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

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-o n-Nb bilayer, Nb-Si-Nb and Si-Nb-Si trilayer, and Nb/Si multilayer structur es. In each case, films having layer thicknesses of 35 Angstrom were deposi ted on float glass and Si(100) substrates under ultrahigh vacuum conditions using an electron beam evaporation technique. EDP determined in as-deposit ed bilayer films shows that the widths of Si-on-Nb and Nb-on-Si interfaces are 20 and 40 Angstrom, respectively. The large difference observed in the widths is attributed to the different growth morphology of 35 Angstrom Nb a nd 35 Angstrom Si single layer films as revealed by atomic force microscopy investigations. In situ dc resistance measurements carried out on 35 Angst rom single layer Nb films during growth show percolation at a thickness muc h less than the layer thickness. In case of as-deposited Nb-Si-Nb trilayer film, EDP shows a width of 21 Angstrom 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 Angstrom, resp ectively. The EDPs obtained from bilayer and trilayer films are used to det ermine layer-by-layer electron density variation in Nb/Si multilayer struct ures. The results corresponding to the as-deposited multilayer structure in dicate that interdiffusion is larger in the bottom layers of the stack. To study the role of kinetic and thermodynamic factors in the interfacial reac tions, the bilayer, trilayer, and multilayer samples were isochronally anne aled in vacuum up to a temperature of 300 degrees C in steps of 50 degrees C for 1 h. EDP of annealed bilayer and trilayer films show an increase in i nterfacial width due to interdiffusion of Nb and Si and samples annealed at 250 and 300 degrees C show Nb-rich and Si-rich intermixed regions. In addi tion to this, plateau regions having an electron density of 1.8 e/Angstrom( 3) are observed in the EDP of Nb-Si-Nb and Si-Nb-Si trilayer structures ann ealed at 300 degrees C which indicates the formation of a Nb3Si phase. Stru ctural parameters obtained from EDP are extended to interpret the results i n 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 dif fraction measurements. Consequently, the multilayer structure is fully dest royed between 250-300 degrees C. (C) 2000 American Institute of Physics. [S 0021-8979(00)00611-3].