Epitaxial Growth of Metastable hcp-Ni and hcp-NiFe Thin Films on Au(100)fcc Single-Crystal Underlayers and Their Structure Characterization

Abstract

Metastable hcp-Ni and hcp-NiFe epitaxial thin films are prepared on Au(100)fcc single-crystal underlayers by molecular beam epitaxy. The epitaxial growth and the transformation from metastable hcp to more stable fcc phase are studied by in-situ reflection high-energy electron diffraction. In an early stage of film growth, hcp(1120) crystal is stabilized through hetero-epitaxial growth. The epitaxial orientation relationship between the film and the underlayer is determined to be hcp(1120)[0001], hcp(1120)[1100] ∥ Au(100)[001]fcc. With increasing the film thickness, the hcp structure starts to transform into fcc structure. High-resolution transmission electron microscopy shows that the film consists of a mixture of hcp and fcc crystals and that a large number of stacking faults exist parallel to the close-packed plane. The results suggest that the hcp structure starts to transform from these stacking faults into fcc structure in the lateral direction by atomic displacement parallel to the hcp(0001) close-packed plane. The crystallographic orientation relationships between the hcp and transformed fcc crystals are determined to be fcc(110)[111], fcc(110)[111] ∥ hcp(1120)[0001] and fcc(110)[112], fcc(110)[112] ∥ hcp(1120)[1100].