Low Atomic Density Research Articles

Nitric acid oxidation of silicon at ∼120°C to form 3.5-nm SiO2∕Si structure with good electrical characteristics

3.5-nm-thick SiO2 layers can be formed at 120 °C by immersion of Si in 40 wt % nitric acid (HNO3) followed by immersion in an azeotropic mixture (i.e., 68 wt % HNO3). The former immersion produces a 1.1-nm SiO2 layer with a low atomic density of 2.19×1022∕cm2, where the layer acts as a catalyst for the decomposition of HNO3. The latter immersion results in a 3.5-nm SiO2 layer with a higher atomic density of 2.22×1022∕cm2. When the postmetalization annealing treatment at 250 °C in hydrogen is performed on the ⟨Al∕3.5-nmSiO2∕Si(100)⟩ metal-oxide semiconductor diodes, interface states are passivated and a low leakage current density (e.g., 8×10−4A∕cm2 at the forward gate bias of 1.5 V) is achieved.

Processes in condensed inert gases involving excess electrons

Drift of an excess electron in dense and condensed inert gases in external electric field and excitation of atoms by electron impact in these systems are analyzed. The effective potential energy surface for an excess electron at a given electric field strength consists of wells and hills, and the actions of neighboring atoms are therefore separated by saddles of the potential energy. At such atomic densities that the difference of interaction potentials for an excess electron between neighboring wells and hills of the potential energy surface becomes small, the electron mobility is large. This is realized for heavy inert gases (Ar, Kr, Xe) with a negative scattering length of an electron on individual atoms. In these cases, the average potential energy of the electron interaction with atoms corresponds to attraction at low atomic densities and to repulsion at high densities. The transition from attraction to repulsion at moderate atomic densities leads to a maximum of the electron mobility. A gas model for electron drift in condensed inert gases is constructed on the basis of this character of interaction. Due to high electron mobility, condensed inert gases provide high efficiency of transformation of the electric field energy into the energy of emitting photons through drifting electrons. It is shown that, although the role of formation of autodetaching states in the course of electron drift is more important for condensed inert gases than for rare gases, this effect acts weakly on exciton production at optimal atomic densities. The parameters of a self-maintained electric discharge in condensed inert gases as a source of ultraviolet radiation are discussed from the standpoint of electron drift processes.

Transport and disruption of Bose-Einstein condensates in optical lattices

We present theoretical and experimental studies of the dynamics of Bose-Einstein condensates in a one-dimensional optical lattice and a three-dimensional harmonic trap. For low atom densities and inertial forces, the condensate performs regular Bloch oscillations, and its center-of-mass motion closely follows semiclassical single-particle trajectories, shaped by the lowest-energy band. But in other regimes, the center-of-mass motion disrupts the internal structure of the condensate by generating solitons and vortex rings, which can trigger explosive expansion of the atom cloud. We use images of the atom cloud to provide experimental evidence for this internal disruption, and find that the process occurs most readily in high-density condensates undergoing slow Bloch oscillations.

Focusing of atoms with spatially localized light pulses

We theoretically study the focusing of atoms using strongly localized light pulses. It is shown that when inhomogenously polarized light is focused at high angular apertures, one may obtain useful potentials for atom focusing. Here we analyze the case of pulsed light potentials for red- and blue-detuned focusings of atoms. In particular, we show that the atomic beam aperture must be stopped considerably down in order to reduce the sidelobes of the atomic density, which is similar to the situation often encountered in conventional optics. It is suggested that an annular aperture in front of the atomic beam could be useful for increasing the resolution, at the cost of a lower atomic density.

Transport of uranium, thorium, and lead in metamict zircon under low-temperature hydrothermal conditions

Understanding the stability of radiation-damaged zircon under low-temperature hydrothermal conditions is crucial to the application of zircon for U–Pb geochronology and as a host phase for the disposal of plutonium waste. We report the results of an investigation of the stability of partially metamict zircon by leaching experiments at 175 °C with a 2 M AlCl 3 and a 1 M HCl–CaCl 2 solution as hydrothermal fluids for 1340 h. Cathodoluminescence (CL) and backscattered electron (BSE) images show that the zircon grains have developed a reaction rim several micrometers thick or deeply penetrating reticulated alteration zones with sharp boundaries to unaltered metamict zircon. These zones have experienced severe loss of Si, U, Th, and Pb, and gain of Al or Ca, and a water species as revealed by electron microprobe, sensitive high-resolution ion microprobe (SHRIMP) analyses, and infrared spectroscopy. Micro-Raman and infrared measurements on the altered areas show that disordered crystalline remnants of the partially metamict zircon structure were partially recovered, whereas recrystallization of the embedding amorphous phase was not observed. No detectable structural or chemical changes were detected inside the unaltered areas. Intensive fracturing, which was most intense in the HCl–CaCl 2 experiment, occurred inside the altered areas due to the volume reduction associated with the recovery of the disordered crystalline material and probably with the leaching reactions. We explain the formation of deep penetrating alteration zones by a percolation-type diffusion model, which is based on the assumption that percolating interfaces or areas of low atomic density between crystalline and amorphous regions as well as between amorphous domains exist along which fast chemical transport is possible. The idea of the existence of fast diffusion interfaces is supported by the sharp chemical gradients at the margin to unreacted zircon. The model was used to estimate for the first time diffusion coefficients for U, Th, and Pb diffusion in amorphous zircon at 175 °C by assuming that volume diffusion inside the amorphous domains is the loss rate-limiting process. These estimates show that volume diffusion in metamict zircon can cause significant loss of U, Th, and especially loss of radiogenic Pb over geological time scales, even at temperatures as low as 175 °C. Our results show that recent Pb loss discordias can be generated (1) by predominate Pb loss from metamict zircon through volume diffusion at low temperatures where thermal healing of the structure is insignificant, and (2) by leaching of Pb (and U and Th) from metamict zircon through an external fluid.

Polarized x-ray absorption spectra ofCuGeO3at the Cu and GeKedges

Polarized x-ray absorption near edge structure (XANES) spectra at both the Cu and Ge K edges of ${\mathrm{CuGeO}}_{3}$ are measured and calculated relying on the real-space multiple-scattering formalism within a one-electron approach. The polarization components are resolved not only in the unit cell coordinate system $(\ensuremath{\varepsilon}\ensuremath{\Vert}a,$ $\ensuremath{\varepsilon}\ensuremath{\Vert}b,$ $\ensuremath{\varepsilon}\ensuremath{\Vert}c)$ but also in a local frame attached to the nearest neighborhood of the photoabsorbing Cu atom. In that way, features which resist a particular theoretical description can be identified. We have found that it is the out-of-${\mathrm{CuO}}_{4}$-plane ${p}_{{z}^{\ensuremath{'}}}$ component which defies the one-electron calculation based on the muffin-tin potential. For the Ge K edge XANES, the agreement between the theory and the experiment appears to be better for those polarization components which probe more compact local surroundings than for those which probe regions with lower atomic density.

Extreme ultraviolet emission from Xe clusters excited by high-intensity lasers

The property of extreme ultraviolet (EUV) generation from Xe clusters irradiated with intense lasers was studied. The Xe cluster jet was well characterized by the interferometric method. In order to obtain the adequate irradiation condition for strong EUV generation, EUV spectra were taken with various laser systems. Then, the wavelength, the pulse width, and the pump energy were widely varied. Through this survey, even with the comparatively low-density Xe jet of ⩽5×1018 cm−3 average atomic density, the highest conversion efficiency of over 10% from laser energy to EUV (5–18 nm) was obtained with a subpicosecond KrF laser pulse, where a 4π source was assumed. This EUV source is considered to be attractive as an EUV lithography light source because of its low average atomic density and small Xe cluster.

Depth of origin of atoms sputtered from crystalline targets

Recently, V.I. Shulga and W. Eckstein (Nucl. Instr. and Meth. B 145 (1998) 492) investigated the depth of origin of atoms sputtered from random elemental targets using the Monte Carlo code TRIM.SP and the lattice code OKSANA. They found that the mean depth of origin is proportional to N −0.86, where N is the atomic density; and that the most probable escape depth is ∼ λ 0/2, where λ 0 is the mean atomic distance. Since earlier molecular dynamics simulations with small crystalline elemental targets typically produced a most probable escape depth of zero (i.e., most sputtered atoms came from the topmost layer of the target), we have carried out new molecular dynamics simulations of sputtered atom escape depths with much larger crystalline targets. Our new results, which include the bcc targets Cs, Rb and W, as well as the fcc targets Cu and Au predict that the majority of sputtered atoms come from the first atomic layer for the bcc(1 0 0), bcc(1 1 1), fcc(1 0 0) and fcc(1 1 1) targets studied. For the high-atomic density targets Cu, Au and W, the mean depth of origin of sputtered atoms typically is less than 0.25 λ 0. For the low-atomic density targets Cs and Rb, the mean depth of origin of sputtered atoms is considerably larger, and depends strongly on the crystal orientation. We show that the discrepancy between the single-crystal and amorphous target depth of origin values can be resolved by applying a simple correction to the single-crystal results.

Morphology and composition of layered anodic films on InP

The composition and morphology of the double-layered anodic film formed at 5 mA cm−2 on InP in aqueous sodium tungstate electrolyte is examined by transmission electron microscopy (TEM), atomic force microscopy (AFM), Rutherford backscattering spectroscopy and nuclear reaction analysis. The outer layer is composed mainly of In2O3 of relatively low atomic density, about 50% that of crystalline In2O3. The low density is probably due partly to the presence of numerous cavities in the layer, revealed by TEM and AFM. The inner layer is enriched in phosphorus relative to the composition of the substrate, with a P : In atomic ratio of about 2.17. From the measured ratio of phosphorus to indium, the composition of the inner layer can be expressed as either In2O3.2.17P2O5 or In(PO3)2.17. The average nm V−1 ratio is 1.99 ± 0.07. However, there are wide variations of the local film thickness, associated with roughness at the substrate/film and film/electrolyte interfaces and local variability in the ionic current density due to cavities in the film. The film forms with a small loss of indium species to the electrolyte. The outer layer represents about 32% of the total thickness of a film formed at 100% efficiency.

Mechanochemical Synthesis and Structure of New Phases in the Pb-V-O System

The products of mechanochemical synthesis in the Pb–V–O system were characterized by x-ray diffraction and magnetic resonance spectroscopy techniques. The mechanical processing was found to yield only new crystalline compounds, Pb2.67V1.33O5.96and Pb3.5V4.5O14.75 . Structural analysis of these phases revealed a low atomic density, mixed-valent states of the constituent cations, the presence of three of four types of vanadium polyhedra, and splitting of crystallographic sites, in line with the views on the mechanisms of mechanochemical reactions developed in the model for the reaction zone.

Properties of the Molecular Clouds in NGC 205

The nearby dwarf elliptical galaxy NGC 205 offers a unique opportunity for high-resolution studies of the interstellar medium in an elliptical galaxy. The present paper investigates the distribution of molecular gas, molecular line ratios, and the relationships between atomic gas, molecular gas, and dust in this galaxy. The line ratios 12CO (2–1)/(1–0) and 12CO (1–0)/13CO (1–0) in one of the molecular clouds in NGC 205 are consistent with the ratios found in other elliptical galaxies and in Galactic giant molecular clouds; they suggest that the CO in this cloud is probably subthermally excited. Atomic gas, molecular gas, and dust are very closely associated on scales of ∼100 pc; the atomic gas can be understood as a photodissociated envelope around the molecular clouds. The atomic column densities in this galaxy are quite low (≈1020 cm-2) because the interstellar UV field is relatively low. The total gas-to-dust column density ratios are consistent with Galactic gas-to-dust ratios. In short, the molecular gas in NGC 205 seems to have properties very similar to the familiar molecular clouds in our own Galaxy except for the low atomic column densities.

Two-photon spectroscopy of atomic hydrogen in high-temperature environments

Two-photon polarisation spectroscopy and two-photon laser-induced fluorescence have been used as sensitive techniques for the measurement of high and low atomic hydrogen number densities, respectively, in high-temperature environments far away from local thermodynamic equilibrium. In addition, Stark-broadened spectral line profiles of the 1S-2S transition of atomic hydrogen were measured with Doppler-free two-photon polarisation spectroscopy in order to test Stark broadening theory at low electron densities. The paper expounds briefly some of the basic properties of these techniques and presents experimental results.

Sub-Doppler spectroscopy in a thin film of resonant vapor

We report on the spectroscopy in a resonant vapor film, in a regime in which the inhomogeneous transit time induces a velocity selection. For cell thickness (10--1000 \ensuremath{\mu}m) smaller than the transverse dimensions and for low enough atomic density, the atomic trajectories are essentially free flights from wall to wall. Hence, a single laser beam irradiation is sufficient to observe sub-Doppler resonances because of the transient atomic response, which enhances the response of the slowest atoms flying nearly parallel to the wall. The peculiar features of this velocity selection are presented in the linear regime as well as in the case of saturation induced by the optical pumping. Also, we report, in the case of the Cs ${D}_{2}$ line (852 nm), on the passage from a two-level linear-response regime to an optical pumping regime when increasing the incident intensity. We also describe the influence of several other parameters, including cell thickness and laser beam size and shape, on the experimental line shapes. An extension to Doppler-free polarization spectroscopy in a thin cell with a single beam, and to saturated absorption, is also reported.

Vibrational relaxation of trapped molecules

Vibrational relaxation of trapped molecules due to collisions with cold atoms is investigated using the results of quantum-mechanical scattering calculations. Trap loss is analyzed using an exactly solvable kinetic model that includes direct collisional quenching and an indirect process of vibrational predissociation. At low atom density, the relaxation is due primarily to collisional quenching. At high atom density, the relaxation involves additional time scales due to the formation and decay of van der Waals complexes. It is shown that the most weakly bound state of the van der Waals complex for a given diatomic vibrational level controls the relaxation at all atom densities. Possible experiments using trapped molecules are discussed.

Optical properties of diamond-like carbon synthesized by plasma immersion ion processing

Hard and transparent diamond-like carbon (DLC) films have been prepared on low temperature substrates by using the C2H2-Ar plasma immersion ion processing (PIIP) method. The optical properties of the DLC films deposited on poly(methylmethacrylate) (PMMA), silicon wafers, and glass plates were systematically examined. It was found that careful control of substrate bias was needed for favorable growth of DLC films with low atomic hydrogen contents, high hardness and mass densities, and excellent optical properties. The resultant DLC films exhibited a high hardness of above 22 GPa, an optical gap energy larger than 1.9 eV, a low refractive index of 1.87, and very high optical transmittance in the visible light range. The study confirmed that C2H2-Ar PIIP with low negative bias voltages and suitable C2H2/Ar gas ratios can process optically transparent and hard protective DLC films on PMMA plastics.

Odd oxygen measurements during the Noctilucent Cloud 93 rocket campaign

The NLC‐93 campaign at Esrange, Sweden, addressed the chemical, dynamical and electrical environment of the summer mesopause in the presence of noctilucent clouds (NLC). A major topic was the detailed investigation of odd oxygen abundances and their structure in the vicinity of the cloud layer. Applying independent in situ measurements, we have obtained consistent profiles of atomic oxygen, ozone, and mesospheric total density. Atomic oxygen was measured on two rocket flights by means of a newly developed probe utilizing the 1304 Å resonance fluorescence / absorption technique. Detailed Monte Carlo simulations of the radiative transfer and the instrument aerodynamics are included in the data analysis. We find relatively low atomic oxygen densities with a peak of 2 × 1011 cm−3 at 96 km. An airglow radiometer recorded the O2 IR‐atmospheric band emission, allowing the retrieval of ozone densities by means of an inversion model that considers both direct and indirect sources of O2 (a1Δg). The odd oxygen measurements are compared to a steady state photochemical model. While good agreement is found in the mesosphere, the results deviate close to the cold mesopause. Significant vertical structures are found in the atomic oxygen profiles; sharp gradients and distinct minima coincide with the observed NLC layer. Possible influences of heterogeneous chemistry on the odd oxygen abundance are discussed.

Oxide and interface characteristics of oxidized silicon oxynitride ceramics – an investigation by electron microscopy

Hot-isostatically pressed silicon oxynitride (Si2N2O) ceramics free from sintering aids were oxidized in 1 atm dry oxygen at 1100 and 1300°C. The structural and chemical characteristics of the oxide and the nature of the oxide–Si2N2O interface were determined using cross-sectional transmission electron microscopy in conjunction with small-probe energy dispersive X-ray analysis and selected-area electron diffraction. Oxidation of Si2N2O resulted in the formation of amorphous SiO2. The oxide–Si2N2O interface was chemically abrupt. The interface was very flat when parallel to low-index, high atomic density Si2N2O crystal planes but became notably undulated if oriented to high index, low atomic density planes. About 6 vol% residual SiO2 phase was present in the bulk of the Si2N2O ceramics. Current results have provided an important baseline for the understanding of the oxidation behaviour of Si2N2O. © 1998 Kluwer Academic Publishers

Differences in microstructural characteristics of dense HA and HA coating.

Two implant types of hydroxyapatite (HA) currently are available for dental implants: dense HA-cemented titanium (Ti) and HA-coated. It has been shown in previous reports that there are differences in the chemical and mechanical stabilities between the dense HA and HA coated. The differences are thought to be due to structural differences between the two ceramic types. The aim of this study was to investigate the differences in microstructural characteristics of currently available dense HA and HA coated implants before implantation and at periods of 3 weeks and 10 months after implantation in canine bone. X-ray diffractometry, infrared analysis, transmission electron microscopy, and energy dispersive X-ray analysis were used. The dense HA is composed of crystal grains, with a well crystallized structure of HA, closely bound to each other and approximately 0.4-0.6 micron in size. Implantation did not change the original sintered structure of the dense HA. The HA coating was composed of an amorphous phase with a Ca/P ratio of 1.46 and a crystal phase consisting of oxyhydroxyapatite, tricalcium phosphate, tetracalcium phosphate, and CaO, with a Ca/P ratio of 1.57. In the amorphous phase, compared to other portions in the amorphous phase, there were some layers with lower atomic density and with no significant difference in Ca/P ratio. After implantation, the crystallization of super fine crystals of approximately 4-5 nm in thickness occurred in the amorphous phase, and with time it progressed and spread from the surface to the deeper portion of the HA coating. A Ca/P ratio of 1.58 in the crystallized portion was close to the ratio (1.60) in the dense HA, suggesting that the super fine crystals were HA. This crystallization cannot significantly decrease the solubility of the amorphous phase portion and poses risks of stress accumulation within the coating and a decrease of binding strength between the HA coating and the substrate.

Propagation of solitary compression waves in heterogeneous materials under high-speed loading

Based on the method of molecular dynamics, we investigated the special features of the passage of soliton-like waves, initiated by high-speed loading, through regions with a low atomic density. It is shown that the shape of these waves is mainly determined by the structure of a material region in which they propagate. The decrease in the amplitude of solitary waves is shown to be determined by the defect concentration. This effect is of interest from the viewpoint of developing the methods of monitoring the microdamage build-up in materials.

HREM analysis of a cubic-hexagonal (ZrBZrB 2) interface using the coincidence approach

The structure of a cubic-hexagonal ZrBZrB 2 interface has been investigated using high resolution electron microscopy and image processing. The interface is in an unusual and complex orientation relationship with an exact parallelism of the (011̄) ZrB, (0001) ZrB 2 planes and a misorientation of 3° between the (111) ZrB and (11̄00) ZrB 2 planes. Apparently flat at a low magnification, the mean plane of the interface is lying parallel to a high index plane for the two crystals. A structural analysis at an atomic scale of the interface reveals that the interface is in fact built up over two types of facets; one is characterised by a low atomic density and a small misfit whereas the other one is characterised by a high atomic density and a large misfit. An analysis based on the coincidence approach relates the complex structure of this interface to an energetically favourable configuration due to a compromise between elastic strain minimisation and misfit accommodation.