Crystallization in Metal-Metalloid Multilayers
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2. EXPERIMENTAL PROCEDURE Multilayers and trilayer thin films were produced by magnetron sputtering at room temperature onto oxidized silicon wafers or slide glasses coated with photoresist. The latter allow easy removal of the film for differential scanning calorimetry (DSC). The former are useful for structural characterization, particularly by transmission electron microscopy (TEM) in both through-foil and cross-section orientations. Besides conventional furnace treatments, the reactions were also followed by in situ TEM including high-resolution observations, from which direct deduction of the atomic diffusion mechanisms is possible.
3. RESULTS AND DISCUSSION The systems chosen for investigation are primarily binary eutectics, with no equilibrium compounds. Thus, although metastable phase formation might occur,
Mat. Res. Soc. Symp. Proc. Vol. 382 ©1995 Materials Research Society
the elements tend not to react with one another chemically. The metalloids are those from group IV in the periodic table: i.e., carbon, silicon and germanium which are all in an amorphous state when sputter-deposited. The metals were selected using a compilation of phase diagrams and are typically fine-grained polycrystalline. As will be seen later, the restriction to eutectics is not completely necesary. 3.1
Metal-Silicon Systems
3.1(a) Aluminum-silicon DSC profiles of Al-Si multilayers, with typical periodicities of about 20 rim, show well-defined exothermic peaks close to 200°C. The exact peak temperature depends on the proportion of metal, decreasing slightly as the metal fraction increases [6], but the heat of reaction is consistently 11 ± 1 kJ mo1- 1 . TEM, and x-ray diffraction, demonstrate that the multilayer structure is destroyed and that an intimate mixture of crystalline silicon and crystalline aluminum is produced. In situ bright field microscopy shows that the silicon nucleates inside the aluminum and grows into, and along, the Al layers. Direct conversion of amorphous to crystalline Si is never seen, which is in accord with the much higher temperature (600'C) required for this reaction. From the in situ recordings, a model was suggested for the transformation (see Fig. 1) which requires diffusion of Si atoms through the solid Al lattice from amorphous to crystalline phases [6]. The silicon growth rate and the activation energy of the behavior are quantitatively consistent with this idea [6].
SFig.
Al
Sa-Si
c-Si
1. Schematic diagram
showing the proposed diffusion mechanism for growth of Si crystals in Al-Si multilayers.
a-Si
3.1(b) Silver-silicon Like Al-Si, Ag-Si is also a simple binary eutectic. In order to effectively study the reaction mechanism, Si/Ag/Si trilayers were made and examined by in situ TEM at high-resolution. The DSC exotherm is at about 410'C and is also the same value as that for silicon crystallization. In the trilayer, it is even more clear that the Si nucleates inside the metal layer, which in turn disintegrates into individual crystals
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migrating apparently through the amorphous Si
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