Positron annihilation in Cr, Cu, and Au layers embedded in Al and quantum confinement of positrons in Au clusters

Defect-sensitive and element-selective measurements on ultrathin chrome, copper, and gold layers embedded in aluminium are presented using coincident Doppler broadening spectroscopy (CDBS) with a monoenergetic positron beam. The amounts of positrons implanted in the layers of different thicknesses are calculated and compared with the experimentally gained fractions of positrons annihilating in the buried layers. A high sensitivity was already reached at an Au layer of only 2 nm thickness embedded below 200 nm Al, which was attributed to the highly efficient positron trapping in the Au layer and Au clusters. An implantation and diffusion model describes this high sensitivity for positron trapping layers. A quantum-well model of the positron wave function limits the trapping to gold clusters of a radius larger than 0.23 nm. This result was confirmed experimentally and validated with complementary TEM measurements.