Ultrahigh vacuum dc magnetron sputter-deposition of epitaxial Pd(111)/Al2O3(0001) thin films.

ABSTRACT

Pd(111) thin films, ∼245 nm thick, are deposited on Al2O3(0001) substrates at ≈0.5Tm, where Tm is the Pd melting point, by ultrahigh vacuum dc magnetron sputtering of Pd target in pure Ar discharges. Auger electron spectra and low-energy electron diffraction patterns acquired in situ from the as-deposited samples reveal that the surfaces are compositionally pure 111-oriented Pd. Double-axis x-ray diffraction (XRD) ω-2θ scans show only the set of Pd 111 peaks from the film. In triple-axis high-resolution XRD, the full width at half maximum intensity Γω of the Pd 111 ω-rocking curve is 630 arc sec. XRD 111 pole figure obtained from the sample revealed six peaks 60°-apart at a tilt angles corresponding to Pd 111 reflections. XRD ϕ scans show six 60°-rotated 111 peaks of Pd at the same ϕ angles for 11[Formula see text]3 of Al2O3 based on which the epitaxial crystallographic relationships between the film and the substrate are determined as [Formula see text]ǁ[Formula see text] with two in-plane orientations of [Formula see text]ǁ[Formula see text] and [Formula see text]ǁ[Formula see text]. Using triple axis symmetric and asymmetric reciprocal space maps, interplanar spacings of out-of-plane (111) and in-plane (11[Formula see text]) are found to be 0.2242 ± 0.0003 and 0.1591 ± 0.0003 nm, respectively. These values are 0.18% lower than 0.2246 nm for (111) and the same, within the measurement uncertainties, as 0.1588 nm for (11[Formula see text]) calculated from the bulk Pd lattice parameter, suggesting a small out-of-plane compressive strain and an in-plane tensile strain related to the thermal strain upon cooling the sample from the deposition temperature to room temperature. High-resolution cross-sectional transmission electron microscopy coupled with energy dispersive x-ray spectra obtained from the Pd(111)/Al2O3(0001) samples indicate that the Pd-Al2O3 interfaces are essentially atomically abrupt and dislocation-free. These results demonstrate the growth of epitaxial Pd thin films with (111) out-of-plane orientation with low mosaicity on Al2O3(0001).