Microstructural study of epitaxial platinum and permalloy/platinum films grown on (0001) sapphire

This paper reports a detailed transmission electron microscopy study of epi taxial Pt films grown by sputtering on single-crystal sapphire (0001) subst rates. The orientation relationship between the Pt film and the substrate w as determined to be. 111 + parallel to (0001)Al2O3, [110](Pt) parallel to [ 10 (1) over bar0](Al2O3). As a result of this relationship there are two or ientation variants, related to each other by 60 degrees rotation about the [111] axis. Microstructural analysis using rose plots showed a large fracti on (82%) of the grain boundaries oriented parallel to preferred facets alon g [110] directions. Although facet orientations occur every 60 degrees, fac et junctions prefer 120 degrees angles. The films exhibited the mazed bicry stal structure, with larger grains displaying increasingly convoluted shape s, a behaviour that was quantified using the external form factor to descri be grain morphologies. From high-resolution imaging of cross-section sample s, the preferred boundaries were found to be perpendicular to the substrate and were identified as Sigma = 3, {112} symmetrical tilt boundaries. Quant itative analysis of high-resolution micrographs using the geometric phase t echnique shows a rigid shift of 0.3d({111}) parallel to the interface where the boundary is not constrained by the rigid substrate. Lattice images tak en along the sapphire [1 (2) over bar 10] and [10 (1) over bar0] zone axes revealed a sharp interface between the Pt and the sapphire with no intermed iate phases. The possible reasons for twinning in these films are discussed in detail, and it is concluded that the twin variants nucleate at the init ial stages of island growth. Finally, we also report the growth of epitaxia l Ni-Fe films on Pt. It is shown that the Ni-Fe film grows as a bicrystal w ith the twin boundaries correlated to those of Pt. These results clearly sh ow the importance of being able to control the microstructure of the seed l ayer to grow defect-free multilayers.