Copper structure and L-edge x-ray absorption in copper/X multilayers (X = chromium, molybdenum, tungsten, tantalum, rhenium)

Interfacial electronic effects between Cu and the transition metals Cr, Mo, W, Ta, and Re were investigated by determining the strength of the white line absorption resonances at the L3 and L2 edges of Cu in multilayers, and their structure investigated by x-ray diffraction. Near-Edge X-ray Absorption Fine Structure (NEXAFS) was performed to study the white lines, which are directly related to the unoccupied states of Cu. The metallic multilayers are 20 A in period and 2000 A in total thickness. Each period contains 5 monolayers of Cu and 5 monolayers of the transition metal. These immiscible material pairs are ideal structures to investigate electronic effects at the interface as approximately 40% of the atoms are at interfaces and these materials form no compounds and exhibit terminal solid solubility. In the L-edge NEXAFS of bulk Cu metal, only weak white lines are observed since all the d-orbitals are filled. In the L-edge NEXAFS of Cu in the Cu/X multilayers, however, enhanced Cu white lines are observed. This is attributed to charge transfer from the interfacial Cu d-orbital to the transition metal layers. Analysis of the experimentally observed white line enhancement enabled quantitative calculation of the charge transfer from the Cu to the transition metal. Theoretical analysis of the Cu/Cr multilayer showed an interfacial electron transfer from Cu to Cr in agreement with this interpretation. Cu shows a charge transfer of about 0.03 electrons/interfacial Cu atom in Cu/Cr, 0.064 in Cu/Mo, 0.35 in Cu/Ta, 0.17 in Cu/W, and 0.23 in Cu/Re.