S. V. Gorbunov

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.

Electronic excitations and defects in nanostructural Al2O3

A time-resolved cathodo-and photoluminescence study of nanostructural modifications of Al2O3 (powders and ceramics) excited by heavy-current electron beams, as well as by pulsed synchrotron radiation, is reported. It was found that Al2O3 nanopowders probed before and after Fe+ ion irradiation have the same phase composition (the γ-phase/δ-phase ratio is equal to 1), an average grain size equal to ∼17 nm, and practically the same set of broad cathodoluminescence (CL) bands peaking at 2.4, 3.2, and 3.8 eV. It was established that Al2O3 nanopowders exhibit fast photoluminescence (PL) (a band at 3.2 eV), whose decay kinetics is described by two exponential stages (τ1 = 0.5 ns, τ2 = 5.5 ns). Three bands, at 5.24, 6.13, and 7.44 eV, were isolated in the excitation spectrum of the fast PL. Two alternate models of PL centers were considered, according to which the 3.2-eV luminescence either originates from radiative relaxation of the P− centers (anion-cation vacancy pairs) or is due to the formation of surface analogs of the F+ center (FS+-type centers). In addition to the fast luminescence, nano-Al2O3 was found to produce slow luminescence in the form of a broad band peaking at 3.5 eV. The excitation spectrum of the 3.5-eV luminescence obtained at T = 13 K exhibits two doublet bands with maxima at 7.8 and 8.3 eV. An analysis of the luminescent properties of nanostructural and single-crystal Al2O3 suggests that the slow luminescence of nanopowders at 3.5 eV is due to radiative annihilation of excitons localized near structural defects.

Optical studies of self-trapped holes and excitons in beryllium oxide

Time-resolved spectra, decay kinetics, degree of polarization of the luminescence and transient optical absorption induced by irradiation of BeO with an electron, synchrotron and subsequent laser pulses have been studied. It is found that the two pairs of slowly and fastly decaying intrinsic luminescence bands at 6.7, 4.9 and 4.4 eV are associated with the two different types of the self-trapped excitons. The 1.7 eV transient optical absorption band and the ultra-violet absorption band at 3.6 eV are attributed to the electron and hole components of the self-trapped excitons, respectively. The transient optical absorption band with decay time of 6.5 ms is found to be due to the self-trapped holes. The models of self-trapped excitons and holes are discussed on the basis of the present experimental results.

Metastable optical absorption of relaxed electronic excitations in BeO-Zn crystals

Spectra of metastable optical absorption and its relaxation kinetics have been studied in zinc-doped BeO crystals by time-resolved pulsed absorption spectroscopy. A comparison of the observed induced optical absorption of self-trapped excitons and small-radius excitons bound to the zinc impurity suggests that their hole components have similar structures and reveals distinctive features of “forbidden” optical transitions in the electronic components. Metastable optical absorption in Zn+ centers has been discovered. It is shown that the small-radius excitons bound to the zinc impurity form in the hole stages of thermally stimulated tunneling recombination processes involving Zn+ electronic centers. It has been found that the high recombination probability of the electronic and hole centers created in BeO-Zn crystals by an electron beam may be due to the high degree of their spatial correlation.

Transient optical absorption of excited F -centres in BeO

Time-resolved spectra, decay kinetics and polarization of the transient optical absorption induced by irradiation of additively colored BeO crystals with electron pulses have been studied. It has been established that the two bands at 3.8 and 4.3 v eV of the transient optical absorption are due to the transitions between triplet and singlet excited states of F -centres in BeO. The polarization of excited F -centres absorption is discussed on the basis of analysis of the splitting of singlet and triplet states in crystalline field of the C 3v symmetry.

Excitation of self-trapped-exciton luminescence in the recombination of frenkel defects in BeO

Polarized luminescence and transient optical absorption (TOA) induced by pulsed electron irradiation in beryllium oxide crystals were studied. Exponential stages with decay times τ = 6.5 ms were observed to exist in luminescence bands at 4.0, 5.0, and 6.7 eV, which coincide in spectral composition and polarization characteristics with the luminescence of self-trapped excitons (STEs) of two types. The formation efficiency of centers with a 6.5-ms decay time is comparable to that of triplet STEs. The general characteristics of the kinetics and the decay times of the TOA of these centers do not depend on electron fluence and are governed by the monomolecular recombination process. The spectra of TOA centers with a decay time of 6.5 ms were found to be similar to those of V-type hole centers and STE hole components. The mechanism by which recombination of closely spaced, spatially correlated Frenkel pairs, Be+ and V− centers, brings about an exponential component with a 6.5-ms decay time in the luminescence of STEs of two types in BeO is discussed.

Metastable optical absorption of exited F-centers in BeO crystals

The metastable optical absorption in additively colored BeO crystals is studied. It is found that the optical absorption is induced by transitions between the excited triplet and singlet states of an F-center.

Peculiarities of a radiation defect creation in beryllium oxide crystals

Abstract The radiation defect formation has been studied in beryllium oxide crystals irradiated with reactor neutrons in a wide range of fluences (1015-1020 cm−2). It has been found that at high dose irradiation the saturation and reduction of anion and cation vacancies (the simplest point radiation defects) occurs in contrast to another alkaline earth oxides where only saturation takes place. A reduction of the point radiation defects under high dose irradiation has been caused not only by achieving a dynamic balance between creation and annihilation of Frenkel pairs but also by aggregation of the defects created. The mechanism of defect aggregation is discussed for the simplest aggregate defect-P- -center (double cation-anion vacancy trapping an electron). A new aggregate paramagnetic centers have been registered in high-dose neutron irradiated BeO crystals.