Low-temperature solid-state reactions between Ni and Si were studied using in situ
transmission electron microscopy (TEM). In the experiments thin amorphous silicon
(a-Si) films were laid on Ni micro-grids and heated up to 973 K. In our approach the
supporting Ni-grid serves as an unlimited source of nickel to successively form the
whole range of Ni-silicide phases while diffusing into amorphous silicon. Unlike other
thin film experiments where Ni and Si are layered on top of each other, our arrangement
enables lateral diffusion of Ni along the Si layer and therefore enables the formation
and study of successive Ni-Si phases side by side. That allowed us to observe in situ
α-NiSi2 as the first reaction product, in contrast to most studies that had reported
either δ-Ni2Si or θ-Ni2Si as the first phase to form. α-NiSi2 was continuously present
at the reaction front propagating into the a-Si film. The phase sequence followed
the increasing Ni concentration from a-Si towards the Ni-grid: α-NiSi2, NiSi, Ni3Si2,
δ-Ni2Si, γ-Ni31Si12 and Ni3Si. Almost all known Ni-silicide phases were found to form
at relatively low temperatures except the θ-Ni2Si, β-NiSi2 and β3-Ni3Si. The dominant
phase was γ-Ni31Si12 which appeared in three structural modifications, differing in
lattice periodicity along the c-axis. The periodicity of the basic γ-Ni31Si12 structure
along the c-axis is ~12 Å (c0 = 12.288 Å) and that of the other two modifications
were ~18 Å and ~36 Å, denoted by S12, S18 and S36 respectively. Of the three, only
S12 has a structural model, S18 had been previously observed by Chen, but S36 had
not been documented in previous works. During our in situ heating experiments, in
addition to the Ni-silicide layer formation a new phenomenon was observed, namely
the appearance, growth and transformation of Ni-silicide whiskers which was attributed
to the accumulation of compressive stress in the thin layer.
