velocity selection mechanism

S. Alexander, R. Bruinsma, R. Hilfer, G. Deutscher, Y. Lereah

Physical Review Letters 60, 1514 (1988)
10.1103/PhysRevLett.60.1514

submitted on
Tuesday, May 26, 1987

We present a theory for a new mode of phase separation discovered recently in thin layers of amorphous Al-Ge alloys. Phase separation and crystallization occurs in colonies developing from Al nuclei. Their growth is controlled by diffusion of atomic Ge inside crystalline Al, and by the nucleation and growth of Ge crystallites on the Al-Ge interface. We find that the growth velocity is constant as a consequence of the interaction between the ramified Al-Ge interface and the smooth boundary of the colony with the amorphous phase. Diffusion occurs only in a narrow strip controlled by a length scale related to the width of the Ge dendrites. Solution of the growth equations leads to a velocity selection mechanism as long as the Ge concentration is above a critical threshold. The basic length scale is argued to be controlled by a competition between nucleation and growth of the Ge crystallites.

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R. Hilfer, S. Alexander, R. Bruinsma

in: Time Dependent Effects in Disordered Materials
edited by: R.Pynn and T. Riste
Plenum Press, New York, 417 (1987)
https://doi.org/10.1007/978-1-4684-7476-3_43
ISBN 978-1-4684-7478-7, ISBN 978-1-4684-7476-3

submitted on
Tuesday, March 31, 1987

Recently Deutscher and Lareah discovered a new mode of phase separation in thin films of Al/Ge alloys. They observe the growth of circular “colonies” whose densely packed appearance has been called “dense branching morphology”. The colonies consist of a highly branched starlike “island” of polycrystalline Ge inside a “lake” of monocrystalline Al which is only slightly larger than the Ge island. Thus the Al forms a thin but essentially uninterrupted rim around the Ge peninsulas. The whole colony is embedded in the amorphous phase having an overall composition of 40 percent Al and 60 percent Ge. As these colonies grow into the metastable amorphous surrounding they preserve their more or less circular shape. This immediately raises the question why on the one hand the Al/Ge-interface shows an instability, while on the other the Al/amorphous boundary does not. We investigate this question first. We then present the theoretical description of the new growth morphology. We outline the solution of our equations and indicate how a unique growth velocity is selected. We finally compare our results with experiment.

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