PHASE-TRANSITIONS DURING SOLID-STATE FORMATION OF COBALT GERMANIDE BYRAPID THERMAL ANNEALING

Phase transitions that involve solid-state reactions between cobalt an d thin films of germanium have been investigated. Germanides are forme d by reacting Co (300 angstrom thick) with thin layers of Ge (approxim ately 2000 angstrom thick) deposited on silicon substrates. Germanium was deposited on Si by rapid thermal chemical-vapor deposition and cob alt was deposited onto Ge by evaporation. The Co/Ge/Si stacked structu re samples were then rapid thermally annealed at atmospheric pressure in an inert ambient consisting of Ar. Using x-ray-diffraction spectros copy, Co5Ge7 and CoGe2 are identified as the phases which form at 300 and 425-degrees-C respectively. The sheet resistance was found to be a strong function of the annealing temperature and a minimum resistivit y of approximately 35 muOMEGA cm is obtained after annealing at 425-de grees-C. The minimum resistivity material corresponds to the CoGe2 pha se with an orthorhombic crystal structure. Above 600-degrees-C, the re sistivity increases due to an instability of the solid-phase reaction between Co and thin Ge layers deposited on Si. This instability is att ributed to rapid Co diffusion at the temperatures which are required t o form CoGe2 along with structural defects in the Ge layer.