Positron Annihilation Spectroscopy Techniques Research Articles

Characterization of residual defects in cubic silicon carbide subjected to hot-implantation and subsequent annealing

Defects introduced in epitaxially grown cubic silicon carbide (3C-SiC) by implantation of nitrogen (N2+) and aluminum ions (Al+) at a wide temperature range from room temperature to 1200 °C were studied using electron spin resonance (ESR), photoluminescence (PL), and positron annihilation spectroscopy (PAS). It is found that while hot-implantation reduces paramagnetic defects and improves the crystallinity of implanted layers, it causes the simultaneous formation of vacancy clusters. These results can be explained in terms of the migration and combination of point defects during hot-implantation. The formation and reduction of defects by hot-implantation are discussed in connection with implantation temperature, dose, and ion species. Postimplantation annealing of the defects in hot-implanted 3C-SiC was also examined by the ESR, PL, and PAS technique. The influence of residual defects on the electrical properties of implanted 3C-SiC layers is also discussed.

The anisotropic electron - positron momentum distribution in diamond

We have performed a high-resolution, high-sensitivity study of the electron - positron momentum distribution in diamond, using the two-detector coincidence Doppler broadening technique of positron annihilation spectroscopy. The measurements were carried out with the , and axes each separately aligned collinearly with the detectors so that the anisotropy of the momentum distribution could be mapped out. Small differences in the momentum profiles were enhanced with difference spectra, and compare well with results of a density-functional theory calculation of the electron - positron momentum distribution in the diamond lattice. There has been much debate regarding previously unexplained anomalies in the annihilation characteristics of the bulk positron configuration in diamond. These new results strongly support a model describing the bulk positron as delocalized and inhomogeneous, but with sufficient amplitude at the intra-bond region such that the annihilation characteristics are determined by the bond electron density and momentum distribution.

Intrinsic Defects in Cubic Silicon Carbide

Irradiation of fast particles like 1 MeV electrons and 2 MeV protons was made for single crystalline cubic silicon carbide (3C-SiC) grown epitaxially on Si by chemical vapor deposition in order to introduce point defects in the material. Intrinsic point defects in 3C-SiC have been characterized by electron spin resonance (ESR), positron annihilation spectroscopy (PAS), Hall and photoluminescence (PL) techniques. The structure and annealing behavior of intrinsic defects, e.g. monovacancies at silicon and carbon sublattice sites, are described based on the results obtained by ESR and PAS. The contributions of such point defects to electrical and optical properties of 3C-SiC are discussed using the Hall and PL results, with a brief review of published work.

The effect of light on positron annihilation in natural diamond

The age-momentum correlation (AMOC) technique of positron annihilation spectroscopy has been used to study a gamma-irradiated natural diamond sample. A light-induced reversible change in both the intensity and the lifetime of the trapped positron component in diamond was observed. No signal attributable to positronium was detected in the measurements, supporting the view that positronium formation does not occur in diamond. The measurements demonstrate the sensitivity of positron annihilation to optically active centers in diamond.

Electron-positron momentum density distribution in diamond

We have measured the momentum distribution of the electron-positron annihilating pair in diamond as a function of crystallographic orientation, using the Doppler broadening technique of positron annihilation spectroscopy. A 1% variation in the longitudinal projection of the momentum distribution was observed as the sample was rotated about the 〈110〉 axis. This anisotropic distribution shows an excellent agreement with a theoretical calculation performed based on two component density functional theory in the local density approximation. The results suggest strongly that the bulk positron density, though delocalized and inhomogeneous, has sufficient amplitude at the intra-bond regions such that the annihilation characteristics are dominated by the bond electron density and momentum distribution.

Free volume after cure vs. fractional conversion for a high‐Tg epoxy/amine thermosetting system

AbstractIsothermal glassy‐state properties of cured thermosetting materials pass through maximum and minimum values with increasing fractional chemical conversion. In this work, a diglycidyl ether of bisphenol A (a diepoxide) and trimethylene glycol di‐p‐aminobenzoate (a tetrafunctional aromatic diamine) system was investigated for the purpose of analyzing the complex behavior after cure of the isothermal properties of the glassy state with increasing conversion. The glass transition temperature (Tg) is used as a direct measure of conversion. Dilatometric, differential scanning calorimetry, torsional braid analysis, and positron annihilation spectroscopy techniques were used to monitor the density, Tg, modulus, and free volume of the material after cure with increasing conversion. The specific volume at 25°C after cure passes through a minimum and the modulus passes through a maximum with increasing conversion. The fractional free volume and the average radius of free volume at 25°C after cure pass through minimum values with respect to conversion. The specific volume, modulus, and fractional free volume at 25°C vs. conversion data qualitatively correlate. The anomaly of the increasing specific volume in the glassy state with increasing conversion is thus considered to arise from changes in free volume on a length scale corresponding to angstroms. The increasing free volume with increasing conversion is related to the phenomenon of antiplasticization. © 1995 John Wiley & Sons, Inc.

HighT c superconductivity in YBa2Cu3O7?x studied by PAC and PAS

HighTc superconductivity has been investigated in YBaCuO by both perturbed angular correlation and positron annihilation spectroscopy techniques as a function of temperature from 77 to 300 K. An abrupt change has been observed in the positron lifetime and Doppler broadening and the electric field gradient and its asymmetry parameter acrossTc, indicating a transition of two- to one-dimensional Cu-O-Cu chain structure and a charge transfer from CuO layers to CuO chains. An anomaly of the normal state has been demonstrated around 125 K, which is attributed to the structural instability.

Defects in hard coatings studied by positron annihilation spectroscopy and x-ray diffraction

TiNx (0.5<x<1.1) thin films, grown on stainless steel substrates via reactive sputtering, have been studied using positron annihilation spectroscopy and x-ray diffraction techniques. The deposition parameters investigated include nitrogen partial pressure, deposition power, substrate bias, film thickness, and substrate condition. The synergistic effects of using multiple techniques to characterize the same or similar samples is emphasized.

Positron annihilation spectroscopy of the defect structure of sputter-deposited TiN

Doppler and slow positron annihilation spectroscopy (PAS) techniques were used to investigate defect structures, as a function of composition and deposition sputtering power, in TiNx (0.48 ⩽ x ⩽ 1.11) thin films fabricated by physical vapor deposition methods. The recovery of the defect structures during a 1 h anneal at 900 °C was also studied. The present data, together with previous positron lifetime data (for the same samples), indicate that PAS has the potential to become an extremely useful technique for the characterization of atomic-scale defects in these materials. The conventional bulk PAS techniques yield consistent data if the results are corrected for the substrate contribution to the measured S parameters and the films have thicknesses greater than a few microns. For films with thicknesses on the order of 1 – 2 μm the slow PAS technique must be used.

A positron annihilation spectroscopy study of carbon-epoxy composites

Many positron annihilation spectroscopy (PAS) studies have been carried out during the last several years for characterizing various polymeric materials. In this work, the PAS technique is used to investigate some positron annihilation characteristics of the combination of carbon fibers with epoxy polymers to form carbon-epoxy composites. These composites are receiving wide and increasing application in industrial markets where light-weight materials with high strength and excellent fatigue characteristics are required. We have found that PAS is capable of detecting different concentrations of carbon fiber in these composites and a relationship is shown to exist between fiber fractions found by PAS and apparent volume and weight fractions found by density measurements. Positron annihilation centers have also been found that we believe were created at the carbon-epoxy interface when the composite was prepared. Characterization of this interface is important for the development of composites with improved mechanical properties.