Showing papers on "Fluence published in 1995"

Room‐temperature visible luminescence from silicon nanocrystals in silicon implanted SiO2 layers

Abstract: We report the observation of visible‐light emission at room temperature from high fluence (0.3–3×1017 cm−2) Si+ implanted thermal SiO2 layers grown on silicon substrates. Significant blue‐light emission and an intense broad luminescent band with a peak beyond 750 nm are observed after annealing at high temperature (T≥1000 °C). The red‐light emission, present only in the highest fluence implant, is attributed to the luminescence emitted from silicon nanocrystals produced by silicon precipitation. The presence of silicon nanocrystals is confirmed by transmission electron microscopy. Significant blue‐light emission is visible after thermal annealing in the 1×1017 cm−2 fluence implant. The peak position shifts from 490 to 540 nm by increasing the annealing cycles temperature.

Excimer laser processing of indium‐tin‐oxide films: An optical investigation

Abstract: dc sputtered indium‐tin‐oxide films have been excimer laser irradiated at subablation threshold fluences (<510 mJ/cm2). Optical characterization of irradiated products has been performed aiming at resolving the finer structure appearing in the IR–visible absorption spectra, as a function of laser fluence, and assigning such features to specific electronic defects which are produced upon irradiation. Four individual Gaussian‐like contributions to absorption spectra are identified at 0.7, 1.0, 1.6, and 2.6 eV, the intensity of which is observed to vary with fluence. Being absent in the original films and emerging in optical spectra at fluences exceeding 300 mJ/cm2, the 2.6 eV contribution is most characteristic to excimer laser processing and is responsible for the darkening of the film. Thermal model calculations reveal that such defects are produced only upon melting and fast resolidification of the film. The evolution of the chemistry actually taking place in the film upon irradiation is followed by x‐ra...

Scanning Electron Microscope Observations of CO2 Laser Effects on Dental Enamel

Abstract: Studies of the effects of carbon dioxide (CO2) lasers on dental enamel have demonstrated that surface changes can be produced at low fluences (< 10 J/cm2) if wavelengths are used which are efficiently absorbed by the hard tissues. In this study, scanning electron microscopy (SEM) was used to characterize the wavelength dependence of surface changes in dental enamel after exposure to an extensive range of CO2 laser conditions. Bovine and human enamel were irradiated by a tunable, pulsed CO2 laser (9.3, 9.6, 10.3, 10.6 microns), with 5, 25, or 100 pulses, at absorbed fluences of 2, 5, 10, or 20 J/cm2, and pulse widths of 50, 100, 200, 500 microseconds. SEM micrographs revealed evidence of melting, crystal fusion, and exfoliation in a wavelength-dependent manner. Crystal fusion occurred at absorbed fluences as low as 5 J/cm2 per pulse at 9.3, 9.6, and 10.3 microns, in contrast to no crystal fusion at 10.6 microns (< or = 20 J/cm2). Longer pulses at constant fluence conditions decreased the extent of surface melting and crystal fusion. The total number of laser pulses delivered to the tissue did not significantly affect surface changes as long as a minimum of 5 to 10 pulses was used. Within the four easily accessible wavelengths of the CO2 laser, there are dramatic differences in the observed surface changes of dental hard tissue.

Enhancement of material ablation using 248, 308, 532, 1064 nm laser pulse with a water film on the treated surface

Abstract: Many industrial laser processes, such as surface cleaning, require the removal of small thicknesses of matter, often on large samples. An experimental study has been performed in order to characterize and enhance the ablation of materials by means of the interaction between a pulsed laser beam and matter using common industrial laser sources, particularly at 248, 308, 532, and 1064 nm. Ablation was achieved on a static sample with one or several successive pulses and for different energy densities. These parameters enabled us to control the depth of the ablation in the micrometer range. Experiments have been carried out in ambient air, under gaseous cover, and then under a flowing water film on the material surface at atmospheric pressure. The material was a stainless‐steel alloy. The experiments made it possible to determine the ablation fluence threshold for each wavelength and the alteration of the surface by the successive laser pulses. In order to avoid oxidation during the process, an argon gaseous medium was used, but peripheral oxide deposits always appeared and the efficiency of the ablation did not seem to increase. However, when the water flow on the surface was employed, the efficiency of the ablation increased by a factor of 2–15 for a certain incident power density. Peripheral oxide redeposition was absent using this method. Experiments have demonstrated that the confinement of the plasma within a transparent layer such as water at the given laser wavelength is an effective method for increasing ablation yield. This technique prevents the expansion of the plasma upstream of the target and the mechanical impulse communicated to the material increases. Calculation of ablated mass confirms that ablation yield could be increased by 15 for higher power densities. Similar results have been found using other materials such as alumina or silica. This means that this ablation technique can be employed in various types of industrial laser applications, for example, for cleaning oxidized surfaces or removing paint coatings.

Laser sputtering of highly oriented pyrolytic graphite at 248 nm

Abstract: The interaction of excimer laser pulses with a highly oriented pyrolytic graphite (HOPG) target has been studied. HOPG, a close approximation to single crystal graphite, was irradiated along a freshly cleaved basal plane in vacuum by pulses from a KrF excimer laser. The energy fluence was varied between 300–700 mJ/cm2, resulting in material removal rates of <0.01 A/pulse to ∼100 A/pulse. In this near‐threshold regime, neutral carbon atoms, dimers, and trimers account for nearly all of the sputtered flux and collisional and plasma effects are minimized. Time‐of‐flight distributions of the neutral carbon atoms and small carbon clusters were measured and inverted to obtain translational energy flux distributions and relative sputtering yields as a function of fluence. The translational energy distributions are remarkably close to Maxwell–Boltzmann distributions over most of the fluence range studied. However, the mean translational energies are far too high to reconcile with a simple thermal vaporization mod...

Surface femtochemistry of 02 and CO on Pt(111)

Abstract: Desorption of OZ and formation of CO2 were induced with subpicosecond laser pulses on a Pt(111) surface dosed with coadsorbed OZ and CO. We report the fluence dependent yields obtained over a range of laser wavelengths from 267 to 800 nm, and pulse durations from 80 fs to 3.6 ps. The nonlinear dependence of the yield on fluence is different at different wavelengths. Two-pulse correlation measurements show two different time-scales relevant to the desorption. The results show that nonthermalized electrons play a role in the surface chemistry, and that an equilibrated pre-heating of the surface modes leads to enhanced desorption.

Synthesis of GaN by N ion implantation in GaAs (001)

Abstract: Both the hexagonal and cubic GaN phases were synthesized in GaAs (001) by 50 keV N ion implantation at 380 °C and subsequent furnace annealing at 850–950 °C for 10 min–2 h. For a fluence of 1.5×1017 cm−2, transmission electron microscopy revealed that cubic GaN epitaxially crystallizes as precipitates in the GaAs matrix. A cubic‐to‐hexagonal GaN phase transition was observed for extended thermal anneals. By increasing the N fluence to 3×1017 cm−2, a continuous buried layer of randomly oriented hexagonal‐GaN nanocrystals was produced.

Surface microarchitectural design in biomedical applications: preparation of microporous polymer surfaces by an excimer laser ablation technique.

Abstract: In this report we demonstrate a microprocessing method to prepare microporous polymer films by an excimer laser ablation technique, which may enable the fabrication of functional biomedical devices such as advanced artificial organs. The irradiation of a KrF excimer laser pulses (wavelength 248 nm ; fluence 1 J/cm 2 pulse) onto several polymer films was achieved by passing a laser pulse through an optical microscope, resulting in the formation of an etched pit on the irradiated surface due to ablative photodecomposition. The number of pulses and the micropositioning of the irradiation were precisely controlled by a computer-aided control unit. Minimal ablation was observed for polyethylene with very small absorption coefficient (α) at 248 nm. For polymers which absorbed the laser photons, the etch depth increased linearly with number of pulses. The etch depth per pulse decreased with an increase in α values. An excellent structural quality, with micron-order precision of an etched pit, was found for those polymers with larger α values, such as polyimide, segmented polyurethane, and polycarbonate. © 1995 John Wiley & Sons, Inc.

Enhanced photosensitivity in lightly doped standard telecommunication fibre exposed to high fluence ArF excimer laser light

Abstract: The core refractive index of Corning SMF-28 optical fibre exposed to ArF laser pulses increases with the square of the fluence per pulse. Bragg gratings with a refractive index modulation amplitude higher than 10/sup -3/ have been obtained. This is an order of magnitude improvement over previously reported values for this type of fibre in the absence of treatment to enhance the photosensitivity.

Ion-beam-induced amorphization of 6H-SiC

Abstract: We have investigated the amorphization of 6H-SiC during bombardment with Na ions of energy 100 and 155 keV, by means of Rutherford backscattering in channelling geometry, mechanical surface profiling and Raman spectroscopy. The material was found to amorphize at a critical damage rate of about 1/10 displacements per atom. An incubation fluence is needed to initiate the amorphization process. The observed density changes and the results of the Raman measurements also indicate that, at low fluences, an amorphous network is formed, which conserves the chemical short-range order of the crystalline material. Further application of higher fluences drives the system to form an atomically disordered structure without chemical short-range order.

Optical Limiting Property of a Polyacene-Based Oligomer Synthesized under High Pressure

Abstract: The optical limiting property of a new polyacene-based diphenyldiacetylene oligomer was investigated. The oligomer was synthesized by reaction of diphenyldiacetylene under high pressure. Toluene solutions of the oligomer contained within a quartz cell were irradiated with the pulse from a frequency-doubled Nd:YAG laser at 532 nm. At low input fluence, the transmittance of the toluene solution obeyed the Beer-Lambert law. The transmittance was constant and agreed well with that obtained by a spectrophotometer. At high input fluence, the transmittance decreased with input fluence. The oligomer had an optical limiting property. As the concentration of the oligomer increased, the threshold for optical limiting and the saturated output fluence decreased. The inverse of saturated output fluence of the oligomer was about 1/5 of that of C 60 . According to the reverse saturable absorption mechanism, the following equation was obtained between the absorbance log(I 0 /I) and I 0 -I; log(I 0 /I)=K(I 0 -I)+A g , where I 0 is the input fluence, I is the output fluence, K is a parameter depending on the absorption cross section and the relaxation time, and A g is the absorbance of the ground state

Neutral atom and molecule emission accompanying 248-nm laser irradiation of single crystal NaNO3

Abstract: We report quadrupole mass-selected measurements of neutral emissions produced by exposing single crystal sodium nitrate to 248-nm radiation from a pulsed excimer laser. The intensity and identity of the observed products are a strong function of laser fluence as well as previous exposure. In particular, a strong incubation effect is observed, with weak emission intensities during the early stages of irradiation which grow dramatically with continued irradiation. At low fluences ( 2 on laser-damaged surfaces), NO is the only neutral product observed. Thus it appears that defects created by NO emission on more or less perfect NaNO 3 surfaces strongly enhance subsequent emission. At higher fluences (> 100 mJ/cm 2 on laser-damaged surfaces), a visible fluorescent plume is observed and other neutral products (primarily O 2 and Na) are emitted along with NO. Time-of-flight measurements of the anion-derived species (NO, O 2 , NO 2 ) show unusually long emission tails which are inconsistent with emission during the laser pulse only. We attribute this “delayed” emission to dissociative electron attachment due to charge transport from the bulk to the surface similar to electron beam induced emission.

Amorphization kinetics of poly(vinylidene fluoride) on high-energy ion irradiation

Abstract: The crystalline fraction of irradiated semi-crystalline polyvinylidene form II (PVDF α) is measured as a function of dose (or fluence) and energy loss ( d E d x ) using high energy particle beams at room temperature. The dependence of the rate of crystallinity loss on the fluence is different between low d E d x particle- and high d E d x ion-irradiations. In particular, the amorphization is retarded at low dose for electron, O or Ne irradiation, whereas the amorphization is direct with heavier projectiles. The relation between the absorbed dose and the crystalline fraction is investigated. We analyze the effects which can affect the crystallinity in the 1–10 MGy dose range: 1) the collapse of a crystalline region induced by several overlapping damaged tracks; 2) the propagation of disorder from the amorphous interface into the crystallite.

‘‘Clean’’ processing of polymers and smoothing of ceramics by pulsed laser melting

Abstract: Surface stability during laser pulsed melting of polymers and ceramics is studied theoretically. Irradiation conditions and material parameters are found giving rise to the suppression of surface wavy relief of a nonresonant type (with period Λ≫λ, where λ is the radiation wavelength) and thus to the smooth flat irradiation spots. For example, for the polymers considered this process takes place for wavelengths where the absorption coefficient is sufficiently high: α(λ)≳105 cm−1. Thus, it is shown that the formation of such spots, previously referred to as ‘‘clean ablation,’’ can be explained using only a thermal mechanism without reference to the concept of ‘‘photodecomposition.’’ Moreover, laser smoothing and polishing of a surface, if it had roughness before irradiation, can be achieved by appropriate matching of the characteristic size of this roughness along the surface with the values of α(λ) and laser fluence. Methods are proposed to decrease the parasitic influence of droplets on the deposition of ...

A Monte Carlo investigation of electron-beam applicator scatter.

Abstract: An EGS4 Monte Carlo investigation into applicator scatter in clinical electron beams has been undertaken in order to establish the characteristics of electrons incident on the patient surface which have interacted with collimation systems. The applicator scattered component of an electron beam (including that from irregularly shaped cutouts) should be considered when modeling the electron phase space since it represents a component of the beam incident on the patient with widely varying characteristics to those of the primary beam. Scattering off an edge of applicator material is considered in terms of the types and characteristics of incident primary beam, the resulting interactions in the edge, and the fluence and energy characteristics of the emerging particles. Results indicate that the principal component to consider is scattered electrons due to the electron component of the primary beam, and that the fluence and energy characteristics of this component are dependent upon primary beam energy and the configuration of the applicator apertures.

Proton irradiation of various resistivity silicon detectors

Abstract: Future high energy physics experiments at CERN's Large Hadron Collider will use high precision silicon detectors for tracking purposes. The hadronic component of the radiation received threatens the lifetime of these detectors and it is vital to choose the silicon starting material to maximise the performance and lifetime. Ion-implanted silicon detectors with various initial resistivities and germanium concentrations have been irradiated with high energy protons up to a fluence of 10/sup 14/ cm/sup -2/. The change in leakage current and full depletion voltage have been studied both as a function of fluence and of time after irradiation. Measurements were made up to 100 days post-irradiation at room temperature and then using heating techniques to accelerate processes up to the equivalent of over 10 years at room temperature. The leakage-current damage constant is shown to be independent of the starting material while the conduction type inversion point and the long-term annealing of the depletion voltage are sensitive to the initial resistivity and impurity concentrations.

Structural changes during Ar-ion irradiation of laser-deposited Fe/Ag multilayers

Abstract: During 350 keV Ar+ irradiation at 77K, laser-deposited Fe−Ag multilayers first show stress relaxation and demixing processes at the interfaces followed by grain coarsening and a supersaturation of the bcc α-Fe phase with Ag due to ballistic mixing. At high fluence, the fcc γ-Fe(Ag) phase (a=3.65 A) is formed, which can be explained by either chemically guided ballistic short-range relocations or by the occurrence of thermal spikes, where all atoms possess sufficient energy to allow collective structural rearrangements, but only during such a short time that a decomposition due to long-range diffusion is suppressed.

Flight mass-spectrometer calibration in a high-velocity atomic-oxygen beam

Abstract: Calibration and characterization of the quadrupole mass-spectrometer component of the Evaluation of Oxygen Interactions with Materials III space-flight experiment are reported in this paper. A high-velocity atom beam system was used to characterize the response of the flight mass spectrometer to high-velocity oxygen atoms (0.8 to 2.5 eV). The response factor based on oxygen atom flux in the high-velocity beam was found to be logarithmically dependent on the exposure history of the instrument, i.e., the calibration factor was logarithmically dependent on atomic oxygen fluence. This dependence was independent of the background pressure over the range between 10~ and 10~ Torn Subsequent contamination of the instrument restored the instrument sensitivity to the original value before exposure to atomic oxygen. Carbon dioxide, carbon monoxide, and water were observed in the mass spectrometer whenever high-velocity oxygen atoms were present. The intensity of reaction products caused by interaction of atomic oxygen with contaminated surfaces within the instrument decreases with increasing atomic oxygen fluence, whereas C>2 resulting from recombination of atomic oxygen on surfaces increases with fluence.

Photoconductivity evolution due to carrier trapping by defects in 17 MeV-proton irradiated silicon

Abstract: Damage concentration dependence of dark and photoconductivity has been studied in crystalline Si, irradiated with 17 MeV proton. The photoconductivity spectra consist of a broad peak due to a band‐to‐band transition and a tail spectrum on the lower energy side. Two kinds of tail spectra are observed in impurity‐doped and/or damaged specimens, and the latter threshold is deeper than the former. While the main peak of the doped specimen does not greatly change up to a certain irradiation fluence φC, it steeply decreases beyond the φC, which depends on the shallow‐impurity concentration. The tail spectrum of the shallow impurity simultaneously vanishes at the φC and another tail spectrum grows above the φC. As compared to fluence dependence of photocurrent decay time, it is clarified that the drastic decrease in the main peak results from a drop in the decay time or the carrier density, not in the carrier mobility. The defect density for the critical fluence φC has good correlation with the dopant concentrat...

Laser Silicon Ablation Studied by Time-of-Flight Mass Spectrometry:Generation of Si Ions and Neutrals and Their Translational Properties

Abstract: Laser pulses of Nd:YAG (532 and 266 nm) were irradiated on a Si target to investigate the ablation process. Monoatomic ions and neutrals were simultaneously ablated, with ions having higher most probable kinetic energy (MPKE) and broader velocity distribution than neutrals. The two beams showed close ablation fluence threshold, but different exponential relationships of the intensity of the ablated species vs laser fluence, with the 266 nm beam having a higher increasing rate than that of the 532 nm beam. The 266 nm beam also resulted in higher kinetic energy and a higher fraction of ions in the ejected particle stream. The time-of-flight (TOF) results fitted the shifted Maxwell-Boltzmann distribution well. The laser Si ablation mechanism was discussed.

Growth of pure and doped cerium oxide thin film bilayers by pulsed laser deposition

Abstract: Thin films of CeO 2 have been grown on single crystal silicon wafers using pulsed laser deposition. X-ray diffraction was used to examine the effect of deposition parameters such as oxygen partial pressure and substrate temperature. The effect of laser fluence on deposition rate and surface morphology is also presented. The best results were obtained for films deposited at low oxygen partial pressures and high substrate temperatures, from a pre-ablated target using a laser fluence higher than 2 J cm −2 . The growth of multilayers with increasing lattice constants is demonstrated by depositing a CeO 2 layer doped with La over an indoped film, X-ray diffraction and lattice imaging in a transmission electron microscope were used to characterise the films.

Nitrogen ion implantation with energy scanning mode into Zr

Abstract: To study effects of ion implantation with the energy scanning mode on the surface structure and the depth profile, Zr samples were implanted with 15N2 ions, where ion energy was scanned in the range of 70–100 and 70–130 keV using a computer‐controlled power supply at an interval of 2 keV while monitoring the ion current of the sample. After the implantation of a total fluence of 3.5×1017 ions/cm2, the depth profiles of 15N2 ions in Zr were measured by nuclear reaction analysis of 15N(p,αγ)12C at 429 keV and the surface was observed by scanning electron microscopy. It was found that the implanted surface structure strongly depended on the implantation mode, and blistering induced by high fluence implantation of nitrogen could be completely avoided with the implantation mode of increasing energy gradually. The depth profiles were satisfactorily in agreement with the prediction by Monte Carlo simulation.