1 ELECTRONIC EFFECTS AT INTERFACES IN Cu - Cr , Mo , W , Ta , Re MULTILAYERS

Interfacial electronic effects between Cu and the transition metals Cr, Mo, W, Ta, Re, are investigated by determining the strength of the white line absorption resonances on the L3,2 edges of Cu in Cu5/TM5 multilayers. X-ray absorption (XAS) was performed to study the white lines, which are directly related to the unoccupied states of Cu in the multilayers. The metallic multilayers are 2 nm in period and 200 nm in total thickness. Each period contains 5 monolayers of Cu and 5 monolayers of the transition metal: ≈ 40% of the atoms are at interfaces. These material pairs form ideal structures for the investigation of interfacial electronic effects as they form no compounds and exhibit terminal solid solubility. Only weak white lines are observed on the L3,2 edges of Cu since all the d-orbitals are filled. In the Cu/TM multilayers, however, we observed enhancement of the Cu white lines. We attribute this to the charge transfer from the “interfacial Cu atoms” d-orbital to the transition metal layers. Analysis of the white line enhancement enables calculation of the charge transfer from the Cu to the transition metal. 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. This charge transfer is consistent with the enhanced absorption energy of Cu on these materials as observed in thermal desorption experiments. INTRODUCTION AND BACKGROUND In this study we characterize electronic effects in short-period (≈20 Å) metallic multilayer films in which 40% of the atoms are at an interface using near-edge (L3,2) x-ray absorption. This study investigates Cu/TM where TM = Cr, Mo, W, Ta, Re. These immiscible elemental pairs are ideal to study as they form no compounds and exhibit terminal solid solubility. An interest in the charge transfer between elements in alloys and compounds has led to studies using x-ray absorption as described above. Near edge x-ray absorption fine structure (NEXAFS), a technique used for analyzing x-ray absorption near the absorption edge of the element, is especially suited to study the amount of unoccupied states in the conduction band of a metal. 1 The d-metals spectra show large peaks at the absorption edges called “white lines.” These are due to the unoccupied d-states just above the Fermi level in these metals. A study 2 of the white lines in the 3d metals show that as the d-band is increasingly occupied the white lines decrease in intensity. Starting with Ti (3d4s), which has an almost empty d-band and shows strong white lines, the white-line intensities decrease across the Periodic Chart to Cu (3d4s), which has a full d-band and no white lines. Systematic measurement of the L3,2 absorption spectra of bulk elemental Cu and Cu in the Cu/TM multilayers enabled measurement of the charge transfer. NEXAFS on metallic multilayers has received less attention than alloys because of the difficulty in synthesizing multilayers with controllability up to the monolayer level and because there is little difference between the signal from the bulk and from longer period (> 30Å) multilayers . For high-quality short period multilayers, however, the difference is clear. This is highlighted in a study of short period Co/Cu multilayers , where the electronic density of states of Cu in Cu/Co greatly differed from that of bulk Cu. The difference was attributed to both charge transfer and band structure changes of the interface atoms. Short period Cu/Fe was the subject of another NEXAFS study , where the signal from a periodic Cu(3 Å)/Fe(10 Å) multilayer was compared with that of a periodic Cu(10 Å)/Fe(3 Å) multilayer. The difference was attributed to the different structure of the Cu in each sample. Cu was BCC in one and was FCC in the other.