High Resolution Electron Microscopy Study of Cu-Pd Superlattice

Abstract

Considerable interest has been generated in metallic superlattice structures by the discovery of anomalous variations in their physical properties as a function of layer thickness. We have prepared Cu-Pd superlattice samples consisting of 400 alternating layers of Cu and Pd, and with modulation wavelengths in the range 1.4-4.0 nm by dual electron beam evaporation. in a UHV system. Prior to depositing a superlattice film, a Cu buffer layer with a thickness of 130 nm was deposited onto a cleaved mica substrate. In order to study the growth morphology, crystal structure and atomic ordering near the interface, both plan-view and cross-sectional specimens of a Cu-Pd superlattice film have been studied using an Hitachi H-9000 high resolution electron microscope.Fig. 1 is a typical cross-sectional TEM image of the as deposited Cu-Pd superlattice film, showing the morphology of the growth of the crystallites and the crystal structures of the mica substrate, Cu buffer layer and Cu-Pd superlattice. The TEM image clearly shows a structure consisting of alternating layers of Cu and Pd with the expected thicknesses of approximately 1.15 nm. In this image the light and dark contrast regions are associated with the Cu and Pd layers, respectively. Fig. 2 shows a cross-sectional HREM image of the interface between the mica substrate and the Cu buffer layer. The interface, indicated by arrows, is uneven and the Cu buffer layer contains some textured polycrystalline grains, with a mean diameter of 10 nm. Many defects are observed in the Cu buffer layer. Fig. 3 is a cross-sectional HREM image of the Cu-Pd superlattice, along the [110] orientation, showing clear evidence of epitaxial stacking. The interface between the Cu and Pd layers is not atomically sharp, and the degree of abruptness of the interface between the Cu and Pd layers was within two to three monolayers. The structural coherence of the crystal planes was found to extend over several composition modulation periods, and was limited by the presence of structural defects. Many misfit dislocations were observed at the Cu-Pd interface, as indicated by arrows in Fig. 3, and the mean spacing between misfit dislocation is about 10 nm. Since the mismatch between the lattice constants of Cu and Pd is 7.5%, the multilayer structure must be considerably strained in order to accommodate this mismatch; however a significant dislocation density is also observed. As shown in Fig 4, some highly faulted regions were found within the multilayer structure and contained dislocations, stacking faults, microtwins and four differently oriented domain structures. Both the plan-view and cross-sectional HREM observations of the Cu-Pd superlattice have revealed a columnar mode of growth with a mean crystallite diameter of 100 nm, as shown in Figs. 1, 5 and 6. The crystal structure of the boundary region is quite imperfect and the modulation-layer sequence is vague, which might be caused by interdifusion between the Cu and Pd layers. It is obvious that such a growth morphology results in a structure with planar anisotropy.