The earliest stage of phase growth in sharp concentration gradients

Phase nucleation in sharp concentration gradient and the beginning of phase growth is investigated in Ni-Si and Co-Si systems experimentally and by computer simulation. We applied a combination of X-ray diffraction, four-wire resistance, grazing incidence X-ray fluorescence analysis and extended X-ray absorption fine structure spectroscopy in fluorescence detection with X-ray standing waves for the depth profiling of a-Si/Ni/a-Si (and a-Si/Co/a-Si) layers with nanometer resolution. We observed that a mixture of Ni and Si with a 2:1 composition ratio is formed at the interfaces during sample preparation, but its thickness was different at the a-Si/Ni and Ni/a-Si interfaces of opposite stacking. During annealing, the Ni2Si crystalline phase formed, but, surprisingly, we observed that the thicker Ni2Si layer grew faster than the thinner one. Similar peculiar behaviour was observed for the a-Si/Co/a-Si system. To clarify this situation, computer simulations were performed for both systems by using our new conceptual model (Erdélyi et al., 2012). This showed that the chemical thickness of the interfaces and the accelerated diffusion in the intermetallic phase probably play key roles in the earliest stages of growth.