Luminescence characteristics of Tl ions co-doped RbCl:Eu crystals

RbCl:Eu²⁺ single crystals which are co-doped with thallium display characteristic Eu²⁺ emission around 420 nm and additional emission band at 312 nm with a weak shoulder around 390 nm attributable to centers involving Tl⁺ ions. Additional excitation and emission bands observed in Tl⁺ doped RbCl:Eu²⁺ single crystals are attributed to the presence of Eu²⁺ aggregates and complex centres involving both Eu²⁺ and Tl⁺ ions. Inclusion of Tl⁺ ions in RbCl:Eu²⁺ crystals is found to enhance the intensity of Eu²⁺ emission at 420 nm due to an energy transfer from Tl⁺ → Eu²⁺ ions.     

Color center formation in silica glass induced by high energy Fe and Xe ions

Silica glass samples were implanted with 1.157 GeV ⁵⁶Fe and 1.755 GeV ¹³⁶Xe ions to fluences range from 1 × 10¹¹ to 3.8 × 10¹² ions/cm². Virgin and irradiated samples were investigated by ultraviolet (UV) absorption from 3 to 6.4 eV and photoluminescence (PL) spectroscopy. The UV absorption investigation reveals the presence of various color centers (E′ center, non-bridging oxygen hole center (NBOHC) and ODC(II)) appearing in the irradiated samples. It is found that the concentration of all color centers increase with the increase of fluence and tend to saturation at high fluence. Furthermore the concentration of E′ center and that of NBOHC is approximately equal and both scale better with the energy deposition through processes of electronic stopping, indicating that E′ center and NBOHC are mainly produced simultaneously from the scission of strained Si-O-Si bond by electronic excitation effects in heavy ion irradiated silica glass. The PL measurement shows three emissions peaked at about 4.28 eV (α band), 3.2 eV (β band) and 2.67 eV (γ band) when excited at 5 eV. The intensities of α and γ bands increase with the increase of fluence and tend to saturation at high fluence. The intensity of β band is at its maximum in virgin silica glass and it is reduced on increasing the ions fluence. It is further confirmed that nuclear energy loss processes determine the production of α and γ bands and electronic energy loss processes determine the bleaching of β band in heavy ion irradiated silica glass.     

Absorption and photoluminescence study of Al2O3 single crystal irradiated with fast neutrons

Colour centers formation in Al₂O₃ by reactor neutrons were investigated by optical measurements (absorption and photoluminescence). The irradiation’s were performed at 40 °C, up to fast neutron (E_n > 1.2 MeV) fluence of 1.4 × 10¹⁸ n cm⁻². After irradiation the coloration of the sample increases with the neutron fluence and absorption band at about 203, 255, 300, 357 and 450 nm appear in the UV-visible spectrum. The evolution of each absorption bands as a function of fluence and annealing temperature is presented and discussed. The results indicate that at higher fluence and above 350 °C the F⁺ center starts to aggregate to F center clusters (F₂, F₂⁺ and ). These aggregates disappear completely above 650 °C whereas the F and F⁺ centers persist even after annealing at 900 °C. It is clear also from the results that the absorption band at 300 nm is due to the contribution of both F₂ center and interstitial ions.     

Defects creation in sapphire by swift heavy ions: A fluence depending process

Single crystals of sapphire (α-Al₂O₃) were irradiated at GANIL with 0.7 MeV/amu xenon ions corresponding to an electronic stopping power of 21 keV/nm. Several fluences were applied between 5 × 10¹¹ and 2 × 10¹⁴ ions/cm². Irradiated samples were characterized using optical absorption spectroscopy. This technique exhibited the characteristic bands associated with F and F⁺ centers defects. The F centers density was found to increase with the fluence following two different kinetics: a rapid increase for fluences less than 10¹³ ions/cm² and then, a slow increase for higher fluences. For fluences less than 10¹³ ions/cm², results are in good agreement with those obtained by Canut et al. [B. Canut, A. Benyagoub, G. Marest, A. Meftah, N. Moncoffre, S.M.M. Ramos, F. Studer, P. Thévenard, M. Toulemonde, Phys. Rev. B 51 (1995) 12194]. In the fluences range: 10¹³-10¹⁴ ions/cm², the F centers defects creation process is found to be different from the one evidenced for fluences less than 10¹³ ions/cm².     

Photoluminescence study of swift heavy ion (SHI) induced defect centers in sapphire

Single crystals of sapphire (Al₂O₃: Fe, Ti, Cr) were irradiated at room temperature with different fluence of 100 MeV Ni ions. Photoluminescence (PL) spectra of pristine and irradiated sapphires were recorded at room temperature under 2.8 eV blue excitation. A broad emission band consists of two bands centered at 516 nm corresponding to F₂ defect center and 546 nm corresponding to defect center was observed. The intensity of these defect centers was found to vary with the fluence. defect center develops at low fluence reaching maximum at 5 × 10¹⁶ ions/m² and finally decreasing at higher fluence. The behavior is interpreted in terms of creation of defect centers, their clustering and annihilation.     

Ab initio study electronic structures of LiYF4 crystal containing interstitial oxygen atoms

The positions of the interstitial oxygen atoms in LiYF₄ crystal are simulated by computer technologies. It is found that the total energy of cluster is low when interstitial oxygen atoms exist around the Li⁺ ion. Basing on the computer results, the electronic structures of perfect LiYF₄ and the LiYF₄ containing interstitial oxygen atoms with the lattice structure optimized are studied within the framework of the density functional theory. By analyzing the calculated results it can be concluded that an interstitial oxygen atom could combine with formal lattice fluorine ions forming molecular ions, which cause the 260 nm absorption band.     

Substrate temperature effect on F etching of SiC: Molecular dynamics simulation

In this study, we performed molecular dynamics simulations to investigate F⁺ continuously bombarding SiC surfaces at temperatures of 100, 400, 600 and 800 K with the energy of 150 eV. The simulation results show that the etch rate of Si atoms is more than that of C atoms. With increasing temperature, the deposition yield of F atoms decreases, while the etch yields of C and Si atoms increase. In etching products, SiF, SiF₂ and CF species are dominant. Their yields increase with increasing temperature.     

Formation of InAs nanocrystals in Si by high-fluence ion implantation

We have studied the formation of InAs precipitates with dimensions of several nanometers in silicon by means of As (245 keV, 5 × 10¹⁶ cm⁻²) and In (350 keV, 4.5 × 10¹⁶ cm⁻²) implantation at 500 °C and subsequent annealing at 900 °C for 45 min. RBS, SIMS, TEM/TED, RS and PL techniques were used to characterize the implanted layers. The surface density of the precipitates has been found to be about 1.2 × 10¹¹ cm⁻². Most of the crystallites are from 3 nm to 6 nm large. A band at 1.3 μm has been registered in the low-temperature PL spectra of (As + In) implanted and annealed silicon crystals. The PL band position follows the quantum confinement model for InAs.     

Luminescence studies on gamma irradiated RbCl:Sm and RbBr:Smcrystals

This paper presents and discusses the results obtained on the thermoluminescence (TL) and other optical studies of gamma irradiated RbCl:Sm³⁺ and RbBr:Sm³⁺ crystals. Samarium when doped into the RbCl and RbBr is found to enter the host lattice in its trivalent state and act as electron trap during gamma irradiation, there by partially converting itself to Sm²⁺. The photoluminescence (PL) spectra of both RbCl and RbBr crystals doped with Sm exhibit the strong red/orange emissions of Sm corresponding to ⁴G_5/2 → ⁶H_9/2 (red) and ⁴G_5/2 → ⁶H_7/2 (orange) transitions. Z₃ centers are observed in RbBr on F bleaching subsequent to gamma irradiation and a TL glow peak attributable to the same has been identified. The thermoluminescence (TL) glow curve explains the defect annihilation process to be due to the mobilization of two different kinds of traps created as a result of exposure to high energy irradiation in both the cases. Spectral distribution under the thermoluminescence emission (TLE) and optically stimulated emission(OSL), support the idea that defect annihilation process to be due to thermal release of F electron in RbCl:Sm³⁺ and in RbBr:Sm³⁺ crystals. Both Sm³⁺ and Sm²⁺ emissions were observed in the thermoluminescence emission of the crystals.     

Photostimulated luminescence studies of γ-irradiated KBr1−xIx:TlI mixed crystals

Results on photoluminescence and photostimulated luminescence studies of KBr_1−xI_x:TlI (0.01 and 0.05 mol%) mixed crystals grown in vacuum and air are presented. Photoluminescence spectra of the mixed crystals excited in the A-absorption band of KBr:Tl⁺ exhibited the characteristic emission bands of Tl⁺ ions in KBr:Tl⁺. When excited in the low energy tail of A-band absorption, additional emission bands were observed. Compared with earlier reports, excitation bands observed around 4.8, 4.6 and 4.4 eV are attributed to complex thallium centres of the form TlBr_6−nI_n (n = 0, 1, 2, 3). Photostimulated luminescence of γ-irradiated KBr_1−xI_x:TlI mixed crystals showed the presence of emission bands similar to the characteristic photoluminescence of Tl⁺ ions. The photostimulated luminescence slightly shifted towards the low energy side with increasing iodide composition x. The mechanism of emission in these mixed crystals is discussed. Dose response and storage stability (fading characteristics) in these mixed crystals are reported.     

First-principles study on electronic structures and absorption spectra for BaWO4 crystal containing barium vacancy

The electronic structures, dielectric function and absorption spectra for the perfect BaWO₄ (BWO) crystal and the BWO crystal containing barium vacancy () have been studied using density functional theory code CASTEP with the lattice structure optimized. The results indicate that the optical properties of the BWO crystal exhibit anisotropy and its optical symmetry coincide with lattice structure geometry of the BWO crystal. For the BWO crystal containing , there exhibit four absorption bands peaking at 0.71 eV (1751 nm), 1.85 eV (672 nm), 3.43 eV (362 nm) and 3.85 eV (322 nm), respectively. The origins of the 370 nm absorption band should be related to the .     

Theory of He trapping, diffusion, and clustering in UO2

We have performed ab initio total energy calculations to investigate the behavior of helium and its diffusion properties in uranium dioxide (UO₂). Our investigations are based on the density functional theory within the generalized gradient approximation (GGA). The trapping behavior of He in UO₂ has been modeled with a supercell containing 96-atoms as well as uranium and oxygen vacancy trapping sites. The calculated incorporation energies show that for He a uranium vacancy is more stable than an oxygen vacancy or an octahedral interstitial site (OIS). Interstitial site hopping is found to be the rate-determining mechanism of the He diffusion process and the corresponding migration energy is computed as 2.79 eV at 0 K (with the spin-orbit coupling (SOC) included), and as 2.09 eV by using the thermally expanded lattice parameter of UO₂ at 1200 K, which is relatively close to the experimental value of 2.0 eV. The lattice expansion coefficient of He-induced swelling of UO₂ is calculated as 9 × 10⁻². For two He atoms, we have found that they form a dumbbell configuration if they are close enough to each other, and that the lattice expansion induced by a dumbbell is larger than by two distant interstitial He atoms. The clustering tendency of He has been studied for small clusters of up to six He atoms. We find that He strongly tends to cluster in the vicinity of an OIS, and that the collective action of the He atoms is sufficient to spontaneously create additional point defects around the He cluster in the UO₂ lattice.     

Resonant Rutherford backscattering spectrometry for carbon diffusion in silicon

300 keV C⁺ ion implantation onto Si(1 0 0) wafers was carried out at temperatures of 400, 500, 550, 600, 650 and 700 °C. Depth profile of C was determined by resonant Rutherford backscattering spectrometry (RRBS) measurements using ¹²C(α,α)¹²C resonant reaction with the α-particle energy of 4.27 MeV. The concentration of the implanted carbon at the surface as a function of inverse of implantation temperature shows an Arrhenius behaviour. The activation energy for diffusion of carbon in Si was measured and found to be 0.434 eV, which is smaller than the activation energy (0.88 eV) for the C diffusion in Si in equilibrium condition. The possible mechanism of C diffusion in Si during irradiation conditions existing in our experiments where large concentration of vacancies and interstitials are produced is discussed and we find that the C diffusion during irradiation conditions could be due to the drag the carbon towards the surface by the vacancy flux.     

Effect of swift heavy ion irradiation on structure, optical, and gas sensing properties of SnO2 thin films

Swift heavy ion irradiation has been successfully used to modify the structural, optical, and gas sensing properties of SnO₂ thin films. The SnO₂ thin films prepared by sol-gel process were irradiated with 75 MeV Ni⁺ beam at fluences ranging from 1 × 10¹¹ ion/cm² to 3 × 10¹³ ion/cm². Structural characterization with glancing angle X-ray diffraction shows an enhancement of crystallinity and systematic change of stress in the SnO₂ lattice up to a threshold value of 1 × 10¹³ ions/cm², but decrease in crystallinity at highest fluence of 3 × 10¹³ ions/cm². Microstructure investigation of the irradiated films by transmission electron microscopy supports the XRD observations. Optical properties studied by absorption and PL spectroscopies reveal a red shift of the band gap from 3.75 eV to 3.1 eV, and a broad yellow luminescence, respectively, with increase in ion fluence. Gas response of the irradiated SnO₂ films shows increase of resistance on exposure to ammonia (NH₃), indicating p-type conductivity resulting from ion irradiation.     

Growth of silicon nitride films by bombarding amorphous silicon with N ions: MD simulation

In this study, the molecular dynamics simulation method was employed to investigate the growth of silicon nitride films by using N⁺ ions, with energies of 50, 100, 150 and 200 eV, to bombard an amorphous silicon surface at 300 K. After an initial period of N⁺ bombardment, saturation of the number of N atoms deposited on the surface is observed, which is in agreement with experiments. During subsequent steady state deposition, a balance between uptake of N by the surface and sputtering of previously deposited N is established. The Si(N_x) (x = 1-4) and N(Si_y) (y = 1-3) bond configurations in the grown films are analyzed.     

Color center creation in LiF crystals irradiated with Xe, Kr and N ions: Dependence on fluence and beam current density

Single LiF crystals were irradiated with Xe (195 MeV), Kr (117 MeV), and N (18 MeV) ions. Using absorption spectroscopy, color center creation was analyzed as a function of the ion energy loss, fluence, and flux. The concentration of single F centers and F₂ centers versus fluence and flux exhibits a nonlinear evolution with saturation at higher fluences. For LiF irradiated with N ions at high fluence, the concentration of F centers is proportional to the cube root of the flux indicating the strong interaction of primary hole centers. Macroscopic hillocks were observed in all irradiated LiF crystals by atomic force microscopy.     

Structural changes in helium implanted Zr0.8Y0.2O1.9 single crystals characterized by atomic force microscopy and EXAFS spectroscopy

The present work is devoted to investigate the local atomic environment (of Zr, Y and O) as well as surface modifications associated with excess helium in the cubic phase of (1 0 0)-oriented Zr_0.8Y_0.2O_1.9 single crystal substrates. Commercially available oxide crystals have been implanted at various fluences in the range 0.15-2.0 × 10¹⁶ He-atoms/cm² using a 2.74 MeV He⁺ ion beam passing through a 8.0 μm Al foil. The microstructure and surface morphology of the irradiated surface are examined using atomic force microscopy (AFM). The local atomic environments of Zr, Y and O in the implanted layer are studied using synchrotron radiation and by extended X-ray absorption fine structure (EXAFS) measured at glancing angles to probe the implanted layer. From AFM studies it was observed that the surface roughness increases as fluence increases and above a critical fluence stage, small blister-like structures originating from helium bubbles are scattered on the irradiated surface. The radial distribution functions (RDFs), derived from EXAFS data at the Zr K-edge, have been found to evolve continuously as a function of ion fluence describing the atomic scale structural modifications in YSZ by helium implantation. From the pristine data, long range order (beyond the first- and second-shell) is apparent in the RDF spectrum. It shows several nearest neighbour peaks at about 2.1, 3.6, 4.3 and 5.4 Å. In the implanted specimens, all these peaks are greatly reduced in magnitude and their average positions are changed, typical of damaged material. A simple model taking into account only the existence of lattice vacancies has been used for the interpretation of measured EXAFS spectra.     

Alkali ion scattering from Ag(0 0 1) and Ag thin films at low and hyperthermal energies

We have investigated the scattering of K⁺ and Cs⁺ ions from a single crystal Ag(0 0 1) surface and from a Ag-Si(1 0 0) Schottky diode structure. For the K⁺ ions, incident energies of 25 eV to 1 keV were used to obtain energy-resolved spectra of scattered ions at θ_i = θ_f = 45°. These results are compared to the classical trajectory simulation safari and show features indicative of light atom-surface scattering where sequential binary collisions can describe the observed energy loss spectra. Energy-resolved spectra obtained for Cs⁺ ions at incident energies of 75 eV and 200 eV also show features consistent with binary collisions. However, for this heavy atom-surface scattering system, the dominant trajectory type involves at least two surface atoms, as large angular deflections are not classically allowed for any single scattering event. In addition, a significant deviation from the classical double-collision prediction is observed for incident energies around 100 eV, and molecular dynamics studies are proposed to investigate the role of collective lattice effects. Data are also presented for the scattering of K⁺ ions from a Schottky diode structure, which is a prototype device for the development of active targets to probe energy loss at a surface.     

Swelling and optical properties of Si3N4 films irradiated in the electronic regime

Silicon nitride layers of 140 nm thickness were deposited on silicon wafers by low pressure chemical vapour deposition (LPCVD) and irradiated at GANIL with Pb ions of 110 MeV up to a maximum fluence of 4 × 10¹³ cm⁻². As shown in a previous work these irradiation conditions, characterized by a predominant electronic slowing-down (S_e = 19.3 keV nm⁻¹), lead to damage creation and formation of etchable tracks in Si₃N₄. In the present study we investigated other radiation-induced effects like out of plane swelling and refractive index decrease. From profilometry, step heights as large as 50 nm were measured for samples irradiated at the highest fluences (>10¹³ cm⁻²). From optical spectroscopy, the minimum reflectivity of the target is shifted towards the high wavelengths at increasing fluences. These results evidence a concomitant decrease of density and refractive index in irradiated Si₃N₄. Additional measurements, performed by ellipsometry, are in full agreement with this interpretation.     

Dynamic annealing study of SiC epilayers implanted with Ni ions at different temperatures

SiC epilayers grown on 4H-SiC single crystals were implanted with 850 keV Ni⁺ ions with fluences in the 0.5-9 × 10¹⁶ Ni⁺/cm² range. Most of the samples were implanted at 450 °C, but for comparison some implantations were performed at room temperature (RT). In addition, a post-implantation annealing was performed in N₂ at 1100 °C in order to recover the implantation-induced structural damage. The disorder produced by the implantation at 450 °C and the effect of the post-implantation annealing on the recrystallization of the substrates have been studied as a function of the fluence by Backscattering Spectrometry in channeling geometry (BS/C) with a 3.45 MeV He²⁺ beam. RT as-implanted samples showed a completely amorphous region which extends until the surface when irradiated with the highest dose, whereas in the case of 450 °C implantation amorphization does not occur. In general, partial recovery of the crystal lattice quality was found for the less damaged samples, and the dynamic recovery of the crystalline structure increases with the irradiation temperature.