Surface segregation measurements during electron irradiation

Abstract

The growth of Ni3Si surface films on Ni-12.7at%Si alloys has been measured during lMeV electron irradiation. Stereoscopic techniques were used to determine film thickness from dark field images formed from Ni₃Si superlattice reflections. Parabolic growth kinetics are observed at lower temperatures. However, at higher temperatures, deviations from parabolic kinetics are observed after short irradiation times. Such deviations have not been observed in bulk specimens during bombardment with energetic ions and, therefore, may be due to foil thickness effects.     

Photoionization processes in F aggregate centres of LiF crystal

Extensive studies have been carried out on the optical conversion of F and F-aggregate colour centres produced in lithium fluoride single crystal on gamma irradiation. Using 308 nm XeCl laser it has been shown that significantly large population build-up of F ₃ ⁺ centre and reduction in the population of undesirable F₂ centres can be achieved in gamma irradiated crystal at room temperature due to multistep photoionization processes. These and other investigations have provided a scheme for possible laser action based on F ₃ ⁺ colour centres in LiF crystal at room temperature.     

Dielectric changes and thermoluminescence of sodium chloride and lithium fluoride irradiated with X-rays

The dielectric changes exhibited by sodium chloride and lithium fluoride on heating after X-ray irradiation have been measured at a frequency of 500 Kc/sec. The thermoluminescence has also been recorded. It is observed that the dielectric loss of both the samples increases during thermal bleaching. The difference between the dielectric losses of the crystals before and after colouration i.e.Δ tan δ, for different stages of thermal bleaching is found to show steps, the temperature regions of which agree with the temperatures of the glow peaks. It is presumed that this observed excess dielectric loss is due to the large number of vacancies created by the destruction of colour centres at the corresponding glow peak temperatures.     

Color centres in crystalline Lif films

Abstract

Policrystalline LiF thin films were produced on amorphous substrates at different temperatures. Preliminary optical measurements on F₂ and F₃ ⁺ aggregate color centres produced by electron irradiation were performed.     

Lithium fluoride colour centres-based imaging detectors for proton beam characterization at high doses

Proton beams of 7 MeV energy, produced by a linear accelerator, were used to irradiate LiF crystals and thin films thermally evaporated on glass substrates in the dose range from 10³ to 4 × 10⁶ Gy, inducing the formation of stable photoluminescent colour centres (mainly F₂ and F₃⁺), emitting in the visible spectral range. Using a conventional fluorescence microscope, the transversal proton beam intensity was mapped by acquiring the photoluminescence image of the irradiated spots. Image analysis allowed measuring the integrated photoluminescence intensity as a function of the irradiation dose: a linear optical response was obtained up to different maximum dose values, after which a quenching was observed, depending on the nature of the samples (crystals or films). The colour centres formation was investigated by optical absorption spectroscopy at room temperature and the Principal Component Analysis was applied to the absorption spectra of irradiated LiF crystals. In samples irradiated at highest doses, it allowed clearly identifying the formation of more complex aggregate defects, which appears strictly related to the observed photoluminescence quenching effect.     

Structural phase transformation in ZnS nanocrystalline thin films by swift heavy ion irradiation

Zinc sulfide (ZnS) thin films in zinc-blende (ZB) and wurtzite (W) phases have been fabricated by pulsed laser deposition. 150 MeV Ni ion beam irradiation has been carried out at different fluences ranging from 10¹¹ to 10¹³ ions/cm² at room temperature for ion induced modifications. Structural phase transformation in ZnS from W to ZB phase is observed after high energy ion irradiation which leads to the decrease in bandgap. Generation of high pressure and temperature by thermal spike during MeV ion irradiation along the ion trajectory in the films is responsible for the structural phase transformation.     

Assignments and optical properties of X-ray-induced colour centres in blue and orange radiophotoluminescent silver-activated glasses

We have systematically investigated the origin and optical properties of the X-ray-induced colour centres based on the blue and red radiophotoluminescence (RPL) in a silver-activated phosphate glass. The induced-absorption band was decomposed into six Gaussian bands on the basis of its strong analogy with silver-activated sodium chloride. We have ascribed these bands to Ag⁰, Ag²⁺, and other silver ion species by means of optical and thermal measurements such as colour centre formation and dissolution by highly successive femtosecond-pulse irradiation, excited-state lifetime and thermal annealing characteristics. The data confirmed that the blue RPL at 450 nm could be attributed to the 270 and 345 nm bands due to the and Ag⁰ centres, respectively, and that the orange RPL at 560 nm was associated with the 308 nm band due to the Ag²⁺ centres.     

Thermal decay of stable colour centres in Gd3Ga5O12 crystals

This work is devoted to an analysis of thermal decay processes of the stable colour centres (CC) created in gadolinium gallium garnet (GGG) Gd₃Ga₅O₁₂, crystals under irradiation by gamma-quanta (E=1.25 MeV, D=10⁶ Gy). An analysis of the model of CC accumulation under irradiation as well as an isochronous heating model is presented. The decay activation energy (Δ E) for O^? and F centres in GGG crystals is calculated in accordance with this model.     

New features from transparent thin films of EuTiO3

ABSTRACT

The almost multiferroic perovskite EuTiO₃ (ETO) has been prepared as films on substrates of SrTiO₃. For all prepared film thicknesses highly transparent insulating films with atomically flat surfaces and excellent orientation have been grown. They were characterized by X-ray diffraction, magnetic susceptibility and birefringence measurements and found to exhibit bulk properties, namely an antiferromagnetic transition at T_N = 5.1 K and a structural transition at T_S = 282 K. The latter could only be identified due to the high transparency of the samples since the optical band gap is of the order of 4.5 eV and larger than observed before for any bulk and thin film samples.     

Photo- and thermo-stimulated luminescence of CsI—Tl crystal after UV light irradiation at 80 K

Abstract

Thermo- and photo-stimulated luminescence are studied for CsI—Tl crystal after the irradiation with the UV light at 80 K. Creation spectrum of the photostimulated luminescence coincides with the D absorption band of Tl⁺ ions. Nature of the defects created by UV light at low temperatures is discussed basing on the correspondence between the thermostimulated glow curve peaks and thermal evolution of the photostimulation spectra observed after irradiation in the D absorption band. Three bands at 1400, 950 and 580 nm have been observed in the stimulation spectrum at 80 K. The 1400 and 950 nm stimulation bands are presumably explained as the optical transitions in the Tl⁰ centre forming the spatially correlated defect pair with V_k centre while the 580 nm stimulation band is connected with the unperturbed Tl⁰ centres. It is concluded that the Tl⁺ luminescence at low temperature is connected with the electron recombination with the Tl²⁺ centre.     

High-energy heavy-ion induced physical and surface-chemical modifications in polycrystalline cadmium sulfide thin films

The effect of high electronic energy deposition on the structure, surface topography, optical properties, and electronic structure of cadmium sulfide (CdS) thin films have been investigated by irradiating the films with 100 MeV Ag⁺⁷ ions at different ion fluences in the range of 10¹²–10¹³ ions/cm². The CdS films were deposited on glass substrate by thermal evaporation, and the films studied in the present work are polycrystalline with crystallites preferentially oriented along (002)-H direction. It is shown that swift heavy ion (SHI) irradiation leads to grain agglomeration and hence an increase in the grain size at low ion fluences. The observed lattice compaction was related to irradiation induced polygonization. The optical band gap energy decreased after irradiation, possibly due to the combined effect of change in the grain size and in the creation of intermediate energy levels. Enhanced nonradiative recombination via additional deep levels, introduced by SHI irradiation was noticed from photoluminescence (PL) analysis. A shift in the core levels associated with the change in Fermi level position was realized from XPS analysis. The chemistry of CdS film surface was studied which showed profound chemisorption of oxygen on the surface of CdS.     

Reduced graphene oxide foils for ion stripping applications

ABSTRACT

Reduced graphene oxide (rGO) films can be employed as ion strippers in an accelerator. They show some advantages with respect to the graphite foils, due to their high thermal and electrical conductivity, low density, high mechanical resistance and high stability. Thin graphene oxide (GO) films with a sub-micron thickness have been synthesized and transformed into reduced GO (rGO) by ion beam irradiations. Physical characterizations of the pristine and ion irradiated GO films have been performed. Measurements of stripping efficiency have been carried out by using helium, lithium, carbon and oxygen ion beams. The rGO stripper films demonstrate a significantly high charge production, comparable to that of the graphite films but with the advantage of a longer lifetime.     

In-line X-ray lensless imaging with lithium fluoride film detectors

In this work, we present preliminary in-line X-ray lensless projection imaging results at a synchrotron facility by using novel solid-state detectors based on non-destructive readout of photoluminescent colour centres in lithium fluoride thin films. The peculiarities of LiF radiation detectors are high spatial resolution on a large field of view, wide dynamic range, versatility and simplicity of use. These properties offered the opportunity to test a broadband X-ray synchrotron source for lensless projection imaging experiments at the TopoTomo beamline of the ANKA synchrotron facility by using a white beam spectrum (3–40 keV). Edge-enhancement effects were observed for the first time on a test object; they are discussed and compared with simulations, on the basis of the colour centre photoluminescence linear response found in the investigated irradiation conditions.     

LSO-Ce fluorescence spectra and kinetics for UV,VUV and X-ray excitation

Abstract

LSO-Ce fluorescence emission and excitation spectra and decay kinetics have been measured for UV, VUV and X-ray excitation at RT and 80 K. The features of the fluorescence excitation spectra of two types of cerium centres in the region 3 to 6 eV are analysed in the assumption of competitative absorption between them. It is shown that the centres can have similar absorption bands. Forbidden energy bandgap for LSO is evaluated to be not less than 6.5 eV.     

Decay kinetics and thermally stimulated luminescence of Mn4+ in SrTiO3

Abstract

Decay kinetics of Mn⁴⁺ luminescence in SrTiO₃ and thermal stimulation of this luminescence (TSL) after low-temperature irradiation with light from the 355—520 nm region, have been investigated in the temperature range of 4.2—150K and 12—110K, respectively, for the first time. It is concluded that TSL glow peaks are associated with thermal release of carriers from shallow traps followed by Mn⁵⁺→Mn⁴⁺(²E) and/or Mn³⁺→Mn⁴⁺ (²E) charge transfer decay kinetics has been explained by considering trap energy levels taking part in the TSL process.     

Colour centres in microcrystalline powders of alkali halides

Abstract

Powders of twelve alkali halides have been coloured in an electrodeless discharge. Due to the simultaneous production and bleaching of F centres, F aggregate centres could be produced in an adequate concentration. From their study it has been concluded that powders behave as deformed single crystals. It could also be shown from this study that the interaction between colour centres and dislocations is lattice dependent and varies regularly as cation changes from Li⁺ to Rb⁺, and anion changes from F ^?to I^?.     

Optically and thermally driven bistability of FH(OH−) centres in KBr: Structure of the bistable configurations

Abstract

Pairs of F centres and OH^? molecules (F_H(OH^?) centres) exhibit a very interesting optical and thermal bistability in KBr. The analysis of our Electron Nuclear Double Resonance (ENDOR) investigation yields structure models. In both bistable configurations the OH^? molecules reside on a next nearest anion site (4^th shell) relative to the F centre. The difference lies in the orientation of the OH^? dipole with respect to the defect pair axis. The thermal bistability will be explained tentatively as being entropy-driven.     

Defects and localized states in MBE-grown GaAs1−x N x solid solutions prepared by molecular-beam epitaxy

Defects and localized states have been studied for molecular-beam-epitaxy (MBE)-grown high-resistivity and undoped GaAs_1?x N _x films with a N concentration not exceeding approximately 1.0 at.%. The crystalline quality of the films and hence the defects and localized states were determined by high-resolution X-ray diffraction, photoluminescence spectra, capacitance versus voltage measurements and photoinduced current transient spectra of GaAs and GaAs_1?x N _x layers. It was concluded that incorporation of low concentrations of N into MBE-grown GaAs_1?x N _x films promotes the formation of high densities of deep centres similar to EL2 donors, leading to heavy compensation of the films by some unidentified acceptors. GaAs antisite acceptors were believed to be responsible for the said compensation. A prominent defect band near 1.33-1.38 eV also appeared to be associated with these defects. The most prominent centres in dilute GaAs_1?x N _x films with N content less than 0.35 at.% seem to be the EL2 donors and the hole traps located near E _v +0.3 eV.     

Initial stages in the Sm-Si(111) interface formation

The initial stages in the formation of the Sm-Si(111) interface have been studied by thermal desorption, atomic beam modulation, and low-energy-electron-diffraction spectroscopy. The structure of adsorbed films and samarium silicide films, as well as the Sm atom desorption kinetics have been investigated within a broad range of surface coverages and temperatures. The activation energy of desorption from the thermally most stable 3×2 submonolayer structure, as well as the binding energy of a single samarium atom with the substrate, have been measured. The temperature of the onset of silicide decomposition and the activation energy of this process have been determined. It is shown that the Sm-Si(111) interface forms by a mechanism close to that of Stransky-Krastanov. Fiz. Tverd. Tela (St. Petersburg) 40, 371–378 (February 1998)     

Band gap variation in co-evaporated AgInSe2 thin films with 1.26 MeV He+ ion irradiation

Polycrystalline AgInSe₂ thin films have been prepared by co-evaporation of individual elements on glass substrate at a high temperature. The samples were subjected to the irradiation of 1.26 MeV helium ions (He⁺). Structural properties were investigated by X-ray diffraction and optical studies have been carried out from transmittance and reflectance measurements. The effect of irradiation on the structural and optical properties has been investigated for different doses of He⁺ ions. It is observed that the band gap of silver indium selenide thin films decreases gradually from 1.25 eV to 1.07 eV with irradiation dose.