Effect of isotopic mass on the photoluminescence spectra of β zinc sulfide

Zinc sulfide is a wide bandgap semiconductor which crystallizes in either the wurtzite modification (α-ZnS), the zincblende modification (β-ZnS) or as one of several similar tetrahedrally coordinated polytypes. In this work, we report a photoluminescence study of different samples of isotopically pure β-ZnS crystals, and crystals with the natural isotopic abundances, at 15 and 77 K. The derivatives of the free and bound exciton energies on isotopic mass have been obtained. They allow us to estimate the contribution of the zinc and sulfur vibrations to the bandgap renormalization energy by electron-phonon interaction. A two-oscillator model based on the zinc and sulfur renormalization energies has been used to account for the temperature dependence of the bandgap energy in ZnS. The results are compared with those found for other tetrahedrally coordinated semiconductors.     

Characterization and luminescent properties of SiO2:ZnS:Mn and ZnS:Mn nanophosphors synthesized by a sol-gel method

ZnS and SiO₂-ZnS nanophosphors, with or without different concentration of Mn²⁺ activator ions, were synthesized by using a sol-gel method. Dried gels were annealed at 600 °C for 2 h. Structure, morphology and particle sizes of the samples were determined by using X-ray diffraction (XRD), highresolution transmission electron microscopy (HRTEM) and field emission scanning electron microscopy (FESEM). The diffraction peaks associated with the zincblende and the wurtzite structures of ZnS were detected from as prepared ZnS powders and additional diffraction peaks associated with ZnO were detected from the annealed powders. The particle sizes of the ZnS powders were shown to increase from 3 to 50 nm when the powders were annealed at 600 °C. An UV-Vis spectrophotometer and a 325 nm He-Cd laser were used to investigate luminescent properties of the samples in air at room temperature. The bandgap of ZnS nanoparticles estimated from the UV-Vis data was 4.1 eV. Enhanced orange photoluminescence (PL) associated with ⁴T₁→⁶A₁ transitions of Mn²⁺ was observed from as prepared ZnS:Mn²⁺and SiO₂-ZnS:Mn²⁺ powders at 600 nm when the concentration of Mn²⁺ was varied from 2-20 mol%. This emission was suppressed when the powders were annealed at 600 °C resulting in two emission peaks at 450 and 560 nm, which can be ascribed to defects emission in SiO₂ and ZnO respectively. The mechanism of light emission from Mn²⁺, the effect of varying the concentration on the PL intensity, and the effect of annealing are discussed.     

The nature of excitonic complexes in ZnTe

Uniaxial stress experiments were used to investigate the nature of the luminescence lines observed at low temperatures in ZnTe in the vicinity of the absorption edge. The single crystals used in this experiment were grown from solution of ZnTe in tellurium. Both “as-grown” crystals and crystals annealed in Zn vapour were investigated. The most intense line in “as-grown” crystals is attributed to an exciton bound to a neutral acceptor. The binding energy of the exciton in this center is 6 meV. After annealing a new center appears in the same spectral region. Stress experiments as well as the temperature dependence of the intensity of the luminescence indicate that this center is a complex consisting of an exciton and an ionized donor. Splitting of J = 1 (Γ₅) and J = 2 (Γ₃ + Γ₄) levels was found to be 1.2 meV.     

Luminescent thin ZnS : Mn films

Photo-, cathodo-, β-luminescence spectra of the ZnS : Mn films and electroluminescence spectra of highly efficient d.c. diodes with SnO_x-ZnS : Mn-Cu_xS-ZnS : Mn-Al structure have been investigated. Strong dependence of intensity and structure of the luminescence band on applied voltage has been found. Results suggest that collision cannot be the only process causing luminescence of the d.c. diodes investigated. The yield of Mn luminescence in ZnS is found to be strongly electric field-dependent.     

Exciton spectra in SrS1−xSex mixed crystals

Exciton reflection spectra are investigated at 2 K on single crystals of SrS_1?xSe_x over the whole composition range. All the exciton reflection structures are found to show an amalgamation-type behaviour. From the observed dependence of the reflection peaks on composition and temperature, it is concluded that the exciton spectra of these Sr-chalcogenides are composed of two kinds of the exciton multiplets, the lower one being associated with the X-point band-edge and the higher one with the г-point band edge. It is predicted that these crystals have an indirect band-gap for which phonon-assisted dipole transitions are parity-forbidden.     

Optical spectra and energy band structure of AgAsS2 crystals

Ground and excited states of three exciton series are observed in the region of fundamental absorption edge of AgAsS₂ crystals. The contours of exciton reflection spectra are calculated and the main parameters of excitons and energy bands are determined in the center of Brillouin zone. The optical reflection spectra are investigated at 30 K in E∥c and E⊥c polarizations in AgAsS₂ crystals in the region of 2-6 eV. The optical functions are calculated from the reflection spectra and a scheme of electronic transitions responsible for peculiarities of reflection spectra deep into the absorption band is proposed.     

Bound exciton luminescence of Ar- and Al-implanted ZnO

Argon- and aluminum-implanted ZnO single crystals (N_impl=10¹⁶?10¹⁹cm^?3) were investigated at liquid helium temperature by photoluminescence. We obtained highly resolved emission spectra of implanted and thermally annealed samples. Maximum luminescence yield was achieved after annealing with an oxygen ambient at 800°C and an anneal time of 30 min. In Al-implanted ZnO crystals, we observed a strong emission line $\text{I}{}_{\mathrm{<mtext>5</mtext><mtext>6</mtext>}}\text{at λ = 3688}\text{A}\text{?}$. The intensity of this line is correlated with the implanted Al concentration. The I_{$\text{5}\text{6}$} line is interpreted as the recombination of a bound exciton at a polycentric Al complex.     

Optical properties of Tm-doped GaSe single crystals

Optical properties of Tm-doped GaSe single crystals were investigated by measurements of optical absorption and photoluminescence. The single crystals were grown by the Bridgman technique. The X-ray diffraction analysis revealed that the single crystals were in the ε-type GaSe phase. The optical absorption spectra showed a sharp absorption peak at 582 nm near the band edge, which is due to direct free exciton. The temperature dependence of the energy of the exciton absorption peak was well fitted by the Varshni relation. In the photoluminescence spectrum at 10 K, we observed a very weak emission peak at 586 nm, a relatively strong emission peak centered at 613 nm, and several sharp and narrow emission peaks in the 790-840 nm region. The two emission peaks at 586 and 613 nm were associated with intrinsic emission lines due to direct free exciton and indirect bound exciton. The emission peaks in the 790-840 nm region, which were related to extrinsic emission, were assigned as due to the ³F₄→³H₆ transition of Tm³⁺ ions with a low symmetry of D₃ in the host lattice.     

X-ray diffraction and optical studies of CdS1−x Sex solid solutions with stacking faults

The existence in CdS_1?x Se_x crystals (with x=0.10–0.50) of crystalline regions with stacking faults (SF) was first demonstrated by x-ray diffraction and optical methods. X-ray diffraction studies showed SF to be present in all the samples investigated, but in different concentrations. The effect of SF present in CdS_1?x Se_x solid solutions of different compositions on their exciton reflectance and photoluminescence (PL) spectra has been studied. Crystals with high SF concentrations were found to exhibit new exciton bands, which are manifested in reflectance and PL spectra. In the CdS_1?x Se_x phase with SF, resonant emission due to free excitons and the corresponding phonon replicas have been observed. The effect of reabsorption, which can bring about a change in the zero-phonon emission line shape (doublet formation), as well as affect the intensity ratios of the zero-phonon line to the phonon replicas, has been analyzed. It is pointed out that the variation with temperature of the shape of the SF-induced PL exciton line indicates its complex structure, with the constituents of this structure varying in intensity with increasing temperature.     

Letters to the editor

Abstract

Optical absorption spectra of TeO₂ crystals, irradiated at room temperature by up to 2·10¹⁸ cm^?2 10-MeV electrons and subsequently annealed to 575K, are studied. The dependence of Urbach absorption edge parameters in TeO₂ on the electron beam fluence is discussed. The irradiation-induced near-edge broad (2.5–3.5eV) absorption band is shown to be related to oxygen vacancies, annealing at 475–525K.     

Pool-Frenkel thermoelectric modulation of exciton photoluminescence in GaSe crystals

Effect of external field on the exciton photoluminescence of GaSe crystals has been investigated and it has been observed that the PL is quenched with the applied field. The changes observed in the PL spectra have been analyzed with impact exciton, Franz-Keldysh and Pool-Frenkel effects. From the analyses of the experimental data, it has been found that the intensity of direct free, indirect free and bound exciton peaks decreased exponentially with the square root of applied field as I∼exp-β√E. The energy positions of emission peaks were found to shift to longer wavelength with the applied field as ΔE∼β√E. From these findings, the Pool-Frenkel thermoelectric field effect is seen to be the dominant mechanism in the variation of exciton PL with the applied field even though the impact exciton and Franz-Keldysh effects contribute.     

Electronic excitations and luminescence of SrMgF4 single crystals

The electronic and crystal structures of SrMgF₄ single crystals grown by the Bridgman method have been investigated. The undoped SrMgF₄ single crystals have been studied using low-temperature (T = 10 K) time-resolved fluorescence optical and vacuum ultraviolet spectroscopy under selective excitation by synchrotron radiation (3.7–36.0 eV). Based on the measured reflectivity spectra and calculated spectra of the optical constants, the following parameters of the electronic structure have been determined for the first time: the minimum energy of interband transitions E _g = 12.55 eV, the position of the first exciton peak E _n = 1 = 11.37 eV, the position of the maximum of the “exciton” luminescence excitation band at 10.7 eV, and the position of the fundamental absorption edge at 10.3 eV. It has been found that photoluminescence excitation occurs predominantly in the region of the low-energy fundamental absorption edge of the crystal and that, at energies above E _g , the energy transfer from the matrix to luminescence centers is inefficient. The exciton migration is the main excitation channel of photoluminescence bands at 2.6–3.3 and 3.3–4.2 eV. The direct photoexcitation is characteristic of photoluminescence from defects at 1.8–2.6 and 4.2–5.5 eV.     

Oxygen inducedK-shell ionization of selected elements from sulphur to bromine

K-shell X-ray production cross sections for oxygen ions on thin solid targets of 13 selected elements with atomic numbers between 16 and 35 were measured by a Si(Li) detector at incident ion energies from 7 to 24 MeV. Ionization cross sections are compared with calculations assuming Coulomb-ionization. Best agreement is found with theoretical cross sections that include corrections for binding energy and Coulomb deflection effects. Energy shifts ofK _α andK _β X-rays andK _α /K _β intensity ratios were also measured and are used to deduce information about outer shell ionization.     

Pre-equilibrium alpha emission accompanying deep-inelastic16O+58Ni collisions

α particles were measured in coincidence with projectile-like reaction products (oxygen and carbon) produced in deep-inelastic¹⁶O+⁵⁸Ni collisions at about 6 MeV/N bombarding energy. The kinematic analysis of the HI andα energies measured as a function ofΘ _α gives strong evidence for a sequential process: the target-like fragments are excited by the deep-inelastic collision and undergo subsequentα decay. In contrast, the angular correlations show a pronounced forward peak, indicative of direct or pre-equilibriumα emission. The emission time for the latter is estimated to be of the order of 2×10^?21 s. To resolve this conflict of co-existing statistical and direct features of the pre-equilibrium emission, the concept of a hot spot is proposed. From the angular correlation and from theα multiplicities, a local temperature ofT?3.5 MeV is deduced which agrees well with the temperature derived from the shape of theα spectra. The spot size is estimated to be 1/5 of the sphere.     

Triplet exciton in a non-reactive bis-triphenyllead (or tin) diacetylene crystal. A spectroscopic study

The lowest triplet exciton state of crystals formed by (C₆H₅)₃?X?C ≡ C?C ≡ C?X?(C₆H₅)₃ where X is Pb or Sn and an equimolecular amount of CH₂Cl₂ solvent molecules, is studied through the analysis of absorption and of phosphorescence emission and excitation spectra at low temperatures. In the lead compound, the origins of absorption and emission are identical, showing the dominance of free exciton emission and the absence of crystal lattice relaxation in the excited state; the splitting is accounted for by symmetry considerations in the different crystalline phases; the triplet exciton band is several cm^-1 wide, although the nearest diacetylene-diacetylene distance is about 10 Å. There is evidence for triplet exciton motion, through the detection of trap (presumably X-trap) emission in the tin compound, and through the analysis of the excitation spectrum lineshape, showing the effect of exciton-exciton interaction in the lead compound.     

Cathodoluminescence of ZnS single crystals subjected to thermal annealing in vapors of the constituents

The 77°K cathodoluminescence spectra of ZnS single crystals grown from the melt and annealed in vapors of the constituents were studied in the spectral region 360 to 550 nm. The single crystals contained oxygen whose phase state could be changed by thermal annealing. The effect of oxygen in the ZnS lattice on the appearance and intensities of various bands in the zinc sulfide spectrum was investigated. It turned out that the bands at 390–400 nm, 410–430 nm, and 500–525 nm are associated with the luminescence of a solid solution of ZnS· O in the lattice. The intensity of the green luminescence was a function of oxygen concentration in precipitates of the solid solution on dislocations. The luminescence in the 363 to 370 nm region is associated with zinc oxide which separates from ZnS containing various amounts of sulfur.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 5, pp. 84–88, May, 1975.     

Size dependent effective band gap,optical absorption coefficients and refractive index changes in a wide ZnS/Zn1−xMgxS strained quantum well

Exciton binding energy of a confined heavy hole exciton is investigated in a Zn_1−xMg_xS/ZnS/Zn_1−xMg_xS single strained quantum well with the inclusion of size dependent dielectric function for various Mg content. The effects of interaction between the exciton and the longitudinal optical phonon are brought out. The effect of exciton is described by the effective potential between the electron and hole. The interband emission energy as a function of well width is calculated for various Mg concentration with and without the inclusion of dielectric confinement. Non-linear optical properties are carried out using the compact density matrix approach. The dependence of nonlinear optical processes on the well width is investigated for different Mg concentration. The linear, third order non-linear optical absorption coefficients values and the refractive index changes of the exciton are calculated for different concentration of magnesium content. The results show that the exciton binding energy is found to exceed LO phonon energy of ZnS for x>0.2 and the incorporation of magnesium ions and the effect of phonon have great influence on the optical properties of ZnS/Zn_1−xMg_xS quantum wells.     

Exciton structure in luminescence excitation spectra of ZnS phosphors

Dips and peaks in luminescence excitation spectra of ZnS phosphors at 75 K have been identified as due to intrinsic exciton formation. Excitation spectra for Mn²⁺ emission exhibit an oscillatory structure due to hot exciton creation.     

Study of the electronic structure of M(CO)5Cl complexes (M = Mn, Re) using X-ray spectroscopy and density-functional theory

The electronic structure of M(CO)₅Cl metal complexes (M = Mn, Re) has been investigated by X-ray emission spectroscopy. The obtained X-ray emission C Kα, OKα, Cl Kβ₁, MnLα, and MnKβ₅ spectra for Mn(CO)₅Cl and ReLβ₅ spectra for Re(CO)₅Cl have been interpreted on the basis of the quantum-mechanical calculations by the method of density-functional theory, using the Gaussian-98 program. The investigation of Mn(CO)₅Cl and Re(CO)₅Cl showed similarity of the electronic structure of both complexes. Only small differences have been revealed in the energy structure and orbital occupancies of the atomic orbitals of the corresponding molecular orbitals; these differences are caused by the difference of the type of metal ions and the molecule geometry. On the basis of the performed quantum-mechanical calculations, theoretical X-ray emission spectra have been constructed, which reproduce well the characteristic features of the corresponding experimental spectra of M(CO)₅Cl metal complexes (M = Mn, Re).