Near-surface Diffusion Research Articles

Synergistic damage effects of vacuum ultraviolet photons and O2 in SiCOH ultra-low-k dielectric films

Damage incurred during plasma processing, leading to increases in dielectric constant k, is a persistent problem with porous ultra-low-k dielectric films, such as SiCOH. Although most of the proposed mechanisms of plasma-induced damage focus on the role of ion bombardment and radical attack, we show that plasma-generated vacuum ultraviolet (VUV) photons can play a role in creating damage leading to increases in the dielectric constant of this material. Using a vacuum beam apparatus with a calibrated VUV lamp, we show that 147 nm VUV photons impacting SiCOH results in post-exposure adsorption and reaction with water vapour from the atmosphere to form silanol bonds, thereby raising the dielectric constant. Furthermore, the level of damage increases synergistically under simultaneous exposure to VUV photons and O2. The vacuum beam photon fluences are representative of typical plasma processes, as measured in a separate plasma tool. Fourier-transform infrared (FTIR) spectroscopy (ex situ) and mass spectrometry (in situ) imply that O2 reacts with methyl radicals formed from scissioned Si–C bonds to create CO2 and H2O, the latter combining with Si dangling bonds to generate more SiOH groups than with photon exposure alone. In addition, sample near-surface diffusivity, manipulated through ion bombardment and sample heating, can be seen to affect this process. These results demonstrate that VUV photo-generated surface reactions can be potent contributors to ultra-low-k dielectric SiCOH film plasma-induced damage, and suggest that they could play analogous roles in other plasma–surface interactions.

Bidimensional versus tridimensional oxygen vacancy diffusion in SnO2−x under different gas environments

Metal oxides present oxygen defects that induce different chemical and physical properties. Experiments performed in SnO(2-x) sensors show that the dynamics of these vacancies are strongly affected by the presence of different gases in the environment. Experimentally, the electrical resistance of individual metal oxide SnO(2-x) nanowires shows modulation: when the environment is oxygen rich long term drifts (hours) are observed indicating extended vacancy dynamics. Instead, if CO is present, drifts disappear in minutes. Density functional theory indicates that changes in resistance follow the extension of reoxidation. For oxygen-poor environments, oxygen vacancy excorporation and healing are confined to the near-surface layer of SnO(2-x) (bidimensional or near-surface diffusion), and completed in short times. Under oxygen-rich conditions, tridimensional diffusion of oxygen vacancies towards the surface takes place at room temperature. In this case, a push-pull mechanism allows bulk-to-surface diffusion and as a consequence resistance drifts are longer and the vacancy quenching is more extensive.

Research Highlights

Research Highlights

Simulation of near-surface proton-stimulated diffusion of boron in silicon

A quantitative model for near-surface redistribution of doping impurity in silicon in the course of proton-stimulated diffusion is developed for the first time. According to the model, the near-surface peak of the impurity concentration is caused by migration of neutral impurity—self-interstitial pairs to the surface with subsequent decomposition of these pairs and accumulation of the impurity at the silicon surface within a thin layer (referred to as δ-doped layer). The depletion and enhancement regions that are found deeper than the near-surface concentration peak are caused by expulsion of ionized impurity by an electric field from the near-surface region of the field penetration. The field appears due to the charge formed in the natural-oxide film at the silicon surface as a result of irradiation with protons. The diffusion-kinetic equations for the impurity, self-interstitials, vacancies, and impurity—self-interstitial pairs were solved numerically simultaneously with the Poisson equation. It is shown that the results of calculations are in quantitative agreement with experimental data on the proton-stimulated diffusion of boron impurity in the near-surface region of silicon.

Computer Model of Aluminum Agglomeration on Burning Surface of Composite Solid Propellant

The process of agglomeration of aluminum particles on the burning surface of an ammonium perchlorate-based composite propellant is modeled using a computer algorithm. A random pack of particulate ingredients of given size and mass specifications is cast on the computer to simulate the propellant microstructure as reported previously. The aluminum particles are tracked as they emerge at a regressing burning surface, accumulate into filigrees, and get ignited by the near-surface leading-edge oxidizer-binder diffusion flamelets attached to the exposed areas of certain ammonium perchlorate particles. An approximate heat transfer model is incorporated to estimate the ignition delays radially inward and outward from the leading-edge flamelets into the filigrees accumulated over ammonium perchlorate particles and surrounding binder-fine ammonium perchlorate matrix layers. The delay influences the number of parent aluminum particles constituting a filigree, and consequently the size of the agglomerate that the filigree rolls up into. The implementation of the algorithm is validated against experimental results available in the literature, which were specifically obtained to investigate the relationship between the decrease in the agglomerate size and attachment of leading-edge flamelets over the fine ammonium perchlorate particles in the propellant with increase in pressure.

Brush-Sheathed Particles Diffusing at Brush-Coated Surfaces in the Thermally Responsive PNIPAAm System

Phase-contrast microscopy and particle tracking algorithms are used to study the near-surface diffusion of poly(N-isopropylacrylamide) (PNIPAAm) brush functionalized micron-sized silica microspheres after sedimentation from aqueous suspension onto planar substrates coated with a similar polymer brush above and below the lower critical solution temperature (LCST) of PNIPAAm, 32 degrees C. A small negative charge on the wall and the particles (zeta potential = -6 mV) prevents adhesion above and below the LCST. The near-surface translational diffusion coefficient (D(surface)) is compared to the bulk-phase translational diffusion coefficient (D(bulk)), which was measured by dynamic light scattering. We find that D(surface)/D(bulk) is approximately equal to 0.6 at temperatures T < 32 degrees C but rises abruptly to approximately 0.8-0.9 at T > 32 degrees C. Near-surface diffusion is expected to be slower than bulk diffusion owing to hydrodynamic coupling to the wall, implying reduced hydrodynamic coupling at the higher temperatures, perhaps mediated by enhanced electrostatic repulsion above the LCST transition.

In situ XPS study of Pd(1 1 1) oxidation at elevated pressure, Part 2: Palladium oxidation in the 10 −1 mbar range

The oxidation of the Pd(1 1 1) surface was studied by in situ XPS during heating and cooling in 0.4 mbar O 2. The in situ XPS data were complemented by ex situ TPD results. A number of oxygen species and oxidation states of palladium were observed in situ and ex situ. At 430 K, the Pd(1 1 1) surface was covered by a 2D oxide and by a supersaturated O ads layer. The supersaturated O ads layer transforms into the Pd 5O 4 phase upon heating and disappears completely at approximately 470 K. Simultaneously, small clusters of PdO, PdO seeds, are formed. Above 655 K, the bulk PdO phase appears and this phase decomposes completely at 815 K. Decomposition of the bulk oxide is followed by oxygen dissolution in the near-surface region and in the bulk. The oxygen species dissolved in the bulk is more favoured at high temperatures because oxygen cannot accumulate in the near-surface region and diffusion shifts the equilibrium towards the bulk species. The saturation of the bulk “reservoir” with oxygen leads to increasing the uptake of the near-surface region species. Surprisingly, the bulk PdO phase does not form during cooling in 0.4 mbar O 2, but the Pd 5O 4 phase appears below 745 K. This is proposed to be due to a kinetic limitation of PdO formation because at high temperature the rate of PdO seed formation is compatible with the rate of decomposition.

高機能酸化物粉体および多結晶体における酸素拡散：測定法と実測値

The methods to determine oxygen diffusion coefficients in powder, sintered body, and single crystal of metal oxides were summarized and their characteristics were also described. In particular, it was demonstrated that if powder specimens were used, near-surface diffusion coefficient could be obtained in addition to lattice diffusion coefficient. Typical results of diffusion experiments were presented. Finally the summaries of diffusion coefficients in yttrium oxide, nickel oxide, cerium oxide, indium oxide, and mullite were shown.

Atomistic simulations of surface diffusion and segregation in ceramics

We demonstrate two approaches to studying processes of diffusion and segregation on the surface of oxides and ceramics. When simulating surface diffusion the activation barriers are often so high that the timescales required for direct simulation by molecular dynamics are prohibitive. We show that a combination of the Temperature Accelerated Dynamics (TAD) together with Kinetic Monte Carlo (KMC) methods can reach the timescales required without the necessity of guessing (often incorrectly) which are the dominant processes. We illustrate the power of the methods for surface diffusion and growth of rocksalt oxides. We show that, surprisingly, exchange mechanisms and correlated ion motions are important in surface and near-surface diffusion of those ionic systems. Another approach, Exchange Monte Carlo (MCX), is used to study equilibrium segregation in thin ceramic films with application to MgO/MnO. Surface concentrations as a function of temperature and film composition are determined directly from the simulations. For all compositions studied the {001} surface is Mn 2+ rich; the occupancy of sites by Mn 2+ decreases rapidly with depth.

Rare earth diffusion kinetics in garnet: Experimental studies and applications

We determined the diffusion coefficient of Sm in almandine garnet as function of temperature at 1 bar and fO 2 corresponding to that of wüstite-iron buffer, and to a limited extent, that of a few other selected rare earth elements in almandine and pyrope garnets. Both garnets were demonstrated to have metastably survived the diffusion annealing at conditions beyond their stability fields. The experimental diffusion profiles were analyzed by secondary ion mass spectrometry, and in addition, by Rutherford back scattering spectroscopy for two samples. Transmission electron microscopic study of an almandine crystal that was diffusion-annealed did not reveal any near-surface fast diffusion path. Using reasonable approximations and theoretical analysis of vacancy diffusion, the experimental data were used to develop an expression of rare earth element (REE) diffusion coefficient in garnet as a function of temperature, pressure, fO 2, ionic radius, and matrix composition. Calculation of the closure temperature for the Sm-Nd decay system in almandine garnet in a metamorphic terrain shows very good agreement with that constrained independently. Modeling of the REE evolution in melt and residual garnet suggests that for dry melting condition, the REE pattern in the melt should commonly conform closely to that expected for equilibrium melting. However, for much lower solidus temperatures that would prevail in the presence of a H 2O-CO 2 fluid, the concentration of light REE in the melt could be significantly lower than that under equilibrium melting condition. A reported core and rim differences in the REE content of a garnet crystal in a mantle xenolith in kimberlite have been reproduced by assuming that the REE zoning was a consequence of entrapment in a magma derived from an external source for ∼32,000 yr before the eruption.

To the theory of anticorrosion alloying of brasses

The protective effect produced by alloying was interpreted in terms of the gradient energy of the near-surface diffusion zone formed as a result of the anodic dissolution or self-dissolution of ternary phases. The general reasons for a changed tendency of such phases toward selective anodic dissolution and selective corrosion compared to nonalloyed phases were revealed. The method for quantitative estimation of the protective effect produced by small amounts of arsenic added to α-brass was proposed. A number of alloying elements (As, P, Ni, and Au) to α- and β-brasses were found to be similar in the nature of the protective effect.

Lattice diffusion and surface segregation of B during growth of SiGe heterostructures by molecular beam epitaxy: Effect of Ge concentration and biaxial stress

Si 1 − x Ge x ∕ Si 1 − y Ge y ∕ Si ( 100 ) heterostructures grown by molecular beam epitaxy were used in order to study B surface segregation during growth and B lattice diffusion. Ge concentration and stress effects were separated. Analysis of B segregation during growth shows that (i) for layers in epitaxy on (100)Si, B segregation decreases with increasing Ge concentration, i.e., with increased compressive stress; (ii) for unstressed layers, B segregation increases with Ge concentration; (iii) at constant Ge concentration, B segregation increases for layers in tension and decreases for layers in compression. The contrasting behaviors observed as a function of Ge concentration in compressively stressed and unstressed layers can be explained by an increase of the equilibrium segregation driving force induced by Ge additions and an increase of near-surface diffusion in compressively stressed layers. Analysis of lattice diffusion shows that (i) in unstressed layers, B lattice diffusion coefficient decreases with increasing Ge concentration; (ii) at constant Ge concentration, the diffusion coefficient of B decreases with compressive biaxial stress and increases with tensile biaxial stress; (iii) the volume of activation of B diffusion [ΔV=−kT(dlnD∕dP)] is positive for biaxial stress while it is negative in the case of hydrostatic pressure. This confirms that under a biaxial stress the activation volume is reduced to the relaxation volume.

Sb surface segregation during epitaxial growth of SiGe heterostructures: The effects of Ge composition and biaxial stress

Antimony is the most widely used n-type dopant for Si molecular-beam epitaxy (MBE). However, because of surface segregation during growth, the control of doping profiles remains difficult. The case of ${\mathrm{S}\mathrm{i}/\mathrm{S}\mathrm{i}}_{1\ensuremath{-}x}{\mathrm{Ge}}_{x}$ heterostructures is complicated by the existence of stresses, which may affect both the thermodynamics and kinetics of segregation. In this study, we analyze the segregation of Sb resulting from the MBE growth of ${\mathrm{Si}}_{1\ensuremath{-}x}{\mathrm{Ge}}_{x}/\mathrm{Si}(100)$ heterostructures using secondary ion mass spectrometry as a function of (i) growth temperature $(200\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}l~T\ifmmode^\circ\else\textdegree\fi{}l~550\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}),$ (ii) germanium content $(0l~xl~0.2),$ and (iii) stresses (compressively strained and relaxed layers). We show that Sb segregation: (i) increases with temperature, (ii) increases with Ge content in biaxially compressed layers, (iii) decreases with Ge content in relaxed layers. The temperature variation indicates that Sb surface segregation during growth is kinetically controlled. The contrasting behaviors observed as a function of Ge content in stressed and relaxed layers can thus be explained by a decrease of the segregation enthalpy induced by Ge addition and an increase of near-surface diffusion in stressed layers.

Development of Near-Surface Composition Anomaly

Many multi-element alloy specimens have been shown to possess a wide variety of near-surface elemental composition profiles, which are significantly different from the bulk composition. Such composition nonuniformity adversely affects the measurement of basic thermophysical properties in alloys. In this paper is presented a new investigation into the mechanisms by which such depth-dependent near-surface elemental composition develops. Specifically, specimens of a low melting-point metallic alloy, Wood's alloy, as a model system are examined under varying thermal cycling conditions within a chamber of controlled gaseous atmosphere. The near-surface composition and thermal diffusivity are measured as a function of depth. The method of time-resolved spectroscopy of laser-produced plasma plumes emanating from the specimen surface is used. Different surface composition profiles emerge depending on the dynamic range of the thermal cycling forced on a specimen.

CATION/ANION MODIFIED CERIA-ZIRCONIA SOLID SOLUTIONS PROMOTED BY Pt AS CATALYSTS OF METHANE OXIDATION INTO SYNGAS BY WATER IN REVERSIBLE REDOX CYCLES

Ca and/or F-modified fluorite-like Ce-Zr-mixed oxides have been prepared by Pechinis method. The bulk structure of samples was characterized by XRD, EXAFS and FTIRS of the lattice modes. The surface properties were studied by SIMS and FTIRS of adsorbed CO and surface hydroxyls. The specific reactivity of the surface oxygen, its amount, coefficients of bulk and near-surface diffusion, as dependent upon the sample composition and temperature, were estimated using sample reduction by CO in the pulse/flow mode. Insertion of fluorine into the lattice results in decreasing the degree of oxygen polyhedra distortion, thus decreasing the amount of reactive oxygen and diffusion coefficients. Calcium and Pt addition counteracts this effect. At 500oC for Pt-supported Ce-Zr-O samples including those modified by Ca and F, the lattice oxygen is easily removed by methane generating CO and hydrogen with high selectivity. Reoxidation of reduced samples by water or carbon dioxide at the same temperature restores the oxygen capacity producing more hydrogen or carbon monoxide.

Investigation of the penetration of atomic hydrogen from the gas phase into SiO2/GaAs

A comparison experiment has been performed to study the hydrogenation of GaAs samples coated with a thin film of SiO2 (5 nm) and thin-film vanadium samples in a flow of atomic hydrogen produced by a source based on a reflected arc discharge with a hollow cathode and a self-heating electrode. An increase in discharge current I results in an increase in the concentration of atomic hydrogen in the gas phase, NAH, and in a corresponding increase in the atomic hydrogen content (dose), QAH, dissolved in vanadium samples. For GaAs samples, the situation is different. At low discharge currents, an increase in current results in an increase in NAH and in a corresponding increase in QAH. However, beginning from some current I*, further increase in current and in NAH decreases QAH. This suggests that the value of QAH is more dependent on the probability of the penetration of hydrogen atoms into the solid, F, than on NAH. It has been supposed that F decreases substantially as the hydrogen atom energy decreases with increasing discharge current. Another reason for the observed phenomenon may be the formation of a near-surface diffusion barrier of immobile hydrogen molecules occupying interstitials and preventing hydrogen atoms from penetrating the bulk of the sample.

Trench formation around and between self-assembled Ge islands on Si

Investigations on near-surface diffusion during the formation of self-assembled Ge islands on Si(001) are presented. We measure the detailed shape of trenches around islands that are formed due to short range, strain enhanced diffusion. It is found that these trenches have anisotropic shape which we explain in terms of the intrinsic anisotropy of the elastic properties for the Si crystal. At high growth temperatures, long-range depletion of the substrate and trench formation between neighboring islands due to strong in-diffusion of Si into the nominally pure Ge islands is observed. A simple diffusion model which predicts trench depths as a function of island distance fits well to our experimentally observed data. Calculated diffusion lengths from this model are comparable to the average island distance on the surface.

Anneal kinetics of a deposited layer in the presence of thermal desorption and particle diffusion into the substrate

The dependences of the Auger signal and adsorbate desorption flux on the anneal time and rate constants of the near-surface reactions and diffusion into the bulk have been obtained from the theory of heat conductivity. It is shown that the decay of the Auger signal with time t is a linear function of \(\sqrt t\) only under surface-limited diffusion of adatoms into the bulk, exactly what was observed in the annealing of Si/Ta. It is shown also that the initial adatom distribution in the bulk obtained during heating the adlayers of different thickness to the anneal temperature practically does not affect the dependence of the measured signals on time, while becoming manifest in their dependence on the anneal temperature. The relation between the dissolution time of an adlayer during heating and its thickness has been found.

Positron studies of polycrystalline TiC

The mean positron lifetime tau , positron diffusion length L, and the positron and electron work functions ( phi + and phi -) for polycrystalline TiC have been experimentally determined. The results were tau =160(2) ps, L+=138(27) nm and phi -=3.96(0.08) eV; phi + was shown to be almost certainly positive. These results strongly support the suggestion from recent first-principles electronic structure and positron state calculations that positions are trapped by and annihilate in metal vacancies in this material. XPS measurements indicate that the trapping sites may be predominantly in thin carbon-rich layers between grains, a picture which may also explain the long near-surface diffusion length.

Room-temperature oxygen diffusion in a-axis normal YBa 2Cu 3O x thin films

Near-surface oxygen chemical diffusion at room temperature in YBa 2Cu 3O x thin films with a-axis orientation normal to the substrate has been studied using four-probe DC conductivity measurements. The results show that under the same experimental conditions the oxygen out-diffusion coefficient is about two orders of magnitude higher in these films compared to those with the c-axis normal to the substrate. We also present data on oxygen out-diffusion enhancement in these films under ultraviolet (UV) light illumination.