires and dots (∼ 100 nm stressor), the exciton radiative recombination rate increases due to lateral confinement. Exciton localization due to the fluctuation of quantum well thickness plays an important role in the temperature dependence of luminescent decay time and exciton transfer in quantum wire and dot structures up to at least ∼ 80 K. Lateral exciton transfer in quantum wire and dot structures formed by laterally patterning quantum wells strongly affects the dynamics of wire and dot luminescence. The relaxation time of hot excitons increases with the depth of strain confinement, but we find no convincing evidence that it is significantly slower in quasi 1-D or 0-D systems than in quantum wells.     

Ultrafast breathinglike oscillation in the exciton density of ZnSe quantum wells

The spatial density profile of a low-density exciton ensemble in ZnSe quantum wells shows a breathinglike oscillation on a 30-ps time scale. This breathing results from the emission of the first acoustic phonon at the end of the quasiballistic transport phase of the excitons which reverses their direction of propagation. Since the scattering destroys the phase of the excitonic wave function, one can deduce simultaneously the coherence length and the coherence time of excitonic transport by evaluation of the oscillation measured from a single experiment. The breathing, which can be modeled by Monte Carlo simulations, is quenched for rising lattice temperature, i.e., increasing phonon absorption, and in samples with significant disorder. These results were obtained by time-resolved nanophotoluminescence with 5 ps and 250 nm temporal and spatial resolution, respectively.     

Coherence length of excitons in a semiconductor quantum well

We report on the first experimental determination of the coherence length of excitons in semiconductors using the combination of spatially resolved photoluminescence with phonon sideband spectroscopy. The coherence length of excitons in ZnSe quantum wells is determined to be 300-400 nm, about 25-30 times the exciton de Broglie wavelength. With increasing exciton kinetic energy, the coherence length decreases slowly. The discrepancy between the coherence lengths measured and calculated by considering only the acoustic-phonon scattering suggests an important influence of static disorder.     

Exciton-exciton interaction in coupled quantum wells

The exciton-exciton interaction is investigated for spatially indirect excitons in coupled quantum wells. The Hartree-Fock and Heitler-London approaches are improved by a full two-exciton calculation including the van der Waals effect. Using these potentials for the singlet and triplet channel, the two-body scattering matrix is calculated and employed to derive a modified relation between exciton density and blue shift. Such a relation is of central importance for gauging exciton densities on the way toward Bose condensation.     

Barrier-disorder induced exciton relaxation via LO-phonons in GaAs/Al x Ga1−x As multiple quantum wells

Hot exciton relaxation is observed in GaAs/Al _x Ga_1–x As multiple quantum wells. The photolumnescence excitation spectra of the localized exciton emission at low temperatures and excitation densities are composed of narrow equidistant peaks exactly separated by the GaAs LO-phonon energy (36 meV). The relaxation mechanism via LO-phonons is found to be important for localized excitons in multiple quantum wells with GaAs layer thicknesses of about 50 Å, where pronounced alloy fluctuations in the barriers provide a strong additional lateral potential which suppresses the dissociation of hot excitons.     

Influence of acoustic phonons on dephasing of free excitons in quantum wells

A theory of the dephasing rate of quasi-2D free excitons due to acoustic phonon interaction at low exciton densities is presented. Both deformation potential and piezoelectric couplings are considered for the exciton–phonon interaction in quantum wells. Using the derived interaction Hamiltonian obtained recently by us, exciton linewidth and dephasing rate are calculated as a function of the exciton density, exciton temperature, exciton momentum and lattice temperature.     

Temperature and density dependence of exciton lifetimes in GaAs/AlGaAs multiple quantum wells

The photoluminescence linewidths and excition lifetimes of free excitons in GaAs/AlGaAs multiple quantum wells were systematically investigated as a function of temperature, quantum well width, and carrier density. The experimental results showed that the exciton decay processes were strongly related to the linewidth of the exciton and the exciton binding energy.     

Signatures of stimulated bosonic exciton-scattering in semiconductor luminescence

We observe signatures of stimulated bosonic scattering of excitons, a precursor of Bose-Einstein-Condensation (BEC), in the photoluminescence of semiconductor quantum wells. The optical decay of a spinless molecule of two excitons (biexciton) into an exciton and a photon with opposite angular momenta is subject to bosonic enhancement in the presence of other excitons. In a gas of biexcitons and spin polarized excitons the bosonic enhancement breaks the symmetry of two equivalent biexciton decay channels leading to circularly polarized luminescence of the biexciton with the sign opposite to the circularly polarized exciton luminescence. Comparison of experiment and many body theory clearly indicates the existence of stimulated exciton-scattering, but excludes the presence of a fully condensed BEC-like state.     

Picosecond dynamics and stimulated emissions of excitons in Zn1-xCdxSe/ZnSe quantum wells

The dynamics and stimulated emission processes of the exciton luminescence are studied in quantum wells (QWs) of the Zn_1-xCd_xSe/ZnSe system. A multiquantum well (MQW) structure shows an exciton lifetime of 150-280 ps and a stimulated emission effect due to exciton-exciton scattering as well as due to electron-hole plasma recombination. A combined-QW structure in which a single quantum well (SQW) is located adjacent to MQWs shows a tunneling process of the excitons from the MQWs through the barriers to the SQW. The stimulated emission takes place in the SQW due to phase space filling effects of the excitons. These observed stimulated emission processes are highly related to the blue-laser-diode operation at both low and room temperatures.     

Determination of the exciton formation in quantum wells from time-resolved interband luminescence

We present the results of a detailed time-resolved luminescence study carried out on a very high quality InGaAs quantum well sample where the contributions at the energy of the exciton and at the band edge can be clearly separated. We perform this experiment with a spectral resolution and a sensitivity of the setup, allowing us to keep the observation of these two separate contributions over a broad range of times and densities. This allows us to directly evidence the exciton formation time, which depends on the density as expected from theory. We also denote the dominant contribution of excitons to the luminescence signal, and the lack of thermodynamical equilibrium at low densities.     

Time-resolved studies of single quantum dots in magnetic fields

We present time-resolved and time-integrated spectroscopy of single InAs quantum dots grown in a GaAs matrix. We observe a number of interesting features in the spectra, including the zero field splitting of exciton and biexciton lines due to quantum dot asymmetry. By the application of an in-plane magnetic field, the normally optically active and inactive exciton states become mixed, enabling us to optically probe the normally inaccessible ‘dark’ states. Time resolved measurements on the mixed states show decay times several times longer than the exciton lifetime at zero field, which we show to be consistent with a dark exciton lifetime orders of magnitude longer than that for bright exciton.     

Stimulated scattering of indirect excitons in coupled quantum wells: signature of a degenerate Bose-gas of excitons.

We observe and analyze strongly nonlinear photoluminescence kinetics of indirect excitons in GaAs/AlGaAs coupled quantum wells at low bath temperatures, > or = 50 mK. The long recombination lifetime of indirect excitons promotes accumulation of these Bose particles in the lowest energy states and allows the photoexcited excitons to cool down to temperatures where the dilute 2D gas of indirect excitons becomes statistically degenerate. Our main result--a strong enhancement of the exciton scattering rate to the low-energy states with increasing concentration of the indirect excitons--reveals bosonic stimulation of exciton scattering, which is a signature of a degenerate Bose-gas of excitons.     

Exciton–phonon interaction in Al_(0.4)Ga_(0.6)N/Al_(0.53)Ga_(0.47)N multiple quantum wells

The exciton-phonon interaction in Al_(0.4)Ga_(0.6)N/Al_(0.53)Ga_(0.47)N multiple quantum wells(MQWs) is studied by deepultraviolet time-integrated and time-resolved photoluminescence(PL).Up to four longitudinal-optical(LO) phonon replicas of exciton recombination are observed,indicating the strong coupling of excitons with LO phonons in the MQWs.Moreover,the exciton-phonon coupling strength in the MQWs is quantified by the Huang-Rhys factor,and it keeps almost constant in a temperature range from 10 K to 120 K.This result can be explained in terms of effects of fluctuations in the well thickness in the MQWs and the temperature on the exciton-phonon interaction.     

CdTe/CdZnTe多量子阱激子复合动力学性质的研究

报道了分子束外延制备的高质量CdTe/Cd_0.64Zn_0.36Te多量子阱结构的光学性质,由变温光致发光光谱讨论了随温度升高辐射线展宽和辐射复合效率降低的机理.在变密度激发的皮秒时间分辨光谱中,发现不同激发密度下发光衰减时间不同,并研究了它的机理.在高激发密度下观测到n=2的重空穴激子发光. 关键词：     

Exciton polaritons and their one-dimensional localization in disordered quantum-well structures

A theory of the Anderson localization of light in randomly arranged ultrathin layers (quantum wells) uniform in lateral dimensions and possessing intrinsic optical resonances is put forward. To solve the multiple-scattering problem, a model of layers with a δ-function resonance dielectric polarization is proposed. The model is an electromagnetic counterpart of the electronic model of zero-radius potentials. Interlayer disorder is included under the assumption of a low average concentration of identical layers in order to calculate analytically the one-and two-photon characteristics of electromagnetic-radiation transport, in particular, the average energy density and the Anderson localization length of light. The analysis is carried out for a structure with randomly distributed quantum wells in which quasi-two-dimensional excitons of different quantum wells are in resonance while their wave functions do not overlap. It is shown that the average electromagnetic field propagates through this disordered structure in the form of polaritons but are produced in exciton reemission between quantum wells. The localization length of light in the polariton spectral region decreases substantially, because the scattering (reflection) of light by individual quantum wells grows near the excitonic resonance.     

Collective state of interwell excitons in GaAs/AlGaAs double quantum wells under pulse resonance excitation

The time evolution and kinetics of photoluminescence (PL) spectra of interwell excitons in double GaAs/AlGaAs quantum wells (n-i-n structures) have been investigated under the pulse resonance excitation of intrawell 1sHH excitons using a pulsed tunable laser. It is found that the collective exciton phase arises with a time delay relative to the exciting pulse (several nanoseconds), which is due to density and temperature relaxation to the equilibrium values. The origination of the collective phase of interwell excitons is accompanied by a strong narrowing of the corresponding photoluminescence line (the line width is about 1.1 meV), a superlinear rise in its intensity, a long time in the change of the degree of circular polarization, a displacement of the PL spectrum toward lower energies (about 1.5 meV) in accordance with the filling of the lowest state with the exciton Bose condensate, and a significant increase in the radiative decay rate of the condensed phase. The collective exciton phase arises at temperatures T<6 K and interwell exciton densities n=3×10¹⁰ cm^?2. Coherent properties of the collective phase of interwell excitons and experimental manifestations of this coherence are discussed.