Phase formation in co-deposited metallic alloy thin films

Abstract

Phase formation in co-deposited thin films of Pd-Rh, Cu-Ag and Cu-Sn has been examined. Depending on the substrate temperature, either equilibrium or non-equilibrium phases are formed. It is postulated that the formation of multiphase structures in co-deposited alloy thin films is controlled by the diffusional breakdown of fully intermixed depositing atoms, so that three kinetic regimes are observed: (i) at low substrate temperatures the surface mobility is insufficient for the decomposition of the fully intermixed depositing atoms and the films contain non-equilibrium single-phase structures; (ii) with increasing substrate temperature decomposition to non-equilibrium two phase structures is observed; and finally (iii) with a further increase in substrate temperature the atomic mobility at the surface is sufficient to allow the full atomic rearrangements necessary for the formation of equilibrium phases. By relating the distance an atom can move on the surface during deposition to substrate temperature and deposition rate, it hau proved possible to account for the temperature ranges of the above deposition regimes. A hypothesis is put forward that relates the non-equilibrium phases formed in the first kinetic regime to free energy against composition diagrams at the temperature of the substrate, and this provides the basis for understanding the complex non-equilibrium phase formation found in the Cu-Sn system.