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Most applications of positron beams require knowledge of the implantation characteristics for an appropriate interpretation of the experimental data. In this work, the median implantation depth as a function of implantation energy, z 1/2 (E), of 3-18 keV positrons and their implantation profile P(z,E) in a total of 13 thin films of atactic polystyrene, poly͑styrene-co-acrylonitrile͒, and polymethylmethacrylate spin coated onto a silicon substrate were determined from positron lifetime measurements using a pulsed, low-energy positron beam. z 1/2 (E) and P(z,E) were determined from the measurement of the ortho-positronium yield obtained from the intensity I 3 of the long lifetime. z 1/2 (E) was parametrized with the commonly used power-law fit z 1/2 (E) ϭ(A/)E n , with and E in units of g cm Ϫ3 and keV, respectively, yielding Aϭ2.81(Ϯ0.2) g cm Ϫ2 and n ϭ1.71(Ϯ0.05). Excellent agreement between amorphous polymer and literature data on Al and Cu suggests that the median implantation depth of positrons for low-to medium-Z materials in the studied energy range is independent of structure and only a function of mass density. Fitting of the Makhovian implantation profile to the experimental data suggested that the value of the parameter m varies between 1.7 and 2.3, systematically increasing with z at constant implantation energy, but is independent of the implantation energy. Using an equation proposed by Baker et al., the experimental data of 12 of the 13 studied polymer films could be described with a slightly better agreement than the Makhovian equation.

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Vacancy-type defects in Si-doped InN grown by plasma-assisted molecular-beam epitaxy probed using monoenergetic positron beamsWe have used a low-energy positron beam to identify In vacancies in InN layers grown on Al 2 O 3 by molecular beam epitaxy. Their concentration decreases from ϳ5ϫ10 18 to below 10 16 cm Ϫ3 with increasing layer thickness ͑120-800 nm͒. The decrease in the vacancy concentration coincides with the increase in the electron Hall mobility, suggesting that In vacancies act as electron scattering centers.

First lifetime results obtained with a scanning positron microscope will be presented. A pulsed positron beam with a variable energy from 0.5 to 20 keV, with a spot diameter of 2 microm, can be electronically scanned over an area of 0.6x0.6 mm(2). This beam is formed after a double-stage stochastic cooling (moderation) of positrons emitted from a radioactive isotope. Included in the system is a conventional scanning electron microprobe for surface analysis. Three-dimensional positron lifetime spectra of a GaAs sample with a small surface scratch reveal the range due to the mechanical damage.