On the origin of A or B-Type NiSi sub 2 from In-Situ Transmission Electron Microscopy and diffraction during growth.

In-situ transmission electron microscopy and diffraction has been carried out during the growth of NiSi sub 2 on (111) Si by Molecular Beam Epitaxy. The metastable hexagonal phase theta -Ni sub 2 Si is found to grow at ~ 300C on reacting certain room-temperature deposited Ni films. This phase does not occur on (100) Si. Its existence is shown to correlate with the subsequent growth of type-A NiSi sub 2 at 450C. This and observations of the room-temperature structure lead to a simple model for the enigma of A or B-type NiSi sub 2 growth. The free energy of intermetallic chemical phases is radically changed by the large volume fraction of interface in such very thin films, leading to unusual thickness-dependent behavior.