In situ TEM study of Ni-silicides formation up to 973K

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 alpha-NiSi2 as the first reaction product, in contrast to most studies that had reported either delta-Ni2Si or theta-Ni2Si as the first phase to form. alpha-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: alpha-NiSi2, NiSi, Ni3Si2, delta-Ni2Si, gamma-Ni31Si12 and Ni3Si. Almost all known Ni-silicide phases were found to form at relatively low temperatures except the theta-Ni2Si, beta-NiSi2 and beta(3)-Ni3Si. The dominant phase was gamma-Ni31Si12 which appeared in three structural modifications, differing in lattice periodicity along the c-axis. The periodicity of the basic gamma-Ni31Si12 structure along the c-axis is similar to 12 angstrom (c(0) = 12.288 angstrom) and that of the other two modifications were similar to 18 angstrom and similar to 36 angstrom, 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. (c) 2022 Elsevier B.V. All rights reserved.