Thickness Effects in the Reaction of Cobalt with Slicon-Germanium Alloys

Abstract

ABSTRACTThe thickness dependence of the reaction of cobalt with silicon and epitaxial silicongermanium alloys (SiGe) was studied. Cobalt layers 25 Å and thicker deposited on Si(100) and annealed at 600°C formed CoSi2. When the thickness of the Co film was reduced to 10 Å the Co/Si(100) reaction resulted in a mixture of CoSi and CoSi2 even after annealing for 20 minutes at 800°C. The products of the reaction of Co with (100)-oriented Si1−x Gex after annealing at 800°C depended on the thickness of the Co film and the Ge concentration in the SiGe layer. When the thickness of the Co film was below a critical value, the only phase formed during the Co/SiGe reaction was CoSi. A mixture of CoSi and CoSi2 was observed when the thickness of the Co film exceeded the critical value. The critical thickness for CoSi2 nucleation increased superlinearly with Ge concentration in the range 0≤x≤0.25, and did not depend on the doping of the Si(100) substrate or the strain state of the SiGe film. XRD and EXAFS measurements indicated no measurable incorporation of Ge had occurred in either the CoSi or CoSi2. The amount of CoSi2 formed above the critical thickness increased monotonically with the thickness of the as-deposited Co film. The observed thickness effect was attributed to preferential Co–Si bonding in the reaction zone, and was modeled in terms of the energy cost of Ge segregation, which accompanies the formation of CoSi and CoSi2 during the reaction of Co with SiGe.