Temperature dependent time-resolved exciton luminescence in GaAs/AlGaAs quantum wires and dots

Transient photoluminescence of GaAs/AlGaAs quantum wires and quantum dots formed by strain confinement is studied as a function of temperature. At low temperature, luminescent decay times of the wires and dots correspond to the radiative decay times of localized excitons. The radiative decay time can be either longer or shorter than that of the host quantum well, depending on the size of the wires and dots. For small wires and dots (∼ 100 nm stressor), the exciton radiative recombination rate increases due to lateral confinement. Exciton localization due to the fluctuation of quantum well thickness plays an important role in the temperature dependence of luminescent decay time and exciton transfer in quantum wire and dot structures up to at least ∼ 80 K. Lateral exciton transfer in quantum wire and dot structures formed by laterally patterning quantum wells strongly affects the dynamics of wire and dot luminescence. The relaxation time of hot excitons increases with the depth of strain confinement, but we find no convincing evidence that it is significantly slower in quasi 1-D or 0-D systems than in quantum wells.     

Recombination dynamics of free and bound excitons in bulk GaN

We report new data on the transient photoluminescence behaviour of free and donor bound excitons in high quality bulk GaN material grown by HVPE. With 266 nm photoexcitation the no-phonon free exciton has a short decay time, about 100 ps at 2 K, assigned to nonradiative surface recombination. The LO replicas of the free exciton have a much longer decay at 2 K, about 1.4 ns, believed to be a lower bound for the bulk radiative lifetimes of the free excitons at 2 K. The donor bound exciton no-phonon lines exhibit a rather short (about 300 ps) nonexponential decay at 2 K, which appears to be dominated by a scattering process. The corresponding LO replicas and the two-electron transitions have a much longer decay. From the latter, the lower bound of the radiative lifetime of the O- and Si-bound excitons are 1800 ps and 1100 ps, respectively.     

Luminescence enhancement of ZnO nanoparticles on metal surface

We have studied luminescence enhancement of zinc oxide (ZnO) nanoparticles with the average size of 30 nm on several metal surfaces at low temperatures. Bandedge luminescence originated from bound exciton (BE) annihilation is observed at 3.360 eV, and strongly depends on the kind and surface roughness of metal. The luminescence intensity is about 10 times larger for Ag surface than that for quartz surface. Furthermore, the luminescence increases remarkably when the roughness of Ag surface is almost the same as the particle size. The intensity ratio of the fast decay component to the slow one decreases for Ag surface compared with quartz. These results suggest that the luminescence enhancement is partially attributed to suppressing of the nonradiative recombination process in ZnO nanoparticles on metal surface.     

Identification of F+ centers in hafnia and zirconia nanopowders

Luminescence properties of undoped hafnia and zirconia nanopowders prepared by solution combustion synthesis were investigated under photo- and electron-beam excitation in 10–400 K temperature range. Along with the main luminescence band revealed in investigated materials at low temperatures at 4.2–4.3 eV and ascribed to the emission of self-trapped excitons, there are luminescence bands due to defects and impurities introduced during sample preparation. At room temperature the latter emissions dominate in the luminescence spectra as the intrinsic self-trapped exciton emission is quenched. Analysis of decay kinetics of defect centers allowed identification of F⁺ centers emission at 2.8 eV with lifetimes ∼3–6 ns in hafnia and zirconia under intra-center excitation.     

Luminescence spectroscopy of the Bi single and dimer centers in Y3Al5O12:Bi single crystalline films

Luminescence of the Bi³⁺ single and dimer centers in UV and visible ranges is studied in YAG:Bi (0.13 and 0.27 at% of Bi, respectively) single crystalline films (SCFs), grown by liquid phase epitaxy from a Bi₂O₃ flux. The cathodoluminescence spectra, photoluminescence decays, and time-resolved spectra are measured under the excitation by accelerated electrons and synchrotron radiation with energies of 3.7 and 12 eV, respectively. The energy level structure of the Bi³⁺ single and dimer centers was determined. The UV luminescence of YAG:Bi SCF in the bands that peaked at 4.045 and 3.995 eV at 300 K is caused by radiative transitions of Bi³⁺ single and dimer centers, respectively. The excitation spectra of UV luminescence of Bi³⁺ single and dimer centers consist of two dominant bands, peaked at 4.7/4.315 and 5.7/6.15 eV, related to the ¹S₀→³P₁ (A band) and ¹S₀→ ¹P₁ (C-band) transitions of Bi³⁺ ions, respectively. The excitation bands that peaked at 7.0 and 7.09 eV are ascribed to excitons bound with the Bi³⁺ single and dimer centers, respectively. The visible luminescence of YAG:Bi SCF presents superposition of several wide emission bands peaking within the 3.125-2.57 eV range and is ascribed to different types of excitons localized around the Bi³⁺ single and dimer centers. Apart from the above mentioned A and C bands the excitation spectra of visible luminescence contain wide bands at 5.25, 5.93, and 6.85 eV ascribed to the O²⁻→Bi³⁺ and Bi³⁺→Bi⁴⁺ + e charge transfer transition (CTT) in Bi³⁺ single and dimer centers. The observed significant differences in the decay kinetics of visible luminescence under excitation in A and C bands of Bi³⁺ ions, CTT bands, and in the exciton and interband transitions confirm the radiative decay of different types of excitons localized around Bi³⁺ ions in the single and dimer centers.     

Localization of nonequilibrium current carriers close to Cr ions in ZnSe:Cr single crystals

The photoluminescence (PL) of Cr-doped ZnSe single crystals is investigated in a temperature interval from 83 up to 297 K and in a wavelengths region from 440 up to 2700 nm. The doping was carried out during a high-temperature annealing of ZnSe crystals in CrSe vapors and in chrome chlorides medium. It is revealed that the doping results in an appearance of both luminescence bands located at 0.54, 0.97, and 2.15 μm and edge luminescence bands located at 454, 457, and 460 nm at 83 K. It is shown that the PL bands located at 457 and 460 nm are caused by the radiative recombination with the participation of holes located on hydrogen-like orbits close to Cr⁺ centers, having a binding energy of 99 meV. The excitons bound with centers responsible for the radiation located at 0.54 μm and having a binding energy of 65-68 meV are considered. The energy of a lattice relaxation at recharge of centers responsible for green radiation is estimated and equals 40-170 meV.     

Photoluminescence from excitons in ZrS3

The photoluminescence spectrum of the layered ZrS₃ crystal shows several narrow emission lines under the excitation by an Ar⁺ laser in the wavelength range from 605–630 nm at 4.2 K. The excitation spectra for these emission lines and their temperature dependences suggest that the observed narrow emission lines originate from the radiative annihilation of indirect excitons bound to impurities. Some of these emission lines seem to be associated with the radiative annihilation of the indirect bound exciton with emission of phonons.     

Exciton luminescence of YAlO<Subscript>3</Subscript> single crystals and single-crystal films

The paper reports on a study of exciton luminescence in single crystals (SCs) and single-crystal films (SCFs) of YAlO₃, which have substantially different concentrations of vacancy-type and substitutional defects, under excitation by synchrotron radiation near the fundamental absorption edge. The radiative annihilation of excitons in SCFs was shown to occur primarily at regular perovskite lattice sites and to be accompanied by luminescence in a band peaking at λ_max = 295 nm with τ = 5.2 ns. In contrast to SCFs, the radiative exciton decay in YAlO₃ SCs takes place predominantly near vacancy-type defects (F⁺ and F centers) and is accompanied by luminescence in the bands at λ_max = 350 nm (τ = 2.5 ns) and 440 nm (τ₁ = 1.9 ns, τ₂ = 30 ms). Photoexcitation in the 175-nm band of YAlO₃ SCs revealed photoconversion of the centers F → F⁺.     

Luminescence of mercuric iodide crystals

Low-temperature (4.2–130 K) photoluminescence spectra of HgI₂ crystals have been measured in the 540–700 nm region. An analysis of the characteristics (intensity vs temperature and excitation power relations, afterglow times, excitation spectra) of the 560, 620, and 635 nm emission bands suggests the following assignments: the 560 nm band is due to radiative annihilation of excitons bound to mercury vacancies, and the “red” emission originates from recombination of free (620 nm) and donor-localized (635 nm) electrons with a hole-filled acceptor level. The energies of the corresponding donor and acceptor levels have been estimated. New emission bands at 540, 545, and 575 nm have been discovered, and their origin discussed. Fiz. Tverd. Tela (St. Petersburg) 39, 67–73 (January 1997)     

Experimental and theoretical studies of polaron optical properties in KNbO3 perovskite

Time-resolved absorption and luminescence spectra have been measured in KNbO₃ perovskite crystals after pulsed band-gap excitation by 200 fs laser pulses and 10 ns electron pulses. Quantum chemical calculations using the large unit cell periodic model support the interpretation of the observed transient absorption bands at 0.8 and 1.1 eV as the self-trapped electron polarons and bound hole polarons, respectively. The activation energy for the 2.2 eV green luminescence quenching is 0.05 eV. We suggest that the short lifetime (<15 ns) of the luminescence at RT is caused by the radiative recombination of nearest electron and hole polarons.     

Luminescence of dense, octahedral structured crystalline silicon dioxide (stishovite)

It is obtained that, as grown, non-irradiated stishovite single crystals possess a luminescence center. Three excimer pulsed lasers (KrF, 248 nm; ArF, 193 nm; F₂, 157 nm) were used for photoluminescence (PL) excitation. Two PL bands were observed. One, in UV range with the maximum at 4.7±0.1 eV with FWHM equal to 0.95±0.1 eV, mainly is seen under ArF laser. Another, in blue range with the maximum at 3±0.2 eV with FWHM equal to 0.8±0.2 eV, is seen under all three lasers. The UV band main fast component of decay is with time constant τ=1.2±0.1 ns for the range of temperatures 16-150 K. The blue band decay possesses fast and slow components. The fast component of the blue band decay is about 1.2 ns. The slow component of the blue band well corresponds to exponent with time constant equal to 17±1 μs within the temperature range 16-200 K. deviations from exponential decay were observed as well and explained by influence of nearest interstitial OH groups on the luminescence center. The UV band was not detected for F₂ laser excitation. For the case of KrF laser only a structure less tail up to 4.6 eV was detected. Both the UV and the blue bands were also found in recombination process with two components having characteristic time about 1 and 60 μs. For blue band recombination luminescence decay is lasting to ms range of time with power law decay ∼t⁻¹.For the case of X-ray excitation the luminescence intensity exhibits strong drop down above 100 K. such an effect does not take place in the case of photoexcitation with lasers. The activation energies for both cases are different as well. Average value of that is 0.03±0.01 eV for the case of X-ray luminescence and it is 0.15±0.05 eV for the case of PL. So, the processes of thermal quenching are different for these kinds of excitation and, probably, are related to interaction of the luminescence center with OH groups.Stishovite crystal irradiated with pulses of electron beam (270 kV, 200 A, 10 ns) demonstrates a decrease of luminescence intensity excited with X-ray. So, irradiation with electron beam shows on destruction of luminescent defects.The nature of luminescence excited in the transparency range of stishovite is ascribed to a defect existing in the crystal after growth. Similarity of the stishovite luminescence with that of oxygen deficient silica glass and induced by radiation luminescence of α-quartz crystal presumes similar nature of centers in those materials.     

Intrinsic ultraviolet luminescence from Lu2O3, Lu2SiO5 and Lu2SiO5:Ce

Radioluminescence and thermally stimulated luminescence measurements on Lu₂O₃, Lu₂SiO₅ (LSO) and Lu₂SiO₅:Ce³⁺ (LSO:Ce) reveal the presence of intrinsic ultraviolet luminescence bands. Characteristic emission with maximum at 256 nm occurs in each specimen and is attributed to radiative recombination of self-trapped excitons. Thermal quenching of this band obeys the Mott-Seitz relation yielding quenching energies 24, 38 and 13 meV for Lu₂O₃, LSO and LSO:Ce, respectively. A second intrinsic band appears at 315 nm in LSO and LSO:Ce, and at 368 nm in Lu₂O₃. Quenching curves for these bands show an initial increase in peak intensity followed by a decrease. Similarity in spectral peak position and quenching behavior indicate that this band has a common origin in each of the samples and is attributed to radiative recombination of self-trapped holes, in agreement with previous work on similar specimens. Comparison of glow curves and emission spectra show that the lowest temperature glow peaks in each specimen are associated with thermal decay of self-trapped excitons and self-trapped holes. Interplay between the intrinsic defects and extrinsic Ce³⁺ emission in LSO:Ce is strongly indicated.     

二维共价有机骨架中的直接激子和间接激子

本文利用基于GW方法和Bethe-Salpeter方程的第一性原理计算，研究了两种二维共价有机骨架材料(COF)的激发态性质. 单层COF是直接带隙材料，而体相COF呈现间接带隙. 根据直接激子计算的体相COF的光学带隙和吸收光谱与实验一致，而由位于导带底的光生电子和位于价带顶的空穴形成的间接激子能量的理论计算值远低于实验荧光光谱的测量值. 研究表明，可以排除间接带隙COF材料的发光由声子主导的可能性. 研究认为体相COF的发光可能源于缺陷处直接激子的复合. 体相COF的AA堆叠结构导致其带隙是间接的. 如果将堆叠方式由AA变成AB，体相COF将转变成直接带隙材料，它的发光效率可能会增强.     

Hydrostatic pressure dependence of excitonic optical transitions in strained wurtzite GaN/AlN quantum disks

In the framework of the effective mass approximation, the effects of hydrostatic pressure on optical transitions associated with the excitons confined in strained wurtzite (WZ) GaN/AlN quantum disks (QDisks) with the confinement potential of finite depth are investigated by using a variational technique, with considering the influences of the built-in electric field (BEF) and the biaxial strain dependence of material parameters. The Schrödinger equation via the proper choice of the exciton trial wave function is solved. The behaviors of the excitonic optical transition are examined at different pressures for different QDisk sizes. In our calculations, the effective masses of electron and hole, dielectric constants, phonon frequencies, energy gaps, and piezoelectric polarizations are taken into account as functions of biaxial strain and hydrostatic pressure. Numerical results show that the hydrostatic pressure and the QDisk size have a remarkable influence on exciton states. The calculated pressure coefficient of optical transition energy shows a negative value if the QDisk height L > 3.2 nm, in contrast with the positive pressure coefficient of the GaN band gap. The peculiar pressure behavior is related to the pressure-induced increase of the built-in electric field. For a fixed pressure, the optical transition energy has a red-shift if the QDisk height and radius increase and QDisk height has a more obvious influence on E_ph than QDisk radius. Furthermore, the relationship between the radiative decay time and hydrostatic pressure (QDisk height) is also investigated. It is found that the radiative decay time increases with pressure and the increment tendency is more prominent for the large height QDisks. The radiative decay time strongly increases by three orders of magnitude reaching microsecond order if the QDisk height increases from 1 nm to 3 nm.     

Photoluminescence of the ZnSe single crystals doped by thermal diffusion of nitrogen

Photoluminescence (PL) spectra of ZnSe single crystals annealed in different ambients containing molecular nitrogen are investigated. The compensating activity of N impurity in n-ZnSe crystals is shown. It is caused by the formation of N_Se acceptor centers, having 101-108 meV activation energy. The intensity of amplification of both long-wave luminescence spectra bands and the edge luminescence spectra bands caused by the presence of nitrogen in annealing medium is investigated. The presented results allow one to assign the long-wave luminescence to deep acceptors caused by uncontrollable impurities, and the relevant bands of the edge luminescence spectra to the excitons bound with the same deep acceptors. The model explaining the transformations of the luminescent properties of ZnSe crystals by means of nitrogen impurity doping is proposed. The model considers the presence of donors having 75 meV activation energy, acceptors having 220-720 meV activation energy and centers having levels localized near the middle of the band gap.     

Luminescence of different modifications of crystalline silicon dioxide: Stishovite and coesite

Luminescence of very small samples of single crystals of coesite and stishovite has been studied. The spectra were detected under ionizing radiation (X-ray and electron beam) and the decay kinetics of cathodoluminescence in the range of time from 10 ns to 3 ms was measured. The coesite luminescence possesses a broad band at 3 eV with exponential decay about 680 μs at 80 K. The nature of this luminescence was explained as a self-trapped exciton creation in tetrahedron framework. The stishovite luminescence possesses two bands—blue (2.8 eV) and UV (4.7 eV). The UV band intensity grows more than 20 times with irradiation dose from initial level. This shows that the corresponding luminescence centers could be induced by the radiation. The decay of the UV band possesses a fast and a slow component. The determination of the fast decay parameters is beyond the capabilities of our apparatus (less than 10 ns), whereas the slow decay of the UV is non-exponential and takes place in the range of hundreds of microsecond. The blue band decay kinetics can be well approximated by power law ∼t⁻², which may correspond to recombination of defects created by radiation. The stishovite single crystal luminescence is very similar to that of germanium dioxide single crystal of rutile structure. The nature of the stishovite luminescence is explained as recombination of defects created by irradiation in octahedron-structured lattice.     

Radiative recombination of excitons in semiconductors at high excitation intensities

The shape of the exciton luminescence band of the gaseous phase of free excitons in crystal Si is investigated under high level excitation. Only the exciton-exciton interaction is considered and the influence of collisions of excitons with phonons and electrons is not taken into account. The theoretical shape of the exciton luminescence band qualitatively agrees with experiment. The process of the radiative Auger-recombination of two indirect excitons without the participation of phonons is studied in Ge and Si. The expression for the frequency and temperature dependence of the probability of radiation is obtained. The band has the asymmetrical Gaussian form with the steep short-wave tail. The predicted luminescence band in Ge is shifted at the long-wavelength side from the well-known exciton luminescence bands with participation of the phonons. The selection rules for the probability amplitude of the process under consideration are obtained.     

Luminescence processes in CsI doped with Na+ and K+ ions

The luminescence bands around 420 nm and 370 nm in CsI:Na and CsI:K have been studied by measuring the temperature dependence of decay times and luminescence and its excitation spectra. The bands are due to singlet+triplet localized excitons and to triplet localized excitons, respectively, at low temperature. Zero field splitting and life time amount to 0.2 meV and 3.3 μs for the 420 nm band, and to 2.1 meV and 1.7 μs for the 370 nm band. Creation processes of 420 nm excitons may not be the same below 40 K and near room temperature.     

The optical properties of Be,Mg and Zn-diffused gallium phosphide

Interest in the Ga-site acceptors Be and Mg was stimulated by the possibility that they might produce efficient luminescence on association with O, analogous to the well-known red Zn-O luminescence in GaP but at higher transition energy. Attention was directed to diffusion doping by Be and Mg of GaP O-doped during growth because the reactivity of Be and Mg with O renders double doping during crystal growth very difficult. Structured green donor-acceptor pair spectra were observed at 1.6°K from many Be-diffused crystals, yielding an accurate measure of (E_A)_BE, 50 ± 1 meV. Moderately efficient orange-red luminescence was also observed below ∼ 100 °K from these crystals, but the intensity of this luminescence decreased rapidly to negligible levels by ∼ 200°K. This luminescence also contains sharp structure at 1.6°K, of a form characteristic of the decay of excitons bound to complex centres. Many sharp phonon replicas occur, involving local modes as well as characteristic GaP modes. One set of no-phonon lines, at least, near 2.19 eV, shows zero-field splitting, luminescence decay times and behaviour in magnetic and external strain fields characteristic of exciton decay at a centre with <100>; or <111>-type symmetry axes, containing no extra electronic particles. The exciton state is split by 2.4 meV by J-J coupling, and the axial field of the centre splits the hole states by ∼ 1.0 meV. These bound excitons are specifically characteristics of diffused GaP and appear analogous to bound excitons observed below 2.12 eV in Zn-diffused GaP. It is probable that the relevant centres contain diffusion components such as Be or Zn interstitials and improbable that O_P is involved. By contrast, weak orange bound exciton luminescence observed in Mg-diffused GaP does involve O, presumably as O_P. No analysis of the magneto-optical behaviour of this Mg-related bound exciton was possible in our crystals, so its symmetry axis was not established. It is possible that this is the Mg_Ga-O_P bound exciton. If so, the two-fold reductions in the exciton localisation energy from ∼ 0.32 eV to ∼ 0.15 eV and in the mass of the Ga-site substituent has produced dramatic changes in the form of the phonon cooperation between the Zn-O and “Mg-O” excitons. The “Mg-O” exciton luminescence is not dominant in our crystals, even at low temperature. The exciton state is again split by a local crystal field as well as by J-J coupling, but here the former splitting is predominant; 2_∈0 = 3.9 meV, Δ = 0.60 meV.     

X-ray and thermoluminescence of LiKB4O7 single crystals

The thermoluminescent (TL) and X-ray luminescent (XL) spectra of undoped LiKB₄O₇ (LKBO) single crystals had been investigated in the temperature range 80-300 K. It was found that in LKBO crystals, there are two intensive TL peaks at 112 and 132 K. The only one band emission spectra of sharply defined Gaussian shape, confirming the same mechanism of XL and TL by the radiation annihilation of the strongly localized self-trapped excitons (STE), had been observed in the TL and XL spectra. The possible models of these localization centers STE have been discussed.