Positron annihilation spectroscopy (PAS) is a sensitive probe for studying the electronic structure of defects in solids. We show that the high-momentum part of the Doppler-broadened annihilation spectra can be used to distinguish different elements. This is achieved by using a new two-detector coincidence system to examine the line shape variations originating from high-momentum core electrons. Because the core electrons retain their atomic character even when atoms form a solid, these results can be directly compared to simple theoretical predictions. The new approach adds increased elemental specificity to the PAS technique, and is useful in studying the elemental variations around a defect site.[ S0031-9007(96) Positron annihilation spectroscopy (PAS) is a sensitive probe for studying defects in solids [1,2]. The method relies on the propensity of positrons to become localized at open-volume regions of a solid and the emission of annihilation gamma rays that escape the test system without any final state interaction. These gamma rays hold information about the electronic environment around the annihilation site. PAS measurements for defect characterization generally utilize two observables: positron lifetime and the conventional Doppler broadening of the annihilation gamma rays using a single detector. Both of these techniques are not very sensitive to elemental variations around an annihilation site, such as the one occurring when a material is lightly doped with another or when a vacancy is tied with an impurity atom. A third observable, angular correlation of annihilation radiation, can overcome this deficiency. However, this observable is not used routinely in defect spectroscopy owing to the difficulties associated with the low counting rates at many of the existing facilities. Here we present the results from a new two-detector setup that measures the elemental variations around the annihilation site. The new setup improves the peak to background ratio in the annihilation spectrum to ϳ10 5 , and as a result the variations of the Doppler-broadened spectra resulting from annihilations with different core electrons can be mapped. Because the core electrons retain their atomic character even when atoms form a solid, the new results can be easily verified with straightforward theoretical calculations. In the past, Lynn et al. have shown the advantage of using a two-detector setup in a study of thermal generation of vacancies in aluminum [3,4].Upon entering the solid, positrons lose most of their kinetic energy and reach thermal equilibrium with the host material (within about 10 psec). In a crystal, the thermalized positrons experience a periodic repulsive potential that is centered on the ionic cores, and their wave function is confined to the interstitial region. Their subsequent motion is dominated by phonon scattering, and in the absence of an overall electric field in the medium, this motion is nearly an isotropic random walk. Open-volume defects and negative charge centers provide isolated minima in t...

In the past few years, there has been rapid growth in the positron annihilation spectroscopy (PAS) of overlayers, interfaces, and buried regions of semiconductors. There are few other techniques that are as sensitive as PAS to low concentrations of open-volume-type defects. The characteristics of the annihilation gamma rays depend strongly on the local environment of the annihilation sites and are used to probe defect concentrations in a range inaccessible to conventional defect probes, yet which are important in the electrical performance of device structures. We show how PAS can be used as a nondestructive probe to examine defects in technologically important Si-based structures. The discussion will focus on the quality of overlayers, formation and annealing of defects after ion implantation, identification of defect complexes, and evaluation of the distribution of internal electric fields. We describe investigations of the activation energy for the detrapping of hydrogen from SiO2−Si interface trap centers, variations of interface trap density, hole trapping at SiO2−Si interfaces, and radiation damage in SiO2−Si systems. We also briefly summarize the use of PAS in compound semiconductor systems and suggest some future directions.

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The free volume of metallic glasses has a significant effect on atomic relaxation processes, although a detailed understanding of the nature and distribution of free volume sites is currently lacking. Positron annihilation spectroscopy was employed to study free volume in a Zr-Ti-Ni-Cu-Be bulk metallic glass following plastic straining and cathodic charging with atomic hydrogen. Multiple techniques were used to show that strained samples had more open volume, while moderate hydrogen charging resulted in a free volume decrease. It was also shown that the free volume is associated with zirconium and titanium at the expense of nickel, copper, and beryllium. Plastic straining led to a slight chemical reordering.

Reduction of background using a coincidence-detection system in Doppler-broadening spectroscopy of positron-annihilation radiation allows us to examine the contribution of high-momentum core electrons. The contribution is used as a fingerprint to identify chemical variations at a defect site. The technique is applied to study a variety of open volume defects in Si, including decorated vacancies associated with doping.