Edge Of Fundamental Absorption Band Research Articles

Optical properties of Ga-Ge-Sb-Se chalcogenide glasses doped with terbium and dysprosium ions, near the fundamental absorption band edge

In the paper, results of measurements of the optical response of chalcogenide glasses of the Ga5Ge20Sb10Se65 composition doped with rare-earth ions Tb3+ or Dy3+, in the wavelength range of 0.7-1.5 μm by the method of IR spectroscopy are presented. Parameters characterizing the fundamental absorption band edge of the glasses --- optical bandgap energy, Urbach tail parameter and weak absorption tail parameter --- have been evaluated. It has been found that doping in low concentrations (up to 0.3 wt%) does not influence the optical bandgap energy and optical properties of the glasses in the range of the Urbach tail, but in the range of the weak absorption tail, optical response of the glasses depends on the activator concentration. Crystallization of the glasses produced by the direct melting method also depends on the activator concentration. In the glasses doped with Dy3+, absorption bands of the ion are located in the range of the weak absorption tail of the glass that makes possible energy transfer between the ions and the gap states of the charge carriers. Keywords: rear earth elements, chalcogenide glasses, bandgap energy, Urbach tail, weak absorption tail, gap states.

Оптические свойства халькогенидных стекол системы Ga-Ge-Sb-Se, легированных ионами тербия и диспрозия, вблизи края полосы фундаментального поглощения

In the paper, results of measurements of the optical response of chalcogenide glasses of the Ga5Ge20Sb10Se65 composition doped with rare-earth ions Tb3+ or Dy3+, in the wavelength range of 0.7 – 1.5 μm by the method of IR spectroscopy are presented. Parameters characterizing the fundamental absorption band edge of the glasses - optical bandgap energy, Urbach tail parameter and weak absorption tail parameter - have been evaluated. It has been found that doping in low concentrations (up to 0.3 mass%) does not influence the optical bandgap energy and optical properties of the glasses in the range of the Urbach tail, but in the range of the weak absorption tail, optical response of the glasses depends on the activator concentration. Crystallization of the glasses produced by the direct melting method also depends on the activator concentration. In the glasses doped with Dy3+, absorption bands of the ion are located in the range of the weak absorption tail of the glass that makes possible energy transfer between the ions and the gap states of the charge carriers.

Features of determining the optical bandgap of amorphous nanosized composite TiO2:Ag films

The study results of optical properties of amorphous nanosized TiO2:Ag composite films with the structure of an amorphous TiO2 matrix with inclusions of isolated Ag nanoparticles are presented. The TiO2:Ag films were obtained by ion-plasma high-frequency magnetron co-sputtering of a polycrystalline TiO2 and Ag target. The films were 40–50 nm thick, and the maximum silver concentration reached 9.0 at.%. The fundamental absorption band edge was analyzed for different possible absorption laws. It is shown that, in TiO2 and TiO2:Ag films, the edge of the fundamental absorption band is formed mainly due to indirect allowed optical transitions (the quadratic absorption law), and this absorption law does not change with increasing silver concentration. The optical bandgap in TiO2:Ag films depends on the Ag concentration and is determined by the degree of order in the amorphous matrix.

Lattice dynamics and electronic transitions in a structurally complex layered copper borate Cu3(BO3)2

Copper borate Cu$_3$(BO$_3$)$_2$ is a complex compound with a layered crystallographic structure in which the Jahn-Teller active and magnetic copper Cu$^{2+}$ ions occupy sixteen nonequivalent positions in the unit cell displaying controversial magnetic behavior. In this paper, we report on the infrared and Raman spectroscopic studies of the lattice dynamics and the electronic structure of 3$d^9$ copper states below the fundamental absorption band. The lattice dynamics is characterized by a large number of phonons due to a low $P\overline{1}$ space group symmetry and a large unit cell with Z=10. Unusually rich set of phonons was found in the low-energy part of the infrared and Raman spectra below 100 cm$^{-1}$, which we tentatively assign to interlayer vibrations activated by a crystal superstructure and/or to weak force constants for modes related to some structural groups. Several phonons show anomalous behavior in the vicinity of the magnetic phase transition at $T_N$=10 K thus evidencing magnetoelastic interaction. No new phonons were found below $T_N$, which excludes the spin-Peierls type of the magnetic transition. In the region of electronic transitions, a strong broad absorption band centered at $\sim$1.8 eV is observed, which we assign to overlapping of transitions between the 3$d^9$ states of Cu$^{2+}$ ions split by the crystal field in nonequivalent positions. The fundamental charge-transfer absorption band edge has a complex structure and is positioned around $\sim$2.8-3.0 eV.

Measurement of non-linear optical coefficients of chalcogenide glasses near the fundamental absorption band edge

A time-resolved pump-probe method is used for the evaluation of non-linear optical coefficients of chalcogenide glasses from the As-S-Se and Ge-Se systems near their fundamental absorption band edges. The results are analyzed via comparison with the spectral dependencies of the non-linear optical coefficients of crystalline semiconductors; the role of electron transitions through the gap states of chalcogenide glasses is discussed.

Exciton-phonon interaction in PbI2 revealed by Raman and photoluminescence studies using excitation light overlapping the fundamental absorption edge

Enhancement at low temperatures of the Raman scattering excited by laser light situated near the edge of the fundamental absorption band is often encountered when studying nanoscale structures. Theory devoted to this phenomenon has established that it originates in an exciton-phonon interaction process, known as the Frohlich interaction. Such a phenomenon was observed in PbI2. The experimental data conclude that the enhancement of the Raman emission results from: (i) an optical excitation near edge of fundamental absorption band; (ii) it is conditioned by the existence of excitonic PL; (iii) its occurrence is different over stokes and anti-stokes Raman branches as result of the different overlapping of the Raman spectral range and the excitonic PL band profile; and (iv) it appears more intense in micrometric powders or in bulk crystalline material. These data are interpreted as stimulated Raman effect resulting from the mixing of the pump laser light and the excitonic light.

Non-linear optical response of bulk chalcogenide glasses near the fundamental absorption band edge

Magnitudes of the non-linear coefficients of absorption and refraction have been evaluated near the bandgap wavelengths of chalcogenide glasses of the system As-S-Se by using the interferometric pump-probe method and are compared with literature data. Photoexcited plasma dynamics and long-time scale variation of the dielectric constant have been studied by comparison with the results of numerical modelling of the behaviour of the glasses when heated by the ultra-short laser pulses.

Preparation and characterization of nanosized GdxBi0.95−xVO4:0.05Eu3+ solid solution as red phosphor

A complete solid solutions with monophasic zircon-type structure of vanadates of formula GdxBi0.95−xVO4:0.05Eu3+ (x = 0–0.95) are synthesized by combined method of co-precipitation and hydrothermal synthesis. Their microstructures and morphologies are characterized by X-ray powder diffraction and transmission electronic microscope, and the results show that each of all the samples has a monophasic zircon-type structure. The absorption spectrum of the prepared phosphor shows a blue-shift of the fundamental absorption band edge with increasing the gadolinium content. Under UV-light and visible-light excitation, all the prepared phosphors show the typical luminescence properties of Eu3+ in the zircon-type structure. The emission intensity of GdxBi0.95−xVO4:0.05Eu3+ (x = 0.55) is strongest in all samples under UV-light and visible-light excitations. Finally, the mechanisms of luminescence of Eu3+ in the GdxBi0.95−xVO4:0.05Eu3+ (x = 0–0.95) solid solutions are analyzed and discussed.

Continuous zircon-type tetragonal YxBi0.95-xVO4:0.05Dy3+ solid solution: synthesis, characterization, and optical properties

We synthesize continuous solid solutions with monophasic zircon-type structure of vanadates of formula YxBi0.95-xVO4:0.05Dy3+ (x = 0-0.95) using a combined method of co-precipitation and hydrothermal synthesis. The X-ray diffractometer patterns confirm the formation of a solid solution of YxBi0.95-xVO4:0.05Dy3+, and the results show that all the samples have monophasic zircon-type structure. The absorption spectra of the prepared phosphors show a blue-shift of the fundamental absorption band edge with increasing Y3+ content. An intense tunable characteristic emission of Dy3+ is observed with the increasing ratio of Y/Bi. Finally, the mechanism of luminescence of Dy3+ in the YxBi0.95-xVO4:0.05Dy3+ (x = 0-0.95) solid solution is analyzed and discussed.

Stress-induced anomalous shift of optical band gap in ZnO:Al thin films

Thickness-dependent stress relaxation and its unreported effect on optical band gap of Al-doped ZnO thin films have been investigated. The thinnest film (∼84 nm) had a stress of −8.39×109 Nm−2, carrier concentration of 1.73×1019 cm−3 and optical band gap of 3.69 eV, a value significantly higher than the reported ones. With increase in thickness, magnitude of the stress decreased, and correspondingly a redshift of fundamental absorption band edge was observed. A linear dependence of optical band gap on stress in the films with a coefficient of 54.6 meV/GPa has been observed.

A new approach to processing electronic diffuse reflectance spectra

The fundamental absorption band edge in the diffuse reflectance spectra can be represented as the superposition of bands described by the Fermi-Dirac distribution to determine the coordination state of element-containing structures formed on the surface of a matrix. The suggested procedure is compared with the other spectrum processing methods (the determination of the position of the absorption band edge and transformation in the Gurevich-Kubelka-Munk or (αhν)m coordinates). It was shown that the use of the Fermi-Dirac distribution allows not only experimental data to be correctly described but also separation of complex spectra to be performed with isolating the contribution of surface structures containing atom-modifiers in various coordination states.

Absorption Bands of Tl+Centers Doped in Ionic Crystals with the CsCl Structure

We have investigated the absorption spectra of Tl + centers doped in ammonium and cesium halides crystals with the CsCl structure up to the vacuum ultraviolet energy range. In the NH 4 X:Tl + (X = Cl, Br) crystals, the A, B and C absorption bands due to the intraionic transitions in the Tl + ion are observed in the energy region below the fundamental absorption band edge of the host crystals. In the CsX:Tl + (X = Cl, Br, I) crystals, on the other hands, several absorption bands having no extremely different absorption intensity appear below the absorption band edge. The lowest absorption bands are assigned to the A absorption band, and other absorption bands have no feature of the B and C bands. The appearance of the absorption bands is remarkably different between the NH 4 X:Tl + and CsX:Tl + crystals, in spite of the same ion arrangements around the Tl + ion which is surrounded by eight nearest-neighbor halogen ions. The difference of the absorption bands between the NH 4 X:Tl + and CsX:Tl + crystals can be interpreted by that of the next nearest-neighbor cation around the Tl + ion.

Excitons in the layered insulators ZnI2 and CdI2:Zn

The fundamental electronic absorption spectra in the layered compounds ZnI2 and Zn-doped CdI2 are investigated in the spectral interval 3–5.9 eV. The samples are thin grain-oriented films deposited on quartz substrates. It is found that ZnI2, unlike CdI2, is a direct-gap insulator, despite the similarity of the crystal structures of these compounds. The introduction of Zn atoms into the cation sublattice of CdI2 at concentrations x⩾1% leads to the vanishing of the absorption from indirect transitions and to the appearance of a strong exciton band at the fundamental absorption band edge. The parameters of the exciton bands (spectral position, half-width Γ, oscillator strength f) are measured in the temperature interval 80–330 K. The measured temperature dependence of Γ in both compounds is typical for three-dimensional excitons. In ZnI2 the oscillator strength decreases with increasing T because of the Debye–Waller factor, while in CdI2:Zn it increases. This last result is evidence of the forbidden character of the direct optical transition at the interband absorption edge in CdI2, which is partially allowed because of the exciton–phonon interaction.

Excitation Wavelength Dependence of Terahertz Electromagnetic Wave Generation from Quantum Wire

We have investigated terahertz (THz) electromagnetic wave generation from a semiconductor crescent-shaped quantum wire structure. The THz electromagnetic wave was generated by ultrashort optical pulse excitation, and detected with the free-space electrooptic (EO) sampling method. THz electromagnetic wave generation from the quantum wire exhibited excitation wavelength dependence. The amplitude of the THz electromagnetic wave was at a maximum at a slightly lower energy than the photoluminescence (PL) peak, and decreased with reduction of the excitation energy. The amplitude of the THz electromagnetic wave decreased rapidly at the fundamental absorption band edge of the GaAs substrate. Therefore the THz electromagnetic wave might be generated due to instantaneous polarization in the quantum wire.

Charge transfer vibronic excitons and excitonic-type polaron states: photoluminescence in SBN

A theoretical model for two characteristic photoluminescence (PL) bands in SBN, ‘green luminescence’ (GL-band), and ‘red luminescence’ (RL-band) is proposed on the basis of the extended photoluminescence experiments in SBN/Cr, and also in SBN/Ce and in nominally pure SBN systems under different conditions. While the RL-band is suggested to be connected with charge transfer vibronic exciton (CTVE) clusters induced by Cr 3+ impurities in the Nb-sites, the GL-band is connected with Nb 4+ electronic polarons in a new excited state. Here Nb 4+ centers are the cores of the CTVE clusters induced by these charged cores. The PL mechanism is the in-cluster CTVE recombination for both bands under discussion. But the CTVE states are quasi-resonantly mixed here with 4T 2 states of the Cr 3+ core in the RL-band case, and with 5s-states of the Nb 4+ core in the G-band case. The CTVE-clusters induced by Cr 3+ ions produce strong lattice distortion which can be the origin of the uncommonly high red shift of the fundamental absorption band edge detected for the SBN/Cr case.

Spectral Properties of CuGa x In1− x Se2 Solid Solutions

We have investigated the absorption spectra in the region of the fundamental absorption band edge and the photoluminescence spectra of single crystals of the ternary compounds CuInSe2 and CuGaS2, and of solid solutions based on them that were grown by the method of chemical transport reaction. The forbidden band width and the positions of the maxima of the radiation bands are determined. The concentration dependences of the indicated quantities are plotted and they are shown to be linear.

Investigation of avalanche photocurrent at the edge of fundamental absorption band in Schottky diodes

The relative importance of the contribution of two mechanisms, that of the change in the collection of photocurrent carriers and that of the shift of the edge of fundamental absorption of light in strong electric fields in Schottky photodiodes, to the observed spectral dependence of the avalanche photocurrent is asessed.

Position of the edge of fundamental absorption band of solutions of ferroelectric crystals

Position of the edge of fundamental absorption band of solutions of ferroelectric crystals

Perturbation of fundamental band edge absorption of NaClO 3 crystals at high temperatures

The fundamental absorption band edge of NaClO 3 crystal showed a shift towards longer wavelengths with increase in temperature. There is also evidence of formation of a new band at temperature higher than about 120°C. The shift is nearly linear up to about 120°C, but it is much larger than the linear rate as the melting point (≈ 250°C) is approached. The excess absorption is assumed to be due to the thermally produced lattice disorder and analysis yielded 1·40±0·05 eV for the energy of formation of the defect. The anomalous thermal expansion of the crystal reported by earlier workers at temperatures above 120°C could also be analysed in a similar manner and the activation energy of 1·45±0·05 eV so obtained is in reasonable agreement with that obtained from optical absorption studies. The additional absorption may be attributed to the perturbation of the fundamental absorption band due to defects formed at higher temperatures.