Hundreds Of Angstroms Research Articles

Structure and topography of molecular assemblies on solid substrates by infrared spectroscopy and scanning tunneling microscopy

Recently we have combined infrared spectroscopy and atomic resolution scanning tunneling microscopy (STM) to probe the local structure and intermolecular arrangement of molecules within thin films. IR spectroscopy provides spatially averaged information about orientation of the molecules with respect to the surface and about intermolecular arrangement within the crystallographic unit cell. STM data yields a local picture of molecular packing within the film. The requirements of an atomically flat (over distances of hundreds of angstroms) conducting substrate for the STM are fulfilled by an epitaxially grown film of gold on a cleaved mica substrate which also provides a good infrared reflective surface, enabling IR and STM measurements on identical samples. Systems investigated include Langmuir-Blodgett films of cadmium arachidate and self-assembled films of octadecyltrichlorosilane.

Evidence of electron induced interfacial defects in electron-gun-deposited insulator InP structures

Using the photoluminescence and capacitance-voltage measurements, it has been shown that low-energy electron beam evaporation of Al2O3 on n-type InP induces defects in the interfacial region of the InP-Al2O3 structure. These defects seem to be located as fixed charge traps in the insulator as well as confined within a few hundreds of angstroms of the InP in the interfacial region. By introducing a metallic grid in the path of the evaporating dielectic flux, the substrate was effectively protected from the electron bombardment which resulted in a major improvement of the final metal-insulator-semiconductor device characteristics.

Critical fluctuations in RbCaF3. I. A high-resolution X-ray scattering study

High-resolution X-ray scattering measurements have shown that the critical fluctuations at the cubic to tetragonal (196 K) structural phase transition in RbCaF3 exist on two length scales. The shorter length scale (some tens to hundreds of angstroms) corresponds to the soft-phonon mode of R25 symmetry and does not diverge at Tc. The longer length scale (some thousands of angstroms) is associated with the presence of defects and diverges at Tc. Experimental studies of the length scale of the critical fluctuations are complicated by the domain structure of the tetragonal phase. A general, cubic-phase Bragg reflection can split into a cluster of up to 15 tetragonal-phase Bragg reflectors. The arrangement of this cluster of Bragg reflections in reciprocal space and the corresponding domain structure in the sample crystal are discussed in detail.

Pigtailed high-finesse tunable fibre Fabry-Perot interferometers with large, medium and small free spectral ranges

We have made prototypes of three kinds of fibre Fabry-Perot (FFP) tunable filters, each of which can cover a separate free spectral range interval as determined by its resonator length. Free spectral ranges from hundreds of angstroms to tens of megahertz can be realised from the appropriate type. We have built FFPs with free spectral ranges of >1000 A, 25 GHz, 5 GHz and 2.5 GHz. Finesse up to 200 has been demonstrated. The devices are piezoelectrically tuned and are coupled to fibres in-line using standard rotary splices or ST connectors.

Metallic Superlattices: The Study of Materials at Length Scales From a Few to Hundreds of Angstroms

AbstractWe present the possibilities that metallic superlattices offer for the study of materials at length scales ranging from a few to hundreds of angstroms. The materials problems being studied span practically all interesting solid-state phenomena, including superconductivity, magnetism, elastic behavior, development of novel materials, diffusion, ion beam mixing, crystallization, and amorphization. Several applications are also being pursued. We present a few examples of problems that can be studied at different length scales. Emphasis is made that for a proper study of materials properties, extensive structural characterization is imperative.

Radiation induced lattice defects in natural zircon (ZrSiO4) observed at atomic resolution

Fission tracks and point defects in natural zircon are directly observed by a 1 MV electron microscope at atomic resolution for three types of samples adjusted to the 100 orientation. Lattice planes intersecting the fission tracks at high angles are distorted in a rather irregular manner over a wide region up to more than 100 A wide. Diameter of the tracks, ranging from 25 A to 40A, is much narrower than those so far reported for the U-doped synthetic zircon (100–200 A), UO2 thin film (100 A), mica (66 A, 240 A) or fluorophlogopite (150 A). The fact that fairly long tracks thousands of angstroms in length are observed in thin 100-oriented sample hundreds of angstroms in thickness and that some of them are nearly parallel to a low index lattice plane suggest a possible occurrence of channelling in the process of track formation. Parallel tracks often observed in chemically etched specimens support the idea of channelling. Slightly bent tracks are sometimes observed. It is concluded from computer simulation that many contrast anomalies of bright and dark spots in the lattice image are due to point defects of vacancies and interstitial atoms, mainly produced by the direct atomic collision with α-particles or by passage of ionizing nuclear particles. Optimum conditions of the observation of point defects with highest contrast are studied. One interstitial Zr atom or one Zr ion vacancy will give very low contrast and will be not detectable unless the crystal is less than two unit cells thick. A pair of Zr ion vacancies, however, yields extended detectable limit of thickness. Some of the observed defects are in good accordance with those simulated.

Indium exodiffusion in annealed GaAs:In crystals

In this work, we report on the effect of annealing on the distribution of indium in the surface region of In-alloyed GaAs crystals. Auger, secondary ion mass spectroscopy, and photoluminescence were used as the analytical tools. It is shown that In migrates and piles up over a depth of several tens or a few hundreds of angstroms from the surface leaving behind an In-depleted region. The In redistribution results in modifications of the band gap in the surface region of annealed GaAs:In substrates; these modifications take place even at temperatures as low as 400 °C.

Concentration Profile of a Dissolved Polymer near the Air-Liquid Interface: X-Ray Fluorescence Study

The x-ray fluorescence, at the near-total external reflection configuration, was employed to study the concentration profile of a dissolved polymer near the liquid-gas interface. The determine experimental concentration profile conformed with a hyperbolic cotangent squared dependence with a concentration excess ratio of about 100:1 of the surface relative to the bulk. This experiment presents of first direct measurement of that kind at the air-liquid interface and demonstrates the effectiveness of this new method to study nondestructively interface structures on the scale of tens to hundreds of angstroms. 17 references, 3 figures.

Chemical and structural aspects of reaction at the Ti/Si interface

Chemical bonding and reactions at the Ti/Si(111) interface have been studied as a function of Ti overlayer thickness and annealing temperature and time. The chemical properties (composition, electronic structure, and reactivity) were observed under ultrahigh-vacuum conditions using photo-emission (ultraviolet and x ray) and Auger electron spectroscopies; the structural properties (phase and microstructure) were investigated by subsequent transmission electron microscopy; sputterdepth profiling was employed to complete the characterization of the material formed by interface reaction. At room temperature the clean Ti/Si interface remains unreacted, i.e., no intermixing of atoms across the interface occurs; this behavior is similar to that of V/Si but in contrast to the more common initial limited reaction (\ensuremath{\sim}10 \AA{} depth) which takes place for other transition-metal/Si interfaces, especially the near-noble metals. At sufficiently high temperatures (\ensuremath{\ge} 600\ifmmode^\circ\else\textdegree\fi{}C), growth of the silicides TiSi and ${\mathrm{TiSi}}_{2}$ takes place over considerable distances (hundreds of angstroms or more) as expected from previous thin-film investigations. However, initial reaction at the interface can occur at temperatures considerably lower (\ensuremath{\sim}300\ifmmode^\circ\else\textdegree\fi{}C), and this reaction can extend over a considerable distance (\ensuremath{\sim}100 \AA{} or more) from the Si interface. This low-temperature intermixing process leads to structural properties quite different from those induced by silicide compound formation at higher temperatures: The reacted material forms Ti-Si mixtures with very small grains (\ensuremath{\sim}10-30 \AA{} in size), if not nearly amorphous material. Finally, if the low-temperature reaction has already progressed considerably, the kinetics of transformation to well-defined larger grain silicide phases is altered, and notably higher reaction temperatures are required to reach the same structural state. These observations suggest the possibility that the phase kinetics and microstructure associated with the Ti/Si interfacial reaction can be altered in the intermediate reaction stage observed at low reaction temperatures.

Nonuniform surface potentials and their observation by surface sensitive techniques

Various models of Schottky barrier formation have been proposed in the last few years which involve metallurgical interactions at the metal–semiconductor interface. Most of these models involve nonuniform lateral variations in the surface potential. For metallic clusters and/or anion clusters, these variations involve a relatively large size scale (tens to hundreds of angstroms). For interface defect formation, the suggestion of cluster formation energy as the driving force for defect formation could also lead to a nonuniform distribution of pinning sites on a similar size scale. We have studied the effects of various spatial distributions of pinning sites (e.g., surface defects, clusters of anions and/or adsorbed metal atoms) and variations of their energy levels upon surface potentials and their depth distribution via a two-dimensional finite difference program that integrates Poisson’s equation. Our results suggest that surface sensitive spectroscopies provide a less than exact measure of pinning levels in many such cases. For example, for 1018 n-GaAs and pinning site separation of 100 Å, our calculations imply a Kelvin probe band-bending result for the surface potential of ≊0.15 V less than the ‘‘pinning’’ value (for pinning values of 0.8 and 0.6). Furthermore, such nonuniform distributions alter the effect of probe depth. The surface potential determined via e.g., UPS, with a 20 Å mean free path, would differ from the ‘‘true’’ surface averaged value by ≊0.09 eV for a uniform 0.8 V surface; while this difference is ≊0.07 for 100 Å separations, and ≊0.05 for 400 Å separations, the ratio of this λ dependent term to the total measured band bending increases as the distance between pinning sites increases. Note that a pinning level 0.8 eV below the conduction band minimum leads (under the assumptions of 100 Å between sites, probe depth ≊20 Å, and 1018 cm−3 bulk doping) to a ‘‘measured’’ surface potential within 0.6 eV of the conduction band minimum. These results should impact the interpretation of surface dependent studies of Schottky barrier formation, especially for specific models of such formation.

Reduction of silicon-aluminum interdiffusion by improved semiconductor surface ordering

Aluminum overlayers on highly ordered single-crystal silicon (100) and (111) surfaces in ultrahigh vacuum exhibit characteristic interface widths less than tens of angstroms at room temperature and hundreds of angstroms at 400 °C—orders of magnitude more abrupt than conventionally prepared Al-Si contacts. We demonstrate that surface disorder plays a critical role in promoting Si diffusion into the Al overlayer.

Effect of surface forces on fluid flow in thin pores

The structure of fluids near solid surfaces differs from that in the bulk [i]. In the case of isotropic simple fluids with a spherically symmetric intermolecular interaction potential, these variations extend to several molecular layers. For anisotropic fluid crystals the surface effect is manifested at distances of the order of several micrometers. Polar associated fluids, including water, occupy an intermediate position. Boundary layers of water with varying structure reach nearly hydrophilic, well-wettable surfaces of dozens, sometimes hundreds of angstrom. A worsening of the wetting leads to a decrease in the width of boundary layers. In the case of hydrophobic surfaces, breakdown of the sticking condtion becomes possible, and we have slipping of water over the solid surface.

Nb<inf>3</inf>(Al.Ge) superconductors prepared by hot-substrate liquid-quenching and subsequent annealing

The A15 Nb 3 (Al.Ge) and Nb 3 Al superconductors with high critical current density J c have been successfully prepared by the liquid-quenching on a hot-substrate and subsequent annealing. The hot-substrate liquid quenching has a higher cooling rate than that of the conventional liquid-quenching techniques and facilitates the formation of completely supersaturated bcc solid solution of Nb 3 (Al 1-x Ge x ) up to x=0.3. The quenched materials are then transformed to the A15 structure by annealing above 750°C. Following the transformation, the critical temperature T c rises above 18 K, exact values depending on the composition and the heat treatment condition. The A15 phase converted from the bcc phase has an extremely small grain size of hundreds of angstroms, resulting in a high J c in high magnetic fields. The J c of the Nb 3 (Al.Ge) is higher than that of the Nb 3 Al probably due to the smaller grain size. J c of 3.5 × 105A/cm2at 4.2 K and 16 T has been achieved in the Nb 3 (Al 0.8 Ge 0.2 sample annealed at 850°C for 7 hr, which is much higher than the highest value measured in the V 3 Ga superconductors. It is demonstrated that this technique of hot-substmate liquid-quenching process is potentially useful for tape conductor fabrication, because the scaling-up is straightforward and the thin copper substrate which strongly adheres to the A15 layer can serve directly as a stabilizing material.

Ellipsometric Studies on Sputter-Damaged Layer in n-InP

Using multiple incident angle ellipsometry, the complex refractive index and the thickness of the damaged layer formed by SiO2 sputtering deposition were determined. An analysis of a least-squares fit of the experimental values Δi and \\varPsii to the theoretical results is carried out in three steps, where Δi and \\varPsii are the phase change and the azimuth, respectively, at an angle of incidence φi. It is found that a point several hundreds of angstroms from the surface is damaged even though the thickness of the deposited film is only some tens of angstroms. The change in the complex refractive index nd–jkd and the thickness td of the damaged layer does not depend on the deposition time, but these quantities increase with increasing sputtering power. Typical values for nd–jkd and td are 3.684–0.389j and 640 Å, respectively at a sputtering power of 300 W.

Studies of the melting of a 2D solid

The authors have made a detailed numerical analysis of neutron diffraction lineshapes across the constant-coverage melting transition of incommensurate monolayers of methane on graphite. They find that in the liquid phase the liquid lineshape is well fitted with a lorentzian structure factor. While the exact nature of the transition very close to Tm cannot be commented upon, the 'freezing' of liquid appears to be initially continuous, with an increase in correlation length to hundreds of Angstroms before the appearance of a solid phase.

IN SITU TEM STUDIES OF FERROELECTRIC PHASE TRANSITIONS IN Ba2NaNb5O15 AND LiTaO3

In this paper we presented our experimental results of in situ TEM observations of ferroelectric phase transitions in BaBa2NaNb5O15 (Tc =550℃) and LiTaO3 (TC = 660℃). At room temperature the domain patterns are of the scale 0.2-1 μm. Increasing the temperature to near Tc, the micro-domains forming quasi-regular arrays of the scale about several hundreds of angstroms have been observed and interpreted according to a model slightly modified from that of Селок. A strong susceptibility to radiation damage is found in LiTaO3 near phase transition temperature which may be connected with soft modes of phase transitions.

Percolative conduction in microemulsion type systems

In the presence of a suitable combination of surfactants (an alkaline metal soap and a medium chain length alcohol, for example), water and oil type organic liquids can form transparent compounds of low viscosity that have been labelled “microemulsions”, with little or no mechanical agitation (spontaneous emulsification) (refs 1–3). Depending on the chemical nature of the surface active agents and the relative constituent proportions, water-in-oil (w/o) or oil-in-water (o/w) systems can be obtained. (Because of the similarity between tertiary solutions of inverted micelles and w/o microemulsions, the term ‘inverted micellar solutions’ has also been suggested4–7.) Microemulsions have been investigated by many scientists interested in liquid state and surface physicochemistry and by many technologists foreseeing numerous applications in industry8,9. It has been suggested9–13 that conductivity and permittivity studies could provide, along with other techniques, valuable information about the structure and phase behaviour of microemulsions which are considered to consist of dispersions between a few tens and a few hundreds of angstroms in diameter globules made up of an inner spherical core surrounded by a concentric shell of mixed surfactant and cosurfactant1–10. The experiments reported here show that the conductive behaviour of certain microemulsion systems can be accounted for using the percolation and effective-medium theories14–16 that have depicted transport properties and continuous metal–non-metal transitions in disordered materials with microscopic inhomogeneities associated with, for example, density, composition15 or bonding configuration17 fluctuations. This result, which could help in understanding the structural behaviour of micro-emulsions, is considered in connection with the postulated existence in liquid systems of equilibrium bicontinuous structures, a state described by Scriven18 as ‘related to ordinary liquids as porous media are to homogeneous solids’.

Analysis of Metal Corrosion

Ion beam and nuclear techniques are well suited for a variety of measurements on chemical elements contained in surface and near-surface regions of a sample and, as such, are a valuable tool for studying the processes by which metals corrode. For metals and corrosion films, the methods are generally nondestructive so that measurements can be interspersed with sample exposures to the corroding environment or with any other treatments or measurements that may be desired. Rutherford backscattering and nuclear resonance reaction techniques can be used to measure impurity depth concentration profiles with depth resolutions of less than 100 angstroms for the topmost layers of a surface and a few hundreds of angstroms for deeper layers. Microbeams can be used to study impurities and compositional variations in the plane of the surface with spatial resolutions of better than 5 microns. Nuclear reaction methods are capable of measuring not only the amount of a given chemical element present but also of specific isotopes of that element. It is possible with these methods therefore to use stable isotope tracers for studying atomic transport and surface reaction mechanisms. The elements important in metal corrosion processes can all be studied with ion beam and nuclear techniques. The reaction to use for a specific analysis problem will depend on the interferences to be expected from the sample, on the sensitivity and accuracy needed, and on the kind of information required.

A mass spectrometric study of the thermal degradation of ultra-thin polymer films

The mass spectrometric method has been used to study the thermal degradation of thin polymer films (PMMA, poly-α-methylstyrene, PS) adsorbed on metal surfaces from dilute solutions. The activation energy of the thermal degradation as a function of the average film thickness has been determined in the range from a few to several hundreds of Angstroms. The reaction at the metal-polymer boundary can result in a reduction radical degradation reactions in isolated macromolecules (PS, poly-α-methylstyrene).

ESCA surface studies of Pb1−xSnxTe devices

ESCA studies have been carried out on a number of Pb1−xSnxTe surfaces with the goal of correlating surface chemistry with surface leakage currents observed in Pb1−xSnxTe infrared detectors. A diode array structure has been developed which permits I–V and ESCA measurements to be carried out on the same surface. It is possible to reprocess this array to change the surface chemistry without altering the bulk semiconductor characteristics. ESCA results have established the existence of thick (hundreds of angstrom) elemental Te0 layers on several chemically processed device surfaces. These surfaces are shown to produce exceptionally low surface leakage currents at 77 K which suggests that Te0 may be a surface passivant for Pb1−xSnxTe. A dimensional analysis model has been developed which seems capable of estimating surface and bulk contributions to leakage currents.