Room Temperature Deposition and Island Formation of Ultra Thin Layers of Co on Si(111) Investigated with X-ray Standing Waves

The disappearance of the Si(111) 7X7 LEED pattern upon cobalt deposition (0.25 ML-4 ML) at room temperature indicates an immediate reaction of the Co with the Si surface. The X-ray standing wave analysis yielded low coherent fraction (F ~ 0.1) and a coherent position of P = 0.28 +- 0.1angstroms normal to the Si(111) planes for the range of Co coverage investigated. The coherent position and fraction show clearly that the reacted silicide does not form a 5 or 8-fold interface. Upon annealing, the coherent fraction increases and the coherent position approaches the value, characteristic for a 5 or 8-fold Si(111)-CoSi sub 2 interface (P = 0.93, Z = 2.92angstroms) at about 350C-450C. The fraction, however, still remained lower than 0.7, indicating a defect rich overlayer. For increasing annealing temperatures, islands of increasing thickness form, containing highly perfect silicide.