Nanoscale Au-Si eutectic mixtures formed by dewetting of a Au-Ni film on Si3N4.

Formation of a eutectic AuSi compound was observed during the solid state dewetting and agglomeration in a Ni-Au bilayer thin film grown on a Si3N4 substrate using coherent X-ray diffractive imaging, X-ray diffraction, and scanning electron microscopy. During rapid thermal annealing at 850 °C in vacuum, AuNi films first separate into islands that are composed of a mixture of Au-rich and Ni-rich phases. As the dewetting proceeds, Si and nitrogen dissociate due to the catalytic action of Ni, which resulted in the formation of NiN, NiSi, and AuSi. With increasing time of the annealing process, nitrogen atoms in NiN are gradually evaporated by forming N2. The eutectic phenomenon in AuSi alloys results in the migration of Au atoms to form Au5Si2 with a composition near the eutectic point, 18.6 at% of Si. Our findings indicate that nucleation of the Au5Si2 alloy formation does not primarily occur through bulk interdiffusion, but instead initiates through grain boundary diffusion of Au atoms.