Suchergebnisse

This work was supported by the Latvian Science Council Grant No lzp-2018/1-0289. ; The photoelectric response of pure silica glasses excited by excimer lasers has been studied. The samples were made under various conditions. Photoelectric polarization of samples due to the Dember effect has been registered. The signal was recorded under the conditions of a space charge limited current. The space charge resulting from the capture of electrons and holes creates a static electric field that prevents diffusion of released charge carriers. The current registration in the external circuit stops, despite the continuation of photoexcitation. This effect was used as a fact of measuring the photocurrent in the sample volume instead of the parasitic current that is not associated with the sample. The screen has been chosen to prevent the influence of a spurious signal. It has been found that charge carriers are released when excited in the spectral absorption range of localized states of silica. Based on the Dember effect, the sign of the photoelectric response shows the type of charge carriers - an electron or a hole is mobile. Thus, a sample containing aluminum without alkali ions gives a negative signal, which indicates the diffusion of electrons at 290 K, since aluminum is an effective hole trap. An oxygen-deficient sample at 290 K provides a positive signal indicating the diffusion of holes, because the center of oxygen deficiency is an effective electron trap. This sample at 100 K provides a negative signal due to the effective self-trapping of holes. ; Latvian Science Council Grant No lzp-2018/1-0289; Institute of Solid State Physics, University of Latvia as the Center of Excellence has received funding from the European Union's Horizon 2020 Framework Programme H2020-WIDESPREAD-01-2016-2017-TeamingPhase2 under grant agreement No. 739508, project CAMART²

This work was supported by the Latvian Science Council Grant No lzp-2018/1-0289. ; Photoluminescence of nominally pure oxidized and fluorinated silica glasses, which have good optical transparency in the optical gap, has been investigated under excitation by excimer lasers. The data are compared with the data for chlorinated silica glass, as well as glass containing OH. The excitation of localized states provides the release and capture of charge carriers, despite the low absorption coefficient. The holes are self-trapped. The recombination in the thermally activated or tunnel process creates luminescence of oxygen-deficient centers. It is concluded that oxidation and fluorination do not change the concentration of localized states, but they change the electronic transitions of the localized states to higher energies. In type III glass with a high OH content, luminescence is suppressed by the presence of hydrogen and appears after irradiation with 7.9 eV photons of the F2 laser. This observation suggests the existence of localized states even in these glasses. ; Latvian Science Council Grant No lzp-2018/1-0289; Institute of Solid State Physics, University of Latvia as the Center of Excellence has received funding from the European Union's Horizon 2020 Framework Programme H2020-WIDESPREAD-01-2016-2017-TeamingPhase2 under grant agreement No. 739508, project CAMART²

This work was supported by ERANET MYND. Also, financial support provided by Scientific Research Project for Students and Young Researchers Nr. SJZ/2017/2 realized at the Institute of Solid State Physics, University of Latvia is greatly acknowledged. The authors express our gratitude to R.I. Mashkovtsev for help in ESR signal interpretation. The authors are appreciative to T.I. Dyuzheva, L.M. Lityagina, N.A. Bendeliani for stishovite single crystals and to K. Hubner and H.-J. Fitting for stishovite powder of Barringer Meteor Crater. ; An electron spin resonance (ESR) and photoluminescence signal is observed in the as grown single crystal of stishovite indicating the presence of defects in the non‐irradiated sample. The photoluminescence of the as received stishovite single crystals exhibits two main bands – a blue at 3 eV and an UV at 4.75 eV. Luminescence is excited in the range of optical transparency of stishovite (below 8.75 eV) and, therefore, is ascribed to defects. A wide range of decay kinetics under a pulsed excitation is observed. For the blue band besides the exponential decay with a time constant of about 18 μs an additional ms component is revealed. For the UV band besides the fast component with a time constant of 1–3 ns a component with a decay in tens μs is obtained. The main components (18 μs and 1–3 ns) possess a typical intra‐center transition intensity thermal quenching. The effect of the additional slow component is related to the presence of OH groups and/or carbon molecular defects modifying the luminescence center. The additional slow components exhibit wave‐like thermal dependences. Photo‐thermally stimulated creation–destruction of the complex comprising host defect and interstitial modifiers explains the slow luminescence wave‐like thermal dependences. ; ERANET MYND; ISSP UL Nr. SJZ/2017/2 ; Institute of Solid State Physics, University of Latvia as the Center of Excellence has received funding from the European Union's Horizon 2020 Framework Programme H2020-WIDESPREAD-01-2016-2017-TeamingPhase2 under grant agreement No. 739508, project CAMART²

This research is funded by the Latvian Council of Science, project "Research of luminescence mechanisms and dosimeter properties in prospective nitrides and oxides using TL and OSL methods], project No. lzp-2018/0361. ; The photoelectric response spectra were studied in a LiGaO2 crystal. The obtained spectra were interpreted as the release and diffusion of charge carriers, as well as external photoemission of electrons. The charge carrier release starts from excitation with a photon with energy greater than the optical gap of the LiGaO2 crystal (∼6 eV). The efficiency of generation of charge carriers exponentially increases up to 8 eV with saturation beginning from 9 eV. The nature of the photoelectric response is attributed to the Dember effect corresponding to the higher mobility of electrons compared to holes. The low yield of the internal photoelectron in the range of the intrinsic absorption band at 6 eV is associated with its exciton nature. Above 9.5–10 eV, an additional response was revealed that was interpreted as external photoelectron emission with a threshold of about Eth = 10.1 ± 0.2 eV. The relative emission yield of electrons increases as (hν - Eth)3, which corresponds to indirect transitions between the valence and conduction bands. The emission of photoelectrons for the direct band-to-band transitions increases linearly with the photon energy and the corresponding threshold is defined as 12.7 ± 0.2 eV. ; Latvian Council of Science lzp-2018/0361; Institute of Solid State Physics, University of Latvia as the Center of Excellence has received funding from the European Union's Horizon 2020 Framework Programme H2020-WIDESPREAD-01-2016-2017-TeamingPhase2 under grant agreement No. 739508, project CAMART²

Growth of gallium oxide thin film was realized with MOCVD on (0001) sapphire substrate. Structural and compositional properties of thin film were studied employing trimethylgallium and water as precursors, carrier gases were H2 and N2. Obtained film is polycrystalline and predominantly consisted of (201) oriented β-Ga2O3. Sample exhibited blue luminescence which is attributed to oxygen vacancies. H2 gas proved to have beneficial effect on film quality and overall growth process. ; Institute of Solid State Physics, University of Latvia as the Center of Excellence has received funding from the European Union's Horizon 2020 Framework Programme H2020-WIDESPREAD-01-2016-2017-TeamingPhase2 under grant agreement No. 739508, project CAMART²

The support from M-ERANET project "MyND" is acknowledged. A.A., M.M-S., and L.R. were supported by the Research Council of Lithuania (Grant M-ERA.NET-1/2015). The authors thank A. Pasquarello for providing the structures of the amorphous SiO 2 matrix for our computational work and K. Kajihara (Tokyo Metropolitan University) for valuable advice in PL kinetics measurements. ; The lowest excited electronic state of the O2 molecule, a1Δg, the "singlet oxygen", is of utmost importance for photochemistry and photobiology. For O2 trapped in silica glass, the lifetime of this state and the associated a1Δg → X3ςg - photoluminescence (PL) is the longest known for O2 in any condensed medium at room temperature. We studied the temperature dependence, decay kinetics, and polarization anisotropy of this PL with 1064 nm excitation to the a1Δg(v = 1) state as well as with excitation to higher energies. PL at this excitation shows nonzero polarization anisotropy at 295 K, which increases with cooling to 14 K. At variance, excitation to higher energies yields depolarized PL. Polarization data indicate weak electric dipole character of the emission of the spin- and parity-forbidden a1Δg → X3ςg - transition, enabled by O2-SiO2 cage interactions. Density functional theory calculations indicate that at low temperatures the rotation of O2 is partially or fully frozen even in large silica voids. As the temperature increases, PL is increasingly depolarized by libration movement of O2 molecules. Analysis of O2 optical absorption in optical fibers allows one to obtain the absorption cross sections of X → a and X → b transitions of O2 in SiO2 glass and to evaluate both radiative and nonradiative rates of a → X luminescence. ; Research Council of Lithuania (Grant M-ERA.NET-1/2015); Institute of Solid State Physics, University of Latvia as the Center of Excellence has received funding from the European Union's Horizon 2020 Framework Programme H2020-WIDESPREAD-01-2016-2017-TeamingPhase2 under grant agreement No. 739508, project CAMART²