Interfacial reactions for the Co/Ge and Ge/Co contact systems on (001)-oriented n-type GaAs substrates were studied with emphasis on Ge-rich stoichiometries-Co:Ge=1:1 and 2:3. Following annealing at temperatures up to 350°C, intermetallic compounds of cobalt and germanium were formed, depending on the Co:Ge atomic ratio, while the inner interface with the GaAs substrate remained intact. At higher temperatures (up to 600°C, the highest temperature used) a limited reaction with the GaAs substrate was detected. This reaction for both configurations was contained near the interface with the substrate, and did not develop with temperature. The extent of reaction decreased with the decrease in the Co:Ge atomic ratio. No reaction could be detected at the GaAs interface when the Co:Ge atomic ratio was 2:3 even at temperatures as high as 600°C (for 30 min). Contacts produced in these systems were rectifying with a nearly ideal thermionic emission behavior. This is in contrast to a case of a more Co-rich composition previously studied in our laboratory where contacts with low barriers and extensive interfacial reactions were observed after annealing at 400°C or higher temperatures.