It has been demonstrated theoretically and experimentally that the thickness of a very thin, pure Al film put on top of an amorphous Si (a-Si) layer can be used as a very accurate tool to control the crystallization temperature of a-Si. The effect has been explained quantitatively by application of surface-interface thermodynamics. The predictions have been confirmed experimentally by a real-time in situ spectroscopic ellipsometry investigation of the crystallization temperature of a-Si as a function of the thickness of ultrathin Al layers.

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The diffusion processes and accompanying microstructural changes occurring in amorphous Ge (a-Ge)/crystalline Al bilayers upon annealing have been investigated by using a combined approach of Auger microanalysis, selected-area depth profiling (SA-DPR), scanning electron microscopy (SEM) and X-ray diffraction. Upon annealing at 150• C for 10 min, the partially crystallized Al/a-Ge (with the Al layer at the top) and a-Ge/Al (with the a-Ge layer at the top) specimens show reacted and unreacted regions with a lateral size of ∼1-2 µm at their surfaces. SA-DPR at the location of unreacted regions at the surface shows that at such locations the original Al/a-Ge (or a-Ge/Al) bilayer structure is almost preserved. On the other hand, depth profiling at the location of reacted regions shows that at such locations the original a-Ge sublayer had fully crystallized and contains 10-20 at.% Al, and that in the original Al sublayer, crystalline Ge (c-Ge) had formed at the Al grain boundaries (GBs). These observations could be explained in consequence of the initiation of crystallization of a-Ge at the Al GBs followed by the nucleation of crystallization at the a-Ge/Al interface, as supported by thermodynamic calculations of interface energetics.

Aluminium-induced crystallization (ALIC) of amorphous Si and subsequent layer exchange (ALILE) occur in amorphous-Si/polycrystalline-Al bilayers (a-Si/c-Al) upon annealing at temperatures as low as 165 degrees C and were studied by X-ray diffraction and Auger electron spectroscopic depth profiling. It follows that: (i) nucleation of Si crystallization is initiated at Al grain boundaries and not at the a-Si/c-Al interface; (ii) low-temperature annealing results in a large Si grain size in the continuous c-Si layer produced by ALILE. Thermodynamic model calculations show that: (i) Si can "wet" the Al grain boundaries due to the favourable a-Si/c-Al interface energy (as compared to the Al grain-boundary energy); (ii) the wetting-induced a-Si layer at the Al grain boundary can maintain its amorphous state only up to a critical thickness, beyond which nucleation of Si crystallization takes place; and (iii) a tiny driving force controls the kinetics of the layer exchange.