Mehdi Balooch

Clusters formed in laser‐induced ablation of Si, SiC, Pt, UO2 and evaporation of UO2 observed by laser ionization time‐of‐flight mass spectrometry and scanning tunneling microscopy

Cluster formation is traditionally observed by mass spectrometry, which has the disadvantage that the detection sensitivity often decreases with increasing mass. Alternatively, one may collect the clusters onto an atomically flat substrate and identify them by scanning tunneling microscopy (STM). Both techniques were used here. For the first technique, a Nd:YAG laser (frequency quadrupled to 266 nm, 5 ns pulse width) focused onto spots of 4–100 μm diameter was used to ablate refractory materials, and a reflectron time‐of‐flight tube served to mass‐analyze the plumes. The observed mass spectra for Si, Pt, SiC, and UO2 varied in the distribution of ablation products among atoms, molecules, and clusters, depending on laser power density and target material. For the second technique, cleaved surfaces of highly oriented pyrolytic graphite were positioned either 10 cm away from materials ablated at 10−5 Torr by 1–3 excimer laser (308 nm) pulses of 20 ns duration, or 1 m away from materials vaporized at 10−8 Tor...

Surface site specificity on the basal plane of graphite: 1.06 μm laser damage threshold and reactivity with oxygen between 350 and 2300 K

Scanning tunneling microscopy (STM) has been used to document changes in the nanometer‐scale morphology of the basal plane of highly oriented pyrolytic graphite after in situ exposure to 7 ns, 1064 nm Nd:YAG, and to ms Nd:Glass laser pulses producing surface temperatures up to 2300 K, and after ex situ exposure to temperatures as low as 350 K in air. Laser damage produced by the ns pulses was visible by STM at fluences far below those that produce melting and effects visible by other imaging techniques. Damage appears first on step edges and consists of exfoliation of graphite layers and recession of steps through removal of mono‐ or multilayer patches. ms and long‐term heating leads to reaction with oxygen, which proceeds by etching of both crystalline boundaries and step edges. In addition a high concentration of flat‐bottom shallow pits (two monolayers deep) and a smaller number of conical pits (greater than 30 A deep) can clearly be identified. The results give for the reaction at the grain boundaries...

Hydrogen dissolution in and release from nonmetals. II. Crystalline silicon

The solubility of hydrogen in crystalline silicon from 1363–1473 K and from 1.7–9.2 atm has been measured using the method of high‐temperature, high‐pressure infusion followed by high‐temperature vacuum outgassing with mass‐spectrometric detection. The measured solubilities were in the range 1–5×1016 atoms/cm3 and exhibited a temperature dependence consistent with a heat of solution of 30–40 kcal/mole. The pressure dependence of the solubility was consistently smaller than the square‐root dependence characteristic of simple interstitial solution. The release‐rate curves showed multiple peaks, which are incompatible with the classical diffusion model of release. Instead the peaks correspond to trapping of hydrogen at distinct binding sites in the silicon lattice. The release kinetics were modeled as detrapping processes.

Retention and release of water by sintered uranium dioxide

Abstract The release of water and hydrogen upon heating sintered UO 2 pellets was measured by a direct mass spectrometric method of vacuum outgassing. The technique avoids water loss by sample transfer and measures, rather than the cumulative release, the rate of release with a sensitivity of 1 μg of water (as D2O) per hour. Exposure of high-density UO 2 pellets to water (liquid D2O) results in negligible water adsorption. Water in pellets fabricated with especially high open porosity (5%) was driven off by a linear temperature ramp below 200°C. A drying model for this process was developed and applied to the data. Strongly bound water was introduced into high-density UO2by sintering in an atmosphere of D2O and D2. Release of the water or hydrogen began at ~500°C and was complete only at the melting point of UO2(2800°C). The release kinetics are not diffusion-controlled; rather the process is governed by the rates of desorption of bound hydrogen-bearing species from at least three binding sites in the solid characterized by interaction energies between 20 and 50 kcal/mol. The D2O/D2 ratio of the desorbed gas was > 1 and did not correspond to thermodynamic equilibrium with stoichiometric urania. Hydrogen and water release kinetics are comparable below 2000°C, suggesting a common bound precursor. The total hydrogen (as D2O or D2) absorbed in the specimens was between 2 and 4 μg/g UO 2 .

Patterning of C60 films

Lines of C60 films with widths between 3 and 5 μm and heights of 0.4–0.8 μm have been patterned on a Si(100) substrate using photoresist and liftoff techniques. Smooth 50 A root mean square roughness films were detected by atomic force microscopy. Auger depth profiling showed no impurities in the film. Fourier transform infrared spectroscopy confirmed the film was composed of C60. An estimate of the C60 sticking probability onto a growing C60 film is ∼0.2.

SiC film deposited by pulsed excimer laser ablation

Thin films of {Beta} - SiC were grown on Si substrates by excimer laser pulse ablation of bulk SiC. The films were examined by Auger electron, x-ray, and photoelectron spectroscopies. The film was smooth as monitored by scanning electron microscopy. Scanning electron and scanning tunneling microscopy showed inclusions in the deposited SiC film and laser ionization mass analysis detected SiC dimers in the vapor plume emitted from the target. 13 refs., 4 figs.

Time-resolved auger electron spectroscopy combined with modulated molecular beam techniques for gas-surface reaction kinetics studies

An Auger electron spectrometer is combined with a modulated molecular beam system and the time-dependent concentration of adsorbed oxygen on a molybdenum surface subject to a chopped O 2 beam is measured and compared to predictions of a reaction kinetic model derived from modulated molecular beam spectroscopy alone.

Reaction of water vapor and oxygen with liquid uranium

The reactions of water vapor and oxygen with liquid uranium were studied by modulated molecular-beam mass spectrometric methods. Equivalent pressures of the reactant fluxes on the surface ranged from 6×10−6to 2×10−4torr. Temperatures up to 1570 K were investigated. The metal surface was kept clean during reaction by mechanical removal of the oxide formed with a tungsten needle. For the clean liquid uranium surface, a water reaction probability of ∼0.4 was deduced both from measurement of the reaction-product hydrogen signal and by the temperature dependence of the scattered reactant signal. Based solely on the latter measure, a reaction probability of 0.6 was estimated for oxygen. The reaction probabilities on the clean surface were temperatureindependent. They decreased as the coverage of the surface by islands of oxide increased and, for water, appeared to approach a value of 0.08 for a surface completely covered with an oxide estimated to be 500 Å thick. Bombardment of the surface during reaction with argon ions produced an increase in the reactivity on solid uranium but had a negligible effect on the reaction probability for the liquid.

Ion-Enhanced Chemical Reaction of XeF 2 with Silicon by Modulated Molecular Beam Mass Spectrometry

The reaction of XeF/sub 2/ with the Si(100) surface was studied by modulated (10-1000 Hz) molecular beam-mass spectrometry in the temperature range 300-1300K and equivalent XeF/sub 2/ pressure of 5x10/sup -6/ to 10/sup -4/ torr. Simultaneous bombardment of the reacting surface by Ar/sup +/ was used to determine the extent of ion-enhancement of the reaction. In the absence of the ion beam, the main reaction product was SiF/sub 4/, which was formed with a reaction probability of approx. 5x10/sup -2/ at room temperature. In the presence of the ion beam three-products, SiF/sub 4/, SiF/sub 2/ and F/sub 2/ (or F), were detected with formation probabilities of approx. 1x10/sup 1/, 6x10/sup -2/ and 7x10/sup -2/ respectively. Increasing surface temperature reduced theion-enhanced reactivity.

Thermal and ion-assisted reactions of Al/sub 0. 3/Ga/sub 0. 7/As with molecular chlorine

Reaction of Al/sub 0.3/Ga/sub 0.7/As with molecular chlorine was studied with and without simultaneous bombardment by energetic argon ions. The reaction products signals were measured as functions of surface temperature. For the purely thermal reactions, the main products below 600/sup 0/K were AlCl/sub 3/, AsCl/sub 3/ and GaCl/sub 3/. The etching rates were two orders of magnitude lower than those of pure GaAs. With simultaneous ion bombardment, the reaction product signal of AsCl/sub 3/ at room temperature increased by almost a factor of four over the corresponding thermal reaction signal. The comparable enhancement factor for pure GaAs was approx.6.