Characterization of lacunar defects by positron annihilation

Among the means of non-destructive study of matter, positron annihilation enables us to probe the electronic structure of materials by measuring annihilation characteristics that depend on the electron density seen by the positron and the momentum distribution of the electrons that annihilate with the positron [1][2][3][4][5][6][7] . The positively charged positron is sensitive to variations in coulombic potential in a material [3] . It preferentially probes remote regions of atomic nuclei constituting potential minima or wells. Defects of the lacunar type, where nuclei are missing, generally form potential wells deep enough in solids to trap the positron up to near-melting temperatures.

• In a defect-free crystal, positron annihilation enables the study of Fermi surfaces [8] . In a crystal with defects, it can be used to determine the free volume of defects – from the atomic scale (lacuna) to the microscopic scale (cavities) – and, depending on the type of solid, metal or insulator, the charge of these defects characterized by the temperature dependence of the trapping [9] . These properties have made it possible to develop positron annihilation as a unique spectroscopy of lacunar-type or acceptor-type defects on the atomic scale [10] .

  Annihilation in lacunar-type defects was demonstrated experimentally in metals and the trapping process modeled in the early 1970s. It was first developed and used in metals. It was then applied to the detection of neutrally or negatively charged defects in semiconductors from the 1980s onwards.

• In molecular solids, and in polymers in particular, positron-electron pairs can exist in either free or bound states, forming an atom known as positronium (Ps). The properties of positronium and its annihilation characteristics in matter constitute a field of study in their own right, but the theoretical bases describing the influence of free volumes are much less advanced than for positrons. Free-volume spectroscopy in such solids is therefore possible from positronium annihilation, but in most cases relies on empirical foundations. For details...