A study of amorphous Ti–Ni alloys by X-ray photoelectron spectroscopy

Abstract

Amorphous thin films of Ti–Ni alloys with 48, 50, and 51.5% atomic nickel concentration have been studied by X-ray photoelectron spectroscopy (XPS). The films were prepared by rf magnetron sputtering technique. The 2p core levels and the X-ray excited Auger regions of titanium and nickel were investigated. The aluminum anode was employed as the source of X-ray excitation. The experimental shifts were found to be positive for both titanium and nickel, indicating that the criterion of electronegativity cannot be used to determine the direction of charge transfer in these films. The Auger parameters (APs) were determined from the binding energies (BEs) of the 2p 3/2 core levels and the most prominent peak in the Auger regions. The Auger parameter data show that the metallic nature to the films comes mainly from titanium. The shift in the Auger parameter is found to be positive for titanium and negative for nickel, suggesting a charge transfer from nickel to titanium. The chemical shifts have been calculated by subtracting the relaxation shifts from the experimental core level shifts. These shifts were observed to be negative for titanium and positive for nickel, establishing the direction of charge transfer from nickel to titanium. This direction of charge transfer was also confirmed by calculating the charges on the ionized atoms using a simple electrostatic model. These values are observed to be positive for titanium and negative for nickel, in complete agreement with the earlier conclusions. The data on the Auger parameter, the chemical shifts, and the charge on the ions show similar trend. The Shirely and Tougaard backgrounds under the core level peaks were also estimated. The difference between these backgrounds when normalized with respect to the elemental values provides information on the density of states at the Fermi level. A decrease in the density of states at the Fermi level was observed both for titanium and nickel in the films. This is in agreement with the observed increase in the unoccupied density of states at the Fermi level studied in the earlier investigations.