Temperature-dependent top-layer composition of ultrathin Pd films on Cu(100).

Abstract

Positron-annihilation-induced Auger electron spectroscopy (PAES), electron-induced Auger electron spectroscopy, and low-energy electron diffraction have been used to study the composition and structure of vapor-deposited Pd films on Cu(100). Our results show that the PAES intensity from Pd saturates while that from Cu attenuates to near zero by one monolayer of Pd deposition at 173 K. This is confirmation of the top-layer selectivity of PAES in metal-on-metal systems. Changes in the PAES intensities provide a direct indication of the intermixing of Cu and Pd in the topmost layer as a Pd film deposited at 173 K is warmed to 303 K, and of the formation of a Cu overlayer upon further warming to 423 K. Lack of agreement between model calculations which assume delocalized positron states and measured PAES intensities at 173 K suggests that positrons are localized at sites containing Pd atoms.