Heterogeneous Nucleation of the Amorphous Phase and Dissolution of Nanocrystalline Grains in Bilayer Al-Ge Thin Films

摘要

Solid State Amorphization Reaction (SSAR) was first reported in thin film couples of Au-La by Schwarz et al. (1). Since then, many other systems have been shown to undergo SSAR. Various issues involved in SSAR have been extensively investigated and reviewed (2, 3, 4). The existence of a large negative heat of mixing, anomalous fast diffusion of one component, the requirement of heterogeneous nucleation sites such as grain boundaries are found to be some of the key features of solid state amorphization. While the need for defect sites such as grain boundaries was spectacularly demonstrated by Meng et al. (5), there are reports of both heterogeneous as well homogeneous nucleation of the amorphous phase in different systems (6, 7). Even in reports of heterogeneous nucleation, the lateral spread of the amorphous phase after nucleation at grain boundaries, is seen to be fairly rapid. Farther, the availability of a large number of competing intermetallic compounds,severely.restricts the kinetic conditions for the formation of an amorphous phase. There exists, to quote W L Johnson (2) "a kinetic window of opportunity" for the formation of the amorphous phase, outside which the amorphous phase is totally, bypassed. In the case of Al-Ge, this constraint is not stringent because of reasons mentioned below. The. amorphous layer can therefore grow unimpeded to substantial thicknesses. As a result we could study the growth stages with greater ease. Further evolution of the amorphous phase once formed, involves the long range diffusion of germanium and its precipitation. This aspect has been reported in literature (8, 9), Johnson, in a recent review (2) has "phylogenetically" classified most known SSAR system into four categories: (i) late transition - rare earth couples, (ii) late-transition - early transition systems, (iii) simple metal - transition metal systems and (iv) semiconductor - metal couples. By this classification Al-Ge is the first reported case of SSAR in a germanium-metal system falling under the fourth category. It is a simple eutectic with poor mutual solid solubilities. Further, no stable crystalline compounds form under thermal treatment, other than the solid solutions at the extremities of the phase diagram. These features ensure that the nucleation and growth stages of the amorphous phase can be followed without severe kinetic constraints. We present in this paper evidence of heterogeneous nucleation and growth of the amorphous phase followed by the precipitation of germanium from the amorphous phase in Al-Ge bilayer films.