Thermal stability of amorphous Si-rich W silicide films composed of W-atom-encapsulated Si clusters

We demonstrate the excellent thermal stability of an amorphous film composed of W-atom-encapsulated Si (WSin) clusters with n >= 8, formed by thermal deposition using WF6 and SiH4 gas sources. To determine how the structure of the constituting clusters affects the thermal stability of the film, we also prepared films containing unencapsulated WSin clusters in which the W atom was not fully encapsulated in the Sin cage, with n <= 7. The effect of annealing was investigated by Raman scattering and optical absorption measurements with repeated 10-min isochronal annealing in a N-2 atmosphere at temperatures ranging from 500 to 1100 degrees C. The films, composed of WSin clusters completely encapsulating W atoms with a uniform composition of n = 12, remained in the same amorphous structure up to 1000 degrees C, although partial crystallization of Si began at 1100 degrees C. The stability decreased when the film contained unencapsulated WSin clusters, even with an average film composition of n <= 10; partial crystallizations of Si and WSi2 were observed after annealing at 800 degrees C. Density-functional theory calculations indicate that a structure assembled from three encapsulated WSi12 clusters preserves the bonding topology of the constituting clusters in which Si atoms are strongly bonded, accounting for the high thermal stability of the film composed of encapsulated WSin clusters. Published by AIP Publishing.