Initial growth of Pd on MgO(0 0 1)

Abstract

Palladium films with nominal thicknesses varying between 0.5 and 40 nm have been grown by electron beam evaporation on polished MgO(0 0 1) wafers at substrate temperatures, T s, of 100, 300 and 600°C. Grazing incidence X-ray diffraction (GIXRD), using a standard laboratory X-ray source, and transmission electron microscopy (TEM) were used to study microstructure, strain, and orientation of the Pd films. GIXRD showed that the films grew epitaxially on MgO(0 0 1). For all temperatures studied, three-dimensional islands were nucleated. The Pd islands started to coalesce at a nominal film thickness ⩽1 nm. For T s=300 and 600°C, the coalescence was liquid-like, and secondary nucleation was observed from nominal film thicknesses of 2.5 nm. Increasing growth temperature resulted in a decreased Pd island surface coverage. Continuous film formation was obtained at thickness 10 and 20 nm for T s=100 and 300°C, respectively. Using GIXRD, epitaxial Pd was detected for films with a nominal thickness ⩾0.5 nm for films grown at 600°C, ⩾1 nm for films grown at 300°C and ⩾2.5 nm for films grown at 100°C. In a few cases, also four 1 1 1-oriented Pd domains rotated 90° with respect to each other were detected using GIXRD. The average Pd island sizes, calculated from 2 θ peak broadening, were comparable to the island sizes seen in TEM. For nominal film thicknesses <2.5 nm, the lateral Pd d 0 2 0 plane distance was expanded ∼0.8% when deposited at 600°C, whereas thicker films showed bulk Pd lattice spacing.