Amorphous phase formation by solid-state reaction between polycrystalline Co thin films and single-crystal GaAs

The formation of an amorphous phase between Co thin films and GaAs substrates by solid-state reaction has been investigated by transmission electron microscopy and Auger electron spectroscopy. The amorphization is ascribed to the fast diffusion of Co in GaAs. The amorphous layer, after a 300 °C, 4.5 h annealing, is measured to be ∼40 nm in thickness and has a broad composition range. Prolonged annealing at 300 °C or heat treatment at 340 °C results in the crystallization of the amorphous phase. This crystallization is found to initiate from the Co/amorphous phase interface and leads to the formation of a metastable, supersaturated phase. Finally, the equilibrium condition is reached by the decomposition of this intermediate phase into CoGa and CoAs.