D. B. Poker

Dynamics of Oxidation of a Fe2+-Bearing Aluminosilicate (Basaltic) Melt

Rutherford backscattering spectroscopy (RBS) and microscopy demonstrate that the ∼1400°C oxidation of levitated droplets of a natural Fe2+-bearing aluminosilicate (basalt) melt occurs by chemical diffusion of Fe2+ and Ca2+ to the free surface of the droplet; internal oxidation of the melt results from the required counterflux of electron holes. Diffusion of an oxygen species is not required. Oxidation causes the droplets to go subsolidus; magnetite (Fe3O4) forms at the oxidation-solidification front with a morphology suggestive of a Liesegang-band nucleation process.

The mechanism of oxidation of a basaltic glass: Chemical diffusion of network-modifying cations

Rutherford backscattering spectroscopy, in conjunction with optical and scanning electron microscopy, has been used to characterize the oxidation process in a homogeneous, well-annealed glass prepared from a nepheline-normative olivine basalt. Initially melted and annealed at an oxygen fugacity substantially below the quartz-fayalite-magnetite (QFM) buffer, the glasses were oxidized in air under the time and temperature ranges 1–100 h and 550–600°C, respectively. Oxidation causes (1) formation of crystalline CaO and MgO that partially covers the free surface of the glass and (2) an internal reaction zone that is depleted of Ca2+ and Mg2+ but enriched in Na+ The reaction morphology is uniquely consistent with a model in which oxidation occurs by the outward diffusion (to the free surface) of Ca2+ and Mg2+ that is charge compensated by an inward flux of electron holes (polarons): oxidation of the glass occurs as the oxygen/cation ratio increases, not by addition of oxygen, but rather by removal of cations. The flux of Na+ from depth in the glass to the oxidizing region, which is also charge compensated by a counterflux of electron holes, is a response to the thermodynamic driving force seeking to stabilize Fe3+ as a network former, consistent with equilibrium thermodynamic and spectroscopic studies. Growth of the oxidized/transformed glass follows parabolic (chemical-diffusion-limited) kinetics. Using a first-order, Wagnerian approach, the diffusion coefficient and driving force terms of the parabolic reaction-rate constant are separated, giving an average divalent cation diffusion coefficient of DA2+(cm2·s−1) = 9.9 × 10−2exp−210kJ·mol−1RT . The oxidation mechanism seen for the glass, that is, one dominated by diffusion of network modifying cations and not an oxygen species, is anticipated to also occur in iron-bearing aluminosilicate melts: the discrepancy between the kinetics of redox reactions and of oxygen tracer diffusion noted in the literature for melts is most likely explained in this way.

Epitaxial superconducting thin films of YBa2Cu3O7−x on KTaO3 single crystals

We have grown thin YBa2Cu3O7−x films on single‐crystal (001) KTaO3 substrates by means of coevaporation of Y, BaF2, and Cu, followed by an ex situ anneal in wet oxygen. The films are epitaxial in three dimensions, consisting of grains with either the c or the a axis of the YBa2Cu3O7−x unit cell perpendicular to the substrate. Films with a composition close to the 1:2:3 cation ratio exhibit sharp superconducting transitions at 90 K and critical current densities in excess of 0.3 MA/cm2 in zero magnetic field at 77 K. In Cu‐rich films, the Y2Ba4Cu8O16–x phase with a Tc of 80 K is formed in grains with the c axis perpendicular to the substrate.

Growth of highly doped p‐type ZnTe films by pulsed laser ablation in molecular nitrogen

Highly N‐doped (mid‐1019 to ≳1020 cm−3) ZnTe/(001)GaAs epitaxial films have been grown by pulsed laser ablation (PLA) of a stoichiometric ZnTe target in a high‐purity N2 ambient (50 to 200 mTorr) without the use of any assisting dc or ac plasma source. Unlike recent experiments in which atomic N, extracted from dc and rf plasma sources, was used to produce N‐doping during molecular beam epitaxy, spectroscopic measurements performed during PLA of ZnTe in N2 do not reveal the presence of atomic N. This suggests that the high hole concentrations in laser ablated ZnTe are produced by a new mechanism, possibly energetic beam‐induced reactions with excited N2 adsorbed on the film surface, and/or transient formation of Zn–N complexes in the energetic ablation plume.

On the thermodynamical driving force during ion mixing of the Co-Si system

The relative importance between the thermodynamical driving force and kinetics in thermal annealing and ion mixing in the thermally activated regime has not been clarified. To probe the role of the thermodynamical driving force in reactions between metals and silicon, the Co‐Si system was chosen for investigation. In general, three silicide phases are formed during thermal annealing of samples consisting of Co thin films deposited on Si substrates, i.e., Co2Si (the first phase to form with a heat of formation, ΔHf=−9 kcal/g atoms), CoSi (ΔHf=−12 kcal/g atoms), and CoSi2 (the last phase to form, with ΔHf=−8.2 kcal/g atoms). Previous experiments have shown that annealing a sample of Si/CoSi/Co converts CoSi into Co2Si instead of a continuous growth of CoSi. This type of reaction is apparently unrelated to the magnitude of the thermodynamical driving force since ΔHf of CoSi is significantly larger than those of Co2Si and CoSi2, but is kinetically restricted instead. Under ion mixing conditions the kinetic re...

Resolving the electronic and nuclear effects of MeV ions in polymers

Abstract The electronic and nuclear stopping effects produced by MeV ion bombardment in polyvinylidene chloride (PVDC) and polyethylene (PE) were separated by stacking thin films of the polymers. The resulting layered system consisting of each polymer was bombarded with 3.5 and 5.0 MeV alpha particles. A layered system was selected such that the first layers experienced most of the effects of the electronic energy deposited and the last layers received most of the effects of the nuclear stopping. The electrical conductance and the changes in the chemical structure were measured by direct resistivity measurements, Raman microprobe analysis and FTIR. Post-irradiation analyses of the films indicated differences in these properties in the various film layers which are attributed to the individual effects of the stopping powers in the polymer films.

Interface broadening in sputter depth profiling through alternating layers of isotopically purified silicon: I. Experimental results☆

Abstract Samples composed of alternating layers of 30Si and 28Si were sputter depth profiled at near normal incidence using a variety of primary ion beams (Ne+, Ar+, Xe+, N2+, O2+ and Cs+) at energies between 2 and 10 keV. The profiles exhibit the well-known asymmetric interface broadening, i.e. a relatively sharp rise of the signal at the leading edge (described by a characteristic width δ) and a comparatively slow exponential fall-off at the trailing edge (defined by a decay length λ). Both δ and λ increase with increasing beam energy. For inert gas and cesium ion bombardment at a fixed energy, the width δ reveals the qualitatively expected decrease with increasing primary ion mass. By contrast, the decay length λ does not show any dependence on the projectile mass or on the matrix sputtering yield. These findings are discussed in terms of ion ranges, cascade mixing, and possible shapes of the stationary internal mixing profiles. The interface broadening observed under N2+ and O2+ bombardment is much (up to a factor of more than two) smaller than expected on the basis of the data for inert gas and cesium ion bombardment. This effect is attributed to beam induced nitride and oxide formation which results in a reduced depth of mixing in the Si subsystem. Comparison of the present data with previous results shows that the decay length for several impurities in Si is shorter than for Si in Si.

Morphological instabilities and ion beam mixing in Ge

Abstract In light of the recent discovery [11,12] that heavy ion irradiation initiates a morphological instability in the amorphous phase of Ge which erupts to form surface craters, previous studies of ion beam mixing in the metal/Ge system [2,4] are re-examined. Correlated ion scattering/channeling and cross-section electron microscopy are utilized to study the Au/Ge and Al/Ge systems under a variety of mixing conditions. Implications for both the crater formation and ion beam mixing mechanisms are discussed.

Synthesis of metastable carbon-silicon-nitrogen compounds by ion implantation

The feasibility of carbon-silicon nitride formation (β−Si1.5C1.5N4, the homologue of equilibrium β−Si3N4 or hypothetical β−C3N4) has been investigated by high dose N+ implantation into polycrystalline β−SiC (cubic phase). Thin films were formed using 100 keV implantations with varying ion doses and target temperatures. X-ray diffraction with a position-sensitive detector and cross-sectional transmission electron microscopy revealed that the as-implanted surfaces contained ∼0.1 μm thick buried amorphous layers. Rutherford backscattering spectroscopy showed that the peak concentration of nitrogen saturated up to approximately 54 at.% with increasing doses, suggesting formation of a new phase.

Third Order Optical Nonlinearity of Colloidal Metal Nanoclusters Formed by MeV Ion Implantation

We report the results of characterization of nonlinear refractive index of the composite material produced by MeV Ag ion implantation of LiNbO{sub 3} crystal (z-cut). The material after implantation exhibited a linear optical absorption spectrum with the surface plasmon peak near 430 nm attributed to the colloidal silver nanoclusters. Heat treatment of the material at 500 deg C caused a shift of the absorption peak to 550 nm. The nonlinear refractive index of the sample after heat treatment was measured in the region of the absorption peak with the Z-scan technique using a tunable picosecond laser source (4.5 ps pulse width).The experimental data were compared against the reference sample made of MeV Cu implanted silica with the absorption peak in the same region. The nonlinear index of the Ag implanted LiNbO{sub 3} sample produced at five times less fluence is on average two times greater than that of the reference.