Exciton-related Photoluminescence Research Articles

Weak Quantum Confinement and Polaritons in ZnO and ZnO Cu Nanocrystals Prepared by Electrochemical Method

Photoluminescence (PL), X-ray diffraction (XRD), Scanning electron microscopy (SEM) and Raman scattering have been used for the optical and structural characterization of ZnO nanocrystals (NCs) of various sizes. The samples prepared by an electrochemical method have a size of NCs from the range 60 to 600 nm after a heat treatment for 2 hours at 400 oC in ambient air. The Raman scattering technique presents the several active modes including the surface phonon mode. The Raman intensity increases with decreasing NC size and the Raman spectrum shifts to low energy. Simultaneously the X-ray diffraction diagrams present a small shift of XRD peaks that testifies on a change in the lattice parameters in ZnO NCs with decreasing NC size. Photoluminescence spectra show a free exciton and defect-related emission. The intensity stimulation of exciton-related PL bands with NC size decreasing up to 60 nm is attributed to the realization of the week confinement and the exciton-light coupling with the formation of polariton in small size of ZnO NCs. The numerical calculation of recombination times in the frame of this model has confirmed the presented explanation.

Photoluminescence and structural properties of CdSe quantum dot–gelatin composite films

Optical and structural properties of composite films of CdSe quantum dots (QDs) embedded in gelatin matrix have been investigated by photoluminescence (PL), optical absorption and X-ray diffraction (XRD) methods. The optical absorption of the composite in the visible spectral range is found to be determined mainly by light absorption in the QDs. The decrease of the film transparency and the shift of the absorption edge to lower energies observed upon thermal annealing of the films at 140–160°C are ascribed to the formation of chromophore groups in gelatin matrix. XRD patterns of the composite revealed helix to coil transition in gelatin matrix under thermal annealing of the composite at 100–160°C. It is found that PL spectra of the composite are dominated by exciton and defect-related emission of the QDs and also contain weak emission of gelatin matrix. It is found that thermal annealing of the composite at 100–160°C changes PL intensity and produces the shift of the PL bands to lower energies. As the annealed composite was kept in air for several months, the shift of exciton-related PL band position restored partially and the PL intensity increased. It is proposed that the increase of the PL intensity upon the thermal annealing of composite at 140°C can be used for enhancement of the QD-related PL. Changes that occurred in the PL spectra of composite are ascribed to structural and chemical transformations in gelatin matrix and at the QD/gelatin interface.

Size dependent emission stimulation in ZnO nanosheets

Photoluminescence (PL), X ray diffraction (XRD) and Raman scattering have been studied in crystalline ZnO nanosheets (NSs) of different sizes, estimated by scanning electronic microscopy (SEM). ZnO NSs with the size from the range of 60–600nm were created by the electrochemical (anodization) method and followed thermal annealing at 400°C for 2h in ambient air. XRD study confirms the wurtzite structure of ZnO NSs and has revealed that the lattice parameters increase monotonically with decreasing NS sizes. Simultaneously the intensity of a set of Raman peaks increases and Raman peaks shift into the low energy range. The surface phonon has been detected in smallest size ZnO NSs.Two types of PL bands deal with a set of phonon replicas of free excitons and the defect related emission have been detected in ZnO NSs. The intensity enhancement of exciton- and defect-related PL bands with decreasing ZnO NS sizes has been detected. The intensity stimulation of exciton-related PL bands is attributed to the realization of the week confinement and the exciton-light coupling with the formation of polariton in small size ZnO NSs of 67–170nm. The intensity rising of defect-related PL bands is attributed to the concentration enlargement of surface defects when the surface to volume ration increases at decreasing ZnO NS sizes. Numerical simulations of radiative lifetimes and exciton radiative recombination rates in ZnO NSs for different emission wavelengths have been done using the exciton-light coupling model. Then the experimental and numerically simulated PL results have been compared and discussed.

Exciton states in GaAs [formula omitted] systems under magnetic fields and hydrostatic pressure

Excitons in GaAs n-type δ-doped quantum wells are studied taking into account the effects of externally applied magnetic fields as well as of hydrostatic pressure. The one-dimensional potential profile in both the conduction and valence bands is described including Hartree effects via a Thomas–Fermi-based local density approximation. The allowed uncorrelated energy levels are calculated within the effective mass and envelope function approximations by means of an expansion over an orthogonal set of infinite well eigenfunctions and a variational method is used to obtain the exciton states. The results are presented as functions of the two-dimensional doping concentration and the magnetic field strength for zero and finite values of the hydrostatic pressure. In general, it is found that the exciton binding energy is a decreasing function of the doping-density and an increasing function of the magnetic field intensity. A comparison with recent experiments on exciton-related photoluminescence in n-type δ-doped GaAs is made.

Exciton-light coupling and polaritons in SiC nanocrystals

The paper presents the results of SiC:N nanocrystal characterization using the photoluminescence (PL) and X-ray diffraction (XRD) techniques. Photoluminescence study of porous SiC:N (PSiC) layers with different PSiC thicknesses and SiC nanocrystal (NC) sizes reveals the intensity stimulation mainly for the high energy PL bands. The early investigation of temperature dependences of the high energy PL bands has shown that these PL bands related to the free exciton emission in the different SiC polytypes. The SiC polytypes in the original n-type SiC:N wafers and in porous SiC layers were confirmed by XRD study. The intensity enhancement of exciton-related PL bands in big size (50–250nm) SiC NCs is attributed to the realization of the exciton week confinement and exciton-light coupling with the formation of polaritons in the SiC NCs. The numerical simulation of exciton radiative recombination rates and radiative lifetimes for the different exciton emissions has been done using a model of exciton–light coupling in SiC NCs. The experimental and numerically calculated results have been compared and discussed.

CuBr films on KBr exhibiting 102 or 103 times higher efficiency of free-exciton luminescence than those on usual substrates such as Al2O3

CuBr films grown on KBr crystals show 102 or 103 times higher efficiency of free-exciton photoluminescence (PL) than those grown on conventional substrates such as Al2O3 or silica glass. The efficiency exceeds even that of CuCl films by a factor of several tens, despite the general recognition that the free-exciton PL from CuBr is much weaker than that from CuCl. The present result encourages the authors to challenge to rethink the exciton PL in CuBr as well as in CuCl, because the compounds are the model materials for basic and application studies of linear/nonlinear exciton-related PL properties of solids.

Extremely-high-intensity photoluminescence from CuBr films fabricated by film–substrate chemical reaction of CuCl films on KBr-crystal substrate

High quality CuBr film is fabricated by film–substrate chemical reaction of CuCl on KBr, where CuCl is vacuum-deposited film of the order of ∼ 100 nm thickness and KBr is fleshly-cleaved single crystal. The resulting CuBr films show ∼ 10 3 times higher efficiency of free-exciton photoluminescence (PL) than conventional CuBr films. The efficiency exceeds that of conventional CuCl films even by a factor of ∼ 10 2, despite the general recognition that the free-exciton PL from CuBr is much weaker than that from CuCl. The present result encourages us to challenge to rethink the exciton PL in CuBr as well as in CuCl, because the compounds are the model materials for basic and application studies of linear/nonlinear exciton-related PL properties of solids.

Electric field and interface fluctuation effects in CdTe/Cd 1− xMn xTe strained quantum wells

The excitonic properties of CdTe/Cd 1− x Mn x Te strained quantum wells (QWs) are calculated taking into account interface and electric field effects. Numerical results for CdTe/Cd 0.68Mn 0.32 QWs show that the broadening of the exciton-related photoluminescence spectra can be as large as 30 meV in the case of interfacial fluctuations of 3 monolayers in 50 A ̊ wide strained QWs subjected to a 100 kV/cm electric field.

Exciton-based photoluminescence broadening in graded ZnSe/ZnS xSe 1− x strained quantum wells

The role of interfacial fluctuations on the exciton-related photoluminescence broadening in graded ZnS/ZnSe 018S 0.82 strained quantum wells is studied taking into account piezoelectric field effects. The broadening of the exciton-related photoluminescence spectra due to the existence of gradual interfaces is shown to be of the order of 10 meV .

Exciton-related photoluminescence in homoepitaxial GaN of Ga and N polarities

A photoluminescence (PL) study of GaN homoepitaxial layers grown by metal–organic chemical-vapor deposition demonstrates the high optical quality of N-face layers deposited on vicinal (0001̄) GaN substrates. In contrast to broad PL emission in exact (0001̄) layers, narrow-bound (0.9 meV) and free- (A and B) excitonic transitions are observed. By following the PL spectra as a function of temperature and excitation power, the main optical transitions in the Ga- and the misoriented N-face layers are found to be the same. Observed differences are related to the distinct creation of donor and acceptor states in the samples of different polarities.

Photoluminescence of CuGaS2 epitaxial layers grown by metalorganic vapor phase epitaxy

High quality epitaxial layers of CuGaS2 have been grown on GaAs and GaP substrates by low-pressure metalorganic vapor phase epitaxy using cycropentadienylcoppertriethylphosphine, normal-tripropylgallium, and ditertiallybutyl sulphide precursors. Photoluminescence (PL) studies at 8 K have been carried out, and exciton-related PL peaks at 2.489 and 2.477 eV have been observed in addition to PL peaks at 2.43 (free-to-bound), 2.39 (donor-acceptor pair), and broad band at 1.65 eV. Temperature and excitation intensity dependences of the PL peaks have been studied in detail, and PL properties are compared with those for the bulk CuGaS2 crystals grown by the chemical vapor transport (CVT) method. PL spectra for nearly stoichiometric CuGaS2 epilayers exhibited the intense free exciton-related PL peak over the entire temperature range (8–300 K), which are in contrast to PLs dominated by defect-related deep peaks for Ga-rich or Cu-rich epilayers and the impurity related near-band edge PL for the CVT bulk crystal. These PL results show that the nearly stoichiometric CuGaS2 epilayers grown in this study are the highest quality reported so far.

Changes in the Exciton-Related Photoluminescence of 4H- and 6H-SiC Induced by Uniaxial Stress

Changes in the exciton-related photoluminescence of 4H- and 6H-SiC induced by uniaxial stress

Exciton-Related Photoluminescence in 4H-SiC Grown by Step-Controlled Epitaxy

The temperature dependence of exciton-related photoluminescence in 4H-SiC grown by step-controlled epitaxy has been studied in detail. In the low-temperature region, luminescence by recombination of free excitons and excitons bound to neutral nitrogen donors could be observed. From the energy of luminescence peaks by bound excitons, the temperature dependence of the exciton energy gap in 4H-SiC was obtained. The luminescence spectra of free excitons could be explained using a theoretical Maxwellian distribution of the kinetic energy of free excitons, based on a law for indirect-band semiconductors. In thermal quenching properties, the intensity of luminescence by bound excitons decreased with temperature. The quenching energy for luminescence by bound excitons was about 25 meV which was almost coincident with the previously reported binding energy of 20 meV. On the other hand, the intensity of luminescence by free excitons increased to 50 K, and after showing the maximum, it became quenched. From a rate equation analysis using transfer processes among free excitons and bound excitons, the above behavior is explained semiquantitatively.

Magnetophotoluminescence measurement of the formation time of an exciton in AlxGa1-xAs/GaAs quantum-well structures.

A magnetophotoluminescence technique is adopted to study the exciton formation processes in ${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As/GaAs multi-quantum-well structures, with the cyclotron periods of carriers in the quantum wells as measures of time. It is found that exciton-related photoluminescence intensities decrease with increasing external magnetic field along the growth direction. Detailed experimental and theoretical analyses revealed that the formation time of an exciton and hence the amount of carriers taking part in radiative recombination decreases with increasing magnetic field. It is found that the exciton formation time is dominated by the time during which the electron and hole move toward each other under the Coulomb force between them. Through the fitting of experimental and theoretical results, exciton formation time is directly obtained. In addition, it is found that the formation time of an exciton is a function of carrier density, and that excitons are formed after the carrier system is cooled down.

Growth of Cu(AlxGa1−x)SSe pentenary alloy crystals by iodine chemical vapor transport method

Single crystals of Cu(AlxGa1−x)SSe pentenary alloys were successfully grown by the vapor transport technique using iodine as a transport agent. Systematic measurements of the basic crystal properties were carried out using powder X-ray, photoluminescence (PL), photoreflectance (PR), and Van der Pauw methods. Dependence of the lattice constants, exciton transition energies, and PL peak energies on Al composition, x, were examined in detail. The lattice constants obeyed Vegard's law. A quadratic dependence of exciton transition energies on x was found. The broadening parameters in the PR spectrum increase monotonically with increasing x, the results showing that the crystal quality degrades with increasingx. Exciton-related PL peaks were found for 0 ≤ x ≤ 0.77 at 77 K.

Photoluminescence and Photoreflectance of CuGa(S1-xSex)2 and Cu(Ga1-xInx)S2 Alloys

Systematic photoluminescence (PL) and photoreflectance (PR) measurements have been carried out at 77 K on single crystals of CuGa(S1-xSex)2 and Cu(Ga1-xInx)S2 alloys grown by iodine-transport method. Energies of A, B and C excitons have been determined as a function of composition. An exciton-related PL peak has been observed for both alloys over the whole composition range.

Quantum size effects in optical properties of CdS-glass composites.

The optical properties of extremely small isolated semiconductors provide a sensitive probe of the developing electronic structure in the materials. Cadmium sulfide and ${\mathrm{CdS}}_{\mathrm{x}}$${\mathrm{Se}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$ crystallites are precipitated in an insulating glass matrix during a secondary heat-treatment procedure. Variation in the heat treatment significantly alters the final crystallite size, enabling the examination of a size-dependent change in the observed optical-absorption edge and exciton-related photoluminescence peak energies. A diffusion-limited coarsening behavior is exhibited by the CdS crystallites precipitated from a Zn-free base glass, indicating a high degree of purity in these crystals. Growth behavior of mixed crystallites, however, indicates some stoichiometric variation with heat-treatment time. Quantum size effects measured using the CdS precipitates in the size range from 40--400 A\r{} reflect confinement primarily in the translational motion of the Wannier exciton with some slight modification of its internal electron and hole orbits.

Effect of elastic strain on the energy band gap in heteroepitaxially grown ZnSe

Low-temperature photoluminescence spectra have been measured for a series of ZnSe films of varying thicknesses grown by molecular-beam epitaxy on GaAs, and variations in the spectra have been correlated with changes in the lattice parameter in the epilayer. Elastic strain resulting from in-plane compression of the epilayer is found to cause a displacement of exciton-related photoluminescence features to higher energies with decreasing layer thickness. This behavior reflects an increase in the band gap of ZnSe under increasing strain, similar to the behavior observed in several other zinc-blende semiconductor epilayer systems, but in contrast to the behavior predicted for this particular system by some authors.

Impact ionization of excitons and shallow donors in InP

Quenching of neutral-donor- and exciton-related photoluminescence in high-purity liquid-phase-epitaxial and vapor-phase-epitaxial InP samples is studied as a function of applied weak electric fields at liquid-He temperatures. Field-dependent suppression of neutral-donor-to-acceptor and free- and bound-exciton recombination peaks in the spectrum is attributed to impact ionization of the shallow donors and excitons by hot electrons which were accelerated by the applied field. More rapid quenching of the neutral-donor-bound and neutral-acceptor-bound exciton peaks with increasing field strength as compared to neutral-donor and free-exciton peaks is attributed to the dissociation of free excitons from the neutral centers by impact ionization.