V. S. Kortov

An interactive process in the mechanism of the thermally stimulated luminescence of anion-defective α-Al2O3 crystals

The role played by deep traps in the appearance of specific features of the thermally stimu- lated luminescence (TL) of anion-defective a-Al2O3 single crystals is reported. The existence of deep traps has been proved by direct observations of the TL peaks associated with these traps. From the ex- perimental evidence it is adduced that the degree of occupancy of the deep traps aAects the basic fea- tures of the main TL peak at 450 K. A model of an interactive trap system is proposed. The main diAerence of this model from those described in the literature is that it allows for the temperature dependence of the probability of the charge carriers to be captured at the deep traps. The proposed model was used to calculate the basic TL parameters of the main peak (the TL output and the shape of the glow curve) of the crystals in question as a function of the heating rate and the occupancy of the deep traps. The calculated values are similar to those observed experimentally. # 1998 Published by Elsevier Science Ltd. All rights reserved

Effects of structural disorder and Urbach's rule in binary lead silicate glasses

Abstract The behavior of optical spectra of PbO–SiO2 glasses with different concentrations of lead near the UV absorption edge was analyzed at temperatures from 80 to 470 K. A generalized formulation of modified Urbachs rule (MUR), which is applicable to glassy materials within a broad temperature interval, was proposed for interpretation of the experimental spectral dependences. The effective energy of phonons responsible for the temperature shift of the Urbach edge was calculated in terms of the approach used. It was shown that spectral and temperature parameters of MUR are structure-sensitive and their concentration behavior reflects the change in the type of short-range order in the glassy matrix.

Luminescence of F and F^+ centers in corundum upon excitation in the interval from 4 to 40 eV

Abstract Time-resolved excitation spectra of luminescence from F + - and F-centers in α -Al 2 O 3 were analyzed with subnanosecond resolution in the interval from 4 to 40 eV at different orientations of the C 3 optical axis relative to the intensity vector of an electric field E of the exciting light. A new excited state of the F + -center near 6 eV was examined. The time-resolved excitation spectra had a doublet structure at the edge of the fundamental absorption. The structure was interpreted as the manifestation of a Frenkel exciton. It was found that the luminescence decay kinetics of F + -centers changed considerably with increasing excitation energy ( E ⩾ E g , where E g is the band gap energy).

Applicability of the empirical Varshni relation for the temperature dependence of the width of the band gap

We have carried out a comparison of relations used to describe the temperature dependence of the width of the band gap in crystals. It is shown that for kT≫ℏω the well-known Varshni relation can be obtained from the non-empirical Fan expression in explicit form taking account of the phonon statistics. We have calculated the temperature coefficient bof the width of the band gap for a number of materials in the range where the high-temperature condition is not met. We have found that the Varshni relation overestimates β, whereas calculations based on the Fan expression agree with experiment.

Specific features of luminescence properties of nanostructured aluminum oxide

Aluminum oxide nanopowders are prepared by the gas phase method and characterized according to the particle sizes and the phase composition. Samples of the nanostructured ceramic material are produced by pressing and annealing in air. The photoluminescence and cathodoluminescence spectra of the Al2O3 nanostructured ceramic material and α-Al2O3 anion-defect single crystals are investigated under comparable conditions. The luminescence bands of centers formed by oxygen vacancies are revealed in the spectra of two types of samples. The nanostructured ceramic material is characterized by the appearance of a new luminescence band at 3.4 eV and a decrease in the luminescence decay time. The inference is made that the characteristic features of the luminescence of the nanostructured ceramic material can be associated with the presence of non-equilibrium phases and the specific features of relaxation processes.

Thermal and optical ionization of F-centers in the luminescence mechanism of anion-defective corundum crystals

A model describing the luminescence mechanism of anion-defective single crystals of aluminum oxide was developed including thermal ionization of excited states of F-centers. The model was validated by experimental studies on the effect of deep traps on specific features of thermoluminescence, thermally stimulated exoelectron emission, and thermally stimulated electric conductivity of test crystals.

Luminescent defects in nanostructured silica

The spectral and kinetic properties of excited states of luminescent defects (oxygen-deficient centers) in SiO2 ceramics are studied using pulsed cathodoluminescence and time-resolved photoluminescence. It is found that, in nanostructured samples prepared by thermal decomposition of polysilazane in air, there can exist modifications of oxygen-deficient centers in the form of surface analogs of either neutral oxygen monovacancies ≡Si-Si≡ (≡Ge-Ge≡) or twofold-coordinated silicon atoms =Si: (=Ge:). Photoluminescence of these centers is efficiently excited in the optical absorption bands of E′s surface centers and silicon clusters ≡SiSiSi≡ and can be associated with the intercenter energy transfer in the course of their nonradiative relaxation. The photoluminescence and excitation spectra indicate thermally induced conversion of different types of oxygen-deficient centers. The specific features of the thermally induced changes in the luminescence characteristics of the defects due to the transformation of the structure of the silica samples from amorphous to partially crystalline are revealed from analyzing the spectral composition and decay kinetics of pulsed cathodoluminescence.

Mechanism of F-center luminescence in anion-defective aluminum oxide single crystals

New experimental data illustrating the effect of deep traps on the luminescence properties of anion-defective α-Al2O3 single crystals are presented. It was established that deep traps have electronic nature and their filling occurs through photoionization of F centers and is accompanied by F → F+-center conversion. Model concepts were developed that describe the luminescence mechanism in anion-defective aluminum oxide single crystals with inclusion of thermal ionization of the excited F-center states. The validity of the model was supported by experimental data obtained in a study of thermoluminescence, thermally stimulated exoelectron emission, and thermally stimulated electrical conductivity.

The Urbach rule for the PbO-SiO2 glasses

An analysis is made of the behavior of optical spectra of lead-silicate glasses, with variable lead content near the UV absorption edge, and within the 80–470-K range. A generalized formulation of the modified Urbach rule, applicable to glassy materials within a broad temperature range, is proposed for the interpretation of experimental spectral relations. Within this approach, the effective energies of the phonons responsible for the temperature-induced shift of the Urbach edge have been calculated. It is shown that the spectral and temperature parameters of the modified Urbach rule are structure-sensitive, and that their concentration behavior reflects the change of the type of short-range order in the glassy matrix.

Photoelectron Spectroscopy of E ' Centers in Crystalline and Glassy Silicon Dioxide

Radiation-induced E′ centers in SiO2 were studied to test the possibility of applying optically stimulated electron emission (OSEE) to the spectroscopy of excited states of point defects in dielectrics. The spectral responses of the OSEE of crystalline α quartz and silica glass irradiated by 10-MeV electrons were measured and studied. It was established that volume E′ centers in the crystalline and glassy SiO2 modifications are dominant emission-active defects. Surface E’s (1) centers were also detected in glassy SiO2. A model of the energy structure of E′ centers accounting for the absence of luminescence and taking into account the presence of two nonradiative (intracenter and ionization) relaxation channels is proposed. This model was used to explain the mechanism of photothermal decay of the E′ centers and to determine the ionization activation barriers and quantum yields of these centers. The emission, spectral, and kinetic parameters of the volume and surface E′ centers in glassy SiO2 were obtained, showing the excited states of these defects to have identical atomic configurations.