Charge carrier recombination mechanisms in Sb-containing quantum well laser structures

The dynamics of interband luminescence in structures with InGaAsSb-based quantum wells and barriers of different types was studied at different temperatures and excitation levels. The lifetime of optically injected charge carriers in quantum wells at different temperatures and optical excitation levels was determined. An increased recombination rate in structures with deep electron quantum wells was discovered; it is associated with the occurrence of resonance Auger recombination. It was concluded that the application of quinary solid solutions as barriers in laser structures for a 3—4 fum wavelength range is to be preferred.     

Effect of the excitation level on the thermal quenching and dynamics of the photoluminescence of GaAs/AlGaAs shallow quantum well structures

The effect of the excitation level on the dynamics of heavy-hole exciton photoluminescence in tunneling-isolated GaAs/Al _x Ga_{1 − x} As (x = 0.05) shallow quantum wells at temperatures of 5 to 70 K is investigated. It is shown that the exciton lifetimes depend strongly on the excitation level, while the activation energies characterizing the thermal escape of nonequilibrium charge carriers from the wells virtually do not.     

Temperature dependence of photoluminescence intensity of self-assembled CdTe quantum dots in the ZnTe matrix under different excitation conditions

The temperature dependence of the integrated photoluminescence intensity of nanometer-sized ZnTe/CdTe/ZnTe quantum wells has been investigated under different excitation conditions. It has been shown that the character of thermal decay of the luminescence intensity depends on the frequency of the exciting light and, under the above-barrier excitation, strongly depends on the optical excitation power density. It has been found that an increase in the excitation intensity leads to a saturation of thermal quenching of the luminescence in the low-temperature range. The conclusion has been drawn that this behavior reflects the saturation of nonradiative recombination centers with photoexcited carriers.     

Excitation of the spin density and current by coherent light pulses in quantum wells

We study the orbital and spin dynamics of charge carriers induced by non-overlapping linearly polarized light pulses in semiconductor quantum wells. It is shown that such an optical excitation with coherent pulses leads to a spin orientation of photocarriers and an electric current. The effects are caused by the interference of optical transitions driven by individual pulses. The distribution of carriers in the spin and momentum spaces depends on the crystallographic orientation of quantum wells and can be efficiently controlled by the pulse polarizations, time delay and phase shift between the pulses, as well as an external magnetic field.     

Kinetics of an exciton-trion system in shallow GaAs/AlGaAs quantum wells

The formation of three-particle charged exciton complexes (trions) in shallow GaAs/AlGaAs quantum wells in the temperature range 1.7–15 K has been investigated by luminescence spectroscopy and resonant light scattering. The effect of the photon energy and the intensity of additional above-barrier illumination on the trion formation kinetics has been analyzed. It is established that, upon intrawell excitation, illumination leads to the formation of trions when the light photon energy corresponds to the regions of effective formation of trions in the photoluminescence excitation spectra. It is shown that, with an increase in the illumination level, the trion concentration first increases and then reaches a plateau since the quantum well acquires an electric charge whose field equalizes the electron and hole capture rates.     

Low-temperature photoluminescence in SiGe single quantum wells

1-x Ge_x single quantum wells (x=0.19) grown by rapid thermal chemical vapor deposition at 625 °C. A well-resolved strong excitonic luminescence with TO-phonon and no-phonon transitions with a full width at half-maximum as low as 6 meV is observed for a quantum well of 98 Å. The photoluminescence emission shows a significant blue shift and a broadening with excitation intensity. The results are analysed in terms of localization of photoinduced charge carriers at the heterointerfaces. Received: 11 September 1996/Accepted: 15 August 1997     

Defects with bright contrast in the induced-current mode in GaN-based light-emitting structures

InGaN/GaN-based light-emitting structures with multiple quantum wells have been studied by the electron beam induced current method. It is shown that the investigation of the dependence of the collected current on the beam energy allows us to evaluate the fraction of excess charge carriers recombining in quantum wells. Two types of defects with bright contrast have been found. It is shown that bright contrast is associated with the formation of enhanced conductivity channels across the structure with quantum wells. The dependence of the contrast on the beam current has been investigated.     

Photoinduced charge redistribution and its influence on excitonic states in Zn(Cd)Se/ZnMgSSe/GaAs quantum-well heterostructures

The photoinduced charge redistribution in Zn(Cd)Se/ZnMgSSe/GaAs quantum-well heterostructures under different conditions of optical excitation has been investigated using scanning probe microscopy and optical spectroscopy in the temperature range from 5 to 300 K. Excitation of the samples by radiation with a photon energy greater than the band gap of Zn(Cd)Se leads to the accumulation of electrons in quantum wells, which is detected using scanning spreading resistance microscopy. For moderate excitation densities (up to 25 W/cm²) and at temperatures ranging from 80 to 100 K, the density of a quasi-two-dimensional electron gas formed in quantum wells is several orders of magnitude higher than the density of electron-hole pairs generated by the excitation radiation. The excess electron concentration in the quantum well leads to a broadening of the exciton resonances and to an increase in the relative intensity of the donor-bound exciton emission line and also determines the increase in the luminescence quantum yield with increasing excitation intensity. An additional illumination with a photon energy less than the band gap of Zn(Cd)Se decreases the concentration of excess electrons in quantum wells. The influence of the additional illumination is observed at a temperature of approximately 100 K and almost completely suppressed at 5 K. The obtained results are explained in terms of the formation of a potential barrier for electrons at the ZnMgSSe/GaAs interface and by the specific features of recombination processes in the electron-hole system containing impurity centers with different charge states.     

Charge and energy transfer in double asymmetric quantum wells with quantum dots

The luminescence and luminescence excitation spectra of CdSe/ZnSe quantum dots are studied in a set of double quantum wells with the ZnSe barrier of width 14 nm, the same amount of a deposited CdSe layer forming a deep well and shallow wells with different depths. It is found that for a certain relation between the depths of shallow and deep wells in this set, conditions are realized under which the exciton channel in the luminescence excitation spectrum of a shallow well dominates in the region of kinetic exciton energies exceeding 10 longitudinal optical phonons above the bottom of the exciton band of the ZnSe barrier. A model is developed for the transfer of electrons, holes, and excitons between the electronic states of shallow and deep quantum wells separated by wide enough barriers. It is shown that the most probable process of electronic energy transfer between the states of shallow and deep quantum wells is indirect tunneling with the simultaneous excitation of a longitudinal optical phonon in the lattice. Because the probability of this process for single charge carriers considerably exceeds the exciton tunneling probability, a system of double quantum wells can be prepared in which, in the case of weak enough excitation, the states of quantum dots in shallow quantum wells will be mainly populated by excitons, which explains experimental results obtained.     

Excitonic state in quantum wells formed from “above-barrier” electronic states

Lines corresponding to localized excitonic states formed from “above-barrier” electron and/or hole states (specifically, excitation lines of excitons formed by an electron localized in a QW and a free heavy hole) have been observed in the photoluminescence excitation spectra of GaAs/Al_0.05Ga_0.95As structures with quantum wells (QWs), each containing one single-particle size-quantization level for charge carriers of each type. A computational method is proposed that permits finding the binding energy and wave functions of excitons in QWs taking the Coulomb potential into account self-consistently. The computed values of the excitonic transition energies agree quite well with the experimental results. Pis’ma Zh. éksp. Teor. Fiz. 70, No. 9, 613–619 (10 November 1999)     

Direct Observation of Carrier Transportation Process in InGaAs/GaAs Multiple Quantum Wells Used for Solar Cells and Photodetectors

The resonant excitation is used to generate photo-excited carriers in quantum wells to observe the process of the carriers transportation by comparing the photoluminescence results between quantum wells with and without a p-n junction.It is observed directly in experiment that most of the photo-excited carriers in quantum wells with a p-n junction escape from quantum wells and form photocurrent rather than relax to the ground state of the quantum wells.The photo absorption coefficient of multiple quantum wells is also enhanced by a p-n junction.The results pave a novel way for solar cells and photodetectors making use of low-dimensional structure.     

Mechanisms for spontaneous and stimulated recombination in multiple quantum wells of InGaN/GaN heterostructures on silicon substrates

With the aim of establishing the mechanisms for spontaneous recombination and lasing, we have studied InGaN/GaN multiple quantum well heterostructures emitting in the 450 nm region and grown by organometallic vapor-phase epitaxy on silicon substrates using several mechanical stress-reducing AlN/AlGaN inserts. Photoluminescence (PL) excitation spectroscopy, the non-monoexponential nonequilibrium carrier relaxation kinetics, and x-ray diffractometry data indicate significant inhomogeneity of the InGaN solid solution in quantum wells of these structures. The dependences of the position of the photoluminescence spectra on the excitation level and the temperature, the large shift in the photoluminescence, gain, and lasing spectra relative to the absorption edge allow us draw the conclusion that the dominant contribution to spontaneous and stimulated recombination comes from nonequilibrium charge carriers localized in indium-rich InGaN clusters. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 75, No. 1, pp. 94–101, January–February, 2008.     

Rate of intersubband transitions of charge carriers in semiconductor quantum wells as a result of their Coulomb interaction

The intersubband scattering of charge carriers in semiconductor quantum wells as a result of their Coulomb interaction has been theoretically investigated. Analytical expressions for the rate of intersubband transitions in the process of electron—electron and electron—hole collisions have been derived in the Born approximation. The theoretical and experimental data on the photoluminescence decay time, obtained for the case of a nondegenerate distribution of charge carriers, were in qualitative agreement. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 72, No. 6, pp. 782–787, November–December, 2005.     

Relaxation of hot excitons in CdZnSe/ZnSe quantum wells and quantum dots

The relaxation dynamics of hot excitons was studied in (Zn,Cd)Se/ZnSe quantum wells and quantum dots. A fast population of the radiative excitonic ground state occurs for an excitation excess energy corresponding to an integer number of optical phonon energies. This is indicated by a spectrally narrow photoluminescence peak observed immediately after the exciting laser pulse. Spatial diffusion of excitons, controlled by the interaction between excitons and acoustic phonons, causes a distinct linewidth broadening with increasing delay time in quantum wells. In contrast, this process is found to be strongly suppressed in quantum dots.     

Ring-shaped spatial pattern of exciton luminescence formed due to the hot carrier transport in a locally photoexcited electron-hole bilayer

A consistent explanation of the formation of a ring-shaped pattern of exciton luminescence in GaAs/AlGaAs double quantum wells is suggested. The pattern consists of two concentric rings around the laser excitation spot. It is shown that the luminescence rings appear due to the in-layer transport of hot charge carriers at high photoexcitation intensity. Interestingly, one of two causes of this transport might involve self-organized criticality (SOC) that would be the first case of the SOC observation in semiconductor physics. We test this cause in a many-body numerical model by performing extensive molecular dynamics simulations. The results show good agreement with experiments. Moreover, the simulations have enabled us to identify the particular kinetic processes underlying the formation of each of these two luminescence rings.     

Heating and cooling of a two-dimensional electron gas by terahertz radiation

The absorption of terahertz radiation by free charge carriers in n-type semiconductor quantum wells accompanied by the interaction of electrons with acoustic and optical phonons is studied. It is shown that intrasubband optical transitions can cause both heating and cooling of the electron gas. The cooling of charge carriers occurs in a certain temperature and radiation frequency region where light is most efficiently absorbed due to intrasubband transitions with emission of optical phonons. In GaAs quantum wells, the optical cooling of electrons occurs most efficiently at liquid nitrogen temperatures, while cooling is possible even at room temperature in GaN heterostructures.     

Effect of electron-phonon interaction on the thermoelectric properties of superlattices

The electron relaxation time on acoustical phonons, the electrical conductivity, and the phonon-drag thermopower of a semiconductor superlattice with quasi-two-dimensional quantum wells are calculated. The inelasticity of the scattering of charge carriers is taken into account. It is shown that the phonon-drag thermopower of a superlattice can be an order of magnitude greater than the corresponding thermopower of a bulk semiconductor. Pis’ma Zh. éksp. Teor. Fiz. 69, No. 4, 290–295 (25 February 1999)     

Effect of free electron-hole pairs on the saturation of excitonic absorption in GaAs/AlGaAs quantum wells

The pump-probe experimental method is used to investigate the effect of photoexcited carriers on the dynamics of the exciton absorption spectra of GaAs / Al_xGa_1–x As-multilayer quantum wells. Use of the method of moment analysis for processing the results makes it possible to identify the simultaneous contribution of changes in oscillator strength and width of the exciton lines in the saturation of exciton absorption. It was found that the oscillator strength recovers its initial value in the course of the first 100–130 ps, whereas broadening and energy-shift of the exciton lines is observed for 700–800 ps. These are the first experimental measurements of the excitation densities at which the oscillator strength of the excitonic state saturates when the latter is perturbed only by free-electron-hole pairs, and when it is perturbed only by other excitons. Fiz. Tverd. Tela (St. Petersburg) 40, 1130–1133 (June 1998)     

Bistable charge states in a photoexcited quasi-two-dimensional electron-hole system

The luminescence spectra of GaAs/AlGaAs quantum wells (QWs) with low-density quasi-two-dimensional electron and hole channels were studied. It was demonstrated that, at temperatures below some critical value (T _c ～30 K) and for an excitation power lying in a certain temperature-dependent range, two metastable charge states with two-dimensional charge densities differing in both magnitude and sign can occur in the system under the same conditions. The obtained experimental data agree well with the mathematical model allowing for the transfer of photoexcited carriers to the barrier followed by their tunneling into QW.     

Modulation of the resonant Rayleigh light scattering spectrum of GaAs/AlGaAs structures with quantum wells under above-barrier illumination

It is found that additional illumination by photons with energies above the band gap width in barrier layers leads to a strong (up to 40% in depth at the values of the illumination power used in this work) modulation of the light intensity elastically scattered upon resonant excitation of exciton states in quantum wells of GaAs/AlGaAs structures. Evidently, the effect observed is associated with the redistribution of oscillator strengths of exciton transitions due to the formation of three-particle exciton complexes (trions). These complexes arise through preferred capture of nonequilibrium like charge carriers (in our case, holes).