E. Feldbach

Defect-related photoluminescence of hexagonal boron nitride

Photoluminescence of polycrystalline hexagonal boron nitride (hBN) was measured by means of time- and energy-resolved spectroscopy methods. The observed bands are related to donor-acceptor pair transitions, impurities, and structural defects. The excitation of samples by high-energy photons above 5.4 eV enables a phenomenon of photostimulated luminescence (PSL), which is due to distantly trapped conduction band electrons and valence band holes. These trapped charges are metastable and their re-excitation with low-energy photons results in anti-Stokes photoluminescence. The comparison of photoluminescence excitation spectra and PSL excitation spectra allows band analysis that supports the hypothesis of Frenkel-type exciton in hBN with a large binding energy.

Excitonic and recombination processes in CaWO4 and CdWO4 scintillators under synchrotron irradiation

Excitation spectra of intrinsic (IL) and extrinsic luminescence (EL) from CaWO 4 and CdWO 4 crystals, measured using synchrotron radiation (4-30 eV) at 8 K, have been analysed. In CaWO 4 the IL (2.9 eV) is efficiently excited by photons of 5.5-7.0 eV at the creation of molecular excitons. The EL is efficiently excited in interband transitions (hv>6.8 eV). The energy threshold for the multiplication of electronic excitations is E t 15 eV in CaWO 4 and E t 10 eV CdWO 4 . In CdWO 4 , a fraction of the molecular excitons undergoes autoionization due to an overlap with the continuum of the interband transitions. Fast emission (<2 ns) is found at 80 K under the pulse excitation by electrons with an energy of 300 keV. The continuous temperature-independent emission of CaWO 4 crystals in the region of 2-5 eV is interpreted as an intraband luminescence. The emission at 5.5 eV is ascribed to a metastable molecular excitons.

Energy transfer in ZnWO4 and CdWO4 scintillators

A systematic spectroscopic study of oriented single ZnWO4 crystals was performed using UV and synchrotron excitation. Emission and phosphorescence spectra in the energy region 1-4 eV as well as reflection spectra and excitation spectra of various emissions and phosphorescence in the energy region 3.5-32eV were studied at 4.2-300K using synchrotron radiation. The results are compared to those available for CdWO4. The peculiarities of the band structure and those of excitonic and electron-hole processes in the systems investigated are discussed.

Multiplication of electronic excitations in CaO and YAlO3 crystals with free and self-trapped excitons

Using synchrotron radiation of 6-32 eV the reflection spectrum and excitation spectra for 5.7, 4.6 and 2.8 eV emissions have been measured for a freshly cleaved CaO crystal at LHeT. In CaO, indirect band-to-band transitions take place in the region of hv>or=6.4 eV. The excitation spectra for intrinsic emissions of 5.9 and 4.2 eV were measured in a YAlO3 crystal at LHeT as well. As in many other dielectrics the electron-hole mechanism of multiplication of electronic excitations connected with the creation of secondary electron-hole pairs by hot photoelectrons also occurs in CaO and YAlO3 crystals. The creation of secondary excitons by hot photoelectrons has been detected in YAlO3 for the first time. The excitonic mechanism of multiplication of electronic excitations has a high efficiency in crystals with self-trapping excitons of small radii.

Low-temperature excitonic, electron–hole and interstitial-vacancy processes in LiF single crystals

The emission spectra and the excitation spectra of various emissions have been measured in LiF crystals at 9 K using VUV radiation of 10–33 eV. In contrast to the luminescence of self-trapped excitons (3.4 eV), the efficiency of several extrinsic emissions (4.2, 4.6 and 5.8 eV) is very low in the region of an exciton absorption (12.4–14.2 eV). A single exciting photon of 28–33 eV is able to create a primary electron–hole (e–h) pair and a secondary exciton. The tunnel phosphorescence has been detected after the irradiation of LiF by an electron beam or x-rays at 6 K, and several peaks of thermally stimulated luminescence (TSL) at 12–170 K appeared at the heating of the sample. It was confirmed that the TSL at 130–150 K is related to the diffusion of self-trapped holes (VK centres). The TSL peak at ∼160 K is ascribed to the thermal ionization

Excitonic and electron-hole processes in NaCl and NaCl:Ag crystals under conditions of multiplication of electronic excitations

The excitation spectra of and emissions of self-trapped excitons (3.4 eV and 5.4 eV, respectively) as well as the excitation spectra of 5.17 eV luminescence of Ag+ impurity centres were measured in NaCl and NaCl:Ag crystals using synchrotron radiation of 5 - 38 eV. Fast and slow components of these emissions were detected. An analysis of the differences in the excitation spectra measured at 8 and 295 K allowed us to separate the excitonic and electron-hole (e-h) mechanisms of the multiplication of electronic excitations. A photon of 17 - 19 eV forms an e-h pair and a secondary exciton, while the absorption of a 21 - 27 eV photon causes the creation of two e-h pairs. Using luminescent and photoelectric methods, it was shown that a 2p3s Na+ cation exciton, formed at the absorption of a 33.4 eV photon, decays with the creation of an anion exciton with a 3p hole component and two e-h pairs. Three e-h pairs are formed after the absorption of a 31 eV photon by a chlorine ion.

Mechanisms of Intrinsic and Impurity Luminescence Excitation by Synchrotron Radiation in Wide-Gap Oxides

Abstract The excitation spectra of intrinsic and impurity luminescence have been measured in the spectral region of 12 to 30 eV for SrO, CaO and YAlO s crystals at 8 K. Processes of exciton selftrapping as well as electron-hole and excitonic mechanisms of multiplication of electronic excitations have been analyzed for wide-gap oxides (BeO, MgO, CaO, SrO, Y 2 O 3 , YAlO 3 ).

Radiative decay of intrinsic electronic excitations in a wide-gap CsCl crystal

Abstract The coexistence of various kinds of intrinsic luminescence has been observed in a high-purity CsCl crystal. In addition to crossluminescence and emission of self-trapped excitons a low-temperature emission of free exciton-polaritons as well as intraband luminescence inside a conduction band and a split core cation band have been detected.

Multiplication of electron–hole pairs in MgO crystals and ceramics

Abstract The excitation spectra for various impurity emissions have been measured in MgO:Al, MgO:OH, MgO:Cr crystals and MgO:F, MgO:LiF ceramics at 8 K using synchrotron radiation of 5–32 eV. Processes of radiative recombination at the impurity centres of various structure differ significantly for two stages of multiplication of electron–hole (e–h) pairs with threshold photon energies at 20 and 25 eV. The multiplication processes have been analysed taking into account the peculiarities of the electronic structure of MgO.

Investigation of possible replacement of protective magnesium oxide layer in plasma display panels by barium ternary oxides

The firing voltage (FV) of gas discharge in a test cell of plasma display materials was investigated for standard protective layers of MgO deposited by electron beam, and ternary BaY2O4 and BaGa2O4 oxides grown by pulsed laser deposition on the special dielectric coated glass substrates. The determined FVs for MgO (160 V), BaY2O4 (210 V) and BaGa2O4 (257 V) lead to the conclusion that a replacement of MgO by these ternary oxides is not expedient for plasma display panels. Using results from luminescence spectroscopy, values for the energy gap Eg ≈ 6.2 and 5.8 eV were estimated for BaY2O4 and BaGa2O4, respectively. The main reason for the observed high FVs is attributed to strong electron affinities χ, where χ (BaY2O4) < χ (BaGa2O4).