Yinghua Piao

Extensive Raman spectroscopic investigation of ultrathin Co1−xNixSi2 films grown on Si(100)

Ultrathin silicide films were formed by starting from 1–8 nm thick Co1−xNix (x = 0, 0.25, 0.5, 0.75, and 1) at 350 °C–900 °C. For each composition x, there exists a critical thickness above which the transition temperature from monosilicides CoSi and NiSi to a disilicide-like phase increases with increasing film thickness. Below this thickness, the disilicide phase seems to form without exhibiting the monosilicides within the detection resolution limits of transmission electron microscopy and Raman spectroscopy. Raman spectroscopic analysis seems to indicate that Ni could be dissolved in the CoSi lattice to a certain fraction despite the fact that CoSi and NiSi are distinct with different crystallographic structures. Moreover, the disorder-induced Raman scattering in NiSi2 is found to be enhanced by Co incorporation. The observed annealing behaviors are attributed to variations in free energy change for phase transition caused by differences in metal thickness.

Phase formation and film morphology of ultrathin Co1−xNixSi2 films

The formation of Co1−xNixSi2 films was investigated using Co1−xNix layers (1–8 nm thick, 1 ≥ x ≥ 0) sputter-deposited onto Si(100). The critical Co1−xNix thickness below which Co1−xNixSi2 films directly grow is found to be x-dependent; it increases from 1–2 nm for Co and Co0.75Ni0.25 to 4–6 nm for Co0.5Ni0.5, and from 3–4 nm for Ni to 6–8 nm for Co0.25Ni0.75. The Co1−xNixSi2 growth tends to occur at lower temperatures with decreasing t and/or increasing x. Although ultrathin, the Co1−xNixSi2 films can remain morphologically stable at 900 °C. Entropy of mixing coupled with lattice matching is discussed as being responsible for the enhanced Co1−xNixSi2 growth and stability.

Formations and morphological stabilities of ultrathin CoSi2 films

In this paper we investigate the formations and morphological stabilities of Co-silicide films using 1-8-nm thick Co layers sputter-deposited on silicon (100) substrates. These ultrathin Co-silicide films are formed via solid-state reaction of the deposited Co films with Si substrate at annealing temperatures from 450 °C to 850 °C. For a Co layer with a thickness no larger than 1 nm, epitaxially aligned CoSi2 films readily grow on silicon (100) substrate and exhibit good morphological stabilities up to 600 °C. For a Co layer thicker than 1 nm, polycrystalline CoSi and CoSi2 films are observed. The critical thickness below which epitaxially aligned CoSi2 film prevails is smaller than the reported critical thickness of the Ni layer for epitaxial alignment of NiSi2 on silicon (100) substrate. The larger lattice mismatch between the CoSi2 film and the silicon substrate is the root cause for the smaller critical thickness of the Co layer.