Defects in heteroepitaxial structures studied with monoenergetic positrons: Large-lattice-mismatch systems Cu/Ag(111) and Ag/Cu(111).

Abstract

In positron-beam experiments the backdiffusion probability of positrons is analyzed as a function of implantation energy to nondestructively extract depth distributions of open-volume defects in the near-surface region. We are reporting on the first quantitative studies of this type for metallic heteroepitaxial structures. Silver and copper layers have been grown on Cu(111) and Ag(111) substrates in situ under ultrahigh-vacuum conditions with an effusion-cell system. The overlayer thickness varies from 250 to 2800 \AA{}. Epitaxial growth was confirmed with low-energy electron diffraction and Auger-electron spectroscopy measurements. Large lattice mismatch between silver and copper is expected to result in a large amount of lattice defects. In Cu/Ag(111) structures a vacancy concentration of the order of 1000 ppm or, alternatively, a dislocation density 1\ifmmode\times\else\texttimes\fi{}${10}^{11}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}$, is observed near the interface. When silver is grown on Cu(111), the corresponding densities are smaller by a factor of 3. As the lattice constant of Cu is smaller than that of Ag, it is more likely that open volume defects exist in the Cu/Ag(111) system. The defect concentration reduces to half in both systems at the distance of \ensuremath{\sim}1000 \AA{}, and the defect density decreases roughly exponentially as a function of distance from the interface.