Positron Diffusion and Surface Interactions in Solids

摘要

Positron diffusion in solids and interactions at solid surfaces have beenexperimentally studied using positron beams. The positron diffusion coefficients have been extracted at 20-1400 K in metals and at 130-1000 K in semiconductors. The theoretical prediction that positron diffusion is limited by scattering from longitudinal acoustic phonons is experimentally verified. In Si also optical phonons contribute to positron scattering at high temperatures and in GaAs polar-optical phonons are as important as acoustic phonons. The effect of ionized impurity scattering is demonstrated in heavily doped Si. Positron emission and Ps formation at metal surfaces are investigated. At clean negative workfunction surfaces the yields are strongly depleted at low temperatures, in accordance with wave mechanical reflection of the positron from the surface barrier. The elemental composition of the Ge surface was monitored using PAES and better surface sensitivity than by conventional electron-impact-induced Auger electron spectroscopy was attained. The positron beam technique was applied to study defects in epitaxial GaAs layers on Si. Both vacancy and Si clusters are detected and their relations to the dislocation density and Si diffusion are established.