0000000000034442
AUTHOR
I. Tale
showing 54 related works from this author
Defect Luminescence of LiBaF3 Perovskites
2000
Blue and red luminescence of undoped LiBaF3 crystals was studied. A broad, isotropic photoluminescence band centered at 410 nm can be excited by 210 nm — 275 nm light in as grown crystals. After X-irradiation at RT a new narrower, anisotropic luminescence band appears at 425 mn which has an additional excitation band at 290 nm. The X-irradiation also creates the F- type centres and anisotropic centres with an absorption band at 630 nm and a luminescence band at 700 nm. No F- centre luminescence is observed. All the other centres mentioned act as radiative recombination centers as well. It is speculated that the origin of the blue luminescence is due to oxygen defects and that the red lumine…
Characterization of Crystalline Structure and Morphology of Ga<sub>2</sub>O<sub>3</sub> Thin Film Grown by MOCVD Technique
2016
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.
Interaction of a Liquid Gallium Jet with ISTTOK Edge Plasmas
2008
The use of liquid metals as plasma facing components in tokamaks has recently experienced a renewed interest stimulated by their advantages in the development of a fusion reactor. Liquid metals have been proposed to solve problems related to the erosion and neutronic activation of solid walls submitted to high power loads allowing an efficient heat exhaust from fusion devices. Presently the most promising candidate materials are lithium and gallium. However, lithium has a short liquid state range when compared, for example, with gallium that has essentially better thermal properties and lower vapor pressure. To explore further these properties, ISTTOK tokamak is being used to test the inter…
Carrier localization effect in polarized InGaN multiple quantum wells
2005
Carrier localization effects in polarized InGaN/GaN multiple quantum wells (MQWs) were investigated as a function of well width, d, and In content, x. Using photoreflectance (PR), photoluminescence (PL), PL excitation (PLE), selective excitation of PL, PL excitation power, and time-resolved PL spectroscopy, the dominance of the localization effect against the built-in field effect on carrier recombination dynamics in InxGa1–xN MQWs of different well width (d = 2.0–4.0 nm, x ≈ 0.15) and In content (x ≈ 0.22–0.27, d = 2.5 nm) was revealed. Based on the modeling of the PL spectra by Monte Carlo simulation of exciton hopping and the spectroscopic reference provided by PR, increased In content a…
Colour centres in LiBaF3 crystals
1998
Abstract The origin of the absorption bands in LiBaF 3 created by X-ray irradiation at RT has been investigated. It is found that three absorption bands at 270, 320 and 430 nm represent different electron transitions within a radiation defect effectively created in LiBaF 3 single crystals. Following the Mollwo–Ivey relation we discuss investigations of optical dichroism, magnetic optical dichroism, as well as the assumptions regarding the accumulation kinetics of these absorption bands, the F centre being the main radiation defect created by X-rays in undoped LiBaF 3 crystals at RT.
Processing of amorphous Si by pulsed laser irradiation at different wavelengths
2012
Amorphous Si thin films deposited on thermally oxidized Si wafers have been processed by the 2 nd and 3 rd harmonics of Nd:YAG laser. Surface modification of amorphous silicon layers have been investigated by scanning electron microscopy before and after chemical etching of processed silicon films. The super-lateral crystal growth regime was achieved with laser fluence of 280 mJ/cm 2 for the 2 nd harmonics and 155 mJ/cm 2 for the 3 rd harmonics. The grain size in polycrystalline Si samples prepared by successive crystallization in the lateral growth regime is about 0.5 - 1 μm.
Ion diffusion-controlled processes in fluoride crystals
2001
Ionic and ion diffusion-controlled thermally stimulated relaxation (TSR) processes in CaF 2 , BaF 2 and LiBaF 3 crystals (X-ray irradiated at 290 K) have been investigated by means of ionic conductivity and the correlated ionic thermally stimulated depolarisation current (TSDC), and radiation-induced optical absorption band thermal bleaching techniques at 290-650 K. It was found that under a DC field fluorides store large ionic space-charge. In CaF 2 , BaF 2 and LiBaF 3 , by using the ionic TSDC technique we were able to detect a series of the interstitial anion and/or anion vacancy delocalisation stages in the extrinsic ionic conductivity region. At least 4-6 wide and overlapping ionic TSD…
Silicon dioxide thin film luminescence in comparison with bulk silica
1998
Abstract The luminescence of the self-trapped exciton (STE) in SiO2 films was measured at low temperatures on the background of defect luminescence under cathodoexcitation and compared with bulk silica luminescence. The defect luminescence is mainly caused by non-bridging oxygen centers (a red luminescence band at 1.8 eV) and twofold coordinated silicon centers (blue and ultraviolet luminescence with 2.7 and 4.4 eV bands, respectively). The STE luminescence with a band at 2.3 eV is uniformly distributed within SiO2 film volume. Contrary to defect luminescence, whose intensity increases with irradiation time, the STE luminescence decreases almost to zero in a few seconds of irradiation time.…
X‐irradiation induced photo‐ and thermostimulated luminescence of CsCdF 3 :Mn crystals
2005
Photo- and thermostimulated luminescence (PSL and TSL respectively) of previously X-irradiated CsCdF3 crystal doped with Mn were investigated. After X-irradiation of CsCdF3 crystal at 8 K PSL bands at about 300 nm and 550 nm appear. Several stimulation bands can be revealed for luminescence at 300 nm and 550 nm. The stimulation band at 340 nm is related to an F-type centre absorption band in accordance with the Mollwo-Ivey relation for halide crystals. Subsequent heating of the crystal after X-irradiation at 8 K shows TSL peaks in the temperature regions 8 K – 90 K and 200 K – 300 K. The spectral composition of the TSL involves both bands at 300 nm and 550 nm. Experiments performed allow us…
Growth temperature influence on the GaN nanowires grown by MOVPE technique
2011
GaN nanowires (NWs) were successfully grown by Vapor-Liquid-Solid (VLS) growth mechanism on GaN template using metal-organic vapor phase epitaxy (MOVPE) with diameters ranging from 20 to 200 nm and length up to few microns. The characterization by scanning electron microscopy (SEM) reveals an optimum growth temperature at 790°C and X-ray diffraction (XRD) investigations indicates oriented crystallinity of grown NWs.
Luminescence and electron transport properties of GaN and AlN layers
2001
Abstract The transport properties of free charge carriers, photo- and cathodo-luminescence (CV) in GaN and AlN films obtained by MOCVD technique on sapphire and Si substrates, are investigated. The concentration of free charge carriers in GaN is of order 10 17 – 10 19 cm −3 whereas AlN thin films are insulating. The Hall mobility of electrons are 80– 140 cm 2 / V s ). In undoped GaN films the spectral composition of CL is close to photoluminescence (PL) when excited in the region of band–band transitions. The decay time constant of the 3.44 eV UV emission attributed to the bound exciton is considerably less than 1 ns , whereas the 3.26 eV violet (VI) band shows a slow hyperbolical decay ove…
<title>Annealing of radiation defects in x-irradiated LiBaF<formula><inf><roman>3</roman></inf></formula>&l…
2003
Results of the glow rate technique application for analysis of the activation energy of thermostimulated annealing of X-ray created F-type color centers in pure and containing oxygen centers LiBaF3 crystals are presented. It is shown that depending on the impurity composition two alternative mechanisms could be involved in the annealing of color centers. It is proposed that either the anion vacancy governed migration of F-centers resulting in recombination with complementary defects, or the thermal delocalization of radiation created fluorine (Fi) interstitials captured by anti-structure defects followed by recombination with all kinds of complementary F-type centers are responsible for the…
Trap Spectroscopy by the Glow Rate Technique using Bleaching of Colour Centres
2002
An application of the glow rate technique (GRT) for analysis of the parameters of thermostimulated decay of colour centres is presented using the data on the decay of radiation defects in LiBaF 3 :Fe crystals created by X rays at 300 K. The GRT offers a procedure for evaluation of the mean activation energy as a function of temperature in the case of arbitrary thermostimulated relaxation kinetics represented by the trap distribution function. The experimental procedure involves at least two subsequent measurements of thermostimulated decay kinetics at different heating rates. It is shown that the decay of the F type centres is governed by interaction of mobile anion vacancies with F A and F…
Some aspects of pulsed laser deposition of Si nanocrystalline films
2009
International audience; Nanocrystalline silicon films were deposited by a picosecond laser ablation on different substrates in vacuum at room temperature. A nanocrystalline structure of the films was evidenced by atomic force microscopy (AFM), optical and Raman spectroscopies. A blue shift of the absorption edge was observed in optical absorption spectra, and a decrease of the optical phonon energy at the Brillouin zone centre was detected by Raman scattering. Early stages of nanocrystalline film formation on mica and HOPG substrates were studied by AFM. Mechanism of nanocrystal growth on substrate is discussed.
Ab initio calculations of charged point defects in GaN
2005
The Generalized Gradient Approximation (GGA) of a Density Functional Theory (DFT) in LCAO basis is used to determine electronic structure of charged point defects in hexagonal GaN. Impurities of MgGa, ZnGa, SiN and CN (the most common dopants) were considered for charge states 0, ±1, –2. The vacancies of VGa and VN were considered for charge states 0 and 0, ±1, ±2 respectively. The defects in consideration are compared from the point of view of the one-electron states localization. All the calculations were performed for a supercell containing 96 atoms. The divergence due to periodically repeated charges has been eliminated by the use of a uniform background charge of an opposite sign. (© 2…
Ion diffusion-controlled thermally stimulated processes in x-ray irradiated halide crystals
2003
The ionic and ion diffusion-controlled thermally stimulated relaxation (TSR) processes in CaF2, BaF2, LiBaF3 and KBr crystals were investigated above 290 K by means of the ionic conductivity, ionic thermally stimulated depolarisation current (TSDC) and thermal bleaching techniques. Under a DC field the halide crystals store large ionic space charge. We were able to detect in CaF2, BaF2, LiBaF3 and KBr in the extrinsic ionic conductivity region a series of the ionic defect (the interstitial anion and/or anion vacancies - in fluorides; the cation vacancies - in KBr) release stages: 3-6 wide and overlapping ionic TSDC peaks. The correlated data of the ionic TSDC and the F band thermal show tha…
Luminescent detectors of ionising radiation
2003
Abstract At present in slow neutron imaging an active layer of an imaging plate IP contains a mixture of storage phosphors, usually BaFBr:Eu 2+ used for imaging of X-rays, and a neutron converter material, usually Gd 2 O 3 , LiF. A novel Li-containing luminescent material perspective for a direct neutron conversion and storage is discussed. Irradiation of LiBaF 3 crystals results in generation of Frenkel defect pairs and creation of F-type centres responsible for three absorption bands in UV-and visible spectral region. Because photo-stimulation in each of these absorption bands leads to bleaching of induced absorption, the F-type colour centres are convenient for storage of radiation dose.…
Ab initio calculation of wurtzite‐type GaN nanowires
2007
Ab initio calculations of wurtzite-type GaN nanowires have been performed using density functional theory. Different shapes of nanowires of with similar diameters of around 2 nm have been considered to determine the stability of the structures. The quantitative similarities in the local properties obtained and dangling bond energies of nanowires and bulk surfaces have lead to a simple model model for a calculation of effective Young's modulus of nanowires of arbitrary diameters and shapes. The size dependence of the Young's modulus reveals a softening of GaN nanowires with the decrease of the diameter. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
Dynamics of exciton creation and decay processes in composition – disordered InGaN thin films
2011
In the GaN-based ternary alloys, InGaN crystals have been recognized as key materials for e-h plasmas-exciton dynamics, because of large exciton binding energies (24.8 meV in GaN). We report investigations of creating and recombination dynamics of excitons in commercially important InxGa1-xN composition range from x = 0.1 to 0.18 in which nanoscale indium composition fluctuation occurs and formation of indium rich clusters acting as quantum dots (QD) can be expected. Three MOCVD grown samples having x = 0.1; 0.14 and 0.18 have been investigated using 3D picosecond transient PL spectroscopy. It has been found that the band to band photo excitation at 15 K in whole composition range results i…
Stimulated Emission in InGaN/GaN Multiple Quantum Wells with Different Indium Content
2005
1. IntroductionLight-emitting and laser diodes for the green to near-UV region are basedon quantum structures with the InGaN active region [1]. The indium contentin the wells is a technological parameter that provides a straightforward accessfor the change in the color of the light-emitting device. However, the redshift ofthe emission peak with increase in indium content is caused not only by a simplealloying, but also by carrier localization due to difierence in In and Ga atomic
Annealing of color centers in LiBaF 3
2002
Results of the glow rate technique to analyze the activation energy of thermostimulated annealing of X-ray created F -type color centers in LiBaF 3 crystals are presented, showing pure and containing oxygen centers. It is shown that depending on the impurity composition two alternative mechanisms are involved in the annealing of color centers. It is proposed that either the anion vacancy governed migration of F -centers resulting in recombination with complementary defects, or the thermal delocalization of radiation created fluorine ( F i ) interstitials captured by anti-structure defects followed by recombination with all kinds of complementary F -type centers are responsible for the recom…
Interaction of a liquid gallium jet with the tokamak ISTTOK edge plasma
2008
Abstract The interaction of a liquid gallium jet with plasma has been investigated in the tokamak ISTTOK. This paper presents a description of the conceived experimental setup, a detailed characterization of the produced jets and the first experimental results related to the gallium–plasma interaction. A stable jet has been obtained, which was not noticeably affected by magnetic field transients. ISTTOK has been successfully operated with the gallium jet without degradation of the discharge or a significant plasma contamination by liquid metal. This observation is supported by spectroscopic measurements showing that gallium radiation is limited to the region around the jet. Furthermore, the…
Structure and characteristics of laser crystallized thin amorphous Si films
2011
Abstract Pure amorphous Si thin films deposited on oxidized crystalline Si surface (111) were crystallized by picosecond UV laser pulses. The Raman scattering spectra show that pulse energy of 330 mJ/cm2 is enough to fully crystallize Si film and further increase of the energy does not improve crystallinity. A large grained polycrystalline Si was obtained as revealed by surface analysis. A significant increase in carrier mobility was observed after laser crystallization.
Glow Rate Technique for Spectroscopy of Complex Thermostimulated Relaxation Processes
1996
The glow rate technique (GRT) developed for the analysis of complex trap spectra, is the extension of the known heating rate method to the full glow curve. It is shown that using two glow curves measured at different heating rate constants, a special mean activation energy of simultaneously emptied traps can be obtained as a function of temperature or emitted light sum to a good approximation. Expressions have been derived for the calculation of the mean activation energy function from data measured using a linear, logarithmic or a hyperbolic heating mode. Theoretical expressions for the mean activation energy function have been obtained and discussed for arbitrary trap distribution functio…
Optical and magnetic resonance spectroscopy of stimulated recombination processes in defect studies
2004
Optical and magnetic resonance spectroscopy is widely used in the investigation of radiation-induced processes in wide-gap solids. This paper discusses the present understanding of applications of the experimental methods of optical and thermoactivation spectroscopy in the research of new materials, for applications in radiation dosimetry and digital imaging and in the basic research into the fundamental physics and chemistry of radiation. The advantages of the simultaneous use of optical and magnetic resonance techniques for the investigation of stimulated processes are considered.
Object size effect on the contact potential difference measured by scanning Kelvin probe method
2010
International audience; Contact potential difference (CPD) was measured by macroscopic Kelvin probe instrument and scanning Kelvin probe microscope on Al, Ni and Pt on ITO substrates at ambient conditions. CPD values measured by scanning Kelvin probe microscope and macroscopic Kelvin probe are close within the error of about 10-30% for large studied objects, whereas scanning Kelvin probe microscope signal decreases, when the object size becomes smaller than 1.4 m. CPD and electric field signals measured using many-pass technique allowed us to estimate the influence of electrostatic field disturbance, especially, in the case of small objects.
Radiation Defects in LiBaF3 Perovskites
2000
We investigated the electron paramagnetic resonance (EPR), recombination afterglow, thermostimulated luminescence (TSL) and absorption of LiBaF3 perovskites crystals X-irradiated at 80 K The self-trapped hole centre VK(F2-) oriented along the [110] axis and electron F type (FA) centres are identified. X-irradiation at temperatures below 200 K results in creation of a long-term temperature-independent afterglow-tunnelling luminescence (TL), with main emission bands at 300, 370 nm and 430 nm. The short wavelength TL bands are associated with the tunnelling recombination of the F type centre with the VK centre. The thermal stability of VK centre is estimated to be about 130 K.
<title>Electronic excitations and defects in fluoroperovskite LiBaF<formula><inf><roman>3</roman></inf></formu…
2003
A survey of the present situation with respect to knowledge of lattice defects, electronic excitations, such as excitons and localized excitons, as well as energy storage and transfer phenomena in LiBaF3 crystals is given. Both phenomenological models and experimental interpretations of optical absorption bands, tentatively associated with F-type (electron) centers created by X-ray or electron irradiation, is reviewed. Interpretation of three radiative processes (super-fast core-valence transitions, slow trapped exciton luminescence and luminescence of structure defects) observed in undoped LiBaF3 crystals is analyzed with respect to practical application. Attention is paid to the behavior …
EPR hyperfine structure of F‐type centres in pure LiBaF 3 crystal
2007
We studied EPR spectra of pure LiBaF3 sample of high quality. For EPR measurements LiBaF3 sample was X-irradiated at room temperature, however spectra could be observed at low temperatures - at 77 K. All hyperfine structure lines of F-type centre could be well resolved contrary to our earlier studies of Fe doped LiBaF3 crystal. Qualitative analysis with g-tensor parameters derived from magneto-optical measurements showed that the F-type centre observed earlier by MCD-EPR techniques is the same F-type centre we observe with the EPR. We analyse origin of the hyperfine structure of the EPR spectra and their angular dependencies and discuss the structure of F-type centres in the LiBaF3 crystal.…
Structure and luminescence of GaN layers
2001
Abstract GaN films grown on 〈1 1 1〉 Si substrate by means of low pressure MOCVD technique in a horizontal flow quartz reactor are characterized by different thin layer analysis methods. The polycrystalline hexagonal structure of the GaN layers has been checked by means of grazing incidence X-ray diffractometry and IR spectroscopy. Cathodoluminescence (CL) spectra and their time kinetics are studied. The mean decay time of the 3.44 eV UV bound exciton transition is below 1 ns, whereas the 3.26 eV violet band shows a slow hyperbolical decay over about 1 μs. A third yellow band appears at 2.12 eV due to transitions via localized states.
Thermal relaxation of colour centres in LiBaF3crystals
2001
Abstract Processes in LiBaF3 crystals caused by the thermal decay of F-type centres created by X-irradiation at room temperature have been examined. It is shown that the thermal decay of F-type centres results in the formation of two kinds of electron centres peaking at 630 nm and 740 nm differing in thermal stability. Weak TSL intensity, accompanying the decay of F-centres, also observed as well as the low value of the process activation energy suggest that due to the presence of moving anion vacancies a random walk of the F-centres occur. We propose that in course of the random walk of the F-centres both the aggregate F-centres are created and the annihilation with some complementary radi…
Advanced trap spectroscopy using the glow rate technique based on bleaching of color centers
2001
Abstract The glow rate technique (GRT) is the extension of the known heating rate method to the full glow curve. The GRT like the fractional glow technique (FGT) offers a procedure for evaluation of the mean activation energy as a function of temperature in the case of arbitrary thermostimulated relaxation kinetics represented by trap distribution function. The experimental procedure involves at least two subsequent measurements of thermostimulated recombination kinetics at different heating rates. The extension of the GRT to the direct measurements of thermostimulated bleaching of the radiation-induced color centers is presented. The experimental procedure involves measurements of the deca…
Self-trapped holes and recombination luminescence in LiBaF3 crystals
2001
Abstract We investigated electron paramagnetic resonance (EPR) angular dependencies, recombination afterglow and thermostimulated luminescence of undoped LiBaF3 crystals, X-irradiated at low temperatures. EPR parameters of the F2− molecule oriented along the [1 1 0] direction have been obtained. Based on the value of the g-shift Δg=g⊥−gII=0.02, characteristic for the VK-centres in similar perovskites, we propose that we indeed observed the VK-centres, not the H-centres. X-irradiation below 170 K results in creation of a long-time temperature-independent afterglow due to the tunnelling recombination between close electron and hole centres. The F-type electron centres and the VK as well as pr…
<title>Localized excitons in fluoroperovskite LiBaF<formula><inf><roman>3</roman></inf></formula> crystals&…
2003
Two radiating processes in LiBaF3 crystals, fast valence-core transitions (5.4 - 6.5 eV) and slow, so called self-trapped exciton luminescence (about 4.3 eV), are important for practical application. Here we present a study of 4.3 eV luminescence under X-ray excitation and photoexcitation as well as under photostimulation after X-irradiation of undoped and Ag-doped LiBaF3 crystals at various temperatures. It is shown that 4.3 eV luminescence appears under X-ray excitation at least from 85 K to 400 K in both undoped and doped crystals. In all samples studied the excitation spectra of 4.3 eV luminescence contain both the main exciton like band at the edge of fundamental absorption at about 10…
LCAO calculation of neutral defects in GaN
2005
Four well known HF, LDA, GGA and B3LYP Hamiltonians in LCAO approximation have been used in band structure calculations to obtain the main properties of the perfect GaN crystal with hexagonal lattice (C space group). Calculated lattice parameters, elastic constants and the band gap have been compared with the experimental data and the results of other calculations. As a consequence, the GGA Hamiltonian has been chosen, giving the lattice parameters a = 3.20 A, c = 5.20 A, u = 0.377, the bulk modulus B = 206 GPa and the energy gap Eg = 2.7 eV. These results reasonably reproduce the experimental data. For the point defects calculation (VGa, VN, MgGa, ZnGa, CN, and SiN) the supercell model was…
Trap spectroscopy and tunnelling luminescence in feldspars
1998
Abstract Comparative studies of tunnel luminescence, thermally stimulated luminescence (TSL), and trap energy spectra by the fractional glow technique (FGT) have been made both with sanidine and microcline feldspars. After X-irradiation at LNT these feldspars show intense tunnelling afterglow. As the temperature is raised, several TSL glow peaks may be observed, starting at 135 K. An intense peak at 250 K is common to all four investigated microclines. In samples of sanidine, tunnelling afterglow overcomes TSL up to 250–260 K. With sanidine and microcline samples, the distributions of trap activation energies vs temperature obtained by the FGT are continuous and quasi-linear. Maxima in the …
<title>Formation of deep acceptor centers in AlGaN alloys</title>
2008
AlGaN alloy thin film materials are of high interest for light emitting diodes (LED of the ultraviolet (UV) spectral region. Origin of the deep intrinsic and impurity Si states in the AlxGa1-xN (0 < x < 0.35) epilayer structures grown by metalorganic chemical vapor deposition (MOCVD) technique have been considered. Effects of the lattice mismatch and Si-doping in the heterostructures of epilayers with different alloy composition are investigated using time resolved photoluminescence (PL) of donor – deep acceptor (DA) pairs. It is shown that the undoped AlGaN alloys, grown on a GaN buffer layer, due to the lattice mismatch contain the increased concentration of cation vacancy (Vcation) defec…
Annealing of Radiation Defects in X-Irradiated LiBaF3
2002
AbstractResults of application of the glow rate technique GRT for analysis of the activation energy of thermostimulated annealing of X-ray created F-type color centers in LiBaF3 crystals, pure and containing hetero-valence oxygen centers are presented. It is shown that depending on impurity composition in crystal two alternative mechanisms are involved in annealing of color centers. It is proposed that either the anion vacancy governed migration of F- centers resulting in recombination with complementary defects, or the thermal delocalization of radiation created fluorine (Fi) interstitials captured by anti-structure defects followed by recombination with all kinds of complementary F- type …
<title>Ion and electron trapping: release and relaxation processes in fluoride crystals</title>
2001
The thermally stimulated relaxation (TSR) processes in CaF2, BaF2 and LiBaF3 crystals (X-ray irradiated at LNT or RT) have been investigated by means of the ionic conductivity, thermally stimulated (TS) ionic depolarization current (TSDC), TS current (TSC), TS luminescence (TSL) and thermal bleaching techniques. The ionic TSDC measurements evidence that under DC field fluorides accumulate large ionic space-charge (thermoelectric state is formed) as a result of the migrating anion interstitial and/or vacancy capture on defects. In the ionic conductivity region (290 - 650 K) the thermoelectric state anneals, and several wide and overlapping anionic TSDC peaks are detected. The ionic TSDC stag…
Colour centres in LiBaF3
1997
Abstract The X-ray-induced optical absorption (OA) and the spectra of magnetic circular dichroism (MCD) followed by optically detected EPR (ODEPR) have been investigated in undoped LiBaF3 crystals. According to ODEPR, the absorption bands in the spectral region 470–770 nm correspond to the F-type centres, whereas the absorption band at 420 nm is assigned to the hole centres. The redistribution of electrons to the thermal more stable F-type centres indicates that all types of colour centres are annealed in the course of the thermal release of electrons from F-type centres, accompanied by a thermostimulated luminescence.
Photo- and thermostimulated processes in α-Al2O3
1995
Abstract We reported on the recombination processes determined by the release of electrons from defects connected with the dosimetric 430 K thermostimulated luminescence (TSL) peak as well as with the 260 K TSL peak. These TSL peaks appear in thermochemically reduced α-Al 2 O 3 crystals containing hydrogen and emission of these TSL peaks corresponds to luminescence of the F-center. The X-ray exposure or UV excitation in the absorption band of F-centers at 6.0 eV of reduced α-Al 2 O 3 crystals doped with acceptor impurities results in the appearance of a broad anisotropic complex absorption band in the spectral region 2.5–3.5 eV and in the appearance of a predominant TSL peak at 430 K. Above…
Liquid gallium jet–plasma interaction studies in ISTTOK tokamak
2009
Abstract Liquid metals have been pointed out as a suitable solution to solve problems related to the use of solid walls submitted to high power loads allowing, simultaneously, an efficient heat exhaustion process from fusion devices. The most promising candidate materials are lithium and gallium. However, lithium has a short liquid state temperature range when compared with gallium. To explore further this property, ISTTOK tokamak is being used to test the interaction of a free flying liquid gallium jet with the plasma. ISTTOK has been successfully operated with this jet without noticeable discharge degradation and no severe effect on the main plasma parameters or a significant plasma conta…
Ex situ investigations of MOCVD-grown gallium nitride nanowires using reflection high energy electron diffraction
2011
Vertically oriented nanowires (NWs) of single-crystalline wurtzite GaN have been fabricated on sapphire substrates, via metal organic chemical vapor deposition (MOCVD). We present ex situ investigations on orientation and structure of grown GaN nanowires on GaN(0001) surface using reflection high energy electron diffraction (RHEED). Both ordered and randomly oriented GaN crystalline structures have been detected.
Plasma-wall interaction studies within the EUROfusion consortium: Progress on plasma-facing components development and qualification
2017
This work has been carried out within the framework of the EUROfusion Consortium and has received funding from the Euratom research and training programme 2014-2018 under grant agreement No 633053. The views and opinions expressed herein do not necessarily reflect those of the European Commission.
Degradation features of BaF2 scintillators under radiation and magnetic fields
1996
Degradation features of BaF2 scintillators have been examined in combined γ- and n-radiation and strong magnetic fields. Created radiation defects have been investigated by optical magnetooptical, and ODMR techniques. It is shown that the magnetic field has no selective action on defects of different structure. The magnetic field slightly diminishes the efficiency of creation of impurity color centers. The decrease of the creation rate at a magnetic field of B = 1 T does not exceed 10%. The nature of these radiation defects in BaF2 scintillators obtained by different technologies is discussed. Degradations-Eigenschaften von BaF2-Szintillatoren wurden in kombinierter γ- und Neutronen-Strahlu…
Thermally and optically stimulated radiative processes in LiBaF3 crystals
2004
Abstract In LiBaF3 crystals both valence–core transitions (5.4– 6.5 eV ) and so-called self-trapped exciton luminescence (about 4.3 eV ) are important for practical application. Here, we present a study of 4.3 eV luminescence under photo- and thermostimulation after X-irradiation of undoped LiBaF3 crystals at various temperatures. Optically stimulated luminescence as a result of electron recombination with both self-trapped holes and holes localized at some defects, were observed after X-irradiation below 130 K and that of electron recombination with defect-localized holes was observed after X-irradiation above 130 K . The spectra of thermo-stimulated luminescence (TSL) contain a broad band…
Thermally Stimulated Ionic and Electronic Processes and Radiation-Induced Defect Annealing in LiBaF3 Crystals
2000
The electronic, ionic and ion-diffusion controlled thermally stimulated relaxation (TSR) processes in X-ray irradiated (at 80 K or 290 K) nominally pure LiBaF3 fluoroperovskite crystals have been investigated in the 90–550 K range by means of the ionic conductivity, ionic thermally stimulated depolarization current (TSDC), as well as the thermally stimulated current (TSC), thermally stimulated luminescence (TSL) and the X-ray induced optical absorption spectra thermal bleaching techniques. The role of the thermoactivated ionic and ionic-electronic processes in the TSR (thermal bleaching, TSC and TSL) of X-ray irradiated crystals is studied above 250 K. The TSL efficiency (ratio TSL/TSC) ver…
Magnetooptical studies of defects and recombination luminescence in LiBaF3
2004
Abstract Optically detected EPR investigations have been performed on the recombination luminescence (RL-EPR) of LiBaF3 crystal, X-irradiated at T=4.2 K . RL-EPR lines of VK- centres were found, as well as further lines of a defect with S= 1 2 and an axial g-tensor with its main axis along a [1 0 0] direction of the crystal. Measurements of the magnetic circular dichroism of the absorption (MCDA) have been performed on LiBaF3 crystals X-irradiated at two temperatures ( 4.2 K and RT). After irradiation at T=4.2 K , the main MCDA bands peak at 453 and 500 nm , but after irradiation at T=300 K , the main bands peak at 444 and 390 nm , there is a change of the sign between the peaks in both cas…
<title>Photostimulated recombination processes in x-irradiated CsCdF<formula><inf><roman>3</roman></inf></form…
2005
Fluoride crystals with the perovskite structure doped with rare-earth ions and other activators are interesting materials for laser hosts, scintillators, and detectors of ionizing radiation. Therefore, an actual task is to clarify the structure of the radiation-induced defects and recombination processes in these crystals. Compared to other fluoroperovskites, considerably less information is available concerning to the radiation-induced processes in the CsCdF3 crystals. We present a study of photostimulated luminescence (PSL) in the previously x-irradiated CsCdF3 crystal doped with Mn (0.05%). After the x-irradiation of the crystal, optical stimulation at 320 nm leads to the appearance of 3…
EPR of radiation defects in LiBaF 3 crystals
2002
EPR spectra of LiBaF 3 crystals have been investigated after X-irradiation at RT. A spectrum consisting of approximately 35 nearly equidistant EPR lines has a strong angular dependence on the line intensities. The spectrum is caused by a hyperfine interaction (hfs) of a spin S=1/2 with neighbouring groups of nuclei. The observed large number of hfs lines required Li nuclei being in the first shell and fluorine nuclei in the more distant second shell. We analysed the spectrum in the F-centre model, taking reduced hfs values of the F-centre in LiF and found qualitative explanation of the number of hfs lines. The angular dependence of the line intensities could be explained by an anisotropy of…
Nature of the blue luminescence bands in PbWO4
1997
Abstract The photoluminescence spectrum of PbWO 4 is composed of blue and green bands, previously attributed to the regular WO 4 group and to the defect-related WO 3 group, respectively. Untill now only green emission was observed in the thermostimulated luminescence (TSL) above 77 K. Investigation of the TSL spectra starting from 20 K indicates that the blue band, being definitely present at least in the 28 K TSL peak, is also due to recombination emission at defect sites.
ODEPR of indium colour centres in the X- irradiated storage phosphor KBr:In
1995
The results of measurements of the magnetic circular dichroism of the optical absorption (MCDA) and optically detected electron paramagnetic resonance (ODEPR) of X-irradiated KBr :In crystals are presented. The MCDA bands and ODEPR parameters of In°(1) centers and In 2+ centres have been measured. The mechanism of the energy storage in KBr :In crystals is found not to be simply the formation of correlated F centre-In 2+ centre pairs as was assumed previously. Considerable similarities to the storage phosphor BaFBr :Eu 2+ were found for the photostimulated emission and read-out properties.
Thermostimulated recombination processes in LiBaF3 crystals
2004
Abstract The creation of radiation defects in LiBaF 3 crystals at 10 K and the processes of their thermostimulated recombination are investigated. The methods of optical absorption, thermal bleaching of color centers, thermostimulated luminescence and fractional glow technique are used. The radiation defects anneal in a multi-stage process accompanied with thermo-luminescence at 20, 46, 105, 130, 170, 210 and 270 K . Differences in the optical absorption spectra measured before and after the TSL peaks are obtained and recombination parameters are determined. The TSL peak at 20 K arises from the delocalization of H-centers. The presence of two TSL peaks related to V K -centers at 105 and 130…
PATTERNED LASER CRYSTALLIZATION OF a-Si
2009
PATTERNED LASER CRYSTALLIZATION OF a-SiThin films of amorphous Si on glass were crystallized by pulsed nano- and picosecond lasers. Two methods for creating the desired patterns of crystallized regions were used. In the former, the pattern is produced by a focused laser beam, and in the latter it is made using a prefabricated mask. The electric conductivity of crystallized films increases by more than 4 orders of magnitude in comparison with untreated amorphous films.