Showing papers on "Fluence published in 1984"

Laser-induced periodic surface structure. III. Fluence regimes, the role of feedback, and details of the induced topography in germanium

Abstract: The mechanisms responsible for the growth of periodic surface structure on Ge irradiated by 1.06-\ensuremath{\mu}m laser pulses from a single beam are investigated. Time-resolved diffraction of a weak cw probe beam from the developing structures, coupled with electron-microscope pictures of the resulting morphology, is used to identify four distinct regimes of ripple formation at different incident laser fluences. At low fluences, the structure develops when thin (1 \ensuremath{\mu}M wide) isolated, molten strips resolidify on the solid substrate, while at high fluences, the structure results from freezing of capillary waves which are generated on the surface that the laser pulse has melted uniformly. Models are presented which clearly demonstrate how the incident electromagnetic field interacts with the evolving structures (in different fluence regimes) to provide feedback mechanisms which sustain their growth.

Continuous-wave probe laser investigation of laser vaporization of small soot particles in a flame.

Abstract: Pulsed laser vaporization of small soot particles (<100-nm radius) in a flame is observed with a cw probe laser. Both light scattering and absorption along the pulsed laser beam are measured. Above a threshold of 0.2 J/cm2, both quantities decay exponentially with increased fluence for submicrosecond pulses. These two measurements demonstrate that the number of particles remains constant, but the mean size decreases: that is, small particles vaporize rather than fragment or photophorese. The vaporization process is modeled including transport across the first gas mean free path (Langmuir layer). Numerical integration of the time-dependent conservation equations demonstrates that a simple analytic treatment is adequate. The threshold and fluence dependence are predicted to within experimental uncertainty, assuming soot has the thermal properties of graphite. Laser vaporization of soot has possible application to laser beam profiling, gas velocity measurements, flow visualization, and point measurement of soot absorption.

Emission spectra and etching of polymers and graphite irradiated by excimer lasers

Abstract: Emission spectra in the visible and ultraviolet range, scanning electron microscopy of various polymers [polyimide, Mylar, and polymethyl‐methacrylate (PMMA)], and graphite exposed to 193‐, 248‐, and 351‐nm laser radiation were used to investigate the laser etching process. Measurements were performed under vacuum and in air and He environments. At low laser fluence, which depends on the sample and laser wavelength, an unresolved continuum emission was observed. At higher fluences clear C2, C, and in some cases also CN emissions were found in addition to the continuum. At still higher fluences ionic carbon emissions became dominant. It was found that laser‐produced plasma processes are responsible for most of the observed emissions. Rough and smooth etched surfaces were obtained at different laser fluences depending on the linear absorption coefficient of the sample at the laser wavelength. In polyimide, a rough‐to‐smooth surface transition versus the laser fluence was found to occur exactly when the C2 and C emissions appeared on top of the continuum. It is suggested that the laser etching mechanism is mostly a statistical thermodynamic process, but without complete energy randomization.

Laser produced plasma in crystalline α‐Al2O3 and aluminum metal

Abstract: A comparative study of the laser produced plasma (LPP) in vacuum at the surface of transparent crystalline α‐Al2O3 (sapphire) and Al metal generated by 248‐nm excimer laser pulses of 15‐ns duration is reported. It was found that the threshold fluence for strong LPP emission in sapphire is 0.55 J/cm2, which is significantly lower than that needed for the same process in clean Al (1.7 J/cm2). At fluences higher than 4 J/cm2 the LPP emission from both targets is similar. The results obtained in sapphire require that its absorption coefficient increase by many orders of magnitude during the laser pulse. A possible explanation for the observed threshold difference is that the greater thermal diffusivity of Al more than compensates for its greater volatility; hence, a greater fluence is necessary to create the LPP in Al.

Single- and multiple-shot laser-damage properties of commercial grade PMMA

Abstract: In this paper, we describe a facility to study laser damage in plastics and present single- and multiple-shot bulk damage results obtained in commercial polymethylmethacrylate with 1.06-μm wavelength, 8-nsec FWHM, 45-μm spot diameter pulses. The measured single-shot peak fluence damage threshold is 41 J/cm2. Multiple-shot damage was studied at fluence levels as low as 6% of the single-shot damage threshold. Scattered collinear He–Ne laser light was found to be a very reliable indicator of damage, especially at the lower fluence levels. Histograms of damage statistics and 100× microscopy of the material after illumination with fewer than the critical numbers of pulses required for damage illustrate the cumulative nature of the damage process.

Depth distributions of sulfur implanted into silicon as a function of ion energy, ion fluence, and anneal temperature

Abstract: Depth distributions for sulfur (S) implanted in (100) and (111) silicon (Si) measured by secondary ion mass spectrometry are reported for S ion energies from 40 to 600 keV and ion fluences from 4×1012 to 4×1016 cm−2. The results of Pearson IV fits are tabulated for the energy and fluence series; the range parameters given are peak depth (Rm) and μ(Rp), σ(ΔRp), γ1, β2, and β′2 for both the peak of the distribution and the full curve (including the influence of the channeling tail). The S densities are measured from a maximum of 1021 down to <1015 cm−3 where the detection limit is probably caused by background oxygen (mass 32). Sulfur profiles are shown for annealing temperatures from 100 to 950 °C in (100) and (111) Si orientations. Sulfur is retained in the region of damage for densities above about 1018 cm−3, until an annealing temperature of 900 or 950 °C is reached, when the S is largely rejected from the Si crystal; the remaining S density is <1015 cm−3, in agreement with solid solubility.

C 1s binding-energy shift of pyrolytic graphite bombarded by keV-deuterium ions.

Abstract: As a basic study of plasma-wall interactions, chemical state of graphite basal face after deuterium bombardment are studied by means of X-ray photoelectron spectroscopy. As the fluence of deuterium ions with energy of 1 or 4 keV is increased from 1014 to 1018 ions/cm2, the C is line is observed to shift towards the lower binding energy at first, down to the minimum by about 0.2 eV and then towards the higher energy up to an asymptotic value of about 0.2 eV higher than the initial position. The first negative shift is due to lattice dis-placement at the target surface, and the subsequent positive shift is due to the deuterium trapping at the graphite surface. The fluence dependence of the observed C is shift is ex-plained by a simple saturation model.

Ion beam processes in Si

Abstract: Observation of the effects of implants of energetic ions at high dose rates into Si have produced some exciting and interesting results. The mechanism whereby displacement damage produced by ions self-anneals during high dose rate implantation is discussed. It is shown that ion beam annealing (IBA) offers in certain situations unique possibilities for damage annealing. Annealing results of the near surface in Si with a buried oxide layer, formed by high dose implantation, are presented in order to illustrate the advantages offered by IBA. It is also shown that ion irradiation can stimulate the epitaxial recrystallization of amorphous overlayers in Si. The nonequilibrium alloying which results from such epitaxial processes is discussed as well as mechanisms which limit the solid solubility during irradiation. Finally, a dose rate dependency for the production of stable damage by ion irradiation at a constant fluence has been observed. For low fluence implants, the amount of damage is substantially greater in the case of high flux rather than low flux implantation.

Optimization of parameters for dynamic recoil mixing of gold films deposited on silicon

Abstract: In a technique known as dynamic recoil mixing, a film of constant mass of a particular material can be maintained on a substrate while it is being bombarded by energetic ions. The technique has been applied to study recoil implantation of gold into silicon. Rutherford back-scattering and chemical etch methods have been used to determine the transmitted flux of gold as a function of 10 keV Ar+ ion fluence for a 10 nm gold film. A near-linear increase in recoil-implanted gold is found but, above a fluence of 2·5 × 1016 ion cm−2 the chemical etch method gives low values. This appears to be related to a fluence-dependent etch rate which could have important application in ion-beam lithography. Further experiments are reported on the recoil sputtering ratio of gold in silicon as a function of film thickness. A recoil ratio of more than one gold atom per incident ion is measured. The results are compared with a MARLOWE-based computer simulation of transmission sputtering, and good agreement is found.

Atomic mixing of silver into a silicon substrate using a 45 keV beam of Ar+ ions

Abstract: In separate experiments, Si substrates were coated with Ag layers of thickness ranging from 219 A to 371 A. These were then irradiated with 45 keV Ar+ ions, producing Ag-Si mixing, together with surface erosion by sputtering. The successive internal Ag profiles were measured by RBS techniques, and are presented graphically up to a maximum fluence of 1.85 × 1016 ions cm−2. A corresponding set of profiles has been computed using a diffusion formalism for atomic mixing developed earlier by the authors. A critical comparison between the theoretical and experimental results is given.

Magnetic properties of iron‐implanted graphite

Abstract: We have measured the magnetic properties of highly oriented pyrolytic graphite (HOPG) implanted with fluences of 25‐keV iron atoms ranging from 1016 to 1017 atoms/cm2. The lowest fluence specimen was paramagnetic down to 2 K, with evidence for clusters of only a few spins, while the highest fluence specimen was clearly ferromagnetic, with magnetization curves resembling those of a set of randomly oriented soft magnetic planes. The critical fluence for formation of a ferromagnetic state appears to be between 1 and 3×1016 atoms/cm2 at 25 keV. These results can be qualitatively understood based on the critical density for percolation of near neighbor exchange interactions.

One- and two-frequency multiple-photon dissociation (MPD) of SF6 under molecular-beam conditions: Effect of fluence and temperature

Abstract: One- and two-frequency IR multiple-photon dissociation (MPD) of SF6 has been performed under molecular-beam conditions at different rotovibrational temperatures. The dependence of MPD on CO2 laser fluence has been studied at different frequencies. The experimental structures have been interpreted on the basis of accurate calculations of the vibrorotational structure ofν 3 overtone states.

Microstructure and properties of TiB2 implanted with 1 MeV nickel

Abstract: Polycrystalline samples of TiB2 were implanted at ambient temperatures with 1 MeV Ni+ ions of a fluence of 1 × 1017 ions cm 2 . This fluence corresponds to a calculated nickel-to-titanium ratio, within the half width of the ion distribution, of approximately 0.12. Analytical electron microscopy was used to study the resultant microstructure. The starting microstructure was modified to a depth of approximately 750 nm, significantly deeper than the calculated peak in the deposited nickel profile of 389 nm. The results also show a change in the character of the microstructure, from one exhibiting a moderate density of tangled dislocations to a high density of small defects, as well as a change in the concentration of nickel as a function of depth from the implanted surface. There was no evidence of nickel precipitation. Surface mechanical properties such as hardness and wear resistance were significantly increased.

Pulsed CO2 laser induced chemistry of vanadium oxytrichloride (VOCl3)

Abstract: Pulsed CO2 laser multiple photon excitation of VOCl3, under collisional and collision‐free conditions, resulted in dissociation via its ground electronic state and in a visible fluorescence. The photodissociative pathways leading to product formation were found to be both pressure and laser fluence dependent. Low pressure and laser fluence led to the formation of VOCl2 while high pressure and laser fluence flavored VO. The prompt fluorescence was also studied as a function of pressure and laser fluence, and was shown to arise from a spontaneous one‐photon radiative decay from an electronic state belonging to the VOCl2 fragment. The reaction mechanisms leading to product formation as well as models for populating a fragment’s electronic state are proposed and discussed in conjunction with present multiphoton excitation and dissociation theory. In addition, CO2 laser induced dielectric breakdown of VOCl3, in the neat and with scavenger gases, resulted in the production of various vanadium oxides. The products are finely divided particles of very large surface area and high purity.

A measurement of absorbed dose to water per unit incident 7 MeV photon fluence

Abstract: After isolating the effects due to 7 MeV photons, measurements were made of the ratio of the 60Co exposure equivalent charge from a Baldwin-Farmer ion chamber in a water phantom, divided by the on-axis fluence of incident 7 MeV photons. This quantity is the ratio between the conversion factor from fluence to absorbed dose to water and the chamber calibration factor specified by various dosimetry protocols. The measured value is a test of the Monte Carlo codes and dosimetric theory used to derive these two factors. A value of 4.13*10-13 C kg-1 cm2+or-1.6% was measured. In order to assess the ion chamber charge due to scattered 7 MeV photons, a subsidiary experiment measured the dose due to scattered photons originating in a series of iron plates. Agreement was obtained between the measured dose as a function of iron plate thickness and that calculated using the EGS Monte Carlo simulation systems and also between the measured and calculated absorbed dose-depth curves. The results may be viewed as an experimental measurement, with 2-3% uncertainty, of the absorbed dose to water per unit incident 7 MeV photon fluence, or of the ion chamber calibration factor specified by dosimetry protocols.

Radiation damage and annealing of amorphous silicon solar cells

Abstract: Amorphous silicon solar cells were irradiated with 1 MeV electrons at the Space Environmental Effects Laboratory of the NASA Langley Research Center. The cells accumulated a total fluence of 10 to the 14th, 10 to the 15th, and 10 to the 16th electrons per square centimeter and exhibited increasing degradation with each irradiation. This degradation was tracked by evaluating the I-V curves for AM0 illumination and the relative spectral response. The observed radiation damage was reversed following an anneal of the cells under vacuum at 200 C for 2 hours.

Size of Crystalline Particle in Si-Implanted GaAs Estimated by Laser Raman Spectroscopy

Abstract: The dependence of the peak intensity, full-width at half maximum (FWHM), and peak frequency of the LO phonon in Si implanted GaAs on the dose of Si and on the depth were studied by laser Raman spectroscopy. Raman spectra from almost crystalline particles were observed in all the samples that Si was implanted into at a current fluence between 2 and 400 nA cm-2 even if the dose is as large as 1×1016 cm-2. The minimum size of the crystalline particle can be estimated to be about 45~50 A. The size and density of the crystalline particles kept the minimum size and a constant value, respectively, from the surface to a depth of about 0.15 µm in spite of an increase in the intensity of a disorder-activated spectra.

Analysis of LED degradation; proton-bombarded GaAs

Abstract: An analysis is given of the degradation of light-emitting, Zn-diffused GaAs diodes after proton bombardment. Use is made of a generally applicable method by which the external bulk quantum efficiency and the injection efficiency of an LED can be determined separately. Owing to the increase of non-radiative recombination being larger in the bulk than in the space-charge region, the injection efficiency at constant current first starts to increase and then decreases as a function of irradiation fluence. Furthermore, it is shown that the apparent bulk quantum efficiency decreases superlinearly with the irradiation fluence. This is consistent with the theory for a linear-graded pn junction and the assumption that the concentration of additional killer centres is directly proportional to the irradiation fluence.

Light emission from hydrogen-copper interaction at grazing incidence

Abstract: The optical emission of excited H reflected from clean Cu(110) after impingement of H + and H + 2 in the energy range of 250 eV to 20 keV per nucleon at 70° angle of incidence to the surface normal was measured. For incident 10 keV H + 2 , the highest excited hydrogen state detected was the n = 10 level. The H α yield was found to be fluence and energy dependent. This effect is attributed either to fast sputtered hydrogen, surface roughness or to an increase with hydrogen concentration in electron states of p-like symmetry near the Fermi level of copper. The H α yield per reflected nucleon shows approximately an exponential dependence on both projectile energy per nucleon and scattered particle reciprocal velocity perpendicular to the surface.

Structure of ion implanted graphite fibers

Abstract: This paper reports on the effect of ion implantation on the structure of highly ordered benzene-derived graphite fibers (BDGF) using high resolution transmission electron microscopy (TEM). Emphasis is given to the dependence of the in-plane and c-axis crystallite sizes and of the interlayer spacing on ion species and fluence. Our results show a decrease in crystallite size with increasing ion mass and fluence, and an increase in the interlayer spacing with increasing ion mass. Preliminary results on post-ion implantation annealing give activation energies of ~ 0.7 eV/atom for both the in-plane and the c-axis recrystallization processes. Our annealing studies show that the c-axis interplanar spacing is larger than 3.36 A even for annealing temperatures of 2800 °C, which is well above the threshold for three-dimensional ordering.

Picosecond Photon-Solid Interaction Phase Transition in Silicon

Abstract: Four different regimes of photoelectric emission are observed over a vide fluence range of UV-laser pulses irradiating single-crystal silicon samples. The role of the electron-hole plasma in the nonlinear photoemission is demonstrated by temporal correlation measurements. A regime where ion thermal evaporation processes take place is observed above the critical fluence for melting. At higher laser fluences nonlinear ion acceleration is demonstrated by direct time-of-flight measurements.

Equivalent electron fluence for space qualification of shallow junction heteroface GaAs solar cells

Abstract: It is desirable to perform qualification tests prior to deployment of solar cells in space power applications. Such test procedures are complicated by the complex mixture of differing radiation components in space which are difficult to simulate in ground test facilities. Although it has been shown that an equivalent electron fluence ratio cannot be uniquely defined for monoenergetic proton exposure of GaAs shallow junction cells, an equivalent electron fluence test can be defined for common spectral components of protons found in space. Equivalent electron fluence levels for the geosynchronous environment are presented.

Infrared photochemistry with shaped laser pulses

Abstract: A relatively simple design for a hybrid CO2 laser and electrooptic crystal switch is described, which can be used to produce pulses with fast (< 10 ns) rise and fall times with constant power over their duration (50–1000 ns). Gaussian beam profiles allow separate and quantitative determination of both fluence (J cm−2) and intensity (W cm−2) dependences of processes induced by IR multiple photon absorption (MPA). Visible luminescence accompanying MPA of OsO4 is shown to depend on intensity, and the dependences of MPA cross sections in SF6 on fluence and intensity are derived from optoacoustic and long path absorption measurements.

A Study of Electrically Active Defects Induced by Pulsed Electron Beam Annealing In (100) N-Type Virgin Silicon

Abstract: Au/Si Schottky contacts have been used as test structures to investigate defects induced in virgin C.Z (100) N-type silicon after irradiation with a 12 to 20 KeV mean energy electron beam pulse. A thin and highly damaged surface layer was observed from a fluence threshold of 1 J/cm2. In addition electron traps were detected in the PEBA induced melting layer with concentrations in the 1012-1013 cm−3 range. Their depth profiles have been related to the PEBA induced melting layer thickness. Quenching of multidefect complexes is the most probable mechanism for electron trap generation in the processed layer.