Structure and stability of cobalt-silicon-germanium thin films

The phase formation and stability of CoSi₂ on strained epitaxial Si_0.80Ge_0.20/Si (0 0 1) thin films has been investigated. Silicide films prepared via direct deposition of cobalt (Co/SiGe), and via co-deposition of silicon and cobalt (Co+2Si/SiGe), were compared. EXAFS, XRD, and sheet-resistance measurements indicated that co-deposited Co+2Si films annealed at 400-700°C exhibit the expected low-resistivity CoSi₂ structure but were susceptible to roughening, pinhole formation, and agglomeration. In contrast, the Co/SiGe structure formed CoSi₂ only after annealing at 700°C and suicide formation was accompanied by Ge segregation in the contact region. In situ RHEED experiments indicated that growth of CoSi₂ co-deposited on SiGe at 400-500°C results in immediate island formation. Template methods, which are often used to enhance the quality of co-deposited Co+2Si/Si structures, did not lead to two-dimensional growth in the Co+2Si/SiGe system. In situ EXAFS measurements of 2 A Co films deposited on SiGe substrates and annealed at 450°C suggested that the failure to achieve two-dimensional growth may be due to preferential bonding of Co to Si atoms at the interface, which prevents the formation of a continuous CoSi₂ template.