Electron density fluctuations at interfaces in Nb/Si bilayer, trilayer, and multilayer films: An x-ray reflectivity study

Abstract

A grazing incidence x-ray reflectivity technique has been used to determine electron density profile (EDP) as a function of depth in Nb-on-Si and Si-on-Nb bilayer, Nb–Si–Nb and Si–Nb–Si trilayer, and Nb/Si multilayer structures. In each case, films having layer thicknesses of 35 Å were deposited on float glass and Si(100) substrates under ultrahigh vacuum conditions using an electron beam evaporation technique. EDP determined in as-deposited bilayer films shows that the widths of Si-on-Nb and Nb-on-Si interfaces are 20 and 40 Å, respectively. The large difference observed in the widths is attributed to the different growth morphology of 35 Å Nb and 35 Å Si single layer films as revealed by atomic force microscopy investigations. In situ dc resistance measurements carried out on 35 Å single layer Nb films during growth show percolation at a thickness much less than the layer thickness. In case of as-deposited Nb–Si–Nb trilayer film, EDP shows a width of 21 Å at both the interfaces viz. Si-on-Nb and Nb-on-Si whereas in the case of as-deposited Si–Nb–Si trilayer films, the widths of Si-on-Nb and Nb-on-Si interfaces are 21 and 42 Å, respectively. The EDPs obtained from bilayer and trilayer films are used to determine layer-by-layer electron density variation in Nb/Si multilayer structures. The results corresponding to the as-deposited multilayer structure indicate that interdiffusion is larger in the bottom layers of the stack. To study the role of kinetic and thermodynamic factors in the interfacial reactions, the bilayer, trilayer, and multilayer samples were isochronally annealed in vacuum up to a temperature of 300 °C in steps of 50 °C for 1 h. EDP of annealed bilayer and trilayer films show an increase in interfacial width due to interdiffusion of Nb and Si and samples annealed at 250 and 300 °C show Nb-rich and Si-rich intermixed regions. In addition to this, plateau regions having an electron density of 1.8 e/Å3 are observed in the EDP of Nb–Si–Nb and Si–Nb–Si trilayer structures annealed at 300 °C which indicates the formation of a Nb3Si phase. Structural parameters obtained from EDP are extended to interpret the results in as-deposited and annealed multilayer structures. The observed contraction in a bilayer period of an annealed multilayer structure is interpreted in terms of formation of a dense Nb3Si phase confirmed by wide angle x-ray diffraction measurements. Consequently, the multilayer structure is fully destroyed between 250–300 °C.