Structure and stability of cobalt-silicon-germanium thin films

The phase formation and stability of CoSi ₂ 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 CoSi ₂ structure but were susceptible to roughening, pinhole formation, and agglomeration. In contrast, the {Co}/{SiGe} structure formed CoSi ₂ only after annealing at 700°C and silicide formation was accompanied by Ge segregation in the contact region. In situ RHEED experiments indicated that growth of CoSi ₂ 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 Å 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 CoSi ₂ template.