Time resolved study on Co/Ni/a-Si phase transition during isothermal annealing at 400 °C

An approach based on in situ sheet resistance analyses during isothermal annealing processes is proposed to find out the critical stages of Co/Ni phase transition on amorphous silicon. Unlike the case of conventional Co/Si systems, it was found that Co/Ni/Si reaction produces a double-peak in the resistance versus time curve. This behaviour was studied by energy filtered transmission electron microscopy (EFTEM), energy dispersive X-ray spectroscopy (EDS) and selected area electron diffraction (SAED) analyses. It was found that cobalt atoms prefer to diffuse through the grain boundaries of the underlying Ni2Si layer that forms at very low temperature in contact with silicon. The diffusion process stops when cobalt atoms reach the deeper NiSi layer which is located at the interface with the substrate. Finally, CoSi and NiSi phases form separately, and Co(Ni)Si-2 grains nucleate in contact with silicon differently from what is known for thick layer systems.