Exciton Radiative Lifetime Research Articles

Photoluminescence studies of ZnO thin films on porous silicon grown by plasma-assisted molecular beam epitaxy

ZnO thin films were grown on porous silicon (PS) by plasma-assisted molecular beam epitaxy (PA-MBE). The optical properties of the ZnO thin films grown on PS were studied using temperature-dependent photoluminescence (PL). At room temperature, the full width at half maximum (FWHM) of the near-band-edge emission (NBE) from the ZnO thin films was 98 meV, which was much smaller than that of ZnO thin films grown on a Si substrate. This value was even smaller than that of ZnO thin films grown on a sapphire substrate. The Huang–Rhys factor S associated with the FX emission from the ZnO thin films was found to be 0.124. The Eg(0) value obtained from the fitting was 3.37 eV, with α = 3.3 × 10−2 eV/K and β = 8.6 × 103 K. The activation energies of the low- and high-temperature region were 9 and 28 meV, respectively. The exciton radiative lifetime in the ZnO thin films showed a non-linear behavior, which was established using a quadratic equation.

Excitonic lifetimes in single GaAs quantum dots fabricated by local droplet etching

The time-dependent optical emission of GaAs quantum dots (QDs) is studied using single-dot photoluminescence (PL) spectroscopy with quasi-resonant excitation into the QD d-shell. The QDs are fabricated with a very recently developed method, i.e. by local droplet etching of self-assembled nanoholes in an epitaxial AlAs/AlGaAs heterostructure surface and subsequent filling with GaAs. The PL data are interpreted in terms of a three-level rate model, which yields lifetimes of 390 and 426 ps for the excitons and biexcitons, respectively. The strong dependences of both the PL peak intensities and decay times on the excitation power are quantitatively reproduced by the model. The comparison with various other types of self-assembled QDs shows the trend of a decreasing exciton lifetime with increasing emission energy. This behavior and the short lifetime of the GaAs QDs are discussed on the basis of common models of the exciton radiative lifetime.

Semipolar {nn¯01} InGaN/GaN ridge quantum wells (n = 1−3) fabricated by a regrowth technique

Semipolar {nn¯01} InGaN/GaN quantum wells (QWs) (n = 1−3) are fabricated on top of GaN microstructures, which consist of semipolar {11¯01} facets. Semipolar planes are obtained via regrowth of three-dimensional structures on (0001) GaN templates under controlled growth conditions. Compared to QWs on {11¯01} facets, {nn¯01} ridge QWs show an intense emission at ∼440 nm. Time resolved photoluminescence reveals that the radiative lifetime of excitons in {nn¯01} InGaN ridge QWs at 13 K is 310 ps, which is comparable to that in {11¯01} QWs. The estimated internal quantum efficiency at room temperature is as high as 57%.

Fundamental temperature-dependent properties of the Si nanocrystal band gap

Ever since the first reports about low-temperature photoluminescence (PL) measurements of Si nanocrystals Si (NCs), a severe deviation from the equations commonly used to describe the temperature dependence of the band-gap energy for bulk materials was reported. Our analysis reveals that the formerly observed deviation is solely attributed to too high excitation power densities that cause a preferential emission enhancement of the smaller NCs within a size distribution, due to the temperature dependence of the radiative exciton lifetime. We report on the successful fit of the temperature-dependent band-gap energy of quantum-confined silicon. By means of four size-controlled Si NC samples (1.5 to 4.5 nm), we are able to prove the validity of these equations down to liquid He temperatures, if sufficiently low excitation power densities (500 $\ensuremath{\mu}$W/cm${}^{2}$) are used. Thereby, it is shown that the characteristics of the band-gap widening of quantum-confined nanocrystalline Si obeys to the same physical law as the bulk Si crystal. In addition, the previously observed decrease of the PL intensity for decreasing temperatures is demonstrated to have its origin in the same measurement artifact caused by excitation saturation.

Spin-Flip Limited Exciton Dephasing inCdSe/ZnSColloidal Quantum Dots

The dephasing time of the lowest bright exciton in CdSe/ZnS wurtzite quantum dots is measured from 5 to 170 K and compared with density dynamics within the exciton fine structure using a sensitive three-beam four-wave-mixing technique unaffected by spectral diffusion. Pure dephasing via acoustic phonons dominates the initial dynamics, followed by an exponential zero-phonon line dephasing of 109 ps at 5 K, much faster than the ~10 ns exciton radiative lifetime. The zero-phonon line dephasing is explained by phonon-assisted spin flip from the lowest bright state to dark-exciton states. This is confirmed by the temperature dependence of the exciton lifetime and by direct measurements of the bright-dark-exciton relaxation. Our results give an unambiguous evidence of the physical origin of the exciton dephasing in these nanocrystals.

Optical Properties of ZnO Soccer-Ball Structures Grown by Vapor Phase Transport

ZnO soccer balls were grown on an Au-catalyzed Si(100) substrate by vapor phase transport (VPT) with a mixture of zinc oxide and graphite powders. Temperature-dependent PL was carried out to investigate the mechanism governing the quenching behavior of the PL spectra. From the PL spectra of the ZnO soccer balls at 10 K, several PL peaks were observed at 3.365, 3.318, 3.249, and 3.183 eV corresponding to excitons bound to neutral donors (DoX), a donor–acceptor pair (DAP), first-order longitudinal optical phonon replica of donor–acceptor pair (DAP-1LO), and DAP-2LO, respectively. The mixed system composed of the free exciton (FX) and DoX and the DAP radiative lifetimes were estimated with a theoretical relation between the lifetime and the spectral width. The exciton radiative lifetimes were observed to increase linearly with temperature.

Donor binding energy and radiative life time of exciton in a strained InGaN/GaN quantum wire

Donor binding energies of positively and negatively charged impurities in a strained InGaN/GaN cylindrical quantum wire are investigated. The interband optical transition with and without the exciton is computed as a function of wire radius. The exciton oscillator strength and the exciton lifetime for radiative recombination as a function of wire radius have been computed.

Energetics and dynamics of exciton–exciton interactions in compound colloidal semiconductor quantum dots

The energetics and dynamics of multiply excited states in single material colloidal quantum dots have already been shown to exhibit universal trends. Here we attempt to identify similar trends in exciton-exciton interactions within compound colloidal quantum dots. For this end, we thoroughly review previously available data and also present experimental data on several newly synthesized systems, focusing on core/shell nanocrystals with a type-II band alignment. A universal condition for the transition from binding to repulsion of the biexciton (type-I-type-II transition) is established in terms of the change in the exciton radiative lifetime. A scaling rule is also presented for the magnitude of exciton-exciton repulsion. In contrast, we do not identify a clear universal scaling of the non-radiative Auger recombination lifetime of the biexciton state. Finally, a perspective on future applications of engineered multiexcitonic states is presented.

Major impacts of point defects and impurities on the carrier recombination dynamics in AlN

Impacts of point defects and impurities on the carrier recombination dynamics in AlN are revealed by time-resolved spectroscopy and positron annihilation measurements. Intrinsically short low-temperature excitonic radiative lifetime (τR∼10 ps) was elongated with the increase in Al-vacancy concentration up to 530 ps, irrespective of threading dislocation density. A continuous decrease in τR with temperature rise up to 200 K for heavily doped samples revealed the carrier release from the band-tail formed due to impurities and point defects. Because room-temperature nonradiative lifetime was equally short for all samples, high temperature growth with appropriate defect management is necessary in extracting radiative nature of AlN.

Spectral and Dynamic Properties of Excitons and Biexcitons in Type-II Semiconductor Nanocrystals

Using time-resolved photoluminescence spectroscopy, we analyze single and multiexciton emission energies and decay dynamics of CdS/ZnSe core/shell nanocrystals (NCs). The NCs exhibit a characteristic type-II band alignment that leads to spatially separated charges; this effect is at the origin of long radiative exciton lifetimes, repulsive biexciton interaction energies, and reduced Auger recombination efficiencies. We determine these properties as a function of ZnSe shell thickness and find that the exciton emission energies and the decay times depend little on this parameter. In contrast, the spectral and dynamic properties of biexcitons vary strongly with the shell thickness. The considerable shell dependence of the biexciton decay lets us conclude that the associated Auger process involves the excitation of holes localized in the ZnSe shell. In NCs with thick shells, the large blue shift of the biexciton emission energy is mainly caused by Coulomb repulsion between electrons localized in the CdS core. The different sensitivity of exciton and multiexciton characteristics on the ZnSe shell thickness provides a unique opportunity to tune them independently.

Exciton magnetic polaron in CdMnSe/CdMgSe quantum wells

AbstractWe study exciton magnetic polaron (EMP) formation in (Cd,Mn)Se/(Cd,Mg)Se diluted magnetic semiconductor quantum wells (QWs) using time‐resolved photoluminescence (PL). Magnetic field dependence of the transients allows us to separate the non‐magnetic and magnetic contributions of the exciton localization. We find binding energy of 18 meV and formation time of 500 ps for the EMP. We propose that long polaron formation time is related to auto‐localization process, accompanied with the squeezing of the heavy‐hole envelope wave function. This conclusion is supported by a strong reduction of the exciton radiative lifetime from 600 to 200 ps with increase of magnetic field.

Formation of nanoscale spatially indirect excitons: Evolution of the type-II optical character of CdTe/CdSe heteronanocrystals

In this work, the evolution of the optical properties of nanoscale spatially indirect excitons as a function of the size, shape, and composition of the heteronanostructure is investigated, using colloidal CdTe/CdSe heteronanocrystals (2.6 nm diameter CdTe core and increasing CdSe volume fraction) as a model system. Emphasis is given to quantitative aspects such as the absorption cross section of the lowest-energy exciton transition $({\ensuremath{\mu}}_{SS})$, Stokes shift, linewidths, and the exciton radiative lifetime. The hole wave function remains confined to the CdTe core while the electron wave function is initially delocalized over the whole heteronanocrystal ($type\text{\ensuremath{-}}{I}^{1/2}$ regime), and gradually localizes in the CdSe segment as the growth proceeds, until the spatially indirect exciton transition becomes the lowest-energy transition (type-II regime). This results in a progressive shift of the optical transitions to lower energies, accompanied by a decrease in the oscillator strengths at emission energies and an increase in the exciton radiative lifetimes. The onset of the type-II regime is characterized by the loss of structure of the lowest-energy absorption band, accompanied by a simultaneous increase in the Stokes shift values and transition linewidths. This can be understood by considering the dispersion of the hole and electron states in $k$ space. The ${\ensuremath{\mu}}_{SS}$ values decrease rapidly in the ${\text{type-I}}^{1/2}$ regime but only slightly in the type-II regime. This shows that the indirect exciton formation leads primarily to redistribution of the oscillator strength of the lowest-energy transition over a wider frequency range. The total absorption cross section per ion-pair unit (i.e., integrated over all the exciton transitions) remains essentially constant during the heteronanocrystal growth, demonstrating that ${\ensuremath{\mu}}_{SS}$ is redistributed from higher-energy transitions of both the CdTe and the CdSe segments, in response to the reduction in the electron-hole wave-function overlap. Two radiative decay rates are observed and ascribed to exciton states with different degrees of localization of the electron wave function (an upper state with a faster decay rate and a lower state with a slower decay rate). The results presented here provide fundamental insights into nanoscale spatially indirect exciton transitions, highlighting the crucial role of a number of parameters (viz., electron-hole spatial correlation, exciton dispersion and exciton degeneracy, shape effects, and electronic coupling).

Theoretical Studies of the Structural, Electronic, and Optical Properties of Phosphafluorenes

Phosphafluorenes have drawn increasing attention recently in the applications of organic electronic devices due to their particular optoelectronic properties. To reveal their molecular structures, optoelectronic properties, and structure-property relationships of the newly emerged functional materials, an in-depth theoretical investigation was elaborated via quantum chemical calculations. The optimized geometric and electronic structures in both ground and exited states, the mobility of the hole and electron, the absorption and emission spectra, and the singlet exciton generation fraction of these novel phosphors-containing materials have been studied by density functional theory (DFT), single excitation configuration interaction (CIS), time-dependent density functional theory (TDDFT) methods, and the polarizable continuum model (PCM). The results show that the highest occupied molecular orbitals (HOMOs), the lowest unoccupied molecular orbitals (LUMOs), triplet energies ((3)E(g)), energy gaps (E(g)), as well as some other electronic properties including ionization potentials (IPs), electron affinities (EAs), reorganization energies (lambda), the singlet exciton generation fraction, radiative lifetime, and absorption and emission spectra can be easily tuned by chemical modifications of the phosphorus atom via methyl, phenyl, oxygen, sulfur, or selenium substitution, indicating that the phosphafluorenes are interesting optoelectronic functional materials, which have great potential in the applications of OLEDs, organic solar cells, organic storage, and sensors.

Positive binding energy of a biexciton confined in a localization center formed in a singleInxGa1−xN/GaNquantum disk

We report microphotoluminescence spectroscopy performed on individual and ensemble InGaN/GaN quantum disks (Q-disks). The typical spectrum of a single Q-disk exhibited the contribution of localization centers (LCs) formed in the InGaN active layer of the Q-disks, characterized by sharp lines appearing on the low energy side of the spectra. In addition, a broader emission peak identified as the luminescence of the quasi-two-dimensional (2D) InGaN active layer surrounding the LCs appears systematically at higher energy. Time-resolved photoluminescence experiment performed on single Q-disks exhibited the excitonic transfer, from the 2D InGaN active layer to LCs, at the submicroscopic scale. Excitation power dependence studies and linear polarization analysis allowed us to identify a biexciton complex confined in a LC in a single Q-disk with a surprising positive binding energy of 13 meV. The absence of screening effect by increasing the excitation power density and the fast excitonic radiative lifetime of a few hundred picoseconds that we measured on several individual Q-disks indicate that the absence of internal electric field in the structure can explain the observed positive biexciton binding energy.

Size Dependence of the Spontaneous Emission Rate and Absorption Cross Section of CdSe and CdTe Quantum Dots

In this paper, the size dependence of the band gap, of the spontaneous emission rate, and of the absorption cross section of quantum dots is systematically investigated over a wide size range, using colloidal CdSe and CdTe QDs as model systems (diameters ranging from 1.2 to 8 nm and from 2 to 9.5 nm, respectively). The size dependence of the band gap is well-described by theoretical models, and is dominated by the quantum confinement contribution (1/d2 scaling). The spontaneous emission rate increases linearly with the emission frequency for both CdSe and CdTe QDs, in good agreement with theoretical predictions. By extrapolating the frequency dependence of the emission rates to the bulk band gap values, the exciton radiative lifetime in bulk CdSe and CdTe could be estimated for the first time (viz., 18 and 20 ns, respectively). Comparison between the empirical trends and theoretical predictions provides new fundamental insights into the size dependence of the 1S(e)1S3/2(h) oscillator strengths of QDs, both for emission and absorption. The results highlight the importance of the balance between quantum confinement and coulomb interaction contributions to the size dependence of the exciton properties in QDs and offer an explanation to the long-standing discrepancies observed between the empirical size-dependent trends and the theoretical predictions. The difference between the size dependence of the radiative decay rates and of the absorption cross sections is shown to be due to the fundamental differences between the emission and absorption transitions (viz., spontaneous versus stimulated). The results are also relevant from a practical viewpoint, since they show that the molar extinction coefficients at energies far above the band gap are better suited for analytical purposes. Moreover, more extended and accurate sizing curves are provided for CdSe and CdTe QDs.

Low-energy exciton states in a ZnO cylindrical nanodisk

We consider the electron-hole pair confined in a simplified infinite potential. The low-lying excition states in a ZnO cylindrical nanodisk are calculated based on effective-mass theory. To further understand the optical properties, we calculate the linear optical susceptibilities χ(w) and the radiative recombination lifetime τ of excitons in a ZnO nanodisk. The exciton radiative lifetime in a cylindrical nanodisk is of the order of tens of picoseconds, which is small compared with the lifetime of bulk ZnO material.

Influence of recapture on the emission statistics of short radiative lifetime quantum dots

AbstractSingle photon emission is investigated for fast radiative quantum dots. A simple model including capture process shows that the probability that a second photon is emitted increases when decreasing the exciton radiative lifetime. This effect is experimentally demonstrated through photon correlation measurements on single GaAs quantum dots presenting various radiative lifetime. The same model is used to estimate the second order autocorrelation function for an InAs quantum dot experiencing strong Purcell effect. The calculation shows the limitations of using Purcell effect to realize efficient single photon sources. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

The effects of crystallographic orientation and strain on the properties of excitonic emissionfrom wurtzite InGaN/GaN quantum wells

We have examined in detail crystal orientation effects on the properties of excitonic emission fromwurtzite InxGa1−xN/GaN quantum wells (QWs) with piezoelectric polarization using exciton binding andtransition energy calculations based on a single-band effective-mass theory. Weshow numerical results for the bandgaps, effective heavy-hole masses, piezoelectricpolarizations and fields, exciton wavefunctions, exciton binding and transitionenergies and radiative lifetimes of excitonic emission as a function of the QWcrystallographic growth planes. Band-edge and effective-mass parameters for a continuumof GaN crystallographic orientations, on which InGaN/GaN QWs are grown, wereobtained from In-composition- and strain-dependent calculation for wurtzite InxGa1−xN, using the Hamiltonian in appropriate {hkil} representations. We have performed calculations for a continuum of technologically relevantQW growth planes oriented at various angles θ relative to the (0001) c-plane. The excitonic ground- and first-excited-state energies and wavefunctions were calculatedusing an effective potential method. A strong reduction of average in-plane heavy-holeeffective mass and normal to the plane piezoelectric polarization and field is observed asθ varies fromθ = 0° (i.e. thec-axisdirection) to θ = 49.5°, where the piezoelectric polarization and electric field reverse their orientationwith respect to the plane of the QW. The decrease of the electric field in theInGaN/GaN QW growth direction leads to an increased exciton transitionenergy and oscillator strength, which results in the increase of the excitonbinding energy and decrease of the excitonic radiative lifetime. For anglesθ>49.5° only small variationson the order of ∼10% in the exciton binding and transition energies and excitonic radiative lifetime are observed for narrowIn0.12Ga0.88N/GaN QWs that havewidths less than ∼ 3.5 nm. The average in-plane heavy-hole effective mass reaches its minimum forθ = 90°,i.e. m-plane growth. These results indicate that InGaN/GaN QW structuresgrown on non-(0001)-oriented planes in a wide variety of angles49.5°≤θ≤90° can be used for optimized operation of optoelectronic devices.

Short exciton radiative lifetime in submonolayer InGaAs∕GaAs quantum dots

The exciton radiative lifetime in submonolayer (SML) InGaAs∕GaAs quantum dots (QDs) grown at 500°C was measured by using time-resolved photoluminescence from 10to260K. The radiative lifetime is around 90ps and is independent of temperature below 50K. The observed short radiative lifetime is a key reason for the high performance of SML QD devices and can be explained by the theory of Andreani et al. [Phys. Rev. B 60, 13276 (1999)] calculating the radiative lifetime of QDs formed at the interface fluctuations of a quantum well, as the SML QDs are 20–30nm in diameter and embedded within the lateral InGaAs QW.

Correlating exciton localization with compositional fluctuations in InGaN∕GaN quantum wells grown on GaN planar surfaces and facets of GaN triangular prisms

We have used spatially and temporally resolved cathodoluminescence (CL) to study the carrier recombination dynamics of InGaN quantum wells (QWs) grown on (0001)-oriented planar GaN and {11¯01}-oriented facets of GaN triangular prisms prepared by lateral epitaxial overgrowth in a metal-organic chemical vapor deposition system. The effects of In migration during growth on the resulting QW thickness and composition were examined. We employed a variable temperature time-resolved CL imaging approach that enables a spatial correlation between regions of enhanced exciton localization, luminescence efficiency, and radiative lifetime with the aim of distinguishing between excitons localized in In-rich quantum dots and those in the surrounding Ga-rich QW regions.