Stabilisation and phase transformation of hexagonal rare-earth silicides on Si(111)

Epitaxial, hexagonal rare-earth silicides, such as ErSi1.7, can be formed u sing channeled ion beam synthesis. In the case of Gd-silicide, an orthorhom bic GdSi2 phase exists at high temperature; the transition temperature is r elated to the thickness and crystalline quality of the silicide. In the cas e of the lightest rare-earth metals, such as Nd, silicides only exist in a tetragonal or orthorhombic phase, which cannot grow epitaxially on Si(111). However, introduction of a fraction of yttrium (YSi1.7 also possesses the aforementioned hexagonal lattice) drives the Nd-Si system into a hexagonal lattice structure. A combined backscattering and channeling spectrometry (R BS/C), X-ray diffraction (XRD) and transmission electron microscopy (TEM) s tudy shows that an epitaxial, continuous ternary silicide is formed land no t a mixture of binaries) with a hexagonal structure, which is stable up to 950 degrees C. Further annealing, however, results in a gradual transformat ion into polycrystalline phases. The experimental results are compared to t otal energy calculations of these (meta-)stable rare-earth silicides, using the density functional theory (DFT). (C) 1999 Elsevier Science B.V. All ri ghts reserved.