Surface Segregation in Cu/Pd and Ni/Pd Observed by Positron Annihilation Induced Auger Electron Spectroscopy

Abstract

The main advantages of positron annihilation-induced Auger-electron spectroscopy (PAES) are the missing secondary electron background in the energy region of the Auger transitions and its extraordinary surface sensitivity for elemental analysis. The variation of the Auger intensities obtained by PAES can be determined quantitatively allowing the observation of changes in the elemental composition of the topmost atomic layer. In the system Cu/Pd, the migration of Cu atoms to the second atomic layer of Pd could be observed in situ for the first time using time-dependent PAES. The characteristic time constant for the segregation process was determined to be 83(13)min. The temperature and orientation dependence of the surface concentration of Ni adatoms deposited on Pd was studied by PAES and X-ray-induced photo electron spectroscopy. A strong tendency for Pd segregation to the surface, that is, the migration of Ni atoms to the second atomic layer, was observed for all samples. The activation temperatures were experimentally determined to 172°C, 261°C, and 326°C for Pd(111), Pd(100), and Pd(110), respectively. As demonstrated, PAES can be applied as a powerful tool for the investigation of various surface specific phenomena such as surface segregation and surface alloying.