Reactive diffusion in AI/Ge multilayers

Abstract

It may be mentioned in this context that there is no evidence of the hexagonal and orthorhombic phase reported in e-beam annealed amorphous Al-Ge films in the present investigations. It is also interesting to note that the metastable phases are stable only up to a temperature of ~523 K. This may be one of the reasons that earlier investigations have failed to report the presence of these phases. The narrow temperature range over which these phases are produced attests to their metastable nature. In fact, in specimens annealed at temperatures of ~623 K, these phases are completely bypassed. Thus, there exists only a narrow kinetic window available for the formation of these phases. In specimens annealed at temperatures above 573 K, intermixing of the layers is followed by the rapid segregation of Ge to the surface. Eventually, there is a complete phase segregation of Al and Ge consistent with thermodynamic requirements. While the present results are largely consistent with earlier reports on rapidly quenched specimens, they still do not answer the question as to why these phases are at all formed and how they are related to the equilibrium phase diagram. These are rather difficult questions to address. Even so, some progress has however been made by pursuing some of the arguments put forward by T. Laoui et al. [9], who have considered the relationship between the microstructure and phases obtained on rapidly quenched alloys in some detail. In their investigations, these authors find that the formation of the metastable phases occurs easily in compositions rich in ?-Al whereas they are difficult to produce in the ?-Ge rich compositions. Further, they invariably find that the metastable phases are surrounded by the phase rich in ?-Al. This is also the case with ?-rich Ge precipitates which are surrounded by a rim of nearly pure Al. Our own observations also indicate this to be the case. Similarly, they also never find two metastable phases contiguous to each other. In our specimens, this feature cannot be clearly seen because of the refined microstructure present in the films. Based on these observations, it is argued that the ?, ? and the metastable phases cannot be simultaneously in equilibrium.