Formation of Cu x Au1- x phases by cold homogenization of Au/Cu nanocrystalline thin films.

Alona Tynkova,Gabor L Katona,Dezso L Beke,Sergey I Sidorenko,Svetlana M Voloshko,Gabor A Langer

doi:10.3762/bjnano.5.162

Abstract

It is shown, by using depth profiling with a secondary neutral mass spectrometer and structure investigations by XRD and TEM, that at low temperatures, at which the bulk diffusion is frozen, a complete homogenization can take place in the Cu/Au thin film system, which leads to formation of intermetallic phases. Different compounds can be formed depending on the initial thickness ratio. The process starts with grain boundary interdiffusion, which is followed by a formation of reaction layers at the grain boundaries that leads to the motion of the newly formed interfaces perpendicular to the grain boundary plane. Finally, the homogenization finishes when all the pure components have been consumed. The process is asymmetric: It is faster in the Au layer. In Au(25nm)/Cu(50nm) samples the final state is the ordered AuCu3 phase. Decrease of the film thicknesses, as expected, results in the acceleration of the process. It is also illustrated that changing the thickness ratio either a mixture of Cu-rich AuCu and AuCu3 phases (in Au(25nm)/Cu(25nm) sample), or a mixture of disordered Cu- as well as Au-rich solid solutions (in Au(25nm)/Cu(12nm) sample) can be produced. By using a simple model the interface velocity in both the Cu and Au layers were estimated from the linear increase of the average composition and its value is about two orders of magnitude larger in Au (ca. 10−11 m/s) than in Cu (ca. 10−13 m/s).

Highlights

Solid-state reactions in nanostructured thin film systems are interesting and challenging from the point of view of pure fundamental research, but are important for technological applications
By using depth profiling with a secondary neutral mass spectrometer and structure investigations by XRD and TEM, that at low temperatures, at which the bulk diffusion is frozen, a complete homogenization can take place in the Cu/Au thin film system, which leads to formation of intermetallic phases
The process starts with grain boundary interdiffusion, which is followed by a formation of reaction layers at the grain boundaries that leads to the motion of the newly formed interfaces perpendicular to the grain boundary plane

Summary

Introduction

Solid-state reactions in nanostructured thin film systems are interesting and challenging from the point of view of pure fundamental research, but are important for technological applications. In addition the GB diffusion coefficients can cover a range of several orders of magnitude, depending on the type of GB structure (low or high angle GBs [5], triple junctions [6]) It can be observed at very low temperatures that the morphology of the formation and the growth of the new phase(s) can be different from the usual planar growth of a reaction layer [7,8,9]: The new phase(s) can be formed at grain boundaries, GBs, and can grow further by the motion of the new interfaces perpendicular to the original GB plane [10]

Results

Discussion

Conclusion