Effect of bilayer geometry on the diffusion of Ni in amorphous Si and the consequent growth of silicides

The deposition and annealing of nickel-amorphous silicon (a-Si) bilayer thin films on fused silica substrates is reported. Two bilayer geometries that consisted of an a-Si layer and the Ni layer over and below it, respectively, are investigated. The bilayers were deposited at temperatures between 200 and 550 degrees C, and in each case, postdeposition annealed in vacuum at the temperature of deposition to study the effect of geometry on the diffusion process. The diffusion process is illustrated using cross-sectional scanning electron microscopy in conjunction with energy dispersive spectrometry. The study reveals that, independent of geometry, the top surface of Ni/Si bilayers is abundant in Ni after annealing. In the geometry when Ni is at the top, x-ray diffraction and Raman spectroscopy studies reveal that, at temperatures <400 degrees C, there is no reaction between the Ni and the Si. The Ni-Si reaction occurs at temperatures >400 degrees C to form NiSi2. However, when Ni is at the bottom of the Si layer at a temperature of 200 degrees C itself, a nickel rich silicide Ni31Si12 forms. This is in addition to nanocrystallization of Ni. In the case when Nickel is at the bottom, most of the energy is taken up in diffusion and crystallization of Ni. The remaining energy is then used to react with the Si, leading to the formation of Ni31Si12 and its crystallization. At temperatures >500 degrees C, Ni reacts with Si to form NiSi2. When Ni is at the top, since it is already at its preferred location most of the energy is used up in crystallization of Ni and reaction of Ni with Si and crystallization of NiSi2. Importantly, in both cases, the energy is insufficient to cause crystallization of Si. (C) 2012 American Vacuum Society. [http://dx.doi.org/10.1116/1.4757134]