Study of the Co-Ge/GaAs contact system

The interfacial reactions between thin films of cobalt and germanium and (001)-oriented GaAs substrates in two configurations, Co/Ge/GaAs and Ge/Co/GaAs, were studied. The microstructure and phase formation, as analyzed by transmission electron microscopy, x-ray diffraction, and Auger electron spectroscopy, were correlated with the electrical properties of these contacts, as determined by current-voltage and capacitance-voltage measurements. At low temperatures, 250≤T<325 °C, the only reaction that was monitored was the formation of Co₅Ge₇ at the outer interface, while the GaAs substrate remained intact. The growth of Co₅Ge₇ was diffusion limited with an activation energy of ∼0.7 eV. At the temperature range of 325-400 °C for both metalizations epitaxial Co₂GaAs was formed on top of the GaAs beneath the Co₅Ge₇ layer. For the Ge/Co/GaAs metalization this was accompanied (at 400 °C) by solid-phase epitaxial growth of Ge precipitates on the GaAs surface. Contacts produced in this annealing regime were rectifying with nearly ideal thermionic emission behavior. The Co₂GaAs phase was unstable at higher temperatures (500-600 °C), and the reaction products were two ternary phases, with compositions of Co₂GeGa and Co₂GeAs. These compounds were spatially separated - the Co₂GeGa layer on top of the Co ₂GeAs phase. Contacts produced at the high-temperature regime (>400 °C) had very low effective barriers and on an n⁺ GaAs substrate became ohmic.