Optical spectra of CdSe nanocrystals under hydrostatic pressure

Optical spectra of CdSe nanocrystals are measured at room temperature under pressure ranging from 0 to 5.2 GPa. The exciton energies shift linearly with pressure below 5.2 GPa. The pressure coefficient is 27 meV GPa(-1) for small CdSe nanocrystals with the radius of 2.4 nm. With the approximation of a rigid-atomic pseudopotential, the pressure coefficients of the energy band are calculated. By using the hole effective-mass Hamiltonian for the semiconductors with wurtzite structure under various pressures, we study the exciton states and optical spectra for CdSe nanocrystals under hydrostatic pressure in detail. The intrinsic asymmetry of the hexagonal lattice structure and the effect of spin-orbit coupling on the hole states are investigated. The Coulomb interaction of the exciton states is also taken into account. It is found that the theoretical results are in good agreement with the experimental values.

Temperature quenching mechanisms for photoluminescence of MBE-grown chlorine-doped ZnSe epilayers

We report on a detailed investigation on the temperature-dependent behavior of photoluminescence from molecular beam epitaxy (MBE)-grown chlorine-doped ZnSe epilayers. The overwhelming neutral donor bound exciton ((ClX)-X-0) emission at 2.797 eV near the band edge with a full-width at half-maximum (FWHM) of similar to 13 meV reveals the high crystalline quality of the samples used. In our experiments, the quick quenching of the (ClX)-X-0 line above 200 K is mainly due to the presence of a nonradiative center with a thermal activation energy of similar to 90 meV, The same activation energy and similar quenching tendency of the (ClX)-X-0 line and the I-3 line at 2.713 eV indicate that they originate from the same physical mechanism. We demonstrate for the first time that the dominant decrease of the integrated intensity of the I, line is due to the thermal excitation of the "I-3 center"-bound excitons to its free exciton states, leaving the "I-3 centers" as efficient nonradiative centers. The optical performance of ZnSe materials is expected to be greatly improved if the density of the "I-3 center" can be controlled. The decrease in the luminescence intensity at moderately low temperature (30-200 K) of the (ClX)-X-0 line is due to the thermal activation of neutral-donor-bound excitons ((ClX)-X-0) to free excitons. (C) 2000 Published by Elsevier Science B.V.

Steady and transient optical properties of cubic InGaN epilayers

Photoluminescence (PL) and time-resolved PI, were employed to study the steady and transient optical properties of cubic InxGa1-xN epilayers grown by MBE. The results suggest that the PL transitions in InGaN epilayers are mainly from localized exciton states. The localization energies are estimated to be 60 meV, independent of In composition. The PL decay is characterized by a hi-exponential dependence. The fast process (50 ps at 12K) is related to the fast relaxation of excitons, while a slower contribution (200-270 ps at 12K) is attributed to the decay process of localized excitons.

Effective-mass theory for InAs/GaAs strained superlattices

By using the recently developed exact effective-mass envelope-function theory, the electronic structures of InAs/GaAs strained superlattices grown on GaAs (100) oriented substrates are studied. The electron and hole subband structures, distribution of electrons and holes along the growth direction, optical transition matrix elements, exciton states, and absorption spectra are calculated. In our calculations, the effects due to the different effective masses of electrons and holes in different materials and the strain are included. Our theoretical results are in agreement with the available experimental data.

Effective-mass theory for InAs/GaAs strained coupled quantum dots

In the framework of effective-mass envelope-function theory, the optical transitions of InAs/GaAs strained coupled quantum dots grown on GaAs (100) oriented substrates are studied. At the Gamma point, the electron and hole energy levels, the distribution of electron and hole wave functions along the growth and parallel directions, the optical transition-matrix elements, the exciton states, and absorption spectra are calculated. In calculations, the effects due to the different effective masses of electrons and holes in different materials are included. Our theoretical results are in good agreement with the available experimental data.

Electronic structure and optical property of semiconductor nanocrystallites

Semiconductor nanostructures show many special physical properties associated with quantum confinement effects, and have many applications in the opto-electronic and microelectronic fields. However, it is difficult to calculate their electronic states by the ordinary plane wave or linear combination of atomic orbital methods. In this paper, we review some of our works in this field, including semiconductor clusters, self-assembled quantum dots, and diluted magnetic semiconductor quantum dots. In semiconductor clusters we introduce energy bands and effective-mass Hamiltonian of wurtzite structure semiconductors, electronic structures and optical properties of spherical clusters, ellipsoidal clusters, and nanowires. In self-assembled quantum dots we introduce electronic structures and transport properties of quantum rings and quantum dots, and resonant tunneling of 3-dimensional quantum dots. In diluted magnetic semiconductor quantum dots we introduce magnetic-optical properties, and magnetic field tuning of the effective g factor in a diluted magnetic semiconductor quantum dot. (C) 2004 Elsevier B.V. All rights reserved.

Spin-Polarized Localized Exciton Photoluminescence Dynamics in GaInNAs Quantum Wells

We have investigated spin polarization-related localized exciton photoluminescence (PL) dynamics in GaInNAs quantum wells by time-resolved PL spectroscopy. The emission energy dependence of PL polarization decay time as well as polarization-independent PL decay time suggests that the acoustic phonon scattering in the process of localized exciton transfer from the high-energy localized states to the low-energy ones dominates the PL polarization relaxation. By increasing the excitation power from 1 to 10 mW, the PL polarization decay time is changed from 0.17 to more than 1 ns, which indicates the significant effect of the trapping of localized electrons by nonradiative recombination centers. These experimental findings indicate that the spin-related PL polarization in diluted nitride semiconductors can be manipulated through carrier scattering and recombination process. (C) 2009 The Japan Society of Applied Physics

Observation of intershell and hybridized energy states in InAs/GaAs quantum dots

We have investigated the evolution of exciton state filling as a function of excitation power density in InAs/GaAs quantum dots (QDs). In addition to the emission bands of exciton recombination corresponding to the atom-like S, P, and D, etc. shells of quantum dots, it was observed that some extra states, P-' (between the S and P shells) and D-' (between the P and D shells), appear in the spectra with increasing number of excitons occupying the QDs. The emergence of these intershell excitonic levels is an experimental demonstration of strong exciton-exciton exchange interaction and coupling as well as state mixing and hybridization of a multiexciton system in quantum dots.

Optical properties and exciton localization in GaNas/GaAs

GaNAs/GaAs single quantum wells (SQWs) and dilute GaNAs bulk grown by molecular beam epitaxy(MBE) were studied by photoluminescence (PL), selectively-excited PL, and time-resolved PL. Exciton localization and delocalization were investigated in detail. Under short pulse laser excitation, the delocalization exciton emission was revealed in GaNAs/GaAs SQWs. It exhibits quite different optical properties from N-related localized states. In dilute GaNAs bulk, a transition of alloy band related recombination was observed by measuring the PL dependence on temperature and excitation intensity and time-resolved PL, as well. This alloy-related transition presents intrinsic optical properties. These results are very important for realizing the abnomal features of III-V-N semiconductors.

Electronic states in InAs quantum spheres and ellipsoids

The eight-band effective-mass Hamiltonian of the free-standing narrow-gap InAs quantum ellipsoids is developed, and the electron and hole electronic structures as well as optical properties are calculated by using the model. The energies, wave functions and transition probabilities of quantum spheres as functions of the radius of quantum sphere R is presented. It is found that the energy levels do not vary as 1/R-2, which is caused by the coupling between the conduction and valence bands, and by the constant terms correspond to the spin-orbit splitting energy. The blueshifts of hole states depend strongly on the coupling from electron states, so that the order of hole states changes as has been predicted in experiment. The exciton binding energies are calculated, the calculated excitonic gaps as functions of the ground exciton transition energy are in good agreement with the photoluminescence measured spectra in details. Finally, the hole energy levels and the linear polarization factors in InAs quantum ellipsoids as functions of the aspect ratio are presented. The state 1S(Z up arrow)((1/2)) becomes the hole ground state when e is larger than 2.4. The saturation value of the linear polarization factors of the InAs long ellipsoids of diameter 2.0 nm is 0.86, in agreement with the experimental results.

Temperature and power-density-dependent inter-shell energy states in InAs/GaAs quantum dots

We have investigated the evolution of exciton state filling in InAs/GaAs quantum dot (QD) structures as a function of the excitation power density by using rnicro-photoluminescence spectroscopy at different temperatures. In addition to the emission bands of exciton recombination corresponding to the atom-like S, P and D, etc. shells of QDs, it was observed that some extra states V between the S and P shells, and D' between the P and D shells appear in the spectra with increasing number of excitons occupying the QDs at a certain temperature. The emergence of these inter-shell excitonic levels is power density and temperature dependent, which is an experimental demonstration of strong exciton-exciton exchange interaction, state hybridization, and coupling of a multi-exciton system in QDs. (c) 2006 Elsevier B.V. All rights reserved.

Recombination property of nitrogen-acceptor-bound states in ZnO

The recombination property of nitrogen (N)-related acceptor-bound states in ZnO has been investigated by photoluminescence (PL), time-resolved PL, and selective PL. Several possible recombination processes were discussed by analyzing the relaxation and recombination properties under large Coulomb interaction. It is strongly suggested that bound exciton emission dominates the recombination process related to the N acceptor. The recombination lifetime is 750 ps and the binding energy is 67 meV for N-acceptor-bound exciton at low temperature. (c) 2006 American Institute of Physics.

Localized exciton dynamics in AlInGaN alloy

Carrier recombination dynamics in AlInGaN alloy has been studied by photoluminescence (PL) and time-resolved PL (TRPL) at various temperatures. The fast red-shift of PL peak energy is observed and well fitted by a physical model considering the thermal activation and transfer processes. This result provides evidence for the exciton localization in the quantum dot (QD)-like potentials in our AlInGaN alloy. The TRPL signals are found to be described by a stretched exponential function of exp[(-t/,tau)13], indicating the presence of a significant disorder in the material. The disorder is attributed to a randomly distributed QDs or clusters caused by indium fluctuations. By studying the dependence of the dispersive exponent beta on temperature and emission energy, we suggest that the exciton hopping dominate the diffusion of carriers localized in the disordered QDs. Furthermore, the localized states are found to have 0D density of states up to 250 K, since the radiative lifetime remains almost unchanged with increasing temperature. (C) 2003 Elsevier Science Ltd. All rights reserved.

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.

Mechanism on exciton-mediated energy transfer in erbium-doped silicon

Exciton-mediated energy transfer model in Er-doped silicon was presented. The emission intensity is related to optically active Er concentration, lifetime of excited Er3+ ion and spontaneous emission. The thermal quenching of the Er luminescence in Si is caused by thermal ionization of Er-bound exciton complex and nonradiative energy back-transfer processes, which correspond to the activation energy of 6.6 and 47.4 meV, respectively. Er doping in silicon introduces donor states, a large enhancement in the electrical activation of Er (up to two orders of magnitude) is obtained by co-implanting Er with O. It appears that the donor states are the gateway to the optically active Er. (C) 2000 Elsevier Science B.V. All rights reserved.