Phase transformations in ion-mixed metastable (GaSb)_{1 -x}(Ge₂)_x semiconducting alloys

Low energy (75-175 eV) Ar⁺ ion bombardment during film deposition has been used to produce well-mixed amorphous GaSb/Ge mixtures which, when annealed, transform first to single phase polycrystalline metastable (GaSb)_1-x(Ge₂)_x alloys before eventually transforming to the equilibrium two-phase state. At 500 °C, for example, the annealing time t_a required for the amorphous to crystalline metastable (ACM) transformation was ∼10 min, while t_a for the crystalline metastable to equilibrium (CME) transformation was >6 h. The exothermic enthalpy of crystallization and the onset temperature of the ACM transition were determined as a function of alloy composition using differential thermal analysis. The thermodynamic data was then used to calculate the surface energy per unit area σ of the amorphous/metastable-crystal interface. σ was found to exhibit a minimum between x=0.3 and 0.4. The driving energy for the transition from the crystalline metastable state to the equilibrium two-phase state was of the order of 0.12 kJ cm^-3 while the activation barrier was ∼19 kJ cm^-3. Thus, the metastable alloys, which had average grain sizes of 100-200 nm and a lattice constant which varied linearly with x, exhibited good thermal and temporal stability.