Search

Results of a calculation of the Doppler broadening of the positron-electron annihilation radiation and positron lifetimes in a large number of elemental defect-free materials are presented. A simple scheme based on the method of superimposed atoms is used for these calculations. Calculated values of the Doppler broadening are compared with experimental data for a number of elemental materials, and qualitative agreement is obtained. These results provide a database which can be used for characterizing materials and identifying impurityvacancy complexes.

Dopants, vacancies, and impurity-vacancy clusters have a substantial impact on the properties of perovskite-type metal oxides (general formula $\mathrm{AB}{\mathrm{O}}_{3}).$ In order to determine synthesis and processing conditions that optimize the desirable properties of these materials a careful study of these defects is required. It is essential to identify the defects and to map the defect densities. Positron annihilation spectroscopy has often been used to identify vacancy-type defects. Calculations of the positron lifetime and Doppler-broadened profiles of the positron-electron annihilation radiation in undoped and doped ${\mathrm{LaCoO}}_{3},$ ${\mathrm{PbTiO}}_{3},$ and ${\mathrm{BaTiO}}_{3}$ are reported, and compared with available experimental data. The results show that these positron techniques are excellent for studying open-volume defects, vacancy-impurity complexes, and for identifying the sublattice occupied by the dopants.

Positron-lifetime and depth-resolved Doppler-broadening spectroscopy were used to investigate vacancies formed in bulk and laser ablated thin film La{sub 1{minus}x}Sr{sub x}CoO{sub 3}. In bulk samples, metal ion vacancies, which are most likely situated in the La sublattice, show a lifetime of 219 ps and a Doppler-broadening {ital S} parameter 1.06 times higher than the bulk. The metal-ion vacancy concentration increases with increasing Sr content. For x{ge}0.3, oxygen vacancies are detected, yielding a positron lifetime of 149 ps, only 10 ps longer than the bulk lifetime; the {ital S} parameter is 4{percent} higher than for the bulk. For a given Sr concentration the films show a higher defect density than the bulk samples. The introduction of oxygen vacancy related defects is observed for increasing Sr doping concentration for both film and bulk samples. In films these defects probably consist of larger clusters and/or form complexes with metal ion vacancies. {copyright} {ital 1999} {ital The American Physical Society}

Aluminum oxide films used for corrosion protection of iron and nickel aluminides were generated by substrate oxidation as well as plasma and physical vapor depositions. The films grown by oxidation were crystalline. The others were amorphous. Defect structures of the films were studied by positron spectroscopy techniques. Lifetimes of the positrons, and Doppler broadening of the $\ensuremath{\gamma}$ photons generated by their annihilation, were measured as functions of the energies with which they were injected. In this manner, densities and sizes of the defects were determined as functions of depths from the outer surfaces of the films. Alumina films generated by oxidation had high densities of open volume defects, mainly consisting of a few aggregated vacancies. In the outer regions of the films the structures of the defects did not depend on substrate compositions. Positron lifetime measurements, and the S and W parameters extracted from Doppler broadening spectra, showed uniform distributions of defects in the crystalline ${\mathrm{Al}}_{2}{\mathrm{O}}_{3}$ films grown on nickel aluminide substrates, but these data indicated intermediate layers of higher defect contents at the film/substrate interfaces of oxides grown on iron aluminide substrates. Amorphous films generated by plasma and physical vapor deposition had much larger open volume defects, which caused the average lifetimes of the injected positrons to be significantly longer. The plasma deposited film exhibited a high density of large cavities.

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. \textcopyright{} 1996 The American Physical Society.

Positron-annihilation spectra are used to identify interface states in a 110-nm-thick, thermally grown (dry, no HCl) SiO{sub 2}/Si(100) system. A normalized shape parameter ({ital S} parameter) is used to characterize the positron-annihilation spectra. The interface-state-density variation under a low-temperature annealing (20 {degree}C--500 {degree}C) is shown to be correlated with the variation in the intensity of the interface shape parameter. Activation and passivation of interface states by atomic hydrogen are demonstrated by repeated {ital in} {ital situ} hydrogen exposure and vacuum anneal. The present study shows that characterizing the interface states with positron-annihilation techniques opens an avenue for studies involving interface states without the need for a gate electrode.

Defects in a silicon-on-insulator structure formed by high-energy (200-keV) oxygen implantation has been studied utilizing a variable-energy positron beam. The positron-based probe is found to be especially sensitive to the condition of the top Si layer. Open-volume defects (cavities) are detected in the top 80-nm Si layer in the as-irradiated state. The majority of these defects are removed by high-temperature annealing (\ensuremath{\sim}1300 \ifmmode^\circ\else\textdegree\fi{}C) after which the positron response correlates with the density of dislocations observed by transmission electron microscopy. Variations in dislocation density across a wafer were probed with positrons, demonstrating the potential of positrons in defect topology.

Variable-energy positron-beam studies have been made on well-annealed polycrystalline Ni samples implanted with 30-, 90-, and 180-keV $^{4}\mathrm{He}$ ions. The positron-annihilation characteristics were measured with a solid-state Ge detector at a number of different incident-positron energies and after isochronal annealing at various temperatures. The Doppler broadening of the annihilation photons was found to be strongly influenced by the $^{4}\mathrm{He}$ implantations. The data indicate that trapping of the positrons occurred predominantly at small He bubbles. The variation of the broadening with incident-positron energy was sensitive to the depth distribution of the traps. A diffusion model assuming a square concentration-defect profile was developed and analytically fitted to the parametrized momentum data. These fitted results were compared to Monte Carlo range calculations for $^{4}\mathrm{He}$ in Ni, and fairly good agreement was found. This investigation demonstrates the capabilities of positron annihilation for nondestructive depth profiling in ion-implanted systems. In addition, it establishes parallels between the trapping behavior of positrons and that reported elsewhere for hydrogen, thereby augmenting the present level of understanding of the technologically important trapping of hydrogen by the bubbles.

Monoenergetic positrons with incident energies of less than 25 eV were employed as a nondestructive surface and near-surface sensitive probe. Positron reflectivity and the positronium emission were measured as a function of incident energy for various oxygen coverages ranging from submonolayer to 3500 A of SiO/sub 2/ on Si. The characteristics of these spectra change systematically with oxidation and are in good agreement with the known development of the surface electronic structure. Different oxidation stages are identified.

Data are presented for the positron diffusion coefficient, ${D}_{+}$, in single crystals of Mo, Nb, Al, and Cd. The data clearly demonstrate that the temperature dependence of ${D}_{+}$ is much stronger than the ${T}^{\mathrm{\ensuremath{-}}0.5}$ that is theoretically predicted, ranging from about ${T}^{\mathrm{\ensuremath{-}}0.8}$ to a maximum of ${T}^{\mathrm{\ensuremath{-}}2.6}$. In addition, our results shown an anisotropy for the temperature dependences of ${D}_{+}$ in two principal directions of Cd (${T}^{\mathrm{\ensuremath{-}}1.0}$ along an a axis versus ${T}^{\mathrm{\ensuremath{-}}2.6}$ along the c axis) which cannot be described by a simple diffusion model.

In a study of neutron-irradiated Mo containing voids, Schultz et al. (Phys. Rev. Lett. 44, 1629 (1980)) presented data which showed a small fraction of positronium atoms being formed in the voids and decaying by three photons. This effect was observable at temperatures above roughly-equal650 K. It was stated at that time that the positronium may be forming through the same type of high-temperature positron desorption mechanism that had previously been observed for various metallic surfaces with the use of low-energy positron beams. In this Brief Report we show that the temperature at which thermal positronium is activated from a clean Mo(110) surface is 1000 K, which decreases significantly with adsorbed oxygen contamination. These results are consistent with the previous conclusions if the Mo voids were covered with oxygen.