Evolution of Cubic FeSi 2 in Si upon Thermal Annealing
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Evolution of Cubic FeSi 2 in Si upon Thermal Annealing X.W. Lin*, J. Desimoni+,a), H. Bemas+, Z. Liliental-Weber*, and J. Washburn* *Materials Science Division, Lawrence Berkeley Laboratory, CA 94720 +Centre de Spectrometrie Nuclaire et Spectrometrie de Masse, Bat. 108, 91405 Orsay Campus, France Abstract Cubic FeSi 2 precipitates were produced in Si (001) wafers by Fe implantation at room temperature, followed by ion beam-induced crystallization at 320*C, and their stability upon thermal annealing was examined by transmission electron microscopy. We found that the cubic phase remains relatively stable for a 650 0 C anneal, but the precipitates tend to change from an aligned to a twinned orientation with respect to the Si matrix. For higher temperature (800 and 900*C) anneals, most of the precipitates are transformed into O3-FeSi 2, accompanied by substantial precipitate coarsening. For platelet-shaped precipitates, the coarsening activation energy was determined to be 3.48 eV. Introduction In the early stage of FeSi 2 epitaxy on Si, e.g., the formation of an ultra-thin epilayer (< - 2 nm thick)[1] or a small coherent precipitate (< - 5 nm in diameter)[2, 3] by solid phase epitaxy, FeSi 2 tends to adopt a metastable cubic phase with a lattice parameter nearly identical to that of Si, in order to minimize its interfacial energy with Si. Despite its relatively lower volume free energy, the thermodynamically stable P-FeSi 2 phase is not formed, because it exhibits a relatively poor lattice matching with Si that can significantly raise the total free energy of the system.[3] As FeSi 2 increases in dimension (thickness or diameter), the contribution of interfacial energy becomes less important and a cubic to 13 phase transition is expected.fl] Recently, we have successfully synthesized cubic FeSi 2 precipitates in Si by ion beam-induced epitaxial crystallization (IBIEC).[2, 3] In this work, we use transmission electron microscopy (TEM) to study the stability of the cubic FeSi 2 phase and the kinetics of precipitate coarsening during thermal annealing. The latter is of particular interest for ion beam synthesis of buried semiconducting 1-FeSi 2 layers in Si by high dose implantation, since the formation of a uniformly buried layer generally involves a coarsening and coalescence process.[41 Experimental Procedure The starting material was a piece of Si (001) wafer (p-type with 1 0-cm) that contained cubic FeSi 2 precipitates over a depth of - 150 nm below the surface. It was prepared by the following procedure: The Si wafer was first implanted at room temperature with Fe+ ions of energies 50, 100, and 180 keV to doses of 3.5, 6, and 15x10 15 cm- 2, Mat. Res. Soc. Symp. Proc. Vol. 311. 01993 Materials Research Society
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respectively, in order to produce a broad Fe distribution; the peak concentration was = 2 at.% Fe. Subsequently, the implanted amorphous layer was crystallized at 3201C by an IBIEC process, i.e., by 500-keV Si+ irradiation to a dose of 1.6xl0 17 cm- 2 ; the dose rate was 1 gtA cm- 2 . Finally, a therma
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