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.

Characterization of defects in Si and SiO2−Si using positrons

Influences of point defects on the nonradiative processes in ZnO were studied using steady-state and time-resolved photoluminescence (PL) spectroscopy making a connection with the results of positron annihilation measurement. Free excitonic PL intensity naturally increased with the increase in the nonradiative PL lifetime (τnr). Density or size of Zn vacancies (VZn) decreased and τnr increased with increasing growth temperature in heteroepitaxial films grown on a ScAlMgO4 substrate. Use of homoepitaxial substrate further decreased the VZn density. However, τnr was the shortest for the homoepitaxial film; i.e., no clear dependence was found between τnr and density / size of VZn or positron scattering centers. The results indicated that nonradiative recombination processes are not solely governed by single point defects, but by certain defect species introduced by the presence of VZn such as vacancy complexes.

Correlation between the photoluminescence lifetime and defect density in bulk and epitaxial ZnO

In recent years, there has been a wealth of new science involving low-energy antimatter (i.e., positrons and antiprotons) at energies ranging from 10 to less than 10 eV. Much of this progress has been driven by the development of new plasma-based techniques to accumulate, manipulate and deliver antiparticles for specific applications. This article focuses on the advances made in this area using positrons. However many of the resulting techniques are relevant to antiprotons as well. An overview is presented of relevant theory of single-component plasmas in electromagnetic traps. Methods are described to produce intense sources of positrons and to efficiently slow the typically energetic particles thus produced. Techniques are described to trap positrons efficiently and to cool and compress the resulting positron gases and plasmas. Finally, the procedures developed to deliver tailored pulses and beams (e.g., in intense, short bursts, or as quasi-monoenergetic continuous beams) for specific applications are reviewed. The status of development in specific application areas is also reviewed. One example is the formation of antihydrogen atoms for fundamental physics [e.g., tests of invariance under charge conjugation, parity inversion and time reversal (the CPT theorem), and studies of the interaction of gravity with antimatter]. Other applications discussed include atomic and materials physics studies and study of the electron-positron many-body system, including both classical electron-positron plasmas and the complementary quantum system in the form of Bose-condensed gases of positronium atoms. Areas of future promise are also discussed. The review concludes with a brief summary and a list of outstanding challenges. ∗ Contact information: jrdanielson@ucsd.edu † Contact information: ddubin@ucsd.edu ‡ Contact information: rgreaves@fpsi.edu § Contact information: csurko@ucsd.edu

https://link.aps.org/accepted/10.1103/RevModPhys.87.247

Plasma and trap-based techniques for science with positrons

Nano-scale properties of defects in compound semiconductor surfaces

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.

Intrinsic defects in cubic silicon carbide

We applied slow positrons to the as‐etched GaAs and/or the (NH4)2Sx‐treated GaAs. The results show that a thin oxide film is easily formed on the surface of as‐etched GaAs as soon as the etched surface is exposed to air for several minutes before the measurement. On the other hand, the monolayer of chemisorbed sulfur atoms in the (NH4)2 Sx‐treated GaAs is effective in protecting the clean surface from the adsorption of oxygen atoms. The mean diffusion length of positrons is not affected by the conditions of the surface treatment. This implies that the centers for the trapping of a positron are not created below the free surface by those treatments.

Evidence for the passivation effect in (NH4)2Sx‐treated GaAs observed by slow positrons

Vacancy-type defects in 150-keV P+-implanted SiO2 (43 nm)/Si(100) specimens were studied by a variable-energy positron beam. The results show that the damaged layer induced by P+-implantation extends far beyond the stopping range of phosphorus ions. From isochronal annealing experiments, it was found that oxygen atoms recoiled from the SiO2 layer into the Si substrate, forming stable oxygen-vacancy complexes in the substrate near the interface. The final recovery of vacancy-type defects was observed at 1200°C.

Defect Production in Phosphorus Ion-Implanted SiO2(43 nm)/Si Studied by a Variable-Energy Positron Beam

A high-efficiency positron moderator using electro-polished tungsten meshes

The impurity effects on the creation of Ga vacancies in Si‐doped GaAs grown on a Be‐doped epilayer by molecular‐beam epitaxy were investigated through slow positron beam measurements. Doping of Si impurities enhances the creation of Ga vacancies in GaAs. The experimental results support the theoretical prediction of the creation of Ga vacancies in terms of the change in the Fermi‐level position by the Si doping into GaAs, and also suggest that a Si atom diffuses in GaAs as a neutral complex of SiGa‐VGa rather than that of SiGa‐SiAs. The change in the S parameter distribution at the interface between Si and Be‐doped regions is explained by the Be carryforward phenomenon which occurs during the growth of Si‐doped GaAs on a Be‐doped epilayer.

Impurity effect on the creation of Ga vacancies in a Si-doped layer grown on Be-doped GaAs by molecular-beam epitaxy

The annihilation characteristics of positrons in SiO2 films grown on Si substrates were studied by using monoenergetic positron beams. Doppler broadening profiles of the annihilation radiation and lifetime spectra of positrons were measured as a function of incident positron energy for SiO2 (166 nm)/Si specimens fabricated by thermal oxidation. From the measurements, it was found that about 90% of positrons implanted into the SiO2 film annihilate from positronium (Ps) states. This fact was due to the trapping of positrons by open‐space defects and a resultant enhanced formation of Ps in such regions. For the SiO2 film grown at 650 °C, the lifetime of ortho‐Ps was found to be shorter than that in the film grown at 1000 °C. This result suggests that the volume of open‐space defects in the SiO2 film decreased with decreasing the growth rate of the SiO2 film.

Long Wei

Papers

Evidence for the passivation effect in (NH4)2Sx‐treated GaAs observed by slow positrons

Defect Production in Phosphorus Ion-Implanted SiO2(43 nm)/Si Studied by a Variable-Energy Positron Beam

A high-efficiency positron moderator using electro-polished tungsten meshes

Impurity effect on the creation of Ga vacancies in a Si-doped layer grown on Be-doped GaAs by molecular-beam epitaxy

Positronium formation in SiO2 films grown on Si substrates studied by monoenergetic positron beams