Orientation relationships of Au5Sn/Au interfaces and formation sequence of Au5Sn and AuSn at the Au/Sn interface

•A plane-view thin film method is developed to study the Au5Sn/Au interfaces.•The orientation relationships and planes of the interfaces are determined by TEM.•The Au5Sn/Au interfaces on the Au (001) and (111) surfaces are analyzed.•The Au5Sn/Au and AuSn/Au interfacial energies are compared by structures.•That Au5Sn forms after AuSn is explained by nucleation theory.

The orientation relationships and interfaces between ζ′-Au5Sn and Au have been studied by transmission electron microscopy. Epitaxial Au thin films were grown on the single-crystal NaCl surfaces and Sn was evaporated to form ζ′-Au5Sn. Results showed that on both the (1¯11)Au and (001)Au surfaces the (0001)ζ′ plane is the common interface surface with a common orientation relationship of (1¯100)ζ′//(110)Au, which has a small misfit of 1.9%. The Au atoms have good matches across both interfaces.That δ-AuSn forms before ζ′-Au5Sn in the solid-state Au/Sn interfacial reactions was discussed by heterogeneous nucleation theory. Both phases have good orientation relationships to Au and therefore low interfacial energies. Analysis of the interfaces indicated that the ζ′-Au5Sn/Au interfacial energies should be less than δ-AuSn/Au, which favors the nucleation of ζ′-Au5Sn. However, the energy of formation of δ-AuSn is much larger, about 2.3 times, than that of ζ′-Au5Sn by data from references. Therefore δ-AuSn forms before ζ′-Au5Sn.