Extinction Distance Research Articles

On the electron microscope contrast of doped semiconductor layers

Abstract The origin of the electron microscope contrast of doped semiconductor layers viewed in cross-section has been investigated. It has been demonstrated that the built-in elastic displacements associated with misfitting dopant atoms is the principal source of electron scattering contrast when imaging layers of relatively low dopant concentration. In the absence of atomic misfit, any observed contrast must be associated with the difference in atomic scattering factors of the dopant and matrix atoms. Diffraction contrast experiments on substitutionally doped B–Si multilayers indicate an increase in the two-beam normal and anomalous absorption coefficients and extinction distance which are quantitatively shown to be a manifestation of the relatively large atomic misfit between B and Si atoms. Furthermore, the image contrast is a sensitive function of the configuration of the dopant atoms; a reduction in the dopant atom misfit strain via clustering or precipitation will result in lower contrast levels as...

Extinction of tubular premixed laminar flames with complex chemistry

In this paper we study the extinction of tubular premixed methane-air and propane-air flames with detailed chemistry and complex transport. By seeking a similarity solution of the two-dimensional conservation equations in cylindrical coordinates, we are able to model the tubular flame system with a two-point boundary value problem along the radial coordinate. Extinction solutions are obtained by applying arclength continuation methods to the governing equations. Excellent agreement is obtained when the computed extinction distances and the extinction velocity gradients for both methane-air and propane-air systems are compared with the experimental work of Kobayashi and Kitano. Significant differences are obtained, however, when the tubular flame results are compared with corresponding twin counterflow flames in which curvature effects are absent.

Strain contrast of coherent precipitates in bright-field images under zone axis incidence

Abstract It has been proposed that bright-field imaging of coherent spherical precipitates under zone axis incidence would give improved resolution in the strain contrast compared with the weak-beam dark-field imaging. We observed the strain contrast from coherent Co-precipitates in Cu-Co alloys, and simulated the images on the basis of many-beam dynamical electron diffraction, taking into account the elastic anisotropy. Zone axis incidence such as [001] or [001] considerably reduces the effective extinction distance of the transmitted beam, and suppresses the intensity oscillation with foil thickness, in comparison with a conventional two-beam condition. The contrast in the bright-field images is straightforwardly related to the precipitate geometry. Individual precipitates are well resolved in the image when the number density of precipitates is fairly high. It has been demonstrated that the imaging under zone axis incidence is quite useful to understanding of microstructures in phase transformation in ...

Determination of Static Displacements of Atoms by Means of Large-Angle CBED

The extinction distance ξ g for electrons becomes large with the increase in an index of excited reflection g. The quasi-kinematical treatment would be applicable if the crystal thickness t is smaller than ξ g/π . When large-angle convergent electron beam is illuminated on a small area of crstal, many reflections may simultaneously satisfy the Bragg conditions and the thickness of the area is considered to be uniform. We can record the intensities of the many reflections on a negative film. If the crystal is tilted so that the high index reflections may fulfill the Bragg conditions, their intensities are sensitive to atom positions in the crystal. In the present paper we will show our recent study on the determination of the static displacement of Al atom in α Al2O3. Our interest is placed on the applicability and accuracy of the quqsi-kinematical treatment.In αAl2O3 (corundum structure) oxygen atoms ideally form a hexagonal close packed layer and aluminum atoms occupy 2/3 of centers of octahedra formed by oxygen atoms.

Thickness Measurement in the AEM by Macintosh-Based Analysis of Two-Beam Convergent Beam Patterns

A method has been developed that allows on-line measurement of the thickness of crystalline materials in the analytical electron microscope. Two-beam convergent beam electron diffraction (CBED) patterns are digitized from a JEOL 2000FX electron microscope into an Apple Macintosh II microcomputer via a Gatan #673 CCD Video Camera and an Imaging Systems Technology Video 1000 frame-capture board. It is necessary to know the lattice parameters of the sample since measurements are made of the spacing of the diffraction discs in order to calibrate the pattern. The sample thickness is calculated from measurements of the spacings of the fringes that are seen in the diffraction discs. This technique was pioneered by Kelly et al, who used the two-beam dynamic theory of MacGillavry relate the deviation parameter (Si) of the ith fringe from the exact Bragg condition to the specimen thickness (t) with the equation:Where ξg, is the extinction distance for that reflection and ni is an integer.

Analysis by Electron Diffraction of a Rhombohedral Al-Ge Phase

Although we have shown in previous work that large angle CBED (LACBED) patterns excited around zone axes include kinematic intensities suitable for determination of unknown structures, the data collection and processing required is quite considerable, especially for crystals with moderately large unit cells. As a preliminary guide to a structure, we have investigated the utility of approximate structure factors and phases obtained from zero layer reflections excited around a major zone axis, followed by a Fourier or Patterson transform to estimate projected atom positions. Naturally, dynamical effects cannot be avoided but the use of focussed LACBED patterns implies that local perturbations from non-systematic reflections are not an important factor that limit the accuracy of intensity measurements. Furthermore, it is convenient here to separate reflections into two sets, where the majority are essentially two-beam in character, whereas the remainder lie within strong systematic rows. Even within these rows, only the extinction distances of weaker reflections are likely to be seriously perturbed, and therefore affect the accuracy of structure factors.

CBED study of strain in InGaAs/GaAs strained layer superlattices

In this paper, we have studied the strain in InGaAs/GaAs superlattices by using CBED. Some authors have started similar reseach in SLSs and epi-layers. Our samples are grown by MOVCVD on exact [001] GaAs substrates with alternate InGaAs layers and GaAs layers. Plan-viewed specimens are prepared by ion-milling from the back side. All our experiments are carried out in regions without dislocations, which can be considered as free superlattices. Phillips EM 430 is used to take the CBED ZAP at 100kV.Using CBED technique, we obtain distribution of intensity of sidebands in two kinds of SLSs InGaAs(100Å)/GaAs(200Å) (Sample A) and InGaAs(150Å)/GaAs(150Å)(Sample B) all with 6% In content. Intensity of HOLZ lines can be got by kinetic calculation because of large extinction distance of these reflections. ϕg is expressed as folows:

Synchrotron X-radiation plane-wave topography for imaging microdefects in thinned silicon crystals

Synchrotron X-radiation plane-wave topography for thinned silicon crystals is shown to be effective and sensitive for imaging minute strains around microdefects. Visibility of microdefects in crystals thinned down to a thickness (about 100 μm) smaller than several times the extinction distance is compared with that in crystals as thick (0.6 mm for Mo Kα 1) as μ -1 (μ: linear absorption coefficient). Various kinds of microdefects reveal themselves much more in the thinned crystals. Observed microdefects are classified into five morphological categories. One of the observed microdefects has an image extinction rule suggesting a specified strain field around it. The morphology of microdefects is described in detail.

Morphology of Microdefects in As-Grown Thinned Silicon Crystals Observed by Synchrotron X-Radiation Plane-Wave Topography

In order to reveal more minute microdefects in as-grown CZ silicon crystals, thinned crystals as thin as the order of the extinction distance have been successfully observed by ultra plane-wave topography using synchrotron X-radiation. Various kinds of microdefects reveal themselves only in the thinned region. Using thinned crystals for plane-wave topography is essential for the present experiment. Observed morphologies of microdefects are classified into five categories. One of the observed microdefects has an image extinction rule suggesting a specified strain field around it. The morphology of microdefects has been described in detail.

Effect of the Extinction Distance in X-Ray Rocking Curve Analyses of II-VI Compounds

ABSTRACTDouble crystal x-ray rocking curves from single crystal CdTe and CdZnTe of various degrees of crystalline perfection were recorded under diffraction conditions in which the extinction distance varied from under 10 microns to more than 100 microns. In crystals of intermediate quality the rocking curves recorded under conditions of long extinction length showed structure not evident in those recorded under conditions of short extinction length. Integrated reflectivity measurements were performed at both long and short extinction lengths. The results have been interpreted within the framework of a transition from kinematic to dynamical diffraction.

Ultrathin silicon membrane thickness determination by CBED and measurement of plasmon mean free path

The lower limit of accurate thickness measurement using two beam CBED patterns is usually about one extinction distance for the reflection used. We have used (111) symmetric CBED patterns to measure thin silicon membrane thicknesses as low as 130 Å by comparing experimental patterns with computer simulated CBED patterns. The silicon plasmon mean free path was also measured using these thickness values.Symmetric convergent beam diffraction patterns of (111) silicon in the thickness range of 130 Å to 1300 Å were obtained in a VG HB501A STEM with a 5 Å probe (8 mrad convergence angle) scanning a 100 Å × 100 Å area. The sample was made by ion implantation, anodic etching and annealing. Subsequent photolithography and reactive ion etching thinned the specimen down further to as thin as 100 Å. The sample was chemically cleaned by the Shiraki method before being loaded into the microscope.

Convergent-beam electron diffraction determination of the extinction distance of the quasicrystalline icosahedral phase

The convergent-beam electron diffraction (CBED) method for determining the extinction distance ϵ g and the foil thickness t of the crystal has been extended to the case of quasicrystals. The correctness of this extension is justified by the fact that the calculated rocking curves are in good agreement with the experimental CBED patterns.

On the measurement of population density and size of platelets in type I a diamond and its implications for platelet structure models

This paper is concerned with the way that nitrogen impurity in type Ia diamond is distributed between the defects responsible for the principal impurity-dependent infrared absorption maxima in the 7-11 µm wavelength range. The defects involved are the platelets on diamond {100} planes, which produce the B´ absorption peak at 7.3 µm, and the A and B defects identified by their absorption maxima at 7.8 µm and 8.5 µm, respectively. The relation between the strength of the B´ absorption and the platelet area per unit volume measured electron-microscopically is found to be A p /µm -1 = (9.0 ± 2.1) x 10 -3 I (B´)/cm -2 , where A p is the platelet area per unit volume, expressed in square micrometres per cubic micrometre, and I (B´) is the integrated absorption, i. e. the area under the B´ peak, expressed in reciprocal centimetres squared. Features of the experimental methods employed include application of weak-beam microscopy for accurate measurement of platelet size and shape, counting a large sample platelet population in specimen thicknesses accurately determined in terms of extinction distance and measurement of infrared absorption through the same thin slice in which at a closely adjacent point platelet counting was performed. The constraints that the relation between A p and I (B´) imposes on platelet structure models, when taken in conjunction with the correlations between A, B and B´ absorptions established by G. S. Woods ( Proc. R. Soc. Lond. A 407, 219-238 (1986)), are analysed. Currently accepted models of nitrogen-containing A and B defects are assumed, and the analysis presupposes that platelets are formed by the conversion of A defects into platelets and B defects. It is found that conversion of A defects cannot supply sufficient nitrogen to form platelets containing four nitrogen atoms per area unit a 2 0 ( a 0 is the diamond face-centred cubic unit cell edge), but could form platelets with two nitrogen atoms per unit area a 2 0 ;. Another constraint on platelet structure appears out of the analysis when considering the known expansion of the platelet cell by ca . 0.36 a 0 normal to the platelet plane. It is that the atomic packing density in the platelet structure cannot be more than about three-quarters that of the perfect diamond structure, and this restriction applies whether the platelet is nitrogen-free or contains all the nitrogen released from A defects.

Neutron diffraction by perfect crystals excited into mechanical resonance vibrations

Our contribution presents results of investigations concerning neutron diffraction by vibrating perfect crystals of Si and SiO 2. They were excited in longitudinal, flexural, thickness and thickness-shear modes of vibration with resonance frequencies ranging from 1 to 1500 kHz. In this case, when the acoustic wavelength is much larger than the extinction distance the following phenomena caused by the deformation and Doppler shift were observed: broadening of the perfect crystal rocking curve, enhancement of the integrated reflectivity by up to two orders of magnitude and time modulation of the diffracted beam. When neutron time-off-flight through the vibrating crystal is comparable to the vibration period, multiple reflections having influence on the reflectivity take place. While investigating the forbidden reflections in the crystal excited into vibrations, a strong multiple diffraction phenomena resulting in “umweganregung” effect was observed. Also the vibrating crystals as neutron choppers with 1 μs pulses and a repetition rate of up to 100 kHz were treated.

Observation of Pendellösung fringes induced by X-ray resonant scattering

A new type of Pendellösung fringe has been observed which is induced by X-ray resonant scattering very near the absorption edge. Integrated intensities of the 600 reflection from GaAs(100) across the Ga K absorption edge, at which X-ray resonant scattering is expected to be significant, were measured. The integrated intensities showed modulations which were caused by the change of the extinction distance due to X-ray resonant scattering or to the change of anomalous scattering factor.

Near limit flame spread over thick fuels in a concurrent forced flow

The influence of the ambient level of oxygen concentration on the flow assisted flame spread over thick solid fuels and the extinction of the flame is studied by means of numerical modeling. The pyrolysis spread rate decreases with the oxygen concentration, showing qualitative agreement with experimental data. In fact, as the oxygen level decreases, the flame temperature decreases, causing lower heat fluxes at the fuel surface and lower pyrolysis mass rates so that the spread process is slowed. The effects due to finite kinetics are of increasing importance as extinction is approached. These effects appear mainly at the upstream flame leading edge, where the extinction length (distance of the flame leading edge from the edge of the fuel slab) increases. However, the spread process continues, that is, the flame and pyrolysis lengths increase with time, until the pyrolysis spread rate is greater than the upstream extinction rate. Complete extinction occurs when the extinction distance extends to the position of the pyrolysis front.

X-ray and electron powder diffraction data for Y 2BaCuO 5

We report the experimental X-ray diffraction pattern of Y 2BaCuO 5 for ready identification of this phase as a-means of quality control during preparation of YBa 2Cu 3O 7 based ceramic superconductors. The experimental patterns are in close agreement with that calculated from the published single-crystal data. We also calculated the extinction distances for electron diffraction at different voltages and these data are useful for transmission electron microscopy analyses of ceramic superconductors.

Extinction and Stabilization of a Diffusion Flame on a Flat Combustible Surface with Emphasis on Thermal Controlling Mechanisms

Abstract The process of extinction and stabilization of a diffusion flame on a flat combustible surface in a mixed convective, oxidizing, gas flow parallel to the fuel surface is studied theoretically in detail and in a more simplified approach experimentally. The Navier-Stokes elliptic set of conservation equations, including finite rate kinetics, is solved numerically to analyse the structure of the flame near its leading edge and to calculate the extinction or stabilization distance as a function of the velocity and oxygen concentration of the gas flow. A global one-step chemical reaction is used to describe the chemical kinetic mechanisms. The analysis, therefore, emphasizes the thermo-physical mechanisms controlling the extinction process. The calculations are performed for a representative solid combustible (PMMA) and liquid fuel (heptane). The experiments are performed in a small scale combustion tunnel. The extinction distance for a Heptane flame, stabilized on a porous, vertical surface, is measu...

Tem analysis of multiple-energy arsenic implanted and Rta'd silicon

Rapid thermal annealing (RTA) of ion-implanted Si is important for device fabrication. The defect structures of 2.5, 4.0, and 6.0 MeV As-implanted silicon irradiated to fluences of 2E14, 4E14, and 6E14, respectively, have been analyzed by electron diffraction both before and after RTA at 1100°C for 10 seconds. At such high fluences and energies the implanted As ions change the Si from crystalline to amorphous. Three distinct amorphous regions emerge due to the three implantation energies used (Fig. 1). The amorphous regions are separated from each other by crystalline Si (marked L1, L2, and L3 in Fig. 1) which contains a high concentration of small defect clusters. The small defect clusters were similar to what had been determined earlier as being amorphous zones since their contrast was principally of the structure-factor type that arises due to the difference in extinction distance between the matrix and damage regions.

Convergent beam electron diffraction

AbstractIn convergent-beam electron diffraction (CBED) a highly convergent electron beam is focussed on to a small (⩽50 nm) area of the sample. Instead of the diffraction spots that are obtained in the back focal plane of the objective lens with parallel illumination in conventional selected-area electron diffraction, CBED produces discs of intensity. The point group can be determined uniquely from the symmetry within the individual discs and the overall pattern. In order to determine the point group, it is usually necessary to record a number of CBED patterns with the electron beam aligned along different zone axes, but sometimes only one, high-symmetry pattern is required. The positions of reflections in higher-order Laue zones can be used to identify the crystal system and lattice type and to detect the presence of certain glide planes. The repeat along the zone axis that is parallel to the beam can be calculated from the diameters of the Laue zones. Hence the presence ofpolymorphs can be detected. Doubly-diffracted discs in CBED often contain a ‘line of dynamic absence’, the orientation of this line with respect to the symmetry seen in the bright field disc allows the symmetry element responsible for it (glide plane or screw diad) to be identified. This allows 191 of the 230 space groups to be uniquely identified. The measurement of specimen thickness, extinction distance and cell parameters are also briefly discussed.