Free Exciton Luminescence Research Articles

PHOTON YIELD FROM SOLID KRYPTON AND XENON AT THE EDGE OF EXCITON ABSORPTION

The spectra of photon yield from solid Xe and Kr were measured in the energy range of absorption of Γ(3/2) n = 1 excitons. Using combination of time-resolved spectroscopy with selective photoexcitation by synchrotron radiation, the creation of free excitons was experimentally separated from direct population of molecular emitting centers. For the first time the threshold of photon absorption by molecular trapped centers is observed in excitation spectra of free exciton luminescence.

Enhancement of Exciton and Biexciton Luminescence in CuCl QDs on Dielectric Multilayers

We investigated photoluminescence properties of the exciton and the biexciton in CuCl quantum dots embedded in an SiO 2 matrix on (λ/4) TiO 2/(λ/4) SiO 2 dielectric multilayers of the distributed Bragg reflector. The intensities of the free exciton luminescence band and the biexciton luminescence bands are enhanced by factors 2 and 6 times in comparison with those in only CuCl quantum dots embedded in an SiO 2 matrix. The origin of this luminescence behavior is partly reflection effects of the luminescence and excitation laser lights but additional effects need to be considered. One possible candidate is the strong exciton-photon interaction due to the confinement effect of the photons in the active layer with CuCl quantum dots.

Strong anti-Stokes photoluminescence of GaAs free excitons inGaAs/Ga1−xAlxAsheterojunctions

In heterojunctions, the strong electric field present at the interface drives the carriers vertically and therefore opens the way to several unique features observed in the optical spectra and transport experiments. In this work, we report the observation of GaAs free-exciton luminescence by exciting ${\mathrm{G}\mathrm{a}\mathrm{A}\mathrm{s}/\mathrm{G}\mathrm{a}}_{1\ensuremath{-}x}{\mathrm{Al}}_{x}\mathrm{As}$ heterojunctions below the energy of the free excitons in GaAs. This anti-Stokes photoluminescence has a long rise time and decay time. We attribute the anti-Stokes photoluminescence to the recombination of the excitons formed bimolecularly from free carriers excited from ionized donor and acceptor levels. This mechanism is enabled by the bending of the band structure across the heterointerface and yields the emission of photons at energies above the injected photons.

Physics of halide scintillators

The paper reviews the results obtained within the last 5–6 years in the field of the physics of some halide crystals which have led to a better understanding of their characteristics. The luminescence and scintillation characteristics of caesium iodides, widely used in the existing detectors, as well as the properties of some other selected materials are discussed in view of their possible applications. For all the systems concerned, the emission important for their practical applications arises from the radiative decay of excitons of various types. The following features are considered in more detail: (1) the excited states structure, the mechanism of excitation of the localized exciton emission in the impurity-induced absorption bands, the processes of defects creation, and the scintillation mechanism in CsI : Tl and CsI : Na scintillators; (2) the luminescence of on-centre excitons in CsI and RbI crystals; (3) the luminescence of free cation excitons in the CsPbX 3-type aggregates thermally created in CsX : Pb single crystals (X=Cl, Br); (4) the model of the excited states responsible for the cation exciton luminescence of lead halides.

Resonant sharp hot free-exciton luminescence in 6H- and 4H-SiC due to inhibited exciton-phonon interaction

Experimentally observed sharp luminescence lines from hot free-exciton recombination in high-purity 6H- and 4H-SiC are presented. The phenomenon is explained in terms of inhibition of the exciton-phonon scattering, prohibited for excitons created resonantly near the bottom of the lowest exciton band at low temperatures. This gives rise to the hot, sharp luminescence. The model is in agreement with the observed quenching of the hot luminescence at higher temperatures (>5 K) and in more highly doped samples, as well as with the dispersion of the exciton band obtained from the measured electron and hole effective masses.

Effect of temperature on longitudinal optical phonon-assisted exciton luminescence in heteroepitaxial GaN layer

First-order and second-order longitudinal optical (LO) phonon-assisted luminescence lines of neutral donor-bound excitons and free excitons in heteroepitaxial GaN film have been investigated in temperature range from 3.5 to 50 K. The energy spacing between the exciton resonant lines and their corresponding LO phonon replicas is found to be strong temperature dependent. Permogorov’s theory on LO phonon-assisted luminescence of free excitons could be employed to explain the main experiments. It is also found that the interaction strength of the free exciton with LO phonon is stronger than that of the bound exciton with LO phonon.

Excitons in a single two-dimensional semiconductor crystal of H 3N(CH 2) 6NH 3PbI 4

We have measured the reflection and luminescence spectra of synthetic two-dimensional semiconductor crystals of H 3N(CH 2) 6NH 3PbI 4 for E∥〈010〉 and E∥〈100〉 polarizations at low temperatures. There appear two absorption bands related to excitons with polarization vectors parallel and oblique to the inorganic layer. From the temperature dependence of the free exciton luminescence intensity, the exciton binding energy is obtained as 0.33 eV, the value of which is consistent with the energy distance between the n=1 and n=2 exciton absorption bands. This very large binding energy is characteristic of layered perovskite crystals.

Luminescence of free and self-trapped excitons in wide-gap oxides

Behaviour of the intrinsic emissions of 7.6, 3.45 and 3.6 eV in Al 2O 3, Y 2O 3 and Sc 2O 3, respectively, has been studied under a crystal excitation by 5–35 eV photons at 8–80 K. In Al 2O 3 and Y 2O 3, electrons and holes do not undergo transformation into a self-trapped state, while self-shrunk excitons are formed at the direct optical creation of excitons or at the recombination of electrons with holes under the conditions of a high excitation density. In line with the theoretical assumption made by Sumi, manifestations of the photocreation of spatially correlated self-trapped d-electrons and p-holes are revealed in the spectra of the tunnel phosphorescence of Sc 2O 3.

Optical properties of CdS layers grown by MOVPE on (211) B and (100) GaAs

CdS layers were grown by MOVPE on (100) GaAs and (211)B GaAs substrates. The last substrate orientation yields homogeneous wurtzite CdS layers with sharply defined free excitonic structures seen in optical reflectivity and photoluminescence. Free exciton luminescence dominates the spectrum at T>50 K. Structural studies indicate that the layers grown on (100) GaAs substrates are a mixture of hexagonal and cubic CdS phases spatially arranged in a quasi-periodic fashion. The luminescence spectrum reflects this situation.

Photo-and X-ray luminescence spectra of CsPbX3 microcrystals dispersed in a PbX2 (X=Cl, Br) matrix

The CsPbX3 (X=Cl, Br) microcrystals are obtained in a PbX2 (X=Cl, Br) matrix doped with Cs ions (CCs=0.1–1 mol %). Spectral and kinetic parameters of the intrinsic luminescence of the CsPbX3 single crystals are presented. For the CsPbCl3 microcrystals, a quantum-confinement effect is detected, which reveals itself as a decrease in the lifetime of the free exciton luminescence. A mechanism of formation of a CsPbX3 microcrystal in a PbX2 matrix is proposed.

Biexciton luminescence from CuGaS 2 bulk single crystals

New photoluminescence from CuGaS 2 appeared at the lower energy side of the free exciton luminescence under high excitation. The time-resolved spectrum of this luminescence at 0–20 ps after the excitation fits quite well with the calculated spectrum of the luminescence from the biexciton with the binding energy of 4.5±0.5 meV. Its time-integrated luminescence intensity was proportional to the excitation intensity to the power of ∼2.

Self-Trapped Excitons in Ultrathin PbI2Nanocrystals Embedded in a Polymer

We have found broad band luminescence due to a self-trapped exciton (STE) and disappearance of a free exciton luminescence in ultrathin nanocrystals of PbI 2 embedded in ethylene-methacrylic acid c...

Observation of Biexciton Luminescence from CuGaS2 Bulk Single Crystals

Excitation intensity dependences of time-integrated and picosecond time-resolved excitonic photoluminescence from CuGaS2 bulk single crystals were studied. Under high excitation broad and fast decay luminescence appeared at the lower energy side of free exciton luminescence. The new luminescence is attributed to the radiative recombination of biexcitons. The shape of the time-resolved spectrum of this luminescence at 0–20 ps after excitation fits quite well with “reverse” Boltzmann distribution for the biexciton temperature of ∼54 K at the excitation level of 42 MW/cm2 and binding energy of the biexciton of 4.5±0.5 meV. This is, to the best of our knowledge, the first observation of biexciton luminescence in CuGaS2.

Polarized luminescence of CsPbBr3 nanocrystals (quantum dots) in CsBr:Pb single crystal

Polarization of the free exciton luminescence of CsPbBr3-type semiconducting nanocrystals of perovskite structure dispersed in CsBr:Pb crystal has been observed for the first time. The polarization spectra, temperature and angular dependences of the degree of polarization as well as the dependence of polarization characteristics on size and shape of CsPbBr3 nanocrystals have been studied. The conclusions have been made about the orientation of anisotropic CsPbBr3 nanocrystals with respect to CsBr crystal axes, the origin of optical transitions responsible for the exciton emission as well as about the mechanism of the production of the nanocrystals in the process of thermally stimulated diffusion of Pb2+ ions in CsBr:Pb crystal lattice.

Temperature quenching of photoluminescence intensities in undoped and doped GaN

This work discusses the temperature behavior of the various photoluminescence (PL) transitions observed in undoped, n- and p-doped GaN in the 9–300 K range. Samples grown using different techniques have been assessed. When possible, simple rate equations are used to describe the quenching of the transitions observed, in order to get a better insight on the mechanism involved. In undoped GaN, the temperature dependence of band edge excitonic lines is well described by assuming that the A exciton population is the leading term in the 50–300 K range. The activation energy for free exciton luminescence quenching is of the order of the A rydberg, suggesting that free hole release leads to nonradiative recombination. In slightly p-doped samples, the luminescence is dominated by acceptor related transitions, whose intensity is shown to be governed by free hole release. For high Mg doping, the luminescence at room temperature is dominated by blue PL in the 2.8–2.9 eV range, whose quenching activation energy is in the 60–80 meV range. We also discuss the temperature dependence of PL transitions near 3.4 eV, related to extended structural defects.

An Analysis of Electron-Hole Recombination in Solid Xenon with Time-Resolved Luminescence Spectroscopy

The formation of free excitons in solid Xe from photogenerated electron-hole pairs was investigated with time-resolved luminescence spectroscopy. The initial kinetic energy of the photocarriers was varied over a large range by an appropriate choice of photon energy of excitation. The decay curves of the free-exciton luminescence yield a cascade-type behaviour. The experimental results were reproduced with model calculations including thermalization via scattering on acoustic phonons and a recombination cross-section which depends on the actual carrier temperatures. Good agreement between experiment and theory was found.

Creation of electronic polaron complexes in solid xenon observed in free-exciton luminescence under selective photon excitation

Abstract Time-resolved excitation spectra of the free exciton luminescence of solid xenon are measured in the energy range up to 30 eV at 7.1 K and 9.3 K. The pronounced structures observed near band gap and two band gap energies are assigned to primary and secondary excitons formed via various mechanisms. For the first time, the creation of free secondary excitons is directly observed in the excitation spectra. The results are assigned to creation of electronic polaron complexes and to inelastic scattering of photocarriers.

Exciton formation and relaxation in GaAs epilayers

Exciton formation and relaxation in GaAs bulk epilayers have been studied by means of time-resolved photoluminescence techniques. It is found that the time evolution of the free exciton luminescence, nonresonantly excited at low temperature and low intensity, is extremely slow, with a rise time of the order of 1 ns and a decay time of several ns. Simulations based on Monte Carlo solution of the set of coupled Boltzmann-like equations for free carriers and excitons show a nice agreement with the experimental data, and suggest a dominant role played by acoustic phonons in the exciton relaxation.

Microphotoluminescence of oval defects in a GaAs layer grown by molecular beam epitaxy

Using a low-temperature microphotoluminescence method, we have investigated the optical properties of oval defects in a GaAs layer grown by molecular beam epitaxy. The photoluminescence (PL) spectra of oval defects exhibited new distinct peaks, which had a narrow width (0.5 meV) and a strong intensity comparable to the exciton luminescence from a defect-free region of the epilayer. The excitation-power dependence and our PL image measurements suggest that the peaks are due to the recombination of excitons bound to the defects. The PL image of the free-exciton luminescence clearly revealed the features of a pair of asymmetric oval defects, each of which had a pyramidal structure consisting of stacking fault planes. The clear features indicate effective carrier confinement within the pyramid, where the stacking fault planes functioned as a potential barrier.

Time-resolved study of free exciton luminescence in KI and RbI

Temporal behaviours of free exciton luminescence in KI and RbI have been studied with picosecond light pulses under excitation in the n = 1 exciton absorption region. The decay curves are explained by one-dimensional diffusion and self-trapping of free excitons, which are represented by t −1 2 and an exponential function, respectively. The exponential decay constant is 0.35 ns for KI and 1.7 ns for RbI at around 10 K.