Nonradiative recombination effect on photoluminescence decay dynamics in GaInNAs/GaAs quantum wells

The influence of nonradiative recombination on the photoluminescence (PL) decay dynamics in GaInNAs/GaAs quantum wells is studied by time-resolved photoluminescence under various excitation intensities. It is found that the PL decay process strongly depends on the excitation intensity. In particular, under the moderate excitation levels the PL decay curves exhibit unusual nonexponential behavior and show a convex shape. By introducing a new parameter of the effective concentration of nonradiative recombination centers into a rate equation, the observed results are well simulated. The cw PL data further demonstrate the nonradiative recombination effect on the optical properties of GaInNAs/GaAs quantum wells. (c) 2006 American Institute of Physics.

Energy spectra of two-electron two-dimensional quantum dots confined by elliptical and bowl-like potentials

The laterally confining potential of quantum dots (QDs) fabricated in semiconductor heterostructures is approximated by an elliptical two-dimensional harmonic-oscillator well or a bowl-like circular well. The energy spectrum of two interacting electrons in these potentials is calculated in the effective-mass approximation as a function of dot size and characteristic frequency of the confining potential by the exact diagonalization method. Energy level crossover is displayed according to the ratio of the characteristic frequencies of the elliptical confinement potential along the y axis and that along the x axis. Investigating the rovibrational spectrum with pair-correlation function and conditional probability distribution, we could see the violation of circular symmetry. However, there are still some symmetries left in the elliptical QDs. When the QDs are confined by a "bowl-like" potential, the removal of the degeneracy in the energy levels of QDs is found. The distribution of energy levels is different for the different heights of the barriers. (C) 2003 American Institute of Physics.

The evolution of InAs/InAlAs/InGaAlAs quantum dots after rapid thermal annealing

We have studied how the optical properties of InAs self-assembled quantum dots (QDs) grown on GaAs substrate are affected when depositing an InAlAs/InGaAs combination overgrowth layer directly on it by rapid thermal annealing (RTA). The photoluminescence measurement demonstrated that the InAs QDs experiences an abnormal variation during the course of RTA. The model of transformation of InAs-InAlAs-InGaAlAs could be used to well explain the phenomena. (C) 2003 Elsevier Science B.V. All rights reserved.

Optical investigation on the annealing effect and its mechanism of GaInAs/GaNAs quantum wells

Thermal annealing of GaInAs/GaNAs quantum wells (QWs) as well as other nitrogen- and indium-contained QW structures grown by molecular beam epitaxy and its effect on optical properties are investigated. The photoluminescence (PL) and photovoltaic (PV) spectra of annealed GaInAs/GaNAs QWs show that the luminescence properties become degraded due to the N diffusion from the GaNAs barrier layers to the GaInAs well layer. Meantime, the annealing-induced blueshift of the PL peak in this QW system is mainly induced by the change of In distribution, suggesting that the In reorganization is greatly assisted by the N-induced defects. The elucidation of annealing effect in GaInAs/GaNAs QW samples is helpful for a better understanding to the annealing effect in the GaInNAs/GaAs QWs. (C) 2003 Elsevier Science B.V. All rights reserved.

Electron ground state energy level determination of ZnSe self-organized quantum dots embedded in ZnS

Optical and electrical characterization of the ZnS self-organized quantum dots (QDs) embedded in ZnS by molecular beam epitaxy have been investigated using photoluminescence (PL), capacitance-voltage (C-V), and deep level transient Fourier spectroscopy (DLTFS) techniques. The temperature dependence of the free exciton emission was employed to clarify the mechanism of the PL thermal quenching processes in the ZnSe QDs. The PL experimental data are well explained by a two-step quenching process. The C-V and DLTFS techniques were used to obtain the quantitative information on the electron thermal emission from the ZnSe QDs. The correlation between the measured electron emission from the ZnSe QDs in the DLTFS and the observed electron accumulation in the C-V measurements was clearly demonstrated. The emission energy for the ground state of the ZnSe QDs was determined to be at about 120 meV below the conduction band edge of the ZnS barrier, which is in good agreement with the thermal activation energy, 130 meV, obtained by fitting the thermal quenching process of the free exciton PL peak. (C) 2003 American Institute of Physics.

Structure and photoluminescence study of InGaAs/GaAs quantum dots grown via cycled (InAs)(n)/(GaAs)(n) monolayer deposition

We report the morphology of an InGaAs nanostructure grown by molecular beam epitaxy via cycled (InAs)(n)/(GaAs)(n) monolayer deposition. Atomic force microscopy images clearly show that varying monolayer deposition per cycle has significant influence on the size, density and shape of the InGaAs nanostructure. Low-temperature photoluminescence spectra show the effect of n on the optical quality, and 1.35mum photoluminescence with a linewidth of only 19.2meV at room temperature has been achieved in the (InAs)(1)/(GaAs)(1) structure.

Temperature dependence of photoluminescence of flat and undulated SiGe/Si multiple quantum wells

Photoluminescence (PL) of strained SiGe/Si multiple quantum wells (MQW) with flat and undulated SiGe well layers was studied at different temperature. With elevated temperature from 10K, the no-phonon (NP) peak of the SiGe layers in the flat sample has firstly a blue shift due to the dominant transition converting from bound excitons (BE) to free excitons (FE), and then has a red shift when the temperature is higher than 30K because of the narrowing of the band gap. In the undulated sample, however, monotonous blue shift was observed as the temperature was elevated from 10 K to 287 K. The thermally activated electrons, confined in Si due to type-II band alignment, leak into the SiGe crest regions, and the leakage is enhanced with the elevated temperature. It results in a blue shift of the SiGe luminescence spectra.

Symmetry in the diagonal self-assembled InAs quantum wire arrays on InP substrate

Diagonal self-assembled InAs quantum wire (QWR) arrays with the stacked InAs/In0.52Al0.48As structure are grown on InP substrates, which are (001)-oriented and misoriented by 6degrees towards the [100] direction. Both the molecular beam epitaxy (MBE) and migration enhanced epitaxy (MEE) techniques are employed. Transmission electron microscopy reveals that whether a diagonal InAs QWR array of the stacked InAs/InAlAs is symmetrical about the growth direction or not depends on the growth method as well as substrate orientation. Asymmetry in the diagonal MEE-grown InAs QWR array can be ascribed to the influence of surface reconstruction on upward migration of adatoms during the self-assembly of the InAs quantum wires.

Electronic structure and optical properties of quantum rods with wurtzite structure

The Hamiltonian of the wurtzite quantum rods with an ellipsoidal boundary is given after a coordinate transformation. The energies, wave functions, and transition possibilities are obtained as functions of the aspect ratio e with the same method we used on spherical dots. With an overall consideration of both the transition matrix element and the Boltzmann distribution we explained why the polarization factor increases with increasing e and approaches a saturation value, which tallies quite well with the experimental result. When e increases more and more S-z states are mixed into the ground, second, and third states of J(z)=1/2, resulting in an increase of the emission of z polarization. It is just the linear terms of the momentum operator in the hole Hamiltonian that cause the mixing of S and P states in the hole ground state. The effects of the crystal field splitting energy, temperature, and transverse radius to the polarization are also considered. We also calculated the band gap variation with the size and shape of the quantum rods.

Anomalous temperature dependence of photoluminescence in GaInNAs/GaAs multiple quantum wells

Photoluminescence (PL) spectra of GaInNAs/GaAs multiple quantum wells grown on a GaAs substrate by molecular beam epitaxy are measured in a range of temperatures and excitation power densities. The energy position of the dominant PL peak shows an anomalous S-shape temperature dependence instead of the Varshni relation. By careful inspection, especially for the PL under lower excitation power density, two near bandedge peaks are well identified. These are assigned to carriers localized in nitrogen-induced bound states and interband excitonic recombinations, respectively. It is suggested that the temperature-induced switch of such two luminescence peaks in relative intensity causes a significant mechanism responsible for the S-shape shift observed in GaInNAs. A quantitative model based on the thermal depopulation of carriers is used to explain the temperature dependence of the PL peak related to N-induced bound states.

Collective and single-particle excitations in coupled quantum wires in magnetic fields

The full spectra of magnetoplasmons and single-particle excitations are obtained of coupled one-dimensional electron gases in parallel semiconductor quantum wires with tunneling. We show the effects of the interwire Coulomb interaction and the tunneling, as well as the magnetic-field-induced localization on the elementary excitations in symmetric and asymmetric coulped quantum wire structures. The interacton and resonance between the plasmon and the intersubband single-particle excitations are found in magnetic fields.

Impact of polarization on quasi-two-dimensional exciton and barrier-width dependence of the exciton associated transition in wurtzite III-V nitride quantum wells

Excitonic states in AlxGa1-xN/GaN quantum wells (QWs) are studied within the framework of effective-mass theory. Spontaneous and piezoelectric polarizations are included and their impact on the excitonic states and optical properties are studied. We witnessed a significant blue shift in transition energy when the barrier width decreases and we attributed this to the redistribution of the built-in electric field between well layers and barrier layers. For the exciton the binding energies, we found in narrow QWs that there exists a critical value for barrier width, which demarcates the borderline for quantum confinement effect and the quantum confined Stark effect. Exciton and free carrier radiative lifetimes are estimated by simple argumentation. The calculated results suggest that there are efficient non-radiative mechanisms in narrow barrier QWs. (C) 2002 Elsevier Science Ltd. All rights reserved.

Optical and electrical characterizations of ZnSe self-organized quantum dots grown by molecular beam epitaxy

Optical and electrical properties of ZnSe self-organized quantum dots were investigated using photoluminescence, capacitance-voltage, and deep level transient Fourier spectroscopy techniques. The temperature dependence of photoluminescence was employed to clarify the mechanism of photoluminescence thermal quenching processes in ZnSe quantum dots. A theoretic fit on considering a two-step quenching processes well explained the experimental data. The apparent carrier concentration profile obtained from capacitance-voltage measurements exhibits an accumulation peak at the depth of about 100nm below the sample surface, which is in good agreement with the location of the quantum dot layer. The electronic ground state of ZnSe quantum dots is determined to be about 0.11 eV below the conduction band of ZnS, which is similar to that obtained by simulating the thermal quenching of ZnSe photoluminescence.

Binding energy of positively and negatively charged excitons in GaAs/AlxGa1-xAs quantum wells

Using a simple two-parameter wavefunction, we calculate variationally the binding energy of positively and negatively charged excitons in GaAs/AlxGa1-xAs quantum wells for well widths from 10 to 300Angstrom. We consider the effect of effective mass, dielectric constant mismatch in the two materials, and the whole correlation among the particles. The results are discussed and compared in detail with previous experimental and theoretical results, which show fair agreement with them.

Photoluminescence study of CdSexS1-x quantum dots in a glass spherical microcavity

The photoluminescence (PL) of CdSexS1-x semiconductor quantum dots (QDs) in a glass spherical microcavity is investigated. The CdSexS1-x semiconductor clusters embedded in a glass matrix are fabricated by using the heat treatment method. Periodical structures consisting of sharp spectral lines are observed in the PL spectra of CdSexS1-x QDs, which can be well explained by the coupling with the whispering gallery modes of the spherical microcavity based on Mie scattering theory.