Strain in coherent cobalt silicide islands formed by reactive epitaxy

We have used surface X-ray diffraction (XRD) to measure strain in coherent islands of cobalt silicide formed by depositing 0-5 ML of cobalt on Si(1 1 1)-7 × 7 at various temperatures. Silicide structure is determined from truncation rod scans, while average island dimensions (width and height) and strain are determined from diffraction lineshapes and verified with TEM. At 800°C, the islands are fully coherent B-type CoSi ₂ , approximately 8 nm thick and >300 nm wide, with atomically flat top and bottom surfaces. At 500°C, the islands are CoSi ₂ , approximately 6 nm thick and 35 nm wide. The lattice parameter is ∼50% relaxed, primarily due to strain relief at the island edges. At 300°C and coverage of 2 ML, the islands are ∼3 nm high and ∼3 nm wide and show long-range position correlation due to registry with the substrate. At higher coverage, they are ∼4 nm high and ∼40 nm wide and commensurate, but lack long-range correlation. It appears that the silicide formed at 300°C is pseudomorphic Co ₂ Si-θ for coverage below 2 ML, but is a mixture of CoSi ₂ and metastable CoSi(CsCl) at higher coverage.