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To experimentally identify the character of radiative transitions in trigonal Cu2BaSnS4, we conduct temperature and excitation intensity dependent photoluminescence (PL) measurements in the temperature range of 15–300 K. The low-temperature near band edge PL spectrum is interpreted as the free exciton at 2.11 eV and the bound exciton at 2.08 eV, coupled with associated phonon-assisted transitions. In the low energy region, we assign the dominant defect emission at 1.96 eV to donor–acceptor-pair recombination and the weak broad emission at 1.6 eV to the free-to-bound transition. The activation energies and temperature shift for the radiative transitions are determined and discussed. Above 90 K, the free exciton recombination becomes the dominant radiative transition, with its energy shift mainly governed by the contribution of optical phonons. [ABSTRACT FROM AUTHOR]

Eu+3 was incorporated into the lattice of a lead-free ferroelectric Na1/2Bi1/2TiO3 (NBT) as per the nominal formula Na0.5Bi0.5-xEuxTiO3. This system was investigated with regard to the Eu+3 photoluminescence (PL) and structural behaviour as a function of composition and electric field. Electric field was found to irreversibly change the features in the PL spectra and also in the x-ray diffraction patterns below the critical composition x=0.025. Detailed analysis revealed that below the critical composition, electric field irreversibly suppresses the structural heterogeneity inherent of the host matrix NBT and brings about a long range ferroelectric state with rhombohedral (R3c) distortion. It is shown that the structural disorder on the nano-scale opens a new channel for radiative transition which manifests as a new emission line branching off from the main 5D0→7F0 line along with a concomitant change in the relative intensity of the other crystal field induced Stark lines with different J values. The study suggests that Euþ3 luminescence can be used to probe the relative degree of field induced structural ordering in relaxor ferroelectrics and also in high performance piezoelectric alloys where electric field couples very strongly with the lattice and structural degrees of freedom. [ABSTRACT FROM AUTHOR]

Eu+3 was incorporated into the lattice of a lead-free ferroelectric Na1/2Bi1/2TiO3 (NBT) as per the nominal formula Na0.5Bi0.5-xEuxTiO3. This system was investigated with regard to the Eu+3 photoluminescence (PL) and structural behaviour as a function of composition and electric field. Electric field was found to irreversibly change the features in the PL spectra and also in the x-ray diffraction patterns below the critical composition x=0.025. Detailed analysis revealed that below the critical composition, electric field irreversibly suppresses the structural heterogeneity inherent of the host matrix NBT and brings about a long range ferroelectric state with rhombohedral (R3c) distortion. It is shown that the structural disorder on the nano-scale opens a new channel for radiative transition which manifests as a new emission line branching off from the main 5D0→7F0 line along with a concomitant change in the relative intensity of the other crystal field induced Stark lines with different J values. The study suggests that Euþ3 luminescence can be used to probe the relative degree of field induced structural ordering in relaxor ferroelectrics and also in high performance piezoelectric alloys where electric field couples very strongly with the lattice and structural degrees of freedom. [ABSTRACT FROM AUTHOR]

We herein report on the occurrence of bright and efficient electrophosphorescence from a simple organic light-emitting diode (OLED) with a single organic layer comprised of a small molecular ionic iridium compound, formed using a solution process. The studied small molecular ionic iridium compound is [Ir(dfppy)2(bpy)]+PF6-, which exhibits excellent film-forming properties, bright green photoluminescence, and efficient bipolar carrier transport with balanced electron and hole mobilities of about 10-5 cm2/(V s). A high performance of the device was achieved by using a phosphorescent OLED (PHOLED) that was fabricated using the [Ir(dfppy)2(bpy)]+PF6- compound, with a peak brightness of about 18 000 cd/m2 and a peak current efficiency of 12 cd/A. A peak power efficiency of 2.5 lm/W was measured at 2800 cd/m2. These results suggest that the small molecular ionic iridium compound is a promising material for bright and efficient PHOLEDs manufactured using a simple solution process. [ABSTRACT FROM AUTHOR]

The blue-green photoluminescence emitted by pure and electron-doped strontium titanate under intense pulsed near-ultraviolet excitation is studied experimentally as a function of excitation intensity and temperature. Both emission spectra and time-resolved decays of the emission are measured and analyzed in the framework of simple phenomenological models. We find an interesting blue-to-green transition occurring for increasing temperatures in pure samples, which is absent in doped materials. The luminescence yield and decay rate measured as a function of temperature can be modeled well as standard activated behaviors. The leading electron-hole recombination process taking place in the initial decay is established to be second order, or bimolecular, in contrast to recent reports favoring a third-order interpretation as an Auger process. The temporal decay of the luminescence can be described well by a model based on two interacting populations of excitations, respectively identified with interacting defect-trapped (possibly forming excitons) and mobile charges. Finally, from the measured doping and sample dependence of the luminescence yield, we conclude that the radiative centers responsible for the luminescence are probably intrinsic structural defects other than bulk oxygen vacancies. [ABSTRACT FROM AUTHOR]

We present an experimental study of the radiative recombination dynamics in size-controlled anatase TiO2 nanoparticles in the range 20–130 nm. From time-integrated photoluminescence spectra and picosecond time-resolved experiments as a function of the nanoparticle size, excitation density, and temperature, we show that photoluminescence comes out from a bulk and a surface radiative recombination. The spectral shift and the different time dynamics provide a clear distinction between them. Moreover, the intrinsic nature of the emission is also proven, providing a quantitative evaluation of volume and surface contributions. [ABSTRACT FROM AUTHOR]

We present new results of temperature dependence of photoluminescence spectra carried out on poly-p-phenylene vinylene (PPV) and on PPV composite films with single-walled carbon nanotubes. By performing studies at different temperatures (87 and 300 K), we show that a distribution of conjugated PPV segments is needed to interpret experimental data. At the microscopic scale, such a distribution corresponds to the morphological picture of poorly packed short chain segments and well-packed ordered long chain segments. Within this scheme, a new interpretation emerges for explaining the specific behavior of the photoluminescence bands. In particular, the two most intense components of the photoluminescence spectra of PPV thermally converted at 300 °C (2.23 and 2.43 eV at 300 K) change drastically their relative intensity when the observation temperature decreases. This effect is interpreted as due to the inhibition of charge migration to longer segments and to radiative recombination occurring mainly on n=5 conjugated segments. [ABSTRACT FROM AUTHOR]

Focuses on a study which examined the influence of the compensation ratio and temperature on the decay time associated with radiative transition in the local impurity concentration in silicon-doped aluminum gallium arsenide. Description of radiative transitions; Details of experimental set-up and procedure; Information on silicon concentrations in liquid phase and carrier concentrations; Features of photoluminescence spectra.

Presents a study that examined the radiative transitions of A-oriented three-step asymmetric coupled quantum wells in an electric field by photoluminescence. Number of transitions observed in the spectra; Features of the transitions; Origin of the transitions.

Recent defect calculations suggest that the open circuit voltage of CuGaSe2 solar cells can be limited by deep intrinsic electron traps by GaCu antisites and their complexes with Cu-vacancies. To gain experimental evidence, two radiative defect transitions at 1.10 eV and 1.24 eV are characterized by steady-state photoluminescence on epitaxial-grown CuGaSe2 thin films. Cu-rich samples are studied, since they show highest crystal quality, exciton luminescence, and no potential fluctuations. Variations of the laser intensity and temperature dependent measurements suggest that emission occurs from two deep donor-like levels into the same shallow acceptor. At 10K, power-law exponents of 1 (low excitation regime) and 1/2 (high excitation regime) are observed identically for both transitions. The theory and a fitting function for the double power law is derived. It is concluded that the acceptor becomes saturated by excess carriers which changes the exponent of all transitions. Activation energies determined from the temperature quenching depend on the excitation level and show unexpected values of 600meV and higher. The thermal activation of non-radiative processes can explain the distortion of the ionization energies. Both the deep levels play a major role as radiative and non-radiative recombination centers for electrons and can be detrimental for photovoltaic applications. [ABSTRACT FROM AUTHOR]

A detailed analysis of the radiative recombination processes in Ag(InGa)Se2 thin films grown by a three-stage method was carried out by photoluminescence. The temperature and excitation dependence of the photoluminescence spectra was used to identify the recombination types and determine the ionization energy of the defects in the films. Significant differences were observed between the spectra of the Ag-rich and Ag-poor samples. The Ag-rich films were dominated by two emission peaks of donor acceptor pairs (DAPs). The DAP at lower energy level is attributed to recombination of donor level 13.8 meV (Agi) with acceptor level 70.3 meV (AgIn), while the one at high energy level is assigned to recombination of donor level 18.5 meV (Agi) with acceptor level 108.9 (Ag

We report on the experimental observation of intrinsic photoluminescence from semiconductors in the terahertz spectral range. The intrinsic terahertz photoluminescence is caused by intraexciton radiative transitions excited during the process of the binding of nonequilibrium electrons and holes into free excitons. The experiments were carried out on high-purity Si crystals at helium temperatures and under interband photoexcitation intensities not exceeding 1W/cm². [ABSTRACT FROM AUTHOR]

Very high polarization degree of 0.98, considerably larger than theoretical predictions, has been measured in (2021) In0.24Ga0.76N/GaN quantum well by low temperature photoluminescence. With increasing temperature, the polarization degree decreases due to thermal population of the excited valence band level. This effect suggests an accurate method to determine the interlevel energy, which, for the studied well, is 32?meV. Time-resolved photoluminescence measurements set radiative recombination times between 2 and 12 ns for temperatures from 3 to 300 K. Nonradiative recombination was found to be slow, over 2?ns at 300 K, taking place via traps with activation energy of 0.19 eV. [ABSTRACT FROM AUTHOR]

The influences of defects and surface roughness on the indirect bandgap radiative transition of Ge were studied. Bulk Ge has 15 times the integrated intensity of photoluminescence of Ge-on-Si. However, for Ge-on-Si sample, the direct transition related photoluminescence intensity is higher than the indirect transition related one. We affirm that the defects in the Ge-on-Si are responsible for the weak indirect transition and relatively strong direct transition. The scattering of electrons by roughness at Ge/oxide interface can provide extra momentum of the indirect band transition of Ge, and thus enhance the indirect radiative transition. [ABSTRACT FROM AUTHOR]

Low-temperature photoluminescence is studied in detail in GaN nanocolumns (NCs) grown by plasma-assisted molecular beam epitaxy under various conditions (substrate temperature and impinging Ga/N flux ratio). The relative intensities of the different emission lines, in particular those related to structural defects, appear to be correlated with the growth conditions, and clearly linked to the NC sample morphology. We demonstrate, in particular, that all lines comprised between 3.10 and 3.42 eV rapidly lose intensity when the growth conditions are such that the NC coalescence is reduced. The well-known line around 3.45 eV, characteristic of GaN NC samples, shows, however, a behavior that is exactly the opposite of the other lines, namely, for growth conditions leading to reduced NC coalescence, this line tends to become more prominent, thus proving to be intrinsic to individual GaN NCs. [ABSTRACT FROM AUTHOR]

Both direct and indirect transitions of photoluminescence and electroluminescence are observed in a Ge n+p diode. The relative intensity of direct radiative recombination with respect to indirect radiative recombination increases with the increase in the optical pumping power, injection current density, and temperature. The increase in electron population in the direct valley is responsible for the enhancement. The spectra can be fitted by the combination of direct and indirect transition models. The direct radiative transition rate is ∼1600 times of the indirect transition, estimated by electroluminescence and photoluminescence spectra near room temperature. [ABSTRACT FROM AUTHOR]

The authors have employed variable temperature photoluminescence (PL) and time-resolved PL spectroscopy to probe the exciton recombination in high density and vertically aligned ZnO nanorods grown on p-type GaN/sapphire template. The low-temperature PL characterizes the dominant near-band-edge excitonic emissions from such nanorod arrays. At 4.3 K, a PL decay time of 432 ps reveals improved crystalline quality. The PL decay time shows irregular behavior due to different types of excitonic transitions dominating the PL spectra at different temperatures and a competitive effect of radiative recombination and nonradiative relaxation processes. [ABSTRACT FROM AUTHOR]

We have measured the photoluminescence (PL) lifetime of a freestanding GaN film using one-photon and two-photon excitations to demonstrate the dramatic difference in exciton recombination dynamics at the surface and in the bulk. An ultralong exciton PL lifetime of 17.2 ns at 295 K is observed from a GaN freestanding film using two-photon excitation, whereas less than 100 ps lifetime is observed for one-photon excitation, suggesting that nonradiative processes from surface defects account for the short PL lifetime measured. A monotonic increase in two-photon excited PL lifetime with increasing temperature and the linear dependence of the exciton lifetime with emission wavelength show good agreement with the theoretical predictions, indicating that radiative recombination dominates for bulk excited state relaxation processes. [ABSTRACT FROM AUTHOR]

We have performed time-resolved resonant pump-probe experiment to study the dynamic response of InAs/InP quantum dot transitions. A 72-stacked InAs/InP quantum dot layer sample is grown on (311)B substrate. Photoluminescence at high excitation power reveals ground and excited transitions. Carrier radiative lifetimes and differential transmission are determined under strong excitation powers. The variation of measured carrier radiative lifetimes with increasing excitation powers is attributed to the exciton and biexciton lifetimes difference. The implications of such a difference on differential transmission are discussed, and finally exciton and biexciton lifetimes are measured to be about 1720 and 530 ps, respectively. [ABSTRACT FROM AUTHOR]

We demonstrate electroluminescence (EL) with an external efficiency of more than 0.1% at room temperature from glide dislocations in silicon. The key to this achievement is a considerable reduction of nonradiative carrier recombination at dislocations due to impurities and core defects by impurity gettering and hydrogen passivation, respectively, which is shown by means of deep-level transient spectroscopy. Time-resolved EL measurements reveal a response time below 1.8 μs, which is much faster, compared to the band-to-band luminescence of bulk silicon. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We studied time-resolved photoluminescence PL over a temporal range 10[SUP-6]- 10[SUP3]s in high-purity freestanding GaN templates. Red, yellow, green, blue, and shallow donor-acceptor emission bands can be resolved in the PL spectrum. Observation of luminescence long after the excitation is switched off is a striking feature of our study. The persistent PL observed for all above bands, except for the green band, is primarily attributed to the donor-acceptor-pair-type recombination. An unusually slow, nonexponential decay of radiative transitions from the conduction band to the shallow acceptor was also observed, pointing to some additional mechanism for the persistent PL. Possible role of the surface states in this effect is discussed. [ABSTRACT FROM AUTHOR]

Temperature dependent photoreflectance (PR) and photoluminescence experiments of the InAs/GaAs quantum dot (QD) structures were performed. At 20 K, effective band-gap transitions due to the InAs QDs, wetting layers, and GaAs buffer and cap layers were identified. Transition energies of the ground state and four excited states with nearly equal interlevel spacings (75–80 meV) were observed. The linewidth of the ground-state transition decreased as the temperature increased from 20 K to 100 K while the linewidth became broader at temperatures above 100 K. Energy features of the PR spectra originating from QDs and relating to the in-plane parabolic potentials were discussed. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Recombination processes in GaP/GaNP core/shell nanowires (NWs) grown on a Si substrate by molecular beam epitaxy are examined using a variety of optical characterization techniques, including cw- and time-resolved photoluminescence and optically detected magnetic resonance (ODMR). Superior optical quality of the structures is demonstrated based on the observation of intense emission from a single NW at room temperature. This emission is shown to originate from radiative transitions within N-related localized states. From ODMR, growth of GaP/GaNP NWs is also found to facilitate formation of complex defects containing a P atom at its core that act as centers of competing non-radiative recombination. [ABSTRACT FROM AUTHOR]