Positron annihilation in near-surface regions and layered structures

Positrons are very sensitive probes for vacancy-type defects of atomic dime nsions, e.g. vacancies, vacancy agglomerates, dislocations or inner surface s. It is well established that positrons can be trapped at these defects an d, because of the locally reduced electron density, the lifetime of the pos itron localized at the defect increases. This lifetime has characteristic v alues for each defect type and therefore it is possible to separate out var ious atomic defect con figurations and their relative abundance with very h igh sensitivity ( similar to 1 ppm) and in a nondestructive way. With a pul sed positron beam ( similar to 3 mm diameter) of variable energy, lifetime studies can be performed as a function of the positron energy and hence the penetrations depth. Results on silicon and silicon carbide subjected to di fferent treatments will be discussed. For many applications in materials sc ience a positron beam in the micrometer range is desirable. This leads to a scanning positron microscope consisting of a pulsed positron beam of micro meter dimension with a scanning facility. The design and the performance of this first system of its kind will be presented. (C) 2000 Elsevier Science Ltd. All rights reserved.