Metastable Au-Si alloy formation induced by ion-beam interface mixing

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Abstract
An amorphous Au-Si alloy with composition Au-28·5 at. % Si (Au5Si2) has been formed by inplanting energetic Xe, Ar or Ne ions through a thin layer of Au deposited on a Si substrate. The ion-induced amorphous Au-Si alloy is uniform in composition and in thickness as indicated by 4He+ backscattering measurement. The Au-Si alloys so obtained were studied by resistivity measurements and by transmission electron microscopy and diffraction. Two distinct stages of phase transformation are observed as revealed by a sudden change of resistivity with increasing annealing temperature. In the first stage, a transformation from amorphous to a metastable crystalline Au5Si2 phase occurs in a narrow temperature range around 100°C. The second stage, which appears broader near 180°C, involves the transformation to the equilibrium two-phase state of Au and Si. The kinetics of the amorphous to metastable crystalline transformation has been determined by isothermal annealing over the temperature interval 85-101°C. An apparent activation energy of 1·1 eV (25 kcal) is found for crystallization in this temperature interval. The atomic mechanisms involved in the interface mixing process as to the formation of amorphous phase are discussed. The present results are compared with those obtained previously by rapid quenching techniques and thermal treatment of thin Au films on single-crystal Si substrates.